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Clinical and Experimental Applications ofSodium PhenylbutyrateTommaso Iannitti

1 and Beniamino Palmieri2

1 Department of Biological and Biomedical Sciences, Glasgow Caledonian University, Glasgow, UK

2 Department of General Surgery and Surgical Specialties, University of Modena and Reggio Emilia

Medical School, Surgical Clinic, Modena, Italy

Contents

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

2. Review Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

3. Phenylbutyrate and Cancer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229

4. Motor Neuron Disorders. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

4.1 Phenylbutyrate and Spinal Muscular Atrophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235

4.2 Phenylbutyrate and Amyotrophic Lateral Sclerosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236

4.3 Phenylbutyrate and Ischemia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

4.4 Phenylbutyrate and Urea Cycle Disorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238

4.5 Sickle Cell Disease and Thalassemias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2394.6 Phenylbutyrate and Cystic Fibrosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240

4.7 Phenylbutyrate and Huntington Disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

5. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243

Abstract Histone acetyltransferase and histone deacetylase are enzymes responsiblefor histone acetylation and deacetylation, respectively, in which the histonesare acetylated and deacetylated on lysine residues in the N-terminal tail andon the surface of the nucleosome core. These processes are considered themost important epigenetic mechanisms for remodeling the chromatin struc-ture and controlling the gene expression. Histone acetylation is associatedwith gene activation. Sodium phenylbutyrate is a histone deacetylase in-hibitor that has been approved for treatement of urea cycle disorders and isunder investigation in cancer, hemoglobinopathies, motor neuron diseases,and cystic fibrosis clinical trials. Due to its characteristics, not only of histonedeacetylase inhibitor, but also of ammonia sink and chemical chaperone, theinterest towards this molecule is growing worldwide. This review aims toupdate the current literature, involving the use of sodium phenylbutyrate inexperimental studies and clinical trials.

1. Introduction

The developmental biologist Conrad H.Waddington (190575) is considered to have

coined the term epigenetics in 1942[1] when

he defined it as the branch of biology whichstudies the causal interactions between genes andtheir products which bring the phenotype into

REVIEW ARTICLEDrugs R D 2011; 11 (3): 227-249

1179-6901/11/0003-0227

2011 Iannitti & Palmieri, publisher and licensee Adis Data Information BV. This is an open access article published under

the terms of the Creative Commons License Attribution-NonCommercial-NoDerivative 3.0(http://creativecommons.org/licenses/by-nc-nd/3.0/) which permits non-commercial use, distribution,

and reproduction, provided the original work is properly cited and not altered.


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being.[2] The word epigenetics derives fromepi- (Greek: epi- over, above) and -genetics(Greek: genetiko B genetikos, genitive and thatfrom genesiB genesis, origin). Nowadays DNAmethylation and histone modifications, i.e. ace-tylation, methylation, and phosphorylation, canbe defined as epigenetic mechanisms importantto determine when and where a gene will beexpressed. Several inhibitors and activators ofenzymes that catalyze DNA or histone modifica-tions are considered useful as therapeutic agents,since the imbalance of epigenetic networks is

known to cause several pathologic conditions.[3]

The human genome is packaged so as to formthe chromatin, which is made up of repetitiveunits called nucleosomes. A single nucleosomecore structure can be described as a fragment ofDNA, wrapped around a histone octamer formedby four histone partners (one H3-H4 tetramerand two H2A-H2B dimers).[4] Histone lysine andarginine residues are subject to post-translationalmodifications i.e. methylation, citrullination,acetylation, ubiquitination, and sumoylation and

the combinatorial action of these modificationsregulates the critical DNA processes, includingreplication, repair, and transcription. In addition,the enzymes that modify histone lysine and argi-nine residues have been connected to a variety ofhuman diseases, including arthritis, cancer, heartdisease, diabetes mellitus, and neurodegenerativedisorders.[5] In particular, histone acetyl trans-ferases (HATs) and histone deacetylases (HDACs)are responsible for the histone being acetylated ordeacetylated (removal of an acetyl group from

the e-amino groups of the lysine side-chains) andhave been related to transcription, cell cycle pro-gression, gene silencing, lymphocyte and muscledifferentiation, regulation of neuronal pheno-type, DNA replication, and the response to DNAdamage.[6] HATs acetylate the specific lysineresidues in the amino terminals of histones, whichare thought to neutralize the positive charge gen-erating a more open DNA conformation, thusconnecting to nucleosome remodeling and tran-scription regulation. The change of the charge

induces a reduction in the affinity between thehistones and DNA generates a more open DNAconformation. The histone deacetylation by HDACs

is thought to restore the positive charges on his-tones by removing the acetyl groups and is re-sponsible for transcriptional repression throughchromatin condensation (hyperacetylated his-tones are linked to transcriptionally active domains,while hypoacetylated histones are generally as-sociated with transcriptionally silent loci).[4,7-9]

Histone deacetylase inhibitors (HDACIs) havebeen classified in different classes: short-chain fattyacid (as sodium butyrate, valproic acid, etc.), ep-oxides (as depudecin and trapoxin), cyclic peptides,benzamides, and hydroxamic acids (as trichostatin

A [TSA], suberoylanilide hydroxamic acid [vor-inostat; SAHA], and LAQ824).[10] Apart fromthese classic HDACIs, today, novel compounds,such as hydroxamic acids and benzamides, arealso available.

Sodium phenylbutyrate (figure 1), a HDACI,is an aromatic fatty acid that is converted/oxidized in vivo into phenylacetate (PAA) byb-oxidation.[11] In humans the so formed PAAis eliminated by conjugation with glutamine toform phenacetylglutamine, which is excreted in

the urine. This metabolic pathway is the mecha-nism by which phenylbutyrate acts as an ammo-nia scavenger in patients with urea cycle disorders(UCDs) and hyperammonemia.[12] It has beenshown that 1 g of phenylbutyrate would allow theremoval of the equivalent of 1 g of protein.[13]

Plasma PAA and urinary phenacetylglutamineare also endpoints of normal tyrosine metabo-lism. In vitro, phenylbutyrate shows the ability toinduce differentiation by various mechanisms,[14]

and the US FDA has approved its clinical use in

patients with hyperammonemia. Phenylbutyratetherapy for infants, children, and adults with UCD,which leads to nitrogen accumulation in the formof ammonia, must be undertaken daily and life-long, and it is generally well tolerated and asso-ciated with improvements in ammonia and liverfunction.[15] Phenylbutyrate is relatively stable,with a 0.81 hour half-life in human serum.[16] It

ONa+

O

Fig. 1. Chemical structure of sodium phenylbutyrate (sodium4-phenylbutanoate).

228 Iannitti & Palmieri

2011 Iannitti & Palmieri, publisher and licensee Adis Data Information BV. Drugs R D 2011; 11 (3)


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has been shown that phenylbutyrate is able tolower very-long-chain fatty acid levels in thebrain of mice with x-linked adrenoleukodys-trophy, suggesting that phenylbutyrate can crossthe blood-brain barrier.[17] An experimental studyhas shown its penetration into the cerebrospinalfluid after intravenous administration in nonhu-man primates.[18] In the US it has been proposedas an alternative pathway for waste nitrogen ex-cretion in female patients with ornithine trans-carbamylase (OTC) deficiency, which is the mostcommon UCD.[19] Phenylbutyrate has also been

proposed during pregnancy of heterozygoteOTC-deficient females, without leading to dele-terious consequences for the fetus.[20] The abilityof phenylbutyrate to inhibit HDACs makes ita good choice as an anti-tumor agent able to en-tice cellular differentiation through modificationof chromatin and reprogramming the gene ex-pression.[21] It has been reported that phenyl-butyrate exerts a potent anti-tumor effect in vitro,causing growth inhibition and differentiation invarious human cancer cells such as colon carci-

noma, Burkitt lymphoma, primary acute myeloidleukemia, retinoblastoma, prostate cancer, U138MG, T98G, U373 MG and A-172 glioma cells,medulloblastoma, and hepatocellular carcinoma.Phenylbutyrate has been safely used in phase Iand II clinical trials for solid tumors and high-grade astrocytoma, and has shown efficacy in thetreatment of different kinds of tumors, includinghormone refractory prostate cancer, hematologicmalignancy, and high grade astrocytoma.[22]

Moreover, phenylbutyrate has reduced the neu-

roinflammation and overall disease process inmultiple sclerosis cases,[23] has attenuated cere-bral infarction and neuronal apoptosis, and im-proved the neurologic status in a mouse modelfeaturing cerebral hypoxia-ischemia.[24] Its brainpermeability remains a major limitation for drugtreatment of CNS disorders, and it has been dem-onstrated that several HDACIs, such as vorino-stat, sodium butyrate, phenylbutyrate, MS275and valporic acid, are able to cross the blood-brain barrier. That is why they are considered

good candidates for treating brain and spinalcord disorders.[25] Interestingly, a study, con-cerning PAA and phenylbutyrate intravenous

administration in three Rhesus monkeys, hasshown high cerebrospinal fluid penetration ofboth drugs, suggesting that their activity in braintumors, and especially in meningeal malignancy,should be investigated; phenylbutyrate seems tobring some advantages, since it results in expo-sure in plasma and cerebrospinal fluid to bothactive compounds.[18]

An experimental in vitro study has been con-ducted to test sodium butyrate, phenylbutyrateand TSA in relation to fragile X syndrome, themost common form of inherited mental retarda-

tion. It is characterized by an abnormal expan-sion of the repeated trinucleotide sequence CGG,which is contained in the regulatory region of theFMR1 gene and causes transcriptional inactiva-tion.[26] These compounds lead to a consistent,but modest (12% of wild-type levels), reactivationof the FMR1 gene in fragile X lymphoblasts usingreverse transcription polymerase chain reaction.[27]

2. Review Criteria

We searched the literature (all articles listedin PubMed Central up to June 2011) for bothclinical trials and experimental studies in vitroand in vivo, involving the use of sodium phenyl-butyrate and derivative compounds, using thekeywords triButyrate, sodium phenylbutyrate,and 4-phenylbutyric acid sodium salt. The resultshave been divided into different categories, basedon the way in which these compounds are able tointeract with different pathologic conditions.

