Patient-Reported Outcomes and Functional Assessments of Patients with Alkaptonuria in a 3-year Nitisinone Treatment Trial

Abstract

Alkaptonuria is a rare disorder of tyrosine catabolism caused by deficiency of homogentisate 1,2-dioxygenase that leads to accumulation of homogentisic acid (HGA). Deposition of HGA-derived polymers in connective tissue causes progressive arthropathy of the spine and large joints, cardiac valvular disease, and genitourinary stones beginning in the fourth decade of life. Nitisinone, a potent inhibitor of the upstream enzyme, 4-hydroxyphenylpyruvate dioxygenase, dramatically reduces HGA production. As such, nitisinone is a proposed treatment for alkaptonuria. A randomized clinical trial of nitisinone in alkaptonuria confirmed the biochemical efficacy and tolerability of nitisinone for patients with alkaptonuria but the selected primary outcome did not demonstrate significant clinical benefit. Given that alkaptonuria is a rare disease with slow progression and variable presentation, identifying outcome parameters that can detect significant change during a time-limited clinical trial is challenging. To gain insight into patient-perceived improvements in quality of life and corresponding changes in physical function associated with nitisinone use, we conducted a post-hoc per protocol analysis of patient-reported outcomes and a functional assessment. Analysis revealed that nitisinone-treated patients showed significant improvements in complementary domains of the 36-Item Short-Form Survey (SF-36) and 6-minute walk test (6MWT). Together, these findings suggest that nitisinone improves both quality of life and function of patients with alkaptonuria. The observed trends support nitisinone as a therapy for alkaptonuria.

1. Introduction

Alkaptonuria is a rare, autosomal recessive disorder of tyrosine degradation (Fig. 1) characterized by early-onset debilitating spondyloarthropathy, cardiac valvular disease, and genitourinary stones [1]. Alkaptonuria is caused by deficiency of homogentisate 1,2-dioxygenase, the third enzyme of the tyrosine catabolic pathway, resulting in accumulation of homogentisic acid (HGA), whose aldehyde derivatives form polymers that bind to connective tissue, turning them black (ochronosis), disrupt collagen assembly, and damage the tissue [2, 3]. Progressive spine and joint degeneration lead to mobility loss, disability, and pain. Degenerative symptoms typically begin in the fourth decade of life and progress over the following decades, although symptom onset, progression, and severity are variable [4]. The only feature of alkaptonuria present from birth is the presence of urine that develops a dark color on standing, as the excreted homogentisic acid is oxidized to form pigmented polymers that resemble melanin.

Figure 1. Tyrosine catabolism pathway.

4-HPPD, 4-hydroxyphenylpyruvate dioxygenase; HGD, homogentisate 1,2-dioxygenase.

Nitisinone has been proposed as a treatment for alkaptonuria [5]. Nitisinone is a potent inhibitor of 4-hydroxyphenylpyruvate dioxygenase enzyme, the second enzyme in tyrosine catabolism, whose product is HGA [5]. Small doses of nitisinone lower HGA levels by >95%, but information regarding its clinical efficacy is limited [4, 6]. In our previous study, total hip range of motion used as a primary endpoint failed to reach significance, possibly due to substantial hip disease already present in study participants. Secondary outcome measures also failed to reach significance, but some exploratory measures such as patient-reported outcomes were not included in the original analysis.[7]

Patient-reported outcomes provide direct insight into symptom burden, physical function, and physical, mental, and emotional wellbeing. The Food and Drug Administration (FDA) defines a patient-reported outcome as “any report of the status of a patient’s health condition that comes directly from the patient, without interpretation of the patient’s response by a clinician or anyone else” [8]. Often pertaining to quality-of-life, function, symptomatology, and treatment satisfaction, patient-reported outcomes aid in the holistic evaluation of clinical efficacy and are often used in chronic disorders where the goal is to manage quality-of-life concerns [9, 10]. Patient-reported outcomes are also accepted clinical endpoints for treatment trials [10].

Given the slow and heterogeneous progression of alkaptonuria, identifying outcome measures that can detect a clinically significant change over the course of a time-limited clinical trial has proven challenging. Since pain is a core feature of alkaptonuria and impacts patients’ quality of life and function, patient-reported outcomes may provide further insights into response to treatment. In this study, we conducted post-hoc per protocol analyses on patient-reported outcome measures from our 3-year randomized nitisinone trial in combination with a functional assessment to better elucidate the longer-term potential of nitisinone therapy in alkaptonuria.

