Correlation of baseline ENPP1 and cGAS expression in advanced solid tumors with intratumoral immune activation and clinical outcomes after treatment with the first-in-class oral ENPP1 inhibitor RBS2418, alone or in combination with pembrolizumab.

2597 Background: ENPP1 is a nucleotide pyrophosphatase and phosphodiesterase. Its expression is associated with poor prognosis in cancer. ENPP1 inhibition protects cGAMP and ATP from hydrolysis and reduces adenosine levels in the TME, activates APCs and increases T-cell infiltration promoting anticancer immunity. RBS2418, a first-in-class, potent ENPP1 inhibitor, is evaluated as monotherapy and combination with pembrolizumab in advanced/metastatic solid tumors. Methods: Phase 1 dose escalation with 100, 200, 400 and 800 mg BID dose and selected expansion cohorts of RBS2418 alone or in combination with pembrolizumab (200 mg IV q3w) in patients who have failed all approved treatments including immunotherapy. Primary outcome measures are safety and PK. Tumor biopsy, and blood samples are collected to determine PK/PD and immune profiles using LC/MS, ENPP1 inhibition, immunofluorescence, IHC, flow cytometry, and TCR/RNAseq analysis. Results: All dose levels evaluated to date were safe and well tolerated with no DLTs. Median plasma concentrations of RBS2418 increased in a dose-dependent manner. Plasma and tumor concentrations of RBS2418 were above the ENPP1 inhibition EC90 level in all patients at all-time points tested. Preliminary analyses support the premise that ENPP1 and cGAS co-expression (EG+ phenotype) in tumors correlates with RBS2418 treatment associated immune activation and clinical benefit. Analysis of all currently available paired pre- and on-treatment biopsies showed on-treatment immune activation profiles in 100% (n=6) with EG+ phenotype and 0% (0/6) with EG- phenotype at baseline. Immune activation profiles include an increase in tumor-infiltrating T-cells and M2 to M1 macrophage switch, consistent with the mechanism of action. EG +/- phenotype and immune activation profile also correlated with observed clinical outcomes. Conclusions: Oral RBS2418 alone or with pembrolizumab has been safe, well tolerated with no DLTs at all dose levels evaluated to date in the ongoing Ph1 dose escalation and expansion study. The robust PK profile shows plasma levels enabling complete ENPP1 inhibition at all time points. Immune activation and observed clinical benefit of treatment in late line treatment of refractory metastatic solid tumors such as CRC and HCC was associated with baseline ENPP1 and cGAS protein expression in tumors. These results warrant further development of RBS2418 in advanced tumors and enrollment continues in expansion cohorts on this phase 1 study. Clinical trial information: NCT05270213 .