Pharmacogenetics as a Predictor of Toxicity in Pre-Menopausal Women Receiving Doxorubicin and Cyclophosphamide in Early Breast Cancer

There is a clear survival benefit with the use of adjuvant cytotoxic therapy for most women with invasive breast cancer, even in those who have hormone receptor positive disease and receive adjuvant hormonal therapy with tamoxifen.1 In addition, several trials have shown a benefit for anthracycline based regimens over the more classic combination of cyclophosphamide, methotrexate, and 5-fluorouracil.1-4 The improved efficacy with taxanes in the adjuvant setting has more recently been demonstrated for patients with lymph-node positive disease.5-8 Despite clear survival benefits with cytotoxic therapy, the 10 year-disease specific mortality remains suboptimal at 69-78% and 49-53% for patients with and without lymph node involvement, respectively.9

Of the 180,000 women diagnosed with breast cancer in the United States, about one-fourth are pre-menopausal.10-13 Breast cancer clearly represents one of the most commonly diagnosed malignancies in this patient population. With the common use of adjuvant chemotherapy, long-term sequelae of treatment are becoming increasingly important. In addition to the acute toxicities of anthracycline and cyclophosphamide-based regimens,5 one side effect with both psycho-social and physical implications is pre-mature menopause.13-17 The frequency of menopause induced by poly-agent chemotherapy ranges from 34-89%.16,18,19 Multiple factors (both patient and drug-related) play a role in explaining this large variability. The age of the patient (at time of therapy),13,19,20 type of chemotherapy drugs,18,21 and duration and intensity22 of therapy all influence the overall likelihood of a patient prematurely entering menopause after therapy. In a previously reported study, age and systemic therapy were important variables in determining menopause in women with loco-regional breast cancer in multivariate analysis.19 Women with advancing age had a higher rate of menopause as expected. Hormonal therapy, and to a much greater degree, systemic therapy predicted early menopause. The combination of systemic and hormonal therapy appeared to have an additive effect on induction of menopause. Of importance, however, the added impact of hormonal therapy (when added to cytotoxic therapy) appears to play a minimal role in the induction of menopause when compared to cytotoxic therapy alone. It is also likely that intrinsic host genetic variability may also play a role as well. The variable ability to metabolize and clear a drug may, in part, affect efficacy and toxicity of these drugs and may ultimately impact the effect of the drug on ovarian function. One important example of this relates to polymorphisms in enzymes important in the clearance of the described drugs. To date, little work has been done to understand the importance of inter-individual, host specific variability on the risk of a breast cancer patient experiencing drug-induced, pre-mature menopause.