Anti-thyroid cancer properties of a novel isoflavone derivative, 7-(O)-carboxymethyl daidzein conjugated to N-t-Boc-hexylenediamine in vitro and in vivo

The incidence of thyroid cancer is up to 3 folds higher in women than in men, suggesting that estrogenic effects may be involved in the pathogenesis of this malignancy. Here, we explore whether or not human thyroid cancer cell growth can be curbed by a novel isoflavone derivative generated in our laboratory, the N-t-Boc-hexylenediamine derivative of 7-(O)-carboxymethyl daidzein (cD-tboc). With the exception of the follicular cancer cell line WRO, estrogen receptor (ER)α mRNA was only marginally expressed in cell lines derived from papillary (NPA), follicular (MRO), anaplastic thyroid carcinoma (ARO) such that the expression of estrogen receptor (ER) βmRNA was more abundant than that of ERα mRNA in these cell types. Estradiol-17β (E2; 0.03-300nmol/l) per se increased proliferation in all four cell-types. The ERβ-specific agonist DPN increased [(3)H]-thymidine incorporation in all four thyroid cancer cell lines, whereas the ERα-specific agonist PPT increased growth only in NPA and WRO. By contrast, cD-tboc, derived from the weak estrogen daidzein, did not cause cell growth and dose-dependently diminished cell growth in all four cell lines via apoptosis and not necrosis, as detected by the release of histone-DNA fragments. The cytotoxic growth inhibitory effect of cD-tboc in these cells was modulated by E2 and the general caspase inhibitor Z-VAD-FMK, and the magnitude of this salvage was cell type-and dose-dependent. When nude mice carrying ARO thyroid xenografts were treated with cD-tboc, tumor volume decreased significantly, and no apparent toxicity was observed. These results suggest that cD-tboc may be a promising agent for therapy of thyroid carcinoma either alone or in combination with existing cytotoxic drugs.