Abstract 2997: Longitudinal circulating tumor DNA (ctDNA) analysis predicts response and reveals the resistance mechanisms of ensartinib in ALK+ NSCLC patients (pts) progressed on crizotinib: Updated analysis of a phase II clinical trial

Introduction: In the previous phase II clinical trial (NCT03215693), ensartinib, a next generation (NG) ALK TKI, showed comparable efficacy to other NG ALK TKIs in the post-crizotinib setting. In the preplanned biomarker analysis, we investigated the efficacy and the resistance mechanisms of ensartinib via longitudinal ctDNA analysis. Methods: Pts with stage IIIB/IV ALK+ NSCLC, and had progressive disease (PD) after crizotinib were eligible. Plasma samples from this trial were prospectively collected for ctDNA analysis at baseline (BL), cycle 3 day 1 (C3D1) and the end of treatment (EOT). Plasma DNA was analyzed using a 212-gene sequencing panel. ctDNA amount was defined as the sum of variant allele fraction (VAF) of ALK fusions and mutations. Cox model was applied for multivariate analysis. Results: Between 9/2017 and 7/2019, 182 pts were enrolled in the trial. In total, 178 were included in the full analysis set (FAS), with a median PFS (mPFS) of 9.8 months (95% CI, 7.5 to 11.7 months). 169 pts were evaluable by independent review committee (IRC), with an ORR of 57.7% (95% CI, 50.2% to 65.3%). A total of 431 ctDNA samples were analyzed, including 168 at BL, 165 at C3D1, and 106 at EOT (8 overlappings between C3D1 and EOT). 168 (99.4%) pts had evaluable ctDNA at BL. The median ctDNA amount was 0.00704. The higher ctDNA amount was associated with liver metastases, bone metastases, and TP53 mutations. Tumor burden, as measured by the sum of the target lesions' longest diameters, also positively correlated with ctDNA amount (Pearson correlation 0.261; p < 0.001). Multivariable Cox regression revealed that high level of ctDNA amount and TP53 mutations at BL was significantly associated with poor PFS independently (p =0.020 and p < 0.001) (Table). 106 (62.7%) pts had evaluable ctDNA at both BL and EOT. Compared to baseline, the frequency of certain ALK mutations significantly increased at EOT, such as G1269A (7.5% vs 21.7%), G1202R (3.8% vs 17.0%) and E1210K (0% vs 10.4%). TP53 statusctDNA amountTP53 Wildtype TP53 Mutations Low amountHigh amount(n = 34)(n = 134)(n = 46)(n = 47)mPFS, months (95% CI) 11.7 (9.8, 14.0) 4.2 (3.0, 5.6)9.3 (5.6, 11.7) 4.2 (2.8, 5.5)HR (95% CI); p value2.50 (1.65, 3.79) ; p < 0.0012.34 (1.52, 3.61) ; p < 0.001 Consistent with the change in frequency, the abundance of G1269A, G1202R, and E1210K also trended to increase at EOT, while the abundance of C1156Y and I1171X had a downward trend in most pts. Conclusions: Consistent with previo