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Using the X-ray structure of human group X secreted phospholipase A2 (hGX), we carried out structure-based design of indole-based inhibitors and prepared the compounds using a new synthetic route. The most potent compound inhibited hGX and the mouse orthologue with an IC50 of 75 nM. This compound is the most potent hGX inhibitor reported to date and was also found to inhibit a subset of the other mouse and human sPLA2s.
Secreted phospholipases A2 (sPLA2s) are a group of enzymes with conserved active sites and calcium-binding loops that catalyze the hydrolysis of the sn-2 ester of glycero-phospholipids to release free fatty acids and 2-lysophospholipids.1–4 The sPLA2s are low molecular weight (~14–20 kDa) enzymes with 5–8 disulfide bonds and require Ca2+ as a catalytic cofactor.1–4 To date, there are 10 distinct mammalian sPLA2s (groups IB, IIA, IIC, IID, IIE, IIF, III, V, X, XIIA), with group IIC present as a pseudogene in humans.1,2 Biomedical interest in this class of enzymes stems from the observation that one or more plays a role in the liberation of arachidonic acid from cellular phospholipids for the biosynthesis of eicosanoids. Mammalian cells also contain a cytosolic phospholipase (cPLA2-α).5 Recent evidence shows that in macrophages and other cells, sPLA2s and cPLA2-α work together to liberate arachidonic acid in agonist-stimulated mammalian cells.6–9 The molecular basis for the cross-talk between these two enzymes is not known. Potent and selective inhibitors of the sPLA2 enzymes would help resolve some of the questions surrounding the mechanism of arachidonate release as well as probe other physiological functions of sPLA2s.
For the past decade, the group IIA sPLA2 has received the most attention among the sPLA2s because it was the first nonpancreatic sPLA2 to be discovered, and it is found in high levels in patients suffering from inflammatory diseases including rheumatoid arthritis10 and acute pancreatitis.11 The group IIA sPLA2 has been the focus of medicinal chemistry efforts at several pharmaceutical companies.12–16 Workers at Lilly Labs and Shionogi & Co. Ltd. have reported on substituted indoles, exemplified by compound I, as potent inhibitors of group IIA sPLA2 (Figure 1).12–15 Among the various reported inhibitors of sPLA2,17 these compounds appear to be the most potent and also appear to have the most drug-like properties. With the discovery of additional sPLA2 enzymes, we have been interested in exploring these indole analogues as inhibitors of all of the members of the sPLA2 enzyme class.18 We are particularly interested in the group X sPLA2 because it appears to have the highest specific activity in promoting arachidonic acid release from mammalian cells.7,19,20 To date, very few inhibitors of the group X sPLA2 have been reported, with the general sPLA2 inhibitor YM-26734 being the most potent at 0.20 μM against the group X enzyme.21 To this end, we recently determined the X-ray structure of the indole-based inhibitor Me-Indoxam bound to human group X sPLA2 (hGX).18 We now report the development of a potent indole-based inhibitor of hGX that was designed based on this structural information.
In our first attempt to develop potent indole-based inhibitors of sPLA2s other than the group IIA enzyme, we made a library of analogues in which the substituent attached to N1 of the indole ring was varied.18 The range of IC50 values for this library was not very broad, and potent inhibitors of hGX were not obtained. Subsequent X-ray structural studies showed that this substituent largely points out of the active site of the sPLA2, toward what would be the membrane plane when the enzyme is bound to the phospholipid bilayer.18 Thus, we turned to chemical modifications of other areas of the indole ring.
