The antitumor cryptophycins are synthetic derivatives of the desipeptide cryptophycins isolated from the cyanobacterium Nostoc sp. Cryptophycin 52 that is currently in clinical trial in solid tumors, is prepared by total synthesis of four key fragments that are coupled to form the final product. The mechanism of anticancer activity of the cryptophycins has been associated with their destabilization of microtubules and with their induction of bcl-2 phosphorylation leading to apoptosis. The cryptophycins maintain activity against ovarian and breast carcinoma cells that overexpress the multidrug resistance efflux pump P-glycoprotein. Cryptophycin 52 has demonstrated a broad range of antitumor activity against both murine and human tumors. In the human MX-1 breast carcinoma xenograft cryptophycin 55 produced greater-than- additive tumor response in combination with 5-fluorouracil. In human non-small cell lung carcinoma and human small cell carcinoma xenografts, administration of the cryptophycins alon g with gemcitabine, cisplatin or carboplatin resulted in antitumor activity greater than either agent alone. The cryptophycins appear to be additive with fractionated radiation therapy in the human H460 non-small cell lung carcinoma. In the human HCT116 colon carcinoma, the cryptophycins resulted in a greater than additive tumor response when administered sequentially with 5-fluorouracil or irinotecan. Treatment of animals bearing intraperitoneal human OVCAR-2 ovarian carcinoma with cryptophycin 52 resulted in survival times that were greater than those achieved with docetaxel or paclitaxel. Cryptophycin 52 is currently in early clinical testing.