Efficacy of Osimertinib in Patients with Lung Cancer Positive for Uncommon EGFR Exon 19 Deletion Mutations.

Purpose: The uncommon EGFR exon 19 deletion (ex19del), L747_A750>P, demonstrates reduced sensitivity to osimertinib compared with the common ex19del, E746_A750del in preclinical models. The clinical efficacy of osimertinib in patients with non-small cell lung cancer harboring L747_A750>P and other uncommon ex19dels is not known.

Experimental Design: The AACR GENIE database was interrogated to characterize the frequency of individual ex19dels relative to other variants, and a multicenter retrospective cohort was used to compare clinical outcomes for patients with tumors harboring E746_A750del, L747_A750>P, and other uncommon ex19dels who received osimertinib in the first line (1L) or in second or later lines of therapy and were T790M+ (≥2L).

Results: ex19dels comprised 45% of EGFR mutations, with 72 distinct variants ranging in frequency from 28.1% (E746_A750del) to 0.03%, with L747_A750>P representing 1.8% of the EGFR mutant cohort. In our multi-institutional cohort (N = 200), E746_A750del was associated with significantly prolonged progression-free survival (PFS) with 1L osimertinib versus L747_A750>P [median 21.3 months (95% confidence interval, 17.0-31.7) vs. 11.7 months (10.8-29.4); adjusted HR 0.52 (0.28-0.98); P = 0.043]. Osimertinib efficacy in patients with other uncommon ex19dels varied on the basis of the specific mutation present.

Conclusions: The ex19del L747_A750>P is associated with inferior PFS compared with the common E746_A750del mutation in patients treated with 1L osimertinib. Understanding differences in osimertinib efficacy among EGFR ex19del subtypes could alter management of these patients in the future.