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Acute Intermittent Porphyria

Recent research efforts aimed at curing Acute Intermittent Porphyria.

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Acute Intermittent Porphyria

Overview

Acute intermittent porphyria (AIP) is an inherited disorder in which the liver has too little activity of the enzyme hydroxymethylbilane synthase (HMBS, also called porphobilinogen deaminase). This can cause buildup of neurotoxic heme-making intermediates, especially aminolevulinic acid (ALA) and porphobilinogen (PBG), leading to intermittent attacks of severe abdominal pain, vomiting, fast heart rate, high blood pressure, weakness, seizures, and mental or neurologic symptoms. AIP is inherited in an autosomal-dominant pattern, but most people carrying an HMBS variant never develop attacks; attacks are more common in women and usually begin after puberty. GeneReviews: Acute Intermittent Porphyria (ncbi.nlm.nih.gov)

With prompt recognition and treatment, recovery from an attack is often good, although severe attacks can cause prolonged nerve injury and recurrent disease can contribute to kidney and liver complications. Standard care includes avoiding triggers, rapidly treating significant attacks with intravenous hemin, supportive treatment for pain and electrolyte disturbances, and using givosiran to prevent recurrent attacks. Liver transplantation is an established cure for the rare group with life-threatening, treatment-refractory disease, because replacing the liver removes the main source of toxic precursor overproduction. GeneReviews: Acute Intermittent Porphyria (ncbi.nlm.nih.gov)

Scope of Recent Research (2020–present)

Research since 2020 has been active but concentrated in a small number of liver-directed strategies: suppressing the overactive heme-production pathway with RNA interference, restoring HMBS enzyme activity with messenger RNA (mRNA), improving liver gene delivery, and stabilizing the remaining normal HMBS protein with small molecules. AIP is especially attractive for etiologic treatment because it is a single-gene, liver-centered disease; however, no experimental genetic or RNA-based treatment has yet demonstrated a durable human cure comparable to liver transplantation. Update on the Porphyrias (ncats.nih.gov)

Major Breakthroughs and Emerging Therapies

The major clinical advance has been givosiran, a small interfering RNA (siRNA) medicine that lowers liver production of aminolevulinic acid synthase 1 (ALAS1), the first rate-limiting enzyme in heme synthesis. This does not repair the faulty HMBS gene, so it is not a cure, but it addresses the biochemical chain that produces ALA and PBG. In the Phase 3 ENVISION trial, participants with AIP receiving givosiran had a mean annualized attack rate of 3.2 compared with 12.5 with placebo, a 74% reduction, along with lower ALA/PBG levels, less hemin use, and improved pain outcomes. ENVISION Phase 3 trial (nejm.org)

The most directly corrective experimental approach is HMBS mRNA replacement. Researchers package laboratory-made human HMBS mRNA in lipid nanoparticles, microscopic fat-like carriers that preferentially deliver the mRNA to liver cells. Those cells then temporarily produce the missing enzyme. In a 2025 nonhuman-primate AIP model, repeated HMBS mRNA dosing restored liver HMBS activity and protected animals from induced biochemical attacks, liver metabolic abnormalities, and central nervous system changes. This is strong translational evidence, but it remains preclinical: repeated dosing would probably be needed, and human safety, dose selection, durability, and clinical benefit remain unproven. HMBS mRNA therapy in nonhuman primates (pubmed.ncbi.nlm.nih.gov)

AAV gene addition aims for a more durable solution by delivering a working HMBS gene to hepatocytes using an adeno-associated virus (AAV) vector. The first-in-human rAAV2/5-PBGD study showed that this strategy could be administered safely, but it did not lower urinary ALA or PBG at the tested doses, indicating that too few liver cells received enough functional gene. All treated participants developed neutralizing antibodies to the AAV5 capsid, an important obstacle because such antibodies can limit repeat dosing. Phase I AAV5-PBGD study (sciencedirect.com)

A complementary small-molecule strategy is pharmacological chaperoning. A pharmacological chaperone is a compound designed to stabilize a protein so that it survives longer and functions better. In a 2020 proof-of-concept study, candidate compounds increased HMBS protein abundance and activity in cultured human liver cells and in an HMBS-deficient mouse model, with lower liver porphyrin-precursor levels in treated mice. This could be particularly useful because many people with AIP retain one normal HMBS allele, but the approach remains an early discovery-stage program rather than a clinical therapy. Pharmacological chaperone therapy for AIP (pmc.ncbi.nlm.nih.gov)

Clinical Trials and Experimental Approaches

The most important recent completed trial is ENVISION (NCT03338816), a completed randomized Phase 3 trial sponsored by Alnylam Pharmaceuticals that enrolled 94 people with acute hepatic porphyria, most of whom had AIP. Its 36-month analysis found sustained attack reduction with ongoing monthly givosiran: 86% of participants continuously receiving givosiran and 92% of placebo-crossover participants had no attacks by the end of the open-label extension. These results establish disease control, not correction of the underlying HMBS defect. ENVISION trial record 36-month ENVISION analysis (clinicaltrials.gov)

