Consequences and Prevention of Elevated Circulating Tyrosine during Nitisinone Therapy in Alkaptonuria.

Abstract

Alkaptonuria (AKU) is an ultra-rare, autosomal recessive disorder of tyrosine catabolism due to mutations within the homogentisate 1,2-dioxygenase (HGD) gene. The resulting enzyme deficiency leads to accumulation of homogentisic acid (HGA) and deposition of melanin-like pigment polymers in the connective tissues of the body in a process called ochronosis. This leads to debilitating early onset osteoarthropathy, renal damage and aortic valve disease. As a multisystem disorder, AKU results in progressive and chronic pain and severe morbidity. Most management approaches for AKU are palliative and rely largely on analgesia and arthroplasty. Several therapeutic approaches have been tested with low degrees of clinical effectiveness. Nitisinone is a promising drug that blocks the enzyme catalysing the formation of HGA and thus lowers its plasma concentration. HGA lowering therapy has been widely used in another rare inborn error of metabolism, Hereditary Tyrosinemia type 1 (HT-1) for over 20 years. Nitisinone is highly efficacious in terms of its metabolic effect as it decreases HGA to very low levels, but there is limited toxicology data available for its use in AKU. There are also concerns relating to the adverse side effects of elevated tyrosine and potential neurotoxicity if treatment was implemented in children. The work presented within this thesis presents novel findings to inform the future licensing process for the use of nitisinone in AKU and investigates the safety of implementing treatment in younger patients. Nitisinone treatment had no detrimental effect on learning, memory or motor function in young AKU or wild type mice. The thesis also includes new data from mouse dosing studies concerning the correlation between plasma HGA and ochronotic pigmentation and reveals that plasma HGA must be lowered to a critical level before pigmentation is beneficially reduced. Finally, this thesis reports on the lability of the arteriovenous metabolome relating to AKU and initiates a discussion relating to the HPPA to HPLA excretory conversion pathway along with important considerations for collection, analysis and comparison of blood samples in future studies.