Inclusion Criteria: * Patient's written Informed Consent * Age between 18 and 64 years, inclusively. * Patients with confirmed diagnosis of Acute Intermittent Porphyria(AIP), as confirmed by clinical, biochemical data and genetic confirmation of porphobilinogen deaminase (PBGD) gene mutation. The patient must have a severe AIP condition, with at least two hospitalizations during the previous year due to acute attacks (clinical manifestations of acute porphyria), or at least four hospitalizations during the previous year due to the requirement of hospital treatment administration (including day-hospital and home hospital program) * Previous participation in the "Observational study of acute intermittent porphyria patients" for at least six months. * Ability to follow instructions and cooperate during the study conduct Exclusion Criteria: * Pregnant women, as confirmed by a positive urine pregnancy test, or with intention of becoming pregnant * Female subjects of childbearing potential who are not using barrier methods of contraception, at least during the study. * Male subjects with partners of child bearing potential who are not using barrier contraceptive methods, at least during the study * Acute or chronic liver disease of viral, autoimmune or metabolic causes * History of acute or chronic severe gastrointestinal dysfunction (different than those typical gastrointestinal symptoms associated with an acute attack of AIP), in the opinion of the principal investigator * Kidney disorder (renal impairment defined as plasma creatinine \> 2 mg/dl (150 µmol/l)), severe respiratory disease, severe autoimmune disease or severe acute active infection * Evidence of active Hepatitis B virus (HBV) or Hepatitis C virus (HCV) infection as reflected by HBs antigen or HCV-antibodies positivity (in case of HCV-antibodies positivity a HCV-RNA test should be performed in order to confirm active viral replication) * Positive human immunodeficiency virus (HIV) serological test * History of drug use (cannabis, cocaine, amphetamines, barbiturates) or alcohol abuse or addiction, during the three months preceding the selection visit * Presence of neutralizing antibodies against adeno-associated serotype 5 (AAV5) * Current or previous (within the previous 12 months) participation in a gene therapy trial. * Previous participation (at any time) in a gene therapy trial using AAV vectors * Any other disease or condition that, in the opinion of the principal investigator, contraindicates the participation in the study because it can expose the patient to a risk or because it disqualifies the patient to complete the schedule of the study.