Abstract 2215: Age and indoleamine 2,3-dioxygenase 1 (IDO1) interact to affect gut microbiome homeostasis in subjects with glioblastoma

Objective: This study aimed to determine how age and IDO1 interact to affect the gut microbiome, microbial metabolite levels, and survival outcomes in subjects undergoing IDO1-targeted immunotherapy against glioblastoma (GBM). Methods: Serum and colon contents from younger 16-20- or older 92-118-week-old wild-type (WT), IDO1 knockout (IDO1-KO), and IDO1 enzyme null (IDO1-H350A) mice were analyzed for 16S rRNA gut microbiome composition and microbial-derived aromatic amino acid metabolites via LC/MS/MS. Fecal samples from younger 7-8 or older 81-85 week-old WT mice with intracranial syngeneic GL261 GBM cells treated with or without brain radiation (RT) and PD-1 mAb were also analyzed. Fecal and plasma samples from 12 newly diagnosed IDHwt/MGMT promoter unmethylated GBM patients who received RT plus PD-1 mAb and IDO enzyme inhibitor were also analyzed. Results: Young and old IDO1-KO and IDO1-H350A mice demonstrated a unique gut microbiome signature with an elevated abundance of Bifidobacterium and Helicobacter (p<0.05) and a reduced abundance of Ventriosum and Sireaum compared to WT mice (p<0.001). Prevotellaceae, Muribaculum, Alistipes, Enterohabdus, Clostridia, and Akkermansia were different in IDO1-KO and IDO1-H350A mice compared to WT - but only in older mice. Serum microbiome-derived aryl-lactates phenyllactate (PLA), indolelactate (ILA), and 4-hydroxyphenyllactate (4HPLA) were increased in IDO1-KO mice compared to WT and IDO1-H350A mice (p<0.001) - but only in older mice. Older WT mice with GL261 GBM showed lower fecal aryl-lactates (PLA, 4HPLA, and ILA) compared to younger counterparts. Treatment with RT + PD-1 mAb also showed decreased serum indole propionic acid (IPA) levels in young but not old mice with GL261 GBM (p<0.05). Analyses from GBM patient baseline fecal samples revealed that higher abundance of Massilioclostridium coli, GGB3819-SGB5184, Dysosmobacter welbionis, and Phocaeicola plebeius was associated with longer survival (p<0.004), while higher levels of fecal ILA and lower levels of plasma IPA also correlated with elongated survival (p<0.04 and p<0.01 respectively). Combined treatment of RT + PD-1 mAb + IDO1 inhibitor decreased plasma IPA when compared to baseline (p<0.01). Conclusions: IDO1 possesses both enzyme- and non-enzyme-dependent effects that change gut microbiota composition and bioactive microbial aromatic amino acid metabolite levels, which is also influenced by aging. Certain gut microbiota species and metabolites may serve as prognostic markers for predicting outcomes to immunotherapy for GBM. These findings warrant further investigation into how IDO1 interacts with age to affect immunotherapeutic efficacy in subjects with GBM. Citation Format: Lijie Zhai, Akriti Shrestha, Kristen L. Lauing, Manon Penco-Campillo, Oluwatomilayo Odum, Prashant Bommi, Taylor Koch, Robert H. McCusker, Douglas E. Anderson, Anand Germanwala, Vikram C. Prabhu, Jigisha P. Thakkar, Jacob M. Allen, Rimas V. Lukas, Derek A. Wainwright. Age and indoleamine 2,3-dioxygenase 1 (IDO1) interact to affect gut microbiome homeostasis in subjects with glioblastoma [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 2215.