Purpose: Although cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors significantly extend tumor response in patients with metastatic estrogen receptor–positive (ER+) breast cancer, relapse is almost inevitable. This may, in part, reflect the failure of CDK4/6 inhibitors to induce apoptotic cell death. We therefore evaluated combination therapy with ABT-199 (venetoclax), a potent and selective BCL2 inhibitor. Experimental Design: BCL2 family member expression was assessed following treatment with endocrine therapy and the CDK4/6 inhibitor palbociclib. Functional assays were used to determine the impact of adding ABT-199 to fulvestrant and palbociclib in ER+ breast cancer cell lines, patient-derived organoid (PDO), and patient-derived xenograft (PDX) models. A syngeneic ER+ mouse mammary tumor model was used to study the effect of combination therapy on the immune system. Results: Triple therapy was well tolerated and produced a superior and more durable tumor response compared with single or doublet therapy. This was associated with marked apoptosis, including of senescent cells, indicative of senolysis. Unexpectedly, ABT-199 resulted in Rb dephosphorylation and reduced G1–S cyclins, most notably at high doses, thereby intensifying the fulvestrant/palbociclib–induced cell-cycle arrest. Interestingly, a CRISPR/Cas9 screen suggested that ABT-199 could mitigate loss of Rb (and potentially other mechanisms of acquired resistance) to palbociclib. ABT-199 did not abrogate the favorable immunomodulatory effects of palbociclib in a syngeneic ER+ mammary tumor model and extended tumor response when combined with anti-PD1 therapy. Conclusions: This study illustrates the potential for targeting BCL2 in combination with CDK4/6 inhibitors and supports investigation of combination therapy in ER+ breast cancer.

中文翻译：

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CDK4/6 和 BCL2 通路的双重靶向增强了雌激素受体阳性乳腺癌的肿瘤反应。

目的：虽然细胞周期蛋白依赖性激酶 4 和 6 (CDK4/6) 抑制剂显着延长了转移性雌激素受体阳性 (ER+) 乳腺癌患者的肿瘤反应，但复发几乎是不可避免的。这可能部分反映了 CDK4/6 抑制剂未能诱导细胞凋亡。因此，我们评估了 ABT-199（venetoclax）的联合治疗，这是一种有效的选择性 BCL2 抑制剂。实验设计：在内分泌治疗和 CDK4/6 抑制剂 palbociclib 治疗后评估 BCL2 家族成员表达。功能测定用于确定在 ER+ 乳腺癌细胞系、患者来源的类器官 (PDO) 和患者来源的异种移植 (PDX) 模型中将 ABT-199 添加到氟维司群和 Palbociclib 的影响。使用同系 ER+ 小鼠乳腺肿瘤模型研究联合治疗对免疫系统的影响。结果：与单药或双药治疗相比，三联疗法具有良好的耐受性，并产生了更好、更持久的肿瘤反应。这与显着的细胞凋亡有关，包括衰老细胞，表明衰老。出乎意料的是，ABT-199 导致 Rb 去磷酸化和 G1-S 细胞周期蛋白减少，尤其是在高剂量时，从而加剧了氟维司群/palbociclib 诱导的细胞周期停滞。有趣的是，CRISPR/Cas9 筛选表明 ABT-199 可以减轻 Rb（以及潜在的其他获得性耐药机制）对 palbociclib 的损失。ABT-199 在同基因 ER+ 乳腺肿瘤模型中并没有消除 palbociclib 的有利免疫调节作用，并在与抗 PD1 治疗联合时延长了肿瘤反应。结论：本研究说明了靶向 BCL2 与 CDK4/6 抑制剂联合的潜力，并支持对 ER+ 乳腺癌联合治疗的研究。