Despite intensive research into pain mechanisms and significant investment in research and development, the majority of analgesics available to prescribers and patients are based on mechanistic classes of compounds that have been known for many years. With considerable ingenuity and innovation, researchers continue to make the best of the mechanistic approaches available, with novel formulations, routes of administration, and combination products. Here we review some of the mechanisms and modalities of analgesics that have recently entered into clinical development, which, coupled with advances in the understanding of the pathophysiology of chronic pain, will hopefully bring the promise of new therapeutics that have the potential to provide improved pain relief for those many patients whose needs remain poorly met.
IntroductionDrug discovery and development continue to be a challenge, with increasingly high investments in R&D and increased numbers of submissions failing to translate into the delivery of novel chemical entities onto the market. The wide availability of generic and overthe-counter analgesics based on non-steroidal antiinflammatories (NSAIDs), acetaminophen, and "weak" opiates (and their combinations) provides many individuals with an accessible source of relief for mild to moderate pain. However, many patients with chronic conditions such as osteoarthritis remain poorly treated (1, 2).Opiates continue to provide an important choice for the treatment of moderate to severe pain but are associated with a number of unwanted side effects such as constipation, nausea, vomiting, itching, and somnolence (3), and negative effects on patients' wellbeing, including sleep quality and ability to concentrate, which can be of such significance that they result in treatment discontinuation (4). Indeed, a recent Internet survey indicated that patients taking opiates for pain (and prescribing physicians) are willing to "trade-off " pain relief for a better toleration profile, with 50% of patients and physicians reporting that improved side effects represent the biggest unmet need (5).Neuropathic pain has a complex pathophysiology and is difficult to treat (6). It has been estimated that about one-third of patients are likely to achieve 50% pain relief with monotherapy (7); for example, in patients treated with the first-line therapy pregabalin, the patient global impression of change rating of much improved or very much improved was about 35% in postherpetic neuralgia, 50% in painful diabetic neuropathy, and 40% in fibromyalgia (8). A recent review identified a 66% increase in published randomized, placebo-controlled trials in a range of neuropathic pain populations over the last five years and concluded that only a limited improvement in the relief of pain had been achieved and that a large proportion of patients remained poorly treated (9).The challenge for drug discovery and development can therefore be very simply stated: we need to bring forward analgesics that will provide more effective pain relief, are safe...