Drugs 46 (5): 834-846. 1993 0012-6667/93/0011-0834/$06.5010 © Adis International Limited. All rights reserved.

Nonsteroidal Anti-Inflammatory Drugs and Chondroprotection A Review of the Evidence

Peter Ghosh

Raymond Purves Bone & Joint Research Laboratories, Royal North Shore Hospital of Sydney, St Leonards, New South Wales, Australia

The remarkable ability of diarthrodial joints to articulate and bear weight without appreciable wear or tear is made possible by the presence of articular cartilage and synovial fluid on their con­tact surfaces. The mechanical properties required to support these functions are determined by the unique composition and structure of articular car­tilage.

Cartilage is composed essentially of chondro­cytes embedded in a proteoglycan-rich hydrated gel which is constrained by a 3-dimensional net­work of type II collagen fibrils. The immobilised negatively charged proteoglycans are strongly hy­drophilic and water molecules diffuse into carti­lage because of this, generating a high osmotic pressure within the tissue (Maroudas et al. 1986). Since the collagen fibrils are anchored in the calci­fied cartilage adjacent to the subchondral bone and are highly cross-linked and inextensible, the influx of water molecules inflates the network until an eqUilibrium is reached between the tension in the fibres and the swelling pressure (Mow et al. 1992). Hence, it is the entrapment of the hydrated macro­molecular proteoglycans within a collagen net­work which imparts to cartilage its visco-elastic properties and resilience. Cartilage is also pro­tected and lubricated by synovial fluid, the major component responsible being hyaluronic acid (hyaluronan) [Balazs 1982].

1. Cartilage and Arthritic Disease Processes

While osteoarthritis and chronic rheumatoid ar­thritis have clearly different aetiologies, in both diseases articular cartilage is invariably degraded. One of the earliest events in this process appears to be the loss of proteoglycans from the extracellular matrix (Bayliss 1987). In the early stages of osteo­arthritis, abnormally high focal stresses acting on cartilage result in chondrocyte hypertrophy and cell death accompanied by loss of matrix proteo­glycans (Ghosh et al. 1990a, 1993a,b,c,d,e; Vignon et al. 1983). In rheumatoid arthritis the proliferat­ing pannus can release, directly into cartilage, pro­teinases, oxygen-derived free radicals, cytokines and prostanoids from resident leucocytes, causing inhibition of proteoglycan synthesis and resorption of the extracellular matrix (Chu et al. 1992; Haraoui et al. 1991; Henderson & Pettipher 1985). The early depletion of proteoglycans from carti­lage in osteoarthritis and rheumatoid arthritis joints reduces its stiffness and functional properties. On joint loading, high impulse stresses are transferred to cartilage and subchondral bone cells and, in ac­cordance with Wolffs' Law, they respond by re­modelling the tissue.

The breakdown products from cartilage re­leased into synovial fluid are immunogenic and can promote synovitis in osteoarthritic joints and con-

NSAIDs and Chondroprotection

tribute further to local inflammation in chronic rheumatoid arthritis (GIant et al. 1992; Kresina et al. 1988). The proteinases, cytokines, prostaglan­dins and free radicals produced by inflamed syno­vial tissues can suppress proteoglycan synthesis and stimulate the release of serine and metallo­proteinase by chondrocytes (Dingle 1991; Lowther et al. 1991). Cartilage-derived antigens can also activate blood monocytes to express pro-coagulat­ing activity and cytokines, which can promote the deposition of fibrin in synovial tissues and the sub­chondral vasculature (Ghosh & Smith 1993g). The occlusion of subchondral capillaries by thrombi (and lipids) in osteoarthritic joints is well known (Bullough & DiCarlo 1990; Kiaer 1987), and leads to osteocyte necrosis and bone sclerosis. This sub­chondral pathology in osteoarthritic joints reduces bone compliance and subjects the overlying carti­lage to additional mechanical stresses on loading. This sequence of events is summarised in figure 1.

835

By definition nonsteroidal anti-inflammatory drugs (NSAIDs) could be expected to modulate the synovial inflammation present in osteoarthritic and rheumatoid arthritic joints. However, as high­lighted in figure 1, synovial inflammation is but one component of a complex series of events which are responsible for joint destruction in arthritis.

The question thus arises whether NSAIDs can positively influence the progression of arthritic disorders by modifying the underlying pathology. Furthermore, as cartilage failure is considered to play a pivotal role in the pathogenesis of osteo­arthritis, the effects that NSAIDs may have on the integrity of this tissue has been of particular concern.

