Objective. In vitro studies have indicated that levels of neutral metalloproteinases in osteoarthritic (OA) cartilage are elevated and that doxycycline (doxy) inhibits collagenolytic and gelatinolytic activity in extracts of OA cartilage. The purpose of the present study was to test the effect of oral doxy administration on the severity of cartilage degeneration in OA.Methods. OA was induced in 12 adult mongrel dogs by transection of the anterior cruciate ligament (ACL) 2 weeks after dorsal root ganglionectomy. Six dogs received doxy orally from the day after ACL transection until they were killed 8 weeks later; the other 6 served as untreated OA controls.Results. The unstable knee of each untreated dog exhibited extensive full-thickness cartilage ulceration of the medial femoral condyle. In sharp contrast, cartilage on the distal aspect of the femoral condyle of the unstable knee was grossly normal in 2 doxy-treated dogs, and exhibited only thinning andlor surface irregularity in the others. Degenerative cartilage lesions on the medial trochlear ridge, superficial fibrillation of the medial tibia1 plateau, and osteophytosis were, however, unaffected by doxy treatment. Collagenolytic activity and gelatinolytic activity in cartilage extracts from OA knees of untreated dogs were 5-fold and 4-fold greater, respectively, than in extracts from dogs given doxy.Conclusion. Prophylactic administration of doxy markedly reduced the severity of OA in weight-bearing regions of the medial femoral condyle. It remains to be determined whether administration of doxy after OA changes have developed is also effective.Tetracyclines inhibit the activity of neutral matrix metalloproteinases (MMPs) (1-3). This effect has been believed to be due, at least in part, to chelation of zinc and/or calcium (1,4), which maintain the normal structural conformation and hydrolytic activity of the MMPs (5).In osteoarthritis (OA), the activities of MMP-1 (collagenase), MMP-2 (gelatinase), and MMP-3 (stromelysin) in the degenerating cartilage are increased (6-8). We recently showed that doxycycline (doxy), in a concentration approximating that found in human serum after a 200-mg oral dose, inhibited type XI collagenolytic activity in homogenates of human OA cartilage (9). Doxy similarly inhibited the activity of purified kidney epithelial cell gelatinase, a metalloproteinase that yields digestion products from type XI collagen identical to those produced by the OA cartilage homogenates (9). In both cases, we showed that the inhibition could be overcome by addition of excess zinc or calcium. We now present evidence that oral administration of doxy ameliorates cartilage destruction in a canine model of experimentally induced OA.

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Homogenates of 6 samples of human osteoarthritic cartilage were shown to degrade exogenous type XI collagen. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the cleavage products generated by each homogenate were similar, and they were identical to those obtained by cleavage of the substrate with purified gelatinase. Enzyme activity, which was inhibited by EDTA, was greater in extracts of fibrillated osteoarthritic cartilage than in extracts of grossly normal cartilage from the same joint or in extracts of cartilage from joints with osteonecrosis. Activation with APMA enhanced digestion, but breakdown was apparent in extracts of fibrillated osteoarthritic cartilage even without APMA. 28, 1990. nent of all types of cartilage in which type I1 collagen forms the main collagen framework (2). The function of type XI collagen in organization of the extracellular matrix is unknown, but its unique properties (3-6) and its presence in all types of cartilage suggest that it plays a major role in preserving the integrity of the tissue.Collagenolytic activity has been demonstrated in human osteoarthritic cartilage (7,s). Mankin et al described the presence of a collagenase in osteoarthritic cartilage that could not be detected in normal cartilage (9). Dean et al showed that the total metaldependent collagenolytic activity, as measured by the release of hydroxyproline from the tissue, is significantly elevated in human osteoarthritic lesions (10). Cartilage contains an inhibitor of metalloproteinases whose level may remain constant (10) or may decline (1 1) in osteoarthritis (OA), resulting in a stoichiometric imbalance between collagenase and the inhibitor, and leading to degradation of the extracellular matrix.Neither classic mammalian collagenase (matrix metalloproteinase 1 [MMP-I]) nor stromelysin (matrix metalloproteinase 3 [MMP-31) degrades type XI collagen, but this collagen is cleaved by another metalloproteinase, gelatinase (MMP-2), that selectively degrades gelatin and has been purified from various sources (12-14).MMP-2 is released from cells in a latent form and can be activated in vitro with the same reagents that activate the classic collagenase (e.g., trypsin, organomercurials) (15). It degrades denatured types I, 11, and 111 collagen (16) and native type V collagen (17), and markedly potentiates the action of interstitial collagenase ( 12).

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