2. Methods

2.1. Trial information

Data were collected from patients enrolled on protocol 05-HG-0076, “Long-term Clinical Trial of Nitisinone in Alkaptonuria” (clinicaltrials.gov #NCT00107783) approved by the National Human Genome Research Institute’s Institutional Review Board at the National Institutes of Health [11]. All patients provided written informed consent. Forty patients were enrolled between 2005–09 and were randomized to either treatment or control (“untreated”) group (Figure 2). Twenty patients in the treatment group were administered 2 mg of nitisinone orally each day. Since nitisinone resolves the urine color change seen in alkaptonuria, patients could not be blinded to their nitisinone status; patients in the control group received neither placebo nor surrogate medication.

Figure 2.

Flow diagram depicting trial participation and data inclusion.

Participants were evaluated at visits every four months throughout the trial. At each evaluation, patients completed questionnaires including the 36-Item Short Form Survey (SF-36) and the Human Activity Profile (HAP). Patients also completed the 6-minute walk test (6MWT) at these visits.

2.2. Outcome Measures

The RAND 36-Item Health Survey 1.0, known as the 36-Item Short Form Survey (SF-36), and the Human Activity Profile (HAP) are validated patient surveys used to assess quality of life and physical activity levels, respectively [12, 13].

The SF-36 assesses quality-of-life measures in eight domains/scales: physical functioning, bodily pain, role limitations due to physical health problems, role limitations due to personal or emotional problems, emotional well-being, social functioning, energy/fatigue, and general health perceptions. Responses were categorized by domain, scored, and averaged to create a domain score according to Version 1.0 of the Scoring Rules for the RAND 36-Item Health Survey [14]. Scores range from 0 to 100; higher scores indicate better health. When patients selected two responses or an intermediate response, the response with the greater deviation from midline was selected. Unanswered questions or questions from which a single response could not be reasonably identified were excluded from the domain score average.

The HAP assesses the physical activity level of patients. The 94-item activity list is arranged from low-energy to high-energy requirements based on metabolic equivalents. For each item, patients indicate if they still participate in that activity, have stopped doing that activity, or have never participated in that activity. The maximum activity score (MAS) is the most energy-intensive item number a patient still participates in. The adjusted activity score (AAS) adjusts the MAS by subtracting the number of activities below the MAS that the respondent has stopped doing. Both maximum and adjusted scores range from 0 to 94 with higher scores indicating greater activity level [13].

The 6-minute walk test (6MWT) was used as a functional measure of mobility. The 6MWT measures the distance in feet a patient can walk within six minutes [15].

2.3. Statistical Analysis

For all analyses, a per-protocol approach was used. Paired and unpaired t-tests and repeated measures ANOVA, or their nonparametric equivalents, were performed.

Initial and 3-year visit scores were compared within nitisinone-treated and untreated groups to determine if there were changes in patient-reported outcomes from trial onset. The normality of the data was checked using a Shapiro-Wilk test. If data were normal (Shapiro-Wilk p>0.05), a paired t-test was used. If data were non-normal (Shapiro-Wilk p≤0.05), a nonparametric Wilcoxon signed-rank test was used.

To validate changes observed between initial and 3-year visit scores over the course of the trial, scores within nitisinone-treated and untreated groups were compared at initial, 1-year, 2-year, and 3-year visits. The normality of the data was checked using a Shapiro-Wilk test; the sphericity of the data was checked using Mauchly’s test; and outliers were checked within visit groups. If data were normal (Shapiro-Wilk p>0.05) and there were no outliers, a repeated measures ANOVA was used; if data violated the sphericity assumption (Mauchly p≤0.05), the Greenhouse-Geisser correction was applied. If the data were non-normal (Shapiro-Wilk p≤0.05) or there were outliers, a nonparametric Friedman test was used. For post-hoc testing, pairwise paired t-tests with Bonferroni correction were used for normal data and pairwise Wilcoxon signed-rank tests with Bonferroni correction were used for non-normal data.

Changes in outcome measures were compared between nitisinone-treated and untreated groups to determine if treatment was associated with observed changes. An F-test was used to check that variances were equal, and a Shapiro-Wilk test was used to determine the normality of the data. If data had equal variances (F-test p>0.05) and were normal (Shapiro-Wilk p>0.05), a two-tailed t-test was used. If data had unequal variances (F-test p≤0.05) and/or were non-normal (Shapiro-Wilk p≤0.05), a nonparametric Wilcoxon rank-sum test was used.