The purpose of this article is to review some recent literature on this subject. It is not intended to be exhaustive, and generally does not refer to earlier studies which have been reviewed else­where (Brandt 1987; Burkhardt & Ghosh 1987; Ghosh 1988; Hess & Herman 1986).

Fig. 1. Diagrammatic representation showing the interrelationships between mechanical injury, chondrocytes, bone cells, synoviocytes and activation of the blood clotting system (from Ghosh & Smith 1993, with permission).

836

2. Effect of NSAIDs on Cartilage

The NSAIDs naproxen, ketoprofen, piroxicam, tiaprofenic acid, diclofenac and tenidap have been selected as representative of this drug class. How­ever, since misoprostol has been proposed as an adjunct to NSAID therapy and has exhibited some useful effects on cartilage, it has also been in­cluded.

2.1 N aproxen

The effects of naproxen on the in vitro biosyn­thesis and turnover of cartilage proteoglycans has recently been examined (Brandt & Albrecht 1990; Glazer et al. 1993).

Normal canine cartilage explants were utilised by Brandt and Albrecht (1990), employing a media concentration of naproxen of 30 mgIL which was considered by these authors to be comparable to that achieved in synovial fluid of patients taking the drug. Under these conditions, cartilage proteo­glycan content and biosynthesis, as determined by the incorporation of 35S into these sulphated spe­cies, were unaffected by naproxen. A similar result was obtained for protein synthesis. On the other hand, when naproxen concentrations of 150 mg/L were used, incorporation of 35S into proteoglycans was depressed. This concentration was considered not to have any clinical relevance (Brandt & Al­brecht 1990).

In a study reported by Dingle (1991), naproxen at concentrations of 100 and 300 mg/L was ob­served to suppress proteoglycan synthesis in por­cine articular cartilage explants relative to control cultures by 28 and 62%, respectively. A less pro­nounced effect of the drug on proteoglycan synthe­sis was found using human articular cartilage.

Naproxen at in vitro concentrations of 10 to 200 mg/L was found to reduce, but not abrogate, the release of sulphated proteoglycans and neutral metalloproteinases into media of bovine explant cultures in a dose-dependent manner (Glazer et al. 1993). This anti-catabolic effect of naproxen was also demonstrable when the cultures were main­tained in the presence of recombinant human inter-

Drugs 46 (5) 1993

leukin-la (IL-la) over 4 days. Since IL-l has been implicated as an important mediator of cartilage destruction in arthritis (Chu et al. 1992; Dingle 1991; Pelletier et al. 1991), the ability of naproxen to modulate its effects on cartilage catabolism was considered to be of potential benefit in arthritic joints.

Naproxen has also been shown to downregulate the expression of IL-l receptors in cultured syno­vial cells derived from rheumatoid joints. Expo­sure of these cells to naproxen 10-4 mollL for 96 hours decreased IL-l receptor expression by 20 to 50% (Pronost et al. 1992).

The in vivo effects of naproxen on canine cartil­age composition, proteoglycan metabolism and metalloproteinase activities has been examined (Ratcliffe et al. 1993). It was found that cartilage from animals administered naproxen orally at 3 mg/kg daily for 4 weeks was indistinguishable from non-drug-treated controls, in terms of pro­teoglycan, collagen and water content. However when cartilages from these animals were main­tained in culture, the release of sulphated glycos­aminoglycans into culture media over 5 days was less in the drug-treated group than in the control group. In addition, the biosynthesis of proteo­glycans in the cartilage obtained from animals given active drug and placebo was similar. These data were interpreted to indicate that naproxen re­duced the normal turnover of cartilage proteo­glycans (Ratcliffe et al. 1993).

Some support for this view was obtained by quantification of the metalloproteinase activities in the same cartilages. Neutral metalloproteinase, collagenase and gelatinase activities were all less than in the corresponding control cartilages, but tissue inhibitor of metalloproteinase (TIMP) levels remained unaltered. From this study, it was sug­gested that naproxen might provide some protec­tive effect to cartilage in arthritic joints.

This conclusion was consistent with an earlier study by Ackerman et al. (1979), who examined the effects of naproxen on connective tissue changes in joints of rats with adjuvant arthritis. N aproxen 7 mg/kg was administered orally for 28 days, and

NSAIDs and Chondroprotection

markedly suppressed the int1ammatory response. However, bone and cartilage destruction was re­duced, as determined by radiographic and histolog­ical methods. It was concluded that this sparing effect on bone and cartilage resorption was medi­ated by the anti-prostaglandin effect of naproxen.

