Abstract 1331: Deciphering PSMA7 modulation by RT and TMZ for enhanced GBM treatments

Abstract Objective: Glioblastoma (GBM, WHO grade 4) is one of the most lethal malignancies, with limited advancements since temozolomide (TMZ) and radiation therapy (RT) became the standard of care nearly two decades ago. A critical challenge and overarching goal of our lab is to understand and overcome the resistance mechanisms that undermine these therapies. Using multi-omics profiling, we identified proteasome subunit alpha type-7 (PSMA7), a non-catalytic subunit of the 20S proteasome, as overexpressed in GBM. Elevated PSMA7 correlates with poor survival, and work by our group demonstrated its potential as a therapeutic target. This study investigated how RT and TMZ modulate PSMA7 expression, shedding light on its role in resistance and its feasibility as a target for novel interventions. Materials/Methods: Patient-derived primary human GBM cells (08-387 and 3359) and normal human astrocytes (NHAs) were used. Protein expression was quantified by Western blot analysis. Cell viability was assessed by ATP assays. TMZ (in vitro: Sigma T2577; in vivo: SelleckChem S1237). A Rad Source 2000 irradiator was used to treat cells with ionizing radiation in vitro. In vivo, mice anesthetized with isoflurane were irradiated using a Clinac linear accelerator, with a 1.7-cm x 1.7-cm copper collimator targeting radiation to the tumor. Dosimetry was performed using GafChromic films placed under the irradiated area. Results: GBM and NHA cell viability decreased with increasing TMZ dose in vitro. Western blots showed that PSMA7 expression did not change when GBM or NHA cells were treated once daily for three days with 300 or 600 μM TMZ. However, cleaved PARP expression increased in GBM cells treated with TMZ, but not in NHAs. In vivo, PSMA7 expression increased in flank tumors of mice treated with 50 mg/kg TMZ for five days starting at day 7 post-implantation compared to vehicle-treated mice. NHAs treated once daily for three days with 0, 2, 3, or 4 Gy RT had no change in PSMA7 expression in vitro, while GBM cells had decreased PSMA7 expression with increasing RT dose. We also performed an in vivo study where GBM cells were implanted in the flank of mice and tumors were treated with up to 14.70 Gy RT. Tumors were collected at mouse removal and PSMA7 expression examined by Western blot. Conclusion: Our findings underscore the selective regulation of PSMA7 by RT and TMZ, highlighting its potential as a therapeutic target. RT’s ability to downregulate PSMA7 supports the rationale for integrating PSMA7-targeted small molecule inhibitors into the standard of care following validation in preclinical and clinical trials. Such inhibitors, particularly blood-brain barrier-permeable derivatives actively under development in our lab, offer a promising strategy to synergize with RT and enhance the therapeutic landscape for GBM. By targeting PSMA7, we may overcome the limitations of current treatments and revolutionize GBM therapy by improving outcomes for this aggressive malignancy. Citation Format: Christian A. Showalter, Priyani V. Rajasekera, Ramasamy Santhanam, Ashok Kumar, Karnika Singh, Xiaomei Meng, Sagarika Jain, Heather R. Manring, Saikh J. Haque, Arnab Chakravarti. Deciphering PSMA7 modulation by RT and TMZ for enhanced GBM treatments [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 1331.