Supplementary Table S1 details the efficacy of MRTX0902 in a panel of cancer cell lines.

Supplementary Table S2 shows the antitumor activity of MRTX0902 and adagrasib in a panel of human KRAS G12C-mutant tumor cell line-derived and patient-derived xenograft models.

Supplementary Table S5 details the antitumor activity of MRTX0902 and avutometinib in human KRAS-MAPK pathway mutant tumor cell line-derived xenograft models.

Supplementary Figure S4 shows levels of KRAS-MAPK pathway modulation associated with coadministration of MRTX0902 and adagrasib in the MIA PaCa-2 (KRAS G12C-mutant) model.

Supplementary Figure S6 details the improved antiproliferative activity observed with the MRTX0902/osimertinib combination in EGFR mutant models, PC9 (EGFR ex19del) and NCI-H1975 (EGFR L858R/T790M).

Supplementary Figure S5 shows levels of KRAS-MAPK pathway modulation associated with coadministration of MRTX0902 and adagrasib in the murine CT26 (KRAS G12C-mutant) model.

Supplementary Figure S3 details the antitumor effects and gene set enrichment analysis data associated with coadministration of MRTX0902 and adagrasib in KRAS G12C-mutant human tumor xenograft models.

Supplementary Materials and Methods section includes detailed protocols for the following: Experimental Preparation of MRTX0902 and Avutometinib, SOS1 Biochemical Binding Assay, SOS1 and SOS2 Functional Assays, KRAS-SOS1 Protein-Protiein Interaction (PPI) HTRF Assay, In-Cell Western Assay, 3D Ultra-Low Attachment (ULA) Viability Assay, Immunoblotting and Densitometry Analysis, DUSP6 Quantification from Naive and Tumor-Bearing Mouse Blood, Bioanalysis and Pharmacokinetic Analysis, Synergy Analysis, CRISPR/Cas9 Screening and Data Analysis Methodology, RNAseq Pre-Processing, RNAseq Data Analysis, and Whole Genome Sequencing.

Supplementary Table S3 displays the various genetic co-alterations present in human KRAS G12C-mutant tumor cell line-derived and patient-derived xenograft models evaluated.

Supplementary Figure S7 shows the improved antiproliferative activity observed with the MRTX0902/avutometinib combination in KRAS-MAPK pathway mutant models, LN229 (PTPN11 A72S) and NCI-H1435 (NF1 K615N).

Supplementary Figure S8 details the antitumor effects associated with coadministration of MRTX0902 and adagrasib in the MIA PaCa-2 (KRAS G12C-mutant) model.

Supplementary Figure S2 shows the antitumor activity of MRTX0902 in the LN229 (PTPN11 A72S-mutant) model, with tumor growth inhibition data displayed in Figure S2A and ERK phosphorylation graphed in Figure S2B.

Supplementary Figure S1 details the genetic vulnerabilities and modifiers of response associated with combination treatment of MRTX0902 with adagrasib in the KRAS G12C-mutant MIA PaCa-2 (S1A, S1B, S1D, and S1E) and LU99 (S1C) cell lines in vitro and in vivo.

Supplementary Table S4 shows the antitumor activity of MRTX0902 and osimertinib in human EGFR-mutant tumor cell line-derived xenograft models.

Co-culture experiments with MTAP-expressing fibroblasts

KRAS is the most frequently mutated oncogene in human cancer and facilitates uncontrolled growth through hyperactivation of the receptor tyrosine kinase (RTK)/mitogen-activated protein kinase (MAPK) pathway. The Son of Sevenless homolog 1 (SOS1) protein functions as a guanine nucleotide exchange factor (GEF) for the RAS subfamily of small GTPases and represents a druggable target in the pathway. Using a structure-based drug discovery approach, MRTX0902 was identified as a selective and potent SOS1 inhibitor that disrupts the KRAS:SOS1 protein-protein interaction to prevent SOS1-mediated nucleotide exchange on KRAS and translates into an anti-proliferative effect in cancer cell lines with genetic alterations of the KRAS-MAPK pathway. MRTX0902 augmented the antitumor activity of the KRAS G12C inhibitor adagrasib when dosed in combination in eight out of 12 KRAS G12C-mutant human non-small cell lung cancer and colorectal cancer xenograft models. Pharmacogenomic profiling in preclinical models identified cell cycle genes and the SOS2 homolog as genetic co-dependencies and implicated tumor suppressor genes (NF1 and PTEN) in resistance following combination treatment. Lastly, combined vertical inhibition of RTK/MAPK pathway signaling by MRTX0902 with inhibitors of EGFR or RAF/MEK led to greater downregulation of pathway signaling and improved antitumor responses in KRAS-MAPK pathway-mutant models. These studies demonstrate the potential clinical application of dual inhibition of SOS1 and KRAS G12C and additional SOS1 combination strategies that will aide in the understanding of SOS1 and RTK/MAPK biology in targeted cancer therapy.

