Clinicogenomic analysis of <i>EGFR</i> -mutant lung cancers for identification of Rb inactivation as a hallmark of squamous transformation.

Abstract

8606 Background: Histologic transformation to squamous carcinoma (LUSC) is a recognized mechanism of resistance in EGFR -mutant lung adenocarcinoma (EGFR+ LUAD) and can occur in 5-8% of patients treated with osimertinib. While our prior work identified the AKT pathway as a possible mediator of LUSC transformation, comprehensive clinicogenomic assessment of this process remains lacking. Methods: We performed genomic characterization of EGFR -mutant patient samples undergoing LUSC transformation, including: (1) pre- and post-transformation specimens transformed after TKI treatment, and (2) microdissected distinct LUAD and LUSC components obtained from adenosquamous (LUAS) tumors. Retrospective analysis of clinical outcomes such as time-to-treatment discontinuation (TTD) were evaluated in patients with EGFR+ LUAD on frontline osimertinib who have undergone MSK-IMPACT (n=181). Xenograft models (using PC9 and HCC827) mimicking squamous transformation were treated with osimertinib (5-10 mice/group). Phenotypic markers of LUSC (P40 and CK5/6) were assessed by IHC. Results: Among patients with EGFR+ LUAD undergoing LUSC transformation (n=20), 50% and 60% had alterations in the AKT or Rb pathway, respectively. When compared to a cohort of never-transforming EGFR+ LUAD (n=1515), patients with transforming LUAD had a higher frequency of AKT (44% vs 18%) and Rb (56% vs 32%) pathway mutations. Clinically, patients with EGFR+ LUAD on first line osimertinib harboring baseline Rb/AKT pathway mutations (n=70) experienced shorter TTD (median 18 vs 24 months, p=0.0261) compared to a Rb/AKT wild-type cohort (n=111). In xenograft models of squamous transformation, Rb inactivation through CRISPR deletion of RB1 or upstream regulators CDKN2A/B led to greater in vivo tumor growth in immunodeficient mice treated with osimertinib compared to controls. Histologic assessment revealed induction of squamous markers P40 and CK5/6 in xenografts with Rb inactivation. Conclusions: Genomic alterations in Rb and AKT pathways are detected at higher frequency in patients with EGFR+ LUAD undergoing squamous transformation and are associated with worse clinical outcomes to frontline osimertinib. Rb inactivation in xenograft mouse models led to increased squamous-like phenotype and resistance to osimertinib. Detection of these mutations may help identify patients at high risk of treatment resistance and transformation.