Ricolinostat in Patients With Painful Diabetic Peripheral Neuropathy

Inclusion Criteria: * Type 1 or Type 2 diabetes of at least 6 months with optimized and stable glycemic control during the 3 months prior to Screening * Painful distal symmetric sensorimotor polyneuropathy due to diabetes * Douleur Neuropathique 4 (DN4) score of ≥4 * Satisfactory diary data during the 14-day Pain Observation period determined by an algorithm that includes diary compliance, overall level of pain and day-to-day variability in pain Exclusion Criteria: * Pregnant or lactating * Body Mass Index (BMI) \>40 kg/m2 * Presence of any neuropathy other than DPN and/or significant risk factors for neuropathy other than diabetes * Other pain conditions that could confound the results of this study, or other chronic pain condition(s) that could affect compliance with pain medication restrictions or confound pain assessments * Painful DPN patients who have undergone lower limb amputations, are non-ambulatory, or whose walking is so impaired as to require a walker or other assistance for ambulation * Have met Diagnostic and Statistical Manual of Mental Disorders V (DSM V) criteria for opioid use disorder or alcohol use disorder * Opioid use at a dose of ≥ 30 morphine milligram equivalents on 3 or more days a week during the month prior to Screening * Suicidal ideation/behavior as measured by the Columbia-Suicide Severity Rating Scale (C-SSRS) * The use of marijuana or cannabidiol (CBD) during the 30 days prior to starting study drug * Chronic use of over-the-counter capsaicin on extremities within 3 months of Screening and/or prescription Qutenza use within 6 months of Screening * Corrected QT interval at Screening using QTcF of ≥450 msec (male) or ≥460 msec (female) * Hemoglobin \< 11.5 g/dL (female) or \< 13 g/dL (male), total white blood cell count \< 2500/mm3, neutrophil count \< 1250/mm3, lymphocyte count \< 1000/mm3, or platelet count \< 100,000/mm3 * HIV positive and/or active hepatitis virus (A, B, or C) infection * Current or previous (≤1 month of Screening) enrollment in a clinical trial involving treatment with an investigational product * Any known recent exposure within the 14 days prior to initial Screening to coronavirus disease 2019 (COVID-19) or symptoms of COVID-19 infection or other reason to suspect COVID-19 infection as assessed by the Investigator at the time of initial Screening