Based on these findings, we propose that the observed CDK6/CDK4 selectivity of 4 (PF-07220060) is the result of the rigidity of the CDK6 G-loop imposing a higher strain energy when accommodating the isopropyl alcohol group of 4 (PF-07220060) in the CDK6-ligand complex. In contrast, the flexibility of the CDK4s G-loop allows for a lower strain energy when accommodating this group. We postulate that this difference in strain energy contributes to the observed 26-fold selectivity for CDK4 over CDK6 selectivity of 4 (PF-07220060).

These studies indicated that the methyl group at the 2-position of the benzimidazole could be a major site of oxidation. This observation is also consistent with the metabolism of the benzimidazole structure of abemaciclib

The C5-position of the pyrimidine core of 6 is nicely positioned in the binding pocket for C5-substituents to extend toward GSK3β (Leu132)

The two proteins share a high degree of sequence identity in the ATP-binding site (92%) with just four differences at positions equivalent to Glu21, Lys26, Thr106, and Gln149 of CDK6

the hydroxy piperidine 6 possessed greatly improved selectivity over CDK2 (557x)

This strategy is effective because CDK2 has a phenylalanine at this position (Phe82) and thus cannot form a productive hydrogen bond to the pyridyl moiety, resulting in a desolvation penalty and, thus, reduced binding affinity to CDK2

The mechanism behind the development of neutropenia with CDK4/6 inhibitors is related to the role of CDK6 in hematopoiesis.