Third-generation EGFR and ALK inhibitors: mechanisms of resistance and management

Patients with non-small-cell lung cancers (NSCLCs) harbouring oncogenic EGFR or ALK alterations can benefit from therapies targeting these alterations, although acquired resistance to these agents is common. Third-generation inhibitors have extended the response durations of many patients with NSCLCs harbouring these alterations, albeit with differing patterns of resistance to those associated with earlier-generation agents. Here, the authors describe the mechanisms of acquired resistance to third-generation EGFR and ALK inhibitors and provide insights into future research directions in this area. The discoveries of EGFR mutations and ALK rearrangements as actionable oncogenic drivers in non-small-cell lung cancer (NSCLC) has propelled a biomarker-directed treatment paradigm for patients with advanced-stage disease. Numerous EGFR and ALK tyrosine kinase inhibitors (TKIs) with demonstrated efficacy in patients with EGFR-mutant and ALK-rearranged NSCLCs have been developed, culminating in the availability of the highly effective third-generation TKIs osimertinib and lorlatinib, respectively. Despite their marked efficacy, resistance to these agents remains an unsolved fundamental challenge. Both 'on-target' mechanisms (largely mediated by acquired resistance mutations in the kinase domains of EGFR or ALK) and 'off-target' mechanisms of resistance (mediated by non-target kinase alterations such as bypass signalling activation or phenotypic transformation) have been identified in patients with disease progression on osimertinib or lorlatinib. A growing understanding of the biology and spectrum of these mechanisms of resistance has already begun to inform the development of more effective therapeutic strategies. In this Review, we discuss the development of third-generation EGFR and ALK inhibitors, predominant mechanisms of resistance, and approaches to tackling resistance in the clinic, ranging from novel fourth-generation TKIs to combination regimens and other investigational therapies.