The effect of preactivated
merocyanine 540 (pMC540) and one of its chemically synthesized active isolate
merodantoin on established human MCF-7 human
breast tumor xenografts was investigated. Preactivation is a novel photochemical method for the production of chemotherapeutic compounds that exert their
biological effects independent of light. These compounds thus produced, are cytotoxic to human
breast cancer cells in vitro and in vivo but only minimally cytotoxic towards normal cells. Nude mice bearing established
breast tumors (with or without exogenous
estradiol) received
injections of pMC540 (250 mg/kg) or
merodantoin (75 mg/kg) with or without concurrent treatment with
tamoxifen. Treatment with pMC540 and
merodantoin caused a 74% and 84% inhibition of
tumor growth respectively. Combination of these drugs with
tamoxifen did not produce a significant enhancement of growth inhibition. In the absence of exogenous
estradiol, identical treatment with pMC540 and
merodantoin resulted in 41% and 25% inhibition of
tumor growth respectively. Both agents caused a significant (59%) inhibition of growth of
estrogen independent human
breast tumors established from MDA-MB-435 cells. These results show that photochemically generated novel compounds in pMC540 are effective in suppressing the growth of established human MCF-7 and MDA-MB-435
breast tumor xenografts.