In a 3-month study involving patients with os­teoarthritis awaiting femoral head replacement surgery, it was found that oral administration of naproxen I g/day reduced the level of natural metalloproteinase (stromelysin) activity in their cartilage when examined after surgery. Further­more, proteoglycan biosynthesis, determined by glycosyltransferase activity, was unaffected by the dosage of drug used (Balblanc et al. 1993).

2.2 Ketoprofen

Investigations to determine the effects of ketoprofen on cartilage metabolism have been largely confined to in vitro studies.

Collier and Ghosh (1989, 1991) used immature lapine chondrocyte and explant cultures to study the effects of a variety of NSAIDs on the incorpo­ration of 35S into proteoglycans. While inhibition of proteoglycan synthesis could be shown at high concentrations of the drug (50 and 100 mg/L) using cell cultures, this did not occur in cartilage ex­plants. Moreover, a stimulatory effect of keto­profen 0.1 mg/L was exhibited on proteoglycan synthesis in explants at all time-points examined over the 8-day culture period (Collier & Ghosh 1991). These data suggest that diffusion of ketoprofen through the extracellular matrix, a nec­essary prerequisite to allow interaction with chondrocytes, was modified in explant cultures, the intracellular concentration of the drug being less than that achieved with monolayer cultures.

Wilbrink et al. (1991) studied the in vitro effects of ketoprofen on normal and osteoarthritic human cartilage explants. The normal tissue included both young and adult specimens using concentrations of 2 x 10-5 and 10-4 mollL. These were considered to correspond to drug synovial t1uid concentrations found clinically. The drug did not int1uence pro­teoglycan synthesis in osteoarthritic or normal

837

adult explant cultures, but at 10-4 mollL it stimu­lated synthesis in the immature human cartilages. This finding was therefore consistent with the re­port of Collier and Ghosh (1991) who used imma­ture lapine cartilage. Ketoprofen was found to have no effect, in vitro, on proteoglycanase or collagen­ase activity in human osteoarthritic cartilage when used at concentrations up to 2.8 mg/L (Vignon et al. 1991).

2.3 Piroxicam

The int1uence of piroxicam in long term cul­tures of human explants has been examined by Ver­bruggen and co-workers (1989a,b). Fibrillated young human cartilage released more newly-syn­thesised nonaggregatable proteoglycans into me­dia than intact cartilage, even though synthesis was higher, probably ret1ecting a high turnover rate. When piroxicam was added to the media at 6 mg/L (equivalent to serum concentration achieved after oral therapeutic dosage in humans), more 35S_la_ belled proteoglycans were retained in the matrix compared with non-drug-treated controls (Ver­bruggen et al. 1989a). This was probably due to inhibition of catabolism, since more proteoglycan aggregates were present after 6 weeks in culture.

At high concentrations (50 mg/L), piroxicam stimulated incorporation of 35S into proteoglycans after 4 to 6 days in lapine chondrocyte cultures, but not in explant cultures (Collier & Ghosh 1991). In a similar study, using monolayer chondrocytes de­rived from normal human femoral head cartilage, piroxicam in concentrations of up to 10 mg/L in­hibited cell proliferation and synthesis of pro­teoglycans (Bulstra et al. 1992). Although these effects were not evident with chondrocytes from moderately damaged osteoarthritic cartilage, cell proliferation from severely osteoarthritic joints was significantly inhibited by the drug.

This latter study contrasted with other reports (Brandt 1987; Dingle 1991; Verbruggen et al. 1989a,b). This discrepancy may be explained by differences in the species, donor age and the method used to culture the cartilage (Collier & Ghosh 1991).

838

Piroxicam, in contrast to indomethacin and as­pirin, was shown to suppress the production of ca­tabolism-inducing cytokines in human synovial tissues obtained from rheumatoid joints (Herman et al. 1987). Piroxicam, as well as tenoxicam, in­hibited proteoglycanase and collagenase activities in human osteoarthritic cartilage explants (Vignon et al. 1991). Hyaluronic acid synthesis in osteoar­thritis and rheumatoid arthritis synoviocytes in vitro was also stimulated by the drug at concentra­tions of 5 and 10 mg/L (Ghosh et al. 1990b).

Collectively, these studies suggest that pir­oxicam, when examined in vitro at concentrations comparable to those reached in serum or synovial fluid after oral use of the drug, had a variable effect on cartilage proteoglycan synthesis but appeared to offer some protection to these molecules by blocking their catabolism. This anti-catabolic ac­tivity of piroxicam may be mediated directly or indirectly but appears to be a property of the ox­icam class of NSAIDs.