3. Phenylbutyrate and Cancer

Histone acetylation is associated with gene ac-tivation, while deacetylation, mediated by HDACs,is associated with gene silencing, explaining whyHDACs are considered a powerful drug tar-get.[28] The inhibitors of HDAC modulate thechromatin structure, which results in looseningthe chromatin and changing transcription factorloading to the DNA,[29] modulating the expres-sion pattern of various tumor-relevant genes for

the control of the cell cycle or apoptosis,[30,31] andcausing the inhibition of cell growth and differ-entiation.[32-34] These inhibitors are currently

Sodium Phenylbutyrate: Clinical and Experimental Use 229



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under clinical investigation in patients with dif-ferent types of malignancies,[35] and phenyl-butyrate is one of some HDACIs already tested inclinical trials in treatment of recurrent malignantgliomas or myelodysplastic syndrome (MDS).[36,37]

The following multiple activities have been as-signed to the ability of phenylbutyrate to protectnormal tissues: (i) the activity of an HDACI;[38]

(ii) the activity of a chemical chaperone;[39-41] and(iii) the activity of an ammonia sink.[42,43] HDACIs,such as phenylbutyrate, have shown a wide rangeof abilities: (i) they are able to cause cell cycle

arrest in the G1 and/or G2 phase; (ii) they inducedifferentiation and/or apoptosis in a variety of

cell types;[44] (iii) they induce the transcriptionalactivation of certain genes, such as the one forcyclin-dependent kinase inhibitor p21, which hasbeen related to tumor suppression growth andprevention of cell cycle progression;[45,46] (iv) theyinhibit tumor growth in animal models with littletoxicity in non-tumor cells;[47] and (v) they enhancethe expression of a wide variety of transientlytransfected transgenes in tumors, both in vitro

and in vivo, through their effect on the acetylationof histones.[46,48] The combination of p53 genetherapy with an HDACI, for example, FR901228(romedipsin/depsipeptide) or phenylbutyrate, hasshown enhanced therapeutic efficacy in vitro[46]

and in vivo.[49]

Phenylbutyrate can upregulate the transcrip-tion of epigenetically silenced genes and could betherapeutically useful for treatment of certaintypes of cancer conditions.[50] Differentiatingagents in cancer therapy may potentially alter

tumor growth and progression, slow or inhibitmetastases, and/or effect response to other formsof therapy. Among them, phenylbutyrate hasshown, in experimental tumor model systems, tobe able to exert broad effects on tumor cytostasisand tumor differentiation, altering the gene ex-pression for tumor growth, invasion, angiogenesis,and immunogenicity through mechanisms of ac-tion, such as inhibition of histone deacetylase,hypomethylation, modification of lipid metabo-lism, and activation of peroxisome proliferation

activator receptor.[16] The effect of phenylbutyrateon tumor growth inhibition has been related toseveral mechanisms, including differentiation and

apoptosis induction,[51] expression of silenced genesvia histone deacetylase inhibition,[16] and induc-tion of transforming growth factor-a secretion.[52]

Particularly, the activation of c-Jun N-terminalkinase and extracellular signal-regulated kinase,in the mitogen-activated protein kinase path-way, seems to mediate the pro-apoptotic effectsof phenylbutyrate.[53] Moreover, it has been re-ported that a differentiation therapy for epithelialmalignancies may potentially alter tumor growthand progression, slow or inhibit metastases, inhibitangiogenesis, and/or effect response to other forms

of therapy.[54]

The ability of phenylbutyrate to be apotent cellular differentiating and cytostatic agenthas been related to DNA methylation, a strongassociation with the peroxisome proliferator-activated receptor-g and systemic glutamine de-pletion.[55] Differentiating agents have been usedas therapy for MDS since the bone marrow, inthis condition, is hypercellular, with aberrant dif-ferentiation and concomitant bone marrow fai-lure. It has been reported that these agents havethree potential roles in the treatment of myeloid

neoplasms: (i) terminal differentiation of a ma-lignant clone to clonal extinction, as in retinoicacid induction of acute promyelocytic leukemia;(ii) enforced clonal differentiation leading tofunctional even though clonal hematopoiesis; and(iii) prolongation of remission duration in patientswith acute myelogenous leukemia (AML) or MDSwith residual disease after chemotherapy throughsuppression of proliferation of the malignantclone.[56]

It has been shown that in rats injected with

methylnitrosourea to induce mammary cancers,the efficacy of anti-inflammatory agents such asphenylbutyrate, vitamin C, and diindoylmethaneto prevent mammary carcinogenesis and to in-hibit the growth of established tumors, has beencompared with their potential to inhibit the pro-liferation index and increase the apoptotic indexin tumor cells showing that it is not significantlyeffective (reducing tumor multiplicity by


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induced by this drug. In wild-type male mice ofinbred strain C57BL/6 it has been observed that(i) phenylbutyrate (intraperitoneal 400 mg/kg/dayinjected 1 day before and daily after the adriamycininjection for 2 days) significantly decreases theadriamycin (intraperitoneal injection [20 mg/kg])-associated elevation of serum lactase dehydroge-nase activities; (ii) it diminishes adriamycin-inducedultrastructural damage of cardiac tissue by >70%;(iii) it completely rescues adriamycin-caused re-duction of cardiac function exemplified by ejec-tion fraction and fraction shortening; and (iv) it

increases the cardiac manganese superoxide dis-mutase protein and activity, suggesting that acombination therapy of HDACIs and adriamy-cin could be beneficial for treatment of cancerand simultaneously decrease adriamycin-inducedcardiotoxicity.[58]

In vitro studies have shown that phenylbutyratecan (i) induce differentiation and inhibit proli-feration of AML cell lines and primary leukemiccells;[59,60] (ii) inhibit CFU-L production frombone marrow specimens from patients with

MDS;[60]

and (iii) induce differentiation asso-ciated with induction of p21WAF1/CIP1 expression,hypophosphorylation of retinoblastoma protein,and G1 arrest in the ML-1 myeloid leukemia cellline.[59] At least some of the pharmacodynamiceffects of phenylbutyrate seem to result becauseof its ability to inhibit HDACs,[61] consideringthat histone acetylation contributes to the regu-lation of the gene transcription.[62]

The in vitro effects of decitabine, phenylbutyrateand TSA on clonogenic AML human cells have

been investigated. It has been observed that deci-tabine and HDACIs increase the expression ofthe stem cell marker CD117 for intermediate anderythroid colonies. Decitabine and HDACIs de-crease the expression of the other stem cell markerCD34 and of CD64. These effects are common todifferent colony subsets. The two HDACIs TSAand S4PB differ in their effects on the expressionof some membrane molecules, and this fact isespecially seen in CD33 and CD11b. All threedrugs modulate erythroid differentiation. Both

decitabine and HDACIs decrease CD71 ex-pression of non-erythroid colonies and increaseits expression for mature erythroid colonies. The

authors conclude that decitabine and the twoHDACIs alter AML cell expression of differ-entiation markers, but the drugs do not have anymajor influence on cell cycle distribution.[63]

Burkitt and Ljungman,[64] after demonstratingthat a subset of head and neck cancer cell lineshave a defective Fanconi anemia DNA damageresponse pathway, which correlates with cisplatinsensitivity, have showed that phenylbutyratesensitizes human cells to cisplatin (cisplatin is awidely used chemotherapeutic agent used againstmany different types of tumors, but unfortunately

tumors initially responsive to it later acquire re-sistance). The same authors observe that phenyl-butyrate (sodium phenylbutyrate 2 mmol/L for48 hours) sensitizes cisplatin-resistant head andneck cancer cell lines UM-SCC-1, -6, -25 to cis-platin by inhibiting the Fanconi Anemia (FA)/Breast Cancer (BRCA) pathway through the down-regulation of BRCA1 as well as by an FA/BRCA-independent mechanism. Therefore, they suggestthat the cisplatin-sensitizing effect of phenylbutyrateis related to its role in targeting the expression of the

apoptosis-antagonist B-cell lymphoma extra large(Bcl-XL).[64]

