Discovery of novel tetrahydroisoquinoline derivatives as orally active N-type calcium channel blockers with high selectivity for hERG potassium channels.

N-type calcium channels represent a promising target for the treatment of neuropathic pain. The selective N-type calcium channel blocker ziconotide ameliorates severe chronic pain but has a narrow therapeutic window and requires intrathecal administration. We identified tetrahydroisoquinoline derivative 1a as a novel potent N-type calcium channel blocker. However, this compound also exhibited potent inhibitory activity against hERG channels. Structural optimizations led to identification of (1S)-(1-cyclohexyl-3,4-dihydroisoquinolin-2(1H)-yl)-2-{[(1-hydroxycyclohexyl)methyl]amino}ethanone ((S)-1h), which exhibited high selectivity for hERG channels while retaining potency for N-type calcium channel inhibition. (S)-1h went on to demonstrate in vivo efficacy as an orally available N-type calcium channel blocker in a rat spinal nerve ligation model of neuropathic pain.

Amelioration of neuropathic pain by novel transient receptor potential vanilloid 1 antagonist AS1928370 in rats without hyperthermic effect.

Transient receptor potential vanilloid 1 (TRPV1) is activated by a variety of stimulations, such as endogenous ligands and low pH, and is believed to play a role in pain transmission. TRPV1 antagonists have been reported to be effective in several animal pain models; however, some compounds induce hyperthermia in animals and humans. We discovered the novel TRPV1 antagonist (R)-N-(1-methyl-2-oxo-1,2,3,4-tetrahydro-7-quinolyl)-2-[(2-methylpyrrolidin-1-yl)methyl]biphenyl-4-carboxamide (AS1928370) in our laboratory. AS1928370 bound to the resiniferatoxin-binding site on TRPV1 and inhibited capsaicin-mediated inward currents with an IC50 value of 32.5 nM. Although AS1928370 inhibited the capsaicin-induced Ca²(+) flux in human and rat TRPV1-expressing cells, the inhibitory effect on proton-induced Ca²(+) flux was extremely small. In addition, AS1928370 showed no inhibitory effects on transient receptor potential vanilloid 4, transient receptor potential ankyrin 1, and transient receptor potential melastatin 8 in concentrations up to 10 µM. AS1928370 improved capsaicin-induced secondary hyperalgesia and mechanical allodynia in an L5/L6 spinal nerve ligation model in rats with respective ED50 values of 0.17 and 0.26 mg/kg p.o. Furthermore, AS1928370 alleviated inflammatory pain in a complete Freund's adjuvant model at 10 mg/kg p.o. AS1928370 had no effect on rectal body temperature up to 10 mg/kg p.o., although a significant hypothermic effect was noted at 30 mg/kg p.o. In addition, AS1928370 showed no significant effect on motor coordination. These results suggest that blockage of the TRPV1 receptor without affecting the proton-mediated TRPV1 activation is a promising approach to treating neuropathic pain because of the potential wide safety margin against hyperthermic effects. As such, compounds such as ASP1928370 may have potential as new analgesic agents for treating neuropathic pain.

Assessment of canine sensory function by using sine-wave electrical stimuli paradigm.

The paradigm of sine-wave electrical stimuli has been used for sensory neurological assessment in humans. In the present study, we applied the paradigm to the dog for the quantitative assessment of sensory function. Sine-wave electrical current stimuli at frequencies of 2000, 250, and 5Hz were delivered to bipolar electrodes attached to the skin surface of the hind paws. The stimulation intensity was gradually increased, and the minimum intensity required to elicit the lifting behavior in the stimulated paw was determined as current threshold (CT) for each of the three frequencies. Dogs consistently showed the lifting behavior at CTs without showing aversive behaviors such as vocalization and wriggling. The baseline CTs (mean+/-SEM, n=12) were 4430+/-110microA for CT2000, 2215+/-173microA for CT250, and 2305+/-152microA for CT5. The CTs immediately increased after bolus intravenous injection of fentanyl at 10microg/kg, although the significant increase disappeared within 1h. The time course for the CTs was parallel to that of plasma fentanyl concentration. In conclusion, the present study applied the paradigm of transcutaneous sine-wave electrical stimuli to the dog, and used the hind paw lifting as endpoint behavior. This paradigm is simple, non-invasive, useful in the assessment of sensory function, and can be adapted to investigate the pharmacokinetics/pharmacodynamics relation of drugs. Further studies are needed to give the conclusive interpretation of the endpoint behavior.