Whole body physiologically based modelling of ß-blockers in the rat: events in tissues and plasma following an i.v. bolus dose.

BACKGROUND AND PURPOSE: Whole body physiologically based pharmacokinetic (PBPK) models have been increasingly applied in drug development to describe kinetic events of therapeutic agents in animals and humans. The advantage of such modelling is the ability to incorporate vast amounts of physiological information, such as organ blood flow and volume, to ensure that the model is as close to reality as possible. EXPERIMENTAL APPROACH: Previous PBPK model development of enantiomers of a series of seven racemic ß-blockers, namely, acebutolol, betaxolol, bisoprolol, metoprolol, oxprenolol, pindolol and propranolol, together with S-timolol in rat was based on tissue and blood concentration data at steady state. Compounds were administered in several cassettes with the composition mix and blood and tissue sampling times determined using a D-optimal design. KEY RESULTS: Closed-loop PBPK models were developed initially based on the application of open loop forcing function models to individual tissues and compounds. For the majority of compounds and tissues, distribution kinetics was adequately characterized by perfusion rate-limited models. For some compounds in the testes and gut, a permeability rate-limited distribution model was required to best fit the data. Parameter estimates of the tissue-to-blood partition coefficient through fitting of individual enantiomers and of racemic pair were generally in agreement and also concur with those from previous steady-state experiments. CONCLUSIONS AND IMPLICATIONS: PBPK modelling is a very powerful tool to aid drug discovery and development of therapeutic agents in animals and humans. However, careful consideration of the assumptions made during the modelling exercise is essential.

Species differences in size discrimination in the paracellular pathway reflected by oral bioavailability of poly(ethylene glycol) and D-peptides.

Animal models are frequently used to aid prediction of intestinal absorption in humans. However, there is little comparative quantitative information on species differences in paracellular permeation, which is an important route for oral absorption of small to medium-sized hydrophilic drug molecules. This study addresses this issue by comparing the molecular mass (MM) dependency in oral bioavailability between rat and dog of poly(ethylene glycol) (PEG), a polydispersed model mixture commonly used to characterize paracellular absorption, and of a series of eight D-peptides (based on D-phenylalanine). Fasted rats and dogs received PEG (400/900) and the D-peptides (MM 236-406 Da), orally and intravenously, with total 24-48 h urine collection to estimate oral bioavailability. After HPLC separation, the individual PEG oligomers and D-peptides were determined using radiometric detection, for radiolabeled material, and LC-MS, for unlabeled (PEG) material. All compounds were predominantly excreted unchanged following intravenous administration. After oral administration, the predominant peak in the radiochromatogram was unchanged material, indicating stability of the compounds in the gastrointestinal tract. A clear molecular mass dependency in oral bioavailability was seen with both series, but with absorption much greater in dog than rat. Thus, for PEG in rat, bioavailability decreased sharply from 79 to around 2% with increasing MM between 282 and 591 Da, and then tapered to around 1. 5% up to 1295 Da. Whereas in dog, bioavailability remained around 100% for oligomers up to 600 Da and then decreased quite sharply with increasing MM, tending to plateau around 13% beyond 900 Da. Likewise, for the d-peptides in rat, bioavailability decreased from 30 to 1% with increasing MM between 236 and 406 Da, whereas in dog it was 100%, declining to 16% over the same molecular range. This species difference appears to be due to a larger pore size and greater frequency of pores in the paracellular pathway of dog compared to rat. Furthermore, on the basis of comparison with literature data for PEG and selected drugs, rat would appear to be a better predictor than dog of absorption of hydrophilic compounds in human.

Oral absorption of D-oligopeptides in rats via the paracellular route.

PURPOSE: This study was undertaken to examine the structural determinants of oral bioavailability in the rat of a set of oligopeptides comprising D-amino acids, which were taken to be absorbed paracellularly based on a pronounced sensitivity of permeability to electrical resistance in Caco-2 cell monolayers. METHODS: The study series comprised eleven D-oligopeptides, designed not to be recognised by peptidases or transport proteins, and to have molecular weights between 222 and 406 daltons with different net electrical charges and composition of D-amino acids. All the peptides were [3H]-radiolabelled and analyzed by HPLC with radiometric detection. Bioavailability was estimated based on 24-hr urinary excretion of unchanged peptide after oral and intravenous administrations. RESULTS: As expected, the series proved metabolically stable. Bioavailability was independent of oral dose when varied by a factor of 10,000, suggesting passive absorption. Whereas bioavailability decreased sharply from 30% to 1% with increasing molecular weight, net charge showed little, if any, effect on bioavailability. CONCLUSIONS: This D-oligopeptide model series served as a useful probe for the structural requirements for paracellular absorption in vivo. A critical determinant of bioavailability is molecular size, expressed as molecular weight in this study; net charged appeared of much lesser importance.