Drugs in development: bisphosphonates and metalloproteinase inhibitors

Inflammatory biomarkers and state of the tibiofemoral joint in the osteoarthritic knee: a narrative review

Background

The healing process is initiated by injurious stimuli in response to cellular damage. Upon recruiting proinflammatory biomarkers to the tissue site of injury, the release of additional biomarkers occurs, including the likes of cytokines, matrix molecules, macrophages, neutrophils, and others. This influx of immune system mediators can occur for chronic periods, and though its intention is for healing the original injurious stimuli, it is also suspected of causing long term cartilage impairment following internal structure damage. The objective of this narrative review is to identify which inflammatory factors have the leading roles in the progression of osteoarthritis (OA) following knee injuries and how they fluctuate throughout the healing process, both acutely and chronically.

Methods

This narrative review was performed following a computerized search of the electronic database on PubMed in May 2023. Abstracts related to the inflammatory biomarkers of the post-traumatic knee were included for review.

Key Content and Findings

The chronic low-level inflammation that leads to OA leads to the destruction of the cartilage extracellular matrix, which new and developing orthopedic research is still attempting to find resolve for. Some of this damage is attributed to the biomechanical alterations that occurs following injury, though with most procedures capable of joint biomechanical restoration, focus has rather been shifted toward the environment of inflammatory biomarkers.

Conclusions

Future studies will be aiming to improve the diagnostics of OA, focusing on a consistent correlation of inflammatory biomarkers with imaging. Additionally, biochemical treatments will need to focus on validating reproducible modulation of signaling molecules, in attempts to lessen the chronic elevations of destructive biomarkers.

Matrix Metalloproteinases as Biomarkers and Treatment Targets in Mesothelioma: A Systematic Review

Metalloproteinases (MMPs) have an important role in tissue remodeling and have been shown to have an effect on tumor progression, invasion, metastasis formation, and apoptosis in several tumors, including mesothelioma. Mesothelioma is a rare tumor arising from pleura and peritoneum and is frequently associated with asbestos exposure. We have performed a systematic search of PubMed.gov and ClinicalTrials.gov databases to retrieve and review three groups of studies: studies of MMPs expression in tumor tissue or body fluids in patients with mesothelioma, studies of MMPs genetic variability, and studies of MMPs as potential novel drug targets in mesothelioma. Several studies of MMPs in mesothelioma tissues reported a link between higher expression levels of commonly studied MMPs and clinical parameters, such as overall survival. Fewer studies have investigated genetic variability of MMP genes. Nevertheless, these studies suggested that certain genetic variants in MMP genes can have either protective or tumor-promoting effects on mesothelioma patients. MMPs have been also reported as novel drug targets, but so far no clinical trials of MMP inhibitors are registered in mesothelioma. In conclusion, MMPs play an important role in mesothelioma, but further studies are needed to elucidate the potentials of MMPs as biomarkers and drug targets in mesothelioma.

Calciphylaxis: Successful Management of a Rare Complication of Chronic Kidney Disease in Two Patients

Calciphylaxis, or calcification uremic arteriolopathy, is a rare disease thought to occur due to arteriolar calcifications of the dermis and is responsible for ischemia with cutaneous necrosis and painful panniculitis. Its mechanism remains poorly understood which makes its management challenging and difficult to standardize. We report our management of two patients diagnosed with calciphylaxis. In one patient, calciphylaxis was mentioned upon admission given the context of preexisting secondary hyperparathyroidism and the existence of multiple risk factors. In both patients, the diagnosis was confirmed histologically. Our two observations highlight the difficulty of the diagnosis and the complexity of the therapeutic management that has been personalized according to patient characteristics and clinical evolution. Several therapeutic means can be implemented once the diagnosis is made; nevertheless, its prognosis remains pejorative despite the therapeutic advances. Broad debridement, good phosphocalcic balance control, and the correction of the risk factors top the list of any therapeutic strategy. One of the major challenges of the therapy is normalizing the calcium-phosphate balance. Thus, Cinacalcet and sodium thiosulfate appear to be promising treatments.

Regional gene expression analysis of multiple tissues in an experimental animal model of post-traumatic osteoarthritis

OBJECTIVE

To characterize local disease progression of the medial meniscus transection (MMT) model of post-traumatic osteoarthritis (OA) at the molecular level, in order to establish a baseline for therapeutic testing at the preclinical stage.

