Rustum, Youcef M.

Abstract Background: Clear cell renal cell carcinoma (ccRCC), metastatic castration-resistant prostate cancer (mCRPC), and glioblastoma (GBM) are treatment-refractory tumors marked by high TGF-β1, a master suppressor of antitumor immunity and antigen presentation. Pharmacologic methylseleninic acid (MSA) potently inhibits TGF-β1, PD-L1, and VEGF without significant off-target toxicity. FN14, the TWEAK receptor and a metastasis driver, is selectively overexpressed in these malignancies, making it a high-specificity CAR-T target. Methods: We developed second-generation anti-FN14 CARs with CD28 or 4-1BB costimulation and CD3ζ signaling and lentivirally transduced them into human CD4+/CD8+ T cells. Cytotoxicity against FN14+ ccRCC, mCRPC, GBM lines and patient-derived tumors was measured using LDH release and xCELLigence assays. Effector function was assessed by intracellular cytokines, phenotyping, and multiplex analysis. MSA was tested at physiologic concentrations to evaluate effects on CAR-T viability, metabolism, function, and tumor susceptibility. Results: Anti-FN14 CAR-T cells mediated rapid, antigen-specific lysis across all tumor types, achieving >50% specific lysis at an E:T ratio of 1:1. MSA (2.5-5 µM) reduced tumor PD-L1 and TGF-β by >40% and improved CAR-T metabolic activity, enhancing IL-2 and IFN-γ secretion. Manufacturing CAR-T cells with MSA (2.5 µM) increased cytotoxicity and persistence versus controls (p < 0.01). MSA’s effects were dose- and schedule-dependent at clinically compatible concentrations without detectable off-target toxicity. Conclusion: MSA augments anti-FN14 CAR-T cytotoxicity, metabolic fitness, and sustained effector function by suppressing tumor-derived TGF-β1 and PD-L1 and directly enhancing CAR-T activity. As a clinically relevant TGF-β1 inhibitor and active seleno-L-methionine metabolite, MSA represents a readily translatable strategy to reduce CAR-T exhaustion and improve efficacy in TGF-β-rich solid tumors. Citation Format: Gloria B. Kim, Obed B. Amissah, Casey R. Ager, Yousef Zakharia, Youcef M. Rustum. Methylseleninic acid enhances anti-FN14 CAR-T cell effector function and redirects cytotoxicity against TGF-β-rich kidney, prostate, and brain tumors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 6496.

Abstract Background: Clear cell renal cell carcinoma (ccRCC), metastatic castration-resistant prostate cancer (mCRPC), and glioblastoma (GBM) are highly treatment-refractory tumors characterized by elevated TGF-β1, a key suppressor of antitumor immunity and antigen presentation. Because CAR-T cells infiltrating these tumors experience persistent physiologic TGF-β exposure, assessing CAR-T functional resilience across increasing TGF-β concentrations is critical for determining therapeutic feasibility. Methylseleninic acid (MSA), a pharmacologic selenium compound, potently inhibits TGF-β1, PD-L1, and VEGF with minimal off-target toxicity. FN14, the TWEAK receptor and a metastasis driver, is selectively overexpressed in these malignancies, making it a high-specificity CAR target. Methods: We developed second-generation anti-FN14 CARs containing CD28 or 4-1BB costimulatory domains and a CD3ζ signaling motif, then lentivirally transduced into primary human CD4+ and CD8+ T cells. Cytotoxicity against FN14+ ccRCC, mCRPC, GBM lines and patient-derived cells was assessed using LDH release and xCELLigence assays. Effector function was evaluated through intracellular cytokine profiling, multiplex analysis, and phenotypic characterization by flow cytometry. CAR-T persistence in TGF-β-rich conditions was measured using CCK-8 proliferation assays, and Ki-67 and activation/exhaustion markers (PD-1, CXCR3) in the presence of 0-10 ng/mL TGF-β. MSA’s immunomodulatory and synergistic effects were tested at physiologically concentrations to measure impacts on CAR-T viability, metabolic activity, cytokine secretion, and tumor cell susceptibility. Results: Anti-FN14 CAR-T cells mediated rapid, antigen-specific lysis across all tumor types, achieving >50% specific killing at a 1:1 E:T ratio. MSA (2.5-5 µM) reduced tumor PD-L1 and TGF-β expression by >40% and enhanced CAR-T metabolic activity, IL-2, and IFN-γ secretion. CAR-T cells maintained comparable proliferation and activation in 0-5 ng/mL TGF-β, indicating that antigen-driven signaling counteracted low-to-moderate TGF-β-mediated suppression. Ex vivo CAR-T manufacturing with MSA (2.5 µM) improved cytotoxicity and persistence versus untreated CAR-T cells (p < 0.01). MSA’s effects were dose- and schedule-dependent and remained within human-relevant exposure ranges without detectable toxicity. Conclusion: MSA enhances anti-FN14 CAR-T cytotoxicity, metabolic fitness, and sustained effector activity by suppressing tumor-derived TGF-β1 and PD-L1 and by directly improving CAR-T functional capacity. Consistent with this mechanism, physiological TGF-β levels (0-5 ng/mL) did not significantly impair CAR-T proliferation or activation, demonstrating intrinsic resilience of FN14 CAR-T cells in TGF-β-rich environments. As a clinically relevant TGF-β1 inhibitor and active metabolite of seleno-L-methionine, MSA represents a readily translatable strategy to reduce CAR-T exhaustion and enhance efficacy in TGF-β-rich solid tumors including ccRCC, mCRPC, and GBM. Citation Format: Obed B. Amissah, Casey Ager, Yousef Zakharia, Youcef M. Rustum, Gloria B. Kim. Anti-FN14 CAR-T cells overcome TGF-β-mediated suppression, and their synergy with methylseleninic acid enhances cytotoxicity and effector function in kidney, prostate, and brain tumors [abstract]. In: Proceedings of the AACR Immuno-Oncology Conference (AACR IO): Discovery and Innovation in Cancer Immunology: Revolutionizing Treatment through Immunotherapy; 2026 Feb 18-21; Los Angeles, CA. Philadelphia (PA): AACR; Cancer Immunol Res 2026;14(2 Suppl):Abstract nr C010.

