Many success stories were presented at the recent FBLD 2016 meeting in Boston, some of which are appearing in the literature. A case in point is published in this month’s issue of ACS Med. Chem. Lett.
Fabrizio Giordanetto, Daniel Pettersen, and colleagues at AstraZeneca were interested in finding inhibitors of secreted phospholipase A2 (sPLA2) enzymes, which cleave glycerophospholipids and are implicated in the lipid accumulation and inflammation associated with atherosclerosis. Of the eleven different isoforms, sPLA2-IIa, sPLA2-V, and sPLA2-X are considered particularly good targets, and the researchers sought an inhibitor that would hit all three. Other companies had shown that a primary amide can form multiple hydrogen bonds at the catalytic site, so the AstraZeneca team reanalyzed previous internal screening data to look for fragment-like hits (defined as having 10-18 non-hydrogen atoms) containing a primary amide. They found many, including compound 1.
In addition to being a potent inhibitor of both sPLA2-IIa and sPLA2-X, compound 1 was quite active in human plasma, which is physiologically relevant. A crystal structure of sPLA2-X revealed that the compound bound as expected, and modeling suggested that adding a carboxylic acid moiety could make additional interactions with the catalytic calcium ion. Several molecules were made, the most potent of which turned out to be compound 4, with a satisfying 2000-fold boost in activity against sPLA2-X. Shortening or lengthening the linker connecting the acid with the rest of the molecule reduced affinity, observations which could be rationalized by modeling.
Compound 4 was characterized in some detail, which revealed bioavailability in rats and dogs, good pharmacokinetics, and a fairly clean off-target profile. Unfortunately, it was reasonably active against OATP1B1, which recognizes carboxylic acids. Among other duties, OATP1B1 transports statins to the liver, and since many people with atherosclerosis are taking statins this activity would obviously be a problem. However, crystallography suggested that introducing substitutions very close to the carboxylic acid moiety would likely be tolerated by sPLA2 but not by OATP1B1. Indeed, simply adding a methyl group maintained or increased activity against the three relevant sPLA2 isoforms while completely abolishing OATP1B1 inhibition. Happily, AZD2716 had excellent pharmacokinetics and bioavailability in mice, rats, dogs, and cynomolgus monkeys.
This is a lovely example of what has been called fragment-assisted drug discovery. The researchers explicitly looked for a small, ligand-efficient starting point and relied heavily on structure-based design during optimization. The paper ends by noting that AZD2716 was selected as a clinical candidate, though it does not appear in the AstraZeneca pipeline or in clinicaltrials.gov; if this changes we’ll make a note on our running list.