Compounds in which the alcohol of the YC-1 parent structure was replaced within an amine (15, 58) proved to be poor substrates, despite their cytotoxicity. This implies that these benzylic amines may not serve as substrates for SULT1A1, and their cytotoxicity may be mediated via other mechanisms. However, metabolism identification studies demonstrated that we had also selected inactive acid 14a and indole 55 with intermediate cytotoxicity activity; both analogs had negligible signals. RITA (95) whose activity has been shown to depend on high SULT1A1 expression was the positive control in the assay and exhibited substrate turnover supporting the physiological basis of the colorimetric biochemical assay.