Statistical analysis was conducted in R Studio using R version 4.3.1. For all analyses, a corrected two-tailed p≤0.05 was considered significant.

3. Results

3.1. Baseline Data

This post-hoc analysis was conducted per-protocol and excluded five trial participants who did not finish the trial or were taken off drug before the end of the trial; three were from the nitisinone-treated group for atrial fibrillation, corneal damage, elevated transaminases, respectively, and two from the untreated group withdrew for personal reasons. Two additional patients, one from the nitisinone-treated group and one from the untreated group, were excluded from SF-36 and HAP analyses because they did not return a questionnaire for one of the visits analyzed. Additional patients were excluded from repeated measures testing due to missing questionnaires (Figure 2).

Untreated and nitisinone-treated groups were well-matched for age and gender. Averages of patient-reported outcome subscores and functional scores at baseline showed no significant differences between untreated and nitisinone-treated groups (Table 1).

Table 1.

Demographic composition and initial scores of untreated and nitisinone-treated groups

		Untreated				Nitisinone-treated				P-value
	Normal Population Averages	N	Mean	SD	(Range)	N	Mean	SD	(Range)
Male/Female		14/4				10/7
Age		18	50.72	6.29	(40–63)	17	51.24	8.20	(38–63)	0.84
Urine HGA (mg/day)		17	5224	3480	(900–12138)	15	4140	2585	(917–10161)	0.33
SF-36
SF-36 PF	84.52	17	46.76	22.0	(5–80)	16	38.75	22.84	(5–75)	0.31
SF-36 RLPH	81.20	17	41.18	35.29	(0–100)	16	37.5	37.64	(0–100)	0.70
SF-36 RLEP	81.29	17	70.59	42.30	(0–100)	15	60	44.01	(0–100)	0.51
SF-36 EW	74.84	17	78.76	10.95	(60–92)	16	72.50	12.89	(52–92)	0.14
SF-36 SF	83.60	17	74.26	22.74	(25–100)	16	61.72	26.00	(12.5–100)	0.16
SF-36 EF	61.05	17	52.65	19.61	(5–80)	16	38.75	24.73	(0–80)	0.083
SF-36 P	75.49	17	45.74	17.16	(22.5–67.5)	16	37.03	23.28	(10–77.5)	0.23
SF-36 GH	72.21	17	46.18	23.88	(0–90)	16	45.31	20.04	(5–80)	0.91
HAP
HAP MAS		17	78	6.72	(57–82)	16	74.56	10.04	(49–82)	0.26
HAP AAS		17	65.76	11.27	(38–81)	16	59.19	15.88	(36–81)	0.18
6MWT (ft)	1621 (F) 1890 (M)	18	1595.61	201.56	(1255–1999)	17	1371.24	358.65	(822–1929)	0.062

3.2. Analysis of 36-Item Short Form Survey (SF-36)

Within the nitisinone-treated group, there were significant improvements from initial to 3-year visit in the following SF-36 domains (Supplemental Table 1): energy and fatigue (Figure 3A, p=0.016), pain (Figure 3B, p=0.0025), physical functioning (Figure 3C, p=0.0062), and role limitations from emotional problems (Figure 3D, p=0.035). Year-to-year analyses validated findings from these paired analyses (Supplemental Table 1); repeated measures testing found significance within the nitisinone-treated group in the following domains: energy and fatigue (Figure 3E, p=0.00055); pain (Figure 3F, p=0.0046); physical functioning (Figure 3G, p=0.016); and role limitations from emotional problems (Figure 3H, p=0.0074).

Figure 3. Significant improvements in SF-36 domain scores.

(A-D) Significant improvement within the nitisinone-treated group in A) energy and fatigue domain (p=0.016), B) pain domain (p=0.0025), C) physical functioning domain (p=0.0062), D) role limitations from emotional problems domain (p=0.035). (E-H) Year-to-year analysis validated improvement within the nitisinone-treated group in E) energy and fatigue domain (p=0.00055) with significant improvements between 1-year and 2-year visit scores (p=0.012) and 1-year and 3-year visit scores (p=0.026), F) pain domain (p=0.0046) with significant improvement between initial and 3-year visit (p=0.043), G) physical functioning domain (p=0.016) with significant improvement between initial and 3-year visit (p=0.037), H) role limitations from emotional problems in both the untreated and nitisinone-treated groups (p=0.0057, p=0.0074), with no signficant pairwise differences (n.s.). (I-J) Significant improvement within the untreated group I) in the emotional wellbeing domain (p=0.0036) with J) validation from year-to-year analysis (p=0.016).; however, post-hoc pairwise analysis only revealed a near-significant difference between initial and 3-year scores after Bonferroni correction (p=0.056, n.s.). K) Nitisinone-treated group showed significant improvement in physical functioning domain when compared to untreated group (p=0.045).

