Cyclin-dependent kinase 6 (CDK6) is an important regulator of the cell cycle. Together with CDK4, it phosphorylates and inactivates retinoblastoma (Rb) protein. In tumour cells, CDK6 is frequently upregulated and CDK4/6 kinase inhibitors like palbociclib possess high activity in breast cancer and other malignancies. Besides its crucial catalytic function, kinase-independent roles of CDK6 have been described. Therefore, targeted degradation of CDK6 may be advantageous over kinase inhibition. Proteolysis targeting chimeras (PROTACs) structurally based on the cereblon (CRBN) ligand thalidomide have recently been described to degrade the targets CDK4/6. However, CRBN-based PROTACs have several limitations including the remaining activity of immunomodulatory drugs (IMiDs) on Ikaros transcription factors as well as CRBN inactivation as a resistance mechanism in cancer. Here, we systematically explored the chemical space of CDK4/6 PROTACs by addressing different E3 ligases and connecting their respective small-molecule binders via various linkers to palbociclib. The spectrum of CDK6-specific PROTACs was extended to von Hippel Lindau (VHL) and cellular inhibitor of apoptosis protein 1 (cIAP1) that are essential for most cancer cells and therefore less likely to be inactivated. Our VHL-based PROTAC series included compounds that were either specific for CDK6 or exhibited dual activity against CDK4 and CDK6. IAP-based PROTACs caused a combined degradation of CDK4/6 and IAPs resulting in synergistic effects on cancer cell growth. Our new degraders showed potent and long-lasting degrading activity in human and mouse cells and inhibited proliferation of several leukemia, myeloma and breast cancer cell lines. In conclusion, we show that VHL- and IAP-based PROTACs are an attractive approach for targeted degradation of CDK4/6 in cancer.

中文翻译：

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系统研究不同的E3泛素连接酶：一种有效的和选择性的CDK6降解剂的方法

细胞周期蛋白依赖性激酶6（CDK6）是细胞周期的重要调节剂。它与CDK4一起磷酸化并灭活视网膜母细胞瘤（Rb）蛋白。在肿瘤细胞中，CDK6经常被上调，而CDK4 / 6激酶抑制剂（如palbociclib）在乳腺癌和其他恶性肿瘤中具有较高的活性。除了其关键的催化功能外，还描述了CDK6的激酶非依赖性作用。因此，CDK6的靶向降解可能优于激酶抑制作用。最近已描述了结构上基于大脑（CRBN）配体沙利度胺的蛋白水解靶向嵌合体（PROTAC）可以降解目标CDK4 / 6。然而，基于CRBN的PROTAC具有几个局限性，包括免疫调节药物（IMiD）对Ikaros转录因子的剩余活性以及CRBN失活作为癌症的耐药机制。在这里，我们通过解决不同的E3连接酶并连接它们各自的小分子结合剂，系统地探索了CDK4 / 6 PROTAC的化学空间通过各种链接至palbociclib的链接。CDK6特异性PROTAC的光谱扩展到了von Hippel Lindau（VHL）和细胞凋亡蛋白1（cIAP1）的细胞抑制剂，这些蛋白对于大多数癌细胞都是必不可少的，因此不太可能被灭活。我们基于VHL的PROTAC系列包含对CDK6特异或对CDK4和CDK6表现出双重活性的化合物。基于IAP的PROTAC导致CDK4 / 6和IAP的组合降解，从而对癌细胞的生长产生协同作用。我们的新降解剂在人和小鼠细胞中显示出强大而持久的降解活性，并抑制了几种白血病，骨髓瘤和乳腺癌细胞系的增殖。总之，我们表明基于VHL和IAP的PROTAC是针对CDK4 / 6靶向降解癌症的一种有吸引力的方法。