Celecoxib dose-dependently inhibits glycosaminoglycan release and stimulates proteoglycan synthesis in healthy human articular cartilage explants when exposed to peripheral blood mononuclear cells from rheumatoid arthritis patients or IL-1β and TNF-α 19 . The fact that the decreased proteoglycan synthesis induced by IL-1β and TNF-α is reversed by celecoxib indicates that this drug can also exert its effects directly on activated cartilage. Furthermore, in OA cartilage explants, celecoxib stimulated proteoglycan synthesis and retention of newly formed proteoglycans 20 21 22 . The non-selective COX inhibitors diclofenac and naproxen did not affect proteoglycan turnover in OA cartilage, and indomethacin and an experimental COX-1 selective inhibitor (SC-560) had adverse effects 20 21 . This difference in NSAID effects supports COX-2 involvement in catabolic activity regulation in cartilage, whereas COX-1 activity might have a more physiological role in chondrocytes.

No effect of celecoxib on proteoglycan turnover was observed in healthy cartilage 19 22 . This is in contrast to the protective in vitro effect of celecoxib on end-stage OA cartilage obtained at joint replacement surgery. For the treatment of OA in clinical practice, it would be beneficial if celecoxib could influence proteoglycan turnover in earlier stages of disease. It was shown that in both degenerated (pre-clinical) and late-stage OA cartilage, celecoxib not only stimulated proteoglycan synthesis and retention of newly formed proteoglycans, but also had favorable effects on proteoglycan content. Importantly, proteoglycan content in degenerated cartilage normalized in vitro during celecoxib treatment, suggesting celecoxib treatment in the early stages of OA could slow down or even reverse the destructive process 22 .

Whereas the in vitro effects of celecoxib on OA cartilage are beneficial, results obtained with isolated chondrocytes are not consistent. In a mechanically stretched monolayer of chondrocytes, celecoxib had a positive effect on aggrecan expression and reduced the release of chondroitin sulfate 23 . In contrast, celecoxib had no positive effect on proteoglycan turnover of osteoarthritic chondrocytes cultured in alginate beads 24 , of a monolayer of chondrocytes 25 , nor in an in vitro model of post-traumatic OA 26 . This variation in the effects of celecoxib could potentially be due to differences in chondrocyte culture models, whereas cartilage explants probably better reflect the in vivo situation.

A possible way in which celecoxib exerts its effect on proteoglycan turnover is inhibition of PGE₂ production. PGE₂ is highly expressed in OA cartilage and studies indicate a pivotal role for PGE₂ in OA cartilage metabolism 27 . Expression of PGE₂ and COX-2 in OA cartilage is strongly inhibited by celecoxib 10 21 22 28 29 . PGE₂ enhances IL-1β-/TNF-α-induced proteoglycan release, resulting in decreased proteoglycan content in cartilage explants 28 . The effect of PGE₂ on the synthesis of proteoglycans remains controversial; in OA cartilage, proteoglycan synthesis is inhibited by PGE₂ 17 , whereas PGE₂ does not affect proteoglycan synthesis rate in healthy cartilage 28 . This discrepancy could be due to differences in expression levels of individual members of the EP receptor family (EP1 to EP4) through which PGE₂ exerts its effects. EP4 has been implicated in mediating catabolic effects because it is highly expressed in OA cartilage 17 . IL-1-induced expression of EP4 in cultured OA chondrocytes is decreased by celecoxib 29 , but not consistently 17 . The overall negative effect of PGE₂ on proteoglycan turnover in cartilage might be mediated through the EP4 receptor (Figure 1).

PGE₂ inhibits collagen synthesis and stimulates expression of MMP and ADAMTS-5, proteolytic enzymes involved in the degradation of collagens and proteoglycans 17 30 31 . Theoretically, celecoxib could also prevent cartilage destruction by inhibiting induction of MMP expression in OA cartilage. Both inhibitory and stimulatory effects of celecoxib on IL-1-induced expression of MMP-13 in OA chondrocytes have been reported 10 17 . Also, there is no agreement on the effect of celecoxib on MMP-1 expression in cartilage 10 25 32 . Celecoxib reverses IL-1β-induced ADAMTS-5 expression in OA cartilage explants 17 . As such, it could prevent enhanced proteoglycan turnover in OA by affecting both MMP and ADAMTS-5 expression. But our understanding of the influence of celecoxib on PGE₂-induced cartilage catabolism is clearly far from complete and it would be worthwhile to explore this role in more detail.

