Toward predicting CYP2D6-mediated variable drug response from CYP2D6 gene sequencing data

Pharmacogenomics is a key component of personalized medicine that promises safer and more effective drug treatment by individualizing drug choice and dose based on genetic profiles. In clinical practice, genetic biomarkers are used to categorize patients into *-alleles to predict CYP450 enzyme activity and adjust drug dosages accordingly. However, this approach leaves a large part of variability in drug response unexplained. Here, we present a proof-of-concept approach that uses continuous-scale (instead of categorical) assignments to predict enzyme activity. We used full CYP2D6 gene sequences obtained with long-read amplicon-based sequencing and cytochrome P450 (CYP) 2D6–mediated tamoxifen metabolism data from a prospective study of 561 patients with breast cancer to train a neural network. The model explained 79% of interindividual variability in CYP2D6 activity compared to 54% with the conventional *-allele approach, assigned enzyme activities to known alleles with previously reported effects, and predicted the activity of previously uncharacterized combinations of variants. The results were replicated in an independent cohort of tamoxifen-treated patients (model R2 adjusted = 0.66 versus *-allele R2 adjusted = 0.35) and a cohort of patients treated with the CYP2D6 substrate venlafaxine (model R2 adjusted = 0.64 versus *-allele R2 adjusted = 0.55). Human embryonic kidney cells were used to confirm the effect of five genetic variants on metabolism of the CYP2D6 substrate bufuralol in vitro. These results demonstrate the advantage of a continuous scale and a completely phased genotype for prediction of CYP2D6 enzyme activity and could potentially enable more accurate prediction of individual drug response.

Effect of Genetic Variability in 20 Pharmacogenes on Concentrations of Tamoxifen and Its Metabolites

Background: Tamoxifen, as a treatment of estrogen receptor positive (ER+) breast cancer, is a weak anti-estrogen that requires metabolic activation to form metabolites with higher anti-estrogenic activity. Endoxifen is the most-studied active tamoxifen metabolite, and endoxifen concentrations are highly associated with CYP2D6 activity. Associations of tamoxifen efficacy with measured or CYP2D6-predicted endoxifen concentrations have been inconclusive. Another active metabolite, 4-OHtam, and other, less active metabolites, Z-4′-endoxifen and Z-4′-OHtam, have also been reported to be associated with tamoxifen efficacy. Method: Genotype for 20 pharmacogenes was determined by VeriDose® Core Panel and VeriDose®CYP2D6 CNV Panel, followed by translation to metabolic activity phenotype following standard activity scoring. Concentrations of tamoxifen and seven metabolites were measured by UPLC-MS/MS in serum samples collected from patients receiving 20 mg tamoxifen per day. Metabolic activity was tested for association with tamoxifen and its metabolites using linear regression with adjustment for upstream metabolites to identify genes associated with each step in the tamoxifen metabolism pathway. Results: A total of 187 patients with genetic and tamoxifen concentration data were included in the analysis. CYP2D6 was the primary gene associated with the tamoxifen metabolism pathway, especially the conversion of tamoxifen to endoxifen. CYP3A4 and CYP2C9 were also responsible for the metabolism of tamoxifen. CYP2C9 especially impacted the hydroxylation to 4-OHtam, and this involved the OATP1B1 (SLCO1B1) transporter. Conclusion: Multiple genes are involved in tamoxifen metabolism and multi-gene panels could be useful to predict active metabolite concentrations and guide tamoxifen dosing.

Reduction in Tamoxifen Metabolites Endoxifen and N-desmethyltamoxifen With Chronic Administration of Low Dose Cannabidiol: A CYP3A4 and CYP2D6 Drug Interaction

Background: Cannabidiol (CBD) serves as a promising medicine, with few known adverse effects apart from the potential of drug interactions with the cytochrome P450 system. It has been hypothesized drug interactions may occur with chemotherapeutic agents, but no supporting evidence has been published to date. Case: A 58-year-old female with a history of bilateral breast carcinoma in remission, was treated with tamoxifen for breast cancer prevention for over 6 years. CBD was instituted to treat persistent postsurgical pain, inadequately managed by alternate analgesics. It was postulated that CBD may diminish tamoxifen metabolism by CYP3A4 and 2D6 to form active metabolite endoxifen, which exerts the anticancer benefits. Endoxifen, tamoxifen, N-desmetyltamoxifen and 4-hydroxytamoxifen levels were collected while the patient chronically received CBD 40 mg/day, and after a 60-day washout. Upon discontinuation of CBD 40 mg/day, it was observed that endoxifen levels increased by 18.75% and N-desmethyltamoxifen by 9.24%, while 4-hydroxytamoxifen remained unchanged. Conclusion: CBD at a low dose of 40 mg/day resulted in the potential inhibition of CYP3A4 and/or CYP2D6. Patients receiving CBD and interacting chemotherapeutic drugs, such as tamoxifen, require monitoring to identify possible subtherapeutic response to treatment. Further pharmacokinetic studies are required to ascertain the dynamics of this drug interaction.

Interactions between antidepressants, sleep aids and selected breast cancer therapy

Depression and insomnia are very significant pathologies in cancer patients as they contribute to the patient’s overall cure and quality of life. Moreover, untreated depression and ongoing insomnia are associated with decreased immune responses and lower survival rates. With all disease states and especially with cancer, close attention to drug-drug interactions and the potential impact on the efficacy of therapy is paramount. One area of particular interest due to the lack of well-done clinical trials is drug-drug interaction(s) between antidepressants and cancer treatment. Pharmacokinetics of a certain drug allows for prediction of certain drug interactions based on chemical properties of the agents involved. If the agents depend on their metabolites for activity, active drug level will be decreased through this enzyme inhibition. In this paper, we looked at the cytochrome-P450 drug interactions between antidepressants and sleep aids with Selective Estrogen Receptor Modulators (SERM). Newer SERM metabolisms are less influenced by interactions with medications used to treat depression. However, tamoxifen metabolism could be severely altered by several antidepressants. This has direct consequences as patients on tamoxifen and antidepressant can have double the risk of relapse to cancer in two years. We discussed those interactions and made recommendations for clinical use.

Interactions between antidepressants, sleep aids and selected breast cancer therapy

Depression and insomnia are very significant pathologies in cancer patients as they contribute to the patient's overall cure and quality of life. Moreover, untreated depression and ongoing insomnia are associated with decreased immune responses and lower survival rates. With all disease states and especially with cancer, close attention to drug-drug interactions and the potential impact on the efficacy of therapy is paramount. One area of particular interest due to the lack of well-done clinical trials is drug-drug interaction(s) between antidepressants and cancer treatment. Pharmacokinetics of a certain drug allows for prediction of certain drug interactions based on chemical properties of the agents involved. If the agents depend on their metabolites for activity, active drug level will be decreased through this enzyme inhibition. In this paper, we looked at the cytochrome-P450 drug interactions between antidepressants and sleep aids with Selective Estrogen Receptor Modulators (SERM). Newer SERM metabolisms are less influenced by interactions with medications used to treat depression. However, tamoxifen metabolism could be severely altered by several antidepressants. This has direct consequences as patients on tamoxifen and antidepressant can have double the risk of relapse to cancer in two years. We discussed those interactions and made recommendations for clinical use.