[Development of a novel aromatase inhibitor, anastrozole (Arimidex)--its basic and clinical studies]

Anastrozole (JAN), developed by Zeneca UK (presently AstraZeneca UK), is a new aromatase inhibitor which belongs to triazole. Anastrozole shows selective aromatase inhibition and negligible effects on other steroid hormone biosyntheses. The daily dose of 3 mg/kg anastrozole inhibited the growth of DMBA mammary tumours significantly. Anastrozole inhibited the tumor growth at 5 micrograms/mouse/day (s.c.) in androstenedione-treated ovariectomized nude mice inoculated with MCF-7CA cells transfected with aromatase gene. In the Japanese Phase IIa study, the response rate was 27.8% (10/36) in the 0.5 mg group, and 38.2% (13/34) in the 1 mg group. The most common adverse drug reactions were leucopenia in the 0.5 mg group, and LDH increased and leucopenia in the 1 mg group. There were no early deaths, other serious adverse events, or adverse drug reactions of grade 3 or above in this trial; anastrozole was well tolerated. In the Japanese Phase IIb study, the response rates were 33.3% (117/351 patients) and 32.8% (114/348 patients) for anastrozole and tamoxifen, respectively, showing non-inferiority of anastrozole. The median time to disease progression (TTP) was 251 days and 252 days for anastrozole and tamoxifen, respectively. Group comparison using a Cox regression model revealed non-inferiority of anastrozole. Therefore, anastrozole was found to be at least as effective as tamoxifen (in the response rate and TTP). In US study compared anastrozole with tamoxifen, TTP with anastrozole is a significantly longer than that of tamoxifen (p = 0.005, two-sides). Anastrozole has shown to be at least as effective as tamoxifen, standard endocrine therapy for breast cancer, with good safety profiles. Thus, anastrozole is a promising first-line endocrine therapy in the treatment of postmenopausal breast cancer.