2.4 Tenidap

Although not considered as an NSAID by its manufacturer on structural grounds, tenidap never­theless exhibits many pharmacological properties which would assign it to this classification. How­ever, the drug also possesses activities which are not characteristic of NSAIDs.

The effects of tenidap, naproxen and diclofenac on in vitro preservation of proteoglycan synthesis in cartilage explants exposed to human recombi­nant IL-l a have been compared. While tenidap ex­hibited some protection at 5 and 10 mg/L, diclo­fenac ~1 mg/L and naproxen ~100 mg/L did not (Dingle et al. 1993).

In this study tenidap was also shown to attenu­ate the IL-l a-mediated loss of sulphated pro­teoglycans from porcine cartilage explants over the concentration range 2.5 to 20 mg/L, in contrast to diclofenac and naproxen (Dingle et al. 1993). These findings were consistent with the decreased synthesis ofIL-l and its receptor proteins by activ­ated osteoarthritic synovial macrophages cultured in the presence of tenidap and piroxicam (Martel-

Drugs 46 (5) 1993

Pelletier et al. 1992; Pelletier & Martel-Pelletier 1992; Pelletier et al. 1993; Sipe et al. 1992). IL-l and IL-l receptor expression by normal and osteo­arthritic chondrocytes was also downregulated by tenidap at in vitro concentrations of 20 mg/L. This effect was accompanied by a reduction in the production of collagenase and stromelysin by chondrocytes when exposed to the drug (Martel­Pelletier et al. 1992; Pelletier & Martel-Pelletier 1992).

2.5 Tiaprofenic Acid

Recent in vitro studies on the effects of tiaprofe­nic acid on chondrocyte biosynthesis of proteo­glycans have indicated inhibition at high concen­trations (>50 mg/L) but not at levels comparable with synovial fluid concentrations achieved after oral administration of 600 mg/day in humans (5 to 10 mg/L) [Collier & Ghosh 1989, 1991].

Using a 3-dimensional culture system of clus­tered human chondrocytes as a model of cartilage repair, Bassleer et al. (1992) compared the effects of aspirin and tiaprofenic acid on cell proliferation, and production of proteoglycans, type II collagen and prostaglandin E2 (PGE2) at 5, 30 and 100 mg/L. While aspirin caused some inhibition of thy­midine incorporation into chondrocytes and de­pressed biosynthesis of proteoglycans, tiaprofenic acid had no effect over the concentration range used. Neither drug influenced the synthesis of type II collagen. However, as was to be expected, PGE2 release into media from chondrocyte cultures was markedly inhibited by tiaprofenic acid in a concen­tration-dependent manner.

Proteoglycan metabolism in explant cultures of human osteoarthritic cartilage has been determined in the presence of tiaprofenic acid, sodium salicy­late and hydrocortisone (Pelletier et al. 1989). Tiaprofenic acid and hydrocortisone had similar inhibitory effects on proteoglycan catabolism, largely due to reduced production of neutral metalloproteinases. Using the same culture sys­tem, proteoglycan biosynthesis was inhibited by salicylate and hydrocortisone but not by tiaprofen­ic acid at concentrations ~26 mg/L. This ability of

NSAIDs and Chondroprotection

tiaprofenic acid to block the catabolism of cartilage proteoglycans without concomitantly affecting their biosynthesis when tested in vitro at therapeu­tic concentrations was also demonstrated histolog­ically (Martel-Pelletier & Pelletier 1989). Signifi­cantly, the in vitro protective effect of tiaprofenic acid on cartilage proteoglycans could be abrogated by the addition of exogenous PGE2 or dibutyryl cyclic adenosine monophosphate (cAMP) to the cultures, clearly suggesting that the anti-catabolic effects of this drug were mediated via its ability to inhibit the synthesis of PGE2 by chondrocytes (Martel-Pelletier & Pelletier 1989).

Tiaprofenic acid was also shown to stimulate the in vitro synthesis of hyaluronic acid by syn­oviocytes derived from human osteoarthritic and rheumatoid joints (Ghosh et al. 1990b). The stim­ulation was concentration-dependent, the maximal effect occurring at 0.5 mglL. Diclofenac showed a similar effect in the osteoarthritis cell lines (Ghosh et al. 1990b).

