Inhibition of p97 with the First-in-Class Small Molecule CB-5083: A Novel Strategy for Acute Myeloid Leukemia Therapy

Acute myeloid leukemia (AML) is one of the most prevalent and aggressive forms of leukemia. Currently utilized therapeutic agents do not achieve long-term survival for the majority of patients with this disease and new approaches are urgently needed to improve outcomes. Protein homeostasis is essential for many critical cellular processes including cell cycle progression, signal transduction, and cell death, but frequently becomes disrupted in cancer and this contributes to disease progression and drug resistance. Considering this, key regulators of protein degradation pathways are very attractive anticancer targets. The AAA ATPase p97 (VCP) is a master regulator of protein turnover that has been implicated in oncogenesis and malignant progression. However, its specific role(s) in AML biology and potential value as a novel ant-leukemic target have not been previously investigated. Our preliminary data suggested that p97 may be required for the proliferation and survival of AML cells, thus inhibition of its activity may be an effective approach to antagonize AML pathogenesis. CB-5083 is a first-in-class selective and potent inhibitor of p97 that has entered Phase I clinical trials. We investigated the efficacy and pharmacodynamic (PD) activity of CB-5083 in human AML cell lines, primary AML specimens, and a FLT3-ITD+ xenograft mouse model of AML. Here we report that CB-5083 potently diminished the viability of AML cell lines and primary CD34+ blasts obtained from patients. Notably, cellular sensitivity to CB-5083 was similar in 3 different paired sensitive and resistant cell line models of cytarabine resistance, suggesting that this novel agent may be effective for patients that are relapsed/refractory to conventional therapy. Acute exposure to CB-5083 ablated clonogenic survival, triggered the accumulation of ubiquitin-conjugated proteins, activated the unfolded protein response (UPR), disrupted STAT5 signaling, and induced apoptosis. The pro-apoptotic effects of CB-5083 were associated with activation of the endoplasmic reticulum (ER) resident initiator caspase-4. RNA sequencing (RNASeq) of AML cells following treatment with CB-5083 revealed the unique PD signature of this novel agent. The in vivo anti-leukemic activity of CB-5083 was investigated in a xenograft mouse model of AML established with the FLT3-ITD+ MV4-11 cell line. Oral administration of 100 mg/kg CB-5083 (once daily, 4 days on, 3 days off) was well tolerated, led to pathway inhibition as evidenced by poly-ubiquitin accumulation in tumor tissue, and induced disease regression. Our collective findings indicate that disruption of p97 activity with CB-5083 is a promising new approach for AML therapy that warrants further investigation.