Resistance of Lung Cancer to EGFR-Specific Kinase Inhibitors: Activation of Bypass Pathways and Endogenous Mutators

SIMPLE SUMMARY: Genome-based cancer medicine is becoming the standard of care: the patient’s tumor DNA is first analyzed to identify driver mutations, and this permits later selection of the most effective drugs. Treatment of lung cancer offers many examples. Activating mutations in the epidermal growth factor receptor (EGFR) gene, as well as in other receptor tyrosine kinases (e.g., ALK), are considered actionable candidates, and the respective drugs, called tyrosine kinase inhibitors, are relatively effective. Unfortunately, despite initial activity, the emergence of new, on-target mutations, along with adaptive processes, preempt the anti-cancer effects and necessitate switching to next-generation drugs. This review highlights recent progress in resolving the mechanisms that underlie acquisition of resistance. Specifically, we focus on the endogenous mutators that initiate emergence of new mutations and the potential clinical benefits that may be derived from this new understanding. ABSTRACT: Epidermal growth factor receptor (EGFR)-specific tyrosine kinase inhibitors (TKIs) have changed the landscape of lung cancer therapy. For patients who are treated with the new TKIs, the current median survival exceeds 3 years, substantially better than the average 20 month survival rate only a decade ago. Unfortunately, despite initial efficacy, nearly all treated patients evolve drug resistance due to the emergence of either new mutations or rewired signaling pathways that engage other receptor tyrosine kinases (RTKs), such as MET, HER3 and AXL. Apparently, the emergence of mutations is preceded by a phase of epigenetic alterations that finely regulate the cell cycle, bias a mesenchymal phenotype and activate antioxidants. Concomitantly, cells that evade TKI-induced apoptosis (i.e., drug-tolerant persister cells) activate an intrinsic mutagenic program reminiscent of the SOS system deployed when bacteria are exposed to antibiotics. This mammalian system imbalances the purine-to-pyrimidine ratio, inhibits DNA repair and boosts expression of mutation-prone DNA polymerases. Thus, the net outcome of the SOS response is a greater probability to evolve new mutations. Deeper understanding of the persister-to-resister transformation, along with the development of next-generation TKIs, EGFR-specific proteolysis targeting chimeras (PROTACs), as well as bispecific antibodies, will permit delaying the onset of relapses and prolonging survival of patients with EGFR(+) lung cancer.