CLINICAL PHARMACOLOGY
Mechanism Of Action
Uridine triacetate is an acetylated form of uridine. Following oral administration, uridine triacetate is deacetylated by nonspecific esterases present throughout the body, yielding uridine in the circulation (Figure 1).
Figure 1: Uridine Triacetate Conversion to Uridine
XURIDEN provides uridine in the systemic circulation of patients with hereditary orotic aciduria who cannot synthesize adequate quantities of uridine due to a genetic defect in uridine nucleotide synthesis.
Pharmacodynamics
Hereditary orotic aciduria (uridine monophosphate synthase deficiency) is a rare congenital autosomal recessive disorder of pyrimidine metabolism caused by a defect in uridine monophosphate synthase (UMPS). The UMPS gene encodes uridine 5′monophosphate synthase, a bifunctional enzyme that catalyzes the final two steps of the de novo pyrimidine biosynthetic pathway in mammalian cells.
The defect in UMP synthase in hereditary orotic aciduria has two primary biochemical consequences. First, the blockade of de novo UMP synthesis results in a systemic deficiency of pyrimidine nucleotides, accounting for most clinical consequences of the disease. Second, orotic acid from the de novo pyrimidine pathway that cannot be converted to UMP is excreted in the urine, accounting for the common name of the disorder, orotic aciduria. Orotic acid crystals in the urine can cause episodes of obstructive uropathy.
XURIDEN delivers uridine into the circulation, where it can be used by essentially all cells to make uridine nucleotides, compensating for the genetic deficiency in synthesis in patients with hereditary orotic aciduria. When intracellular uridine nucleotides are restored into the normal range, overproduction of orotic acid is reduced by feedback inhibition, so that urinary excretion of orotic acid is also reduced.
Pharmacokinetics
Absorption
XURIDEN delivers 4- to 6-fold more uridine into the systemic circulation compared to equimolar doses of uridine itself. Maximum concentrations of uridine in plasma following oral XURIDEN are generally achieved within 2 to 3 hours, and the half-life ranges from approximately 2 to 2.5 hours.
A study in patients with hereditary orotic aciduria included an assessment of plasma uridine pharmacokinetics in 4 patients. Three of the patients were previously treated with oral uridine. On Day 0 (baseline), these three patients received their usual daily dose of oral uridine as a single dose (150 to 200 mg/kg once daily) and on Day 1, initiated oral XURIDEN treatment (60 mg/kg once daily). A fourth patient was enrolled who was naïve to uridine replacement therapy. The dose of XURIDEN was increased on Day 116 to 120 mg/kg once daily in two patients (Patients 3 and 4) and plasma uridine concentrations were assessed on Day 160 (44 days after the dose increase).
Plasma uridine levels in all four patients are depicted in Figure 2. Pharmacokinetic parameters are summarized in Table 3. Mean exposure to plasma uridine as assessed by Cmax and AUC was greater after oral XURIDEN than after oral uridine (approximately 4-fold on an equiweight basis, and 6-fold on an equimolar basis), although individual differences in relative bioavailability were noted. Plasma concentrations of the uridine catabolite uracil were generally below the limit of quantitation in all patients.
Table 3: Pharmacokinetic Parameters for Plasma Uridine
Pharmacokinetic Parameters (Plasma Uridine) |
Day 0 (Baseline)
(Oral Uridine, 150 to 200 mg/kg once daily)
N = 3 a |
Day 1
(Oral XURIDEN, 60 mg/kg once daily)
N = 4 |
Day 28
(Oral XURIDEN, 60 mg/kg once daily)
N = 4 |
Day 160
(Oral XURIDEN,
120 mg/kg once daily)
N = 2b |
Cmax (ìM)
mean ± SD |
56.0 ± 16.6 |
91.3 ± 32.2 |
88.7 ± 43.2 |
80.9 ± 20.0 |
Tmax (hours)
median (rangec) |
2.0 (1.0, 4.0) |
2.0 (1.2, 2.1) |
1.3 (1.0, 2.5) |
3.0 (2.0, 4.0) |
t1/2 (hours)
mean ± SD |
1.6 ± 0.7 |
1.6 ± 0.6 |
2.3 ± 1.6 |
8.2 ± 6.8 |
AUC(0-8) (μM•hr)
mean ± SD |
238.0 ± 163.2 |
311.2 ± 153.3 |
278.7 ± 148.5 |
465.6 ± 95.3 |
a Data shown are from patients previously treated with oral uridine
b The dose of XURIDEN was increased on Day 116 to 120 mg/kg per day. Serial plasma samples were taken on Day 160 (44 days after the dose increase) for plasma uridine levels.
cTmax range is expressed as the minimum and maximum values obtained |
Figure 2: Plasma Uridine Following Oral Administration of Uridine (Day 0) or XURIDEN (Days 1, 28 and 160) in Patients with Hereditary Orotic Aciduria
Food Effect on Uridine PK
A study in healthy adult subjects receiving a slightly different formulation of uridine triacetate granules (6 gram dose) under fed and fasted conditions showed no difference in the overall rate and extent of uridine exposure.
Distribution
Circulating uridine is taken up into mammalian cells via specific nucleoside transporters, and also crosses the blood brain barrier.
Excretion
Uridine can be excreted via the kidneys, but is also metabolized by normal pyrimidine catabolic pathways present in most tissues.
Drug Interaction Studies
In vitro enzyme inhibition data did not reveal meaningful inhibitory effects of uridine triacetate or uridine on CYP3A4, CYP1A2, CYP2C8, CYP2C9, CYP2C19, CYP2D6, and CYP2E1. In vitro enzyme induction data did not reveal an inducing effect of uridine triacetate or uridine on CYP1A2, CYP2B6, or CYP3A4.
