Carboxy derivatives of isoflavones that exhibit oestrogenic/anti-oestrogenic properties were used as carriers for affinity drug targeting to H295R adrenocortical carcinoma cells that express transcripts of oestrogen receptor (ER) alpha and beta. These derivatives were prepared by introducing a carboxymethyl group at the 6-position of genistein and of biochanin A, yielding 6CG and 6CB respectively. In transactivation assays, 6CG displayed mixed agonist/antagonist activity for ERalpha, whereas 6CB displayed only weak antagonist activity. Low concentrations of oestrogen, 6CG and 6CB were capable of inducing proliferation in H295R cells and of stimulating creatine kinase (CK) specific activity, suggesting that these cells were sensitive to oestrogenic compounds. In in vivo experiments, both 6CG and 6CB were capable of inhibiting oestrogen-induced CK specific activity in rat tIssues. For affinity drug targeting, the cytotoxic drug daunomycin was coupled to 6CB and 6CG, yielding 6CB-Dau and 6CG-Dau respectively. These conjugates were tested for their antiproliferative ability to inhibit DNA synthesis as assessed by incorporation of [(3)H]thymidine in H295R cells. A dose-dependent cytoxicity was observed with both conjugates. At 0.3-3 nM, both conjugates were 10 to 30 times more potent than daunomycin. At 30 nM these conjugates were two to three times more potent than daunomycin. At concentrations ranging between 300 and 3000 nM, no difference in cytotoxicity was observed between the conjugates and daunomycin. When the cells were treated over a wide range of concentrations with a combination of 6CG plus daunomycin, the observed cytotoxicity was less than with daunomycin alone. When non-transformed rat enterocytes, which do not express ER, were treated with 6CG-Dau or daunomycin, the antiproliferative effect of 6CG-Dau was the same as that of daunomycin over the concentration range tested. These pilot studies suggest that the ready availability of oestrogenic binding sites in H295R cells can be exploited for site-directed chemotherapy.

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