J. Steroid Biochem. Molec. Biol. Vol.44, No. 4-6, pp. 633~i36, 1993 0960-0760/93$6.00 + 0.00 Printed in Great Britain. All rights reserved Copyright© 1993 Pergamon Press Ltd ANTI-TUMOR AND ENDOCRINE EFFECTS OF NON-STEROIDAL AROMATASE INHIBITORS ON ESTROGEN-DEPENDENT RAT MAMMARY TUMORS K. SCHIEWECK,* A. S. BHATNAGAR, CH. BATZLand M. LANG Research Department, Pharmaceuticals Division, CIBA-GEIGYLimited, 4002 Basel, Switzerland Summary--The non-steroidal aromatase inhibitor CGS 20 267, at maximally effective doses in non-tumor bearing adult female rats, elicits endocrine effects mimicking those seen after surgical ovariectomy and thus induces a "medical" ovariectomy. We now report on studies characterizing the anti-tumor and endocrine effects of three orally active non-steroidal aromatase inhibitors, CGS 20 267, CGP 45 688 and CGP 47 645, in adult female rats bearing estrogen-dependent DMBA-induced mammary tumors. Doses ranging from 3 to 3000 gg/kg were given by gavage once daily for 6 weeks. After 6 weeks of treatment, the EDs0 for suppression of tumor volume was 10-30, 100 and 3-10/tg/kg for CGS 20 267, CGP 45 688 and CGP 47 645, respectively. The maximally effective dose for anti-tumor efficacy was 300, 1000 and 100/~g/kg for each of the three inhibitors, respectively. The observed potent anti-tumor efficacy was accompanied by potent endocrine effects. Thus, disruption of ovarian cyclicity (at maximal doses rats remained in constant diestrus) was observed in all animals from the 2nd or 3rd week of treatment to the end of the 6-week treatment period. Uterine weight, at the maximally effective doses for each of the three inhibitors, was suppressed to between 42 and 28% of pre-treatment levels. This suppression was similar to the suppression of uterine weight (27% of pre-treatment) seen after ovariectomy. Serum estradiol concen- trations in rats treated with 300 #g/kg CGS 20 267 were significantly suppressed to 12% of pre-treatment levels and serum luteinizing hormone (LH) concentrations were elevated 3 to 4-fold over pre-treatment levels. Thus the potent anti-tumor efficacy seen with each of the three non-steroidal aromatase inhibitors was accompanied in each case by a variety of endocrine effects corresponding to those seen after ovariectomy. INTRODUCTION Aromatase, a microsomal cytochrome /45o- dependent enzyme is a key enzyme in the bio- synthesis of estrogens [1]. Thus inhibition of this enzyme should lead to an attenuation of estro- gen biosynthesis and consequently to an inhi- bition of estrogen-dependent effects. In non-tumor bearing animals these effects, amongst others, would be characterized by a reduction in uterine weight, a disruption of ovarian cyclicity and an increase in serum luteinizing hormone (LH) concentrations. In rats bearing estrogen-dependent mammary tu- mors induced by the chemical carcinogen 7,12- dimethylbenz[a]anthracene (DMBA), an inhibition of estrogen biosynthesis should lead a suppression in tumor volume. We have reported previously on the anti-tumor effects of Fadro- zole hydrochloride (CGS 16 949A) [2]. We have Proceedings of the Third International Aromatase Confer- ence. Basic and Clinical Aspects of Aromatase, Bologna, Italy, 14-17 June 1992. *To whom correspondence should be addressed. also reported previously on the endocrine efficacy of CGS 20 267 and have presented preliminary data on its anti-tumor efficacy [3]. In this report, we present data on the anti-tumor efficacy and the associated endocrine effects of CGS 20 267 and two new non-steroidal aroma- tase inhibitors, CGP 45 688 and CGP 47 645. The chemical structures of these three com- pounds are shown in Fig. 1. ANTI-TUMOR EFFICACY The tumor model used for these studies was the estrogen-dependent DMBA-induced mam- mary carcinoma in adult female Sprague-Dawley rats. Details of the experimen- tal methodology have been reported pre- viously [2]. In brief, animals bearing 1-3 tumors of 10-12 mm dia are divided into groups of 15 animals. They are treated with aromatase in- hibitor orally, once daily for 42 days (6 weeks). Tumor size, numbers of tumors and bodyweight are determined once weekly. During the 6-week treatment schedule with each of the three 633