Cell cycle as therapeutic target – CDK4/6 inhibition

One of the most important decision of a cell: to live or die. If survival is the choice, there are three options: proliferate, to stay in sleeping state for a while, or differentiate in order to perform its specifi c function. These decisions are under a very strict molecular regulation infl uenced by internal and external factors. Tumor cells more and more disregard the regulations, and move into independency for a continuous proliferation, which has a very similar program in normal and tumor cells. The main route towards mitosis is the cell cycle, under the supervision of positive and negative regulators, forming checkpoints, telling to the cell – under the infl uence of mitogenic signals – to go or to stop. The most critical checkpoint is at the border of G1 and S phases where the main players are cyclinD, CDK4/6 and RB1. It turned out that the best targets to inhibit cell proliferation are the CDKs, but this approach, when used unselected targets, was unsuccessful due to the toxicity.  To improve the clinical results, the selection of CDK4/6 as a therapeutic target seems to fulfi l most of the hopes. Today three drugs are the most promising: palbociclib (with an acceptance by FDA and EMA to treat breast cancer patients), abemaciclib and ribociclib (underclinical trials). Now, most of the data concern breast cancer, especially the combinations of CDK4/6 inhibitors and endocrine therapy, but many other malignancies are studied (e.g. liposarcoma, mantel cell lymphoma, melanoma, renal cancer, lung cancer, pancreatic cancer, ovarian cancer, teratomas etc.). The key points are the side-effects, the most frequently observed is neutropenia, but so far it is managed without serious toxicity.