PRMT5 Inhibition Hits a Nerve (Sheath Tumor): A Targeted Strategy for MPNSTs

Summary

PRMT5 inhibitors represent a promising therapeutic approach for multiple cancers, but their clinical development is hindered by toxicity to normal cells. Synthetic lethality approaches using MTAP-cooperative PRMT5 inhibitors offer a compelling strategy that selectively targets cancer cells in nerve sheath sarcomas and other MTAP-deleted tumors.

In this issue of Clinical Cancer Research, Zhang and colleagues (1) present a compelling preclinical study of MTA-cooperative PRMT5 inhibitors in a comprehensive panel of Malignant Peripheral Nerve Sheath Tumor (MPNST) patient-derived xenografts (PDX). MPNSTs are aggressive, highly metastatic sarcomas with limited treatment options. The current therapeutic regimen of surgery, chemotherapy, and radiation provides only marginal benefit, with 5-year survival rates lingering around 20–50%. Efforts to improve outcomes with molecularly targeted agents have been largely unsuccessful, and Phase II clinical trials report overall survival of less than five months for patients with unresectable disease (2).

MPNSTs frequently exhibit loss of the tumor suppressor genes NF1 and CDKN2A. Due to its genomic proximity to CDKN2A, the MTAP gene is frequently co-deleted, occurring in 10–15% of all human cancers. MTAP encodes methylthioadenosine phosphorylase, an enzyme involved in recycling intracellular methionine pools. These pools are critical to the function of PRMT5 (Protein Arginine Methyltransferase 5), the cell’s primary symmetrical arginine methyltransferase that is essential for methylation of histone and non-histone targets (3). By demonstrating that MTAP loss can synergize with inhibition of the PRMT5 pathway, the study by Zhang and colleagues (1) represents an important step toward new treatment strategies for MPNSTs. The authors offer both mechanistic insight and preclinical validation for biomarker-driven therapies in a cancer where targeted approaches have limited clinical benefit. This study also highlights the power of interrogating a genetically diverse panel of PDX models that reflect the heterogeneity of human tumors, enabling robust biomarker-dependent therapeutic investigations. These findings support stratifying patients based on MTAP status and lay the groundwork for incorporating PRMT5 inhibitors into MPNST treatment paradigms.

The methionine salvage pathway is a critical metabolic process that recycles methionine from methylthioadenosine (MTA), a byproduct of polyamine synthesis. This recycling, mediated in part by MTAP, is essential for maintaining levels of S-adenosylmethionine (SAM), the cell’s main methyl donor. Maintenance of SAM pools is critical to many cellular processes, and PRMT5 requires SAM for enzymatic function. PRMT5 activity is elevated in many cancers, including melanoma, glioma, colorectal carcinoma, leukemia, lymphoma, non-small cell lung cancer, pancreatic, and prostate cancers. Thus, PRMT5 inhibition has emerged as an attractive therapeutic strategy (4). First-generation PRMT5 inhibitors—including GSK3326595, PF-06939999, PRT811, and JNJ-64619178—showed early signs of durable responses in Phase I/II trials (3). These include partial responses in several patients with adenoid cystic carcinoma, ER-positive breast cancer, and glioblastoma, with complete responses observed in two IDH-positive gliomas. However, these inhibitors also impair PRMT5 activity in healthy tissues, particularly those in the bone marrow, leading to hematologic dose-limiting toxicities. As a result, this narrow therapeutic window limits the broad clinical use of these first-generation inhibitors.

Enhanced selectivity of PRMT5-targeted inhibitors is essential to expanding their clinical utility. The MTAP enzyme catalyzes the first step of methionine regeneration required for SAM production. Loss of MTAP leads to MTA accumulation, which partially blocks PRMT5 activity through competition with SAM. This creates a specific vulnerability in MTAP-deficient tumor cells where PRMT5 preferentially forms complexes with MTA rather than SAM (Figure 1). This metabolic vulnerability has spurred development of small molecules that selectively bind PRMT5-MTA complexes while sparing PRMT5-SAM complexes in normal cells (4). These MTA-cooperative PRMT5 inhibitors have a larger therapeutic index compared to non-selective PRMT5 inhibitors by exploiting synthetic lethality in MTAP-deleted cancers. Several PRMT5/MTA complex inhibitors have shown promise in preclinical and early-phase clinical studies. The study by Zhang and colleagues (1) shows that two of these newer agents, TNG908 and TNG462 (4, 5), have MTA-selective activity and minimal toxicity in MTAP-deficient MPNSTs. Both compounds are in active clinical trials, with TNG462 showing promising early signs of target engagement and tolerability in MTAP-deleted solid tumors. The brain-penetrant TNG908 has demonstrated durable clinical activity in pancreatic and NSCLC tumors, and ongoing Phase I/II trials are extending this strategy into CNS malignancies, including gliomas, where MTAP loss is prevalent.

