Previously, we demonstrated that the peroxisome proliferator-activated receptor gamma (PPAR gamma) agonist troglitazone mediated the repression of cyclin D1 in MCF-7 breast cancer cells by facilitating proteasome-facilitated proteolysis. This PPAR gamma-independent mechanism provided a molecular basis for using troglitazone as scaffold to develop a novel class of cyclin D1-ablative agents. The proof of principle of this premise is provided by Delta 2TG, in which the introduction of a double bond adjacent to the thiazolidinedione ring abrogated the PPAR gamma activity while retaining the activity in cyclin D1 repression. Structural optimization of Delta 2TG led to STG28 [(S)-5-(4-{[6-(allyloxy)-2,5,7,8-tetramethylchroman-2-yl]methoxy}-3-methoxybenzylidene)-thiazolidine-2,4- dione], which exhibited low micromolar potency in ablating cyclin D1 and inhibiting MCF-7 cell proliferation. It is noteworthy that STG28 mediated the proteasomal degradation of cyclin D1 with a high degree of specificity. Exposure to STG28 did not cause any appreciable change in the expression levels of a series of other cyclins and CDK-dependent kinases. In light of the pivotal role of cyclin D1 in promoting tumorigenesis and drug resistance, this novel cyclin D1-ablating agent may have therapeutic relevance in cancer therapy.
