The cyclin groove is an important recognition site for substrates of the cell cycle cyclin dependent kinases and provides an opportunity for highly selective inhibition of kinase activity through a non-ATP competitive mechanism. The key peptide residues of the cyclin binding motif have been studied in order to precisely define the structure-activity relationship for CDK kinase inhibition. Through this information, new insights into the interactions of peptide CDK inhibitors with key subsites of the cyclin binding groove provide for the replacement of binding determinants with more druglike functionality through REPLACE, a strategy for the iterative conversion of peptidic blockers of protein-protein interactions into pharmaceutically relevant compounds. As a result, REPLACE is further exemplified in combining optimized peptidic sequences with effective N-terminal capping groups to generate more stable compounds possessing antitumor activity consistent with on-target inhibition of cell cycle CDKs. The compounds described here represent prototypes for a next generation of kinase therapeutics with high efficacy and kinome selectivity, thus avoiding problems observed with first generation CDK inhibitors.