DESIGN

Weight-matched male Lewis rats underwent MMT or sham surgery on the left limb with the right leg as contralateral control. At 1 and 3 weeks post-surgery, tissues were harvested from different areas of the articular cartilage (medial and lateral tibial plateaus, and medial osteophyte region) and synovium (medial and lateral), and analyzed separately. RNA was extracted and used for microarray (RT-PCR) analysis.

RESULTS

Gene expression changes due to surgery were isolated to the medial side of the joint. Gene changes in chondrocyte phenotype of the medial tibial plateau cartilage preceded changes in tissue composition genes. Differences in inflammatory markers were only observed at the osteophyte region at 3 weeks post-surgery. There was surgical noise in the synovium at week 1, which dissipated at week 3. At this later timepoint, meniscal instability resulted in elevated expression of matrix degradation proteins and osteogenic markers in the synovium and cartilage.

CONCLUSION

These results suggest feedback interactions between joint tissues during disease progression. Regional tissue expression differences found in MMT joints indicated similar pathophysiology to human OA, and provided novel insights about this degeneration model. The examination of gene expression at a localized level in multiple tissues provides a well-characterized baseline to evaluate mechanistic effects of potential therapeutic agents on OA disease progression in the MMT model.

Bisphosphonates for the preservation of periprosthetic bone mineral density after total joint arthroplasty: a meta-analysis of 25 randomized controlled trials

The present meta-analysis aimed to evaluate the long-term efficacy of bisphosphonates (BPs) on preservation of periprosthetic bone mineral density (BMD) after joint arthroplasty. It confirmed the protective effect of BPs in a long-term follow-up, and found the influence factors on this effect.

INTRODUCTION

Periprosthetic bone loss is believed to cause aseptic loosening and failed prosthetic fixation in joint arthroplasty. This meta-analysis which included high-quality randomized controlled trials aimed to analyze the effect of bisphosphonates on maintaining periprosthetic bone mineral density after total joint arthroplasty.

METHODS

Twenty-five RCTs were included and the total number of participants was 1163 by computerized searches of bibliographic databases. The weighted mean differences with 95% confidence interval were calculated to evaluate the efficacy of BPs on total periprosthetic BMD and the BMD of different Gruen zones. Subgroup analyses identified the potentially influencing factors such as surgical site, cement fixation, and generation of BPs. A descriptive review was conducted for BP-related adverse effects.

RESULTS

The BPs group presented significantly higher total periprosthetic BMD in the BPs group than that in the control group at 3, 6, 12 months, 2-4 years, and 5-10 years after arthroplasty (P < 0.05). The BPs group presented significantly higher periprosthetic BMD in femoral Gruen Zone 1 and 7 than that in the control group at 3, 6, 12 months, 2-4 years, and 5-10 years (P < 0.05). The heterogeneity was minimized by dividing THA and TKA into two subgroups. Subgroup analyses revealed that the effect of BPs on preservation of BMD was significantly greater in arthroplasty with cemented component than in that with uncemented component at 12 months and 5-10 years (P < 0.05), and the administration of the second and third generation BPs was significantly more effective than the first-generation BPs at 6 and 12 months (P < 0.05). None of the included studies described severe or fatal adverse effects related to BPs.

CONCLUSIONS

BPs have significantly long-term efficacy on the preservation of periprosthetic BMD after joint arthroplasty. To obtain a better efficacy, the cemented components and the second and third generation BPs are recommended.

Effect of bisphosphonates on periprosthetic bone loss after total knee arthroplasty: a meta-analysis of randomized controlled trials

Background

Aseptic loosening and osteolysis are the most common indications after TKA for revision surgery. This meta-analysis which included high-quality randomized controlled trials (RCTs) aimed to analyze the effect of bisphosphonates (BPs) on maintaining periprosthetic bone mineral density (BMD) after total knee arthroplasty.

Methods

PubMed, AMED, EMBASE, the Cochrane library, ISI Web of Science, and China National Knowledge Infrastructure were systematically searched, five RCTs were included and the total number of participants was 188. The weighted mean differences with 95% confidence interval were calculated to evaluate the efficacy of BPs on total BMD of knee and the BMD of different periprosthetic regions. A descriptive review was performed for BP-related adverse effects.

