The antiprogestins mifepristone and onapristone reduce cell proliferation in the canine mammary carcinoma cell line CMT-U27.

Canine mammary tumours (CMTs) represent nearly half of all tumours in female dogs and some 50% have malignant behaviour. Simple epithelial carcinomas have shorter disease free periods after surgery and a higher reduction of the proliferation index reduction after antiprogestin aglepristone treatment in vivo related to the expression of progesterone receptors (PR). These findings make simple carcinomas good candidates for endocrine therapy. To further explore this possibility, the effects of the antiprogestins mifepristone (RU486) and onapristone (ZK299) on cell viability and PR expression of the canine mammary carcinoma cell line isolated from a simple epithelial carcinoma CMT-U27 were studied. Twenty five percent of CMT-U27 control cells expressed PR. RU486 (p<0.05) and ZK299 (p<0.05) reduced the number of viable cells (WST-8 test) at 24h but only the latter treatment reduced significantly PR expression in viable tumour cells at 24h of incubation. The results suggest that both RU486 and ZK299 induce a decrease in the number of viable CMT-U27 tumour cells with different effects on PR expression. The canine mammary carcinoma cell line CMT-U27 is sensitive to the effects of antiprogestins and may serve to further explore the role of these drugs in canine mammary carcinomas.