Abstract
1036
Introduction: Breast cancer is a worldwide health issue with about 1 million new cases every year. Dysregulated cellular proliferation, a common feature to all human cancers, is a key player of aberrant proliferative signaling and is therefore considered as a “hallmark of cancer”. In breast cancer, a lot of attention focuses on particular members of the cell cycle machinery, the D-type cyclins and their partner cyclin-D kinases 4 and 6 (CDK4/6). CDKs are serine/threonine kinases key in regulating cell cycle progression by associating with cyclins. Several studies have identified alterations of cell cycle regulators in human breast cancer and provide a strong rationale for a therapeutic role for CDK4/6 inhibition in this tumor type. Amplification of the cyclin-D1 occurs in about 20% of human breast cancers, while overexpression of the protein is above 60%. The subtype for which CDK4/6 inhibition has the strongest rationale is estrogen receptor (ER)-positive disease. These subtypes almost always retain Rb function, thereby CDK4 and CDK6 targeting agents can block pRb phosphorylation (in low nanomolar concentration) and induce G1 arrest in sensitive cell lines. It is often controversial whether CDK4/6 inhibitors are capable to prolong overall survival. Moreover, it is urgent to find a way to select which patients are most likely to benefit from these drugs and to monitor, non-invasively, the progress of the disease and the overall treatment response. To this aim, we developed a PET imaging agent ([18F]-CDKi) as an in vivo PET reporter of CDK4/6 status with the final aim to improve efficacy of breast cancer therapy. To generate our fluorine-18 inhibitor, we introduced an F-18 prosthetic group (18F-fluorobenzoic acid, [18F-FBA]), on the terminal piperazine and transformed palbociclib (Inbrance®, Pfizer) into a different PET active functional molecule. The first in vitro experiment aiming to analyze pharmacokinetics (PK) and in vitro activity revealed that [18F]-CDKican be a successful PET agent with nearly ideal imaging characteristics. Moreover, we demonstrated that [18F]-CDKi is stable in vitro and in vivo (>98% at 4h post injection) and maintained a potent targeting affinity to CDK4/6. Cellular uptake experiments performed in MCF-7 breast cancer cell line (ER-positive/HER2-negative) demonstrated specific uptake. Similar significant uptake values were also observed in biodistributed MCF-7 bearing mouse models. The strong activation of CDK4/6 in cancer cells in concert with its low activation in untransformed healthy cells makes [18F]-CDKi a nearly ideal imaging agent for the early detection of malignant growth of the breast. Moreover, we also hypothesize it could be an excellent PET imaging agent for metastatic breast cancer due to their high proliferative rate. CDKi represents the first of a new generation of PET imaging agents critical to study how cancer cells escape the cell cycle arrest and develop resistance to conventional treatment.