Mammalian nonpancreatic secretory phospholipase A2 (PLAz) splits the 2-acyl bond in 1,2-diacylpho~phatide.~ This enzyme has been found in high concentrations in the synovial fluid of patients with rheumatoid arthritis: and it has been suggested that inhibitors of this enzyme may have therapeutic value. The three-dimensional structure of human synovial fluid PLAz (HSF-PLAz) is known both in its native form3 and in a complex with the transitionstateanalogue (TSA) L-1-0-octyl-2-heptylphosphonyl-snglycero-3-phosphoethanolamineP 1. The present work is a part of our program to develop PLAz inhibitors and describes the successful rational modifications introduced into 1 aimed at enhancing its affinity toward HSF-PLA2, based on the combined use of biochemical information, molecular graphics analysis? molecular orbitale and molecular mechanics calculations,' and the GRID8 and LUDI? programs. Hydrocarbon chain length is a critical factor for the activity of potential PLA2 inhibitors. Studies with phospholipid analogues demonstrated that 10 carbons are required in the sn-2 acyl chain for optimum binding to cobra venom PLA2,lO whereas the optimal length for the sn-1 alkyl chain is four carbons in the case of porcine pancreatic PLA2." These findings can be rationalized in terms of the observed number of contacts between the phospholipid analogs and the enzyme in known PLA2inhibitor complexes.12 Analysis of the HSF-PLA2 structure with the GRID program suggests similar structureactivity relationshipsl3 (Figure 1). The capacity of TSAs to bind with high affinity has been shown by Gelb et al. who introduced a phosphonate group into compound l.14 In contrast, the substitution of acyl by sulfonyl, which is extensively used as a TSA of an ester group undergoing hydrolysis, has been reported by de Haas et ~ 1 . ~ 5 not to improve inhibitory properties. Despite this discouraging data, we decided to introduce the sulfonamide group on the basis of the following rationale: Yu and Dennislea showed that the pKa of the catalytically active His-48 is 6.1. Therefore, this residue is predominantly unprotonated under physiological conditions. Thus, in order for a TSA to function effectively at physiological pH, the bioisostere of the ester should be chosen so that, in addition to possessing tetrahedral features to resemble the transition state, it has a proton available to form a hydrogen bond to the N6 atom of His-