Doxorubicin is one of the cornerstone therapies in adjuvant chemotherapy in early stage breast cancer. Cumulative doses of 240mg/m2 (4 cycles of doxorubicin/cyclophosphamide) and 300mg/m2 (6 cycles of doxorubicin/cyclophosphamide) are typically administered in the adjuvant setting, which is associated with \<1% chance of severe cardiotoxicity, and are thus considered 'safe dose ranges'. However, Blanco et al recently reported childhood cancer survivors with the CBR3 (a metabolizing enzyme of doxorubicin) V244M homozygous G genotypes to be at increased risk of cardiomyopathy following exposure to anthracyclines doses as low as 101-150mg/m2, suggesting that there is no safe dose threshold for individuals with certain genotypes. We have previously studied several genes in the doxorubicin pharmacology pathway, including CBR1, CBR3, and AKR1C3, and found correlation between functional variants in CBR3 and AKR1C3 with doxorubicin-induced myelosuppression. The CBR3 G allele is present in about 50-60% of the Singapore population, and we postulate that these common variants may similarly modify the risk of anthracyclines-induced cardiomyopathy in adult breast cancer patients. Post-treatment echocardiography is not routinely performed in patients who complete adjuvant anthracyclines-containing chemotherapy. We believe that some of these high-risk individuals may have subclinical reduced left ventricular ejection fraction that may in the future increase the risk of congestive cardiac failure in the presence of other risk factors (eg hypertension, anemia, serious infection, etc). Identifying these individuals could therefore be important as early treatment with ACE inhibitors may improve cardiac function. Confirming the correlation between genetic variants including the CBR3 V244M can also help to develop a predictive algorithm in the future to identify patients in whom anthracyclines should be avoided.
