Safety and Tolerability Study of ST-503 for Refractory Pain Due to Peripheral Neuropathy (Small Fiber Predominant, SFN)

Inclusion Criteria 1. Diagnostic characterization of Small Fiber Neuropathy (SFN) according to the Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities and Networks (ACTTION) criteria. 2. Medical record documentation that pain is refractory to 2 of 3 categories of first line medical therapy for at ≥ 6 months prior to screening. 3. Serum sample negative for pre-existing anti-AAV9 antibodies determined by assay detection limit Exclusion Criteria 1. Drug- and alcohol-related: 1. Persons using opioid analgesics for under 3 months or persons who are not on a stable dose of opioids; if on a stable dose, the dose may decrease over the course of the study but should not be increased. 2. History of known alcohol abuse, opioid analgesic abuse, or illicit drug abuse within 2 years of Screening. 3. Positive urine test for drugs of abuse (including opiates, benzodiazepines, amphetamines, cocaine, barbiturates, and phencyclidine) without prescription and investigator approval, at Screening and Day -1. 4. Use of cannabinoids is not permitted. 2. Persons with Fabry's disease, with erythromelalgia, with peripheral neuropathies due to alcohol or drug toxicity, or with diagnosed channelopathies 3. Procedure-related: 1. Contraindications to LP, general anesthesia or sedation 2. Any medical disorders that, in the opinion of the Investigator, could interfere with LP including but not limited to evidence for a pressure gradient between supratentorial and infratentorial compartments, Arnold-Chiari malformation, bleeding diathesis, clinically significant coagulopathy, thrombocytopenia, increased intracranial pressure, or spine disease or past surgical procedures involving the spine 4. Infectious disease-related: 1. Active viral infection or bacterial 2. A severe infection (e.g., pneumonia, septicemia, central nervous system infections \[e.g., meningitis, encephalitis\]) within 12 weeks prior to Screening 5. Hepatic disease- and hepatotoxic medication-related: 1. Presence of clinically relevant liver disease 2. Hepatic dysfunction as indicated by one or more of the following: i. Albumin ≤ 3.5 g/dL ii. Total bilirubin \> 1.5 x ULN and direct bilirubin ≥0.5 mg/dL iii. Alkaline phosphatase (ALP) \> 2 x ULN iv. Alanine transaminase (ALT) or aspartate transaminase (AST) \> 1.5 x ULN 3. Hepatotoxic medications should be avoided during the study period including acetaminophen exceeding 4 gm/day unless essential to patient's treatment, approved by investigator, and hepatic dysfunction is not identified 4. Hepatotoxic supplement use during the study period 6. Cancer-related: a. History of cancer, including B-cell cancers, within 5 years of Screening i. Exceptions to this exclusion are fully excised non-melanoma skin cancers, non-metastatic prostate cancer, and fully treated ductal carcinoma in situ of the breast, provided subject has been stable for at least 6 months b. Previous autologous or allogeneic bone marrow transplant, peripheral stem cell transplant or solid organ transplantation 7. Previously received gene or cellular therapy