Somatic alterations in the epidermal growth factor receptor gene (EGFR) result in aberrant activation of kinase signaling and occur in ∼15% of non-small cell lung cancers (NSCLC). Patients diagnosed with EGFR-mutant NSCLC have good initial clinical response to EGFR tyrosine kinase inhibitors (EGFR TKIs), yet tumor recurrence is common and quick to develop. Mechanisms of acquired resistance to EGFR TKIs have been studied extensively over the past decade. Great progress has been made in understanding two major routes of therapeutic failure: additional genomic alterations in the EGFR gene and activation of alternative kinase signaling (so-called "bypass activation"). Several pharmacological agents aimed at overcoming these modes of EGFR TKI resistance are FDA-approved or under clinical development. Phenotypic transformation, a less common and less well understood mechanism of EGFR TKI resistance is yet to be addressed in the clinic. In the context of acquired EGFR TKI resistance, phenotypic transformation encompasses epithelial to mesenchymal transition (EMT), transformation of adenocarcinoma of the lung (LUAD) to squamous cell carcinoma (SCC) or small cell lung cancer (SCLC). SCLC transformation, or neuroendocrine differentiation, has been linked to inactivation of TP53 and RB1 signaling. However, the exact mechanism that permits lineage switching needs further investigation. Recent reports indicate that LUAD and SCLC have a common cell of origin, and that trans-differentiation occurs under the right conditions. Options for therapeutic targeting of EGFR-mutant SCLC are limited currently to conventional genotoxic chemotherapy. Similarly, the basis of EMT-associated resistance is not clear. EMT is a complex process that can be characterized by a spectrum of intermediate states with diverse expression of epithelial and mesenchymal factors. In the context of acquired resistance to EGFR TKIs, EMT frequently co-occurs with bypass activation, making it challenging to determine the exact contribution of EMT to therapeutic failure. Reversibility of EMT-associated resistance points toward its epigenetic origin, with additional adjustments, such as genetic alterations and bypass activation, occurring later during disease progression. This review will discuss the mechanistic basis for EGFR TKI resistance linked to phenotypic transformation, as well as challenges and opportunities in addressing this type of targeted therapy resistance in EGFR-mutant NSCLC.

Mutation in the tyrosine kinase (TK) domain of the epidermal growth factor receptor (EGFR) gene drives the development of lung cancer. EGFR tyrosine kinase inhibitors (EGFR TKIs), including erlotinib and afatinib, are initially effective in treating EGFR mutant nonsmall cell lung cancer (NSCLC). However, drug resistance quickly develops due to several mechanisms, including induction of the epithelial-mesenchymal transition (EMT). No effective therapies are currently available for patients who develop EMT-associated EGFR TKI resistance. 1,25-Dihydroxyvitamin D3 (1,25D3) promotes epithelial differentiation and inhibits growth of NSCLC cells. 1,25D3 thus represents a promising agent for the treatment of EMT-associated EGFR TKI resistance. However, 1,25D3 induces the expression of 24-hydroxylase (24OHase), which decreases 1,25D3 activity. CTA091, a potent and selective 24OHase inhibitor, has been developed to attenuate this adverse effect. CTA091 also suppresses renal 24OHase activity and so may promote hypercalcemia. To exploit favorable effects of 1,25D3 plus CTA091 in tumor cells while avoiding problematic systemic effects of 24OHase inhibition, we developed EGFR-targeted, liposomal nanoparticles (EGFR-LP) to offer tumor-targeted co-delivery of 1,25D3 and CTA091. We then established an EMT-associated model of EGFR TKI resistance, and showed that such nanoparticles improved cellular uptake of 1,25D3 and CTA091, drove pro-epithelial signaling by upregulating E-cadherin (CDH1), and significantly inhibited the growth of EGFR TKI resistant cells. Our results demonstrated that the delivery of vitamin D-based drug payloads via tumor-targeted EGFR-LP has promise as a new therapy for EFGR TKI resistant lung cancer. Future studies will focus on in vivo evaluation of biological activity, therapeutic benefits, and systemic toxicity prior to clinical translation.

