Abstract 1295: Small molecule reactivators of Y220C mutant p53 modulate tumor infiltrating leukocytes and synergize with immune checkpoint inhibitors
Authors: Puzio-Kuter Anna M. , Mulligan Chris , Russo Brandon , Wiebesiek Amy , Xu Lizhong , Yang Hong , Vu Binh , Dumble Melissa 
Published: June 2022

Abstract
Half of human cancers possess mutations in the TP53 gene, with most clustering as hotspots in the DNA binding domain. A tyrosine to cysteine substitution at amino acid 220 of the p53 protein (Y220C) is one such hotspot mutation (~1% of all solid tumors). Small molecules that reactivate Y220C mutant p53 activity to be like wild type have been developed at PMV Pharmaceuticals and the lead compound PC14586 is in Phase I clinical development. The molecules were designed to bind tightly to a crevice within the Y220C p53 mutant protein and stabilize it to a wild type conformation allowing reactivation of p53 transcriptional activity and expression of target proteins (e.g. p21, MDM2), resulting in tumor regression in Y220C p53 mutant xenograft models grown in immunocompromised mice. p53 has been documented to contribute to immune responses by activating regulators of immune signaling pathways and to result in T-cell exclusion in tumor models. To investigate the role of p53 and the immune system in tumorigenesis, a Y220C human p53 knock-in (HUPKI) mouse was generated. Mice that are homozygous mutant for the Y220C humanized alleles succumbed to lymphomas and sarcomas within 6 months and cell lines generated from these tumors were shown to be sensitive in an in vitro proliferation assay to Y220C p53 reactivators (IC50 ~ 192-722 nM) and used to generate Y220C mutant syngeneic mouse models. Administration of Y220C p53 reactivators in Y220C p53 syngeneic mouse models resulted in a dose responsive anti-tumor effect, with maximally efficacious doses resulting in durable cures in nearly all animals, more than observed in immunocompromised mouse models. Addition of checkpoint agents with sub-efficacious administration of Y220C reactivating compounds resulted in synergistic increases in mean survival time of mice, longer than with either single agent. Immunophenotyping analysis of HUPKI Y220C mutant tumors exposed to p53 reactivator compounds showed modulation of the tumor immune environment. Changes include dose responsive increases in T-cells (CD4+, CD8+), T-regulatory cells, natural killer T cells and dose responsive decreases in macrophages (M2) and g-MDSC cells. The synergy observed with an anti-PD-1 combination appears driven by an increase in CD8+ T cells, given a decrease in efficacy in mice depleted of CD8+ T cells, following exposure to anti-CD8. Further investigation into the role of Y220C p53 reactivation in immune modulation showed decreased cytokines and chemokines involved in inflammation (i.e. CXCL10, Ccl2) and an increase in a Tumor Inflammation Signature (nanoString) in a dose and time dependent manner. Taken together, these data suggest a role for p53 signaling in encouraging an immunologically hot tumor microenvironment and provide support for testing small molecule reactivators of mutant p53 in combination with immune checkpoint inhibitors.
Citation Format: Anna M. Puzio-Kuter, Chris Mulligan, Brandon Russo, Amy Wiebesiek, Lizhong Xu, Hong Yang, Binh Vu, Melissa Dumble. Small molecule reactivators of Y220C mutant p53 modulate tumor infiltrating leukocytes and synergize with immune checkpoint inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1295.