Urinary Excretion of Phenytoin Metabolites, 5-(4′-hydroxyphenyl)-5-Phenylhydantoin and its O-glucuronide in Humans and Analysis of Genetic Polymorphisms of UDP-glucuronosyltransferases

Chemical metabolite synthesis and profiling: Mimicking in vivo biotransformation reactions

Biotransformation was previously viewed as merely the structural characterization of drug metabolites, and it was performed only when drug candidates entered clinical development. The synthesis of drug metabolites is crucial to the drug development process because it generates either pharmacologically active, inactive, or reactive molecules and hence their characterization and comprehensive pharmacological evaluation is necessary. The chemical metabolite synthesis is very challenging due to the complex structures of many drug molecules, presence of multiple stereocenters, poor reaction yields, and the formation of unwanted by-products. Drug metabolites and their chemical synthesis have immense significance in the drug discovery process. The chemical synthesis of metabolites facilitates on- or off-target pharmacological and toxicological evaluations at the easiest. In a broader view metabolite could be a target lead molecule for drug design, toxic reactive metabolites, pharmaceutical standards for bioanalytical methods, etc. Collectively these metabolite information dossiers will aid regulatory agencies such as the EMA and FDA in maintaining strict vigilance over drug manufacturers with regard to the safety of NCE's and their hidden metabolites. Herein, we are presenting a systematic compilation of chemical and biocatalytic strategies reported to date for pharmaceutical drug metabolite synthesis. This review report is very useful for the laboratory synthesis of new drug metabolites, and their preclinical biological evaluation could aid in the detection of early threats (alerts) in drug discovery, eliminate the toxicity profile, explore newer pharmacology, and delivering a promising and safe drug candidate to humankind.

Developmental Toxicity and Biotransformation of Two Anti-Epileptics in Zebrafish Embryos and Early Larvae

The zebrafish (Danio rerio) embryo is gaining interest as a bridging tool between in-vitro and in-vivo developmental toxicity studies. However, cytochrome P450 (CYP)-mediated drug metabolism in this model is still under debate. Therefore, we investigated the potential of zebrafish embryos and larvae to bioactivate two known anti-epileptics, carbamazepine (CBZ) and phenytoin (PHE), to carbamazepine-10,11-epoxide (E-CBZ) and 5-(4-hydroxyphenyl)-5-phenylhydantoin (HPPH), respectively. First, zebrafish were exposed to CBZ, PHE, E-CBZ and HPPH from 5¼- to 120-h post fertilization (hpf) and morphologically evaluated. Second, the formations of E-CBZ and HPPH were assessed in culture medium and in whole-embryo extracts at different time points by targeted LC-MS. Finally, E-CBZ and HPPH formation was also assessed in adult zebrafish liver microsomes and compared with those of human, rat, and rabbit. The present study showed teratogenic effects for CBZ and PHE, but not for E-CBZ and HPPH. No HPPH was detected during organogenesis and E-CBZ was only formed at the end of organogenesis. E-CBZ and HPPH formation was also very low-to-negligible in adult zebrafish compared with the mammalian species. As such, other metabolic pathways than those of mammals are involved in the bioactivation of CBZ and PHE, or, these anti-epileptics are teratogens and do not require bioactivation in the zebrafish.

Allele and genotype frequencies of genes relevant to anti-epileptic drug therapy in Mexican-Mestizo healthy volunteers

AIM

To determine allele and genotype frequencies of genes influencing anti-epileptic drug therapy in Mexican-Mestizo (MM) healthy volunteers, and to evaluate whether these are different from those reported for other populations.

SUBJECTS & METHODS

Thirty-nine variants of CYP3A5, EPHX1, NR1I2, HNF4A, UGT1A1, UGT2B7, ABCC2, RALBP1, SCN1A, SCN2A and GABRA1 were genotyped in 300 MM healthy volunteers.

