Abstract
SGN-40 is a humanized antibody targeting CD40, a TNF receptor family member expressed on normal B cells, non-Hodgkin’s lymphoma (NHL), multiple myeloma, and a variety of carcinomas. Previous studies have shown that SGN-40 triggers proapoptotic signal transduction, mediates effector function (ADCC), and has in vivo antitumor activity in CD40+ lymphoma xenograft models. We now report in vivo efficacy data for SGN-40 in combination with the anti-CD20 monoclonal antibody, rituximab, and approved chemotherapy regimens for the treatment of NHL. The growth of subcutaneous Ramos tumors in SCID mice was delayed following SGN-40 or rituximab treatment. However, the combination of SGN-40 + rituximab (S-R) significantly improved efficacy over either antibody alone. SGN-40 was then tested with ICE (ifosfamide, carboplatin, etoposide) chemotherapy with or without rituximab (S-R-ICE and S-ICE). These studies demonstrated that both S-R-ICE and S-ICE treated mice had lower tumor burden than R-ICE or SGN-40 treated animals. Additionally, the effect of SGN-40 in combination with CHOP (cyclophosphamide, doxorubicin, vincristine, prednisone) chemotherapy with or without rituximab (S-R-CHOP and S-CHOP) was examined. S-R-CHOP and S-CHOP therapies showed a significant delay in tumor growth compared with R-CHOP or SGN-40 alone. Furthermore, the efficacy observed in S-R-ICE and S-R-CHOP treatments exceeded the S-R combination, suggesting that SGN-40 chemosensitizes lymphoma cells by a signaling mechanism in addition to augmenting ADCC when combined with rituximab. To better understand the chemosensitization effect of SGN-40 in xenograft models, signal transduction events triggered by SGN-40 were examined in vitro. SGN-40 treatment caused the sustained degradation of the BCL-6 protooncogene in several lymphoma cell lines, following prolonged MAP Kinase pathway activation. BCL-6 is implicated in lymphomagenesis of germinal center derived lymphomas, and is proteasomally degraded after phosphorylation by ERK1/2 MAPK. Immunohistochemical analyses of Ramos tumors harvested from mice following treatment with SGN-40 or S-CHOP revealed elevated numbers of apoptotic cells versus untreated tumors. A distinct downregulation of BCL-6 staining in Ramos tumor cells was also observed in SGN-40 and S-CHOP treated animals, correlating with increased cell death. Finally, in some NHL lines SGN-40 upregulated the p53 family member TAp63alpha, a chemo-sensitizing transcription factor capable of inducing apoptosis when overexpressed. When combined with cytotoxic agents, SGN-40 caused a greater induction of TAp63alpha compared with chemotherapy alone, a potential mechanism underlying the improved antitumor activity seen in combination studies. Collectively, these data suggest that SGN-40 signaling occurs at the tumor site, likely contributing directly to tumor cell killing and chemosensitization. These preclinical studies support our earlier work suggesting that addition of SGN-40 to standard therapeutic regimens may improve the outcome for patients with NHL.
The C terminus of p73 is essential for hippocampal development