Activation of GCN2 By HC-7366 Results in Significant Anti-Tumor Efficacy As Monotherapy and Overcomes Resistance Mechanisms When Combined with Venetoclax in AML

Venetoclax (anti-BCL2) is approved for elderly patients with acute myeloid leukemia (AML) in combination with hypomethylating agents such as azacitidine or decitabine. However, several resistance mechanisms arise, impeding responses in patients. These resistance mechanisms include TP53 or FLT3-ITD mutations, suppression of pro-apoptotic proteins NOXA or PUMA, increases in oxidative phosphorylation and energy metabolism, or elevated expression of S100A8/A9. Recent reports have highlighted that hyperactivation of the integrated stress response (ISR) pathway overcomes venetoclax resistance through NOXA activation and glycolytic activity inhibition. However, these studies used drugs that indirectly activate the ISR with other off-target effects. General Control Nonderepressible 2 (GCN2) is an ISR kinase that senses and responds to nutrient stress conditions, and activation of GCN2 results in antitumor activity. We are developing HC-7366, a first-in-class, first-in-human direct GCN2 activator, and are currently evaluating this agent in a phase 1 clinical trial in solid tumors (NCT05121948). Here we present a series of studies characterizing the antitumor effects of HC-7366 in AML, and how it impacts venetoclax resistance. AML patients with TP53 mutations are known to have lower response rates to venetoclax.The in vivo efficacy of HC-7366 in both CDX and PDX models was independent of TP53 mutation, resulting in 100% complete response and 100% tumor growth inhibition in TP53-mutated MOLM-16 and KG-1 tumor models, respectively. Analysis of tumors from treated mice by IHC demonstrated activation of the ISR as evidenced by increased expression of the ATF4 targets ASNS and PSAT1. A genome-wide CRISPR screen in KG-1 cells revealed that GCN2 sgRNAs were significantly enriched in HC-7366-treated cells, highlighting the importance of GCN2 activation in mediating the mechanism of action of HC-7366. Interestingly, BCL2 sgRNAs were significantly depleted in HC-7366 treated cells, suggesting a synthetic lethal relationship with BCL2 inhibitors, such as venetoclax, when combined with HC-7366. Indeed, in the MV4-11 FLT3-ITD mutant model (a differentiated subtype of AML that shows limited response to venetoclax), the combination of HC-7366 and venetoclax produced substantial benefit resulting in 26% tumor regression and enhanced activation of the ISR pathway. HC-7366 treatment also increased mRNA and protein levels of NOXA and PUMA while reducing mitochondrial respiration and glycolysis in a GCN2-dependent manner, leading to a low energetic state. Consistent with its impact on cellular energetics, global metabolomic analyses of AML xenograft tumors showed that HC-7366 significantly altered metabolites associated with glycolysis and the TCA cycle. Additionally, HC-7366 reduced protein levels of S100A8/A9, also known to correlate with venetoclax resistance. Our in vitro and in vivo results demonstrate that HC-7366 is a potent GCN2 activator with strong antitumor activity in AML as a single agent and in combination with venetoclax. The potential of HC-7366 to counteract multiple known resistance mechanisms to venetoclax could provide a viable treatment option for patients with relapsed/refractory AML when used in combination with venetoclax.