Results

The BPs group presented significantly higher total BMD in proximal part of the tibia than the control group at 3 and 6 months (P < 0.05), but no significant difference at 12 months (P = 0.09). The BPs group presented significantly higher BMD in the distal aspect of the femur than that in the control group at 3, 6, 12 months. The BPs group presented significantly higher periprosthetic BMD than that in the control group at 3, 6 and 12 months in tibial medial and lateral metaphyseal region, and femoral anterior, central and posterior metaphyseal region (p < 0.05), but no significant difference for tibial diaphyseal region at 3, 6, and 12 months. None of the included studies described severe or fatal adverse effects related to BPs.

Conclusion

BPs have a short-term effect on reducing periprosthetic bone loss after total knee arthroplasty. Compared with diaphyseal region, BPs are more effective on the preservation of BMD in medial lateral metaphyseal regions of proximal tibia and in anterior, central, and posterior metaphyseal region of distal femur.

Low-grade inflammation as a key mediator of the pathogenesis of osteoarthritis

Osteoarthritis (OA) has long been viewed as a degenerative disease of cartilage, but accumulating evidence indicates that inflammation has a critical role in its pathogenesis. Furthermore, we now appreciate that OA pathogenesis involves not only breakdown of cartilage, but also remodelling of the underlying bone, formation of ectopic bone, hypertrophy of the joint capsule, and inflammation of the synovial lining. That is, OA is a disorder of the joint as a whole, with inflammation driving many pathologic changes. The inflammation in OA is distinct from that in rheumatoid arthritis and other autoimmune diseases: it is chronic, comparatively low-grade, and mediated primarily by the innate immune system. Current treatments for OA only control the symptoms, and none has been FDA-approved for the prevention or slowing of disease progression. However, increasing insight into the inflammatory underpinnings of OA holds promise for the development of new, disease-modifying therapies. Indeed, several anti-inflammatory therapies have shown promise in animal models of OA. Further work is needed to identify effective inhibitors of the low-grade inflammation in OA, and to determine whether therapies that target this inflammation can prevent or slow the development and progression of the disease.

Targets, models and challenges in osteoarthritis research

Osteoarthritis is a chronic degenerative disorder of the joint and represents one of the most common diseases worldwide. Its prevalence and severity are increasing owing to aging of the population, but treatment options remain largely limited to painkillers and anti-inflammatory drugs, which only provide symptomatic relief. In the late stages of the disease, surgical interventions are often necessary to partially restore joint function. Although the focus of osteoarthritis research has been originally on the articular cartilage, novel findings are now pointing to osteoarthritis as a disease of the whole joint, in which failure of different joint components can occur. In this Review, we summarize recent progress in the field, including data from novel ‘omics’ technologies and from a number of preclinical and clinical trials. We describe different in vitro and in vivo systems that can be used to study molecules, pathways and cells that are involved in osteoarthritis. We illustrate that a comprehensive and multisystem approach is necessary to understand the complexity and heterogeneity of the disease and to better guide the development of novel therapeutic strategies for osteoarthritis.

Biomechanical parameters of the BP-enriched bone cement

Bisphosphonates (BPs) are well-known substances with very efficient antiresorptive properties. Their beneficial actions are useful not only in achieving better bone mineral density but also in improving bone microarchitecture, strength and, consequently, its quality. Surgical cement, being a polymer composite, is required to be highly biocompatible and biotolerant. The goal of the presented study was to assess whether the enrichment of cement with pamidronate has changed its biomechanical properties. We compared the biomechanical parameters of clean bone cement and BP-enriched bone cement, which were both used formerly in our rat models. Biomechanical properties of BP-enriched bone cement are defined by two basic terms: stress and strain, which are caused by the influence of external force. In the investigatory process of the bone's biomechanical parameters, the compressive test and the three-point flexural tests were used. During the three-point flexural investigation, the sample was supported at both ends and loaded in the middle, resulting in a flexure. After a specific range of flexure, the sample was fractured. In obtained results, there were no significant differences in the values of the stress determined at the point of maximal load and the energy stored in the samples for proportional stress-strain limit (elastic region). There were also no significant differences in the density of the samples. The study shows that the enrichment of bisphosphonates causes yielding of the bone cement material. In the presented data, we conclude that use of pamidronate implanted in bone cement did not have a detrimental effect on its biomechanical properties. Therefore, the obtained results encouraged us to perform further in vivo experiments which assess the biomechanical properties of bones implanted with BP-enriched bone cement.