RESULTS

All studied alleles were presented in MM, except for seven UGT1A1 variants (*6-8, 14, 15, 27 and 29). Allele and genotype frequencies showed interethnic variations when compared with European, Asian and African populations. Allele frequencies of greater than 30% were observed in ten genes.

CONCLUSION

The results presented regarding the frequencies and interethnic differences of these polymorphisms should be taken into account for future pharmacogenetic studies of anti-epileptic drugs in MM patients with epilepsy.

The Impact of Anti-Epileptic Drugs on Growth and Bone Metabolism

Epilepsy is a common neurological disorder worldwide and anti-epileptic drugs (AEDs) are always the first choice for treatment. However, more than 50% of patients with epilepsy who take AEDs have reported bone abnormalities. Cytochrome P450 (CYP450) isoenzymes are induced by AEDs, especially the classical AEDs, such as benzodiazepines (BZDs), carbamazepine (CBZ), phenytoin (PT), phenobarbital (PB), and valproic acid (VPA). The induction of CYP450 isoenzymes may cause vitamin D deficiency, hypocalcemia, increased fracture risks, and altered bone turnover, leading to impaired bone mineral density (BMD). Newer AEDs, such as levetiracetam (LEV), oxcarbazepine (OXC), lamotrigine (LTG), topiramate (TPM), gabapentin (GP), and vigabatrin (VB) have broader spectra, and are safer and better tolerated than the classical AEDs. The effects of AEDs on bone health are controversial. This review focuses on the impact of AEDs on growth and bone metabolism and emphasizes the need for caution and timely withdrawal of these medications to avoid serious disabilities.

The role of glucuronidation in drug resistance

The final therapeutic effect of a drug candidate, which is directed to a specific molecular target strongly depends on its absorption, distribution, metabolism and excretion (ADME). The disruption of at least one element of ADME may result in serious drug resistance. In this work we described the role of one element of this resistance: phase II metabolism with UDP-glucuronosyltransferases (UGTs). UGT function is the transformation of their substrates into more polar metabolites, which are better substrates for the ABC transporters, MDR1, MRP and BCRP, than the native drug. UGT-mediated drug resistance can be associated with (i) inherent overexpression of the enzyme, named intrinsic drug resistance or (ii) induced expression of the enzyme, named acquired drug resistance observed when enzyme expression is induced by the drug or other factors, as food-derived compounds. Very often this induction occurs via ligand binding receptors including AhR (aryl hydrocarbon receptor) PXR (pregnane X receptor), or other transcription factors. The effect of UGT dependent resistance is strengthened by coordinate action and also a coordinate regulation of the expression of UGTs and ABC transporters. This coupling of UGT and multidrug resistance proteins has been intensively studied, particularly in the case of antitumor treatment, when this resistance is "improved" by differences in UGT expression between tumor and healthy tissue. Multidrug resistance coordinated with glucuronidation has also been described here for drugs used in the management of epilepsy, psychiatric diseases, HIV infections, hypertension and hypercholesterolemia. Proposals to reverse UGT-mediated drug resistance should consider the endogenous functions of UGT.

Preliminary investigation of the contribution of CYP2A6, CYP2B6, and UGT1A9 polymorphisms on artesunate-mefloquine treatment response in Burmese patients with Plasmodium falciparum malaria

CYP2A6, CYP2B6, and UGT1A9 genetic polymorphisms and treatment response after a three-day course of artesunate-mefloquine was investigated in 71 Burmese patients with uncomplicated Plasmodium falciparum malaria. Results provide evidence for the possible link between CYP2A6 and CYP2B6 polymorphisms and plasma concentrations of artesunate/dihydroartemisinin and treatment response. In one patient who had the CYP2A6*1A/*4C genotype (decreased enzyme activity), plasma concentration of artesunate at one hour appeared to be higher, and the concentration of dihydroartemisinin was lower than for those carrying other genotypes (415 versus 320 ng/mL). The proportion of patients with adequate clinical and parasitologic response who had the CYP2B6*9/*9 genotype (mutant genotype) was significantly lower compared with those with late parasitologic failure (14.0% versus 19.0%). Confirmation through a larger study in various malaria-endemic areas is required before a definite conclusion on the role of genetic polymorphisms of these drug-metabolizing enzymes on treatment response after artesunate-based combination therapy can be made.