Bandres et al.,[65] after identifying five micro-RNAs (miRNAs) downregulated in patients withcolorectal cancer (CRC) and located around/on acytosine-phosphate-guanine (CpG) island, haveobserved that treatment with a DNA methyl-transferase inhibitor (5-aza-20-deoxycytidine)and phenylbutyrate have restored the expressionof the three miRNAs hsa-miR-9, hsa-miR-129,and hsa-miR-137 in three CRC cell lines. The

expression of hsa-miR-9 is inversely correlatedwith the methylation of their promoter regions asmeasured by methylation-specific polymerase chainreaction (PCR) [MSP] and bisulfate sequencing.Moreover, the methylation of the hsa-miR-9-1,hsa-miR-129-2, and hsa-miR-137 CpG islands isfrequently observed in CRC cell lines and in primaryCRC tumors, but not in normal colonic mucosa.Finally, the methylation of hsa-miR-9-1 is associatedwith the presence of a lymph node metastasis. Thisstudy shows that miRNA-specific hypermethylation

in CRC and histone-deacetylation could be animportant molecular mechanism, as it causes theglobal downregulation of miRNAs. miRNA gene




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methylation could be considered as a useful tumormarker considering the high frequency of miRNAhypermethylation found in CRC cancer.[65]

Hurtubise et al.[66] have observed that phenyl-butyrate, among other HDACIs (MS-275, tri-chostatin-A, LAQ824, and depsipeptide), hasenhanced the antineoplastic action of 5-aza-20-deoxycytidine (5AZA-CdR; a potent inhibitor ofDNA methylation that has been approved fortreatment of hematologic malignancies and is ableto reactivate the expression of silenced tumor sup-pressor genes [TSGs]) on Ewing sarcoma cells.

Connexin (Cx) protein deregulation is believedto be involved in the carcinogenesis process andto play a key role in gap junction intercellularcommunication,[67] and it is essential for bothproliferation and activation of differentiationpathways.[68] It has been observed that highlyproliferating malignant cells have less gap junc-tions than the most differentiated ones.[69] Inparticular, connexin 43 (Cx43) is a tumor sup-pressor,[69] and its expression is reduced in vari-ous tumors.[70,71] The forced expression of the

Cx43 gene in several Cx43-deficient tumor celllines attenuates their malignant phenotype,[72,73]

although the mechanisms by which the Cx43 geneinhibits the tumor growth remain unclear. Therole of Cx43 alone and combined with an HDA-CI in tumor growth inhibition has shown that(i) trasfecting Cx43 plasmid DNA (pCMV-Cx43)into human nasopharyngeal cancer KB cells,using folate-linked nanoparticles, induces inhibi-tion of cell growth (Cx43 induced a tumor sup-pressive effect via a gap junctional intercellular

communication-independent mechanism); and(ii) the transfection of pCMV-Cx43 along withphenylbutyrate greatly enhances Cx43 expressionin vitro, and significantly inhibits the tumorgrowth of KB cells and xenografts compared withthat of pCMV-Cx43 alone. Phenylbutyrate in-duces an increased expression of genes of DNAdamage checkpoints and of apoptosis via the down-regulation of anti-apoptotic b-cell lymphoma(bcl-2) messenger RNA (mRNA) expression and up-regulation of the activity of the apoptosis-associated

enzyme caspase-3/7.[74]

The combined treatment 50-azacytidine/phenylbutyrate results in a partial but efficient

demethylation of the miR-203 upstream regionboth in the two T-cell leukemia cell lines, KAR-PAS-45 and PEER, and in the two Ph-positivechronic myelogenous leukemia (CML) cell lines,K562 and KCL-22, restoring miR-203 expres-sion (a tumor suppressor miRNA controlling theexpression levels of ABL1, a classic oncogeneextensively characterized in hematopoietic ma-lignancies, and the expression levels of the BCR-ABL1 translocation protein, produced by thePhiladelphia chromosome, which is a hallmark ofCML and some B-cell leukemias in children). The

re-expression of this miRNA dramatically reducesthe proliferation of tumor cells and strongly cor-relates with a significant reduction in both ABL1and BCRABL1 protein levels. This fact suggeststhat epigenetic drugs, in addition to re-expressingother methylated complementary DNAs, can resultin oncogene downregulation mediated by thechemical restoration of miRNA function.[75]

It has been observed that phenylbutyrate(i) induces up to 70% apoptosis in pancreaticcarcinoma cell lines Panc 1, T4M-4, COLO 357,

and BxPc3, while it leads to cell cycle arrest inonly T3M-4 and Colo357 cells, which have beenfound to express p21 to a higher extent thanPanc1 and BxPc3 cells; (ii) it increases gap junc-tion communication between adjacent T3M-4cells in a concentration-dependent manner andefficiently inhibits cellular export mechanisms inpancreatic carcinoma cell lines Panc 1, T4M-4,COLO 357, and BxPc3 cells; (iii) in combinationwith gemcitabine, phenylbutyrate shows an over-additive effect on induction of apoptosis in BxPc3

and T3M-4 cells (up to 4.5-fold compared withsingle drug treatment) with accompanied activa-tion of caspase 8, BH3 interacting domain deathagonist (Bid) and poly (ADP-ribose) polymerasefamily, member 1 (PARP) cleavage; (iv) in con-centrations exceeding 2.0 mmol/L it sensitizesH6c7 cells (a pancreatic ductal epithelial cell linenot of malignant origin, but characterized by im-mortality and a high proliferation rate), whichsuggests that phenylbutyrate does not act on ma-lignant cells specifically, but also on other highly

proliferating immortalized cells; and (v) it does notaffect low proliferating primary human fibroblastsand peripheral blood mononuclear cells.[76]




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A pilot study of five African patients withBurkitt lymphoma, has shown that phenylbutyrate,administered before cyclophosphamide, inducedshrinkage of cyclophosphamide-resistant tumorsin two patients, at least temporarily. A compar-ison of the responses of these two patients sug-gests that phenylbutyrate pretreatment for6 days, before cyclophosphamide, is more effec-tive than pretreatment for 2.5 days.[77]

It has been shown that phenylbutyrate signif-icantly reduces the number of inclusions in glucoseoxidase and acetyl-leucyl-leucyl-norleucinal-treated

cells (human hepatoma cell line established froma hepatocellular carcinoma [Huh7 cells] and ahighly differentiated immortalized human hepa-tocyte cell line [OUMS-29 cells]), indicating thatendoplasmic reticulum (ER) dysfunction, in-duced by glucose oxidase and acetyl-leucyl-leucyl-norleucinal, influences the formation of inclusions.These phenomena are reversed by a co-treatmentwith phenylbutyrate, which, due to its properties ofchemical chaperone, reduces the ER stress.[78]

The effect of a combination treatment with

the DNA methyltransferase inhibitor 5-aza-20

-deoxycytidine (5-aza-CdR) and the HDACIphenylbutyrate on a set of CCD-1070SK (humannormal fibroblasts) and cancer cell lines (T24[bladder transitional carcinoma cells], CFPAC-1[pancreatic carcinoma cells], CALU-1 [lung car-cinoma cells], NCCIT [embryonal carcinomacells]) has been studied and the results have shownthat (i) low doses of the drug combination causecell cycle arrest, whereas high doses induce apo-ptosis in T24 bladder carcinoma cells; (ii) both

p16 (CDKN2A/INK4) and p21 (CIP1/SDI1/WAF1) expression is induced to similar levelsin normal and cancer cells in a dose-dependentfashion after combination treatments; (iii) a dis-tinct increase of histone H3 acetylation at lysine9/14 near the transcription start sites, in bothLD419 normal fibroblasts and T24 bladder car-cinoma cells, has been observed, and instead theacetylation changes in the p21 locus are less ap-parent; (iv) the levels of trimethylation of histoneH3 on lysine 9 do not change after drug treat-

ments and there is evidence that the remethylationof the p16 promoter CpG island in T24 cells after5-aza-CdR treatment cannot be halted by a sub-

sequent continuous phenylbutyrate treatment; and(v) the p16 gene is resilenced with kinetics similar to5-aza-CdR-only-treated cells, which is also markedby a localized loss of histone acetylation at thetranscription start site.[79]

Cyclo-oxygenase-2 is frequently overexpressedin non-small cell lung cancer (NSCLC) and resultsin increased levels of prostaglandin E2, an im-portant signaling molecule implicated in tumori-genesis, with levels that have increased followingcyclo-oxygenase-2 overexpression in NSCLC. Itexerts its effects through the E prostanoid (EP)

receptors (EPs14). In NSCLC cell lines (Beas-2B[transformed normal human bronchoepithelial],H460 [large cell], H647 [adenosquamous carci-noma], A549 [adenocarcinoma] and SK-MES-1[squamous cell carcinoma]), histone acetylationhas been found to be a critical regulator of EPexpression with TSA, phenylbutyrate, and sub-eroylanilide hydroxamic acid, as they induce in-creased expression of EPs24. In addition, directchromatin remodeling has been demonstrated atthe promoters for EPs24.[80]