In the untreated group, there was only significant improvement from the initial to 3-year visit in the emotional wellbeing domain (Supplemental Table 1, Figure 3I, p=0.0036). While the nitisinone-treated group also showed numerical improvement, it did not reach significance (Supplemental Table 1, Figure 3I, p=0.29). Year-to-year analysis validated the significant improvement detected within the emotional wellbeing domain of the untreated group (Supplemental Table 1, Figure 3J, p=0.016) and found additional significance within the untreated group in the role limitations from emotional problems domain (Supplemental Table 1, Figure 3H, p=0.0057).

There were no significant score changes within untreated or nitisinone-treated groups in the following SF-36 domains when compared using paired analysis and year-to-year analysis (Supplemental Table 1, Supplemental Figure 1); role limitations due to physical health problems, social functioning, and general health.

When nitisinone-treated and untreated groups were compared, there was a significant difference in change in physical functioning domain scores (Table 2, Figure 3K, p=0.045). While not statistically significant, positive trends were also seen in the pain and energy and fatigue domains (Table 2, p=0.067 and p=0.057, respectively). Further, the nitisinone-treated group showed average improvement in all eight domains of the SF-36 as compared to the untreated group which showed average improvement in only five of the eight domains (Table 2).

Table 2.

Average change in SF-36 domain score and p-values of analyses between nitisinone-treated and untreated groups.

	Physical Functioning	Role Limitations from Physical Health Problems	Role Limitations from Emotional Problems	Energy and Fatigue	Emotional Wellbeing	Social Functioning	Pain	General Health
Nitisinone-treated	14.69	12.50	26.67	13.44	3.25	14.06	19.38	0.31
Untreated	3.24	11.76	17.65	−0.69	7.59	−2.21	5.74	−2.94
P-value	p=0.045	p=0.75	p=0.64	p=0.057	p=0.25	p=0.089	p=0.067	p=0.62

3.3. Analysis of Human Activity Profile (HAP)

Within the nitisinone-treated group, there was numerical improvement in both scores of the HAP (Supplemental Table 1; Figure 4A), although not reaching significance. On average, the nitisinone-treated group experienced a 1.6-point improvement in MAS and a 6.4-point improvement in AAS. Within the untreated group, there is a loss of 2.7 points in MAS and of 2.1 points in AAS. Year-to-year analysis did not reveal any significant changes within the untreated or nitisinone-treated groups in AAS or MAS (Supplemental Table 1, Figure 4B, 4E). The nitisinone-treated group showed a nonsignificant improvement in AAS and MAS when compared to the untreated group (Figure 4C, p=0.13 and Figure 4F, p=0.17).

Figure 4. HAP adjusted (AAS) and maximum activity score (MAS) and 6-minute walk test.

(A-C) HAP AAS: A) Non-significant improvement within the nitisinone-treated group from initial to 3-year visit (p=0.16). B) No significant changes within untreated or nitisinone-treated groups from year to year (p=1.00, p=0.73). C) Nitisinone-treated group showed non-significant improvement when compared to the untreated group (p=0.13). (D-F) HAP MAS: D) No significant change within untreated or nitisinone-treated groups (p=0.39, p=0.36). E) No significant changes within untreated or nitisinone-treated groups from year to year (p=0.67, p=0.36). F) No significant change between untreated or nitisinone-treated groups (p=0.17). (G-I) 6MWT: G) Significant improvement in distance walked within nitisinone-treated group (p=0.006). H) Year-to-year analysis validated significant improvement found in distance walked within nitisinone-treated group (p=0.021). Post-hoc pairwise analysis revealed a significant difference between initial and 3-year scores after Bonferroni correction (p=0.036). I) Nitisinone-treated group showed a non-significant improvement in distance walked when compared to the untreated group (p=0.2).

