3,5,7-Trisubstituted pyrazolo[4,3-d]pyrimidines
have been identified as potent inhibitors of cyclin-dependent kinases
(CDKs), which are established drug targets. Herein, we describe their
further structural modifications leading to novel nanomolar inhibitors
with strong antiproliferative activity. We determined the crystal
structure of fully active CDK2/A2 with 5-(2-amino-1-ethyl)thio-3-cyclobutyl-7-[4-(pyrazol-1-yl)benzyl]amino-1(2)H-pyrazolo[4,3-d]pyrimidine (24) at 1.7 Å resolution, confirming the competitive mode of inhibition.
Biochemical and cellular assays in lymphoma cell lines confirmed the
expected mechanism of action through dephosphorylation of retinoblastoma
protein and RNA polymerase II, leading to induction of apoptosis.
Importantly, we also revealed an interesting ability of compound 24 to induce proteasome-dependent degradation of cyclin K
both in vitro and in a patient-derived xenograft in vivo. We propose
that 24 has a dual mechanism of action, acting as a kinase
inhibitor and as a molecular glue inducing an interaction between
CDK12 and DDB1 that leads to polyubiquitination of cyclin K and its
subsequent degradation.