Evaluation of Mcl-1 Inhibitors in Preclinical Models of Multiple Myeloma

Mcl-1, an anti-apoptotic Bcl-2 family member, is a known resistance factor to many cancer therapies, and a historically difficult target to drug. Recently however, we have made significant progress optimizing the potency and characterizing the mechanism of action of a novel class of selective Mcl-1 inhibitors. Here, we characterize the activity of one of our lead compounds and confirm on-target mechanistic activity in vitro and in vivo.

Using an Mcl-1 binding assay we demonstrate very potent activity (IC₅₀ < 3nM), while maintaining selectivity over other anti-apoptotic Bcl-2 family proteins, Bcl-2 (IC₅₀ > 33 µM) and Bcl-xL (IC₅₀ > 33 µM). In an Mcl-1 dependent multiple myeloma cell line, MOLP-8, we demonstrate rapid Mcl-1/Bak complex disruption with 10 nM inhibitor by 15 min followed by caspase 9-dependent apoptosis by 1 hr (EC₅₀= 50 nM), consistent with an Mcl-1 dependent mechanism of action. Importantly, by depleting Bak from cells using siRNA, we also confirm that the observed apoptosis is through a Bak-dependent mechanism. Moreover, our Mcl-1 inhibitor exhibits broad submicromolar activity across a panel of multiple myeloma cell lines.

In the absence of a validated selective Mcl-1 reference compound to benchmark in vivo activity, we engineered MOLP-8 cells to express the pro-apoptotic protein, Noxa, using a Tet-on expression system and grew these as subcutaneous tumors on the flanks of SCID mice. Within 24 hours of switching the mice to a diet containing doxycycline, we observe significant upregulation of Noxa, leading to an increase in cleaved caspase 3 (CC3) and cleaved PARP in tumor lysates, resulting in rapid tumor regression. In the same MOLP-8 xenograft model, we demonstrate that our Mcl-1 inhibitor induces rapid dissociation of Bak from Mcl-1 in tumors leading to accumulation of CC3 and cleaved PARP by 1 hr. Complete tumor regression was observed after a single 60 mg/kg or 100 mg/kg dose, while partial regression was observed after a single 30 mg/kg dose, and tumor growth inhibition was observed after a single 10 mg/kg dose. All doses were well tolerated with no significant body weight loss. Together, these data reinforce the potential utility of selective Mcl-1 inhibitors in hematological malignancies.