HOW DOES THE IN VIVO BILIARY ELIMINATION OF DRUGS CHANGE WITH AGE? EVIDENCE FROM CLINICAL DATA USING A SYSTEMS PHARMACOLOGY APPROACH

There is little information on the development of biliary drug elimination (BE) with age. The aims of this study were to collate literature data on the pharmacokinetics of biliary excreted drugs used in paediatrics and to apply a Physiologically Based Pharmacokinetic model to predict their systemic clearance (CLiv) across this age range.Drug parameters for azithromycin, ceftriaxone and digoxin were collated from the literature and validated against adult clinical data in Simcyp (V14R1). The change in CLiv with age was simulated in the paediatric model and compared to the observed data; the ontogeny function associated with BE was optimised in order to recover the age-related CLiv.For azithromycin (79% BE) a fraction of adult biliary excretion activity of 15% had to be assumed to be able to predict accurately the CL of the drug in neonates (24 to 28 weeks GA) whilst 100% activity was apparent by 7 months. For ceftriaxone (51% BE) full biliary excretion activity appeared to be present at full term birth. Finally, for digoxin (25% BE), a fraction of adult biliary excretion activity of 10% had to be assumed to predict the CL of the drug at birth whilst 100% activity was present by The ontogeny of BE for all three drugs appears to be rapid and reach adult levels at birth or in the first few months of postnatal age. More research is required in this area particularly on the ontogeny of specific canalicular transporters in humans.

Role of cysteine residues in the function of human UDP glucuronosyltransferase isoform 1A1 (UGT1A1)

Bilirubin glucuronidation, catalysed by UGT1A1 [UGT (UDP glucuronosyltransferase) isoform 1A1, EC 2.4.1.17], is critical for biliary elimination of bilirubin. UGT1A1 deficiency causes CN-1 (Crigler–Najjar syndrome type 1), which is characterized by potentially lethal unconjugated hyperbilirubinaemia. Nucleotide sequence analysis of UGT1A1 in two CN-1 patients revealed that patient A was homozygous for a nt 530 G→A (where nt 530 G→A means guanine to adenine transition at nucleotide 530) mutation, predicting a C177Y substitution, and patient B had a nt 466 T→C mutation on one allele and a nt 1070 A→G mutation on the other, predicting a C156R and a Q357R substitution respectively. All 11 cysteine residues of mature human UGT1A1 are highly conserved in other human UGT isoforms and in rat, mouse and Rhesus monkey UGT1A1, suggesting their functional importance. Expression of mutagenized UGT1A1 plasmids showed that substitution of any of the seven cysteine residues located within the endoplasmic reticulum cisternae (including those mutated in patients A and B) abolished UGT1A1 activity or markedly increased its apparent Km for bilirubin. Substitution of the three cysteine residues within the C-terminal cytosolic tail had minimal effect on basal UGT1A1 activity, but prevented UGT1A1 activation by UDP-GlcNAc. N-Ethylmaleimide did not inhibit UGT1A1 activity in native microsomes, but prevented UGT1A1 activation by UDP-GlcNAc and inhibited the activity in digitonin-permeabilized microsomes. Dithiothreitol did not affect UGT1A1 activity in human liver microsomes. Together, the results suggested that free thiol groups, but not disulphide bonding, of seven cysteine residues within the intracisternal region of human UGT1A1 are important for its catalytic activity, while cysteine residues in the cytosolic domain may be involved in its physiological activation by UDP-GlcNAc.