Ansamitocin p-3 (Maytansinol isobutyrate, NSC292222)

Ansamitocin p-3 (Maytansinol isobutyrate, NSC292222) at 5 μM completely inhibits the polymerization of tubulin isolated from bovine brains, but in contrast to VCR, this compound at a high concentration of 80 μM does not leads to the aggregation of tubulin. It at 16 μM also potently depolymerizes the polymerized tubulin (IC50 = 3.8 μM). The addition of Ansamitocin p-3 to culture cells blocks the morphological alteration of AC cells from fibroepithelioid to a glial cell type caused by the exposure to a certain concentration of dibutyryl cyclic adenosine 3':5'-monophosphate. In addition, treatment with it at 16 nM causes the well-defined network of cytoplasmic microtubules of A31 cells rapidly dispersed. Short-term Ansamitocin p-3 treatment also inhibits the synthesis of DNA in A31 cells or KB cells. These results confirm that it acts by interfering with the microtubule assembly system, thus resulting in an inhibition of mitotic spindle fiber formation and, ultimately, cytokilling. ^[1] This compound displays potent cytotoxicity against A-549, HT-29, and MCF-7 cells in a dose-dependent manner with ED50 of 4 ×10^-7, 4 × 10^-7, and 2 × 10^-6 μg/mL, respectively. ^[2] It also exhibits cytotoxicity against HCT-116 cells with a much low EC50 of 0.081 nM. ^[3] Ansamitocin p-3 enhances the effect of radiation both in Drosophila cells and human cancer cells in a p53 dependent manner. ^[4]

Cells are synchronized, and then exposed to various concentrations of Ansamitocin p-3 (Maytansinol isobutyrate, NSC292222) for ~24 hours. After pulse-labeling at 37 °C for 1 hour with [³H]thymidine (5 Ci/mM, 1 μCi/mL) in 1 mL of the medium, the cells on coverslips are fixed with methanol:acetic acid (3:1). The acid-soluble fraction is washed out from the cells, and the radio activity of each coverslip is determined by a liquid scintillation counter.

Ansamitocin p-3 (Maytansinol isobutyrate, NSC292222) treatment (>1 μg) significantly suppresses the growth of leukemia SN36, and induces an increased arrest in metaphase of P388 leukemia cells. At 25 μg/kg/day, it significantly prolongs the survival time of mice bearing i.p. B16 melanoma by 130%. This compound also significantly prolongs the survival time of mice bearing Ehrlich ascites carcinoma, Sarcoma 180, and P815 mastocytoma, while slightly prolonging the survival time of mice bearing ascites MOPC-104E myeloma, leukemia L1210, and leukemia C1498. ^[1]