Emerging therapies for acute intermittent porphyria.

Acute intermittent porphyria (AIP) is an autosomal dominant metabolic disease caused by hepatic deficiency of hydroxymethylbilane synthase (HMBS), the third enzyme of the heme synthesis pathway. The dominant clinical feature is acute neurovisceral attack associated with high production of potentially neurotoxic porphyrin precursors due to increased hepatic heme consumption. Current Standard of Care is based on a down-regulation of hepatic heme synthesis using heme therapy. Recurrent hyper-activation of the hepatic heme synthesis pathway affects about 5% of patients and can be associated with neurological and metabolic manifestations and long-term complications including chronic kidney disease and increased risk of hepatocellular carcinoma. Prophylactic heme infusion is an effective strategy in some of these patients, but it induces tolerance and its frequent application may be associated with thromboembolic disease and hepatic siderosis. Orthotopic liver transplantation is the only curative treatment in patients with recurrent acute attacks. Emerging therapies including replacement enzyme therapy or gene therapies (HMBS-gene transfer and ALAS1-gene expression inhibition) are being developed to improve quality of life, reduce the significant morbidity associated with current therapies and prevent late complications such as hepatocellular cancer or kidney failure in HMBS mutation carriers with long-standing high production of noxious heme precursors. Herein, we provide a critical digest of the recent literature on the topic and a summary of recently developed approaches to AIP treatment and their clinical implications.