Osteoarthritic and rheumatoid synovium has also been studied with respect to the effect of tiaprofenic acid on plasminogen activator produc­tion. Plasmin has been implicated in the destruc­tion of cartilage by its ability to activate latent metalloproteinases in rheumatoid synovium (Werb et al. 1977). Plasmin is derived from plasminogen by specific plasminogen activators which belong to the serine class of proteinases. Synovial mem­brane cultures synthesised plasminogen activators corresponding to the urokinase (uPA) and tissue (tPA)types which were present in culture media in both bound and free forms (Pelletier et al. 1992). While both uPA and tPA can convert plasminogen to plasmin, it has been suggested that uPA is the predominant activator in arthritic joints (Pelletier et al. 1992). At very high concentrations (260 mg/L) tiaprofenic acid inhibited production of uP A by synovium from osteoarthritic joints but not from patients with rheumatoid arthritis.

Tiaprofenic acid did not modulate the produc­tion of the inhibitor of uPA (PAl -2) in osteoarthritic tissues, but did in rheumatoid arthritic synovium. On the other hand the activity of PAI-1, the inhib-

839

itor of tPA, was decreased in osteoarthritic tissues by therapeutic concentrations of tiaprofenic acid (Pelletier et al. 1992).

These data indicate that tiaprofenic acid may be beneficial in reducing levels of activated metalloproteinases in osteoarthritic synovium. However, as fibrin deposition in subchondral bone capillaries and synovial tissues may contribute to joint disorders (Ghosh & Smith 1993), a drug-in­duced reduction in fibrinolysis could be counter­productive.

IL-1 stimulates the production of tPA by chondrocytes, and tPA is present in osteoarthritic cartilage (Martel-Pelletier et al. 1991). The ability of tiaprofenic acid to reduce the expression of this protein could contribute to its anti-catabolic effects by attenuating the activation of latent metallo­proteinases by plasmin. However, the question of whether the plasmin precursor, plasminogen, is normally present in articular cartilage to facilitate this process is presently unresolved.

Tiaprofenic acid 5 mg/kg/day administered orally for 8 weeks to normal dogs or dogs whose anterior cruciate ligament (ACL) had been tran­sected, or whose hind limbs had been immobilised (atrophy model) was reported to not provide any protective effects on cartilage (Brandt et al. 1990). In the non-drug-treated ACL-deficient group, mar­ginal osteophytes and pitting of the cartilage sur­face was evident. Chondrocyte cloning and mild surface fibrillation was also identified histologi­cally. In the ACL-deficient tiaprofenic acid-treated group, these joint abnormalities remained un­changed. Biochemical analysis of cartilage from the tiaprofenic acid and untreated ACL-deficient or atrophic groups showed no statistically significant difference compared with corresponding controls.

The ex vivo synthesis of 35S-proteoglycans was determined in the experimental and contralateral joint femoral condylar cartilages, and synthesis in the experimental cartilage was expressed as a per­centage of that in the contralateral joint. Under these conditions, the synthesis ratio decreased in the atrophic canine model but remained similar for the ACL-deficient animals. The corresponding

840

data for the tiaprofenic acid-treated animals was not presented, but it was reported that the drug had no effect on proteoglycan metabolism (Brandt et al. 1990).

These findings of Brandt and co-workers (1990) contrasted with similar studies of others using higher doses of tiaprofenic acid. Tiaprofenic acid 5 mg/kg administered orally twice daily to ACL­deficient dogs commencing 1 day after surgery re­duced the grade and size of cartilage lesions on the tibial plateau relative to untreated controls (Pellet­ier & Martel-Pelletier 1991). It was concluded that tiaprofenic acid, when used prophylactically, could reduce cartilage damage in traumatically induced osteoarthritis. In addition to improvement in the histological score of cartilage damage, ACL­deficient dogs given tiaprofenic acid 15 mg/kg/day orally also maintained higher levels of hyaluronic acid in articular cartilage (Howell et al. 1991). Os­teoarthritic cartilage is reported to be deficient in hyaluronic acid (Thonar et al. 1978) and, since this glycosaminoglycan is necessary for proteoglycan aggregation and retention in the matrix, mainte­nance of levels in cartilage in the tiaprofenic acid­treated animals could be considered to be benefi­cial to matrix integrity. Whether this preservation of cartilage hyaluronic acid levels by tiaprofenic acid was a result of inhibition of its degradation or stimulation of its synthesis is presently unknown.

Although proteoglycan biosynthesis in carti­lages of ACL-deficient joints was reported to be unaffected by tiaprofenic acid in the animal model used by Brandt et al. (1990), a stimulatory effect of this NSAID was observed using a sheep meniscectomy model of early osteoarthritis (Ghosh et al. 1993e).