Modulation of bone turnover in orchidectomized rats treated with raloxifene and risedronate

Osteoporosis is a reduction in bone mineral density (BMD). It develops less often in men than in women. This study aimed to evaluate the bone protective effects of raloxifene (RAL), risedronate (RIS), and their combination on osteoporotic male rats. Forty male Wister rats (12 weeks) were randomly divided into five groups: sham-operated group (n = 8), orchidectomized (ORX) group (n = 7), RAL group (n = 9), RIS group (n = 7) and RAL + RIS group (n = 7). RAL was orally administered at 3 mg/kg three times/week, and RIS was given subcutaneously at 5 μg/kg, twice weekly. After 6 weeks of treatment, serum cathepsin-K, alkaline (ALP) and acid phosphatase activities, serum osteocalcin, serum Ca²⁺, and Pi were determined. Urinary Ca²⁺ and deoxypyridinoline levels, BMD, and Ca²⁺ content of femur ash were estimated. Histochemical localization of ALP activity of tibia and histomorphometry was examined. As compared to sham, ORX rats showed a significant increase in bone turnover markers, and histochemical activity of ALP was increased markedly in proximal tibia of ORX rats, whereas BMD and Ca²⁺ content of femur ash were reduced after ORX. These changes were modulated after treatment with RAL and RIS or both to ORX rats; BMD of femur was improved by each treatment, and bone turnover markers were reduced as compared to ORX vehicle group. We concluded that orchidectomy induced osteoporosis and increased bone turnover in male rats because of withdrawal of sex hormones. Both RAL and RIS could treat osteoporosis in ORX rats; they reduced bone turnover markers and maintained BMD.

Bisphosphonates for post-menopausal osteoporosis: are they all the same?

The primary goal of treatment for post-menopausal osteoporosis (PMO) is reduction in fracture risk. Therefore, clinicians must recommend therapies that are safe and have proven anti-fracture efficacy. Bisphosphonates have long been established as first-line therapy for osteoporosis and several of these drugs significantly reduce osteoporotic fracture risk. However, choosing among different bisphosphonates can represent a difficult clinical decision. This review outlines the pharmacology of various bisphosphonates, discusses how their pharmacological characteristics affect their efficacy, and summarizes clinical safety and efficacy data. Clinical trial data and the opinions of expert bodies suggest that alendronate, risedronate, ibandronate and zoledronic acid all provide fracture protection for patients with PMO. However, there are differences among these agents. For example, all four agents have demonstrated efficacy in preventing vertebral fractures, but only zoledronic acid and risedronate significantly reduce non-vertebral fracture risk in pivotal trials. Moreover, reduction in hip fracture risk has only been established for alendronate, risedronate and zoledronic acid. Current data suggest that ibandronate and zoledronic acid have the most persistent antifracture effect. Bisphosphonates have been associated with a number of side effects, the evidence for which is summarized in this review. The most pertinent of these when choosing a bisphosphonate for a particular patient are the well-documented associations between gastrointestinal adverse events and oral administration, and between acute phase reactions and intravenous administration. Ultimately, selection of a specific bisphosphonate for treatment of PMO should be based on efficacy, risk profile, cost-effectiveness and patient preference.

Changes in serum and synovial fluid biomarkers after acute injury (NCT00332254)

INTRODUCTION

Acute trauma involving the anterior cruciate ligament is believed to be a major risk factor for the development of post-traumatic osteoarthritis 10 to 20 years post-injury. In this study, to better understand the early biological changes which occur after acute injury, we investigated synovial fluid and serum biomarkers.

METHODS

We collected serum from 11 patients without pre-existing osteoarthritis from a pilot intervention trial (5 placebo and 6 drug treated) using an intra-articular interleukin-1 receptor antagonist (IL-1Ra) therapy, 9 of which also supplied matched synovial fluid samples at presentation to the clinic after acute knee injury (mean 15.2 ± 7.2 days) and at the follow-up visit for reconstructive surgery (mean 47.6 ± 12.4 days). To exclude patients with pre-existing osteoarthritis (OA), the study was limited to individuals younger than 40 years of age (mean 23 ± 3.5) with no prior history of joint symptoms or trauma. We profiled a total of 21 biomarkers; 20 biomarkers in synovial fluid and 13 in serum with 12 biomarkers measured in both fluids. Biomarkers analyzed in this study were found to be independent of treatment (P > 0.05) as measured by Mann-Whitney and two-way ANOVA.

