The metabolism of tamoxifen by human liver microsomes is not mediated by cytochrome P450IID6

Cytochrome P-450 2D6 (CYP2D6) Genotype and Breast Cancer Recurrence in Tamoxifen-Treated Patients: Evaluating the Importance of Loss of Heterozygosity

Tamoxifen therapy for estrogen receptor-positive breast cancer reduces the risk of recurrence by approximately one-half. Cytochrome P-450 2D6, encoded by the polymorphic cytochrome P-450 2D6 gene (CYP2D6), oxidizes tamoxifen to its most active metabolites. Steady-state concentrations of endoxifen (4-hydroxy-N-desmethyltamoxifen), the most potent antiestrogenic metabolite, are reduced in women whose CYP2D6 genotypes confer poor enzyme function. Thirty-one studies of the association of CYP2D6 genotype with breast cancer survival have yielded heterogeneous results. Some influential studies genotyped DNA from tumor-infiltrated tissues, and their results may have been susceptible to germline genotype misclassification from loss of heterozygosity at the CYP2D6 locus. We systematically reviewed 6 studies of concordance between genotypes obtained from paired nonneoplastic and breast tumor-infiltrated tissues, all of which showed excellent CYP2D6 genotype agreement. We applied these concordance data to a quantitative bias analysis of the subset of the 31 studies that were based on genotypes from tumor-infiltrated tissue to examine whether genotyping errors substantially biased estimates of association. The bias analysis showed negligible bias by discordant genotypes. Summary estimates of association, with or without bias adjustment, indicated no clinically important association between CYP2D6 genotype and breast cancer survival in tamoxifen-treated women.

Prostate cancer: germline prediction for a commonly variable malignancy

What's known on the subject? and What does the study add? Prostate cancer is a heterogeneous disease and biomarkers to predict its incidence and subsequent clinical behaviour are needed to tailor screening, prevention and therapeutic strategies. Rare mutations in genes such as BRCA1, BRCA2 and HOXB13 can affect prostate cancer incidence and/or clinical behaviour. Genome wide association studies (GWAS) have identified more common genetic variations that explain an estimated 20% of familial prostate cancer risk. In this review, we focus on the potential of germline genetic variation to provide biomarkers for prostate cancer screening, prevention and management. We discuss how germline genetics may have a role in treatment selection if reliable pharmacogenetic predictors of efficacy and toxicity can be identified. We have outlined possible mechanisms for including germline investigation in future prostate cancer clinical trials.

OBJECTIVES

• Prostate cancer is a heterogeneous disease and biomarkers to predict its incidence and subsequent clinical behaviour are needed to tailor screening, prevention and therapeutic strategies. • In this review we focus on the potential of germline genetic variation to provide these biomarkers.

METHODS

• We review the published literature on germline genetics in prostate cancer and examine the possibility of including germline genetic biomarkers in future prostate cancer clinical trials.

RESULTS

• Rare mutations in genes such as BRCA1, BRCA2 and HOXB13 can affect prostate cancer incidence and/or clinical behaviour. • Genome-wide association studies (GWAS) have identified more common genetic variations that explain an estimated 20% of familial prostate cancer risk. • Germline genetics may have a role in treatment selection, if reliable pharmacogenetic predictors of efficacy and toxicity can be identified.

CONCLUSION

• This rapidly emerging area of prostate cancer research may provide answers to current clinical conundrums in the prostate cancer treatment paradigm. We have outlined possible mechanisms for including germline investigation in future prostate cancer clinical trial design.

Variable contribution of cytochromes P450 2D6, 2C9 and 3A4 to the 4-hydroxylation of tamoxifen by human liver microsomes

