Abstract P039: A phase 1 dose escalation study of protein arginine methyltransferase 5 (PRMT5) inhibitor PRT543 in patients with advanced solid tumors and lymphoma
Authors: McKean Meredith , Patel Manish R. , Wesolowski Robert , Ferrarotto Renata , Stein Eytan M. , Shoushtari Alexander N. , Mauro David , Viscusi John , Scherle Peggy , Bhagwat Neha , Sun William , Chiaverelli Rachel , Mintah Eric , Jauhari Shekeab , Finn Laura , Palmisiano Neil D. , Baiocchi Robert A. 
Published: December 2021

Abstract
Background: PRMT5 catalyzes symmetric arginine dimethylation of protein substrates with important roles in cancer cell growth/survival. PRT543 is a potent, selective, oral PRMT5 inhibitor with robust preclinical efficacy (Bhagwat AACR 2020). An open-label Phase I study of PRT543 in unselected patients (pts) with advanced solid tumors and hematologic malignancies (NCT03886831) is ongoing. Dose escalation results from pts with solid tumors and lymphoma are presented herein. Materials and Methods: This study assesses safety, pharmacokinetics, pharmacodynamics, and preliminary tumor response (REClST v1.1) of PRT543 administered at varying schedules beginning with twice weekly (BIW) and increasing to once daily (QD) with doses ranging from 5-50 mg in 28-day cycles. Dose escalation followed a 3+3 design. Serum symmetric dimethylarginine (sDMA) and intron retention, a marker of PRMT5-mediated mRNA splicing fidelity, were assessed as measures of PRMT5 target engagement and function, respectively. Results: As of 18 June 2021, 49 unselected pts with measurable disease refractory to established therapies had enrolled (17 pts BIW, 11 pts 5x/week [wk], 21 pts QD). Median number of prior lines of systemic therapies was 3. Six of 21 pts dosed at 25-50 mg QD experienced dose limiting toxicity (DLT) of thrombocytopenia, and 1 pt of 6 dosed at 45 mg 5x/wk had DLT of fatigue. Most frequent treatment-related adverse events (TRAEs), any grade, in all regimens were fatigue (n=20, 41%), nausea (n=14, 29%), thrombocytopenia (n=13, 27%), and anemia (n=12, 24%). Grade ≥3 TRAEs in all regimens occurring in ≥5 pts included thrombocytopenia (n=10, 20%) and anemia (n=6, 12%). Cytopenias were reversible and managed with dose modifications. 44 pts discontinued treatment, mainly due to disease progression, with 2 due to AEs (Grade 3 thrombocytopenia and Grade 4 cholangitis). The 45 mg 5x/wk regimen was selected as the expansion dose. PRT543 demonstrated dose-dependent increases of Cmax (nM) and AUC (nM.hr). At the expansion dose, T½ was 15 hrs, and plasma drug exposures (Cmax, 2142 nM; AUC, 26,293 nM.hr) exceeded those for preclinical efficacy models. Serum sDMA decreased in a dose-dependent manner reaching 77% reduction with 50 mg QD and 69% reduction with the expansion dose. Increased intron retention was seen in peripheral blood mononuclear cells. A complete response (CR) has been maintained for &gt;1 yr in a pt with homologous recombination deficiency-positive (HRD+) ovarian cancer who remains on study therapy. An additional 5 pts exhibited stable disease ≥6 mo. Conclusions: PRT543 was well tolerated with a favorable safety profile. Dose-dependent inhibition of target engagement and functional activity of PRMT5 were observed. PRT543 demonstrated encouraging clinical activity with a CR in HRD+ ovarian cancer and prolonged stable disease in multiple pts. An expansion phase in biomarker-selected solid tumor cohorts is ongoing.
Citation Format: Meredith McKean, Manish R. Patel, Robert Wesolowski, Renata Ferrarotto, Eytan M. Stein, Alexander N. Shoushtari, David Mauro, John Viscusi, Peggy Scherle, Neha Bhagwat, William Sun, Rachel Chiaverelli, Eric Mintah, Shekeab Jauhari, Laura Finn, Neil D. Palmisiano, Robert A. Baiocchi. A phase 1 dose escalation study of protein arginine methyltransferase 5 (PRMT5) inhibitor PRT543 in patients with advanced solid tumors and lymphoma [abstract]. In: Proceedings of the AACR-NCI-EORTC Virtual International Conference on Molecular Targets and Cancer Therapeutics; 2021 Oct 7-10. Philadelphia (PA): AACR; Mol Cancer Ther 2021;20(12 Suppl):Abstract nr P039.