AML cells (cell lines K562, CMK, HL-60,KG-1a, and SK-Hep-1) have been cultured inthe presence of phenylbutyrate, valproate, sub-eroylanilide hydroxamic acid, or TSA and ana-lyzed for drug transporter expression and functionas well as sensitivity to anticancer drugs. Phenyl-butyrate has induced P-glycoprotein and breastcancer resistance protein expression and the effluxof drugs as determined with labeled substrates.KG-1a cells, treated with phenylbutyrate, have de-veloped resistance to daunorubicin, mitoxantrone,

etoposide, vinblastine, paclitaxel, topotecan, gem-citabine, and 5-fluorouracil, resulting in an im-pairment of drug-induced apoptosis.[81]

p21waf1/Cip1 (p21) is a tumor suppressorgene involved in apoptosis in many cancer celltypes induced by different agents. In hepato-cellular hepatocarcinoma, Hep3B cells trans-fected by EGFP-p21 anti-sense or sense plasmids,apoptosis induced by phenylbutyrate, and TSAhas been assessed by terminal transferase dUTPnick end labeling (TUNEL) assay. The results have

shown that the p21 anti-sense construct preventsapoptosis induced by HDACIs in Hep3B cells.The obtained results suggest an important role




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for p21 in mediating the apoptotic effect ofHDACIs.[82]

An experimental study has investigated theantineoplastic effect of TSA and phenylbutyrateon the human glioblastoma cell lines GBM-29,U-343 MG, and U-343 MGa Cl. 2 : 6, giving thefollowing results: (i) TSA and phenylbutyratehave induced apoptosis in the three cell lines in adose- and time-dependent manner, and caspase-3activation has been detected in all three cell lines;(ii) U-343 MG cells have been more sensitive tothe apoptotic effect of HDACI compared with

U-343 MGa Cl. 2 : 6; (iii) TSA and phenylbutyratehave induced differentiation in the three cell lines,each cell line developing unique phenotypiccharacteristics; (iv) during long-term treatmentwith a low dose of HDACI U-343 MGa Cl. 2 : 6cells have developed an astrocytic morphologywith expression of glial fibrillary acidic protein(GFAP); (v) GFAP-negative U-343 MG cellshave changed their morphology in response toHDACI and downregulated their expression ofvimentin; and (vi) the nestin and vimentin pos-

itive GBM-29 cells have also shown a morpho-logic differentiation, while the expression of thetwo malignancy markers has decreased.[22]

Treatment of tumor cells (human prostate[PC3, DU-145 (ATCC)] and colon carcinoma celllines [HCT116,HCT116 p21/, HCT116 p53/])with phenylbutyrate and 13-cis-retinoic acid(CRA) and with paclitaxel and doxorubicin hasbeen studied with the following results: (i) in-hibition of tumor cell growth has been greatlyenhanced when compared with phenylbutyrate

+CRA, paclitaxel or doxorubicin alone, with>90% growth inhibition; (ii) when the cells havebeen pretreated with phenylbutyrate +CRA, fol-lowed by paclitaxel or doxorubicin, the enhancedinhibition is abolished; (iii) treatment with phe-nylbutyrate+CRA restores sensitivity to doxo-rubicin in the PC-3 human prostate cancer cellline; (iv) phenylbutyrate +CRA induces p21 ex-pression and cell cycle arrest in the G1 phase,while paclitaxel and doxorubicin induce G2/M

arrest; (v) p21- and p53-deficient colon carcino-ma cell lines are more sensitive to the effect ofphenylbutyrate+CRA and paclitaxel as singleagents and in combination, compared with thewild-type cells; (vi) when the p21-deficient cellsare pretreated with phenylbutyrate+CRA, fol-lowed by paclitaxel, the protective effect is stillobserved; (vii) treatment of tumor cells with acombination of these drugs induces cell cycledelay at multiple mitotic checkpoints before un-dergoing apoptosis; (viii) the tumor growth is sig-nificantly inhibited and delayed in male athymic

nude or female BALBc mice treated with eitherpaclitaxel or concomitantly with paclitaxel andphenylbutyrate+CRA compared with control;and (ix) animals, treated with all three agents,demonstrate further growth inhibition or delaycompared with the paclitaxel alone or phenyl-butyrate+CRA arm.[83]

It has been suggested that phenylbutyrate canproduce p21-independent cytostasis, and en-hance radiation sensitivity in p53 mutant humanglioblastoma cells in vitro (D54, U87-MG, U251,

and SKMG-3), suggesting the potential applica-tion of combined phenylbutyrate and radiother-apy in glioblastoma harboring mutant p53.[84]

An experimental study has shown that treat-ment of different metastatic melanoma cell lines(SK-Mel-19, SK-Mel-29, and SK-Mel-147 mela-noma cells; 1.5 106) with 5-azacytidine-CdR, apotent inhibitor of cytosine methylation, sig-nificantly induces 14-3-3s protein expression1

and additional treatment with phenylbutyratefurther enhances 14-3-3s expression. The induc-

tion of 14-3-3s expression by 5-azacytidine-CdR/phenylbutyrate treatment leads to almost com-plete inhibition of cell proliferation, with cellspredominantly arrested in G2-M. The antiprolif-erative effect of 5-azacytidine-CdR/phenylbutyrateis reversed in 14-3-3s knockdown cells.[85]

A 6-hour pretreatment with phenylbutyrate andvitamin B3 on human promyelocytic leukemia cellsHL-60, before the exposition to all-trans-retinoicacid (RA) alone or in combination with vitamin B3,

1 14-3-3 proteins play an important role in cancer biology by interfering with intracellular signaling pathwaysand cell cycle checkpoints, and in particular the 14-3-3s isoform acts as a tumor suppressor and is often in-activated during the tumor development.




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has shown a significant acceleration and high level ofgranulocytic differentiation. The effects are asso-ciated with a rapid histone H4 acetylation and laterhistone H3 modifications.[86] It is important to un-derline that vitamin B3 inhibits sirtuins, NAD

+-dependent protein deacetylases that are involved intranscriptional regulation, metabolism, apoptosis,differentiation, and aging.[87]

Phenylbutyrate, at different concentrations,has been used to treat hepatocellular carcinomacells Bel-7402 and normal liver cell line L-02showing: (i) a time-dependent growth inhibition

in hepatocellular carcinoma cells Bel-7402; (ii) asignificant decline in the fraction of S phase cellsand significant increase in G0/G1 cells; (iii) anincrease in the expression of P21WAF1/CIP1 andE-cadherin in Bel-7402 and a significant decreasein the level of HDAC4 in Bel-7402; (iv) a signif-icant improvement in the level of acetylatinghistone H4; (v) no distinct changes in the normalliver cell line L-02 suggesting phenylbutyrate tobe safe for clinical treatment; (vi) the inhibitionin the growth of hepatocellular carcinoma cells

Bel-7402 and induction of partial differentiationthrough the enhancement of the acetylating his-tones; and (vii) no significant effect on normalliver cells have been observed. These results un-derline the safety of phenylbutyrate, emphasizingthat it may have great potential as a growth in-hibitor for hepatocellular carcinoma.[88]

The clinical trials involving the use of phenyl-butyrate for treatment of cancer are summarizedin table SI (Supplemental Digital Content 1,http://links.adisonline.com/DRZ/A3).

4. Motor Neuron Disorders

Motor neuron diseases are a group of fatalneurologic progressive disorders, and the mostcommon are spinal muscular atrophy (SMA) andamyotrophic lateral sclerosis (ALS), both char-acterized by the loss of spinal motor neurons. Noeffective cure exists, but there have been majoradvances to understand the molecular and ge-netic mechanisms involved in these pathologies.

Riluzole stops the advance of the disease byblocking the effects of the neurotransmitter glu-tamate and it is thought to extend the lifespan of

ALS patients for few months.[89] In particular,the use of phenylbutyrate and its derivatives hasbeen widely investigated both in vitro and in vivo.Important genes, playing a key role in motorneuron diseases, have been discovered during thepast few years and several pathways have beenrecently implicated in these pathologies, i.e.pathways that affect RNA processing, axonaltransport, and mitochondrial function.[90]

4.1 Phenylbutyrate and Spinal

Muscular Atrophy

SMA is the term for a number of conditionscharacterized by the degeneration of a-motorneurons in the brainstem and spinal cord leadingto hypotonia and muscle weakness.[91] SMA is anautosomal recessive disease and the leading causeof infant deaths caused by the gene survivalmotor neuron 1 (SMN1).[92] SMA has been sub-divided into three clinical groups, based on theage of the onset of symptoms and the achieve-ment of motor milestones, and that are due to

homozygous loss of the functional motor neurongene (SMN1) on chromosome 5q13. The age ofthe onset in patients with SMA I is less than6 months and these patients never acquire theability to sit unsupported. The onset of SMA II isusually between 6 and 18 months and the patientsare able to sit unsupported, but are not able towalk. The onset of SMA III is usually >18 monthsand the patients are able to walk independently.[93]

The fact that SMN genes have a unique organi-zation has led to investigation of how to make

SMN2, a gene existing in all patients, functionlike the missing SMN1 gene.[92] Histone acetyla-tion leads to DNA relaxation, making it acces-sible to the transcriptional machinery, and itsinhibition might enhance the expression in about2% of human genes.[94] Phenylbutyrate, the firstHDACI, found to enhance SMN2 expression,increases exon seven inclusion in lymphoid celllines derived from patients with SMA. However,its half-life of about 6 minutes in human serumhas made it impossible to develop a clinical use

for this compound.[92] Andreassi et al.[95] reportedthat phenylbutyrate treatment caused an increasein full-length SMN2 transcripts in fibroblast cell


http://links.adisonline.com/DRZ/A3http://links.adisonline.com/DRZ/A3


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cultures from 16 SMA patients and it is was alsoeffective in enhancing SMN protein levels and thenumber of SMN-containing nuclear gems. There-fore, this compound could represent a valuablecandidate for pharmacologic treatment of SMA.[95]

