Inclusion Criteria:

Prior to randomization, subjects eligible for enrollment in the study must meet all of the following criteria:

• Outpatient subjects aged 18 years or older, with a documented medical diagnosis of PHN of ≥3 months in duration prior to screening (i.e. pain present for at least 3 months from the healing of a herpes zoster skin rash).
• A female subject is eligible to enter and participate in the study if she:
• Is of non-childbearing potential; or
• Is of child-bearing potential, is not lactating and has a negative pregnancy test <7 days prior to study treatment initiation and agrees to use one of the GSK specified highly effective methods for avoiding pregnancy
• For the purposes of this study, PHN is defined as pain persisting for ≥ 3 months after healing of the shingles rash.

Subjects with the above definition of PHN must satisfy the following criteria to be eligible for the study:

• Subjects currently on a stable dose of 1800 mg/day of gabapentin for 2 weeks or more since the diagnosis of PHN, with inadequate response;
• Subjects not currently treated with gabapentin but previously treated with ≥ 1800 mg/day of gabapentin for 4 weeks or more since the diagnosis of PHN with inadequate response.

Subjects may previously have been on >1800 mg/day of gabapentin since diagnosis of PHN.

• The baseline 24-hour average pain intensity score is >4.0 based on an 11-point PI‑NRS. The baseline score is the calculated mean of the daily scores (based on at least 4 assessments) taken during the 7 days prior to randomization. Subjects are to continue to take gabapentin 1800mg/day provided by GSK during the Baseline Period.
• Subject is compliant with the gabapentin treatment supplied during the 14-day Baseline Period. Compliant is defined as administering >80% of the total supplied treatment over the 14-day Baseline Period.
• Subject is able to provide written informed consent prior to participation in the study. The contents and process of obtaining informed consent will be in accordance with all applicable regulatory requirements.

Exclusion Criteria:

Subjects meeting any of the following criteria must not be enrolled in the study:

• Has other chronic pain conditions not associated with PHN. However, the subject will not be excluded if ALL the following criteria apply:
• The pain is located at a different region of the body.
• The pain intensity is not greater than the pain intensity of the PHN ; and
• The subject can assess PHN pain independently of the other pain condition.
• Is unable to discontinue:
• Prohibited medications, or
• Non-drug therapies or procedures (i.e. nerve blocks, trans-cutaneous electrical nerve stimulation [TENS]) for the relief of pain of PHN for the required washout period and throughout the duration of the study
• Has any of the following medical conditions, laboratory abnormalities or disorders:
• Hepatic impairment defined as alanine aminotransferase (ALT) or aspartate aminotransferase (AST) > 2x upper limit of normal (ULN) or alkaline phosphatase or bilirubin > 1.5x ULN.
• Chronic hepatitis B or C with a positive Hepatitis B surface antigen (HBsAg) or Hepatitis C Core Antigen Antibody (Hep C antibody).
• Impaired renal function defined as either creatinine clearance < 60 mL/min (estimation of creatinine clearance by Cockroft and Gault Method) or renal dysfunction requiring hemodialysis.
• Corrected QT (QTc) interval ≥450 msec (based on single or average QTc value of triplicate electrocardiograms (ECGs) obtained over a brief recording period).
• QTc interval ≥480 msec for patients with Bundle Branch Block.
• Uncontrolled hypertension at screen (sitting systolic blood pressure [SBP] >160 mmHg and/or sitting diastolic blood pressure [DBP] >90 mmHg.
• Current diagnosis of active epilepsy or any active seizure disorder requiring chronic therapy with antiepileptic drug(s).
• Medical condition or disorder that would interfere with the action, absorption, distribution, metabolism, or excretion of XP13512, or, in the investigator's judgement:
• Is considered to be clinically significant and could pose a safety concern or,
• Could interfere with the accurate assessment of safety or efficacy or,
• Could potentially affect a subject's safety or study outcome.

Examples of such medical conditions include:

• Skin conditions at the site of the neuropathy.
• Active infection at the site of the neuropathy.
• Current or chronic history of liver disease (including acute viral hepatitis), or known hepatic or biliary abnormalities (with the exception of Gilbert's syndrome or asymptomatic gallstones).
• Meets criteria as defined by the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) for a major depressive episode or for active significant psychiatric disorders within last year, including dementia, general anxiety disorder, psychosis or bipolar disorder.
• Subjects with a history of depression that is in remission, with or without antidepressant treatment, may participate, unless a stable antidepressant regimen includes a prohibited medication.
• Antidepressant medication may not be changed or discontinued to meet entry criteria and must be stable for at least three months prior to the start of the Baseline Period.
• Has a history of clinically significant drug or alcohol abuse as defined by DSM‑IV‑TR or is unable to refrain from substance abuse throughout the study. Benzodiazepines or atypical benzodiazepines prescribed as hypnotic sleep agents are permitted.
• Is currently participating in another clinical study in which the subject is, or will be exposed to an investigational or non-investigational drug or device.
• Has participated in a clinical study in which the subject was exposed to an investigational or non-investigational drug or device:
• Within the preceding month for studies unrelated to the current illness, or
• Within the preceding six months for studies related to the current illness.
• Has been treated previously with XP13512.
• Has a history of an allergic reaction, or a medically significant adverse reaction to any of the following:
• The investigational products (including gabapentin) or,
• Their excipients or,
• Acetaminophen or,
• Compounds closely related to acetaminophen.