Preclinical evalution of the orally active proteasome inhibitor MLN9708 in preclinical models of human cancer

The ubiquitin-proteasome system processes the majority of cellular proteins and is the principal manner by which cells regulate protein homeostasis. The successful development of VELCADE® (bortezomib) for Injection for multiple myeloma and previously treated mantle cell lymphoma has validated the proteasome as a therapeutic target for the treatment of malignancies. MLN9708 was identified from efforts to develop a proteasome inhibitor that had broad-based antitumor activity in preclinical models. MLN9708 immediately hydrolyzes to MLN2238, the biologically active form, upon exposure to aqueous solutions or plasma. MLN2238 was used for all preclinical studies described below. MLN2238 is a reversible, potent and orally bioavailable proteasome inhibitor with improved pharmacokinetics (PK), pharmacodynamics (PD) and antitumor activity when compared to bortezomib. MLN2238 inhibited 20S proteasome activity in vitro, preferentially binding the 20S \#946;5 site with an IC50 of 3.4 nM and demonstrated potent activity against cultured cancer cells in cell viability assays. MLN2238 achieved exposures that resulted in significant blood and tumor proteasome inhibition and had increased plasma and tumor exposure compared to bortezomib (when dosed at MTD) in mice. PD responses were assessed by evaluating tumor 20S \#946;5 site-specific activity and expression levels of GADD34 and ATF-3, two unfolded protein response (UPR) pathway genes shown to be upregulated in response to proteasome inhibition. In CWR22 (prostate) xenografts, dose response in 20S \#946;5 inhibition and UPR pathway marker elevation were detected following MLN2238 treatment. Efficacy studies in CWR22 xenografts demonstrated strong antitumor activity following intravenous (IV), subcutaneous (SC) or oral (PO) dosing of MLN2238. In WSU-DLCL2 xenografts, greater antitumor activity was seen in mice treated with IV or SC doses of MLN2238 compared to bortezomib. For example, WSU-DLCL2 xenograft efficacy experiments demonstrated that MLN2238 had improved antitumor activity when dosed SC QD (T/C = 0.33, TGI = 66.5%) compared to bortezomib dosed SC QD (T/C = 0.7, TGI = 30.2%). Consistent with the efficacy difference between MLN2238 and bortezomib, MLN2238 demonstrated an improved PD response as compared to bortezomib, showing higher levels of tumor proteasome inhibition and pathway marker elevation. Collectively these data demonstrated that MLN2238 has improved PK, PD and antitumor activity compared to bortezomib in preclinical models and has the potential to be active in both hematologic and nonhematologic malignancies.