Alkaptonuria

Overview

Alkaptonuria (AKU) is a rare inherited metabolic disorder caused by disease-causing changes in both copies of the HGD gene. The missing or poorly functioning HGD enzyme allows homogentisic acid (HGA), a normal by-product of tyrosine breakdown, to accumulate. Urine may darken after standing, while long-term HGA deposition causes bluish-black connective-tissue pigment called ochronosis, early and severe arthritis of the spine and large joints, and sometimes heart-valve disease or kidney and prostate stones. AKU affects an estimated 1 in 250,000 to 1 million people; symptoms commonly become clinically important in adulthood, but life span is not generally shortened. GeneReviews EMA overview

Current care combines monitoring and treatment of complications—pain management, physical and occupational therapy, joint replacement when necessary, and cardiac, kidney, and endocrine surveillance—with HGA-lowering therapy. In the United States, FDA-labeled Harliku (nitisinone) is an oral 2 mg once-daily treatment for reducing urinary HGA in adults with AKU; it is disease-modifying rather than curative because it reduces production of HGA upstream of the defective HGD enzyme rather than restoring HGD. FDA prescribing information EvolvAKUre project

Scope of Recent Research (2020–present)

Research activity since 2020 has been focused primarily on improving the use and safety of nitisinone, measuring whether lower HGA translates into slower multisystem disease, and developing liver-directed genetic approaches that could replace HGD activity. The field remains small, but it has advanced from a 2023 gene-therapy conference report to a 2026 mouse preprint reporting metabolic correction with an AAV-HGD vector; however, there is not yet a human curative gene-therapy trial or an approved treatment that repairs the underlying genetic defect. SONIA 2 trial AAV-HGD preprint 2023 AAV-HGD conference report

Major Breakthroughs and Emerging Therapies

The clearest clinical advance has been nitisinone, a small molecule that inhibits 4-hydroxyphenylpyruvate dioxygenase, an enzyme upstream of HGD in tyrosine metabolism. In the international phase 3 SONIA 2 trial, 10 mg daily reduced 24-hour urinary HGA by 99.7% at 12 months compared with no treatment and was associated with a significantly smaller increase in clinical AKU severity over four years. A separate National Alkaptonuria Centre analysis found that nitisinone treatment was associated with partial reversal of visible ochronotic pigmentation and a 19.1% reduction in pigment measured in paired ear-cartilage biopsies. These findings are important disease-modifying progress, but nitisinone does not replace defective HGD and therefore is not a genetic cure. SONIA 2 trial Ochronosis reversal study EvolvAKUre project

The leading curative strategy is liver-directed gene addition: delivering a functional human HGD gene to liver cells using an adeno-associated virus (AAV) vector. A 2026 preprint reported that AAV2/8 carrying codon-optimized human HGD under a liver-specific promoter substantially corrected biochemical abnormalities in Hgd-deficient mice. At the highest tested dose, reporter studies showed liver-predominant delivery with 89–93% hepatocyte transduction; treated mice achieved HGA lowering comparable to nitisinone without elevation of upstream tyrosine-pathway metabolites. This is a strong preclinical proof of concept, but it remains a mouse study and has not yet undergone peer review or testing in people. AAV-HGD preprint

A complementary strategy, EvolvAKUre, has explored “directed evolution” of HGD: laboratory selection methods intended to produce a more active enzyme that could work at lower AAV doses. The project also proposed combining evolved HGD with a nitisinone-insensitive form of the upstream enzyme HPPD, potentially allowing continued metabolic flow while avoiding nitisinone-associated tyrosine accumulation. Supporting this engineering effort, researchers developed a bacterial high-throughput assay that measures how AKU-associated HGD missense variants consume HGA, helping rank residual activity and identify variants relevant to enzyme design. EvolvAKUre project HGD variant-screening platform

Clinical Trials and Experimental Approaches

The pivotal recent human study was SONIA 2, a completed international, multicentre phase 3 trial sponsored by the University of Liverpool. It enrolled 138 adults aged 25 years or older in the United Kingdom, France, and Slovakia and randomized them to nitisinone 10 mg daily or no treatment for four years. The trial met its biochemical endpoint and showed slower clinical progression by the clinical Alkaptonuria Severity Score Index; 55 of 69 nitisinone-treated participants and 53 of 69 controls completed the study. SONIA 2 trial SONIA 2 registry record

The experimental cure-oriented program is still preclinical. A liver-directed AAV-HGD study was presented as a conference poster in 2023 and was followed by the 2026 mouse preprint; neither source reports dosing of human participants. EvolvAKUre, the related Belgian gene-therapy development project, ran from January 2020 through December 2023 and investigated evolved HGD, AAV delivery, human cell models, and AKU mice. 2023 AAV-HGD conference report AAV-HGD preprint EvolvAKUre project