Meniscectomy is a common orthopaedic proce­dure which is associated with a high incidence of cartilage degeneration and osteoarthritis (Appel 1970; Floman et al. 1980; Ghosh et al. 1983; John­son et al. 1974; Moskowitz et al. 1981). Medial meniscectomy in adult sheep results, after 6 months, in focal cartilage erosions, marginal osteo­phytes and subchondral bone change comparable to those seen in the early stages of osteoarthritis in

Drugs 46 (5) 1993

humans (Ghosh et al. 1990a, 1993a,b,c,d). When sheep were administered tiaprofenic acid 20 mg/kg/day orally for 12 weeks, 3 months after meniscectomy, and the biosynthesis of 35S_pro_ teoglycans in cartilage from the tibial plateau de­termined, a significant increase was found relative to the cartilage from the corresponding joint re­gions of meniscectomised animals given placebo (Ghosh et al. 1993e).

When synovial fluid from these animals was ex­amined after 3 months, levels of keratan sulphate epitopes, a marker of cartilage degradation in early osteoarthritis (Lohmander et al. 1989), were de­creased in the tiaprofenic acid group, compared with the placebo group (Ghosh et al. 1991).

Collectively, these experiments suggest that in early traumatic osteoarthritis, tiaprofenic acid not only reduced the catabolism of cartilage proteo­glycans, but also preserved their biosynthesis. Since Brandt et al. (1990) used tiaprofenic acid dosages of3.3 to 4 mg/kg/day, whereas in the same model Pelletier et al. (1991) used 10 mg/kg/day and Howell et al. (1991) 15 mg/kg/day with in­creasing benefit, it could be reasonably assumed that the higher the dose of tiaprofenic acid used, the better the chondroprotective effect.

However, studies using the atrophy model of osteoarthritis in rabbits have shown that this was not the case. In this model, the rabbit hind limb is immobilised in extension for 4 weeks, after which cartilage resorption, manifested by loss of pro­teoglycans and a deficiency in their aggregation, occurs (Langenskiold et al. 1979). Using this model, tiaprofenic acid 2.5, 5 and 10 mg/kg was administered subcutaneously every 48 hours for 4 weeks (Meyer-Carrive & Ghosh 1992). While a protective effect of tiaprofenic acid on the loss of proteoglycans from cartilage of immobilised joints was demonstrated at 5 mg/kg, this loss was exac­erbated when the 10 mg/kg dose was used (Meyer­Carrive & Ghosh 1992). Since tiaprofenic acid was known to be a potent inhibitor of prostaglandin synthesis (Deraedt et al. 1982) and it had been demonstrated that arachidonic acid metabolites regulate IL-l production (Knudson et al. 1986;

NSAIDs and Chondroprotection

Kunkel & Chensue 1985), it was hypothesised that the high subcutaneous dosage of tiaprofenic acid used in the aforementioned rabbit atrophy experi­ment could be elevating cartilage IL-l levels due to the suppression of prostaglandin production.

In a subsequent pharmacokinetic study of tia­profenic acid distribution in serum and synovial fluid of immobilised rabbit joints, it was found that with the 10 mg/kg subcutaneous dosage the peak concentration of the drug reached in synovial fluid was 2 to 3 times that which could be achieved in human synovial fluid after taking the recom­mended oral dosage of 600mg (Meyer-Carrive et al. 1993). Clearly, therefore, the daily subcutane­ous dosage of 10 mg/kg of tiaprofenic acid used in the rabbit experiments was too high and was not clinically relevant.

2.6 Misoprostol

Misoprostol, a synthetic analogue of PGEI modulates IL-l production by chondrocytes ex­posed to NSAIDs, as shown by its ability to restore proteoglycan synthesis in vitro in a concentration­dependent manner (Dingle 1991). A study was therefore undertaken using the rabbit atrophy arth­ritis model to ascertain if coadministration of miso­prostol with high dose tiaprofenic acid (10 mg/kg) could reverse the deleterious effects of tiaprofenic acid on cartilage proteoglycan metabolism (Ghosh et al. 1993f).

Misoprostol was originally developed for the treatment of peptic ulcers, but more specifically for the prevention and treatment of NSAID-induced gastrointestinal damage (Fenn & Robinson 1991). However, it was also found that the protective ac­tivity of the drug could be beneficial in other tis­sues, including cartilage (Shield 1992).