It has been shown that the SMN2 gene is subjectto gene silencing by DNA methylation, whichmight have major implications for SMA diseaseprogression and upcoming pharmacologic inter-ventions for epigenetic SMA therapy.[96]

It has been found that treatment of SMA-liketransgenic mice with the hyperacetylating agent

phenylbutyrate effectively increases the expres-sion of SMN protein in motor neurons of thespinal cord,[97] and phenylbutyrate significantlyincreases SMN expression in fibroblast culturesfrom SMA patients.[95]

It has been shown that five pan-HDACIs, in-cluding SAHA, as well as the cyclic tetrapeptideromidepsin (FK-228), are able to bypass LT-SMN2gene silencing in SMA fibroblasts and humanhippocampal brain slice cultures (OHSCs), re-sulting in up to 5-fold inductions of total SMN2

transcript levels. In contrast, the HDAC iso-enzyme selective inhibitors MS-275, VPA, andphenylbutyrate display only moderate effects,suggesting that pan-HDACIs possess superiorfunctional capacities to activate SMN2. Amongthe pan-HDACIs, vorinostat appears to be promis-ing, as this molecule has been shown to cross theblood-brain barrier. Moreover, ongoing clinicalresearch in cancer patients has shown a good oralbioavailability and biologic activity.[96] The clin-ical trials involving the use of phenylbutyrate for

treatment of SMA are summarized in table SII(see Supplemental Digital Content).

4.2 Phenylbutyrate and Amyotrophic

Lateral Sclerosis

ALS is a fatal neurodegenerative disorder,characterized by a loss of upper and lower motorneurons in the brain and spinal cord, and leadingto a progressive muscle wasting and limb paral-ysis, dysphagia, dysarthria, and respiratory fail-

ure.[98,99] About 10% of patients report a familyhistory, and mutations in SOD1 (which encodessuperoxide dismutase-1) are the cause in about

20% of these cases; 3% and 4% of familial cases,respectively, have been related to TARDBP (alsoknown as TDP-43, which encodes TAR DNA-binding protein) and FUS (RNA-binding proteinfusion), which have been linked to familial formsof the disease. Sporadic ALS has been associatedwith another gene, ELP3 (elongator protein 3),which encodes the catalytic subunit of the HATcomplex elongator protein 3. ALS also featurescytoplasmic deposits of misfolded proteins, con-sisting of SOD1, TARDBP, or FUS aggregates.[100]

Riluzole (an FDA-approved ALS therapy) provides

a 2- to 3-month prolongation of survival.[101,102]

Todate, no other drug therapies have been shown toslow or abrogate the disease process in ALS. It hasbeen reported that the mean survival of patientsfrom the onset of symptoms is 35 years.[103] Thedifferent types of ALS pathologies are studied intransgenic rodent models of ALS, expressingmutant forms of SOD1.[104] Mice, overexpressinga human SOD1 mutation, are characterized by aglycine, which is substituted by an alanine at aminoacid position 93[105] since these transgenic models

develop clinical and pathologic features that aresimilar to those in human diseases.[105-107] Ap-optotic cell death is a common cellular event inboth animal models and patients with ALS,[108]

as suggested by the observations that increasedexpression of bcl-2, the dominant negative inhi-bition of caspase-1, and the administration of tetra-peptide caspase inhibitors delay the onset of thedisease and prolong survival in ALS mice.[109-111]

Neuroprotective therapies that target specificneurotoxic molecular mechanisms in ALS mice

have the potential to delay the onset and slow theprogression of the disease in ALS, and phenyl-butyrate has been proposed as a good candidatefor this kind of therapy.

Pharmacologic treatment, using phenylbutyrate(the animals received either 200, 400, 600, or800 mg/kg intraperitoneally or phosphate-bufferedsaline [PBS] injection, i.e vehicle; treatment wasstarted at 21 and 70 days, before and after theoccurrence of the symptoms), significantly ex-tends survival and improves both the clinical and

neuropathologic phenotypes in G93A transgenicALS mice. Moreover, it ameliorates histone hy-poacetylation and induces expression of nuclear




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factor-kB (NF-kB) p50, the phosphorylated in-hibitory subunit of NF-kB (pIkB) and bcl-2, andit reduces cytochrome c and caspase expres-sion. The same authors propose that the pharma-cologic induction of NF-kB-dependent transcriptionand bcl-2 gene expression is neuroprotective inALS mice by inhibiting programmed cell death,in particular: (i) phenylbutyrate acts to phos-phorylate pIkB, translocating NF-kB p50 tothe nucleus, or to directly acetylate NF-kB p50;(ii) NF-kB p50 transactivates bcl-2 gene expres-sion; and (iii) upregulated bcl-2 blocks cytochrome

c release and subsequent caspase activation, slow-ing motor neuron death. These transcriptional andpost-translational pathways ultimately promotemotor neuron survival and ameliorate disease pro-gression in ALS mice.[38]

The catalytic antioxidant Mn (III) porphyrinAEOL 10150 (2.5mg/kg/day; systemic administra-tion), phenylbutyrate (400 mg/kg/day; intraperi-toneal administration) and the combination ofphenylbutyrate and AEOL 10 150 have been testedin G93A transgenic familial ALS mice (admin-

istered from the onset of the disease, i.e. after theappearance of motor dysfunction) observing that(i) AEOL 10 150 alone improved motor functionand extended survival by 11%; (ii) phenylbutyratealone significantly improved motor function andextended survival by 13%; and (iii) phenylbutyrateand AEOL 10 150 together increased survival by19%. An increase in histone acetylation was con-firmed by Western blot and quantitative real-time PCR analysis revealed the upregulation ofthe compounds capable of protecting cells against

oxidative stress and apoptosis. Markers of oxi-dative damage were reduced in the lumbar spinalcord when compared with vehicle administration.This study suggests that agents inhibiting apop-tosis and blocking oxidative stress show efficacyin treating mutant-SOD1-associated ALS and thata combination of agents, targeting different dis-ease mechanisms, may exert additive therapeuticeffects.[112]

The combined treatment of riluzole and phe-nylbutyrate significantly extends survival and

improves both clinical and neuropathologic phe-notypes in G93A transgenic ALS mice. In partic-ular, the combination therapy increased survival

by 21.5%, compared with the separate administra-tion of riluzole (7.5%) and phenylbutyrate (12.8%),while improving both bodyweight loss and gripstrength. Riluzole/phenylbutyrate treatment ame-liorates gross lumbar and ventral horn atrophy,attenuates lumbar ventral horn neuronal celldeath, and decreases reactive astrogliosis. In ad-dition, riluzole/phenylbutyrate administrationincreases acetylation at H4 and increases NF-kBp50 translocation to the nucleus in G93A mice.[113]

An open-label study, involving 40 patientswith ALS, has been performed to establish the

safety and pharmacodynamics of escalating dos-ages of phenylbutyrate in 26 participants whocompleted a 20-week treatment phase. It showedthat phenylbutyrate is safe and the majority ofsubjects tolerated higher dosages (21 g/day) ofthis drug, but the lowest dose (9 g/day) was ther-apeutically efficient in improving histone acety-lation levels. No deaths or clinically relevantlaboratory changes occurred with phenylbutyratetreatment. Histone acetylation decreased by ap-proximately 50% in blood buffy-coat specimens

at screening and significantly increased after phe-nylbutyrate administration. In addition, blood lev-els of phenylbutyrate and the primary metabolite(PAA) increased with dosage.[114]

4.3 Phenylbutyrate and Ischemia

Retinal ischemia is a common cause of visualimpairment for humans and animals. Retinal is-chemia has been typically observed as a con-sequence of the presence of diabetic retinopathy,

glaucoma, and retinal-artery occlusion.[115] Is-chemic retinal injury consists of a self-reinforcingcascade of destruction involving neuronal depo-larization, calcium influx, and oxidative stressinitiated because of energy failure, and increasedglutamatergic stimulation.[116,117]

A study has investigated the neuroprotectiveeffects of phenylbutyrate upon retinal ischemicinjury using Sprague Dawley rats. Retinal gan-glion cells (RGCs) were retrograde labeled withthe fluorescent tracer fluorogold applied to the

superior collicoli of test Sprague Dawley rats and,7 days after that, high intraocular pressure andretinal ischemia were induced. The animals received




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either 100 or 400 mg/kg PBA intraperitoneally,30 minutes prior to the induction of retinal is-chemia and 1 hour subsequent to the cessation ofthe procedure inducing retinal ischemia. Histo-logic analysis has revealed that ischemic injurycauses the loss of retinal RGCs and a net decreasein retinal thickness. Moreover, it has been ob-served that in the phenylbutyrate-treated groups,almost 100% of the RGCs have been preserved bya pre-ischemia treatment with phenylbutyrate (ata dose of either 100 or 400 mg/kg), while post-ischemia treatment of RGCs with phenylbutyrate

has not led to the preservation of RGCs fromischemic injury by phenylbutyrate, as determinedby the counting of whole-mount retinas. Pre-ischemia treatment of RGCs with phenylbutyrate(at a dose of either 100 or 400 mg/kg) has signif-icantly reduced the level of ischemia-associatedloss of thickness of the total retina, especially theinner retina and the inner plexiform layer of ret-ina. Besides, phenylbutyrate treatment has sig-nificantly reduced the ischemia-induced loss ofcells in the ganglion-cell layer of the retina.[118]

