Detection of 20-thiarapamycin (A) and 15-deoxo-19-sulfoxylrapamycin (B) by HPLC-UV analysis. (A) Retention times: prolyl-rapamycin, 21.4 min; 20-thiarapamycin, 21.9 min; rapamycin, 22.8 min. (B) Retention times: 15-deoxo-19-sulfoxylrapamycin, 17.2 min; prolyl-rapamycin, 21.6 min; rapamycin, 22.5 min.