Crossover Post-herpetic Neuralgia (PHN)

Allocation: Randomized Stratified

Interventional model: Cross-over Placebo Controlled

Inclusion Criteria:

- Patient with Post-Herpetic Neuralgia (PHN) as defined as pain present for more than 6
months after the onset of a herpes zoster skin rash affecting the trigeminal,
cervical, thoracic, lumbar, or sacral regions.

- Based on patient diary information collected during the Baseline week (day -7 to
randomization Day 1), patient has completed at least 5 diary entries and has an
average weekly pain rating of at least 4 on the 11-point pain rating scale.

- The patient is able to satisfactorily complete, in the Investigator's judgment, the
Cognitive Battery.

- Male or female, 18-85 years of age.

Exclusion Criteria:

- Other severe pain that may potentially confound pain assessment.

- History of complete lack of response to pregabalin (at least 300 mg qd for 4 weeks)
or gabapentin (at least 1800 mg qd for 4 weeks).

- Hemoglobin A1c > 9%

- Hemoglobin ≤ 9 g/dL.

- Active herpes zoster or known viral infection.

- Previous neurolytic or neurosurgical therapy for PHN.

- Estimated glomerular filtration rate (eGFR) according to the re-expressed abbreviated
(four-variable) Modification of Diet in Renal Disease (MDRD) Study equation ≤
40ml/min/1.73m2.

- Patients who have been on a stable dose of anticonvulsant,anticholinergic, antiviral
medications, nicotine replacements, or any other smoking cessation medications for <4
weeks prior to randomization. Patients who are on stable doses for => 4 weeks prior
to randomization are allowed, however, there should be no adjustments to the dose of
these medications during study.

- Patients currently on more than one drug for treatment of neuropathic pain (low dose
opioids, antidepressants, or anticonvulsants). Patients are allowed to participate if
on a stable dose for at least 4 weeks prior to randomization (Day1) and should remain
stable during course of study.