The p53-MDM2 interaction has been proved to be a valuable target to develop effective antitumor agents. Novel p53-MDM2 inhibitors bearing pyrrolidone scaffolds were successfully identified by structure-based design. The nanomolar inhibitor 5 possessed good p53-MDM2 inhibitory activity (K(i) = 780 nM) due to its hydrophobic and hydrogen bonding interactions with MDM2. Further hit optimization led to the discovery of a number of highly potent pyrrolidone derivatives with improved p53-MDM2 inhibitory activity and in vitro antiproliferative potency. Compounds 41 (K(i) = 260.0 nM) and 60a (K(i) = 150.0 nM) showed good and selective activity against tumor cells with deleted p53. In addition, these two compounds also effectively inhibited the tumor growth in the A549 xenograft model. Interestingly, compound 41 was proved to be a potent MDM2/MDMX dual inhibitor. The novel pyrrolidone p53-MDM2 inhibitors represent promising lead structures for the development of novel antitumor agents.

Lactone is a common structural motif in biologically active natural products. However, the metabolic instability of lactone significantly reduces their in vivo potency. In the present investigation, a new strategy to improve the metabolic stability of lactone was provided by the design of α-fluoro ether as a novel bioisostere of lactone. The effectiveness of the α-fluoro ether/lactone replacement was validated by the discovery of (20S,21S)-21-fluorocamptothecins as hydrolytically stable topoisomerase I inhibitors. A highly potent camptothecin derivative, 8l, was successfully identified, which showed excellent in vitro and in vivo antitumor activities and represents a promising lead for the discovery of novel antitumor agents. Interestingly, this study also provided a new structure-activity relationship for the C21-carbonyl group of camptothecin, which has been regarded as an essential pharmacophore. Our results revealed that the conserved C21-carbonyl group can be replaced by a fluorine substituent. α-Fluoro ether may have general application in improving the metabolic stability of lactone.

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Homocamptothecin (hCPT) is a camptothecin (CPT) homologue with the insertion of a methylene (CH₂) spacer between the alcohol moiety and carbonyl group of the classical six-membered α-hydroxylactone ring. This modification provides higher lactone stability and did not impair its activity against topoisomerase I (Topo I), but rather appears to improve it compared to CPT. In an attempt to improve the antitumor activity of homocamptothecins, a series of novel hCPT derivatives conjugating with dihydropyrimidine (DHPM) derivatives was designed and synthesized based on a synthetic route which couples 7-formylhomocamptothecin with different dihydropyrimidine derivates. Most of the synthesized compounds exhibited good antiproliferative activity on tumor cell lines A549, MDA-MB-435 and HCT116. Furthermore, this class of compounds showed superior Topo I inhibition activity comparable to or higher than CPT.

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