Acute and Chronic Administrations of Rheum palmatum Reduced the Bioavailability of Phenytoin in Rats: A New Herb-Drug Interaction

The rhizome of Rheum palmatum (RP) is a commonly used herb in clinical Chinese medicine. Phenytoin (PHT) is an antiepileptic with narrow therapeutic window. This study investigated the acute and chronic effects of RP on the pharmacokinetics of PHT in rat. Rats were orally administered with PHT (200 mg/kg) with and without RP decoction (single dose and seven doses of 2 g/kg) in a crossover design. The serum concentrations of PHT, PHT glucuronide (PHT-G), 4-hydroxyphenytoin (HPPH), and HPPH glucuronide (HPPH-G) were determined by HPLC method. Cell line models were used to identify the underlying mechanisms. The results showed that coadministration of single dose or multiple doses of RP significantly decreased the C_max and AUC_0-t as well as the K₁₀ of PHT, PHT-G, HPPH, and HPPH-G. Cell line studies revealed that RP significantly induced the P-gp-mediated efflux of PHT and inhibited the MRP-2-medicated transport of PHT and HPPH. In conclusion, acute and chronic coadministrations of RP markedly decreased the oral bioavailability of PHT via activation of P-gp, although the MRP-2-mediated excretion of PHT was inhibited. It is recommended that caution should be exercised during concurrent use of RP and PHT.

Influence of drug transporters and UGT polymorphisms on pharmacokinetics of phenolic glucuronide metabolite of mycophenolic acid in Japanese renal transplant recipients

Mycophenolic acid (MPA) is mainly glucuronized by uridine diphosphate-glucuronosyltransferases (UGTs) into the phenolic MPA glucuronide (MPAG). MPAG is excreted by transporters such as organic anion-transporting polypeptide (gene SLCO), multidrug resistance protein 2 (gene ABCC2), breast cancer resistance protein (BCRP, gene ABCG2) or P-glycoprotein (gene ABCB1). This study investigated the association of UGTs, SLCOs, ABCB1, ABCC2, and ABCG2 polymorphisms with MPAG pharmacokinetics in 80 Japanese renal transplant recipients. Eighty recipients were given repeated doses of combination immunosuppressive therapy consisting of mycophenolate mofetil and tacrolimus every 12 hours at a designated time (0900 and 2100). On day 28, after renal transplantation, plasma concentrations of MPA and MPAG were measured by high-performance liquid chromatography. There were no significant differences in the area under the plasma concentration-time curve (AUC) ratio of MPAG/MPA between UGT1A1, UGT1A6, UGT1A7, UGT1A8, and UGT1A9 I399C/T genotypes. On the other hand, the median dose-adjusted AUC0-12 of MPAG in SLCO1B1 1a/1a+1a/1b+1b+1b (n = 53) and 1a/*15 + 1b/*15+*15/*15 (n = 27) were 1549 and 1134 mg.h L g, respectively (P = 0.03004 in multivariate analysis). The median dose-adjusted AUC0-12 of MPAG in SLCO1B3 334T/T+T/G (699G/G+G/A, n = 46) and 334G/G (699A/A, n = 34) was 1191 and 1580 mg.h L g, respectively (P = 0.02792 in multivariate analysis). There were no significant differences in the dose-adjusted AUC0-12 of MPAG between the ABCB1 C3435T and ABCC2 C-24T genotypes. However, the dose-adjusted AUC0-12 of MPAG was significantly lower in recipients with ABCG2 421C/A+A/A (n = 44) than in those with C/C (n = 36) (P = 0.0295). In conclusion, our findings showed that MPAG pharmacokinetics were significantly influenced by SLCO1B1 and SLCO1B3 polymorphisms and not by UGT polymorphisms. BCRP rather than multidrug resistance protein 2 seems to be the transporter associated with biliary excretion of MPAG.

