Rapid clearance of supplemented tetrahydrobiopterin is driven by high-capacity transporters in the kidney

Tetrahydrobiopterin (BH4) is an essential cofactor of aromatic amino acid hydroxylases and NO synthase. Supplementation of BH4 potentially targets cardiovascular dysfunction as well as inherited BH4 deficiencies and BH4-responsive phenylketonuria. However, the high cost/effect ratio of the recommended daily dose of BH4 supplementation acts against further popularization of this therapy. The aim of this study was to attenuate urinary excretion with the intention of improving efficacy of BH4 supplementation. The rapid excretion of BH4 in the urine was confirmed to be the major route of supplemented BH4 loss. In addition to glomerular filtration into the urine, a dominant rapid exclusion by renal secretion was observed in rats (T(1/2) = 16 min) when the plasma BH4 was higher than about 1 nmol/mL (more than10 times higher than normal), due to BH4 supplementation. The rapidity of the process was slowed by prior administration of cyclosporin A, a representative anti-excretory drug, and the excretion decelerated to a moderate rate (T(1/2) = 53 min). By the combined administration of BH4 plus cyclosporin A, the blood BH4 levels were dramatically elevated. It was hypothesized that the drug interfered with kidney excretion of BH4 rather than by attenuating organ tissue distribution by inhibiting biopterin uptake from the plasma. Consistent with this hypothesis, biopterin levels after BH4 administration were elevated in major organs in the presence of anti-excretory drugs without notable change in their BH4 fraction which was consistently 95% or higher regardless of combined administration with the drugs. Targeting these putative transporters would be a promising approach for improving the efficiency of BH4 supplementation therapy.

► In BH4 replacement, blood BH4 levels rose sharply but soon declined.
► BH4 was lost in urine via 2 routes, filtration and tubular secretion.
► Tubular secretion lowered plasma BP levels that exceeded a particular threshold.
► The secretion was transporter-mediated and sensitive to particular inhibitors.
► Finding a suitable inhibitor may improve the efficacy of BH4 supplementation.