NO plays an important role in cartilage destruction in OA - for example, by inhibiting matrix synthesis, activating MMPs, and inducing chondrocyte apoptosis 33 34 35 . Because NO is an attractive target in OA treatment, several studies have addressed the question of whether celecoxib influences NO production, although little agreement has been reached. Several studies found inhibitory effects of celecoxib on NO production in chondrocytes 25 32 36 , whereas others did not 28 37 . These contradictory effects are potentially due to differences in culture models, treatment duration, and celecoxib concentration used.

In articular chondrocytes, NO production is regulated by NF-κB, JunNH₂-terminal kinase (JNK) and p38 32 38 . Celecoxib was shown to suppress NO production by inactivating JNK and NF-κB 32 . An inhibitory effect of celecoxib on NF-κB signaling in OA chondrocytes was reported previously 10 . NF-κB has an essential role in OA pathogenesis, being involved in cytokine stimulation, MMP and ADAMTS expression, and diminished secretion of extracellular matrix proteins by chondrocytes. Inhibition of NF-κB could potentially be beneficial in OA treatment. Interestingly, it was reported that celecoxib reduces expression of IL-1 37 and IL-6 24 , both inflammatory cytokines involved in OA pathogenesis 39 . It is currently unknown how celecoxib mediates its effects on cytokine expression and NF-κB activity.

Celecoxib induced apoptosis in a dose-dependent manner in chondrocytes derived from cartilage from patients with OA 25 , although reduced apoptosis via COX inhibition by celecoxib has also been reported 26 .

In general, celecoxib has favorable effects on cartilage destruction in vitro, thereby theoretically slowing down disease progress in vivo (Figure 1).

Although originally viewed as a non-inflammatory arthropathy, a pivotal role of synovial inflammation in OA progression is now recognized. Imaging studies have shown synovium changes in early and late OA 40 . Histologically, synovium from OA patients shows hyperplasia, increased lining layer thickness, blood vessel formation and mononuclear cell infiltration, mainly consisting of macrophage like cells. IL-1β and TNF-α levels are increased in OA synoviocytes, potentially contributing to disease progression by activating chondrocytes and synovial fibroblasts 41 42 . Enhanced PGE₂ and COX-2 expression in synovial fluid and synovial membrane have been observed 43 44 . Several effects of celecoxib on synovium, with a focus on fibroblasts, have been described. Celecoxib reversed IL-1β-induced PGE₂ and COX-2 protein expression in synovial fibroblasts. Furthermore, celecoxib inhibited IL-1β-induced activation of NF-κB in synovial fibroblasts from OA patients 10 . NF-κB induces expression of large numbers of inflammatory mediators and plays a major role in the initiation and maintenance of synovitis, synovial hyperplasia, and inhibition of synovial apoptosis in rheumatoid arthritis. Although less is known concerning the function of NF-κB in osteoarthritic synovium, it is clear that celecoxib could reduce expression of various inflammatory mediators by down-regulation of NF-κB 45 .

Among the downstream factors of NF-κB are MMPs, which play a crucial role in cartilage degradation in OA. Both MMP-1 and MMP-13 levels are enhanced in OA; MMP-1 is predominantly released by synovial cells, and MMP-13 is highly expressed by chondrocytes 46 . MMP-2 and MMP-9 are also elevated in the osteoarthritic joint. MMP-2 expression is regulated by COX-2. Several NSAIDs, including celecoxib, inhibit MMP-2 secretion in OA synovial fibroblast cultures 47 . Furthermore, celecoxib can decrease the expression of MMP-9 and urokinase-type plasminogen activator (u-PA) and its inhibitor PAI 47 . Alterations in u-PA and PAI expression have been found in osteoarthritic tissue and contribute to a disturbed proteolytic balance 48 .

