We first evaluated the kinetics of YC-1 sulfonation at multiple concentrations by following the pnitrophenolate signal over 145 min (Figure ). We observed concentration-dependent increase in signal and signal saturation at the highest doses of 30 and 100 M kinetics after ~75 and 90 min respectively. Since the throughput of the assay was relatively low, we selected 10 analogs from our SAR exploration with a spectrum of cytotoxic activities and distinct chemical structures to be characterized as SULT1A1 substrates. Figure compares the conversion of these compounds kinetically in the same plot at 100 M, and Figure reports the activity at the assay endpoint (120 min) in a histogram. The benzyl alcohols 79, 92, and 93 appeared to be the strongest substrates in this biochemical assay; these compounds had the best correlation between their cytotoxicity and substrate activity in the SULT1A1 biochemical assay. They are followed by RITA 95, YC-1, and the YC-1 pyridine analog 42; these were quite potent in the RBE assay, but they showed <2-fold response in the SULT1A1 assay compared to the benzyl alcohols.