This is a placebo-controlled, double blind, crossover study to evaluate the efficacy and safety of KHK6188 in postherpetic neuralgia when administered orally for 2 weeks.

Inclusion Criteria:

• Patients with pain for ≥ 3 months and ≤ 1 year following onset of a herpes zoster rash
• Patients with a mean pain intensity score (11-point numerical rating scale) of ≥ 4
• Patients whose rash has been healed
• Patients who are able to fill their patient diary
• Patients who provided written voluntary informed consent to participate in the study

Exclusion Criteria:

• Patients who have other pain or disease which may impair the self assessment of pain
• Patients who have dementia, depression or schizophrenia which may affect the self assessment of pain
• History or presence of severe cardiovascular disease, hepatic dysfunction, renal failure, respiratory disease, blood disease or CNS disorder
• History or presence of a drug allergy
• Patients who have a history of a diagnosis of cancer or a treatment of cancer within 5 years prior to the study entry
• Patients received an investigational medication within 4 months (6 months for biologic medication such as antibody) prior to the informed consent
• Women of child bearing potential who do not agree to avoid pregnancy from the time of providing the consent until 3 months after the end of dosing, or men of reproductive potential who do not agree to avoid pregnancy from the time of the first dose until 3 months after the end of dosing
• Patients who are pregnant, lactating, or possibly pregnant
• Patients judged to be inappropriate to enter the study by a investigator or a subinvestigator