Kinase inhibitor-responsive genotypes in EGFR mutated lung adenocarcinomas: moving past common point mutations or indels into uncommon kinase domain duplications and rearrangements

The most frequent epidermal growth factor receptor (EGFR) mutations found by traditional or comprehensive molecular profiling of lung adenocarcinomas include indels of exon 19 (the exon 19 deletion delE746_A750 being the most common) and the exon 21 L858R point mutation. The current approval labels for first line palliative gefitinib 250 mg/day, erlotinib 150 mg/day and afatinib 40 mg/day for advanced lung cancers require the presence of the aforementioned classical/sensitizing EGFR mutations. Other gefitinib, erlotinib and afatinib sensitizing mutations include exon 18 indels, G719X, exon 19 insertions, A763_Y764insFQEA, S768I and L861Q; for which off-label EGFR kinase inhibitor use is generally agreed upon by thoracic oncologists. The main biological mechanism of resistance to approved first line EGFR inhibitors is the selection/acquisition of EGFR-T790M that in itself can be inhibited by osimertinib 80 mg/day, a 3^rd generation EGFR inhibitor that is bypassed by EGFR-C797X mutations. Another class of de novo inhibitor insensitive mutation includes EGFR exon 20 insertions. More recently, the dichotomy of only point mutations or indels explaining aberrant kinase activation of EGFR plus inhibitor response has been shattered by the discovery of uncommon (<0.5% of all EGFR mutations) genomic events involving exon 18–25 kinase domain duplications (KDD) and rearrangements (EGFR-RAD51 or EGFR-PURB). The latter lead to oncogene addiction, enhanced sensitivity to kinase inhibitors in vitro and clinical responses to approved EGFR inhibitors. The enhanced landscape of EGFR inhibitor-responsive genotypes highlights that comprehensive molecular profiling may be necessary to maximize the identification of all cases that can benefit from precision oncology.