Design, synthesis, and proposed active site binding analysis of monocyclic2-azetidinone inhibitors of prostate specific antigen

A homology derived molecular model of prostate specific antigen (PSA) was c reated and refined. The active site region was investigated for specific in teracting functionality and a binding model postulated for the novel 2-azet idinone acyl enzyme inhibitor 1 (IC50 = 8.98 +/- 0.90 muM) which was used a s a lead compound in this study. A single low energy conformation structure II (Figure 2) was adopted as most likely to represent binding after minimi zation and dynamics calculations. Systematic analysis of the binding import ance of all three side chains appended to the 2-azetidinone was conducted b y the synthesis of several analogues. A proposed salt bridge to Lys-145 wit h 4 (IC50 = 5.84 +/- 0.92 muM) gave improved inhibition, but generally the binding of the N-1 side chain in a specific secondary aromatic binding site did not tolerate much structural alteration. A hydrophobic interaction of the C-4 side chain afforded inhibitor 6 (IC50 = 1.43 +/- 0.19 muM), and pol ar functionality could also be added in a proposed interaction with Gln-166 in 5 (IC50 = 1.34 +/- 0.05 muM). Reversal of the C-4 ester connectivity fu rnished inhibitors 7 (IC50 = 1 59 +/- 0 15 muM), 11 (IC50 = 3.08 +/- 0.41 m uM), and 13 (IC50 = 2.19 +/- 0.36 muM) which were perceived to bind to PSA by a rotation of 180 degrees relative to the C-4 ester of normal connectivi ty. Incorporation of hydroxyl functionality into the C-3 side chain provide d 16 (IC50 = 348 +/- 50 nM) with the greatest increase in PSA inhibition by a single modification. Multiple copy simultaneous search (MCSS) analysis o f the PSA active site further supported our model and suggested that 18 wou ld bind strongly. Asymmetric synthesis yielded 18 (IC50 = 226 +/- 10 nM) as the most potent inhibitor of PSA reported to date. It is concluded that ou r design approach has been successful in developing PSA inhibitors and coul d also be applied to the inhibition of other enzymes, especially in the abs ence of crystallographic information.