Sepiapterin

Sepiapterin is a natural precursor of the enzymatic co-factor BH4, a critical co-factor for phenylalanine hydroxylase (PAH). Sepiapterin acts as a dual pharmacological chaperone (sepiapterin and BH4 each with its own binding affinity to variant PAH), including PAH variants commonly found in PKU and known to be insensitive to BH4, to improve the activity of the defective PAH enzyme, achieving a high concentration of BH4 intracellularly. By enhancing the conformational stability of misfolded PAH enzyme and increasing the intracellular concentrations of BH4, sepiapterin is able to effectively reduce blood Phe levels.

Absorption

Following oral administration, sepiapterin is quickly absorbed, and the peak plasma concentrations occur in approximately 1 to 3 hours and decline to below the limit of quantitation (0.75 ng/mL) rapidly (generally by 12 hours). Maximum plasma sepiapterin concentration (Cmax) was approximately 2.80 ng/mL following the 60 mg/kg/day dose for 7 days with a high-fat high-calorie diet. No accumulation of sepiapterin was observed following repeated dosing.

Plasma sepiapterin is metabolised extensively to form the pharmacologically active metabolite BH4. The apparent terminal half-life for BH4 is approximately 5 hours. Both BH4 C_max and area under the concentration-time curve from time zero to 24 hours postdose (AUC_0-24h) increased with the dose, while the increase was less than dose proportional when the sepiapterin dose was above 20 mg/kg. There is no accumulation of BH4 following repeated doses of sepiapterin up to 60 mg/kg for 7 days.

Effect of food

When sepiapterin was administered with a low-fat, low-calorie meal in the dose range of 20 to 60 mg/kg, BH4 exposures were 1.69- to 1.72-fold higher for C_max and 1.62- to 1.73-fold higher for AUC_0-24h compared to administration under fasted conditions. When sepiapterin was administered with a high-fat, high-calorie meal, BH4 exposures were 2.21- to 2.26-fold higher for C_max and 2.51- to 2.84-fold higher for AUC_0-24h compared to administration under fasted conditions.

Sepiapterin can be taken with any meal at any time of the day at the same time every day.

Distribution

Binding of sepiapterin or BH4 to plasma protein is low, and the majority of sepiapterin and BH4 in plasma are free to exert pharmacological effects. In vitro studies show that sepiapterin is bound (mean 15.4%) to plasma protein in the presence of 0.1% dithiothreitol in the concentration range of 0.1 to 10 μM. BH4 was 41.3% (at 2 μM), 33.0% (at 5 μM), and 24.1% (at 15 μM) bound to protein in human plasma in the presence of 0.5% β-mercaptoethanol.

In healthy subjects, elevated BH4 concentration was observed in the cerebrospinal fluid following repeated sepiapterin oral administration.

Biotransformation

Sepiapterin is metabolised by SR/carbonyl reductase and DHFR in a 2-step unidirectional process to form BH4. The metabolism of BH4 is presumed to follow the same pathway as endogenous BH4, oxidised while acting as coenzymes for aromatic amino acid hydroxylases, such as PAH, tyrosine hydroxylase, tryptophan hydroxylase, and alkylglycerol monooxygenase, and nitric oxide synthase, and some metabolites, like 4α-hydroxy-tetrahydrobiopterin and quinonoid dihydrobiopterin, could be recycled to regenerate BH4 mediated by pterin-4α-carbinolamine dehydratase and dihydropteridine reductase.

Extensive metabolism of sepiapterin was observed in human following a single oral dose of ¹⁴C-sepiapterin. The major metabolic pathway involved oxidation/dehydrogenation, reduction/oxidation, oxidative deamination, dehydration, side chain cleavage, and methylation, etc, alone or in combination.

Elimination

Following oral administration in healthy human participants, sepiapterin was extensively metabolised with the metabolites excreted primarily in faeces. Plasma sepiapterin declined rapidly following C_max to below the limit of quantitation, generally by 12 hours post-dose. Plasma BH4 declined mono-exponentially following C_max. The terminal half-life was approximately 5 hours.

Following a single oral dose of ¹⁴C-sepiapterin to adult healthy subjects, a mean of 6.71% dosed radioactivity was recovered in urine and 26.18% in faeces with the combined total recovery of 32.9% by 240 hours. The majority of those radioactivity was recovered within the first 48 hours post-dose (28.2%). The total renal clearance of radioactivity derived from ¹⁴C-sepiapterin was 1.54 L/h (25.6 mL/min). Formation of volatile metabolites from sepiapterin in the gastrointestinal tract was confirmed in an in vitro study using human intestinal microbiota.

Special populations

Age

PKU patients of all ages had been included in the Phase 3 clinical studies. Except for allometric effect on clearance and volume of distribution, no further age effect was identified in the population PK study.

Ethnicity and race

Higher exposures to BH4 were observed for Asian subjects. In the Japanese ethno-bridging study, 10% to 24% higher AUC_0-last and 14% to 29% higher C_max of BH4 were observed in Japanese compared to non-Japanese subjects at sepiapterin dose range of 20 to 60 mg/kg.

Renal impairment

The PK and safety of sepiapterin have not been studied in patients with renal impairment.

Hepatic impairment

The PK and safety of sepiapterin have not been studied in patients with hepatic impairment.

Drug interactions

In vitro studies

In vitro studies indicate that sepiapterin and BH4 are unlikely to be perpetrators of CYP450-mediated metabolism.

In vitro, sepiapterin did not inhibit CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, or CYP3A4, or induce CYP1A2, CYP2B6, or CYP3A4.

In vivo studies

In healthy subjects, co-administration of sepiapterin (20 mg/kg) with a single dose of the breast cancer resistance protein (BCRP) inhibitor curcumin (2 g) increased the exposures of BH4 slightly. The overall estimated geometric mean ratios (GMRs) (90% CI) for BH4 C_max and area under the concentration-time curve from time zero to time of the last quantifiable measurement (AUC_0-last) when sepiapterin was co-administered with curcumin compared to sepiapterin alone were 1.24 (1.15-1.33) and 1.20 (1.13 1.28), respectively. This modest increase is deemed not clinically relevant.

Co-administration of a single dose of sepiapterin at the maximum therapeutic dose of 60 mg/kg with the BCRP substrate rosuvastatin (10 mg) had no effect on the of rosuvastatin. The overall estimated GMRs (90% CI) for rosuvastatin C_max and AUC_0-last when rosuvastatin was co-administered with sepiapterin compared to rosuvastatin alone were 1.13 (1.00-1.28) and 1.02 (0.93-1.13), respectively.

Non-clinical data reveal no special hazard for humans based on conventional studies of safety pharmacology, genotoxicity, carcinogenic potential and toxicity to reproduction and development.

In rats, following repeated oral administration, sepiapterin-related renal tubule degeneration/regeneration, interstitial inflammation, and fibrosis were noted as a result of crystal deposition in the papillary collecting tubules. These findings were partially reversible after a 4-week recovery period and no kidney toxicity occurred at BH4 exposure levels 2 times the clinical BH4 exposure levels at the maximum recommended human dose (MRHD).