Abstract 4611: Microbiome-derived indole metabolites and response to immune checkpoint inhibitors in patients with hepatocellular carcinoma

Abstract Background: Immune checkpoint inhibitors (ICI) represent the most promising therapy for hepatocellular carcinoma (HCC) to-date; however, responses are heterogeneous and strategies to extend survival are urgently needed. Metabolites of the essential amino acid, tryptophan (Trp) are emerging as potential targets in immune tolerance, inflammation and ICI response. Trp metabolism occurs through: (1) the kynurenine pathway, (2) the serotonin pathway, and (3) the indole pathway—the latter of which is primarily mediated by the gut microbiota. Specific indole metabolites exhibit anti-cancer properties, such as inhibiting cell proliferation and activating CD8+ T cells to target tumor cells. In unresectable, neoadjuvant and adjuvant HCC patients treated with ICI, we aimed to identify metabolite biomarkers of durable response. We further explored microbial sources of these metabolites and validated the presence of relevant gut bacteria in a separate but overlapping cohort of HCC patients prior to ICI therapy. Methods: Ten serum indole metabolites were measured by liquid chromatography-high-resolution mass spectrometry (LC-HRMS) in 38 HCC patients treated with anti-PD1/PL1 (nivo+/-ipi, atezo+bev). Kaplan-Meier analysis assessed time-to-progression (TTP) and recurrence-free survival (RFS) in patients with non-resectable (66%) and resectable disease, with Mann-Whitney U test in landmark analysis (TTP>8 mos, RFS>2 yrs). Finally, we characterized the fecal microbiome of 8 unresectable HCC patients using 16Sv4 rDNA sequencing and compared the profiles to predictions from TrpNet, a web-based tool that uses Bayesian logistic regression to identify potential microbial sources of Trp metabolites. Results: Among unresectable patients with elevated vs. low indolepropionic acid (IPA), 50% vs. 10% reached TTP>8 mos. According to TrpNet, IPA is most likely produced by the Peptostreptococcaceae family, which was observed in our small microbiome cohort. However, the relative abundances of bacterial families Pasteurellaceae and Lachnospiraceae—producers of the IPA precursor indole pyruvate—were the most significantly elevated in patients that initially responded to IC1. No clear associations were observed in the small subset of resectable patients. Conclusion: Elevated IPA levels were correlated with improved outcomes among unresectable HCC patients treated with ICI. These findings further highlight the potential role of circulating indoles and other microbial-derived metabolites in ICI response, warranting further validation in larger cohorts and investigation of their influence on tumor progression and treatment efficacy. Citation Format: Shwetha V. Kumar, Iqbal Mahmud, Yehia I. Mohammed, Xiaotao Zhang, Philip L. Lorenzi, Jennifer A. Wargo, Ahmed O. Kaseb, Manal M. Hassan, Kristi L. Hoffman, Carrie R. Daniel. Microbiome-derived indole metabolites and response to immune checkpoint inhibitors in patients with hepatocellular carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4611.