The Hyperpolarisation-Activated Cyclic Nucleotide-Gated (HCN) Ion Channel Blocker Ivabradine As An Analgesic In Inflammatory And Neuropathic Pain

Ayesha Sengupta, Gareth Young, Elizabeth Mooney, Edward Emery, Peter McNaughton. University of Cambridge, Cambridge, UK

Background and Purpose:

Inflammatory and neuropathic pain conditions represent an unmet clinical need as current therapies are associated with unpleasant side-effects and, in the case of neuropathic pain, have limited efficacy. The HCN2 ion channel has been identified as a key downstream target in inflammatory and neuropathic pain (Emery et al., 2011), suggesting that blocking HCN2 ion channels could alleviate pathological pain. Here, we investigated whether the HCN channel blocker ivabradine, currently used as a bradycardic agent, also has efficacy as an analgesic. We tested ivabradine both in patch-clamp experiments and using in vivo pain models, and we compared its analgesic action to the bradycardia it caused.

Experimental Approach:

Whole-cell patch clamp electrophysiology (Emery et al, 2011) was used to measure Ih current block by ivabradine in mouse DRG neurons. The effect of ivabradine, gabapentin and saline were assessed in two mouse models of pain: a model of inflammatory pain (the formalin test, Tjolsen et al., 1992) and a model of neuropathic pain (chronic constriction injury, CCI, Bennett and Xie, 1988). Inflammatory pain was assessed from the area under the curve (AUC) of time spent attending to the inflamed paw from 10 to 60 minutes after formalin injection. Von Frey mechanical pain thresholds were assessed in the CCI model. The bradycardic effects of ivabradine, governed primarily by block of HCN4, were monitored with pulse oximetry. Gross motor side effects of ivabradine and gabapentin were assessed using the rotarod test.

Key Results: At 30μM, ivabradine blocked 99.9 +/- 0.02 % (n = 9) of Ih in vitro 300s after drug application. Ivabradine reduced pain responses in the second phase of the formalin test. AUC was reduced from 3337 to 1530s2 by 5mg/kg ivabradine (p=0.0054 for 5mg/kg ivabradine [n=6] compared to saline [n=12]; t-test). Both gabapentin and ivabradine had analgesic effects in neuropathic pain. Seven days after CCI, the mechanical pain threshold was raised from 2.0±0.20g to 3.9±0.25g g by 50mg/kg gabapentin and to 3.2±0.33 g by 5mg/kg ivabradine (p=0.0002 for 50mg/kg gabapentin [n=9]:, p=0.0007 for 5mg/kg ivabradine [n=10]:, p=0.6007 for saline [n=11]:, paired t-test). Ivabradine had bradycardic effects at its analgesic doses for inflammatory pain (IC50 for heart rate: 3.4mg/kg; IC50 for inflammatory pain: 2mg/kg).Unlike gabapentin, ivabradine did not cause general motor deficits at analgesic doses. Gabapentin (5mg/kg) reduced time spent on rotarod from 132 to 66 seconds, while ivabradine (5mg/kg) caused no change from 124 to 123 seconds (30 minutes post-drug, p=0.0138 for gabapentin [n=5] vs. saline [n=5]:, p=0.9194 for ivabradine [n=5] vs. saline [n=5]; paired t-test).

Conclusions and Implications:

The HCN channel blocker ivabradine is an efficacious analgesic in mouse models of neuropathic pain, but was shown to cause bradycardia at analgesic doses. HCN channel blockers appear to have fewer sedative side effects than gabapentin, a widely used treatment for neuropathic pain.