Abstract MRTX1719 is an MTA-cooperative PRMT5 inhibitor that leverages the increased concentration of the metabolite MTA in cancer cells harboring a homozygous deletion of the MTAP gene (MTAP del) under investigation in clincial trials. MRTX1719 preferentially binds to the PRMT5•MTA complex to selectively inhibit PRMT5, an essential gene for all cells, in MTAP del cancer cells while sparing PRMT5 activity in normal MTAP-wildtype cells. MTAP del occurs in ~10% of all cancers, and these patients exhibit remarkably poor survival. Among pancreatic cancer, MTAP del occurs in ~ 25% of patients and of these ~30% have co-occurring KRAS G12D mutations. In addition, ~15% of lung adenocarcinoma patients harbor MTAP del and of these ~13% harbor KRAS G12C. While recently approved KRAS G12C mutant selective inhibitors demonstrate anti-tumor activity, strategies to address drug resistance and maximize clinical benefit are warranted. Combination of MRTX1719 with adagrasib or the KRAS G12D inhibitor MRTX1133 in CDX models harboring both a KRAS mutation (G12C or G12D) and homozygous MTAP deletion resulted in increased anti-tumor activity compared to either single agent. This includes the pancreatic KP4 model in which initial tumor regression in response to single agent MRTX1133 was observed, followed by adaptative resistance and tumor progression with monotherapy while co-administration with MRTX1719 continued to result in marked tumor regression. Mechanistically, combination treated animals demonstrated strong inhibition of both PRMT5 and RAS pathway signaling and combinatorial targeting of these pathways converged on enhanced inhibition of RB-1 phosphorylation. We also characterized this combination treatment in the context of resistance to KRAS selective inhibitors or MRTX1719 to evaluate the benefit of combination over sequential treatment. Finally, an NF1 mutant, MTAP del malignant peripheral nerve sheath tumor (MPNST) PDX model was treated with MRTX1719, the SOS1 inhibitor MRTX0902, or the combination, and a similar increase in anti-tumor activity was observed in the combination treatment compared to either monotherapy alone. These results suggest that the combination of an MTA-cooperative PRMT5 inhibitor with a KRAS mutant selective or KRAS pathway inhibitor may lead to deeper and more durable responses in MTAP del cancer patients with co-alterations in the KRAS pathway. Citation Format: Laura Waters, Ruth Aranda, Xousaen Helu, Laura Vegar, Krystal Moya, Andrew Calinisan, Allan Hebbert, Jill Hallin, Darin Vanderpool, David M. Briere, James G. Christensen, Peter A. Olson, Lars D. Engstrom. The MTA-cooperative PRMT5 inhibitor, MRTX1719, demonstrates increased anti-tumor activity in combination with KRAS mutant-selective inhibitors in MTAP del,KRAS-mutant cancers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 3319.

Materials and methods related to supplemental data

The mTOR kinase regulates a variety of critical cellular processes and has become a target for the treatment of various cancers. Using a combination of property-based drug design and Free-Wilson analysis, we further optimized a series of selective mTOR inhibitors based on the (S)-6a-methyl-6a,7,9,10-tetrahydro[1,4]oxazino[3,4-h]pteridin-6(5H)-one scaffold. Our efforts resulted in 14c, which showed similar in vivo efficacy compared to previous lead 1 at 1/15 the dose, a result of its improved drug-like properties.

Abstract Previous studies implicated protein arginine methyltransferase 5 (PRMT5) as a synthetic lethal target for MTAP-deleted (MTAP del) cancers; however, the pharmacologic characterization of small-molecule inhibitors that recapitulate the synthetic lethal phenotype has not been described. MRTX1719 selectively inhibited PRMT5 in the presence of MTA, which is elevated in MTAP del cancers, and inhibited PRMT5-dependent activity and cell viability with >70-fold selecti­vity in HCT116 MTAP del compared with HCT116 MTAP wild-type (WT) cells. MRTX1719 demonstrated dose-dependent antitumor activity and inhibition of PRMT5-dependent SDMA modification in MTAP del tumors. In contrast, MRTX1719 demonstrated minimal effects on SDMA and viability in MTAP WT tumor xenografts or hematopoietic cells. MRTX1719 demonstrated marked antitumor activity across a panel of xenograft models at well-tolerated doses. Early signs of clinical activity were observed including objective responses in patients with MTAP del melanoma, gallbladder adenocarcinoma, mesothelioma, non–small cell lung cancer, and malignant peripheral nerve sheath tumors from the phase I/II study. Significance: PRMT5 was identified as a synthetic lethal target for MTAP del cancers; however, previous PRMT5 inhibitors do not selectively target this genotype. The differentiated binding mode of MRTX1719 leverages the elevated MTA in MTAP del cancers and represents a promising therapy for the ∼10% of patients with cancer with this biomarker. See related commentary by Mulvaney, p. 2310. This article is featured in Selected Articles from This Issue, p. 2293