CTNI-55. EXPLORING THE RELATIONSHIP BETWEEN PATIENT BASELINE CHARACTERISTICS WITH RESPONSES TO MIRDAMETINIB IN THE RENEU TRIAL OF CHILDREN AND ADULTS WITH NEUROFIBROMATOSIS TYPE 1 (NF1)-ASSOCIATED PLEXIFORM NEUROFIBROMA (PN)

Abstract BACKGROUND Mirdametinib, a MEK1/2 inhibitor, is the first FDA-approved medicine for both adults and children (≥2y) with NF1-PN not amenable to complete resection. In the phase 2b ReNeu trial (NCT03962543), confirmed objective response rate (ORR) was 41% in adults and 52% in children. This analysis explored relationships between baseline characteristics and mirdametinib responses in ReNeu. METHODS Confirmed ORR was the proportion of patients with ≥20% reduction from baseline in target PN volume by magnetic resonance imaging on consecutive scans during the 24-cycle treatment phase (“responders”) as assessed by blinded independent central review. Characteristics included age, body surface area (BSA), sex, race, ethnicity, target PN location, PN type (typical, nodular, or solitary nodular), target PN volume, and progression status. Rates of dose reductions and treatment-related adverse events (TRAEs) were evaluated. These factors were descriptively analyzed for potential relationships with confirmed objective response (data cutoff: 20September2023). RESULTS With mirdametinib, there was no clear difference between responders (n=53) and non-responders (n=61) in age (mean, 22.89 and 22.67y), BSA (mean, 1.42 and 1.51 m2), sex (60.4% and 57.4% female), race (83.0% and 68.9% White), or ethnicity (83.0% and 85.2% non-Hispanic/Latino). The majority of responders and non-responders had head/neck for target PN location (50.9% and 47.5%), typical PN type (81.1% and 63.9%), and baseline progression status (60.4% and 55.7%). Mean (95% CI) target PN volume was 283.43 (155.19, 411.66) and 415.74 (211.71, 619.77) mL, respectively. Dose reduction (11.3%; 18.0%) and TRAE rates (96.2%; 96.7%) were comparable. CONCLUSIONS This analysis suggests mirdametinib provides clinical benefit across a broad range of patient and NF1-PN tumor characteristics; a relationship between analyzed baseline characteristics and confirmed objective response was not established. Mean PN volume was higher in non-responders, though CIs overlapped. Dose reductions and emergence of TRAEs did not appear to impact response to mirdametinib. SUPPORT SpringWorks Therapeutics, Inc.