Mantle cell lymphoma (MCL) is an aggressive form of B-cell non-Hodgkin’s lymphoma (NHL), with a mean survival of less than five years and suboptimal therapeutic options. MCL is characterized by a balanced t(11;14)(q13;q32) translocation, resulting in the overexpression of cyclin D1. A member of the G1 cyclins, cyclin D1 plays a key role in controlling cell cycle progression and is regulated by the PI3K/Akt/mammalian target of rapamycin (mTOR) signaling pathway essential for cell survival and proliferation. Given the necessity for improved therapy for MCL and the identification of dysregulation of the PI3K/Akt/mTOR pathway in several cancers, we have evaluated the effects of RAD001, a derivative of the mTOR inhibitor rapamycin, on MCL viability and proliferation in vitro. As a single agent, RAD001 (2,000 nM, 96 hours) inhibited proliferation in three MCL cell lines (Jeko1, SP49, and NCEB1) approximately 40-65% compared to diluent control cells. This inhibition of proliferation was associated with a decrease in cell-cycle progression, as demonstrated by an increased proportion of cells in G0-G1 and decreased percentage in the S phase in all MCL cell lines. MCL cells treated with RAD001 also demonstrated reduced Thr37/46 phosphorylation of the mTOR downstream target, 4E-BP1, as well as decreased levels of the hyperphosphorylated 4E-BP1γ isoform and enhanced levels of hypophosphorylated 4E-BP1α. Under nutrient-poor conditions or mTOR inhibition, hypophosphorylated 4E-BP1 tightly binds initiation factor eIF4E and inhibits cap-dependent translation. Thus, we also found that while RAD001 treatment did not have an impact on overall expression of eIF4E, it did result in increased 4E-BP1-eIF4E interaction, as demonstrated by enhanced levels of 4E-BP1 expression in the RAD001 treated cells. Furthermore, various secondary agents were individually evaluated for antiproliferative effects in combination with RAD001. While some drugs are components of the standard MCL “CHOP” chemotherapy regimen (doxorubicin, vincristine, and the anti-CD20 monoclonal antibody, rituximab), others (paclitaxel, SAHA, and bortezomib) were chosen because they have demonstrated promising results in clinical trials for NHL or in vitro studies. Interestingly, combination drug studies revealed additive cytotoxicity, based on observed-to-predicted ratios calculated by the additive model. Taken together, these encouraging results suggest that clinical trials of RAD001 should be pursued in the treatment of MCL.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]