In vitro data showed that uridine triacetate was a weak substrate for P-glycoprotein. Uridine triacetate inhibited the transport of a known P-glycoprotein substrate, digoxin, with an IC50 of 344 μM. Due to the potential for high local (gut) concentrations of the drug after dosing, the interaction of XURIDEN with orally administered P-gp substrate drugs cannot be ruled out.
In vivo data in humans are not available.
Clinical Studies
The efficacy of XURIDEN was evaluated in an open-label study in 4 patients with hereditary orotic aciduria (3 male, 1 female; age range from 3 to 19 years). Three patients were previously treated with uridine and were switched at study entry to XURIDEN. All patients were administered XURIDEN orally at a daily dosage of 60 mg/kg once daily. The study duration was 6 weeks.
The study assessed changes in the patients’ pre-specified hematologic parameters during the 6-week trial period. The pre-specified hematologic parameters were: neutrophil count and percent neutrophils (Patient 1), white blood cell count (Patient 2), and mean corpuscular volume (Patients 3 and 4).
For patients switched from oral uridine to oral XURIDEN (Patients 1, 2, and 3), the primary endpoint was stability of the hematologic parameter; for the treatment-naïve patient (Patient 4), the primary endpoint was improvement of the hematologic parameter. Secondary endpoints were urine orotic acid and orotidine levels, and growth (height and weight) for all patients.
After six weeks of treatment, Patients 1 and 3 met the pre-specified criteria for stability of the hematologic parameter. When Patient 2 was switched from uridine to XURIDEN treatment, the pre-specified criteria for white blood cell count remained stable; however documentation of a low white blood cell count prior to uridine initiation was not available. Patient 4 did not meet the pre-specified endpoint of improvement of the hematologic parameter.
Table 4 summarizes the primary efficacy results.
Table 4: Primary Efficacy Results for Study 1
Patient |
Pre-specified hematologic parameter
(Age-specific reference range) |
Primary Endpoint |
Baseline
(Day 0) |
Week 6
(Day 42) |
% Change from Baseline |
Patient #1 |
Neutrophil count (1.5 to 8.0 x103/mm3) |
Stable hematologic value |
0.95 |
0.81 |
-15% |
Neutrophil %
(26 to 48%) |
Stable hematologic value |
21 |
23 |
10% |
Patient #2 |
White Blood Cell Count
(3.8 to 10.6 x109/L) |
Stable hematologic value |
7.8 |
7.4 |
-5% |
Patient #3 |
Mean Corpuscular Volume
(75 to 91 fL) |
Stable hematologic value |
109.9 |
108.5 |
-1% |
Patient #4 |
Mean Corpuscular Volume
(72 to 90 fL) |
Stable hematologic value |
114.6 |
113.4 |
-2% |
At baseline, three patients had normal urine orotic acid levels and all four patients had normal urine orotidine levels. Three patients who had achieved normal urine orotic acid levels when they were treated with uridine maintained normal levels 6 weeks after transitioning to XURIDEN. All four patients had normal urine orotidine levels at baseline which remained stable after 6 weeks of treatment with XURIDEN.
During an extension phase of the trial, patients continued to receive XURIDEN. Dosing during the extension phase ranged from 60 mg/kg to 120 mg/kg once daily. After 6 months of treatment, Patient #1’s neutrophil count and neutrophil percent values normalized; hematologic parameters for the other three patients remained stable. Orotic acid and orotidine levels also remained stable for all four patients.
The treatment effect of XURIDEN on growth was assessed in the three pediatric patients (Patients 1, 3, and 4). At baseline, weight and height measurements were at or below the lower limit of normal for age (below 5th percentile for age) for Patients 1 and 4; height and weight measurements were within the normal range for age for Patient 3. After 6 months of treatment, Patients 1 and 3 experienced improved weight growth, as reflected in increases in their weight-for-age percentiles and weight velocity percentiles; Patient 4’s weight growth remained stable (i.e., weight percentile for age and weight velocity percentile for age was unchanged). Height growth remained stable in all three patients (i.e., height percentiles for age and height velocity percentiles for age were unchanged).
Case Reports
Nineteen (19) case reports of patients with hereditary orotic aciduria have been documented in published literature. Eighteen (18) patients were diagnosed as infants or children between the ages of 2 months and 12 years and were treated with exogenous sources of uridine. One patient, diagnosed at age 28, was not treated with exogenous uridine.
All 19 patients presented with significantly elevated levels of urinary orotic acid. Fifteen of 19 had abnormal hematologic parameters at presentation, including 15 with megaloblastic anemia, 8 with leukopenia and at least 2 with neutropenia. Oral administration of exogenous sources of uridine was reported to significantly improve hematologic abnormalities (megaloblastic anemia, leukopenia and neutropenia) within 2 to 3 weeks in almost all documented cases when administered in sufficient amounts. Concentrations of urinary orotic acid were significantly reduced within 1 to 2 weeks of initiating uridine replacement therapy. Some fluctuation in levels of urinary orotic acid were observed, but always at much lower levels than those reported prior to treatment. Improvements in body weight were also documented over time with continued uridine replacement therapy.
The effects of exogenous uridine were maintained over months and years, as long as treatment continued at sufficient doses (with appropriate dose increases based on body weight increases). Most hematologic abnormalities and orotic aciduria reappeared within days up to 2 or 3 weeks when administration of uridine was stopped or the dose was reduced. If treatment was interrupted for longer periods, body weight growth receded. If absolute dosages were not adjusted adequately to compensate for body weight gains, signs and symptoms of hereditary orotic aciduria recurred.