Abstract: Methylthioadenosine phosphorylase (MTAP) deletion is a common genetic alteration in various malignancies, including a significant proportion of pancreatic and gastrointestinal cancers. This deletion leads to the accumulation of methylthioadenosine (MTA), which partially inhibits protein arginine methyltransferase 5 (PRMT5) by competing with its methyl donor, S-adenosyl methionine (SAM). AMG 193 is a novel, orally bioavailable, MTA-cooperative PRMT5 inhibitor designed to exploit this synthetic lethal vulnerability. By selectively binding to the PRMT5-MTA complex, AMG 193 avoids the severe hematologic toxicities associated with earlier, non-selective PRMT5 inhibitors. Preclinical and early clinical data demonstrate that AMG 193 exhibits robust pharmacological activity and manageable safety profiles in patients with MTAP-deleted solid tumors, including pancreatic ductal adenocarcinoma (PDAC), cholangiocarcinoma, and esophageal cancers. This review summarizes the pharmacological activity, molecular mechanism of action, structure-activity relationship, current limitations, and future perspectives of AMG 193, with a specific focus on its application in gastrointestinal and pancreatic cancers.
1. Introduction
The continuous pursuit of novel anticancer agents that balance efficacy with tolerability has led to the identification of cancer-specific biomarkers and synthetic lethal vulnerabilities. One such biomarker is the homozygous deletion of the MTAP gene (MTAP-del), which occurs in approximately 10% of all human cancers due to its chromosomal proximity to the frequently deleted CDKN2A tumor suppressor gene on the 9p21 locus [1]. Gastrointestinal and pancreatic cancers exhibit notable rates of MTAP deletion, with pancreatic cancer showing a prevalence of approximately 22% [1].
In normal cells, MTAP metabolizes MTA into adenine and a methionine precursor. In MTAP-deleted cancer cells, the inability to metabolize MTA leads to its intracellular accumulation. This accumulated MTA outcompetes S-adenosyl methionine (SAM) for binding to protein arginine methyltransferase 5 (PRMT5), partially inhibiting its activity [1]. Because PRMT5 is essential for cell survival, this partial inhibition creates a unique synthetic lethal vulnerability. While first-generation, tumor-nonselective PRMT5 inhibitors caused severe, dose-limiting hematologic toxicities by targeting both healthy and cancerous cells, a new class of MTA-cooperative PRMT5 inhibitors has been developed. AMG 193 is a leading clinical-stage MTA-cooperative PRMT5 inhibitor that selectively targets the PRMT5-MTA complex, offering a promising targeted therapy for MTAP-deleted gastrointestinal and pancreatic cancers [1].
2. Pharmacological Activity
AMG 193 has demonstrated significant pharmacological activity in both preclinical models and early-phase clinical trials. In preclinical studies, AMG 193 exhibited dose-dependent tumor growth inhibition across a variety of MTAP-deleted xenograft models, including pancreatic, esophageal, and melanoma cancers, without causing marked toxicity, weight loss, or significant hematologic toxicity [1].
Clinically, AMG 193 is being evaluated in a Phase I/Ib trial (NCT05094336) involving patients with advanced MTAP-deleted solid tumors, including PDAC, biliary tract cancer, and gastric/esophageal cancer [1]. In the dose-exploration phase, AMG 193 was administered orally at dosages ranging from 40 to 1,600 mg once daily or 600 mg twice daily. The maximum tolerated dose (MTD) was established at 1,200 mg once daily [1]. An efficacy analysis of 74 patients across all dose levels revealed an objective response rate (ORR) of 12.2% and a disease control rate (DCR) of 47.3%. Notably, in 42 patients treated at active dosage levels (800 and 1,200 mg once daily, and 600 mg twice daily), the ORR improved to 21.4% and the DCR to 54.8%, with a median duration of response of 8.3 months [1]. Objective responses were observed across multiple gastrointestinal malignancies, including PDAC, cholangiocarcinoma, esophageal cancer, and gallbladder cancer [1].
