Acute intermittent porphyria (AIP) is a rare genetic disease in which mutations in the porphobilinogen deaminase (PBGD) gene produce insufficient activity of a protein necessary for heme synthesis. This leads to an accumulation of toxic intermediates resulting in a wide variety of problems including acute, severe abdominal pains, psychiatric and neurological disorders, and muscular weakness. Acute porphyric attacks can be life-threatening and the long-term consequences include irreversible nerve damage, liver cancer and kidney failure. AIP affects 1/10,000 people in the EU and the therapies currently available do not prevent the symptoms or consequences of acute porphyric attacks. The only curative therapy is liver transplantation and thus, new curative options are clearly needed. In 2009, the European Medicines Agency granted Orphan Drug Designation to AAV5-AAT-PBGD for the treatment of AIP. AAV is a replication-incompetent virus that has been modified to deliver genes or genetic material into human tissues or cells. AAV5-AAT-PBGD acts by delivering the PBGD expression cassette directly into hepatocytes. In heterozygous AIP patient that show 50% of the normal activity the additional PBGD activity will be sufficient to prevent the accumulation of toxic metabolites and thus, to prevent porphyric attacks. the aim of this project is the clinical development of the orphan drug AAV-AAT-PBGD for use to treat AIP. The project will be performed in three different phases. In the first phase, we will develop a GMP-compliant process to produce sufficient AAV5-AAT-PBGD for clinical trials, and we will constitute the largest possible cohorts to improve the follow-up of patients and determine the clinical criteria to select patients for gene therapy. In the second phase, the safety and efficacy of AAV5-AAT-PBGD will be explored in a dose escalation clinical phase I/II trial.