It was shown that celecoxib, but no other selective COX-2 inhibitors, enhances MMP-1 and MMP-13 protein expression in IL-1β-stimulated synoviocytes 10 49 . This observation does not corroborate the inhibitory effect of celecoxib on MMP-1 expression in rheumatoid arthritis synoviocytes 50 . This discrepancy could be due to different concentrations used, celecoxib being stimulatory at low concentrations (0.5 to 1 μM) and inhibitory at higher concentrations (5 to 10 μM). Evidently, a stimulatory effect of celecoxib on synovial MMP-1 and MMP-13 expression could be detrimental in OA treatment 10 . In conclusion, celecoxib influences the balance of proteolytic enzymes in OA synovium, and although this appears to be generally beneficial (reducing expression of MMP-2, MMP-9, and uPA), adverse effects have been reported as well (increasing expression of MMP-1 and MMP-13).

Recently, it was shown that celecoxib dose-dependently inhibits proliferation and induces apoptosis in synovial fibroblasts obtained from OA patients 51 52 . This is in agreement with findings in rheumatoid arthritis 53 54 55 . Remarkably, various other COX-2 selective inhibitors, including nimesulide and rofecoxib, did not induce apoptosis of synovial fibroblasts, indicating that celecoxib stimulates apoptosis in a COX-2-independent manner 51 . In cancer cells celecoxib has been shown to modulate apoptosis pathways by inhibiting anti-apoptotic proteins, elevating Ca²⁺ concentration and altering NF-kB signaling (reviewed in 56 ). Although the exact pro-apoptotic mechanism of celecoxib in synovial tissue remains to be established, it is evident that anti-proliferative and pro-apoptotic effects of celecoxib on synovium are beneficial in reducing synovial hyperplasia and potentially slow down synovitis-mediated OA disease progress.

Taken together, celecoxib modulates several pathogenic mechanisms of synovial cells that are not always affected by other NSAIDs, suggesting that celecoxib may have additional, COX-2-independent value in the treatment of OA (Figure 2).

Subchondral bone sclerosis and osteophyte formation are radiographic hallmarks of end-stage OA. Several studies suggest that bone remodeling in OA is biphasic: an early decrease in trabecular bone formation, followed by an increase in subchondral bone density and stiffness 57 58 . The initial thinning of the subchondral plate coincides with changes in articular cartilage, suggesting a pivotal role for the cartilage and subchondral bone interaction in OA progression. In established OA, the increased subchondral bone stiffness probably contributes to further cartilage degeneration 59 .

Osteoclasts play a pivotal role in the destruction of subchondral bone 4 14 59 . Osteoclastogenesis and activation of mature osteoclasts are critically regulated by the receptor activator of NF-κB ligand (RANKL). RANKL mediates its function by binding to its cell-surface receptor RANK on osteoclast precursor cells and osteoclasts, thus stimulating differentiation and activation of osteoclasts. It is mainly expressed by osteoblasts and stromal cells, where expression of RANKL is COX-2-dependent 60 . During inflammation RANKL is also produced by T lymphocytes and fibroblast-like synoviocytes. Osteoprotegerin (OPG), a soluble decoy receptor for RANKL, can prevent the biological effects of RANKL, and the ratio between OPG and RANKL determines whether the balance is in favor of bone resorption or bone formation 61 62 . Interestingly, two osteoblast subpopulations were identified in OA, one with a low OPG/RANKL ratio that favors bone resorption, and one with a high OPG/RANKL ratio that promotes bone formation 61 63 . Inhibition of COX-2 by NSAIDs diminishes RANKL production by osteoblasts, and since RANKL is an important inducer of osteoclastogenesis, celecoxib inhibited osteoclast differentiation in co-cultures of osteoblasts and bone marrow-derived cells 12 64 . Besides affecting osteoclastogenesis indirectly through its effect on osteoblasts, celecoxib also directly influenced osteoclast precursor cells by inhibiting COX-2 expression. Adding celecoxib to bone marrow-derived monocyte/macrophage cells, in the absence of stromal cells, suppresses RANKL-induced osteoclast differentiation 65 66 . This celecoxib effect was reversed by PGE₂, indicating that RANKL-induced COX-2 and PGE₂ expression in osteoclast precursors is critically involved in osteoclastogenesis 65 (Figure 3).