As already discussed, cytokines such as IL-l and tumour necrosis factor-a. (TNFa.), released by activated synoviocytes in arthritic joints, not only promote resorption of the cartilage matrix by stim­ulating chondrocyte production of metallo­proteinases and plasminogen activators, but can also suppress the synthesis of proteoglycans by these cells. The latter effect occurs at concentra-

841

tions of IL-l several orders of magnitude less than that required to cause proteoglycan degradation in cartilage (Dingle 1991; Shield 1992).

The ability of NSAIDs to effectively close down prostaglandin production and thereby in­crease tissue IL-l levels in arthritic joints has been proposed as a possible mechanism for the inhibi­tory effects of certain NSAIDs on proteoglycan synthesis in cartilage (Dingle 1991; Shield 1992).

In vitro experiments reported by Brandt et al. (1991), using normal canine cartilage cultured for 24 hours, failed to demonstrate a protective effect of misoprostol on the inhibitory effects of sodium salicylate or aspirin on proteoglycan biosynthesis. However, in similar studies using porcine or hu­man cartilage explants cultured for 4 to 8 days, a concentration-dependent protective effect of misoprostol was observed on proteoglycan synthe­sis in the presence of inhibitory concentrations of NSAIDs (Dingle 1991; Shield 1992).

In vitro cultures of human osteoarthritic carti­lage with misoprostol in the absence of NSAIDs also indicated a direct stimulatory effect on pro­teoglycan synthesis when the drug was used at con­centrations of 10 to 100 IlglL (Shield 1992). This activity was confirmed by Savineau et al. (1993), who reported that type II collagen synthesis was also increased on exposing osteoarthritic cartilage to the drug in culture.

We have used the rabbit atrophy model of arthri­tis to determine the in vivo protective effects of misoprostol on cartilage proteoglycan metabolism and joint inflammation (Ghosh et al. 1993f). Rab­bits with immobilised joints were administered misoprostol 150 Ilg/kg transdermally alone or in combination with daily subcutaneous injections of tiaprofenic acid 10 mg/kg. As previously noted (Meyer-Carrive & Ghosh 1992), the high subcuta­neous dose oftiaprofenic acid exacerbated the loss of proteoglycans from cartilage of immobilised joints, but in the group coadministered miso­prostol, cartilage proteoglycan levels were main­tained to within control values (Ghosh et al. 1993f). While misoprostol alone modified the numbers and types of neutrophils entering the im-

842

mobilised joints as well as reducing PGE2 activity, it failed to restore cartilage proteoglycans to con­trollevels (Ghosh et al. 1993f).

3. Conclusions

In this overview, experimental data have been assembled which provide some support for the contention that certain NSAIDs may be chondro­protective. However, the issue is clouded by the variety of in vitro and in vivo experimental systems used to study these drugs. This renders direct com­parisons between NSAIDs difficult.

Notwithstanding these problems, it seems clear that certain NSAIDs afford protection to cartilage by virtue of their potent inhibitory activities against the cyclo-oxygenase enzyme systems. By suppressing the conversion of arachidonic acid to prostaglandins in inflamed joints, chondrocyte production of stromelysin and collagenase may be attenuated and the concomitant proteolytic break­down of the cartilage matrix reduced. There is no evidence that NSAIDs used at physiological con­centrations could inhibit metalloproteinases or serine proteinase released by activated neutrophils directly.

Cytokines derived from migrating neutrophils, particularly IL-1 and TNFa., play central roles in connective tissue destruction in inflamed joints. Apart from stimulating metalloproteinase produc­tion, IL-1 at concentrations < 1 IlglL can inhibit the synthesis of proteoglycans by chondrocytes (Din­gle 1991; Shield 1992). There is in vitro evidence that piroxicam and tenidap can reduce the expres­sion of IL-l by macrophages, and that naproxen may downregulate tPe cell receptor for this cyto­kine (see section 2). Funhermore, tenidap is dis­tinct from other NSAIDs in that it exhibits inhibi­tory activity against the lipoxygenase enzymes (Blackburn et al. 1990; Carty et al. 1988), reduces neutrophil release of collagenase (Blackburn et al. 1991), and inhibits bone resorption (AI-Humidan et al. 1991).

Misoprostol, while not complying with the NSAID rubric on structural grounds, is now emerging as a molecule with an interesting phar-

Drugs 46 (5) 1993

macological profile which includes a protective ef­fect on chondrocyte anabolic activities.

In principle, combinations of misoprostol with NSAIDs should allow the powerful analgesic and anti-inflammatory properties of NSAIDs to be ex­pressed, while moderating their potential damag­ing effects on mucosal, kidney or cartilaginous tis­sues. Studies cited in this review provide some support for this concept.