It has been observed that, if we target the ER byadministration of intraperitoneal phenylbutyrate(1 hour before and 12 hours after reperfusion)the liver is protected against ischemia/reperfusioninjury in male C57BL/6 mice subjected to warmischemia (70% of the liver mass, 45 minutes), asevidenced by the decreased liver enzymes and re-duction in the number of non-viable cells andPMN infiltrates into the liver. Moreover, thehepatoprotective properties of phenylbutyratehave further been demonstrated by a reduction in

apoptosis after reperfusion. This study shows thatphenylbutyrate reduces mortality in a lethal modelof total ischemia/reperfusion injury to the liver (allvehicle-treated controls died within 3 days afterreperfusion while 50% survival [>30 days] has beenobserved in animals given phenylbutyrate).[119]

4.4 Phenylbutyrate and Urea Cycle Disorders

The urea cycle is the final common pathwayfor the excretion of waste nitrogen in mammals,

and the defects in this cycle, observed in the neo-natal period, are usually associated with severeand rapidly worsening hyperammonemia which,

if not treated, leads to cerebral and pulmonaryhemorrage, leaving the baby severely handicappedor leading to death.[120] It has been reported thatall the UCDs are associated with an accumulationof glutamine and alanine, and are characterizedby a difficult diagnosis since they are associatedwith nonspecific symptoms such as poor feeding,vomiting, lethargy and/or irritability, and tachy-pnea. These babies can show a transient respi-ratory alkalosis, neuromuscular irritability andstridor, which are only transient, with the patientsdeteriorating rapidly, becoming unresponsive,

and requiring intensive care. Other complicationsare changes of tone, with loss of normal reflexes,vasomotor instability and hypothermia, apnoeaand fits, while a disordered liver function could bea secondary complication. Measurement of plas-ma ammonia concentration is the main diagnostictest for the UCD (plasma ammonia, in healthyneonates, is normally200mmol/L inpatients with inborn errors and achieves higherlevels in patients with UCD).[120]

In cases of significant hyperammonemia, thefollowing investigations could be useful: bloodpH and gases, plasma urea and creatinine, elec-trolytes, glucose, liver function tests and clottingstudies, plasma amino acids, urine organic acids,orotic acid and amino acids, and plasma free andacylcarnitines.[120] The therapy for this disease isdirected at decreasing the requirement for ureabiosynthesis by decreasing dietary nitrogen in-take and increasing waste nitrogen excretion. Itis achieved by prescribing phenylbutyrate, a pre-

cursor of PAA that conjugates with glutamineto yield phenylacetylglutamine, a waste nitrogencompound that is rapidly excreted in the urine.[122]

The use of phenylbutyrate to treat UCD is dueto its ability to be oxidized in the liver to PAA,which is then conjugated with glutamine, result-ing in phenylacetylglutamine, which is excreted inthe urine and hence 2 mol of nitrogen is lost foreach mole of the given phenylbutyrate.[123] Anearly diagnosis is fundamental because the prog-nosis often depends on urgent treatment. Family

investigation and genetic counseling play a pre-ventive key role. When elevated plasma ammonialevels are found, a low-protein diet, with supple-




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mental arginine, and with both sodium benzoateand phenylbutyrate to remove excess nitrogen,has been suggested as treatment.[124]

Three cases of patients with hyperammonemiawho received accidental inappropriate doses ofintravenous sodium benzoate and PAA that haveresulted in severe complications have been reported.All the patients showed alteration in mental sta-tus, Kussmaul respiration, and a partially com-pensated metabolic acidosis with an increasedanion gap. Two patients developed cerebral edemaand hypotension, and died; the third patient sur-

vived after hemodialysis, and plasma levels ofbenzoate and PAA were excessively high in all ofthem.[125] During pregnancy, benzoate might bethe best choice because it is usually ingested withsoft drinks and several types of food, while phe-nylbutyrate can affect many metabolic and otherfunctions. Phenylbutyrate tablets are consideredto be dangerous because they can cause serioustissue damage in the esophagus, underlining theimportance of instructions to the patient to en-sure correct swallowing of the drug.[126]

OTC deficiency is the most common urea cycledefect, usually affecting males in the neonatalperiod and causing acute hyperammonemic comaand leading to death if not treated.[124] OTC is anx-linked mitochondrial enzyme that catalyzes thesynthesis of citrulline from carbamoyl phosphateand ornithine. A deficiency in this enzyme leads tohyperammonemia and hyperglutaminemia, hy-poargininemia, hypocitrullinemia, and episodicencephalopathy that, if uncontrolled, results inbrain injury and death. In boys, the disease is

often fatal when its onset occurs during the neo-natal period, but it is milder when the onset oc-curs later in childhood. Heterozygous girls may benormal or may have episodes of hyperammonemicencephalopathy and decline in cognitive func-tion.[19] This condition has also been observedin the later childhood (between 15 months and5 years) and has been often under-recognized.[127,128]

This pathology is characterized, in the neonatalperiod, by feeding difficulties, lethargy, respiratorydistress, impairment of consciousness, vomiting,

convulsions, and coma. In later life the most im-portant symptoms that might occur are headache,vomiting, lethargy, hyperventilation, episodes of

abnormal behavior, and sometimes disorienta-tion and confusion, ataxia, hypotonia, and focalneurologic signs such as hemiplegia[128] (proposedto be due to alterations in vascular endothelialwall integrity or alterations in cerebral perfusionand metabolism with an accumulation of toxicmetabolites).[129] Cases of acrodermatitis andenteropathica-like rashes have been reported,probably associated with a deficiency of arginine,an amino acid that represents the 16% of theamino acid content of the human epidermal ker-atin.[130] The voluntary adoption of a vegetarian

diet has been observed.[131]

Acute pancreatitis hasalso been reported as a complication.[132] Theclinical trials, involving the use of phenylbutyratefor treatment of UCD, are summarized in tableSIII (see Supplemental Digital Content).

4.5 Sickle Cell Disease and Thalassemias

b-Thalassemia is an autosomal recessive geneticdisease, caused by variations in the inactivationmechanism of the b-globin genes,[133] that shows

clinical symptoms ranging from mild symptom-less anemia to transfusion dependence. No ad-equate treatment for the various phenotypes hasbeen established yet. The current management ofthis condition includes the use of regular redblood cell transfusions and iron chelation ther-apy. Bone marrow transplantation could be animportant therapeutic choice, although it is notan option for the majority of the patients. Thedevelopment of an effective therapy to increasehemoglobin levels in homozygous b-thalassemia,

without the use of red blood cell transfusions,could allow normal growth and developmentwhile decreasing or eliminating transfusional ironoverload, which is the leading life-threateningfactor in patients with this disease.[134] Butyrateanalogs are able to induce erythroid differentia-tion[135-137] and stimulate hemoglobin F productionin human erythroid progenitors in vitro.[138-140]

In vivo, they reactivate embryonic globin pro-duction in an avian model,[141] delay the switchfrom fetal to adult globin in ovine fetuses,[142] and

increase fetal hemoglobin (HbF) production inadult primates.[140,143-145] In humans, phenyl-butyrate is able to stimulate fetal hemoglobin




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production.[146,147] For the majority of children withb-hemoglobinopathies and b-thalassemias who donot have a transplant donor, survival is shortenedand morbidity is high. Hydroxyurea, recombinanthuman erythropoietin preparations, phenylbutyrate,arginine butyrate, and 5-azacytidine/decitabine haveshown efficacy in approximately 4070% of sicklecell and b-thalassemia patients.[148]

The clinical trials involving the use of phenyl-butyrate for treatment of sickle cell disease andthalassemias are summarized in table SIV (seeSupplemental Digital Content).