RESULTS

We observed significant decreases in synovial fluid (sf) biomarker concentrations from baseline to follow-up for (sf)C-Reactive protein (CRP) (P = 0.039), (sf)lubricin (P = 0.008) and the proteoglycan biomarkers: (sf)Glycosaminoglycan (GAG) (P = 0.019), and (sf)Alanine-Arginine-Glycine-Serine (ARGS) aggrecan (P = 0.004). In contrast, we observed significant increases in the collagen biomarkers: (sf)C-terminal crosslinked telopeptide type II collagen (CTxII) (P = 0.012), (sf)C1,2C (P = 0.039), (sf)C-terminal crosslinked telopeptide type I collagen (CTxI) (P = 0.004), and (sf)N-terminal telopeptides of type I collagen (NTx) (P = 0.008). The concentrations of seven biomarkers were significantly higher in synovial fluid than serum suggesting release from the signal knee: IL-1β (P < 0.0001), fetal aggrecan FA846 (P = 0.0001), CTxI (P = 0.0002), NTx (P = 0.012), osteocalcin (P = 0.012), Cartilage oligomeric matrix protein (COMP) (P = 0.0001) and matrix metalloproteinase (MMP)-3 (P = 0.0001). For these seven biomarkers we found significant correlations between the serum and synovial fluid concentrations for only CTxI (P = 0.0002), NTx (P < 0.0001), osteocalcin (P = 0.0002) and MMP-3 (P = 0.038).

CONCLUSIONS

These data strongly suggest that the biology after acute injury reflects that seen in cartilage explant models stimulated with pro-inflammatory cytokines, which are characterized by an initial wave of proteoglycan loss followed by subsequent collagen loss. As the rise of collagen biomarkers in synovial fluid occurs within the first month after injury, and as collagen loss is thought to be irreversible, very early treatment with agents to either reduce inflammation and/or reduce collagen loss may have the potential to reduce the onset of future post-traumatic osteoarthritis.

TRIAL REGISTRATION

The samples used in this study were derived from a clinical trial NCT00332254 registered with ClinicalTrial.gov.

Synthesis of functionalized bisphosphonates via click chemistry

An efficient general synthetic approach giving the possibility for facile, rapid and cheap access to a wide range of novel nitrogen-bisphosphonates (N-BPs) as potent drug candidates, based on the reaction of mono- and bis-propargyl-substituted bisphosphonates with a variety of azides under Cu(i) catalysis ("click" methodology), has been developed. The method allows the incorporation of two functionalities into the N-BP molecule simultaneously, as well as to ligate in situ two N-BPs to one another via the one-pot reaction of organic dibromides with propargyl-substituted bisphosphonates, generating both the diazide and Cu(I) moieties.

Changes in MMP-2 and -9 activity and MMP-8 reactivity after amphotericin B induced synovitis and treatment with bufexamac

The objective here was to evaluate the acute effects of induced arthritis on synovial fluid (SF) levels of matrix metalloproteinases (MMP) -2, -8 and -9 in horses. To evaluate MMP-2 and -9 activities and the effect of non-steroidal anti-inflammatory drug (NSAID) bufexamac during remission from acute arthritis. Aseptic arthritis was induced in 24 Standardbred horses using 20 mg of amphotericin B as a single intra-articular (IA) injection in the right intercarpal joint. After 1 week and 2 weeks, horses were treated intra-articularly with 10, 20, or 40 mg of bufexamac suspension or with sterile saline solution as control. SF was sampled prior to induction and at weekly intervals for 5 weeks. Fluids were evaluated for MMP-2 and MMP-9 activity by gelatin zymography or for MMP-8 immunoreactivity by Western Blotting. IA injection of amphotericin B consistently resulted in significant increase in the immunoreactivity of MMP-8 and activity of both the latent and the active forms of MMP-2 and -9, among which the active form of MMP-2 increased the most. MMP-9 levels declined to pre-induction levels within 2 weeks, whereas levels of MMP-2 remained still high after 5 weeks. Treatment with bufexamac did not significantly affect levels of gelatinolytic MMP. Results suggest that after acute arthritis of horses, elevated MMP activity is present in the joint, for several weeks, to a degree that could promote cartilage degradation, and treatment with the NSAID bufexamac is not likely to affect that. Furthermore, analysing levels of MMP-9 activity and especially levels of active forms of MMP-2 activity may be valuable to predict the time of occurrence of arthritis in horses.

