A guide to drug evaluation for chronic pain

Synthesis of 3,6-diazabicyclo[3.1.1]heptanes as novel ligands for neuronal nicotinic acetylcholine receptors

Alpha series of novel 3,6-diazabicyclo[3.1.1]heptane derivatives 4a-f was synthesized and their affinity and selectivity towards alpha4beta2 and alpha7 nAChR subtypes were evaluated. The results of the current study revealed a number of compounds (4a, 4b and 4c) having a very high affinity for alpha4beta2 (K(i) at alpha4beta2 ranging from 0.023 to 0.056 nM) versus alpha7 nAChR subtypes; among these compounds, the 3-(6-bromopyridin-3-yl)-3,6-diazabicyclo[3.1.1]heptane 4c was found to be the most alpha7alpha4beta2 selective term in receptor binding assays (alpha7alpha4beta2=1295). Moreover, compound 4d also had high affinity for the alpha4beta2 nAChR subtype (K(i)=1.2 nM) with considerably high selectivity (alpha7/alpha4beta2=23300).

Disruption of the P2X7 purinoceptor gene abolishes chronic inflammatory and neuropathic pain

The P2X(7) purinoceptor is a ligand-gated cation channel, expressed predominantly by cells of immune origin, with a unique phenotype which includes release of biologically active inflammatory cytokine, interleukin (IL)-1beta following activation, and unique ion channel biophysics observed only in this receptor family. Here we demonstrate that in mice lacking this receptor, inflammatory (in an adjuvant-induced model) and neuropathic (in a partial nerve ligation model) hypersensitivity is completely absent to both mechanical and thermal stimuli, whilst normal nociceptive processing is preserved. The knockout animals were unimpaired in their ability to produce mRNA for pro-IL-1beta, and cytometric analysis of paw and systemic cytokines from knockout and wild-type animals following adjuvant insult suggests a selective effect of the gene deletion on release of IL-1beta and IL-10, with systemic reductions in adjuvant-induced increases in IL-6 and MCP-1. In addition, we show that this receptor is upregulated in human dorsal root ganglia and injured nerves obtained from chronic neuropathic pain patients. We hypothesise that the P2X(7) receptor, via regulation of mature IL-1beta production, plays a common upstream transductional role in the development of pain of neuropathic and inflammatory origin. Drugs which block this target may have the potential to deliver broad-spectrum analgesia.

Evaluation of intravenous parecoxib for the relief of acute post-surgical pain

Parecoxib is a prodrug of valdecoxib, which is a potent and selective inhibitor of COX-2. Intravenous preparation of parecoxib is in Phase III clinical trials for the management of acute and severe post-surgical pain. It is the only COX-2 inhibitor that is available in a parenteral formulation. Clinical results compare parecoxib with ketorolac, a NSAID, which is the only non-narcotic analgesic available in parenteral formulation that can be administered for the relief of moderate to severe acute pain. Pharmacokinetic studies have shown that parecoxib is converted to valdecoxib within a short time following administration by im. or iv. injection. In clinical trials, parecoxib compares favourably with ketorolac and produces less gastric or duodenal ulcers, the predominant adverse effect, than ketorolac. Parecoxib, thus, fulfils some of the desirable characteristics of an ideal non-narcotic analgesic for severe post-surgical pain and has application in other acutely painful conditions. Parecoxib is expected to be filed for approval before the end of 2000 and is expected to be introduced in the market in 2001. It has favourable prospects for a fair share of the post-surgical pain relief market which is valued at approximately US$ 1 billion for the year 2000.

An evaluation of intrathecal ziconotide for the treatment of chronic pain

Ziconotide, the synthetic form of cone snail peptide pi-conotoxin MVIIA, is a neurone-specific N-type calcium channel blocker with an analgesic and neuroprotective effect. Intrathecal ziconotide has been recommended for approval by the FDA for the management of chronic pain. Spinally administered ziconotide produces analgesia by blocking neurotransmitter release from primary nociceptive afferents and prevents the propagation of pain signals to the brain. It has an advantage over intrathecal morphine in that there is no development of tolerance after prolonged use. Systemic toxicity is considerably reduced by administration of smaller doses intrathecally and selective delivery to the site of action in the nervous system. Nevertheless, there are neurological adverse effects due to delay in clearance of ziconotide from the neural tissues. Overall, ziconotide has a favourable risk/benefit ratio with advantages over several currently available intrathecal therapies for pain.