More than an Amide Bioisostere: Discovery of 1,2,4-Triazole-containing Pyrazolo[1,5‑<i>a</i>]pyrimidine Host CSNK2 Inhibitors for Combatting β‑Coronavirus Replication

The pyrazolo[1,5-a]pyrimidine scaffold is a promising scaffold to develop potent and selective CSNK2 inhibitors with antiviral activity against β-coronaviruses. Herein, we describe the discovery of a 1,2,4-triazole group to substitute a key amide group for CSNK2 binding present in many potent pyrazolo[1,5-a]pyrimidine inhibitors. Crystallographic evidence demonstrates that the 1,2,4-triazole replaces the amide in forming key hydrogen bonds with Lys68 and a water molecule buried in the ATP-binding pocket. This isosteric replacement improves potency and metabolic stability at a cost of solubility. Optimization for potency, solubility, and metabolic stability led to the discovery of the potent and selective CSNK2 inhibitor 53. Despite excellent in vitro metabolic stability, rapid decline in plasma concentration of 53 in vivo was observed and may be attributed to lung accumulation, although in vivo pharmacological effect was not observed. Further optimization of this novel chemotype may validate CSNK2 as an antiviral target in vivo.吡唑并[1,5-a]嘧啶（pyrazolo[1,5-a]pyrimidine）母核是一类极具潜力的骨架，可用于开发具有抗β-冠状病毒（β-coronaviruses）活性的强效、选择性CSNK2抑制剂。本文报道了一种1,2,4-三唑（1,2,4-triazole）基团的发现，该基团可替代众多强效吡唑并[1,5-a]嘧啶类抑制剂中用于结合CSNK2的关键酰胺基团。晶体学证据表明，1,2,4-三唑可替代酰胺，与埋于ATP结合口袋（ATP-binding pocket）内的Lys68及水分子形成关键氢键。该等排置换（isosteric replacement）可提升化合物的活性及代谢稳定性，但会以牺牲溶解度为代价。通过对活性、溶解度及代谢稳定性进行优化，我们得到了强效且选择性优异的CSNK2抑制剂53。尽管53具备优异的体外代谢稳定性，但实验观察到其体内血浆浓度快速下降，这一现象可能与肺部蓄积有关，不过目前尚未观察到其体内药理学效应。对该新型化学型的进一步优化，有望验证CSNK2作为体内抗病毒靶点的可行性。