Calithera continues to leverage its discovery engine to build a robust preclinical pipeline of undisclosed synthetic lethality targets with a focus on paralog genes. These will be announced as the company advances preclinical development

Vacuolar protein sorting-associated protein 4A (VPS4A) and 4B (VPS4B) are gene paralogs that show strong synthetic lethal interaction. Synthetic lethal interaction between genes occurs when inhibition of either gene alone does not alter cell viability, but simultaneous inhibition of both genes induces cell death. Paralog genes are pairs of genes that play redundant roles in essential cellular functions In the context of cancer therapy, inhibiting one of the paralogs when there is an existing loss-of-function alteration in the other paralog can be lethal to cancer cells. We have conducted multiple studies to validate the paralog gene pair, VPS4A and VPS4B, demonstrating that cells with VPS4B homozygous or heterozygous loss are sensitive to VPS4A knock down while cells without VPS4B loss are not. In addition, simultaneously knocking down VPS4A and VPS4B consistently resulted in cell death. We subsequently identified a novel series of small molecule inhibitors and detailed the performance of one inhibitor of VPS4A and VPS4B ATPase activity, which outperformed previously reported VPS4 inhibitors, at the AACR annual meeting 2022. We are currently advancing multiple VPS4A inhibitors in the series through lead optimization.

IL4I1, an enzyme that is expressed by tumor cells and antigen presenting cells, metabolizes phenylalanine, tyrosine and tryptophan to produce hydrogen peroxide, an inhibitor of T-cell function. In particular, IL4I1 can metabolize tryptophan to kynurenic acid and other metabolites that lead to immunosuppression in the tumor microenvironment. IL4I1 expression has been correlated with poor outcomes in several tumor types, has a potential role in immune invasion and may decrease the ability of checkpoint therapy to stimulate an anti-tumor immune response. IL4I1 expression is elevated in multiple tumor types with particularly high expression in ovarian and B-cell tumors. We have developed an investigational first-in-class, potent, orally available IL4I1 inhibitor. Preclinical data demonstrated that our novel small-molecule inhibitor of IL4I1 has single-agent antitumor activity in syngeneic mouse tumor models and augments the activity of checkpoint inhibitors.