3. Molecular Mechanism of Action
The molecular mechanism of AMG 193 relies on the selective inhibition of PRMT5 exclusively in the presence of elevated MTA, a condition specific to MTAP-deleted cells. PRMT5 is a type II methyltransferase that normally utilizes SAM to add symmetric dimethylarginine (SDMA) to histones and other target proteins involved in RNA splicing, cell cycle progression, and DNA repair [1]. AMG 193 binds selectively to the PRMT5-MTA complex, further inhibiting PRMT5 activity and inducing synthetic lethality in MTAP-deleted cancer cells while sparing healthy MTAP wild-type cells [1].
Pharmacodynamic analyses show that AMG 193 administration leads to a dose-dependent reduction in SDMA levels. In preclinical models, AMG 193 inhibited SDMA by 86% to 93% in MTAP-deleted tumors compared to only 26% to 76% in MTAP wild-type tumors [1]. In human patients, near-complete SDMA elimination was observed at dosage levels of 480, 800, and 1,200 mg once daily [1]. Downstream, this profound PRMT5 inhibition impairs RNA splicing—predominantly by inducing the retention of detained introns in genes associated with cell cycle defects, apoptosis, and senescence—ultimately leading to tumor cell death [1].
4. Structure-Activity Relationship (SAR)
The discovery and optimization of AMG 193 utilized high-throughput screening techniques aimed at identifying small molecules that bind selectively to PRMT5 only in the presence of MTA [1]. Following initial identification, molecular modeling and structure-based drug discovery approaches were employed to iteratively optimize the compound's potency and selectivity for the PRMT5-MTA complex [1].
X-ray crystallography of the PRMT5-binding structure of AMG 193 revealed that the compound occupies the substrate binding site of the PRMT5-MTA complex. It forms a tight van der Waals interaction with the accumulated MTA molecule while simultaneously interfering with the interaction between PRMT5 and SAM. This structural configuration contributes to a long dissociation half-life and high binding selectivity for the PRMT5-MTA complex over the PRMT5-SAM complex, ensuring that the drug remains highly selective for MTAP-deleted cancer cells [1].
5. Current Limitations
Despite its improved safety profile compared to non-selective PRMT5 inhibitors, AMG 193 is associated with certain clinical limitations and toxicities. In the Phase I trial, 85% of patients experienced treatment-related adverse events (TRAEs), with the most common being nausea (48.8%), fatigue (31.3%), vomiting (30%), and decreased appetite (22.5%) [1]. Grade 3 or higher TRAEs were reported in 13.8% of patients, including fatigue (5%), decreased lymphocyte count (3.8%), and decreased neutrophils (2.5%) [1]. Dose-limiting toxicities (DLTs) occurred in 11.1% of patients at the 1,200 mg MTD, including vomiting, nausea, fatigue, hypersensitivity reactions, hypokalemia, encephalopathy, and palpitations [1].
Additionally, the median time to response for AMG 193 can be relatively long (3.6 months at 800 mg once daily), suggesting that deepening tumor regression occurs over an extended period, which may require prolonged patient tolerance of the drug [1]. Furthermore, while KRAS mutations do not appear to confer resistance to MTA-cooperative PRMT5 inhibition, the broader mechanisms of acquired resistance to AMG 193 remain to be fully elucidated [1].
6. Future Perspectives
The future clinical development of AMG 193 in gastrointestinal and pancreatic cancers is heavily focused on combinatorial strategies to enhance depth and durability of response. Because KRAS alterations frequently co-occur with MTAP deletions in PDAC, AMG 193 is being investigated in combination with the KRAS G12C mutant-selective inhibitor sotorasib in the Phase I NCT06333951 trial, following promising additive activity in PDAC xenograft models [1].
AMG 193 is also being explored in combination with standard chemotherapies relevant to gastrointestinal cancers, including carboplatin, gemcitabine, paclitaxel, pemetrexed, and the fluorouracil/oxaliplatin/leucovorin/irinotecan regimen (NCT06333951 and NCT06360354) [1]. Furthermore, dual targeting of the metabolic and epigenetic axis using AMG 193 alongside MAT2A inhibitors (such as IDE397) has shown synergistic anti-tumor responses in preclinical models of MTAP-deleted pancreatic cancer, inducing earlier and greater PRMT5 inhibition than either agent alone [1]. Finally, combinations of AMG 193 with immunotherapies are under clinical investigation to determine if PRMT5 inhibition can reverse the immunosuppressive microenvironment typically associated with MTAP-deleted tumors [1].