Besides inhibiting osteoclast differentiation, celecoxib is able to almost completely inhibit the activity of human osteoclasts 66 . Slightly lesser effects were observed with indomethacin, and no effects were seen with a selective COX-1 inhibitor, suggesting a COX-2-dependent pathway is involved 66 . However, other mechanisms might be involved in inhibiting osteoclast activity as well. Celecoxib, as well as other sulfonamide-type COX-2 inhibitors, contain an aryl sulfonamide moiety that inhibits carbonic anhydrase II 67 . Abundantly expressed on the inner surface of osteoclasts, carbonic anhydrase II catalyzes conversion of CO₂ and H₂O into bicarbonate and H⁺. Acidification in the resorption pit is required for dissolution of the inorganic matrix of bone 68 . Treatment with celecoxib reduced carbonic anhydrase activity and thereby inhibited osteoclast activity, an effect not observed for COX-inhibitors without this sulfonamide moiety 12 .

Recently, it was found that human chondrocytes express OPG, RANKL and RANK 61 69 . Interestingly, the OPG/RANKL ratio is significantly lower in OA chondrocytes compared to healthy chondrocytes 70 . This shift in OPG/RANKL ratio is mediated by PGE₂ 69 71 , and inhibition of PGE₂ production by celecoxib resulted in a higher OPG/RANKL ratio 71 72 . It was shown that RANKL produced by chondrocytes can stimulate osteoclastogenesis 73 74 and, furthermore, as a chemoattractant for peripheral blood monocytes, it could attract osteoclast precursor cells to the joint 75 . Inhibition of chondrocyte RANKL expression by celecoxib might thus prevent subchondral bone loss (Figure 3).

In vitro experiments have shown a cartilage-sparing effect of celecoxib in OA cartilage; however, in vivo data, from either human or animals, are scarce. Contrary to its positive effects on cartilage degeneration in vitro, no chondroprotective effect of celecoxib in the canine groove model of OA was observed 76 . Although PGE₂ levels in the joint were inhibited, celecoxib did not improve cartilage histopathology or proteoglycan turnover. This lack of chondroprotective effect might have been due to increased loading of the joint in the celecoxib-treated group compared to the placebo-treated group, where no analgesics were given 76 . Conversely, celecoxib was shown to reduce cartilage damage in collagen-induced osteoarthritis in rabbits; histopathological evaluation showed less cartilage erosion, reduced cartilage fibrillation and decreased loss of chondrocytes. Proteoglycan content, determined by Safranin-O staining intensity, was higher than in the placebo-treated group 77 . Next to the direct effects of celecoxib, the anti-inflammatory effects of celecoxib may have caused this chondroprotective effect as the model depends on inflammation and the number of inflammatory cells and the PGE₂ concentration in synovial fluid was significantly reduced by celecoxib.

Few studies have described the in vivo effects of celecoxib on cartilage destruction in OA patients 37 44 78 79 80 . However, these studies generally have limitations with respect to their small size and short duration. A way to study drug effects is to treat patients with severe knee OA waiting for joint replacement surgery and analyze the cartilage ex vivo. In this manner, a beneficial effect of celecoxib on cartilage degradation after 4 weeks of treatment was observed 78 . Although no differences in the histopathological Mankin score were observed, proteoglycan synthesis rate and retention of newly formed proteoglycans was significantly increased in celecoxib-treated OA patients compared to indomethacin-treated or untreated patients. The expression of key players in the destructive process, NO and PGE₂, was inhibited by both celecoxib and indomethacin 78 . Hence, differences in cartilage proteoglycan turnover between celecoxib- and indomethacin-treated patients could result from specific effects of indomethacin-induced COX-1 inhibition on cartilage 20 21 , or from COX-2-independent actions of celecoxib 13 . Using a similar approach, long-term (3 months) effects of celecoxib and aceclofenac were studied in OA patients 37 . It was demonstrated that expression of COX-2, microsomal prostaglandin E synthase-1 (mPGES-1) and inducible NO synthase, an enzyme involved in NO generation, was strongly reduced in both celecoxib- and aceclofenac-treated patients. Only celecoxib was shown to inhibit expression of the PGE₂ receptors EP2 and EP4, as well as TNF-α and IL-1β, in articular cartilage. A positive correlation exists between TNF-α/IL-1β levels and cartilage damage 81 , suggesting a chondroprotective effect of celecoxib in vivo.

