4.5. Cryptophycin-5
It is a chemical analog of cryptophycin-1, which entered into a clinical trial but produced only marginal activity.21 Two other analogs, cryptophycins 249 and 309, with improved stability and water solubility are being considered as second-generation clinical candidates.22 The enhanced efficacy of these compounds compared with that of cryptophycin-52 has prompted new efforts to move these compounds into clinical trials.