LAUSR

Lorlatinib, a third‐generation anaplastic lymphoma kinase (ALK)‐tyrosine kinase inhibitor (TKI) with a broad coverage against ALK mutations, has demonstrated dramatic effects in patients with ALK‐rearranged lung cancer. The mechanisms of acquired resistance to lorlatinib by secondary ALK compound mutations have recently been reported; however, resistance mechanisms other than secondary mutations remain unclear. Here, we investigated the molecular mechanisms of the acquired resistance in ALK‐rearranged lung cancer cells in vitro. We established two different lorlatinib‐resistant ALK‐rearranged lung cancer cell lines (H3122LR and A925LLR) via long‐term administration of lorlatinib. These resistant cells did not harbor the secondary ALK mutations and showed cross‐resistance to the other kinds of ALK‐TKIs (crizotinib or alectinib) compared with the parental cells; however, these resistant cells overexpressed the phosphorylated human epidermal growth factor receptor 3 (HER3) protein and the ligand of HER3 (neuregulin 1; NRG1). Pharmacological inhibition of HER3 with pan‐HER inhibitors or genetic knockdown of HER3 with siRNA resensitized H3122LR and A925LLR cells to lorlatinib in vitro, indicating that H3122LR and A925LLR acquired resistance by NRG1/HER3 activation. These findings demonstrated that targeting NRG1/HER3 is a potential novel therapeutic option for lorlatinib‐resistant ALK‐rearranged lung cancer.