Successful treatment of murine melanoma with bryostatin 1

Bryostatins are a novel class of protein kinase C activators which were isolated from the marine bryozoan Bugula neritina and found to possess both antineoplastic and immunoenhancing properties. In this report, we examined the relationship between the in vivo and in vitro antineoplastic effects of bryostatin 1. The in vivo antitumor activity of bryostatin 1 was demonstrated in a B16 melanoma pulmonary metastases model, in which treatment of tumor-bearing C57BL/6 mice with 5 days of bryostatin 1 resulted in a significant reduction in of the number of lung nodules (control, 87; bryostatin, 7). There was a clear dose-response effect, with the optimal antimelanoma dose being 100 micrograms/kg/day, but even low doses of bryostatin 1 of 1 micrograms/kg/day resulted in a 53% reduction in the number of metastases. Although bryostatin 1 shares many biological properties with the phorbol esters, parallel treatment with 12-O-tetradecanoyl 13-phorbol acetate was ineffective against B16 melanoma in vivo. Using a clonogenic assay, bryostatin 1 was found to have a direct antiproliferative effect against B16 melanoma. This inhibition occurred at relatively high bryostatin 1 concentrations (10(-6) M), in comparison with a sensitive cell line REH (10(-10) M). Treatment of mice with bryostatin 1 or preincubation of normal spleen cells with bryostatin 1 failed to enhance nonspecific cell-mediated cytotoxicity against B16 melanoma in vitro. Moreover, bryostatin 1 was found to inhibit both natural killer cell activity and interleukin 2 generation of lymphokine-activated killer cells. Thus, a role for an in vivo immune enhancement mechanism as the basis for the antimelanoma activity observed with bryostatin 1 cannot be invoked from these experiments. These findings indicate that bryostatin 1 may act directly on the B16 melanoma pulmonary metastases. The precise mechanism whereby bryostatin exerts its antimelanoma effects remains unclear.