Abstract LB123: Poorer outcomes in EGFR L858R-driven NSCLC treated with osimertinib may be addressed with novel combination of BLU-945 and osimertinib

Abstract Background: Osimertinib, a 3rd-generation (gen) EGFR tyrosine kinase inhibitor (TKI), is standard of care in front-line (1L) patients with advanced EGFR mutant NSCLC; however, not all subgroups may benefit equally. In the phase 3 FLAURA study, patients with exon 19 deletions (ex19del) had a median progression-free survival (mPFS) of 21.4 months (mo); patients with L858R had a shorter mPFS of 14.4 mo. Poorer outcomes with L858R have also been reported with other 3rd-gen TKIs aumolertinib and lazertinib. These patients are a potential poor-risk subgroup for these therapies. Here, we explored outcomes of patients with L858R-driven NSCLC using real-world datasets (RWDs), analyzed potential contributors to poorer outcomes, including co-mutation incidence and osimertinib potency for each mutation, and report preclinical proof of concept of combination treatment BLU-945, an investigational next-gen L858R inhibitor, with osimertinib. Methods: Two large RWDs were analyzed for survival outcomes in 1L osimertinib-treated patients with ex19del or L858R from MD Anderson Cancer Center (MDACC; n=105) and the clinical-genomic Guardant INFORM database (EGFR ctDNA baseline positive; n=1386). IC50s of osimertinib on EGFR mutations and wildtype were determined in BaF3 cells. Preclinical studies of combination BLU-945 and osimertinib were performed in L858R-driven BaF3 xenograft models. Results: Both RWDs confirmed poorer prognosis for 1L osimertinib-treated patients with L858R vs ex19del. MDACC cohort showed a 12-mo PFS rate of 63% for L858R (n=45) vs 82% for ex19del (n=60); mPFS was immature. Guardant INFORM cohort (ctDNA baseline positive) had a median time-to-treatment discontinuation of 8 mo for L858R (n=517) and 11.4 mo for ex19del (n=869), P=0.003. Poor prognosis factors (including TP53 mutations and co-mutation number) were not significantly different between L858R and ex19del; both had a similar number of off-target mutations in post-osimertinib samples. Strong association was found between osimertinib cellular IC50 and osimertinib clinical trial outcomes (mPFS). Osimertinib exhibited most clinical and cellular activity on ex19del, followed by L858R, then G719X, and then exon 20 insertions. BLU-945 in combination with osimertinib in a BaF3 L858R xenograft model demonstrated a longer duration of response vs osimertinib monotherapy. Conclusions: In both RWDs, 1L osimertinib-treated patients with L858R-driven NSCLC had poorer outcomes vs ex19del, consistent with osimertinib’s weaker activity on L858R. Preclinically, BLU-945 in combination with osimertinib increased L858R inhibition, resulting in more durable antitumor activity in L858R xenografts vs osimertinib alone, supporting rationale for combination treatment in patients with L858R mutations. This combination is being evaluated in 1L patients with L858R in the SYMPHONY study (NCT04862780). Citation Format: Yasir Y. Elamin, Tyler Rouskin-Faust, Nicole Zhang, Teresa Green, Aditya Dhande, Brenton G. Mar, John V. Heymach, Chiara Conti. Poorer outcomes in EGFR L858R-driven NSCLC treated with osimertinib may be addressed with novel combination of BLU-945 and osimertinib [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB123.