Remodeling of small pulmonary arteries represents a main pathological finding associated with pulmonary arterial hypertension (PAH). Hyper-proliferation of pulmonary vascular cells results in the obstruction of the vessels leading to increased pulmonary artery pressure. These vascular lesions of patients with severe PAH exhibit a tumor-like phenotype with an uncontrolled replicative potential. We hypothesize that an increased activity of cyclin-dependent-kinases (CDK), key regulators of the cell cycle, is responsible for the unlimited cell growth of the pulmonary vascular cells.
We could identify several CDKs to be strongly over-activated in IPAH patient derived PASMCs and confirmed an up-regulation of CDK expression in human specimen and in animal models of P(A)H. Pharmacological CDK inhibitors (CDK-Is) were able to block pulmonary artery smooth muscle cell (PASMC) proliferation in vitro as determined by BrdU incorporation assay to quantify DNA synthesis. Flow cytometric measurements indicated an arrest in cell-cycle progression after incubation with CDK-Is. In cell death detection assays for AnnexinV-positive cells as well as for lactate dehydrogenase release, no signs of cytotoxicity or apoptosis have been observed upon CDK-I administration. Furthermore, we could show a diminished phosphorylation of the direct CDK downstream target and proliferation marker P-Rb (Retinoblastoma protein) in PASMCs which were treated with CDK-Is.
In the monocrotaline rat model, CDK-Is showed a significant improvement of several parameters of right ventricular function/hypertrophy and hemodynamics in vivo. Right ventricular systolic pressure was reduced upon CDK-I therapy while cardiac index and TAPSE were elevated. The analysis of the pulmonary tissue ex vivo showed a targeted effect of CDK-Is as determined by a decrease of the percentage of fully muscularized pulmonary vessels and impaired downstream CDK signaling. In summary, we can conclude that there is rationale in the therapeutic potential of CDK-Is as a new treatment option for PAH.