Topical capsaicin for pain management: therapeutic potential and mechanisms of action of the new high-concentration capsaicin 8% patch

Abstract

Topical capsaicin formulations are used for pain management. Safety and modest efficacy of low-concentration capsaicin formulations, which require repeated daily self-administration, are supported by meta-analyses of numerous studies. A high-concentration capsaicin 8% patch (Qutenza™) was recently approved in the EU and USA. A single 60-min application in patients with neuropathic pain produced effective pain relief for up to 12 weeks. Advantages of the high-concentration capsaicin patch include longer duration of effect, patient compliance, and low risk for systemic effects or drug-drug interactions. The mechanism of action of topical capsaicin has been ascribed to depletion of substance P. However, experimental and clinical studies show that depletion of substance P from nociceptors is only a correlate of capsaicin treatment and has little, if any, causative role in pain relief. Rather, topical capsaicin acts in the skin to attenuate cutaneous hypersensitivity and reduce pain by a process best described as 'defunctionalization' of nociceptor fibres. Defunctionalization is due to a number of effects that include temporary loss of membrane potential, inability to transport neurotrophic factors leading to altered phenotype, and reversible retraction of epidermal and dermal nerve fibre terminals. Peripheral neuropathic hypersensitivity is mediated by diverse mechanisms, including altered expression of the capsaicin receptor TRPV1 or other key ion channels in affected or intact adjacent peripheral nociceptive nerve fibres, aberrant re-innervation, and collateral sprouting, all of which are defunctionalized by topical capsaicin. Evidence suggests that the utility of topical capsaicin may extend beyond painful peripheral neuropathies.

Figures

Fig 1

Efficacy of capsaicin 8% patch in post-herpetic neuralgia patients. Per cent change from baseline in mean numeric pain rating scale (NPRS) score during weeks 2–8 (the primary endpoint) in two similarly designed randomized, double-blind, multicentre trials (C116 and C117). Capsaicin 8% w/w or control (capsaicin 0.04% w/w) patches were applied once for 60 min to the painful areas and patients were followed for 12 weeks. Mean baseline NPRS scores per group ranged from 5.7 to 6.0. *P=0.011, **P=0.001 vs control. Taken from McCormack.

Fig 2

Activation of TRPV1 by capsaicin results in sensory neuronal depolarization, and can induce local sensitization to activation by heat, acidosis, and endogenous agonists. Topical exposure to capsaicin leads to the sensations of heat, burning, stinging, or itching. High concentrations of capsaicin or repeated applications can produce a persistent local effect on cutaneous nociceptors, which is best described as defunctionalization and constituted by reduced spontaneous activity and a loss of responsiveness to a wide range of sensory stimuli.

Fig 3

Multiple mechanisms underlie capsaicin-induced defunctionalization. Inactivation of voltage-gated Na+ channels and direct pharmacological desensitization of plasma membrane TRPV1 receptors may contribute to an immediate reduction on neuronal excitability and responsiveness. More persistent effects may be due to the overwhelming of intracellular Ca2+ buffering capacity by extracellular Ca2+ entering through TRPV1 and being released from intracellular stores, with subsequent activation of calcium-dependent proteases and cytoskeleton breakdown. Microtubule depolymerization may interrupt fast axonal transport. At concentrations far in excess of those required to activate TRPV1, capsaicin can also render mitochondria dysfunctional by directly inhibiting electron chain transport. Thus mitochondria are a key convergence point for defunctionalization.

Fig 4

The site of action of topical capsaicin is in the skin, and pain relief is not mediated by transdermal systemic delivery. Owing to near insolubility in water, capsaicin is not readily absorbed into the microvasculature. When cutaneous nociceptors are hypersensitive and sometimes spontaneously active, localized defunctionalization of capsaicin-responsive nerve fibre terminals in the epidermis and dermis can reduce the afferent barrage which may drive pain syndromes. Inset shows how mitochondrial dysfunction leads to nerve terminal retraction.

Fig 5

Topical capsaicin treatment leads to a reversible loss of ENFs. Human leg (calf) skin biopsies pre-capsaicin treatment (baseline; a, PGP 9.5; d, TRPV1), 1 day post (b, PGP 9.5; e, TRPV1), and 54 days post (c, PGP 9.5; f, TRPV1) capsaicin treatment. Biopsies were immunostained with antibodies to structural nerve marker PGP 9.5, and heat and capsaicin receptor TRPV1. There was a marked loss of ENFs and sub-ENFs after capsaicin treatment for 3 days (day 1 biopsy), with regeneration of a majority of ENFs by day 54. Magnification ×40.

Fig 6

Alterations in skin innervation can be used to categorize neuropathic pain syndromes of diverse aetiologies. (a) Innervation of the skin serves to protect organisms through normal nociception. Growth factors (e.g. NGF and GDNF) are constantly produced in the skin and transported retrogradely to sensory neurone cell bodies. (b) When cell bodies are lost or nerves cut and cannot regrow (‘static’ denervation neuropathies), reduced cutaneous innervation results in intact sensory fibres being exposed to abnormally high levels of neurotrophins; this hypertrophic microenvironment is known to enhance excitability and promote sprouting. (c) In ‘dynamic’ denervation neuropathies, cyclic metabolic or other types of stress render cell bodies unable to maintain their longest axons. During cycles of retraction and regrowth, pro-inflammatory cytokines or other mediators may produce axonal excitation. In addition, intact nerve terminals are subject to a hypertrophic environment. (d) In a class of neuropathies or ‘dynias’ best exemplified by vulvodynia or Gullian–Barré syndrome (GBS), immune system activation (or perhaps other factors) have caused local regions of hyperinnervation by cutaneous nociceptors and these nerve terminals display hyperexcitability.