The effects of celecoxib treatment on disease progression are more ambiguous 79 80 82 . In an observational study, conventional NSAID use was associated with enhanced cartilage destruction compared to selective COX-2 inhibitors. Furthermore, the COX-2 inhibitors rofecoxib and celecoxib showed beneficial effects on tibial cartilage defects in knee OA compared to no medication 82 . Recently, the effect of celecoxib treatment (200 mg daily, 12 months) on cartilage volume loss was studied compared to a historical cohort of patients receiving standard care 79 . Using quantitative magnetic resonance imaging, no protective celecoxib effect on knee cartilage was found. Only one randomized controlled trial has addressed the effects of celecoxib on cartilage degeneration 80 . Patients who met radio-graphic criteria grade 2 and 3 (Kellgren and Lawrence) were blinded and given celecoxib, chondroitin sulfate, glucosamine or placebo. Unexpectedly, no differences in joint space narrowing (measured radiographically) or disease progression between celecoxib- and placebo-treated groups were observed after 2 years follow-up 80 . Less than anticipated loss of joint space width in the placebo-treated group hampered the study and prevented a strong conclusion. Moreover, the results found in these studies were obtained in an un-controlled trial set-up and, as such, could be affected by the selection of patients. Also, the numbers of patients used in most studies is rather limited.

Figure 4 summarizes the suggested in vivo effects of celecoxib. The beneficial in vitro effects and the some-what controversial in vivo effects on cartilage, mostly based upon weak evidence, clearly indicate the requirement for properly designed randomized controlled trials on the potential disease-modifying osteoarthritic drug effects of celecoxib.

Celecoxib has been shown to reduce synovitis, leukocyte infiltration and synovial hyperplasia in different arthritis animal models 83 84 85 . In the synovium of severe knee OA patients, inhibitory effects of celecoxib on IL-1β and TNF-α expression have been demonstrated 44 78 . Furthermore, celecoxib reduced IL-6 concentrations in the synovial fluid of patients with moderately severe OA after 2 weeks of treatment 86 . Interestingly, aceclofenac and indomethacin had no or only moderate effects on cytokine expression in these studies 44 78 .

Reduction of pro-inflammatory cytokines in synovial fluid by celecoxib could be the result of decreased production by chondrocytes, as has been shown in vitro 24 . However, synovial macrophages are also an important source of pro-inflammatory cytokines 42 . Ex vivo analysis of OA synovium after in vivo celecoxib treatment showed a significant reduction in synovial macrophage numbers, which was not observed for aceclofenac 44 . This macrophage depletion might be due to enhanced apoptosis in response to celecoxib, which has a pro-apoptotic effect on synoviocytes and macrophages 51 53 55 . Decreasing macrophage numbers would result in lower pro-inflammatory mediator levels in synovial fluid. Only one study has addressed the influence of celecoxib on MMP activity in synovial tissue; despite controversial results on MMP activity in synoviocytes in vitro, no celecoxib effect on MMP activity was demonstrated in vivo 78 .

In conclusion, under certain conditions pro-inflammatory cytokines play a crucial role in OA pathogenesis by inhibiting proteoglycan synthesis, inducing chondrocyte apoptosis and activating other cells. Preventing enhanced production of these inflammatory mediators by celecoxib will likely slow disease processes. Several lines of evidence indicate that synovial changes can be among the first to occur in OA (reviewed in 87 ), suggesting early treatment could slow or maybe prevent joint damage. As little research has focused on the effects of celecoxib on synovial tissue, further research should elucidate the effects of celecoxib in disease progression.

Various studies have shown a beneficial effect of celecoxib on bone in vivo 12 88 89 90 . Celecoxib, but not other NSAIDs, reduced bone mineral density loss 12 88 90 and enhanced trabecular bone volume in adjuvant- and collagen-induced arthritis in rats 88 89 90 . The increased trabecular bone volume correlated with reduced serum type I collagen C-telopeptide, a bone resorption marker representing osteoclast activity 12 88 , and other bone resorption parameters 89 . Whereas celecoxib did not affect bone formation, it suppressed osteoclast numbers in tibia of arthritic animals 89 90 . These celecoxib effects were partly mediated by RANKL, as celecoxib decreased expression of RANKL in synovial tissue, bone marrow cells and cartilage in vivo 71 89 . As shown in vitro, celecoxib inhibited both osteoclastogenesis and osteoclast activation, thereby directly diminishing bone destruction.

Despite celecoxib being used for treatment of OA for many years, no effects of it on serum markers of bone resorption and formation or on structural changes in bone have been reported. As celecoxib has beneficial effects on bone resorption in vitro and in vivo in animal models, it would be interesting to explore these effects on bone metabolism in OA patients in more detail.