Does cephalothin inhibit matrix metalloproteinases?

A recent paper by Santavirta et al. (5) describes the inhibition by cephalothin of a 2,4-dinitrophenol (DNP)-octapeptidecleaving activity (substrate mimics collagen cleavage site) in tissue around loose hip prostheses. The authors conclude that cephalothin inhibits matrix metalloproteinases (MMPs), a family of zinc-dependent enzymes that degrade all major components of the extracellular matrix (10), in periprosthetic tissue. Interestingly, doxycycline, a known inhibitor of MMPs (7), showed no inhibition in the author’s assay. There are several issues critical to the validity of the conclusions that are not addressed in the paper. First, the synthetic substrate used in the assay is not specific for MMPs. In fact, the original publication on the use of the DNP-octapeptide substrate noted that it is cleaved by several proteinases and cautioned against its use with crude extracts (3). Second, almost all known small molecule inhibitors of MMPs act by chelating the metal at the active site (2). The previously reported MMP inhibitor cefotetan (6) may also act in a similar manner, through its side chain. No such chelating potential exists for cephalothin. Third, there is extensive literature on the inhibition of serine proteinases, such as elastase, by b-lactams (4). Elastase itself, a relatively nonspecific enzyme (1, 8) found in leukocytes and thus probably present in inflamed periprosthetic tissue (9), might hydrolyze the DNP-octapeptide used in the assay. In view of the above, additional experiments are in order before any conclusions can be drawn. At a minimum, the DNPoctapeptide-cleaving activity in periprosthetic tissue should be characterized with known inhibitors of metallo(EDTA, 1,10phenanthroline) and serine (diisopropylfluorophosphate, phenylmethanesulfonyl fluoride, 3,4-dichloroisocoumarin) proteinases. In view of the implied usefulness of cephalothin in disease states characterized by excessive MMP activity, it is important to ascertain whether cephalothin does indeed inhibit MMPs.