PRT543, a protein arginine methyltransferase 5 inhibitor, in patients with advanced adenoid cystic carcinoma: An open-label, phase I dose-expansion study.

• PRT543 is a potent and selective oral inhibitor of PRMT5. • PRT543 catalyzes symmetric arginine dimethylation of protein substrates. • In this phase I dose-expansion trial, patients with R/M ACC received PRT543. • PRT543 was generally well tolerated and yielded tumour regression in some patients. • Median progression-free survival was 5.9 months (95% CI: 3.8–8.3) • Further investigation of PRMT5 inhibitors for R/M ACC treatment is warranted. Currently, no systemic treatments are approved for patients with recurrent and/or metastatic (R/M) adenoid cystic carcinoma (ACC). PRT543, a protein arginine methyltransferase 5 inhibitor that downregulates NOTCH1 and MYB signalling in tumours, is a potential candidate for R/M ACC treatment. We report the safety, tolerability and preliminary efficacy of PRT543 in a dose-expansion cohort of patients with R/M ACC. This phase I multicentre, open-label, sequential-cohort, dose-escalation and dose-expansion study (NCT03886831) enrolled patients with advanced solid tumours and select haematologic malignancies. Dose-escalation study design and results were reported previously. In the dose expansion, patients with R/M ACC received recommended phase II doses of 35 or 45 mg PRT543 orally on days 1–5 of each week. Primary objectives were to establish the safety and tolerability of PRT543. Secondary objectives included efficacy. Between February 2019 and May 2022, 56 patients with ACC were enrolled across 23 US sites and received either 35 mg (n = 28) or 45 mg (n = 28) of PRT543. Overall, 23% of patients experienced a grade 3 treatment-related adverse event, most commonly anaemia (16%) and thrombocytopaenia (9%). No grade 4/5 treatment-emergent adverse events were reported. Median progression-free survival was 5.9 months (95% CI: 3.8–8.3). The clinical benefit rate was 57% (95% CI: 43–70). Overall response rate (per Response Evaluation Criteria in Solid Tumours v1.1) was 2%, with 70% of patients having stable disease. In this analysis, PRT543 was tolerable, and the observed efficacy was limited in patients with R/M ACC. [ABSTRACT FROM AUTHOR]