Antitumor benzothiazoles. 8. Synthesis, metabolic formation, and biological properties of the C- and N-oxidation products of antitumor 2-(4-aminophenyl)benzothiazoles

2-(4-Aminophenyl)benzothiazoles 1 and their N-acetylated forms have been co nverted to C- and N-hydroxylated derivatives to investigate the role of met abolic oxidation in the mode of action of this series of compounds. 2-(4-Am ino-3-methylphenyl)benzothiazole (1a, DF 203, NSC 674495) is a novel and po tent antitumor agent with selective growth inhibitory properties against hu man cancer cell lines. Very low IC50 values (<0.1 mu M) were encountered in the most sensitive breast cancer cell lines, MCF-7 and T-47D, whereas rena l cell line TK-10 was weakly inhibited by la. Cell lines from the same tiss ue origin, MDA-MB-435 (breast), CAKI-1 (renal), and A498 (renal), were inse nsitive to 1a. Accumulation and metabolism of la were observed in sensitive cell lines only, with the highest rate of metabolism occurring in the most sensitive MCF-7 and T-47D cells. Thus, differential uptake and metabolism of 1a by cancer cell lines may underlie its selective profile of anticancer activity. A major metabolite in these sensitive cell lines has been identi fied as 2-(4-amino-3-methylphenyl)-6-hydroxybenzothiazole (6c). Hydroxylati on of 1a was not detected in the homogenate of previously untreated MCF-7, T-47D, and TK-10 cells but was readily observed in homogenates of sensitive cells that were pretreated with 1a. Accumulation and covalent binding of [ C-14]1a derived radioactivity was observed in the sensitive MCF-7 cell line but not in the insensitive MDA-MB-435 cell line. The mechanism of growth i nhibition by 1a, which is unknown, may be dependent on the differential met abolism of the drug to an activated form by sensitive cell Lines only and i ts covalent binding to an intracellular protein. However, the 6-hydroxy der ivative 6c is not the 'active' metabolite since, like all other C- and N-hy droxylated benzothiazoles examined in this study, it is devoid of antitumor properties in vitro.