Two TRPV1 antagonists with distinct profiles on mechanical hyperalgesia and core temperature in rats

Elliot Lilley, Moh Panesar, Victoria Head, Mark Nash, Alyson Fox

Novartis Institutes for Biomedical Research, Horsham, UK

Transient receptor potential vanilloid - 1 (TRPV1) is a non-selective cation channel that is sensitive to noxious heat, protons and some lipid mediators, in addition to capsaicin. TRPV1 channels are expressed on somatic and visceral afferent nerve fibres and antagonists are active in models of somatic pain. It is well established that capsaicin will produce hypothermia in rats. Interestingly, recent reports have suggested that TRPV1 antagonism may lead to an increase in core body temperature. Here, we have assessed the effect of TRPV1 antagonism in models of inflammatory pain and core temperature in rats using the TRPV1 antagonists ‘compound 2’ (2-thioxo-2,3-dihydro-1.H.-pyrido[2,3-d.]pyrimidin-4-one; Culshaw et al, 2006. J Med Chem, 49: 471-474) and AMG517 (N-{4-[6-(4-Triluoromethyl-phenyl)-pyrimidin-4-yloxy]-benzothiazol-2-yl}-acetamide).

Inhibition of mechanical hyperalgesia induced by the TRPV1 agonist capsaicin was assessed with an analgesymeter and used to demonstrate in vivo antagonism of the receptor in male Wistar Han rats (180-200g). Paw withdrawal thresholds were measured in naïve animals prior to drug administration. 30min following drug treatment an intraplantar injection into the left hindpaw of 3 nmol (10 μl) capsaicin was administered and paw withdrawal thresholds were measured again 30 minutes later. Inflammatory hyperalgesia was induced in rats by intraplantar injection into the left hindpaw of 25 μl Freund’s complete adjuvant (FCA). After 3 days paw withdrawal thresholds were measured prior to (predose) and then 1 h following drug administration. Core temperature was measured using a BAT-12 thermometer and a lubricated rectal probe 1h after drug administration. Data is expressed as mean ± SEM for 6 animals per treatment group.

Two TRPV1 channel antagonists: AMG517 and compound 2 with the following in vitro profiles: rat pIC50 vs capscaisin and pH: 7.9 and 7.5 for compound 2 and 9.3 and 8.5 for AMG517, were used for these studies. In an in vivo mechanistic model, compound 2 (3-30mg/kg p.o.) and AMG517 (1-10mg/kg p.o.) inhibited capsaicin-induced inflammatory hyperalgesia in rats. Both compounds at 3mg/kg produced 60.3±2.8% and 57.9±8.0% inhibition, respectively. In a model of inflammatory pain the compounds similarly reversed FCA-induced mechanical hyperalgesia. Compound 2 (3-30mg/kg p.o.) at 10mg/kg produced 50.0±5.6% reversal and AMG517 (1-10mg/kg p.o.) at 10mg/kg produced 59.9±4.3% reversal. Both compounds also induced hyperthermia. AMG517 (0.3-3mg/kg p.o.) produced significant hyperthermia at all doses tested (1mg/kg produced a 1.0±0.1 ºC increase in core temperature). In contrast, compound 2 (3-100mg/kg p.o.) only produced significant hyperthermia at 100mg/kg (100mg/kg produced a 0.6±0.2 ºC increase in core temperature).

We have found that two different TRPV1 antagonists with similar analgesic activity in vivo induce hyperthermia to different extents suggesting that it is possible to separate these two pharmacological effects. We hypothesise that the kinetics of channel antagonism may affect the extent of hyperthermia observed in vivo and further experiments will be performed to investigate this.