Cryptophycin (CP)-52, a synthetic analog of CP-1, possesses potent and selective antiproliferative activity against human solid tumors bothin vitroandin vivo. Based on an algorithm developed in this laboratory using HCT-116 human colon adenocarcinoma cells, CP-52 exhibited a time- and concentration-dependent antiproliferative effect in thein vitroclonogenic assay. Inhibition of both DNA and RNA synthesis was observed in the absence of any effect on protein synthesis following a 24-h exposure to CP-52, at a time when proliferating cells were arrested in the G2/M phase of the cell cycle. In summary, we interpret these data to indicate that the selective inhibition of DNA synthesis may be a major causative factor responsible for the antiproliferative activity of CP-52 and subsequent G2/M arrest.