The study is a randomized, double blind, placebo-controlled, parallel-group comparison (two dose levels of EMA401 versus a placebo group), of safety and efficacy in patients with postherpetic neuralgia.

Screening Period (Up to Four Weeks) to confirm eligibility for the study which will be determined by Screening tests, physical examination/medical history and fulfillment of eligibility criteria including assessment of pain.

Study Period (14 Weeks) Approximately 360 eligible male and female patients will receive double-blind treatment for 14 weeks. Patients will be randomized in a 1:1:1 ratio to treatment with EMA401 100 mg BID, 300 mg BID or placebo.

Patients will attend the study site at the end of the Baseline visit and end of Weeks 1, 3, 5, 7, 9, 11, 13, and 14 for on-study assessments.

Inclusion Criteria:

• Be diagnosed as suffering from PHN defined as pain in the region of the rash persisting for more than six months after onset of herpes zoster rash.
• Be assessed as suffering from moderate to severe pain across the Screening Period. The assessment of moderate and severe pain will be made using an algorithm proprietary to Spinifex. The Investigator/site staff will be informed immediately as to whether the patient is eligible or ineligible on the ePRO website based on the patient entering all relevant pain scores in the eDiary device.
• Women of child bearing potential (WOCBP) must have a negative urine pregnancy test at the Screening Visit (Visit 1) and within 72 hours prior to administration of IP.

Exclusion Criteria:

• Patients taking any topical treatment for their PHN at the time of Screening Visit 2 will be excluded, including lidocaine plaster, capsaicin patch, and any other topical preparations of these or any other topical medications (e.g., aspirin, Non-Steroidal Anti-Inflammatory Drugs (NSAIDs)) for their PHN.
• Have a blood pressure reading, after resting for at least five minutes, outside a systolic blood pressure range of 84-151 mmHg or a diastolic blood pressure > 95 mmHg. If the blood pressure is outside of the range, a repeat measurement can be taken after the patient has rested. The repeat measurement should be used as the screening value.
• Have serum aspartate transaminase (AST) or alanine transaminase (ALT) levels > 1.5 x the upper limit of normal or have total bilirubin concentrations > 1.5 x the upper limit of normal at Screening (Visit 1).
• Patients who have a known diagnosis of diabetes and are stable on medication with a hemoglobin A1c > 8%. Those who do not have a known diagnosis of diabetes with a hemoglobin A1c > 7%.
• Have active herpes zoster upon physical examination at Screening (Visit 1) or during the study.
• Known history of, or positive laboratory result for hepatitis B virus (HBV), hepatitis C virus (HCV) or human immunodeficiency virus (HIV) infection as defined by being seropositive for hepatitis B surface antigen (HBsAg), HCV antibodies or HIV antibodies respectively.