In rheumatic diseases, pharmacotherapies are largely used for control of inflammatory processes or for pain control; such therapies are usually used in conjunction with non-pharmacological therapies such as muscle strengthening. Over the past decade, a greater understanding of the underlying mechanism involved in inflammatory arthritides has facilitated the development of new drugs and raised therapeutic goals. The aims of modern therapy are early diagnosis and prompt initiation of therapy to control inflammation, thereby preventing pain and subsequent joint damage with associated impaired function.

The decision regarding initiation and selection of drug therapy is not only based on the diagnosis, but on a person’s age, past medical history (e.g. previous peptic ulcer), the presence of co-morbidities (e.g. renal impairment) and the presence of concomitant medications. Compliance with medication and patient expectations are other important concepts to be taken into consideration when prescribing. It is worth briefly reviewing the underlying processes for which pharmacotherapies are employed.

The word pain comes from Latin ‘poena’ meaning fine or penalty. Pain is a complex phenomenon involving biopsychosocial interactions, and detailed mechanisms are beyond the scope of this chapter. In brief, pain perception peripherally is mediated by the terminal endings of finely myelinated A delta and of non-myelinated C fibres. Chemicals produced locally as a result of injury produce pain by direct stimulation or by sensitizing the nerve endings. A-delta fibres are responsible for the acute pain sensation, which may be followed by a slower onset, more diffuse pain mediated by the slower conducting C fibres. Most sensory input enters the spinal cord via the dorsal spinal roots. When the signal reaches the spinal cord, a signal is immediately sent back along motor nerves to the original site of the pain, triggering the muscles to contract (Kidd et al 2007). The pain signal is also sent to the brain. Only when the brain processes the signal and interprets it as pain do people become conscious of the sensation. Pain receptors and their nerve pathways differ in different parts of the body and the type of pain felt depends on the stimuli. These stimuli may be mechanical, thermal or chemical.

It is worth noting that the anatomical source of pain in many arthritic conditions is not always clear, and may often be multifactorial in nature. In early rheumatoid arthritis (RA) where synovitis is the primary pathology, the pain is presumably derived from inflammatory mediators in the synovium. In osteoarthritis (OA), the sources of pain are more controversial and may arise from the synovium or subchondral bone. The source of pain in lateral epicondylitis may be different again and relate to specific entheseal pathology. Musculoskeletal pain may be present without the classical signs of inflammation.

Inflammation results in heat, redness, swelling, pain and loss of function. Acute inflammation involves: increased blood supply in the region of injury; an increase in local capillary permeability; exudation of vascular fluid; migration of inflammatory cells out of the blood vessels and into the surrounding tissue; and the release of mediators of inflammation. Acute inflammation may resolve or can lead to chronic inflammation, tissue death (necrosis), scarring or fibrosis.

However, the triggering event for inflammation in many inflammatory arthritides is unclear and both environmental and genetic triggers have been considered i.e. exposure to an environmental antigen in a genetically predisposed individual. It is known that a foreign antigen activates T-cells resulting in an inflammatory response. This process is paramount in the body’s normal defence against infection, when the process is controlled and subject to inhibitory mechanisms. Uncontrolled and autonomous, this process can result in diseases such as rheumatoid arthritis (RA). In RA, activated T-cells secrete inflammatory cytokines that lead to chronic activation of B cells and immunoglobulin synthesis (see Fig. 15.1). The pro-inflammatory cytokines include interleukin (IL)-1 and tumour necrosis factor α (TNF) that in turn stimulate the production of more cytokines such as IL-6 and IL-8 (Jovanovic et al 1998). The result is pain, chronic synovitis and eventually local tissue and bone destruction.

Figure 15.1 Pathogenesis of inflammation in rheumatic disease.

Before prescribing a therapeutic agent, two main questions must be answered:

Pharmadynamics is the study of the biochemical and physiological effects of drugs and the mechanism of drug action, and the relationship between drug concentration and effect.

Pharmacokinetics is a branch of pharmacology dedicated to the determination of the fate of substances administered externally to a living organism. In practice, this discipline is applied mainly to drug substances, though in principle it concerns itself with all manner of compounds ingested or otherwise delivered externally to an organism, such as nutrients, metabolites, hormones, toxins, etc. Pharmacokinetics is often divided into several areas including, but not limited to, the extent and rate of absorption, distribution, metabolism and excretion.

Absorption is the process of a substance entering the body. Distribution is the dispersion or dissemination of substances throughout the fluids and tissues of the body. Metabolism is the transformation of the substance and its daughter metabolites. Excretion is the elimination of the substances from the body. In rare cases, some drugs irreversibly accumulate in a tissue in the body.

When calculating dosage, it is important to know the half-life of the drug. This refers to the time required for the amount in the body to fall to 50% and is influenced by drug clearance and the volume of distribution. Drug clearance is defined as the volume of plasma that would contain the amount of drug excreted per minute. Drug clearance is a measure of the ability of the body to eliminate a drug from the circulation. It determines daily dosage. Drugs may be cleared by the renal system (mostly excreted, e.g. penicillin) or by the hepatic system (mostly metabolised e.g. paracetomol). Some drugs undergo substantial removal from the portal circulation by the liver after oral administration (e.g. Buprenorphine). This ‘first-pass’ effect can significantly reduce the amount of active drug that reaches the systemic circulation. The volume of distribution for a drug is determined by its degree of water or lipid solubility, the extent of plasma- and tissue-protein binding, and the perfusion of tissues. It gives an indication of initial or loading dose e.g. hydroxychloroquine (see later).