Abstract 3999: Combination of T-DXd with the irreversible pan-HER TKI afatinib drives combination benefit in HER2-low gastric and lung tumors

Abstract Background: Trastuzumab deruxtecan (T-DXd) has been approved for advanced or metastatic breast, gastric and lung cancers. It has been postulated that irreversible tyrosine kinase inhibitors (TKIs) increase HER2 internalization, thus enhancing T-DXd efficacy. Here we report studies evaluating the combination of TKIs with T-DXd in preclinical models of HER2-low disease. Methods: Surface HER2 was measured with flow cytometry quantifying fluorescently labelled trastuzumab on the surface of HER2-low cell lines HCC38 and HCC1171 treated with afatinib, neratinib, lapatinib or tucatinib for 12 h. HER2 receptor dynamics on cell surface was also measured in NCI-N87 (HER2 IHC 3+) and HCC38 at 24, 48, 72 and 96 h post-treatment with afatinib, and recovery was measured up to 72 h after drug washout. In vivo efficacy was established with T-DXd alone and in combination with afatinib or tucatinib in two gastric patient-derived xenograft (PDX) models, HD-2 (HER2 IHC 1+) and FU (HER2 IHC 2+), and one lung PDX model, MEDI-NSCLC-04 (HER2 IHC 1+). Results: In both HER2-low cell lines, treatment with irreversible inhibitors neratinib or afatinib led to enhanced internalization with a greater than 50% surface HER2 reduction, while treatment with the reversible inhibitors lapatinib or tucatinib led to 13-35% higher surface HER2 levels. Washout experiments indicated that receptor recovery at the plasma membrane begins between 4-12h post-washout of afatinib, with complete recovery at about 72h. Tumor shrinkage was significant in HER2-low gastric PDXs when T-DXd was combined with afatinib, but not with tucatinib. In HD-2, T-DXd 10mg/kg with afatinib 20mg/kg, and T-DXd 3mg/kg with tucatinib 25mg/kg combinations resulted in 74% and -3% (with respect to T-DXd) tumor growth inhibition (TGI), respectively. In FU, T-DXd 3mg/kg with afatinib 20mg/kg, and T-DXd 3mg/kg with tucatinib 25mg/kg combinations resulted in 80.3% and 24.7% TGI, respectively. Similarly, in MEDI-NSCLC-04 PDX model, benefit was observed in 5mg/kg T-DXd combination with 20mg/kg QD afatinib (95.3% TGI with respect to monotherapy T-DXd), but not with tucatinib (0.37% TGI). Intermittent schedules (QW, MWF) were also tested for afatinib in combination with T-DXd, with the MWF showing 49.2% TGI and QW showing 33.0 %TGI. In tissue samples from MEDI-NSCLC-04 efficacy study, γH2AX was detected by IHC in T-DXd and T-DXd+afatinib combination. In western blots, a decrease in HER2 as well as pHER2 was observed in response to T-DXd and 20mg/kg afatinib treatment, but not with tucatinib. Conclusions: In HER2 low gastric and lung preclinical models, T-DXd combined with irreversible pan-HER inhibitor induced superior antitumor effects compared to single agents or the combination of T-DXd with reversible inhibitors. These data support the rationale to test combination of T-DXd and pan-HER irreversible inhibitors in the clinical setting. Citation Format: Deepa Bhavsar, Laura Kazlauskas, Flavia Michelini, Matt Griffin, Matt Wilson, Fabiola Cecchi, Liz Croydon, Kim Maratea, Elisa de Stanchina, Yelena Y. Janjigian, Maurizio Scaltriti, Theresa Proia, Jerome Mettetal. Combination of T-DXd with the irreversible pan-HER TKI afatinib drives combination benefit in HER2-low gastric and lung tumors. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3999.