Pharmacogenetic testing-guided treatment for oncology: an overview of reviews

Pharmacogenetics is the relationship between an individual's genetic variations and their response to pharmacological treatment. We conducted an overview of reviews on the use of post-treatment pharmacogenetic testing for oncology, based on clinically relevant gene-drug pairs. We conducted a search on Medline, Embase and Cochrane Library, from their inception to 18 June 2020. We selected six eligible systematic reviews. The most studied drug categories were estrogen agonists/antagonists and fluoropyrimidines associated with cytochrome P450 and dihydropyrimidine dehydrogenase genes (CYP2D6 and DPYD), but many studies were classified as being of critically low or low quality. There is a need for more high-quality primary studies and systematic reviews that assess the risk of bias, with consistent definitions of clinical outcomes to consider the benefits of pharmacogenetic testing for oncology.

Initialization of adjuvant hormonal treatment for breast cancer

The introduction of aromatase inhibitors (AI) has resulted in practice change approaches in the treatment of early breast cancer. In this paper, we analyze the most relevant studies including the ATAC, BIG 1-98, TEAM, MA-17, NSABP B-33, and ABSCG-6 studies. Postmenopausal patients with hormone receptor-positive early breast cancer should be treated with AI for 5 years. For patients who have been initiated with tamoxifen (TAM), switching to an AI to complete 5 years of treatment is also recommended. The results of the extended adjuvant therapy studies recommend the use of an AI (anastrozole, letrozole, or exemestane) after the completion of standard TAM treatment. With regards to premenopausal women, TAM is the recommended adjuvant hormonal treatment for pre- and perimenopausal women. There is no indication for the use of AI in these subgroups of patients. Finally, determination of CYP 2D6 polymorphisms could be considered when choosing the best adjuvant hormonal treatment option.

Symptom management in metastatic breast cancer

Approximately 40,000 women die as a result of breast cancer each year and many more live with advanced disease. When breast cancer recurs, the goals of treatment often shift from one of cure to controlling the disease for as long as possible while palliating symptoms interfering with the patient's functional status and quality of life. This requires ongoing discussions with the patient and family about the goals of care. Many symptoms depend on the site of metastasis, with bone being the most frequent, and commonly occur with fatigue, depression, insomnia, and pain. The purpose of this paper is to identify and provide an overview of the management of the most common symptoms in patients with breast cancer metastases.

Genotype-guided tamoxifen dosing increases active metabolite exposure in women with reduced CYP2D6 metabolism: a multicenter study

PURPOSE

We examined the feasibility of using CYP2D6 genotyping to determine optimal tamoxifen dose and investigated whether the key active tamoxifen metabolite, endoxifen, could be increased by genotype-guided tamoxifen dosing in patients with intermediate CYP2D6 metabolism.

PATIENTS AND METHODS

One hundred nineteen patients on tamoxifen 20 mg daily ≥ 4 months and not on any strong CYP2D6 inhibiting medications were assayed for CYP2D6 genotype and plasma tamoxifen metabolite concentrations. Patients found to be CYP2D6 extensive metabolizers (EM) remained on 20 mg and those found to be intermediate (IM) or poor (PM) metabolizers were increased to 40 mg daily. Eighty-nine evaluable patients had tamoxifen metabolite measurements repeated 4 months later.

RESULTS

As expected, the median baseline endoxifen concentration was higher in EM (34.3 ng/mL) compared with either IM (18.5 ng/mL; P = .0045) or PM (4.2 ng/mL; P < .001). When the dose was increased from 20 mg to 40 mg in IM and PM patients, the endoxifen concentration rose significantly; in IM there was a median intrapatient change from baseline of +7.6 ng/mL (-0.6 to 23.9; P < .001), and in PM there was a change of +6.1 ng/mL (2.6 to 12.5; P = .020). After the dose increase, there was no longer a significant difference in endoxifen concentrations between EM and IM patients (P = .84); however, the PM endoxifen concentration was still significantly lower.

CONCLUSION

This study demonstrates the feasibility of genotype-driven tamoxifen dosing and demonstrates that doubling the tamoxifen dose can increase endoxifen concentrations in IM and PM patients.

American Society of Clinical Oncology clinical practice guideline: update on adjuvant endocrine therapy for women with hormone receptor-positive breast cancer

PURPOSE

To develop evidence-based guidelines, based on a systematic review, for endocrine therapy for postmenopausal women with hormone receptor-positive breast cancer.

METHODS

A literature search identified relevant randomized trials. Databases searched included MEDLINE, PREMEDLINE, the Cochrane Collaboration Library, and those for the Annual Meetings of the American Society of Clinical Oncology (ASCO) and the San Antonio Breast Cancer Symposium (SABCS). The primary outcomes of interest were disease-free survival, overall survival, and time to contralateral breast cancer. Secondary outcomes included adverse events and quality of life. An expert panel reviewed the literature, especially 12 major trials, and developed updated recommendations.

RESULTS

An adjuvant treatment strategy incorporating an aromatase inhibitor (AI) as primary (initial endocrine therapy), sequential (using both tamoxifen and an AI in either order), or extended (AI after 5 years of tamoxifen) therapy reduces the risk of breast cancer recurrence compared with 5 years of tamoxifen alone. Data suggest that including an AI as primary monotherapy or as sequential treatment after 2 to 3 years of tamoxifen yields similar outcomes. Tamoxifen and AIs differ in their adverse effect profiles, and these differences may inform treatment preferences.

CONCLUSION

The Update Committee recommends that postmenopausal women with hormone receptor-positive breast cancer consider incorporating AI therapy at some point during adjuvant treatment, either as up-front therapy or as sequential treatment after tamoxifen. The optimal timing and duration of endocrine treatment remain unresolved. The Update Committee supports careful consideration of adverse effect profiles and patient preferences in deciding whether and when to incorporate AI therapy.

Coprescription of tamoxifen and medications that inhibit CYP2D6

Evidence has emerged that the clinical benefit of tamoxifen is related to the functional status of the hepatic metabolizing enzyme cytochrome P450 2D6 (CYP2D6). CYP2D6 is the key enzyme responsible for the generation of the potent tamoxifen metabolite, endoxifen. Multiple studies have examined the relationship of CYP2D6 status to breast cancer outcomes in tamoxifen-treated women; the majority of studies demonstrated that women with impaired CYP2D6 metabolism have lower endoxifen concentrations and a greater risk of breast cancer recurrence. As a result, practitioners must be aware that some of the most commonly prescribed medications coadministered with tamoxifen interfere with CYP2D6 function, thereby reducing endoxifen concentrations and potentially increasing the risk of breast cancer recurrence. After reviewing the published data regarding tamoxifen metabolism and the evidence relating CYP2D6 status to breast cancer outcomes in tamoxifen-treated patients, we are providing a guide for the use of medications that inhibit CYP2D6 in patients administered tamoxifen.

The Impact of CYP2D6 Genotyping on Tamoxifen Treatment

Tamoxifen remains a cornerstone of treatment for patients with oestrogen-receptor-positive breast cancer. Tamoxifen efficacy depends on the biotransformation, predominantly via the cytochrome P450 2D6 (CYP2D6) isoform, to the active metabolite endoxifen. Both genetic and environmental (drug-induced) factors may alter CYP2D6 enzyme activity directly affecting the concentrations of active tamoxifen metabolites. Several studies suggest that germline genetic variants in CYP2D6 influence the clinical outcomes of patients treated with adjuvant tamoxifen. Here, we review the existing data relating CYP2D6 genotypes to tamoxifen efficacy.