Modulation of Adriamycin Resistance in Human Breast Carcinoma MCF‐7 Cells in vitro and in vivo by Medroxyprogesterone Acetate

The combination effect of adriamycin (ADM) and medroxyprogesterone acetate (MPA) was examined in vitro against human breast carcinoma MCF‐7 and its ADM‐resistant line (MCF‐7/ADM). MCF‐7 cells, which are positive for estrogen receptors, progesterone receptors and high‐affinity MPA‐binding activity, were more susceptible to the growth‐inhibitory activity of ADM or MPA than MCF‐7/ADM cells. A combination effect of ADM and MPA was observed against MCF‐7/ADM cells, which are negative for steroid receptors, and furthermore against human nasopharynx carcinoma KB and its ADM‐resistant line KB‐A1. This combination effect of ADM and MPA against MCF‐7/ADM cells was demonstrated to be synergistic by using the median effect plot method. The activity of MPA was almost equivalent to that of chlormadinone acetate or tamoxifen, greater than that of progesterone, and less than that of verapamil. The accumulation of ADM in MCF‐7/ADM cells was enhanced by treatment with 10 μM MPA as well as 10 μM verapamil. The efflux of accumulated ADM from MCF‐7/ADM cells was also partially inhibited by treatment with MPA or verapamil. MPA augmented the growth‐inhibitory activity of ADM against MCF‐7/ADM tumors inoculated into nude mice, although statistical significance was not observed. It is suggested that the clinical advantage of the combination of MPA with ADM against advanced breast cancers may be partly explained by the modulation of ADM resistance by MPA.