3.4. Analysis of 6-Minute Walk Test (6MWT)

There was a significant improvement in distance walked in the 6MWT within the nitisinone-treated group (Supplemental Table 1; Figure 4G, p=0.006) and a nonsignificant change in distance walked within the untreated group (Supplemental Table 1; Figure 4G, p=0.48). The distance walked by the nitisinone-treated group improved by an average of 155 feet from initial to final visit compared to an average improvement of 47 feet from the untreated group. Year-to-year analysis confirmed observed improvement in initial and 3-year 6MWT scores within the nitisinone-treated group across the clinical trial (Supplemental Table 1; Figure 4H, p=0.021). However, this improvement in the distance walked did not reach significance when comparing the nitisinone-treated group to the untreated group (Figure 4I, p=0.20).

4. Discussion

Analysis of a controlled clinical trial of nitisinone in alkaptonuria showed that nitisinone-treated patients reported significant improvements in the SF-36 physical functioning domain when compared to untreated patients, and made significant functional gains in the 6MWT and several SF-36 domains as well as numerical gains on the HAP when compared to baseline. These measures are known to be correlated in older adults and in populations with arthritic and cardiac changes [16–18] and, together, suggest clinical benefit of nitisinone in quality of life and functioning in patients with alkaptonuria.

The SF-36 is known to be responsive to arthritic severity, specifically in the bodily pain and physical functioning domains [19]. Nitisinone-treated patients demonstrate a significant improvement in the physical functioning domain (p=0.045) and near-significant improvement in the bodily pain domain (p=0.067) compared to untreated patients, suggesting that nitisinone may ease the arthritic pain that hallmarks alkaptonuria.

Within the nitisinone-treated group, patients experienced significant improvements in the energy and fatigue (p=0.016), pain (p=0.0025), physical functioning (p=0.0062), and role limitations from emotional problems (p=0.035) domains, all of which maintained significance when analyzed yearly except for the latter. The role limitation domains of the SF-36 have been identified to be more susceptible to floor and ceiling effects in patients with arthritis [20]. In this study, we observed clear ceiling effects in the role limitations from emotional problems domain, limiting interpretation of the data and explaining the inconsistency of the data when examined annually.

Within the untreated group, patients experienced a significant improvement only within the emotional wellbeing domain (p=0.0036). Patients in both control and treatment groups often report satisfaction and psychological benefit stemming from clinical trial participation, which is suggested to be the result of better access to care and an enhanced “sense of hope and purpose” [21]. Although only the untreated group reached significance, both nitisinone-treated and untreated groups demonstrated improvement in the emotional wellbeing domain.

Outside of these domains, nitisinone-treated patients showed numerical improvement across all eight SF-36 domains while the untreated patients showed improvement in only five of the eight domains. The wide-ranging improvements in the nitisinone-treated group indicate broad potential clinical benefit of nitisinone. Though the untreated group showed fewer improvements than the nitisinone-treated group, the observed improvements may be unexpected since alkaptonuria is a degenerative disorder. Improvements within the untreated group can likely be attributed to the more frequent interventions from rehabilitation medicine and pain services received by both patient groups as part of the trial.

The nitisinone-treated group reported numerical improvements in both in the MAS and even more in the AAS. Since the AAS accounts for all the activities that patients have stopped doing, it is regarded to be a more stable measure of an individual’s daily activities [22] and a more sensitive measure of activity level than the MAS [13]. Although not reaching significance, these trends are consistent with the findings in related SF-36 domains such as physical functioning.

Finally, nitisinone-treated patients showed a significant improvement in distance walked in the 6-minute walk test when compared to their initial visit (p=0.006), and a greater improvement than the untreated group although the difference did not reach significance. As a functional measure, the 6MWT confirms patient-reported improvements in physical functioning and activity; the 6MWT is correlated to the physical functioning domain of the SF-36 and the HAP [17, 18]. The 6MWT is also an accepted outcome parameter for studies of progressive loss-of-function disorders [23].