4.6 Phenylbutyrate and Cystic Fibrosis

Cystic fibrosis (CF) is an autosomal recessivedisorder caused by mutations in the CF trans-membrane conductance regulator (CFTR) gene.[149]

The DF508 mutation is caused by a three basepair deletion in the gene for CFTR, resulting inthe loss of a phenylalanine residue at position 508in the CFTR protein and this mutation fails tofold properly in the ER, which leads to ubiquiti-

nation and degradation of the protein in theproteasome.[150,151] The DF508-CFTR protein,which is retained in the ER and degraded ratherthan trafficked to the cell surface, forms a func-tional chloride channel in reconstituted bilayers,but with decreased mean open times, and there-fore decreased conductance compared with wild-type CFTR. This fact underlines how treatmentsthat would promote DF508-CFTR traffickingbeyond the ER might restore partial CFTRchloride channel function at the cell surface.[152]

Two members of the 70 kDa heat shock proteinfamily, HSP70 and HSC70, interact with CFTR,and the regulation of these heat shock protein-CFTR interactions can restore DF508-CFTR traf-ficking as shown in a study proving phenylbutyratemodulates heat shock protein function and restoresDF508 maturation in vitro and in vivo.[15] Heatshock proteins are constitutive and stress-inducibleproteins that can protect normal cells against pro-tein damage by physical interaction during syn-thesis, folding, assembly, and degradation.[153]

In cultured cells, the deregulation of DF508-CFTR by butyrate has been observed.[154] Moreover,phenylbutyrate, at micromolar concentrations,

induces CFTR channel function on the plas-ma membrane ofDF508-expressing CF airway epi-thelial cells (in IB3-1 cells [an immortalized CFcell line containing the mutations DF508 andW1282X]) in vitro.[155] CFTR mutations havebeen categorized into five major classes: (i) thetotal absence of CFTR (class I); (ii) defects inCFTR folding and trafficking, leading to reducedamounts of mature CFTR (class II; the DF508 isthe prototype class II trafficking mutation andhas been found in >70% of patients with CF);(iii) abnormal regulation of chloride conduc-

tion in an otherwise mature CFTR (class III);(iv) decreased chloride conduction in a fully pro-cessed CFTR (class IV); and (v) decreased CFTRsynthesis (class V).[156]

A study investigating the effects of genisteinand phenylbutyrate on CFTR in three humanairway epithelial cell lines expressing wild-type orDF508 CFTR (Calu-3, CFSMEo-, and CFBE41o-cells) loaded with the fluorescent dye N-(ethoxy-carbonylmethyl)-6-methoxyquinolinium bromideto study the chloride efflux, has shown that

(i) forskolin and 3-isobutyl-1-methylxanthine(IBMX) induce chloride efflux in Calu-3 cells butnot in CF lines; (ii) genistein (2.550 mmol/L) isable to induce chloride efflux in all cell lines butdoes not enhance the effect of forskolin andIBMX; and (iii) phenylbutyrate has little or noeffect on genistein-induced chloride efflux.[157]

A study has investigated the ability of phe-nylbutyrate, genistein, and 8-cyclopentyl-1,3-dipropylxanthine (CPX) to activate the defectivechloride channel in the CF bronchial epithelial

cell line CFBE41o-, which expresses the DF508mutation. It was investigated through x-ray micro-analysis. Both genistein and CPX act by stimulatingthe chlorine flow end increasing the transmem-brane conductance in CF. It was observed that8-bromo-cyclic-adenosine monophosphate (cAMP)alone does not induce Cl- efflux in CFBE41o-cells, but, after incubation with phenylbutyrate, asignificant efflux of Cl- occurs showing thatphenylbutyrate allows the DF508 CF transmem-brane conductance regulator to escape degrada-

tion and to be transported to the cell surface. Thestimulation of cells, with a combination of genis-tein and cAMP, also induces Cl- efflux, whereas




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a combination of pretreatment with phenyl-butyrate and a combined stimulation with genis-tein and cAMP induces an even larger Cl- efflux.Cl- efflux could also be stimulated by CPX, butthis effect is not enhanced by phenylbutyratepretreatment.[158]

It has been shown that arginine butyrate (AB)and two novel short-chain fatty acid derivatives,a-methylhydrocinnamic acid (ST7) and 2,2-dimethyl-butyrate (ST-20), functionally correctthe DF508-CFTR defect in airway IB3-1 cells,which are heterozygotes for the CFTR mutations

DF508 and W1282X.[159]

Singh et al.[160] have highlighted a specific subsetof HSP70 system proteins that sense the rescue ofDF508-CFTR from ER-associated degradation(ERAD). The phenylbutyrate-mediated rescue ofDF508-CFTR is associated with changes in thenetwork of ER-resident cytosolic chaperones andproinflammatory responses that mimic patternsof protein interaction observed with wild-typeCFTR expression.

The proteome profiling of IB3-1 CF bronchial

epithelial cells, treated with phenylbutyrate, hasbeen investigated to identify butyrate-responsivecellular chaperones, protein-processing enzymes,and cell-trafficking molecules associated with theamelioration of the chloride transport defect inthese cells (analyzed by two-dimensional gel elec-trophoresis and mass spectrometry). The samestudy has shown that over a pI range of 47 andmolecular weight 20150 kDa, a total of 85 dif-ferentially expressed proteins have beem detected.They were mostly chaperones, catalytic enzymes,

and proteins comprising structural elements, cel-lular defense, protein biosynthesis, traffickingactivity, and ion transport.[161]

Phenylbutyrate decreases the expression ofHSC70 mRNA and protein by inducing cellularadaptations that result in the decreased stabilityof HSC70 mRNA. Phenylbutyrate has been ob-served to stimulate the degradation of HSC70mRNA in IB3-1 cells without significantly alter-ing HSC70 mRNA synthesis, promoter activity,or protein turnover. The authors suggest that,

since this response requires a preincubation withphenylbutyrate, it results from a cellular adapta-tion to phenylbutyrate treatment.[162]

In an in vitro experiment, IB3-1 cells (CF epi-thelial cells genotype; DF508/W1282X) have beentreated with phenylbutyrate 0.055 mmol/L for2 days in culture showing a dose-dependent re-duction in Hsc70 protein immunoreactivity andmRNA levels. Immunoprecipitation with Hsc70-specific antiserum demonstrates that Hsc70 andCFTR are associated under control conditions.Levels of immunoreactive Hsp40, Hdj2, Hsp70,Hsp90, and calnexin were unaffected by phenyl-butyrate treatment. This study suggests that phe-nylbutyrate may improve DF508-CFTR trafficking

by allowing a greater proportion of mutant CFTRto escape association with Hsc70.[163]

Butyrate and phenylbutyrate, using patch clamprecording from CFTR-transfected mammaliancell lines, cause a voltage-dependent block ofCFTR Cl- currents when applied to the cyto-plasmic face of membrane patches, with appar-ent dissociation constants (Kds) [at 0 mV] of29.6 mmol/L for butyrate and 6.6 mmol/L forphenylbutyrate. At the single channel level, boththese fatty acids cause an apparent reduction in

CFTR current amplitude, suggesting a kineticallyfast blocking mechanism. The concentration-dependence of the block suggests that CFTR-mediated Cl- currents in vivo may be affected byboth phenylbutyrate, used in the treatment of var-ious diseases, including CF, and by butyrate pro-duced endogenously within the colonic lumen.[164]

It has been proposed that a combination ofchronic treatment with phenylbutyrate or selectedflavonoids, followed by acute flavonoid ex-posure, may be beneficial in CF as proved by an

increase in Cl- conductance measured by Cl- ef-flux in cells (IB3-1 cells [F508/W1282X]) that aretreated for 24 hours with phenylbutyrate andthen assayed with forskolin and genistein 1 mmol/Lor 5mmol/L, and also with cells treated for 24 hourswith either phenylbutyrate, apigenin 5mmol/L, orquercetin 1mmol/L.[153]

Phenylbutyrate (5mmol/L for 6 hours) increasesthe functional expression of epithelial sodiumchannels (ENaCs) in the apical membrane ofnon-CF and CF nasal epithelial cells by enhanc-

ing exocytosis of ENaC subunits likely via regu-lation of heat shock proteins, suggesting that(i) in non-CF patients, phenylbutyrate treatment




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may be useful to treat airway diseases in whichENaC trafficking may be disrupted, as it has beenobserved during hypoxia or endotoxemia; and(ii) in CF disease, although phenylbutyrate maybe useful to restore functional mutant CFTR atthe apical membrane, it is important to keep inmind that this effect may be counteracted by anincrease in sodium hyperabsorption.[165]

The clinical trials involving the use of phenyl-butyrate for treatment of cystic fibrosis are sum-marized in table SV (see Supplemental DigitalContent).

4.7 Phenylbutyrate and Huntington Disease

The symptomatic Huntington disease (HD)onset occurs typically between 30 and 50 years ofage, and mutant huntingtin has been shown todisrupt activator-dependent transcription in theearly stages of HD pathogenesis, which has beenrelated to transcriptional deregulation and func-tional loss of transcriptional co-activator proteins,leading to neuronal loss and gliosis, particularly

in the cortex and basal ganglia regions of the HDpatients brain.[166] HD is an inherited autosomaldominant neurodegenerative disorder caused bythe expansion of the polyglutamine tract near theamino terminus of the huntington protein (HTT;CAG trinucleotide repeat located in the Hun-tington gene), leading to polyglutamine-dependentinclusions, which have been seen in some HD brains,and some mouse models that express a smallfragment of mutant huntingtin.[167] This diseaseleads to the loss of medium spiny neurons affecting

mainly the striatum (caudate nucleus, putamen,and globus pallidus), and also the frontal andtemporal cortex causing chorea, psychiatric dis-turbances, and cognitive impairments.[168] Thereis evidence that mutant HTT interacts with thetranscription factors, leading to reduced histoneacetylation.[169] In particular phenylbutyrate im-proves survival and attenuates striatal atrophy inthe R6/2 transgenic mouse HD model, when ad-ministered presymptomatically, starting at 21 daysof age.[170]

Administration of the histone deacetylaseinhibitor phenylbutyrate (intraperitoneal injec-tions of phenylbutyrate [100 mL/kg/day, volume