Inspection of the X-ray structure of Me-Indoxam bound to hGX sPLA218 reveals a large hydrophobic pocket near the 2-substituent that makes little interaction with the inhibitor. We hypothesized that a larger 2-alkyl substituent would bind to the hydrophobic pocket and increase the binding affinity of the inhibitor. Also, the 6-position of the indole points out of the enzyme active site and does not contribute to binding affinity. However, modification of the 6-position may be useful for modifying the physiochemical properties of the indole for subsequent use in whole animal studies to affect pharmacokinetics. Docking studies were performed on the hGX enzyme in which the indole inhibitor was modified to include a 2-ethyl and 6-methyl, and the N1 substituent replaced with a benzyl (Figure 2). As suspected, the larger 2-ethyl group contacts the inner wall of the enzyme much better than a 2-methyl group, and a 6-methyl group sticks out of the enzyme active site and should not affect binding. As the reported synthesis12 of substituted 2-ethyl indoles was unsuccessful in our laboratory and the starting material for introduction of the 6-methyl substituent is not commercially available, a novel synthesis for 2-ethyl-6-methyl indoles was developed (Scheme 1). As there are few known literature reactions to functionalize the 6-position of an indole, the indole core had to be built up from pyrrole. Michael addition of nitromethane to tert-butyl crotonate followed by deprotection of this ester and subsequent treatment with thionyl chloride produced the acyl chloride 2. This was then added to benzenesulfonyl protected pyrrole in the presence of aluminum trichloride to give ketone 3. Treatment of 3 with NaOH in MeOH at low temperature followed by concentrated H2SO4 yielded dimethyl acetal 4. Ring closure to form the 4-oxyethanol indole 5 was accomplished by addition of a catalytic amount of acid with refluxing in toluene/ethylene glycol solvent. Conversion to the chloride followed by addition of excess n-butyllithium and benzyl protection yielded indole 6. Addition of n-butyllithium and acetic anhydride produced the desired 2-acetyl compound 7 due to the ortho-lithiating director used to protect the N1-position of the indole. Removal of the benzenesulfonyl protecting group and reduction of the ketone was accomplished in one step by refluxing in excess lithium aluminum hydride. Deoxygenation at the 2-position was accomplished using NaBH4 and trifluoroacetic acid to produce the 2-ethyl indole 9. N1-benzylation and 4-hydroxy deprotection followed by addition of tert-butyl bromoacetate yielded the tert-butyl oxyethanoate 11. Treatment of compound 11 with dilute oxalyl chloride followed by ammonia gas and deprotection of the tert-butyl ester with trifluoroacetic acid yielded the desired substituted 2-ethyl-6-methyl indole (compound A). Compounds B–D (Figure 3) were synthesized similarly (Supporting Information).
To test the indole analogues as sPLA2 inhibitors, we used a fluorometric assay consisting of unilamellar vesicles of 1-hexa-decanoyl-2-(10-pyrenedecanoyl)-sn-glycero-3-phosphoglycerol.22 The sPLA2-catalyzed liberation of 10-pyrenedecanoic acid allows the fluorophore to dislodge from the vesicles and bind to albumin in the buffer phase where it now undergoes monomer fluorescent emission rather than excimer emission. The assay results (Table 1) show the 2-ethyl substituent to have a dramatic affect on binding to the hGX, with IC50 values of 75 nM for compounds A and B. The 2-ethyl compounds (A and B) are 26-fold more potent than the analogous 2-methyl compounds (C and D) against hGX, which have IC50 values of 2 μM. The 6-methyl substituent has no effect on hGX binding; compounds A and B have identical IC50 values. The inhibitors were then screened against a panel of recombinant human and mouse sPLA2s (hGIB, mGIB, hGIIA, mGIIA, hGIIE, mGIIE, hGV, mGV, hGX, and mGX). In all cases the 2-ethyl compounds are more potent than the 2-methyl derivatives, and the 6-methyl group is tolerated (Table 1). Compounds A and B should be useful in distinguishing the groups X and V sPLA2s based on the ~10- fold increased potency for the former. This is significant because current evidence favors a role of these two sPLA2s in arachidonate liberation in mammalian cells. Although these compounds are also potent inhibitors of the group IIA sPLA2s, the original lead compound Me-Indoxam is 50-fold more potent on hGIIA and mGIIA versus hGX and mGX.18 Thus, by carrying out studies with a combination of inhibitors, it should be possible to probe for the role of specific sPLA2s in cellular processes.
In conclusion, the first potent inhibitor against hGX and mGX sPLA2s has been discovered. A new chemical route to these indole-based sPLA2 inhibitors has been developed.