Retreatment and rechallenge with BRAF/MEK inhibitors in patients with metastatic melanoma: Results from the observational study GEM1801

9547 Background: Most patients with BRAF-mutant melanoma eventually develop resistance to therapy with BRAF/MEK inhibitors and immune-checkpoint blockade. Alternative treatment strategies are required in these patients since further therapy options lack survival benefit. Small series have shown promising activity from re-exposure to BRAF/MEK inhibitors after a treatment free interval of targeted therapy (TT), probably due to regression of resistant tumors and proliferation of clones that maintain sensitivity to BRAF/MEKi. However, prospective evidence for this approach is limited. Here we report the results of 30 patients who received a second course of TT at relapse after adjuvant therapy (retreatment) or after first progression on TT in the advanced setting (rechallenge). Methods: GEM1801 is a prospective observational study from the Spanish Melanoma Group (GEM) including 893 patients with melanoma treated in 37 centers. This is a descriptive analysis of basal characteristics, response rates (RR), progression free survival (PFS) and overall survival (OS) from 30 patients that received retreatment or rechallenge with BRAF/MEKi in GEM1801 study. Results: 4 patients received retreatment and 26 rechallenge. For retreatment, BRAF/MEKi median free interval from end of adjuvant TT to subsequent retreatment was 17.8 months (range: 12.4 - 57.7). RR with retreatment were 25%, with a median PFS and OS of 5.7 and 8.5 months. For rechallenge, RR was 38.5% and median PFS and OS 11.1 and 22.2 months respectively. Patients who were selected for rechallenge had higher complete response (CR) rates to first TT (26.9% vs 20.3%; p= 0.026). Patients who experimented deeper responses with 1st TT in the metastatic setting showed a correlation to longer PFS to 2nd TT with 12-m PFS rates of 66.7% versus 35.7% (HR 9.09 [95%CI: 1.42-58]; p= 0.02). OS showed a not statistically significant improvement in patients who reported CR to first TD (24-m OS rate of 75% versus 40.7% respectively). BRAF/MEKi free interval since first progression and retreatment or rechallenge was not associated with prognosis. Prognostic factors as LDH and number of metastatic sites did not correlate with the efficacy of retreatment. Conclusions: Retreatment or rechallenge with BRAF/MEKi in the metastatic setting achieved encouraging efficacy and may be a valid therapeutic strategy in a selected group of patients. Response to first treatment, retreatment and rechallenge with BRAF/MEKi. Clinical trial information: NCT03605771 . [Table: see text]