Development of a Radiolabeled Cyclin-Dependent Kinases 4 and 6 (CDK4/6) Inhibitor for Brain and Cancer PET Imaging

The synthesis, biochemical evaluation and radiosynthesis of a cyclin-dependent kinases 4 and 6 (CDK4/6) inhibitor and radioligand was performed. NT431, a newly synthesized 4-fluorobenzyl-abemaciclib, exhibited high potency to CDK4/6 and against four cancer cell lines with IC50 similar to that of the parent abemaciclib. We performed a two-step one-pot radiosynthesis to produce [F-18]NT431 with good radiochemical yield (9.6 +/- 3%, n = 3, decay uncorrected), high radiochemical purity (>95%), and high molar activity (>370 GBq/mu mol (>10.0 Ci/mu mol). In vitro autoradiography confirmed the specific binding of [F-18]NT431 to CDK4/6 in brain tissues. Dynamic PET imaging supports that both [F-18]NT431 and the parent abemaciclib crossed the BBB albeit with modest brain uptake. Therefore, we conclude that it is unlikely that NT431 or abemaciclib (FDA approved drug) can accumulate in the brain in sufficient concentrations to be potentially effective against breast cancer brain metastases or brain cancers. However, despite the modest BBB penetration, [F-18]NT431 represents an important step towards the development and evaluation of a new generation of CDK4/6 inhibitors with superior BBB penetration for the treatment and visualization of CDK4/6 positive tumors in the CNS. Also, [F-18]NT431 may have potential application in peripheral tumors such as breast cancer and other CDK4/6 positive tumors.