Figure 1: MTAP-cooperative PRMT5 inhibitors induce cancer cell-specific death.

(Left) In healthy cells, MTAP metabolizes MTA into adenine and methionine, which are then shuttled into essential cellular processes. PRMT5 is bound to SAM and catalyzes symmetric methylation of arginine (SDMA) on proteins and histones which is essential for homeostasis. (Right) In cancer cells, MTAP is often lost in tandem with CDKN2A. This leads to a buildup of MTA which competes with SAM for binding to PRMT5. When PRMT5 is bound to MTA, it cannot catalyze SDMA formation and SDMA levels drop dramatically. Non-MTAP-cooperative PRMT5 inhibitors (red) bind to both SAM-bound and MTA-bound forms of PRMT5 and further reduce SDMA levels, causing death in both healthy cells and cancer cells. MTAP-cooperative PRMT5 inhibitors (green) selectively bind to MTA-bound PRMT5, further reduce SDMA levels, and induce death only in cancer cells, while sparing healthy, MTAP-containing cells.

The study by Zhang and colleagues (1) is the first to evaluate these targeted agents in MPNST, where MTAP loss is observed in 25–50% of cases. The authors exploit this therapeutic vulnerability with remarkable precision. A key strength of the work lies in its thoughtful preclinical design. To capture the genetic heterogeneity of MPNSTs, the authors leveraged a diverse panel of patient-derived cell lines and PDX models. They analyzed co-deletion patterns of CDKN2A and MTAP across 13 PDX lines. Homozygous MTAP loss was observed in over half of the tumors with CDKN2A deletion and was never detected independently. Although PRMT5 inhibitors have shown activity across several cancers, their clinical utility has been limited by toxicity in normal tissues. Using MTAP-deleted MPNST cell lines, the authors demonstrated that the PRMT5-MTA inhibitors TNG908 and TNG462 were substantially more selective for MTAP-null cells. These anti-proliferative effects were driven by activation of caspase-3 and/or PARP-1-mediated apoptotic pathways, which were not observed in MTAP-intact cells. In contrast, conventional chemotherapies such as doxorubicin and trabectedin lacked MTAP-specific selectivity and exhibited only additive anti-tumor effects when combined with TNG compounds. These findings underscore the importance of harnessing a synthetic lethal strategy where MTA-cooperative PRMT5 inhibitors selectively target MTAP-null cells to enhance efficacy and spare normal tissue. In vivo, treatment of mice bearing two MTAP-null PDX lines resulted in dose-dependent tumor suppression with TNG908 and even tumor regression with TNG462. Notably, this may be the first report of tumor regression in MPNST PDX models, providing compelling preclinical evidence to support clinical development of MTA-cooperative PRMT5 inhibitors for this cancer. By leveraging this tumor-specific vulnerability, the authors not only identified a novel therapeutic strategy for MPNST but also address a major limitation of conventional cytotoxic therapies by minimizing toxicity in a population that has few effective treatment options.

The implications of this study extend well beyond MPNST. As ~10% of all cancers exhibit concurrent loss of CDKN2A and MTAP, these findings suggest that MTA-cooperative PRMT5 inhibitors may have broad therapeutic relevance across multiple malignancies. The study by Zhang and colleagues (1) not only identifies a promising therapeutic avenue for a rare and aggressive sarcoma, but also highlights an exploitable vulnerability shared by a wide spectrum of MTAP-deficient tumors. Given the high frequency of CDKN2A loss in MPNSTs, there is strong rationale for combining these PRMT5 inhibitors with CDK4/6 inhibitors, a strategy that has proven synergistic in xenograft models of glioblastoma and non-small cell lung cancer (4). Additional rational combinations may include DNA-damaging agents, PARP inhibitors, Ras pathway inhibitors, or MAT2A inhibitors that disrupt the supply of SAM. Furthermore, refined patient stratification based on MTAP status could maximize efficacy and safety. The promising preclinical results in this study offer substantial translational potential for patients with MPNST. By selectively targeting tumor-specific vulnerabilities, MTA-cooperative PRMT5 inhibitors represent a hopeful new direction for patients with limited treatment options and historically poor outcomes.