Estrogen-linked 2-chloroethylnitrosoureas: anticancer efficacy in MNU-induced rat mammary carcinoma, uterine activity in mice and receptor interactions.

A series of estradiol-linked N-(2-chloroethyl)-N-nitrosocarbamoyl(CNC)-L-alanines attached in various positions (positions 3, 6 alpha, 17, 3 + 17 of estradiol) have been synthesized and tested in hormone-dependent MNU-induced rat mammary carcinoma. Compounds were given i.p. on day 1, 8, 22 and 29 after randomization in equimolar dosage. Equimolar mixtures of unlinked agents were tested in comparison. The results show that the 17-linked derivative was significantly superior to the other congeners and to the unlinked equimolar mixture. The 6 alpha-linked analogue unexpectedly was highly toxic and ineffective. Binding affinities to cytosolic estrogen receptors cannot fully explain the findings of the chemotherapy experiments. Especially the 3-linked derivative we found to exhibit much higher values for relative binding affinity than the 17-analogue, which was a distinctly more effective antineoplastic agent. Estradiol liberation by facile cleavage of the phenolic 3-ester bond might be responsible. Estradiol receptor contents in tumours were diminished or disappeared completely during treatment with individual analogues, progestin receptor contents behaved differently. After a single dose of CNC-L-alanine-estradiol-17-ester, a long-lasting disappearance of estradiol-receptors, measured for up to 192 hr, and a strong induction of progesterone receptors was observed with a maximal value reached at 16 hr, and return back to normal at 192 hr. In the Dorfman uterine weight test in mice, all compounds exhibited distinct uterotrophic activity with no clearcut differences. The relevance of estradiol receptor contents in tumours for antineoplastic activity of the most effective analogue, the 17-ester, became evident from the observed reduced responsiveness of MNU-induced rat tumours after ovariectomy. In these hormone-independent tumours the linked compound revealed only the same antitumour efficacy as CNC-L-alanine alone.