Disclosure: Dr. Dingemans has received honoraria for serving on advisory board for BMS, MSD, Roche, Eli Lilly, Takeda, Pfizer, Boehringer Ingelheim (all to her institution) and has received a research grant from BMS (to her institution) outside the submitted work. Dr. Hendriks has received research funding from Roche and Boehringer Ingelheim (both to her institution); received honoraria for serving on advisory boards for Boehringer Ingelheim (to her institution) and BMS (to both her institution and herself); received travel reimbursement from Roche and BMS; participated in a mentorship program with key opinion leaders that was funded by AstraZeneca

Purpose: This phase I expansion study assessed safety, pharmacodynamic effects, and antitumor activity of RO4987655, a pure MEK inhibitor, in selected patients with advanced solid tumor.Experimental Design: We undertook a multicenter phase I two-part study (dose escalation and cohort expansion). Here, we present the part 2 expansion that included melanoma, non-small cell lung cancer (NSCLC), and colorectal cancer with oral RO4987655 administered continuously at recommended doses of 8.5 mg twice daily until progressive disease (PD). Sequential tumor sampling investigated multiple markers of pathway activation/tumor effects, including ERK phosphorylation and Ki-67 expression. BRAF and KRAS testing were implemented as selection criteria and broader tumor mutational analysis added.Results: Ninety-five patients received RO4987655, including 18 BRAF-mutant melanoma, 23 BRAF wildtype melanoma, 24 KRAS-mutant NSCLC, and 30 KRAS-mutant colorectal cancer. Most frequent adverse events were rash, acneiform dermatitis, and gastrointestinal disorders, mostly grade 1/2. Four (24%) of 17 BRAF-mutated melanoma had partial response as did four (20%) of 20 BRAF wild-type melanoma and two (11%) of 18 KRAS-mutant NSCLC. All KRAS-mutant colorectal cancer developed PD. Paired tumor biopsies demonstrated reduced ERK phosphorylation among all cohorts but significant differences among cohorts in Ki-67 modulation. Sixty-nine percent showed a decrease in fluorodeoxyglucose uptake between baseline and day 15. Detailed mutational profiling confirmed RAS/RAF screening and identified additional aberrations (NRAS/non-BRAF melanomas; PIK3CA/KRAS colorectal cancer) without therapeutic implications.

Among 157 patients treated with ICIs, 65 (41.4%) experienced at least one irAE. Median time to the first irAE onset was 28 days (IQR:15-56). Baseline clinicopathologic characteristics were well balanced between patients who developed irAEs and those who did not. Median tumor mutational burden (TMB) was significantly higher among patients with irAEs compared to those without (14.4 vs 8.4 mutations/megabase [mut/Mb], P <0.01). Patients who developed at least one irAE had a significantly higher objective response rate (26.3% versus 3.3%, P <0.001), and significantly longer median progressionfree survival (mPFS, 4.1 vs 1.3 months, HR: 0.30 [0.20-0.43, P <0.001]) and median overall survival (mOS, 14.1 vs 2.9 months, HR: 0.32 [0.21-0.48], P <0.001). The proportion of patients who were progression-free at 6, 9, and 12 weeks was significantly higher in patients who developed early irAEs compared to those who did not develop early irAEs (6 weeks: 89.5% vs 69.5%, P ¼0.01; 9 weeks: 71.1% vs 40%, P ¼0.001; 12 weeks: 65.8% vs. 31.6%, P <0.001). The median TMB was also significantly higher in patients who developed early irAEs (14.5 vs 8.7 mut/Mb, P <0.01). Conclusion: Patients with SCLC treated with ICIs who developed early irAEs had a higher TMB and enhanced antitumor responses compared to those who did not develop irAEs. Whether a higher TMB is associated with the development of irAEs remains to be determined mechanistically.

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