Although nonopioids are generally used for mild to moderate pain, opioids— such as morphine, fentanyl, hydrocodone, and oxycodone—are
used to manage moderate to severe pain and chronic pain. Opioids bind to opiate receptors that are located in the central
nervous system and decrease the transmission of nociceptive input (pain caused by irritation or injury) —reducing the perception
of pain in the brain.
Adjuvants include antidepressants, anticonvulsants, and other therapeutics that are used off-label or in combination with
nonopioid and opioid therapies. (See "Off-Label Controversy,".)
Worldwide, there are few options that effectively address neuropathic pain, with only eight drugs approved for such conditions.
(See "Global Neuropathic Pain Growth" and "Neuropathic Market Share,".) Only three are marketed worldwide: Lidoderm (lidocaine
patch), a topical analgesic, and Neurontin (gabapentin) and Tegretol (carbamazepine), both of which are anticonvulsants with
undefined mechanisms of action. Tegretol is a rarely prescribed drug indicated for trigeminal neuralgia (a rare form of neuropathic
pain), leaving Pfizer's Neurontin and Endo's Lidoderm as the only substantially viable options for most US patients.
Global Neuropathic Growth, Neuropathic Market Share
FDA, however, has approved those two drugs for the limited indication of neuropathic pain associated with herpes zoster (postherpetic
neuralgia), which means most US prescriptions are off-label. In Europe, Neurontin is indicated for the broader category of
neuropathic pain, allowing Pfizer to legally market it there for various types of painful neuropathy.
Hartaj Singh, managing consulting at Navigant Consulting, explains why FDA is so strict. "It is hard to design clinical trials
around neuropathic pain, as clinical endpoints for neuropathic pain are not well defined and are difficult to measure, with
the exception of postherpetic neuralgia." But until FDA becomes more lenient in the indications granted to neuropathic pain
treatments, growth in the US market will be somewhat hindered.
Fortunately, there are products in the pipeline that hold promise for the near future. New drugs entering the pain market
include four products under development specifically for neuropathic pain: Pfizer's Lyrica (pregabalin), Elan's Prialt (ziconotide),
GW Pharmaceuticals' Sativex (cannabis extract), and GSK's Lamictal (lamotrigine), which, like Tegretol, is already on the
market for epilepsy. Of these, Lyrica, a follow-up product to top-selling Neurontin, may hold the most potential. Not only
has Lyrica demonstrated superior clinical efficacy over Neurontin, but Pfizer has sought a broader neuropathic pain indication
for the second-generation drug.
On September 2, FDA issued three approvable letters for Lyrica to treat neuropathic pain associated with diabetic peripheral
neuropathy and postherpetic neuralgia, and as adjunctive therapy in the treatment of partial seizures. (The drug did not receive
approval for treating generalized anxiety disorder.) In addition, Lyrica was recently approved in the European Union for peripheral
neuropathic pain and as an adjunctive therapy in the treatment of partial seizures.