This is a randomized, double-blind, 2-arm, parallel group study of up to 274 evaluable patients designed to evaluate the safety and efficacy of the histone deacetylase 6 (HDAC6) inhibitor ricolinostat for painful DPN.

The study includes an approximately 12 week randomized, double-blind, placebo controlled Treatment period in which patients will receive either ricolinostat or placebo, followed by an approximately 12 week open label Safety Extension period during which all patients will receive ricolinostat 120 mg daily.

Prior to randomization, patients will be enrolled in a baseline Pain Observation period from Day -14 to Day -1, during which the NRS (average and worst pain) will be recorded daily using an electronic daily diary that will be completed by patients to allow patients to familiarize themselves with the pain rating procedures, and to establish a baseline and confirm eligibility to participate. Patients will also initiate daily dosing during this time to evaluate compliance eligibility for participation. A daily diary will be used by the patient to record the pain assessments and rescue medication use. A follow-up phone contact will be conducted at Day -7 to Day -5 to review diary and dosing compliance.

Following the baseline Pain Observation period, patients who meet entry criteria will be randomized in a 1:1 ratio to receive either ricolinostat or placebo. During the 12-week double-blind, placebo-controlled Treatment period, patients will return for assessments in accordance with the schedule of assessments. At the conclusion of the approximately 12 week open label Safety Extension period, patients will enter a Follow-up safety washout and assessment period, which will incorporate 2 visits at approximately 2 and 4 weeks following the final Safety Extension visit, with assessments performed as outlined in the schedule of assessments.

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

double-blind