The rate of cartilage and joint damage in RA is correlated with plasma elevations in inflammatory acute phase reactants, such as C- reactive protein and erythrocyte sedimentation rate, rheumatoid factor positivity and the synovial concentrations of matrix metalloproteinases(MMP), a matrix digesting enzymes directly responsible for joint destruction.
Matrix metalloproteinases (MMP) are a family of zinc-containing endoproteinases that degrade extracellular matrix (ECM) components.The MMPs are thought to play a critical role in the degradation of many components of the extracellular matrix in the synovial joint.
Matrix metalloproteinase-3 (stromelysin-1) is a proteolytic enzyme which is thought to play a pivotal role in joint destruction in RA through breaking down various extracellular components, including collagens (types III, IV, V, IX, and XI), matrix proteins and proteoglycans and activating other pro-MMPs such as pro-MMP-7, pro-MMP-8 and pro-MMP-9.
In RA, MMP-3 is locally produced in the inflamed joint and released into the blood stream. It has been suggested that serum MMP-3 level correlates with levels produced by the synovium, thus reflect the level of activity of rheumatoid synovitis.
MMP-9 has been associated with chronic inflammatory autoimmune diseases. It has been implicated in the pathogenesis of autoimmune diseases.
MMP-9 displays gelatinolytic, elastolytic and collagenolytic activity, thus playing a key role in extracellular matrix turnover, in addition, MMP-9 may also modulate the activity of various biological factors, including other proteinases (e.g., MMP-13), their inhibitors (e.g., α1-antitrypsin) or cytokines.
Matrix metalloproteinases also play key roles in adverse cardiovascular remodeling, atherosclerotic plaque formation and plaque instability, vascular smooth muscle cell (SMC) migration and restenosis that lead to coronary artery disease (CAD) and progressive heart failure.
Matrix metalloproteinases-9 (MMP-9) is mainly found in the most stenotic areas of carotid plaques in humans and is a marker for plaque vulnerability . It can also predict stroke and fatal cardiovascular events . Elevated circulating levels of MMP-9 have been found in subjects with stable angiographic coronary atherosclerosis and intermittent claudication.
The prevalence of athero sclerosis is increased in RA, even in early disease. The earliest vascular change described microscopically in atherosclerosis is intimal media thickening (IMT) , which consists of layers of smooth muscle cells and extracellular matrix. Intimal thickening is more frequent in atherosclerosis-prone arteries such as coronary, carotid, aorta and iliac arteries .
IMT is the "phenotype" of the early phases of atherosclerotic disease and is related to the main traditional risk factors. Moreover, IMT is a marker of organ damage either in the heart or in other vascular districts. The simultaneous measurement of carotid and femoral IMT may improve risk stratification in patients with coronary heart disease.
Atherosclerosis is recognized as a chronic inflammatory condition with key roles for both the innate and adaptive immune systems in the initiation, progression and stability of lesions.
Inflammation is an important trigger of plaque erosion and stability and one focus of secondary prevention of coronary artery disease (CAD) in the general population is the development of anti-inflammatory agents for plaque stabilization.
Many aspects of the pathophysiology of atherosclerosis are mirrored in the inflamed RA synovium, including pronounced infiltration by macro phages and type 1 T-helper cells, collagen degradation and neovascularization . Cytokines such as tumor necrosis factor-α(TNFα), interleukin-6(IL-6) and matrix metalloproteinases are implicated in both processes.
As plaques become more complex, thinning of the fibrous cap occurs. This thinning eventually causes rupture of the plaque, exposing its thrombogenic content to blood, resulting in acute thrombosis and a clinical event .Fibrous cap erosion is thought to be mediated by inflammatory cells, in particular macrophages and T helper cells, via secretion of matrix metalloproteinases.
Doxycycline, a semi-synthetic tetracycline, is a commonly used broad-spectrum antibiotic. It has been shown that it also inhibits the activity of mammalian collagenases and gelatinases, an activity unrelated to its antimicrobial efficacy. Doxycycline not only inhibits MMP-8 and MMP-9 (gelatinase B) activity, but also the synthesis of MMPs in human endothelial cells .
Tetracyclines exhibit multiple anti-inflammatory properties, including the inhibition of T-cell activation and chemotaxis, the downregulation of proinflammatory cytokines, including TNFα and IL-1b and the inhibition of matrixmetalloproteinases.
A study of patients with early disease showed significant efficacy compared with placebo when used in combination with methotrexate .The benefit of minocycline and doxycycline was confirmed in a recent meta-analysis that found clinically significant improvement in disease activity with no increased risk for adverse events. Rheumatologists have not embraced minocycline or doxycycline as primary treatment options for RA and reserve their use primarily in patients with long-standing, refractory disease. Minocycline and doxycycline are safe and moderately effective disease-modifying anti-rheumatic drugs (DMARDs) in the treatment of early rheumatoid arthritis . These drugs are generally well tolerated, with skin complaints, nausea, and dizziness being the most common patient-reported side effects.
Whether RA is caused, triggered, or perpetuated by an infectious agent or agents is still not delineated, it is possible, that suppression of infections in a nonspecific manner, decreasing the stimuli for TNFα production, may play a role in the treatment of RA.
