Study of the Efficacy, Safety, and Tolerability of Serlopitant for the Treatment of Chronic Pruritus of Unknown Origin

Inclusion: * Male or female, age 18 years or older at consent. * The subject must have ongoing chronic pruritus * The subject's pruritus is assessed by the investigator to be of unknown origin at baseline. * Worst-Itch Numeric Rating Scale (WI-NRS) score in the 24-hour period prior to the Screening visit, and average weekly WI-NRS score in each of the 2 weeks prior to Baseline visit indicating an appropriate pruritus level for the study. * The pruritus must have been unresponsive to prior treatment with emollients. * The subject's pruritus must be present on multiple segments of the body * Willing and able to complete daily eDiary entries within a consistent timeframe for the duration of the study * All females who are of childbearing potential must be willing to practice highly effective contraception and not be pregnant or nursing * Willing to comply with study visits and study related requirements including providing written informed consent. * Adequate cognitive and physical ability, in the investigator's opinion, to comply with study visits and study related requirements including providing written informed consent Exclusion * Prior treatment with any NK1-receptor antagonists * Known dermatologic or systemic condition(s), other than dry skin, that is considered by the investigator to be the primary cause of current pruritus. * Untreated or inadequately treated thyroid, adrenal, or pituitary disease or nodules, or history of thyroid malignancy. * Use of an excluded therapy within 3 weeks prior to randomization * Treatment with any investigational therapy within 3 weeks prior to randomization. * Serum creatinine, total bilirubin, alanine aminotransferase or aspartate aminotransferase \> 2.5 times the upper limit of normal during screening. * History of malignancy within 3 years prior to randomization, with the (actinic keratosis, non-metastatic cutaneous squamous cell carcinoma, basal cell carcinoma of skin). * Any known major psychiatric diagnosis that would impact the subject's ability to complete the study * Suicidal ideation within 3 years prior to randomization, or any history of suicide attempt. * Known use of recreational drugs. * Documented history of parasitic infection, including skin parasites such as scabies, within 12 weeks prior to randomization. * Presence of clinically significant dementia, intellectual impairment, or any medical condition or disability that, in the investigator's opinion, could interfere with the assessment of safety or efficacy in this trial or compromise the safety of the subject. * History of hypersensitivity to serlopitant or any of its components. * Planned or anticipated major surgical procedure or other activity that would interfere with the subject's ability to comply with protocol-mandated assessments (e.g. extended international travel) during the subject's participation in the study.