The Novel Proteasome Inhibitor CEP-18770 Inhibits Myeloma Tumor Growth In Vitro and In Vivo and Enhances the Anti-MM Effects of Melphalan

Proteasome inhibitors (PI) have been shown to be effective agents for the treatment of multiple myeloma (MM) and enhance the anti-tumor effects of a variety of chemotherapeutic drugs including melphalan and doxorubicin as well as arsenic trioxide (ATO). The novel proteasome inhibitor CEP-18770 has recently been shown to induce cytotoxic effects across a broad panel of human tumor cell lines including MM in vitro. However, little data exists on the in vivo anti-MM effects of this PI either alone or in combination with other active anti-MM drugs. First, we examined the anti-proliferative effects of treating MM cell lines in vitro with CEP-18770 alone and in combination with melphalan, arsenic trioxide (ATO) and doxorubicin. MM cell lines were cultured without fetal bovine serum and incubated in the presence of CEP-18770 alone and in combination with these agents for 48 hours. Cell growth was then measured using an MTS assay. First, RPMI8226 and U266 cells were tested in vitro using a constant concentration of melphalan or doxorubicin in combination with varying concentrations of CEP-18770 or varying concentrations of the chemotherapeutic agent with constant CEP-18770. Although single agent treatment showed marked anti-proliferative effects, combination indexes as calculated by the Chou-Talalay method showed synergistic anti- MM effects of CEP-18770 with either melphalan or doxorubicin in these MM cell lines. In addition, similar experiments were carried out evaluating the combination of ATO plus CEP-18770 in RPMI8226 cells and also showed synergism with this combination. Next, a series of in vivo studies were conducted using our SCID-hu models of MM including LAGλ-1, LAGκ-1A and LAGκ-1B. Mice receiving CEP-18770 at 0.1, 0.3, 1, and 3 mg/kg were injected twice weekly via intravenous injection throughout the study. CEP-18770 dosed at 10 mg/kg was administered via oral gavage twice weekly and mice dosed with melphalan received injections once weekly via intraperitoneal injection. Mice bearing intramuscularly implanted LAGλ-1 were treated with CEP-18770 or vehicle alone. Mice treated with the PI inhibited tumor growth as determined by human immunoglobulin (hIg) G levels and measurement of tumor volume (P = 0.0008) compared to mice receiving vehicle. A significant inhibition of both human paraprotein secretion and reduction of tumor growth was also observed in LAGk-1A-bearing mice treated with CEP-18770 at 1, 3 and 10 mg/kg (hIgG: P = 0.0001, P = 0.0002 and P = 0.0001, respectively; tumor volume: P = 0.0001, P = 0.0001 and P = 0.0001, respectively) and LAGk-1B-bearing mice treated with CEP-18770 at 3 and 10 mg/kg (hIgG: P = 0.0008 and P = 0.0034, respectively; tumor volume: P = 0.0008 and P = 0.0028, respectively) compared to mice receiving vehicle. Finally, the combination of CEP-18770 (1 mg/kg) plus melphalan (3 mg/kg) was tested in LAGk-1B-bearing mice. Mice treated with the combination showed markedly smaller tumors compared to treatment with vehicle (P = 0.0008) or melphalan alone (P = 0.0204). Mice treated with the PI alone or in combination with melphalan did not show any observed toxicity. Thus, these studies provide promising preclinical data to suggest the potent anti-MM effects of CEP-18770 both in vitro and in vivo and also suggest that this new PI may enhance the anti-MM effects of several active anti-MM agents including melphalan, doxorubicin and ATO.