Genotyping of Cerebrospinal Fluid Associated With Osimertinib Response and Resistance for Leptomeningeal Metastases in EGFR-Mutated NSCLC.

Introduction: Patients with NSCLC with leptomeningeal metastases (LM) presented dismal prognosis. Cerebrospinal fluid (CSF) is suggested as a medium of liquid biopsy of LM. However, the clinical implications of CSF genotyping on treatment outcomes remained elusive.

Methods: Patients with EGFR-mutated advanced NSCLC with LM were included: cohort 1, patients with LM who were treated with osimertinib with CSF and plasma genotyping performed before the first dosing of osimertinib (baseline, n = 45); cohort 2, CSF genotyping on progression on osimertinib and development of LM (the progression event on osimertinib is the diagnosis of LM, n = 35). Circulating tumor DNA in CSF underwent next-generation sequencing.

Results: Sensitivity of CSF genotyping for EGFR-sensitizing mutations was 93.3% (42 of 45) and 97.1% (34 of 35) in cohorts 1 and 2, respectively. In cohort 1, patients with EGFR exon 19 deletion had higher median intracranial progression free survival (iPFS) than those with EGFR exon 21 L858R mutation (11.9 versus 2.8 mo; p = 0.02). Median iPFS was significantly longer in patients with T790M-positive CSF genotyping (15.6 mo) than T790M-negative CSF (7.0 mo, p = 0.04). Concurrent CDK4 (2.8 versus 11.6 mo, p = 0.002) and CDKN2A (2.5 versus 9.6 mo, p = 0.04) mutation with EGFR-sensitizing mutations indicated lower median iPFS. Patients with T790M-negative CSF, EGFR exon 21 L858R mutation, concurrent FGF3 alteration, and over first-line osimertinib had shortened iPFS. In cohort 2, possible EGFR-related and EGFR-independent resistance mechanisms were found including C797S mutation, MET dysregulation, and TP53 plus RB1 co-occurrence. Patients with loss of T790M in CSF had a shorter median iPFS (7.4 mo) compared with those with reserved T790M (13.6 mo, p = 0.01).

Conclusions: Genotyping of CSF indicated heterogeneous response to osimertinib and revealed the genetic characteristic of LM on osimertinib failure in patients with EGFR-mutated NSCLC diagnosed with LM.