4-Hydroxylation is an important pathway of tamoxifen metabolism because the product of this reaction is intrinsically 100 times more potent as an oestrogen receptor antagonist than is the parent drug. Although tamoxifen 4-hydroxylation is catalysed by human cytochrome P450 (CYP), data conflict on the specific isoforms responsible. The aim of this study was to define unequivocally the role of individual CYPs in the 4-hydroxylation of tamoxifen by human liver microsomes. Microsomes from each of 10 human livers catalysed the reaction [range = 0.6-2.9 pmol/mg protein/min (1 microM substrate concentration) and 6-25 pmol/mg protein/min (18 microM)]. Three of the livers with the lowest tamoxifen 4-hydroxylation activity were from genetically poor metabolisers with respect to CYP2D6. Inhibition of activity by quinidine (1 microM), sulphaphenazole (20 microM) and ketoconazole (2 microM), selective inhibitors of CYPs 2D6, 2C9 and 3A4, respectively, was 0-80%, 0-80% and 12-57%. The proportion of activity inhibited by quinidine correlated positively with total microsomal tamoxifen 4-hydroxylation activity (rs = 0.89, P < 0.01), indicating a major involvement of CYP2D6 in this reaction. Recombinant human CYPs 2D6, 2C9 and 3A4 but not CYPs 1A1, 1A2, 2C19 and 2E1 displayed significant 4-hydroxylation activity. Similar inhibition and correlation experiments confirmed that tamoxifen N-demethylation is catalysed predominantly by CYP3A4. These findings indicate that the 4-hydroxylation of tamoxifen is catalysed almost exclusively by CYPs 2D6, 2C9 and 3A4 in human liver microsomes. However, the marked between-subject variation in the contribution of these isoforms underlines the need to study metabolic reactions in a sufficient number of livers that are characterised with respect to a range of cytochrome P450 activities.

Dextromethorphan as an in vivo probe for the simultaneous determination of CYP2D6 and CYP3A activity

Dextromethorphan (DM) is O-demethylated into dextrorphan (DEX) in humans by the cytochrome P450 designated as CYP2D6 and N-demethylated into 3-methoxymorphinan (3MM) via CYP3As. Clinically, DM has been successfully used as an index of CYP2D6 and this paper describes analytical and clinical data that will help evaluate the use of DM hydrobromide as a probe of CYP3A activity. DM and its three demethylated metabolites were measured in a 4-h spot urine sample using a HPLC method employing solid-phase extraction (C(18)), analysis on a phenyl column [mobile phase, methanol-acetonitrile-phosphate buffer (10 mM, pH 3.5, 20:25:55, v/v)] and fluorescence detection (excitation at lambda=228 nm, no emission cut-off filter). The urinary molar ratio DM-DEX was used to assess CYP2D6 activity while DM-3MM was used for CYP3As. The DM-3MM ratios were sensitive to the co-administration of selective CYP3A inhibitors grapefruit juice and erythromycin. In addition, in healthy volunteers and cancer patients, the N-demethylation of DM correlated with the CYP3A-mediated metabolism of verapamil and tamoxifen. DM appears to be a promising way to simultaneously phenotype patients for CYP2D6 and CYP3As.

The active metabolite hydroxytamoxifen of the anticancer drug tamoxifen induces structural changes in membranes

The effects of hydroxytamoxifen (OHTAM) on lipid organization of pure phospholipid liposomes, native sarcoplasmic reticulum (SR) membranes and liposomes of SR lipids were evaluated by intramolecular excimer formation of 1,3-di(1-pyrenyl)propane (Py(3)Py) and by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) and its derivative 3-[p-(6-phenyl)-1,3,5-hexatrienyl]phenylpropionic acid (DPH-PA). OHTAM promotes alterations in the thermotropic profiles of DMPC, DPPC and DSPC. As detected by Py(3)Py and DPH-PA, OHTAM induces an ordering effect in the fluid phase and a fluidizing effect in the temperature range of the cooperative phase transition. In the gel phase, no significant effects are noticed, except for DSPC bilayers, where Py(3)Py and DPH-PA detect a disordering effect. In the hydrophobic region of the above membrane systems probed by DPH, OHTAM induces only a slight fluidizing effect in the range of the phase transition and a small ordering effect in the fluid phase. As detected by all probes, the drug broadens the transition profile of DMPC and shifts the main transition temperature to lower values. However, these effects, and so those observed for the fluid phase, decrease as the fatty acyl chain length increases. Moreover, the drug removes the pre-transitions of DPPC and DSPC bilayers, as probed by Py(3)Py. In fluid SR native membranes and liposomes of SR lipids, OHTAM induces a moderate ordering effect in the outer regions of the lipid bilayer, as monitored by Py(3)Py and by DPH-PA, DPH failing to detect any apparent effect, as observed for the fluid phase of pure phospholipids. Apparently, OHTAM distributes preferentially in the outer region of the lipid bilayer, without significant effect in the bulk lipid organization of the bilayer interior. The changes of OHTAM in the bilayer dynamic properties and the different location across the bilayer thickness relative to its drug promoter (Custódio et al. (1993) Biochim. Biophys. Acta 1150, 123-129) may be involved in the cytostatic activity of tamoxifen.