Preclinical studies of a nonpeptidic small-molecule inhibitor of Bcl-2 and Bcl-XL [(-)-gossypol] against diffuse large cell lymphoma

Overexpression of Bcl-2/Bcl-XL protein has been observed in more than 80% of B-cell lymphomas. Diffuse large cell lymphoma (DLCL) is the most common subtype of non-Hodgkin's lymphoma. (−)-Gossypol, a natural product isolated from cottonseeds, was discovered as a potent small-molecule inhibitor of Bcl-2 and Bcl-XL proteins, with a K i value in the nanomole per liter range for both. In vitro , (−)-gossypol showed significant growth inhibition effect against WSU-DLCL2 lymphoma cell line and fresh cells obtained from a lymphoma patient with no effect on normal peripheral blood lymphocytes. As expected (−)-gossypol induced complete cytochrome c release from mitochondria, increased caspases-3 and -9 activity, and caused apoptotic death without affecting protein levels of Bcl-2, Bcl-XL, Bax, and Bak. The addition of cyclophosphamide-Adriamycin-vincristine-prednisolone (CHOP) regimen to lymphoma cells preexposed to (−)-gossypol enhanced killing significantly. The maximum tolerated dose of (−)-gossypol in severe combined immunodeficient (SCID) mice was 40 mg/kg for three i.v. injections when given alone and 20 mg/kg × 3 when given in combination with CHOP. Using WSU-DLCL2-SCID mouse xenograft model, the tumor growth inhibition, the tumor growth delay, and the log10 kill of mice treated with (−)-gossypol + CHOP were better than CHOP or (−)-gossypol alone. We conclude that adding Bcl-2/Bcl-XL small-molecule inhibitor to standard chemotherapy may prove an effective strategy in lymphoma therapy.