Effects of chronic administration of URB597 in the carrageenan model of inflammatory pain

Leonie Norris, Ian Robinson, Annie Patel, Bright Okine, Maulik Jhaveri, Steve Alexander, Dave Kendall, Dave Barrett, Victoria Chapman

University of Nottingham, Nottingham, UK

Anandamide (AEA), oleoylethanolamide (OEA) and palmitoyl ethanolamide (PEA) are primarily metabolised by fatty acid amide hydrolase (FAAH). Systemic FAAH-inhibitors attenuate inflammatory pain (Jayamanne et al., 2006). AEA is also a substrate for cycloxygenase-2 (COX-2) (Kozak et al., 2004), being metabolised to prostamides. The aim of this study was to investigate the effects of chronic dosing with the FAAH inhibitor URB597 on nociceptive behaviour and levels of ECLs and prostamides in a model of inflammatory pain.

Adult male Sprague-Dawley rats (240-260g) were dosed daily with 0.3 mg/kg (ip) URB597 or vehicle (saline + Tween 80) for 4 days. On the day of behavioural testing, rats received a final dose of URB597 30 minutes prior to intraplantar injection of 2% carrageenan (carra) or saline (100μl). Weight bearing on the left (injected) and right hindpaw was assessed 30 min. prior to injection of carrageenan/saline and at 90, 150 and 210 min. post-injection. Spinal cord and paw tissue were collected for LCMS/MS analysis of endocannabinoids and prostamides. Weight bearing data were analysed using a two-way ANOVA and spectrometric data were analysed with Mann-Whitney non-parametric test.

Intraplantar injection of carrageenan significantly increased the difference in weight bearing between ipsilateral and contralateral hindpaws 2-3 h post-injection, compared to saline injection (p<0.001). Levels of AEA and PEA (0.187±0.01) were decreased in the vehicle-treated carrageenan-inflamed hindpaw, compared to contralateral paw (Table 1). FAAH activity in the liver was substantially decreased (p<0.005) in the chronic URB treated group, compared to vehicle group. Chronic URB597 treatment did not alter the carrageenan-induced changes in weight bearing at any time point or alter levels of AEA or PEA in the carrageenan inflamed hindpaw, compared to the vehicle-treated carrageenan group. In the spinal cord, there was a trend towards chronic URB597 increasing levels of levels of AEA in the ipsilateral spinal cord, compared to the vehicle-treated group. Chronic URB597 signifcantly increased levels of PEA in ipsilateral spinal cord of carrageenan-treated rats, compared to the vehicle-treated group (Table 1). Prostamides were not detected in any of the samples tested.

Chronic treatment with URB597 substantially inhibited FAAH activity in the liver. Despite this neither carrageenan induced pain behaviour or hindpaw levels of AEA or PEA were altered by this treatment. These data suggest that in the presence of FAAH inhibition, ECLs in the periphery are metabolised by other pathways. We were, however, unable to measure prostamides in the hindpaw, The differential effect of URB597 treatment on levels of ECLs in the hindpaw and spinal cord suggest there is a tissue specific role of FAAH at these key sites involved in nociceptive processing.

Jayamanne, A et al (2006), Br J Pharmacol, 147, 281-288

Kozak, KR et al. (2004)Curr Pharm Des, 10, 659-67