Lazertinib (YH25448) is a novel third-generation tyrosine kinase inhibitor (TKI) developed as a treatment for EGFR mutant non-small cell lung cancer. To better understand the nature of lazertinib inhibition, we determined crystal structures of lazertinib in complex with both WT and mutant EGFR and compared its binding mode to that of structurally related EGFR TKIs. We observe that lazertinib binds EGFR with a distinctive pyrazole moiety enabling hydrogen bonds and van der Waals interactions facilitated through hydrophilic amine and hydrophobic phenyl groups, respectively. Biochemical assays and cell studies confirm that lazertinib effectively targets EGFR(L858R/T790M) and to a lesser extent HER2. The molecular basis for lazertinib inhibition of EGFR reported here highlights previously unexplored binding interactions leading to improved medicinal chemistry properties compared to clinically approved osimertinib (AZD9291) and offers novel strategies for structure-guided design of tyrosine kinase inhibitors.

Activation of the epidermal growth factor receptor (EGFR) pathway is an early event in head and neck carcinogenesis. As a result, targeting EGFR for chemoprevention of head and neck squamous cell carcinomas (HNSCC) has received considerable attention. In the present study, we examined the impact of 1,25(OH)2D3, the active metabolite of the nutritional supplement vitamin D on the chemopreventive efficacy of the EGFR inhibitor, erlotinib, against HNSCC. Experimental studies were conducted in patient derived xenografts (PDX) and the 4-nitroquinoline-1-oxide (4NQO) carcinogen-induced model of HNSCC. Short-term treatment (4 weeks) of PDX-bearing mice with 1,25(OH)2D3 and erlotinib resulted in significant inhibition of tumor growth. Non-invasive magnetic resonance imaging (MRI) enabled longitudinal monitoring of disease progression in the 4NQO model with 100% of control animals showing evidence of neoplastic lesions by 24 weeks. Among the experimental groups, animals treated with the combination regimen showed the greatest reduction in tumor incidence and volume (p<0.05). Combination treatment was well tolerated and was not associated with any significant change in body weight. Histopathologic assessment revealed a significant reduction in the degree of dysplasia with combination treatment. Immunoblot analysis of whole tongue extracts showed down regulation of phospho-EGFR and phospho-Akt with the combination regimen. These results highlight the potential of 1,25(OH)2D3 to augment the efficacy of erlotinib against HNSCC. Further optimization of schedule and sequence of this combination regimen along with investigation into the activity of less calcemic analogues or dietary vitamin D is essential to fully realize the potential of this approach.

EGFR tyrosine kinase inhibitors (EGFR TKIs) are the standard of care treatment for patients with EGFR-mutant lung adenocarcinoma (LUAD). Although initially effective, EGFR TKIs are not curative. Disease inevitably relapses due to acquired drug resistance. We hypothesized that vitamin D metabolites could be used with EGFR TKIs to prevent therapeutic failure. To test this idea, we investigated the link between serum 25-hydroxyvitamin D3 (25(OH)D3) and progression-free survival (PFS) in patients with EGFR-mutant LUAD that received EGFR TKIs (erlotinib n = 20 and afatinib n = 1). Patients who were 25(OH)D3-sufficient experienced significantly longer benefit from EGFR TKI therapy (mean 14.5 months) than those with 25(OH)D3 insufficiency (mean 10.6 months, p = 0.026). In contrast, 25(OH)D3 had no prognostic value in patients with KRAS-mutant LUAD that received cytotoxic chemotherapy. To gain mechanistic insights, we tested 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) activity in vitro. 1,25(OH)2D3 promoted epithelial differentiation and restored EGFR TKI sensitivity in models of EGFR TKI resistance that were associated with epithelial-mesenchymal transition (EMT). 1,25(OH)2D3 was ineffective in a non-EMT model of resistance. We conclude that vitamin D sufficiency portends increased PFS among EGFR-mutant LUAD patients that receive EGFR TKIs, and that vitamin D signaling maintains drug efficacy in this specific patient subset by opposing EMT.