733 Immunological mechanisms of resistance to CDK4/CDK6 inhibitors in breast cancer (2021)

<h3>Background</h3> Hormone receptor+ (HR+) breast cancer (BC) is the most frequent cause of BC-related deaths. CDK4/6 inhibitors (CDK4/6i) combined with endocrine therapy (ET) emerged as an effective approach for metastatic HR+ BC. However, &gt;60% women with HR+ BC receiving CDK4/6i+ET ultimately relapse, potentially due to activation of poorly characterized immunosuppressive pathways in the tumor microenvironment (TME).<sup>1</sup> Thus, strategies breaking resistance to CDK4/6i+ET in women with HR+ BC are urgently awaited. Radiation therapy (RT) mediates immunostimulatory effects that only partially overlap with those of CDK4/6i+ET,<sup>2</sup> standing out as a promising therapeutic partner. Consistent with this notion, we recently demonstrated that RT followed by the CDK4/6i palbociclib + ET (RT-P+ET) enables superior tumor control in various immunocompetent mouse models of HR+ BC.<sup>3</sup> These findings have inspired the design of a randomized phase II clinical trial testing P+ET vs. RT-P+ET in patients with oligometastatic HR+ BC (CIMER, NCT04563507). In this context, we set out to dissect the immunological mechanisms underlying sensitivity vs. resistance to treatment in HR+ BC exposed to P+ET vs. RT-P+ET. <h3>Methods</h3> To dissect the impact of these treatments on immune contexture in HR+ BC, we performed single-cell RNAseq on CD45+ cells infiltrating MPA/DMBA (M/D)-driven carcinomas established in immunocompetent mice (a unique model of luminal B BC), coupled to bulk RNAseq, bioinformatic analysis on public patient datasets, functional studies on ex vivo immune cells and efficacy studies. <h3>Results</h3> We observed that (1) RT and P+ET alone mediate partial efficacy correlating with accumulation of immunosuppressive TREG and IL17A-producing γδ T cells, respectively, (2) γδ T cell depletion improves the efficacy of P+ET, (3) RT-P+ET mediates superior (but incomplete) tumor control, which is partially offset by CD4+/CD8+ T cell co-depletion and correlates with limited infiltration by γδ T cells and TREGS, but accumulation of PD-L1 expressing myeloid cells and M2-polarized TREM2+ macrophages, which have been ascribed robust immunosuppressive effects in multiple settings<sup>4</sup>; and (4) that PD-1 blockage does not ameliorate the therapeutic effects of RT-P+ET (not shown), pointing to TREM2+ macrophages as to the main culprits for resistance in this setting. <h3>Conclusions</h3> Our observations suggest that γδ T cells and TREM2+ macrophages support the resistance of HR+ BC to CDK4/6i and RT-CDK4/6i, and hence constitute potential targets to delay disease progression. <h3>References</h3> Pandey et al. Molecular mechanisms of resistance to CDK4/6 inhibitors in breast cancer: a review. <i>Int J Cancer</i> 2019;<b>145</b>(5):1179–1188. Rodriguez-Ruiz et al. Immunological impact of cell death signaling driven by radiation on the tumor microenvironment. <i>Nat Immunol</i> 2020;<b>21</b>(2):120–134. Petroni et al. Radiotherapy delivered before CDK4/6 inhibitors mediates superior therapeutic effects in ER + Breast cancer. <i>Clin Cancer Res</i> 2021;<b>27</b>(7):1855–1863. Xiong et al. A gene expression signature of TREM2 hi macrophages and γδ T cells predicts immunotherapy response. <i>Nat Commun</i> 2020;<b>11</b>(1):5084. <h3>Ethics Approval</h3> Animal experiments were approved by the Institutional Animal Care and Use Committee (IACUC) of Weill Cornell Medical College (n° 2019–2022).