Representation and quantitation of the binding interaction between prednisone, prednisolone and corticosteroid binding globulin

Population pharmacokinetics of clonazepam in saliva and plasma: Steps towards noninvasive pharmacokinetic studies in vulnerable populations

AIM

Traditional studies focusing on the relationship between pharmacokinetics (PK) and pharmacodynamics necessitate blood draws, which are too invasive for children or other vulnerable populations. A potential solution is to use noninvasive sampling matrices, such as saliva. The aim of this study was to develop a population PK model describing the relationship between plasma and saliva clonazepam kinetics and assess whether the model can be used to determine trough plasma concentrations based on saliva samples.

METHODS

Twenty healthy subjects, aged 18-30, were recruited and administered 0.5 or 1 mg of clonazepam solution. Paired plasma and saliva samples were obtained until 48 hours post-dose. A population pharmacokinetic model was developed describing the PK of clonazepam in plasma and the relationship between plasma and saliva concentrations. Bayesian maximum a posteriori optimization was applied to estimate the predictive accuracy of the model.

RESULTS

A two-compartment distribution model best characterized clonazepam plasma kinetics with a mixture component on the absorption rate constants. Oral administration of the clonazepam solution caused contamination of the saliva compartment during the first 4 hours post-dose, after which the concentrations were driven by the plasma concentrations. Simulations demonstrated that the lower and upper limits of agreements between true and predicted plasma concentrations were -28% to 36% with one saliva sample. Increasing the number of saliva samples improved these limits to -18% to 17%.

CONCLUSION

The developed model described the salivary and plasma kinetics of clonazepam, and could predict steady-state trough plasma concentrations based on saliva concentrations with acceptable accuracy.

Prednisone and prednisolone interconversion in the rabbit utilizing unbound concentrations

Prednisolone (POH) and prednisone (PO) were both administered via seven different intravenous infusions (three POH and four PO) in each of six rabbits to obtain steady state. Bolus doses of POH and PO were also administered to four of the rabbits. Plasma samples of POH and PO were analyzed by normal phase HPLC. Unbound concentrations of both compounds were estimated in each rabbit using equilibrium dialysis and computer fit of the nonlinear or linear binding. The unbound fraction of POH ranged between 3 and 15 per cent from tracer to total concentrations of 2000 micrograms-1. The unbound fraction of PO ranged between 23 and 34 per cent and appeared nonlinear for four of the rabbits. The mean apparent unbound clearances of POH ranged from 4.4 to 6.1 lh-1kg-1 and from 4.1 to 5.9 lh-1kg-1 for PO, and were dose-independent for both compounds. The available fraction of POH from PO was 50 to 70 per cent and independent of dose. The available fraction of PO from POH averaged 44 per cent at the low infusion rate decreasing to 16 and 20 per cent at the medium and high infusion rates. Although saturability in the formation of PO from POH was noted, this nonlinear step is apparently not large enough to induce observable changes from linearity in the overall disposition of unbound POH.

Oral absorption of 21-corticosteroid esters: a function of aqueous stability and intestinal enzyme activity and distribution

The intestinal absorption of hydrocortisone and prednisolone are compared with three water-soluble derivatives (succinate, phosphate, and lysinate) in experiments at two levels of biological system complexity. Rates of absorption are compared by measuring permeabilities from rat intestinal perfusions of drugs and derivatives in solution. Extents of absorption are compared over a 10-fold dose range of parent steroid and with the steroid derivatives by measuring plasma levels from solid oral dosage in dogs. While the parent steroids are well absorbed over the entire length of the intestinal tract, variability in plasma levels is observed at higher doses. Limited solubility and resultant dissolution rate variability are likely to be playing a role in the early erratic blood level profiles found at higher doses. While the soluble prodrugs have a dissolution rate advantage which results in a greater concentration gradient, their absorption is limited by their aqueous luminal stability, their polarity and resultant passive membrane permeability, and the distribution and activity of enzyme reconversion sites in the intestinal tract. The unstable lysinate ester, targeted for aminopeptidase, has an absorption profile and permeability similar to that of the parent steroid. The absorption of the moderately stable succinate ester is limited by its polarity and the activity of intestinal esterases. The stable phosphate derivative is well absorbed in the upper intestine, where high levels of alkaline phosphatase exist, while the prodrug polarity and drop-off of enzyme activity limit its absorption from the lower gastrointestinal (GI) tract.