Stereoselective Glucuronidation of 5-(4′-Hydroxyphenyl)-5-phenylhydantoin by Human UDP-Glucuronosyltransferase (UGT) 1A1, UGT1A9, and UGT2B15: Effects of UGT-UGT Interactions

5-(4'-Hydroxyphenyl)-5-phenylhydantoin (4'-HPPH), a major metabolite of phenytoin in human, is exclusively metabolized to a glucuronide. 4'-HPPH has a chiral center. (S)-4'-HPPH is a predominant form produced from phenytoin in humans, and (R)-4'-HPPH is an extremely toxic form with respect to gingival hyperplasia. In the present study, we investigated stereoselective 4'-HPPH O-glucuronide formation in human liver microsomes. Human liver microsomes predominantly formed (S)-4'-HPPH O-glucuronide rather than (R)-4'-HPPH O-glucuronide from racemic 4'-HPPH. Among human UDP-glucuronosyltransferase (UGT) enzymes, UGT1A1, UGT1A9, and UGT2B15 showed 4'-HPPH O-glucuronide formation. Interestingly, UGT1A1 stereoselectively formed (R)-4'-HPPH O-glucuronide, whereas UGT1A9 and UGT2B15 stereoselectively formed (S)-4'-HPPH O-glucuronide from racemic 4'-HPPH. By using UGT1A double-expression systems in HEK293 cells that we previously established, the effects of UGT-UGT interactions on 4'-HPPH O-glucuronide formation were investigated. It was demonstrated that coexpression of UGT1A4 increased the V(max) values of (S)- and (R)-4'-HPPH O-glucuronide formation catalyzed by UGT1A1 but decreased the V(max) values of (S)- and (R)-4'-HPPH O-glucuronide formation catalyzed by UGT1A9. Coexpression of UGT1A6 increased the S(50) values and decreased the V(max) values of (S)- and (R)-4'-HPPH glucuronide formation catalyzed by UGT1A1 and UGT1A9. However, the interaction did not alter the stereoselectivity. In conclusion, we found that 4'-HPPH O-glucuronide formation in human liver microsomes is catalyzed by UGT1A1, UGT1A9, and UGT2B15 in a stereoselective manner, being modulated by interaction with other UGT1A isoforms.

Phenytoin immunoassay measurements in serum samples from patients with renal insufficiency: comparison with high-performance liquid chromatography

The debate continues regarding the possible interference of phenytoin metabolites in phenytoin immunoassays, and its clinical importance for patients with renal failure. The aim of this study was to compare the results obtained using the Abbott fluorescence polarization immunoassay (FPIA), Dade enzyme-multiplied immunoassay technique (EMIT), and high-performance liquid chromatography (HPLC) to establish the significance of the differences in conditions of renal failure. Thirty-six adult patients who had been treated with phenytoin and whose renal function ranged from normal to severely impaired were chosen for this study. In accordance with previously established validation criteria for analytical methods for the determination of drugs, a 15% bias from the HPLC phenytoin values was considered an acceptable limit. The mean (+/-SEM) glomerular filtration rate (GFR) of the patients was 37.5+/-4.6 mL/min (range = 10-102 mL/min). The mean values found using FPIA (10.8+/-1.2 microg/mL) and EMIT (10.8+/-1.3 microg/mL) presented acceptable deviations with respect to HPLC (10.5+/-1.2 microg/mL), and a high correlation was found among the results (N = 36) of the different methods (r > or = 0.987, P < 0.001). An FPIA deviation above the 15% bias limit with respect to HPLC was found only in two cases with very low serum phenytoin concentrations and low GFR values (< 20 mL/min), although it does not appear to be important in terms of adjusting drug dosage. According to our data, FPIA and EMIT gave accurate results for total phenytoin in serum samples from patients with renal failure.