Methodologies and Scientific Approaches

AKU cure research is increasingly designed around the biology of the liver, which is the main organ responsible for metabolizing HGA. The AAV-HGD program uses Hgd-deficient mice, liver-specific AAV2/8 vectors, and a codon-optimized human HGD sequence. Researchers assess delivery and expression with DNA in situ hybridization, quantitative RNA testing, and protein analysis, then measure metabolic correction using targeted liquid chromatography–tandem mass spectrometry and broader untargeted metabolomics. AAV-HGD preprint

Animal models are also clarifying where a therapy must act. A 2026 kidney-specific Hgd knockout study found that mice with intact liver HGD maintained near-normal circulating HGA despite loss of kidney HGD, indicating that the kidney is not a useful target for HGD-restoration therapy. In people, key translational biomarkers include plasma and 24-hour urinary HGA, tyrosine concentrations, eye examinations for tyrosine-related keratopathy, ochronosis assessments, and the clinical AKU severity score used in SONIA 2. Kidney-specific HGD study SONIA 2 trial FDA prescribing information

Leading Institutions and Funding

The University of Liverpool and its clinical partners have led much of the modern clinical evidence base, including the SONIA 2 trial and the United Kingdom National Alkaptonuria Centre. The European Commission funded the DevelopAKUre program, which included SONIA 2, with €6 million through Framework Programme 7; Swedish Orphan Biovitrum supplied nitisinone for the program. DevelopAKUre trial-development review

The gene-therapy effort has been led by Vrije Universiteit Brussel, working with the University of Liverpool and RWTH Aachen University. The Research Foundation–Flanders awarded EvolvAKUre approximately €511,000 to pursue preclinical proof of concept for an AKU cure through gene therapy and protein engineering. The AKU Society has been a prominent patient-led partner in communicating and supporting this work. EvolvAKUre funding announcement EvolvAKUre project

Strengths, Limitations, and Challenges

A major strength is that AKU has a direct, measurable biochemical driver: lowering HGA is straightforward to quantify, and nitisinone has shown that very large HGA reductions can translate into slower clinical progression. The new AAV-HGD mouse work is especially promising because it addresses the cause of AKU and, unlike nitisinone, restored metabolic flux without raising tyrosine-pathway metabolites in the reported mouse experiments. SONIA 2 trial AAV-HGD preprint

The limitations are substantial. Nitisinone can raise plasma tyrosine, requiring dietary management and monitoring for eye symptoms and painful skin changes; it is therefore not a one-time cure. The AAV-HGD findings are preliminary, derived from mice, and will need replication, long-term durability data, dose optimization, immune-safety testing, manufacturing validation, and evidence that biochemical correction prevents or improves clinically meaningful outcomes in people. The EvolvAKUre proposal itself sought higher-activity HGD specifically to avoid requiring high viral-vector doses, illustrating a central translational challenge. FDA prescribing information EvolvAKUre project

Outlook and Future Directions

As of October 8, 2026, AKU is closer to a plausible genetic cure than it was in 2020, but a cure is not yet near clinical availability. The key milestones to watch are peer-reviewed publication and independent replication of the AAV-HGD mouse results; demonstration of durable HGA normalization and safety at clinically feasible vector doses; confirmation that liver-directed HGD replacement avoids nitisinone-associated tyrosine toxicity; and eventual regulatory clearance for a first-in-human trial. Until then, nitisinone remains the evidence-based disease-modifying option, while liver-directed HGD gene therapy is the leading experimental curative approach. AAV-HGD preprint SONIA 2 trial

References

• Alkaptonuria GeneReviews — Introne, Perry, and Chen; University of Washington, 2021.
• EMA overview of first treatment for alkaptonuria — European Medicines Agency, 2020.
• Harliku prescribing information — U.S. Food and Drug Administration, 2025.
• SONIA 2 phase 3 nitisinone trial — Ranganath et al., The Lancet Diabetes & Endocrinology, 2020.
• Ochronosis reversal study — Ranganath et al., JIMD Reports, 2020.
• SONIA 2 registry record — ClinicalTrials.gov, 2026.
• 2023 AAV-HGD conference report — Lequeue et al.; Vrije Universiteit Brussel, 2023.
• AAV-HGD preprint — Lequeue et al.; University of Liverpool Repository, 2026.
• EvolvAKUre project — Vrije Universiteit Brussel, 2020–2023.
• HGD variant-screening platform — Lequeue et al., Scientific Reports, 2022.
• Kidney-specific HGD study — Rutland et al., Human Molecular Genetics, 2026.
• DevelopAKUre trial-development review — Davison et al., Orphanet Journal of Rare Diseases, 2022.
• EvolvAKUre funding announcement — AKU Society, 2020.