Combined cytotoxic and endocrine therapy for human breast carcinoma (Br-10) serially transplanted into nude mice.

Cytotoxic and endocrine therapy on a human breast carcinoma (Br-10) serially transplanted into nude mice was given with reference to the sequence of drug administration. Mitomycin C (MMC) was combined with 2.5 mg/kg of tamoxifen (TAM). MMC was dissolved in 0.2 ml of physiological saline and administered intraperitoneally once weekly. TAM was dissolved in 0.1 ml of sesame oil and administered intramuscularly twice weekly. Both drugs were administered in the reverse sequence for 2 or 3 weeks. Cytosol estrogen receptor (ERc), nuclear estrogen receptor (ERn) and progesterone receptor (PgR), and 3H-thymidine uptake labeling index (L.I.) were assayed after the treatment. When 1.5 mg/kg of MMC was combined with TAM, statistically significant differences were nil between the different sequential administrations. When the MMC administration was reduced to 0.75 mg/kg and 2 weeks, respectively, the MMC----TAM sequence was more effective than the reversed sequential administration. MMC preserved ERc and depressed L.I. to almost half of that of the control tumor. TAM generated the ER systems and slightly depressed L.I. These different modes of action between MMC and TAM on ER systems and L.I. may explain the antitumor effects of different sequential administrations.