Effects of Cytochrome P450 and Transporter Polymorphisms on the Bioavailability and Safety of Dutasteride and Tamsulosin

Effects of CYP2D6 allelic variants on therapy with tamsulosin in patients with benign prostatic hyperplasia

OBJECTIVES

Tamsulosin is a first-line drug for the treatment of lower urinary tract symptoms (LUTS) associated with benign prostatic hyperplasia (BPH). Despite its high ratings for efficacy and safety, these parameters may vary due to genetic polymorphisms of CYP2D6 enzyme, which is involved in the metabolism of the drug. This variability may have great impact on the therapy of LUTS associated with BPH and may require an individualized approach to drug selection. The aim of the study was to assess the impact of genetic polymorphisms in CYP2D6 on the efficacy and safety of tamsulosin therapy in patients with LUTS associated with BPH.

METHODS

The study included 106 patients with LUTS/BPH (N40 according to ICD-10). All patients received monotherapy with tamsulosin 0.4 mg/day for at least 8 weeks. Depending on the severity of symptoms, all patients were divided into 2 groups based on the IPSS score: the first group of patients had moderate symptoms (n=57), and the second group of patients had severe symptoms (n=49). The results of treatment were assessed using the IPSS questionnaire with determination of quality of life (QoL), transrectal ultrasound of the prostate with determination of prostate volume and postvoid residual urine volume, and uroflowmetry. The carriage of allelic variants of CYP2D6 (*3, *4, *9, *10, and *41) were determined by polymerase chain reaction in all patients.

RESULTS

In patients with moderate symptoms who was classified as «intermediate» metabolizers by CYP2D6, a statistically significant greater reduction in symptoms according to the overall IPSS scale at 8 weeks (p=0.046) and the obstructive symptom subscale starting from 4 weeks of treatment (p<0.05) was shown. Allelic variants of the CYP2D6 gene did not affect the frequency of adverse reactions to tamsulosin.

CONCLUSIONS

The results of the study show that in patients with moderate LUTS associated with BPH who are «intermediate» metabolizers by CYP2D6, there is a better therapeutic effect of tamsulosin.

Combined and independent effects of OCT1 and CYP2D6 on the cellular disposition of drugs

The organic cation transporter 1 (OCT1) mediates the cell uptake and cytochrome P450 2D6 (CYP2D6) the metabolism of many cationic substrates. Activities of OCT1 and CYP2D6 are affected by enormous genetic variation and frequent drug-drug interactions. Single or combined deficiency of OCT1 and CYP2D6 might result in dramatic differences in systemic exposure, adverse drug reactions, and efficacy. Thus, one should know what drugs are affected to what extent by OCT1, CYP2D6 or both. Here, we compiled all data on CYP2D6 and OCT1 drug substrates. Among 246 CYP2D6 substrates and 132 OCT1 substrates, we identified 31 shared substrates. In OCT1 and CYP2D6 single and double-transfected cells, we studied which, OCT1 or CYP2D6, is more critical for a given drug and whether there are additive, antagonistic or synergistic effects. In general, OCT1 substrates were more hydrophilic than CYP2D6 substrates and smaller in size. Inhibition studies showed unexpectedly pronounced inhibition of substrate depletion by shared OCT1/CYP2D6 inhibitors. In conclusion, there is a distinct overlap in the OCT1/CYP2D6 substrate and inhibitor spectra, so in vivo pharmacokinetics and -dynamics of shared substrates may be significantly affected by frequent OCT1- and CYP2D6-polymorphisms and by comedication with shared inhibitors.

CYP2C8*3 and *4 define CYP2C8 phenotype: An approach with the substrate cinitapride

Cinitapride is a gastrointestinal prokinetic drug, prescribed for the treatment of functional dyspepsia, and as an adjuvant therapy for gastroesophageal reflux disease. In this study, we aimed to explore the impact of relevant variants in CYP3A4 and CYP2C8 and other pharmacogenes, along with demographic characteristics, on cinitapride pharmacokinetics and safety; and to evaluate the impact of CYP2C8 alleles on the enzyme's function. Twenty-five healthy volunteers participating in a bioequivalence clinical trial consented to participate in the study. Participants were genotyped for 56 variants in 19 genes, including cytochrome P450 (CYP) enzymes (e.g., CYP2C8 or CYP3A4) or transporters (e.g., SLC or ABC), among others. CYP2C8*3 carriers showed a reduction in AUC of 42% and C_max of 35% compared to *1/*1 subjects (p = 0.003 and p = 0.011, respectively). *4 allele carriers showed a 45% increase in AUC and 63% in C_max compared to *1/*1 subjects, although these differences did not reach statistical significance. CYP2C8*3 and *4 alleles may be used to infer the following pharmacogenetic phenotypes: ultrarapid (UM) (*3/*3), rapid (RM) (*1/*3), normal (NM) (*1/*1), intermediate (IM) (*1/*4), and poor (PM) metabolizers (*4/*4). In this study, we properly characterized RMs, NMs, and IMs; however, additional studies are required to properly characterize UMs and PMs. These findings should be relevant with respect to cinitapride, but also to numerous CYP2C8 substrates such as imatinib, loperamide, montelukast, ibuprofen, paclitaxel, pioglitazone, repaglinide, or rosiglitazone.