[Comparison of the cytostatic effect of epirubicin and mitoxantrone on native breast carcinoma cells using the ATP tumor chemosensitivity assay].

This trial was conducted to compare the cytotoxic efficacy of epirubicin (EPI) and mitoxantrone (MX) in clinical tumor samples derived from 34 chemotherapy-native women suffering from breast cancer by utilizing an in vitro ATP tumor chemosensitivity assay (ATP-TCA). An assay evaluability rate of 32/34 (94 %) was achieved. There was no significant difference between EPI and MX when regarding the mean IC90 and IC50 values corresponding to nearly identical overall in vitro response rates observed for both drugs (EPI: 62.5 %; MX 59.4 %). Individual IC90 values showed a weak correlation which was lacking for IC50. Accordingly, a lack of cross-resistance was found in 11/32 tumors (34 %) with 6 tumors showing sensitivity to EPI and resistance to MX and 5 tumors showing resistance to EPI and sensitivity to MX. At lower drug concentrations, a steep slope of the cumulative dose response curves for IC50 could be observed for both cytostatics with an apparent shift to higher concentrations at the IC90 level. For both drugs, a loss of linearity of the IC90 and IC50 dose-response curves was found at maximum concentrations. In conclusion, our results demonstrated an overall equality of EPI and MX in previously untreated breast cancers which already has been found clinically in metastatic disease. Nevertheless, we were able to show that approximately one third of chemotherapy-naive breast carcinomas are not cross-resistant to both EPI and MX. These findings could have implications on planning adjuvant chemotherapy for an individual breast cancer patient.