Nitisinone attenuates progression of aortic stenosis in patients with alkaptonuria: an analysis of the SONIA 2 study

Background/introduction Alkaptonuria (AKU) is a rare metabolic disorder caused by a defective enzyme, resulting in deposition of unmetabolised homogentisic acid in various connective tissues throughout the body (also termed "ochronosis"). Ochronosis of the aortic valve leading to progressive aortic stenosis is a rare but serious complication. Nitisinone decreases urinary and serum homogentisic acid levels and has been shown to improve morbidity and slow disease progression in AKU but the effects of this treatment on the progression of aortic stenosis have not yet been described. This review extrapolated from the data set of the SONIA 2 study, a 4-year multi-centre, randomised controlled trial investigating the effect of nitisinone on a composite clinical measure of AKU disease activity, but looked more specifically at measures of aortic stenosis disease progression. Methods Data was obtained from echocardiograms performed on 138 patients at baseline, 12, 24, 36 and 48 months of follow-up. In measuring the degree of aortic stenosis, the peak trans-aortic velocity (Vmax) was examined. A linear mixed effects regression model was used to assess the association between treatment and Vmax and to ascertain the difference in this measure at baseline and 48 months between the treatment and control groups. The mixed effects model incorporated both fixed effects for population parameters (age, sex, baseline Vmax, follow-up time) and treatment; and random effects, to account for intra-subject correlation of longitudinal observations of Vmax, inter-subject variability of baseline measurements of Vmax, and centre. Results At baseline, 19/138 patients (13.8%) had aortic stenosis, as classified by echocardiogram findings in accordance with the European Society of Cardiology (ESC) guidance on valvular heart disease, with 9/19 having mild aortic stenosis, 6/19 having moderate aortic stenosis and 4/19 having severe aortic stenosis. 25/138 had aortic sclerosis (see figure 1). The prevalence of aortic valve disease increased with age. From the 4-year follow-up period, 613 longitudinal observations of 138 subjects across all sites were obtained. At baseline, the difference in Vmax between the control and treatment groups was 0.063 m/s [95% CI: -0.054 m/s to 0.18 m/s] and did not reach statistical significance (p=0.23). At the end of the 4-year treatment period, the difference in Vmax was 0.10 m/s [95% CI: -0.0007 m/s to 0.20 m/s] and was statistically significant (p=0.05) (see figure 2). Conclusion Nitisinone slowed progression of aortic stenosis in patients with AKU. This may be grounds for timely initiation of nitisinone in those deemed to be at high risk, as identified by echocardiography. Of note, this is the first time that any medical therapy has ever been shown to affect the natural history of aortic stenosis.