**1. Introduction**

210 Rheumatoid Arthritis – Treatment

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Inflammation is a necessary response to infection and injury by which the invading pathogen and/or damaged cells are cleared. Under normal circumstances this is a tightly controlled and transient process. However, in conditions such as rheumatoid arthritis (RA) these regulatory mechanisms appear inactive or ineffective such that inflammation progresses unchecked. This results in the pain, swelling and bone and cartilage destruction that define this disease.

The etiology of RA initiation is still uncertain, but increasing evidence points to a key role for the toll-like receptor (TLR) family in driving aberrant inflammation in the joint. TLRs were originally identified as receptors for exogenous pathogen associated molecular patterns (PAMPs) of bacterial, fungal or viral origin, which initiate inflammation in response to microbial infection. Perhaps of more interest in the context of RA however, is the role that these molecules play in the recognition of endogenous danger signals or DAMPs (damage associated molecular patterns).

DAMPs are generated by both infection-induced and sterile tissue damage. They include a wide range of molecules including intracellular proteins such as high mobility group protein 1 (HMGB1), cell derived nucleic acids and extracellular matrix molecules such as tenascin-C and fibrinogen. High levels of DAMPs are present in both the RA synovium and in the peripheral circulation in RA patients. Accumulating evidence from both human studies and experimental animal models now suggests that these molecules may be critical to the persistence of the inflammatory state in RA. Moreover, targeting TLRs and their downstream signalling pathways is emerging as a potentially tractable means for treating a range of inflammatory conditions, including RA and its associated pathologies.

Here we focus on the current literature that demonstrates a role for DAMPs in driving chronic inflammation in RA. We will discuss the mechanistic differences between PAMP and DAMP mediated activation of TLRs; and highlight how these data have already informed novel pathways to develop improved therapies for RA and how future therapeutic strategies may further evolve.

Targeting DAMP Activation of

(Genovese, 2009).

Vollenhoven, 2009).

Toll-Like Receptors: Novel Pathways to Treat Rheumatoid Arthritis? 213

Knowledge of the processes responsible for disease activity and progression has lead to significant advances in the treatment of RA in the last 30 years. Early treatment, within months of the onset of persistent symptoms, is recommended, and at the present time usually takes the form of disease modifying anti-rheumatic drugs (DMARD) such as methotrexate. In more recent years however, the choice of treatment for RA has expanded significantly, and importantly now utilises agents that are less globally immunosuppressive than methotrexate (Weinblatt et al., 1985) (Table 1). These newer therapies target either specific cell types, such as B and T cells that present within the inflamed synovium, or their products (Genovese et al., 2008; Tedder, 2009; Townsend et al., 2010; Buch et al., 2011). Indeed, anti-cytokine therapies have revolutionised the treatment of RA in recent years. In particular, the use of anti-TNF biologicals such as Adalimumab, Etanercept and Infliximab have become the treatment of choice in those who do not respond to conventional DMARDs (Taylor et al., 2009), although biologicals that target other pro-inflammatory cytokines are also approved for use. These include Tocilizumab (anti-IL-6R) (Fleischmann et al., 2006; Jones et al., 2010) and Anakinra, a recombinant IL-1 receptor antagonist, as well as Bevacizumab, an antibody that targets vascular endothelial growth factor (VEGF) and hence may reduce neovascularisation that pannus formation depends upon. A variety of small molecule inhibitors designed to target critical elements of the B cell receptor, T cell receptor or cytokine signalling pathways such as inhibitors of IKK2, PDE4 and Btk (Bruton's tyrosine kinase), have shown interesting results in some animal models of arthritis, as have clinical trials with the syk inhibitor R788 (Podolin et al., 2005; Lindstrom et al., 2010; Di Paolo et al., 2011). The p38 inhibitors however, which showed such promise in animal models have not lived up to expectations in clinical trials and have not progressed beyond phase II

Despite the significant advances made with this arsenal of therapies, the goal of achieving sustained remission of RA has remained elusive and even with long term DMARD and biologic therapy is relatively uncommon. The efficacy of treatments is also unpredictable. Thus, a significant proportion of patients do not respond adequately to first line DMARD treatment and are then moved on to biologics. Even here, approximately 40% of patients do not respond to anti-TNF therapy for example. Moreover, many of these treatments are accompanied by significant side-effects, ranging from injection site reactions, increased infection rates and neutropenia to the potential for an increased risk of malignancies (van

When taking a global view of all the therapies for RA, either in use in the clinic, in early trials, in animal models, or in *in vitro* studies, it becomes clear that all are designed to target the ongoing process of inflammation. Namely the cells present in the joint during the inflammatory process, or their soluble products (cytokines), rather than targeting a causative agent for RA. However, for RA sufferers a single causative agent has not, and probably will never, be defined. Rather, RA is a complex disease with a multi-factorial etiology. It's prevalence in women (3:1 female to male ratio) suggests a hormonal contribution, and there are clear links to environmental factors such as smoking as well as a predisposition to RA with certain HLA haplotypes (Bax et al., 2011). Genetic and twin studies also suggest a strong environmental influence as well as a genetic link. As a tractable causative agent or single predisposing gene is therefore unlikely to be identified, a therapy that will treat disease in the early stages, that will prevent progression to a chronic state and thereby allow the inflammatory state to resolve, thereby preventing tissue damage, bone and cartilage destruction and progression, remains the holy grail of many researchers.
