Abstract PS10-04: Tucatinib favourably modulates the immune microenvironment and synergises with anti-PD1 therapy in a trastuzumab resistant HER2+ murine model
Authors: Li Ran , Sant Sneha , Brown Emmaline , Caramia Franco , Byrne Ann , Clarke Kylie , Neeson Michael , Neeson Paul J , Darcy Phillip K , Peterson Scott , Loi Sherene 
Published: February 2021

Abstract
Background: The efficacy of PD-(L)1 inhibitors in patients with trastuzumab-resistant advanced HER2+ breast cancer is poor. Although many HER2 targeted therapies are used clinically, their effect on the tumor immune microenvironment (TME) and whether this contributes to efficacy is not understood. Tucatinib is a potent, highly selective, HER2 small molecule tyrosine kinase inhibitor with proven clinical benefit in the advanced setting.
Methods: We used two immunocompetent, HER2+ murine cancer models (trastuzumab-sensitive H2N113 and trastuzumab-resistant fo5) to investigate the effects of tucatinib on tumor growth kinetics, as well as tucatinib anti-tumor efficacy in combination with trastuzumab and PD-1 checkpoint blockade. Effects of tucatinib on the tumor infiltrating lymphocytes were analysed using flow cytometry. To identify genes that were modulated by tucatinib treatment, we performed bulk RNA sequencing on fo5 tumors treated with vehicle or tucatinib. We evaluated healthy donor peripheral blood T cells treated with tucatinib for 96 hours and analysed secreted cytokine levels using cytometric bead array.
Results: Treatment of CD4+ and CD8+ T cells with tucatinib together with TCR stimulation resulted significantly higher levels of IFNγ and TNFα compared with stimulated controls (p &lt; 0.0001), suggesting tucatinib can directly modulate human T cell function. In both murine models, tucatinib significantly inhibited tumor growth in a dose-dependent manner and was observed at doses of 25 mg/kg, 50 mg/kg and 100 mg/kg (p &lt; 0.05 between each dose). Median survival was 16 days in the vehicle group vs 50 days with 100 mg/kg of tucatinib (p &lt; 0.0001). Ex vivo analysis of tumors by flow cytometry showed increased infiltration of NK cells (p=0.01) and, CD8+ T cells with high PD-1 (p = 0.001), TIM-3 (p = 0.009), IFNγ (p = 0.003) and Ki67 (p = 0.0003) expression in Tucatinib treated mice compared to vehicle treated mice. Concomitant with these changes there was a significant reduction in numbers of neutrophils (p = 0.0085) and MHC-II low expressing macrophage populations (p &lt; 0.0001) following tucatininb treatment with an increase in frequency of MHC-II expressing dendritic cells (p &lt; 0.0001) and macrophages (p &lt; 0.0001) suggesting an increase in anti-tumor immunity.
Similarly, in the fo5 model tucatinib treated tumors had significantly higher interferon-γ (IFNγ) produced by CD8+ T cells (p = 0.005). Gene expression profile analysis shows significant enrichment in pathways associated with immune activation, including antigen binding and presentation (p = 0.0002), adaptive immune responses (p = 0.0002) including IFNγ (p = 0.0002) and IFNα (p = 0.0002). In this model, tucatinib in combination with trastuzumab demonstrated significantly better anti-tumor activity (p = 0.03) and survival (p &lt; 0.0001) compared with tucatinib alone, with 33% of mice achieving complete tumour regressions. Tucatinib in combination with PD-1 inhibition also demonstrated significantly greater anti-tumor efficacy compared to tucatinib alone (p = 0.0079) with increased survival (p = 0.05) and 50% of mice achieving complete tumor regression.
Conclusions: This study suggests an anti-tumor immune response may be an important component of the efficacy of tucatinib. This is supported by in vitro data demonstrating tucatinib stimulated human peripheral T cells and in vivo data showing favorable effects on the TME mediated by tucatinib treatment. These changes were associated with improved efficacy when tucatinib was combined with PD-1 inhibition or trastuzumab in the setting of trastuzumab resistance. These findings suggest that the combination of tucatinib and PD-1 inhibition is a rational combination that warrants investigation in the clinical setting, particularly for trastuzumab resistant HER2+ breast tumors.
Citation Format: Ran Li, Sneha Sant, Emmaline Brown, Franco Caramia, Ann Byrne, Kylie Clarke, Michael Neeson, Paul J Neeson, Phillip K Darcy, Scott Peterson, Sherene Loi. Tucatinib favourably modulates the immune microenvironment and synergises with anti-PD1 therapy in a trastuzumab resistant HER2+ murine model [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS10-04.