The 6MWT was analyzed as a secondary outcome parameter in the original analysis of this trial but yielded a negative result alongside other outcome parameters, including the primary outcome parameter of hip range of motion and secondary outcome parameters of Schober’s test of spinal flexion, functional reach, and timed get up and go. The current analysis was approached as a post-hoc analysis of a negative clinical trial and used a per-protocol design instead of the intent-to-treat design; patients who did not complete the trial or stay on treatment for the duration of the trial were excluded from analysis to better assess the biologic efficacy of nitisinone [24]. Given the reasons for withdrawing from the study were unrelated to the outcome measures, it should not skew the analysis. Though intent-to-treat is regarded as the gold standard for trial analysis, issues arise when imputing missing data; last observation carried forward is the most common approach utilized in intent-to-treat analyses but has fallacies of its own that skew data interpretation [25, 26]. For this reason, the current analysis also excluded patients who completed the trial but were missing data from analyzed visits. To better understand how these exclusions may have influenced our findings, full cohort analysis of all forty trial participants was conducted using the last observation carried forward approach. This analysis found that significant improvement of the nitisinone-treated group was maintained in the SF-36 physical functioning domain (Supplemental Figure 2A, p=0.038) when compared to the untreated group. Full-cohort analysis also revealed greater improvement in the HAP AAS and 6MWT in the nitisinone-treated group than in the untreated group, though not to a significant level (Supplemental Figure 2B–C, p=0.59 and p=0.25).

Limitations of this study include choices in the statistical design, introduced to maximize the ability to detect trends in the data with a rare disease population having a slow rate of progression. The statistical methods used in this analysis also differed from the original analysis, which used repeated measures with random coefficients and controlled for age and sex. Groups were well-matched for age and follow-on analyses indicated that age and sex did not influence the findings described here (data not shown).

The symptomatology of alkaptonuria also limits study design. Since the urine of alkaptonuria patients receiving nitisinone does not darken on standing while on nitisinone, this study could not be blinded and controlled for placebo effect. Importantly, alkaptonuria is a disorder with a degenerative cascade that progresses over the span of decades; a 3-year snapshot of nitisinone use may not capture its full benefit. While several patients in this study had joint involvement that required replacements before or during the trial, analyses corrected for occurrence of joint replacements did not influence the patient reported outcomes (data not shown).

As an outcome measure, patient-reported outcomes are inherently limited. Because they rely on patient reporting, surveys are subject to differences in interpretation and errors in reporting.

Patient-reported outcomes also miss nuance and neglect extenuating circumstances since responses are constrained. This restriction, however, is necessary for patient-reported outcomes to be quantitative, and conservative approaches to the interpretation of reporting errors and p-value correction were used to maintain the integrity of the measures. Though limited, patient-reported outcomes provide useful perspective in assessing clinical benefit, providing standardized and quantitative insight into the patient experience: It is valuable to know that patients with alkaptonuria feel their quality of life and physical functioning is improved and report doing more activities on nitisinone.

The patients enrolled in this study had existing, and often extensive, spine and joint degeneration already apparent. These degenerative changes would not be expected to be reversed by introduction of nitisinone at this stage of the disease; though nitisinone dramatically reduces HGA, cumulative effects from HGA deposition leading up to nitisinone intervention are likely lasting, which makes these quality-of-life improvements even more remarkable. The observed gains in multiple complementary metrics within the nitisinone-treated group signal consistent benefit in how patients feel and function. Further clinical benefit of nitisinone would be expected when initiated in patients with alkaptonuria earlier in their disease course to prevent the cumulative effects of homogentisic acid.

5. Conclusions

This analysis of patient-reported outcomes and functional measures indicates that nitisinone significantly benefits quality of life and function of patients with alkaptonuria, as observed via the significant changes in several SF-36 subdomains and 6MWT. The findings of this study suggest that the clinical impact of nitisinone aligns with its biochemical reduction of HGA, pointing to nitisinone as a therapy for alkaptonuria. Ultimately, analysis of patient-reported outcomes offers preliminary insight into the clinical benefits of nitisinone and also allows for further studies characterizing its efficacy.

Highlights.

• Nitisinone benefits quality of life and function of patients with alkaptonuria

• Nitisinone-treated patients report gains in physical functioning and activity

• 6-minute walk improvements support reported gains in nitisinone-treated patients

• Patient-reported outcomes offer novel insight into clinical benefit of nitisinone

Abbreviations:

AAS

Adjusted activity score

EF

Energy & fatigue

EW

Emotional wellbeing

GH

General health

HAP

Human Activity Profile

HGA

Homogentisic acid

MAS

Maximum activity score

P

Pain

PF

Physical functioning

RLEP

Role limitations from emotional problems

RLPH

Role limitations from physical health problems

SF

Social functioning

SF-36

36-Item Short-Form Survey

6MWT

6-minute walk test