Abstract Purpose: Data on clear-cell renal cell carcinoma (ccRCC) xenografts defined the seleno-L-methionine (SLM) dose and the plasma selenium concentrations associated with the enhancement of hypoxia-inducible factor-1α/2α degradation, stabilization of tumor vasculature, enhanced drug delivery, and efficacy of axitinib. The data provided the rationale for the development of this phase I clinical trial of SLM and axitinib in advanced or metastatic relapsed ccRCC. Patients and Methods: Patients were ≥18 years with histologically and radiologically confirmed advanced or metastatic ccRCC who had received at least one prior systemic therapy, which could include axitinib (last dose ≥6 months prior to enrollment). Escalating dose levels of SLM (2,500, 3,000, and 4,000 μg) were administered orally twice daily for 14 days and then once daily concurrently with axitinib 5 mg twice daily using a 3 + 3 design in phase I. Patients were treated at the 4,000 μg dose level in the expansion cohort to obtain preliminary estimates of efficacy. Results: No dose-limiting toxicities occurred at the 4,000 μg SLM dose level. Among the 27 patients treated with 4,000 μg of SLM, the overall response rate was 55.6%, median duration of response was 18.4 months, median progression-free survival was 14.8 months, and median overall survival was 19.6 months. Preliminary results have shown that plasma selenium concentrations, inhibition of TGF-β1, and stabilization of tumor vasculature by SLM are time dependent. Conclusions: SLM (4,000 μg) in sequential combination with axitinib is well tolerated with encouraging efficacy.

This study was carried out to quantitate the expression levels of microRNA-17, -19a, -34a, -155, and -210 (miRs) expressed in nine clear cell renal cell carcinoma (ccRCC) and one chromophobe renal cell carcinoma cell line with and without sarcomatoid differentiation, and in six primary kidney tumors with matching normal kidney tissues. The data in the five non-sarcomatoid ccRCC cell lines—RC2, CAKI-1, 786-0, RCC4, and RCC4/VHL—and in the four ccRCC with sarcomatoid differentiation—RCJ41T1, RCJ41T2, RCJ41M, and UOK-127—indicated that miR-17 and -19a were expressed at lower levels relative to miR-34a, -155, and -210. Compared with RPTEC normal epithelial cells, miR-34a, miR-155, and miR-210 were expressed at higher levels, independent of the sarcomatoid differentiation status and hypoxia-inducible factors 1α and 2α (HIFs) isoform expression. In the one chromophobe renal cell carcinoma cell line, namely, UOK-276 with sarcomatoid differentiation, and expressing tumor suppressor gene TP53, miR-34a, which is a tumor suppressor gene, was expressed at higher levels than miR-210, -155, -17, and -19a. The pilot results generated in six tumor biopsies with matching normal kidney tissues indicated that while the expression of miR-17 and -19a were similar to the normal tissue expression profile, miR-210, -155, -and 34a were expressed at a higher level. To confirm that differences in the expression levels of the five miRs in the six tumor biopsies were statistically significant, the acquisition of a larger sample size is required. Data previously generated in ccRCC cell lines demonstrating that miR-210, miR-155, and HIFs are druggable targets using a defined dose and schedule of selenium-containing molecules support the concept that simultaneous and concurrent downregulation of miR-210, miR-155, and HIFs, which regulate target genes associated with increased tumor angiogenesis and drug resistance, may offer the potential for the development of a novel mechanism-based strategy for the treatment of patients with advanced ccRCC.