Antitumor activity of combretastatin-A4 phosphate, a natural product tubulin inhibitor

The tubulin-binding natural product combretastatin A-4 (CA-4) was tested for antitumor activity against fresh human tumors in vitro and 2 mouse tumors, both in vitro and in vivo. In colony forming assays using 10% fetal bovine serum, CA-4 was inhibitory in 27/40 human ovary cancers with a mean IC₅₀ of 3.18 μg/mL for a 1-hour exposure (n = 35 specimens) and 0.27 μg/mL for a continuous exposure to CA-4 for 11-14 days (n = 5 specimens). Murine B-16 melanoma and P-388 leukemia were also highly sensitive to CA-4 in vitro with an identical IC₅₀ value of 0.0007 μg/mL for continuous drug exposure for 8 days. Comparable in vitro cell culture studies performed in serum concentrations higher than 10%, revealed a significant loss of cytotoxic potency. Using the same reversed-phase HPLC technique as developed for paclitaxel, CA-4 was shown to bind to serum proteins (≤ 30,000 mw) > 99% and to albumin approximately 70%. CA-4 was only marginally active (25% increased lifespan) in DBA/2 mice bearing P-388 leukemia who were given doses of 100 mg/kg IP on either days, 1, 5 and 9 (p = 0.075 by Wilcoxon analysis) or on consecutive days 1-9 (p = 0.19 compared to control). A higher IP dose of 150 mg/kg on days 1, 5 and 9 did not delay subcutaneous B-16 melanoma tumor growth in C57/B1 mice. These findings demonstrate a substantial loss of antitumor efficacy for CA-4 in physiologic serum concentrations in vitro. No consistent antitumor activity was observed in two murine tumor models in vivo.