In terms of cartilage metabolism, tiaprofenic acid has been the most thoroughly investigated NSAID. These studies have, for the most part, in­dicated that tiaprofenic acid is benign to cartilage when administered at dosages equivalent to those recommended for use in humans.

The advantages in using animal models over hu­man volunteers to evaluate the chondroprotective properties of NSAIDs are obvious. However, hu­mans represent the primary target species for NSAIDs and the effects of these drugs on human cartilage must be considered. Unfortunately, the current clinical methods of assessing patient re­sponses to drug treatment are incapable of identi­fying any improvements in cartilage integrity. Tra­ditional radiological methods of determining joint space changes are insensitive and have low repro­ducibility. On the other hand, high definition microfocal radiography can quantify small changes in joint-space width (JSW) [Buckland­Wright et al. 1990]. Using this technique, diclo­fenac was found to decrease JSW for 12 months, but to increase this parameter at 18 months in a group of osteoarthritis patients with initial JSW of <50% (Buckland-Wright et al. 1992).

Magnetic resonance imaging (MRI) offers con­siderable potential as a means of detecting early cartilage changes in osteoarthritis (Braunstein et al. 1990; Gluckert et al. 1990; Hodgeson et al. 1992). However, the use of this expensive tech­nique to identify changes in cartilage structure in osteoarthritic patients before and after NSAID treatment has not been reported.

Real-time ultrasound (sonography, echogra­phy) has been reported to be useful for the assess­ment of the status of cartilage in osteoarthritic knee

NSAIDs and Chondroprotection

joints (Ai sen et al. 1984; Patella et al. 1993). This technique has not been used to evaluate the effects of NSAIDs on cartilage, but chondroitin sulphate administered orally for 12 months to patients with osteoarthritis under triple-blind conditions was re­ported to improve cartilage integrity as assessed by echography (Pipitone et al. 1992).

There has also been considerable interest in the use of biochemical markers released into synovial fluid and serum as a means of assessing the extent of cartilage and bone breakdown in arthritic joints. Synovial fluid would appear to be the most reliable source of these epitopes. Proteoglycan degradation products, oligomeric matrix protein (COMP) and type II procollagen peptides (pColl-II-c) in syno­vial fluid have been found to be useful markers of cartilage damage in arthritic joints (Lohmander et al. 1989; Saxne et al. 1993; Saxne & Heinegard 1992; Shinmei et al. 1992). More recently, stromelysin and TIMP, possibly originating from the synovial tissue, were reported to be sensitive markers of cartilage breakdown in post-traumatic osteoarthritis, primary osteoarthritis and pyro­phosphate arthritis (Lohmander et al. 1993; Shinmei et al. 1992). Once again, however, inves­tigations with NSAIDs have not been reported, even though the effects of intra-articular hyaluro­nic acid (Shinmei et al. 1992) and corticosteroid therapy on these markers have been investigated (Saxne et al. 1987; Shinmei et al. 1992).

Objective data on the response of human carti­lage to NSAID therapy is thus conspicuous by its absence, although numerous laboratory and animal studies would suggest that such investigations may be worthwhile. This could be considered to be an oversight by the pharmaceutical industry in failing to support drugs whose use in osteoarthritis is now being strongly criticised.

We are of the opinion that this is not an over­sight, the issue in reality being more complex. Firstly, there are inherent technical difficulties as­sociated with monitoring patient response to drug or placebo treatments in the long term trials (up to 5 years) required to demonstrate any changes in cartilage integrity. Apart from the high costs of

843

such investigations, patient noncompliance and withdrawal would be unacceptably high, thereby invalidating the study. Secondly, for most of the currently used NSAIDs there is little or no remain­ing patent life. As it is inevitable that generics will eventually replace, or greatly undermine, the sales of the proprietary NSAIDs currently used, it is not surprising that there has been little enthusiasm for such research. Perhaps the concept of chondro­protection arrived too late for many NSAIDs.

Today, however, disease modification and chondroprotection (cytoprotection) are essential requirements for the clear passage of a new anti­arthritic drug from the laboratory to the clinic. While this enlightened approach to drug research has been slow to develop, it will undoubtedly lead to the availability of safer and more efficacious anti-arthritic drugs in the future.

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Correspondence and reprints: Associate Professor Dr Peter Ghosh, Director, Raymond Purves Bone & Joint Research Laboratories, Royal North Shore Hospital of Sydney, St Leonards, NSW 2065, Australia.