Application and challenges of nitrogen heterocycles in PROTAC linker

The absence of effective active pockets makes traditional molecularly targeted drug strategies ineffective against 80 % of human disease-related proteins. The PROTAC technology effectively makes up for the deficiency of traditional molecular targeted drugs, which produces drug activity by degrading rather than inhibiting the target protein. The degradation of PROTAC is not only affected by POI ligand and E3 ligand, but by the selection of suitable linker which can play an important role in the efficiency and selectivity of the degradation. In the early exploring stage of the PROTAC, flexible chains were priorly applied as the linker of PROTAC. Although PROTAC with flexible chains as linkers sometimes perform well in vitro bioactivity evaluations, the introduction of lipophilic flexible chains reduces the hydrophilicity of these molecules, resulting in generally poor pharmacokinetic characteristics and pharmacological activities in vivo. In addition, recent reports have also shown that some PROTAC with flexible chains have some risks to causing hemolysis in vivo. Therefore, PROTAC with flexible chains show less druggability and large difficulty to entering the clinical trial stage. On the other hand, the application of nitrogen heterocycles in the design of PROTAC linkers has been widely reported in recent years. More and more reports have shown that the introduction of nitrogen heterocycles in the linker not only can effectively improves the metabolism of PROTAC in vivo, but also can enhance the degradation efficiency and selectivity of PROTAC. These PROTAC with nitrogen heterocycle linkers have attracted much attention of pharmaceutical chemists. The introduction of nitrogen heterocycles in the linker deserves priority consideration in the primary design of the PROTAC based on various druggabilities including pharmacokinetic characteristics and pharmacological activity. In this work, we summarized the optimization process and progress of nitrogen heterocyclic rings as the PROTAC linker in recent years. However, there were still limited understanding of how to discover, design and optimize PROTAC. For example, the selection of the types of nitrogen heterocycles and the optimization sites of this linker are challenges for researchers, choosing between four to six-membered nitrogen heterocycles, selecting from saturated to unsaturated ones, and even optimizing the length and extension angle of the linker. There is a truly need for theoretical explanation and elucidation of the PROTAC to guide the developing of more effective and valuable PROTAC.