Patient-Reported Tolerability of Selpercatinib Compared to Cabozantinib/Vandetanib: A Secondary Analysis of the LIBRETTO-531 Randomized-Controlled Trial in RET -Mutant Medullary Thyroid Cancer

Background: Progression-free survival (PFS) may not fully capture the impact of treatment on patients, especially in cancers with longer natural histories and thus, could be complemented by robust measures of patient-reported tolerability (PRT). We report the use of a novel, quantifiable PRT metric as a multiplicity-controlled endpoint to support regulatory and clinical decision-making for selpercatinib use. Comparative PRT was assessed in LIBRETTO-531 (NCT04211337), a randomized phase 3 trial of selpercatinib versus vandetanib/cabozantinib (control) in advanced RET -mutant medullary thyroid cancer (MTC). Patients and Methods: Patients were self-administered the single Functional Assessment of Cancer Therapy item GP5: “I am bothered by side effects” weekly, and scores were dichotomized into “low” (0–2) and “high” (3–4) side-effect burden. PRT measured the proportion of time on treatment (PTT) with “high” side-effect burden for each patient. Comparative PRT was tested at a two-sided significance level of 0.05, conditional on achieving significance for efficacy endpoints. Complementary patient-reported outcomes included health-related quality of life (HRQoL) and symptomatic adverse events self-administered at baseline and at different intervals post-baseline during treatment period. Results: In the tolerability evaluable population (N = 242; selpercatinib n = 161 and control n = 81 [56 received cabozantinib, 25 received vandetanib]), patients on selpercatinib had significantly better PRT with lower PTT with “high side-effect burden” than control (8% vs. 24%, p < 0.0001). Post-baseline compliance rates for PRO questionnaires were generally greater than 80% in both treatment groups. Patients on selpercatinib reported significantly less PTT with HRQoL impairment across physical (36% vs. 52%), role (2% vs. 11%), cognitive (4% vs. 8%), emotional (6% vs. 11%), and social (2% vs. 8%) function (all p < 0.01); and significantly less PTT with severe diarrhea (5% vs. 38%), fatigue (6% vs. 21%), taste change (3% vs. 15%), decreased appetite (2% vs. 15%), and hand-foot syndrome (2% vs. 9%) (all p < 0.001). Conclusion: This study demonstrated superior PRT for selpercatinib compared with control in patients with RET -mutant MTC, further supporting selpercatinib use as the first-line treatment for patients with advanced RET -mutant MTC. Comparative PRT deserves further adoption as a complement to traditional endpoints in future randomized-controlled trials.