Anti-breast cancer potential of SS1020, a novel antiestrogen lacking estrogenic and genotoxic actions

Long-term treatment with tamoxifen (TAM) increases the risk of developing endometrial cancer in women. Several antiestrogens developed in last decades have been discontinued from clinical testing because of their undesirable effects on the uterus. To avoid such serious side-effect while increasing the drug's anti-breast cancer potential, new triphenylethylene antiestrogens, 2E-3-{4-[(E)-4-chloro-1-(4-hydroxyphenyl)-2-phenylbut-1-enyl]-phenyl} acrylic acid (SS1020) and 2E-3-{4-[(Z)-4-chloro-1,2-diphenylbut-1-enyl]phenyl}acrylic acid (551010), were designed as safer alternatives. Unlike TAM, 551020 does not present significant uterotrophic potential in rats; in contrast, 551010, a compound removing a 4-OH moiety from 551020, represented weak uterotrophic activity. The structurally related compounds 4-hydroxytamoxifen, toremifene, ospemifene, raloxifene (RAL) and GW5638 all have uterotrophic activity. In addition, 551020 and 551010 exhibit no genotoxic activity to damage hepatic DNA in rats. Therefore, 551020 was selected as a safer antiestrogen candidate and used for evaluating the antitumor potential in animals. At the human equivalent doses of TAM, SS1020 had antitumor potential much higher than that of TAM, RAL and GW5638 against 7,12-dimethylbenz(a)anthracene-induced mammary carcinoma in rats. The growth of human MCF-7 breast cancer xenograft implanted into athymic nude mice was also effectively suppressed by SS1020. SS1020, lacking estrogenic and genotoxic actions and having strong antitumor potency superior to that of TAM and RAL, could be a safer alternative for breast cancer therapy and prevention.