Crystal structures of the catalytic domain of human stromelysin-1 (MMP-3) and collagenase-3 (MMP-13) with a hydroxamic acid inhibitor SM-25453

Crystal structures of the catalytic domain of human stromelysin-1 (MMP-3) and collagenase-3 (MMP-13) with a hydroxamic acid inhibitor SM-25453 have been solved at 2.01 and 2.37A resolutions, respectively. The results revealed that the binding modes for this inhibitor to MMP-3 and -13 were quite similar. However, subtle comparative differences were observed at the bottom of S1' pockets, which were occupied with the guanidinomethyl moiety of the inhibitor. A remarkable feature of the inhibitor was the deep penetration of its long aliphatic chain into the S1' pocket and exposure of the guanidinomethyl moiety to the solvent.

Autoactivation profiles of calcium-dependent matrix metalloproteinase-2 and -9 in inflammatory synovial fluid: effect of pyrophosphate and bisphosphonates

BACKGROUND

The presence of matrix metalloproteinase-2 and -9 (MMP-2, MMP-9), gelatinase A and B, in synovial fluid is typical in inflammatory connective tissue diseases especially rheumatoid arthritis (RA). Because MMPs are synthesized as latent proforms, a pathophysiologic understanding of MMP regulation has focused on mechanisms of activation that remain to date largely unresolved.

METHODS

Synovial fluid was collected by aseptic aspiration from RA patients and incubated with and without physiologic levels of calcium and other modifiers (pyrophosphate, bisphosphonates, and the tissue inhibitors of MMPs (TIMPs), under conditions that activate MMPs. MMP-2 and -9 were then characterized by substrate gel electrophoresis (gelatin zymography) to resolve both latent and activated 'partially proteolyzed' forms.

RESULTS

Gelatin zymography revealed that RA synovial fluid contained latent neutrophil MMP-9 (92, 130, 225 kDa) and fibroblast MMP-2 (72 kDa). A small amount of activated MMP-2 (64 kDa) was also noted. Incubation of synovial fluid without calcium resulted in MMP-9 activation to 87, 116, and 209 kDa forms. MMP-9 activation was, however, substantially delayed in the presence of physiologic calcium (2.5 mmol/l). MMP-2 did not demonstrate any appreciable activation with or without physiologic calcium. MMP-9 activation likely occurred via an autoactivation mechanism since it was susceptible to inhibition by the tissue inhibitor of MMP-9 (TIMP-1). Pyrophosphate and bisphosphonates (alendronate and risedronate) were ineffective in blocking synovial fluid MMP-9 autoactivation. Some early MMP-9 activation was noted with alendronate despite the presence of physiologic calcium.

DISCUSSION

Although RA synovial fluid contained abundant MMP-2 and MMP-9, only MMP-9 underwent autoactivation to lower molecular weight forms. MMP-9 was transiently stable in the presence of physiologic calcium concentration, whereas autoactivation was more pronounced without exogenous calcium. The apparent lack of MMP-2 autoactivation with or without calcium, likely resulted from the coexistence of its bound endogenous inhibitor, TIMP-2. The role of differential autoactivation of MMPs activity in inflammatory arthritic disease is discussed.

Mithramycin downregulates proinflammatory cytokine-induced matrix metalloproteinase gene expression in articular chondrocytes

Interleukin-1 (IL-1), IL-17 and tumor necrosis factor alpha (TNF-α) are the main proinflammatory cytokines implicated in cartilage breakdown by matrix metalloproteinase (MMPs) in arthritic joints. We studied the impact of an anti-neoplastic antibiotic, mithramycin, on the induction of MMPs in chondrocytes. MMP-3 and MMP-13 gene expression induced by IL-1β, TNF-α and IL-17 was downregulated by mithramycin in human chondrosarcoma SW1353 cells and in primary human and bovine femoral head chondrocytes. Constitutive and IL-1-stimulated MMP-13 levels in bovine and human cartilage explants were also suppressed. Mithramycin did not significantly affect the phosphorylation of the mitogen-activated protein kinases, extracellular signal-regulated kinase, p38 and c-Jun N-terminal kinase. Despite effective inhibition of MMP expression by mithramycin and its potential to reduce cartilage degeneration, the agent might work through multiple unidentified mechanisms.