A unified definition of clinical anthracycline resistance breast cancer.

The purpose of the study was to determine the response rates (RR) and duration to second- and third-line chemotherapy programmes in patients with anthracycline-resistant breast cancer, utilizing various definitions of anthracycline resistance. This was a retrospective analysis performed on 1335 patients with metastatic breast cancer who participated in consecutive clinical trials of first line, anthracycline-containing combination chemotherapy (ACCC) at the University of Texas MD Anderson Cancer Center between July 1973 and April 1980. Anthracycline-resistant groups were identified using definitions of anthracycline resistance found in the literature: progressive disease as best response to ACCC (Group 1, n = 56 patients); progressive disease while receiving ACCC after an intervening response to the drug (Group 2, n = 84); progressive disease within 6 months of last dose of ACCC (Group 3, n = 233); and progressive disease within 12 months of last dose of ACCC (Group 4, n = 272). Second- and third-line therapies administered to these patients included methotrexate, doxorubicin, mitoxantrone, bisantrene, vinblastine, vindesine, melphalan, mitomycin, cisplatin, etoposide and others, but not taxanes. The distribution of patients' characteristics was similar between the four groups, as was the use of second- and third-line regimens. Response rate (RR) to second-line chemotherapy were 5% and 7.7% for Group 1 and Group 2 respectively. In contrast, RR to second-line chemotherapy were 21.6% and 15% for Group 3 and 4. The differences in response rate between the combination of Groups 1 and 2 and Groups 3 or 4 were significant (P = 0.005 and P = 0.04 respectively). These results indicate that strictly defined anthracycline resistance as defined in Groups 1 and 2 is associated with resistance to many other cytotoxic drugs. The definitions used in Groups 3 and 4 include many patients with responsive tumours, and a more favourable prognosis.