The notable gene-replacement clinical study is NCT02082860, a completed Phase I, open-label, dose-escalation trial of liver-directed rAAV2/5-PBGD gene therapy in adults with severe AIP. The trial tested a single intravenous infusion across four dose cohorts. It provided important human safety and vector-persistence data, but biochemical correction was not achieved, so it should be viewed as a foundational feasibility study rather than evidence of clinical cure. rAAV2/5-PBGD trial record Phase I AAV5-PBGD study (clinicaltrials.gov)

Methodologies and Scientific Approaches

AIP cure research relies on models that reproduce both the liver biochemical defect and the neurologic consequences of attacks. Researchers measure urinary ALA and PBG as core biomarkers, alongside liver HMBS activity, ALAS1 expression, mitochondrial function, nerve measures, pain-related behavior, and brain imaging or metabolic signals. The recent nonhuman-primate model used liver-specific HMBS suppression followed by porphyrinogenic drug exposure to recreate recurrent attacks and test whether HMBS mRNA could reverse the disease process. HMBS mRNA therapy in nonhuman primates (pubmed.ncbi.nlm.nih.gov)

Delivery is the decisive technical challenge. AAV vectors are designed for longer-lived liver gene expression but can provoke anti-capsid immunity, whereas lipid nanoparticles enable repeatable delivery of temporary mRNA instructions. Researchers are also using pharmacokinetic-pharmacodynamic modeling—mathematical models linking dose, liver HMBS activity, and ALA/PBG reduction—to estimate human dose regimens before first-in-human mRNA trials. mRNA therapy pharmacokinetic-pharmacodynamic modeling (pubmed.ncbi.nlm.nih.gov)

Leading Institutions and Funding

The University of Navarra’s Center for Applied Medical Research (CIMA) and its clinical collaborators have been central to AAV-PBGD and HMBS-mRNA research, including the development of advanced animal models. Moderna contributed to the mRNA-replacement development program, while investigators in European porphyria centers have supplied expertise in clinical phenotyping, metabolic biomarkers, and trial design. HMBS mRNA therapy in nonhuman primates mRNA therapy pharmacokinetic-pharmacodynamic modeling (pubmed.ncbi.nlm.nih.gov)

In the United States, the NIH Rare Diseases Clinical Research Network’s Porphyrias Consortium—supported by the National Center for Advancing Translational Sciences and the National Institute of Diabetes and Digestive and Kidney Diseases—has built natural-history cohorts, biomarker knowledge, and trial infrastructure. Consortium sites, including Mount Sinai and collaborating academic medical centers, helped enable givosiran studies, while Alnylam Pharmaceuticals funded ENVISION and developed givosiran. NCATS Porphyrias Consortium ENVISION Phase 3 trial (ncats.nih.gov)

Strengths, Limitations, and Challenges

AIP has several scientific advantages for cure development: the causal gene is known, the main disease-driving organ is the liver, and ALA/PBG offer rapid measurable biomarkers of biochemical response. Givosiran validates the principle that reducing the liver’s abnormal heme-synthesis drive can greatly reduce attacks, while the nonhuman-primate mRNA data show that restoring HMBS activity can protect against biochemical and neurologic consequences of attacks. ENVISION Phase 3 trial HMBS mRNA therapy in nonhuman primates (nejm.org)

The limitations are substantial. Givosiran requires continuing treatment and has been associated with liver-enzyme elevations, renal-function changes, and injection-site reactions. AAV gene therapy must overcome insufficient liver transduction and the immune response that may prevent re-administration. mRNA replacement is repeat-dose compatible but may require lifelong infusions unless durable delivery is achieved. Small-molecule chaperones must demonstrate sufficient enzyme rescue across diverse HMBS variants and must progress from mouse studies to rigorous human testing. Liver transplantation remains curative but is reserved for severe disease because it requires donor organs and lifelong immunosuppression. ENVISION Phase 3 trial Phase I AAV5-PBGD study GeneReviews: Acute Intermittent Porphyria (nejm.org)

Outlook and Future Directions

As of September 7, 2026, AIP has an established surgical cure—liver transplantation—but no scalable molecular cure proven in people. The closest experimental etiologic strategy is likely liver-targeted HMBS replacement, particularly repeat-dose lipid-nanoparticle mRNA therapy, because it has now shown protection in a clinically relevant nonhuman-primate model. The milestones to watch are a first-in-human HMBS-mRNA trial, improved AAV vectors that achieve adequate liver delivery while addressing anti-AAV immunity, and evidence that restoring HMBS not only lowers ALA/PBG but also prevents attacks, chronic neurologic injury, kidney decline, and liver complications over years. HMBS mRNA therapy in nonhuman primates Update on the Porphyrias (pubmed.ncbi.nlm.nih.gov)

References

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