3.33 mL/kg] or vehicle [PBS, 3.33 mL/kg], 6 daysper week from 75 days of age) after the onset ofsymptoms in a transgenic mouse HD model(transgenic N171-82Q mice maintained on aB6C3F1 background) has shown that (i) a signif-icant attenuation of gross brain atrophy andventricular enlargement, as well as a neuronalatrophy after phenylbutyrate treatment (significantat 120 days) has been observed; (ii) phenylbutyratetreatment significantly reduces striatal neuronatrophy in N171-82Q mice; (iii) the administra-tion of phenylbutyrate for 2 weeks increases im-

munostaining for both acetylated histone H3 andhistone H4 in striatal neurons, and the adminis-tration of phenylbutyrate at 100 mg/kg increaseshistone acetylation in the spleen and brain at2 hours post-administration; (iv) the immuno-cytochemistry shows a marked increase in meth-ylation of histone 3 in the striatum at 120 days ofage, which is markedly attenuated by phenyl-butyrate treatment; (v) the expression of selectedgenes (Gfer, Gstm3, and Psma3) is significantlyupregulated after phenylbutyrate treatment

compared with controls, whereas the other genes(Casp9, Cflar, and Prkce), after phenylbutyratetreatment, have shown a significantly lower ex-pression; and (vi) the caspase 3 immunoactivity(caspase 9 is involved in activation of caspase 3,and has increased in HD patients) at 120 days ofage is markedly increased in the striatum of theN171-82Q mice compared with controls, and theincrease is markedly attenuated by phenyl-butyrate treatment.[169]

Affymetrix GeneChip U133A and Amersham

Biosciences CodeLink (Amersham Biosciences)oligonucleotide microarrays have been used toanalyze global gene expression changes in bloodsamples from 17 HD subjects (12 symptomaticand five late presymptomatic carriers of the HDmutation) and 14 healthy, age- and gender-matchedcontrol subjects. 322 mRNAs, showing significantlyaltered expression in HD blood samples, have beenidentified in this study. Some changes in bloodmRNAs that clearly distinguish HD patients fromcontrols, and that correlate with disease progression

and response to experimental treatment, havealso been identified as follows: (i) a subset of up-regulated mRNAs, selected from this group, is




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able to distinguish controls, presymptomatic in-dividuals carrying the HD mutation, and symp-tomatic HD patients; (ii) early presymptomaticsubjects have shown gene expression profiles sim-ilar to those of controls, whereas late presymp-tomatic subjects have shown altered expressionthat resembles that of symptomatic HD patients;(iii) these elevated mRNAs were significantly re-duced in HD patients involved in a dose-findingstudy of phenylbutyrate; and (iv) the expressionof the marker genes is significantly upregulated inpost mortem HD caudate, suggesting that alter-

ations in blood mRNAs may reflect disease mech-anisms observed in HD brain.[171]

A phenylbutyrate dose-finding study in 21 sub-jects with HD found that phenylbutyrate was safeand well tolerated up to 15 g/day (maximum tol-erated dose), while at 18g/day, dose-limitingtoxicity emerged in 5 of 11 subjects with gait in-stability, which was the most commonly reportedadverse event (observed in one-third of the sub-

jects) at all doses tested, and it was dose-limitingin two subjects. At higher doses, other reported

toxicities were vomiting, lightheadedness, andconfusion. A gene expression assay was able todetect a biologic response to the drug in peri-pheral lymphocytes. These results provide safetyand dose-ranging data to support a phase IIclinical trial with phenylbutyrate in HD and re-lated neurodegenerative disorders.[172]

5. Conclusions

HDACIs have been widely studied since they

seem to give the oppurtunity to manipulate geneexpression and, through it, they seem to bepromising to treat several pathologic conditions.Apart from the previously described conditions,recently, phenylbutyrate has also been inves-tigated in Wilson disease, an autosomal recessivedisorder of copper homeostasis. Wilson disease iscaused by mutations in the gene encoding the cop-per transporting P1B-type adenosine triphos-phatase (ATPase) ATP7B, and, currently, morethan 300 different mutations have been described.

Treatment with phenylbutyrate and curcuminhas partially restored protein expression of mostATP7B mutants (p.G85V, p.R778L, p.H1069Q,

p.C1104F, p.V1262F, p.G1343V, and p.S1363F).[173]

Phenylbutyrate has also been investigated re-cently in deficiency in P-type ATP8B1 (caused byautosomal recessive mutations in the gene en-coding ATP8B1), a severe and clinically highlyvariable hereditary disorder characterized by in-trahepatic cholestasis. This condition is clinicallydivided into progressive familial intrahepaticcholestasis type 1 or intermittent, i.e. benign re-current intrahepatic cholestasis type 1 disease. Inthis study, it has been observed that a large pro-portion of ATP8B1 mutations have resulted in

aberrant folding and decreased expression at theplasma membrane, and, interestingly, these effectshave been partially restored by treatment withphenylbutyrate. Therefore, according to the twopreviously described studies, it has been pointedout that pharmacologic chaperone may be animportant strategy in the future management ofhereditary liver disease in general.[174] A studyreveals that phenylbutyrate possesses chemicalchaperone activity in vitro, which prevents theaggregation of denatureda-lactalbumin and bovine

serum albumin. The same authors investigated theeffects of phenylbutyrate on the accumulation ofParkin-associated endothelin receptor-like receptor(Pael-R), pathologically relevant to the loss ofdopaminergic neurons in autosomal recessive ju-venile parkinsonism, showing that (i) phenyl-butyrate restores the normal expression of Pael-Rprotein and suppresses ER stress induced by theoverexpression of Pael-R; (ii) phenylbutyrate at-tenuates the activation of ER stress-induced signaltransduction pathways and subsequent neuronal

cell death; and (iii) phenylbutyrate restores the vi-ability of yeasts that fail to induce an ER stressresponse under ER stress conditions. These find-ings lead the author to conclude that phenyl-butyrate suppresses ER stress by directly reducingthe amount of misfolded protein, including Pael-Raccumulated in the ER.[175]

A recent study has shown that 5-week admin-istration of phenylbutyrate reverses spatial learn-ing and memory deficits in an established mousemodel of Alzheimer disease, i.e. Tg2576 mice,

without altering b-amyloid burden. In addition,the same authors have observed that transgenicmice have showed a significant decrease in phos-




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phorylated tau in the hippocampus, after phe-nylbutyrate treatment, which may be due to an in-crease in the inactive form of the glycogen synthasekinase 3b (GSK3b). A consistent decrease in brainhistone acetylation in the transgenic mice, whichmay reflect an indirect transcriptional repressionunderlying memory impairment, has also beenobserved. The administration of phenylbutyratehas restored brain histone acetylation levels andactivated the transcription of synaptic plasticitymarkers, such as the selective glutamate recep-tor (GluR)-1 subunit of the a-amino-3-hydroxy-

5-methyl-4-isoxazolepropionate (AMPA) receptor,PSD95, and microtubule-associated protein-2. Ac-cording to these results, phenylbutyrate may rep-resent a new therapeutic agent to restore memoryfunction in AD.[176]

A recent experimental study showed usingboth an in vivo and in vitro approach that phe-nylbutyrate treatment may be successfully used toreduce the plasma levels of neurotoxic branched-chain amino acids and their corresponding a-ketoacids in a subset of maple syrup urine disease pa-

tients encouraging further studies of its long-termefficacy.[177]

A recent study has also analyzed the efficacy ofphenylbutyrate on lipid-induced insulin resis-tance and b-cell dysfunction in eight overweightor obese nondiabetic men who were randomizedto undergo four studies each 46 weeks apart. Twostudies were preceded by 2 weeks of phenylbutyrate(7.5g/day) orally, followed by a 48-hour intravenousinfusion of intralipid/heparin or saline, and twostudies were preceded by placebo treatment, fol-

lowed by similar infusions. It was observed thatlipid infusion reduced insulin sensitivity (S[I]), asassessed by hyperinsulinemic-euglycemic clamps,while it was significantly ameliorated by pretreat-ment with phenylbutyrate. Absolute insulin secre-tion rate (ISR), assessed by hyperglycemic clamps,was not affected by any treatment. Phenylbutyratepartially ameliorated the lipid-induced reductionin the disposition index (DI= ISRS[I]) pointingout how phenylbutyrate prevented lipid-inducedb-cell dysfunction.[178]

Particularly in this review we have reportedthe different clinical uses of phenylbutyrate, bothin vitro and in vivo, in lethal illnesses that do not

have any effective treatment. In our opinion, thisdrug should be more widely and deeply inves-tigated in the clinical area but, unfortunately, theabsence of viable international patents to protectcommercial use of the molecule reduces the chancesof financing new studies. Despite this frustratingposition, there is a wide speculative attention tofurther investigate phenylbutyrate, even on ananedoctical basis, and we definitely support thischallenge.

Acknowledgments

The authors have contributed equally to this work. Thisreview has not been supported by grants.

Competing interests: the authors certify that there is noconflict of interest with any financial organization regardingthe material discussed in the manuscript.

Authors contribution: the authors hereby certify that allwork contained in this review is original work of TommasoIannitti and Beniamino Palmieri. All the information takenfrom other articles, including tables and illustrations, havebeen referenced in the References section. The authors claimfull responsibility for the contents of the article.

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