Lack of effect of central nervous system-active doses of nabilone on capsaicin-induced pain and hyperalgesia.

The aim of the present study was to investigate the effects of nabilone on capsaicin-induced pain and hyperalgesia, as well as on biomarkers of cannabinoid central nervous system (CNS) effects. A randomized, double-blind, placebo-controlled, crossover study was conducted in 30 healthy male volunteers receiving single doses of nabilone (1, 2 or 3 mg). Pain intensity after intradermal capsaicin injections in the forearm was assessed by continuous visual analogue scale (0-100 mm). Capsaicin cream was applied to the calf to induce hyperalgesia. Primary hyperalgesia was assessed by measuring heat pain thresholds, whereas secondary hyperalgesia was assessed by measuring the area where light tactile stimulation was felt to be painful. Pain and hyperalgesia were measured at baseline and 2-3.5 h after dosing. The CNS effects were assessed at baseline and up to 24 h after dosing using visual analogue mood scales for feeling 'stimulated', 'anxious', 'sedated' and 'down'. Plasma samples for pharmacokinetic analysis were obtained up to 24 h after drug administration. Nabilone did not significantly attenuate either ongoing pain or primary or secondary hyperalgesia, whereas dose-dependent CNS effects were observed from 1.5 to 6 h after dosing, being maximal at 4-6 h. Plasma concentrations of nabilone and its metabolite carbinol were maximal 1-2 h after dosing. Adverse events (AE) were common on nabilone treatment. Four subjects withdrew due to pronounced CNS AE (anxiety, agitation, altered perception, impaired consciousness). Although nabilone had marked CNS effects, no analgesic or antihyperalgesic effects were observed.