Abstract. As TrkA, a high-affinity receptor of nerve growth factor (NGF), is a potential target for relieving uncontrolled inflammatory pain, an effective inhibitor of TrkA has been required for pain management. To identify a specific inhibitor of TrkA activity, we designed cell-penetrating peptides combined with amino-acid sequences in the activation loop of TrkA to antagonize tyrosine kinase activity. To select a peptide inhibiting TrkA activity, we examined the effect of cell-penetrating peptides on tyrosine kinase activity of recombinant TrkA in vitro and studied their effects on NGF-stimulated neurite outgrowth and protein phosphorylation in PC12 cells. Thereafter we investigated the effect of the selected peptide on NGF-stimulated TrkA activity and the expression of transient receptor potential channel 1 in PC12 cells. The selected peptide inhibited TrkA activity, but did not inhibit tyrosine kinase activities of other receptor-type tyrosine kinases in vitro. It also suppressed NGF-stimulated responses in PC12 cells. The selected synthetic cell-penetrating peptide antagonizing TrkA function would be a candidate for inflammatory pain therapy.

Abstract. Novel agents that inhibit nerve growth factor signaling are required for the treatment of inflammatory pain. The present study investigated the effect of local administration of inhibitory peptide of TrkA (IPTRK3), a synthetic cell-penetrating peptide that antagonizes TrkA function, in complete Freund's adjuvant (CFA)-induced hyperalgesia in rats. Three hours after subcutaneous injection of CFA into the plantar surface of the rat's left hind paw, 10 mM IPTRK3 was injected at the same site. Thermal and mechanical hyperalgesia were tested in the ipsilateral hind paw until 7 days after CFA injection. The ipsilateral dorsal root ganglion (DRG) was dissected out for immunohistochemical analysis of transient receptor potential vanilloid subfamily member 1 (TRPV1) channels and TrkA. Local injection of this peptide significantly suppressed both thermal and mechanical hyperalgesia produced by CFA and also significantly reduced TRPV1 expression at the DRG. These results suggest that local administration of IPTRK3 is likely effective in the treatment of inflammatory pain in rats.

Abstract. Nerve growth factor (NGF) and its high-affinity receptor, TrkA, are one of the targets in the production of new drugs for the treatment of neuropathic pain. NGF contributes to both the initiation and maintenance of sensory abnormalities after peripheral nerve injury. This study examined the effects of IPTRK3, a new synthetic cell-penetrating peptide that antagonizes TrkA function, on neuropathic pain in mice. Partial sciatic nerve ligation (PSNL) was used to generate neuropathic pain, and we injected IPTRK3 (2 or 10 mg/kg) intraperitoneally on day 7 after PSNL. Effects of the peptide on hyperalgesia, allodynia, and expression of Fos in the spinal cord were examined. Single administration of the peptide on day 7 significantly suppressed both thermal hyperalgesia and mechanical allodynia. Gentle touch stimuli-evoked Fos expression in the lumbar spinal cord was also significantly reduced. Intraperitoneal injection of a cell-penetrating peptide antagonizing TrkA function appears effective for treatment of neuropathic pain in a mouse pain model.

These results suggest that TrkA inhibitory peptide likely suppress melanoma-induced pain with concomitant reduction in the increased paw volume in a mouse skin cancer pain model.

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