Selective estrogen-receptor modulators (SERMs) in the cyclopentadienylrhenium tricarbonyl series:: Synthesis and biological behaviour

A series of organometallic antiestrogens based on the OH-tamoxifen (OH-Tam) skeleton and bearing the (eta(5)-C5H4)Re'(CO)(3) unit has been prepared by using McMurry coupling for the purpose of studying their biological behaviour. The cyclopentadienylrhenium tricarbonyl moiety is indeed stable in biological media, compact, lipophilic and easy to handle. Furthermore, this study allowed us to select the best candidates for subsequent use as radiopharmaceuticals either for imaging or therapy by using appropriate radionucleides, namely Tc-99m and Re-188. In these molecules the beta-phenyl group of OH-Tam has been replaced by the (eta(5)-C5H4)Re(CO)(3) moiety, and the length of the dimethylamino side chain -O(CH2)(n)N(CH3)(2) was varied (n = 2, 3, 4, 5 and 8). The compounds 7a-7e were obtained as mixtures of their Z and E isomers, which could be separated by semipreparative HPLC Unlike their ferrocene homologues, the compounds do not isomerise in solution. Structural identification was carried out with NMR spectroscopy by using the HMBC and NOE techniques and was confirmed by the X-ray structural determination of (E)-7a (n=2). These molecules were more lipophilic than OH-Tam (logP(o/w) = 4.5-6.3) and they were all reasonably well recognized by the two forms of the estrogen receptor (ERalpha and ERbeta). For example,(Z)-7b (n = 3) has high relative binding affinity (RBA) values of 31% for ERalpha and 16.8% for ERbeta. The antiproliferative effects of two pairs of isomers, (Z)- and (E)-7b (n=3) and (Z)- and (E)-7d (n=5), were studied at a molarity of 1 mum on two breast-cancer cell lines, MCF7 (ERalpha positive) and MDA-MB231 (ERalpha negative). These molecules had an antiproliferative effect on MCF7 cells slightly higher than that of OH-Tam and no effect on MDA-MB231 cells. Thus, the antiproliferative effect observed on the MCF7 cells seemed essentially to be linked to an antiestrogenic effect. Molecular modelling studies hove allowed us to rationalise these effects and select the best compounds for future development of a radioactive series.