Circadian rhythms in theophylline disposition: simulations and observations in the dog

Patient variation in veterinary medicine--part II--influence of physiological variables

In veterinary medicine, the characterization of a drug's pharmacokinetic properties is generally based upon data that are derived from studies that employ small groups of young healthy animals, often of a single breed. In Part I of the series, we focused on the potential influence of disease processes, stress, pregnancy and lactation on drug pharmacokinetics. In this Part II of the series, we consider other covariates, such as gender, heritable traits, age, body composition, and circadian rhythms. The impact of these factors with respect to predicting the relationship between dose and drug exposure characteristics within an animal population is illustrated through the use of Monte Carlo simulations. Ultimately, an appreciation of these potential influences will improve the prediction of situations when dose adjustments may be appropriate.

Uncertainty factors for chemical risk assessment: interspecies differences in the in vivo pharmacokinetics and metabolism of human CYP1A2 substrates

The 100-fold default uncertainty factor is used to convert a no-observed-adverse-effect level (NOAEL) from a animal toxicity study, to a "safe" value for human intake. The composite uncertainty factor (100) has to allow for interspecies (10-fold) and interindividual (10-fold) differences in toxicokinetics and toxicodynamics. The aim of the current study was to assess the validity of the interspecies default for toxicokinetics (4.0) for each of the test species (dog, rabbit, rat and mouse), using published data for compounds eliminated by CYP1A2 in humans (caffeine, theobromine, theophylline and paraxanthine). An analysis of the published literature showed that the absorption, bioavailability and route of excretion were generally similar between humans and the test species, for each probe substrate. However, interspecies differences in the route of metabolism, and the enzymes involved in this process, were identified. The magnitude of difference in the internal dose, between species, showed that values for the mouse (10.6) and rat (5.4) exceed the 4.0-fold default, whereas the rabbit (2.6) and dog (1.6) were below this value. This work supports the need to replace the generic default factors by a compound-related value derived from specific, relevant, quantitative data; this would result in more relevant and reliable non-cancer risk assessments.

Factors influencing the bioavailability of peroral formulations of drugs for dogs

The oral route is presently the preferred route of drug delivery. Poor oral bioavailability results in variable concentrations of drugs in the plasma and variable pharmacological responses, in addition to higher product costs. The unique canine physiology, anatomy and biochemistry makes designing canine dosage forms a challenging exercise. This article reviews the physicochemical, physiological, pharmacokinetic, pharmacological and formulation factors that can influence the drug availability of the oral formulations in dogs in an effort to provide a source of data to aid development of canine drug products with superior bioavailability.

Chronokinetics of liposomal encapsulated ampicillin in fasting and nonfasting rats

The chronobiological effect on the pharmacokinetics of liposomal encapsulated ampicillin (LEA) was compared at noon (1200) and midnight (2400) after intravenous injection of 50 mg/kg of LEA in rats. The effects of fasting on the circadian rhythms of LEA were also investigated. Serial blood samples were collected for 2 h. Bile and urine were collected during the entire study. Plasma data was analyzed by noncompartmental methods. Dosing of LEA at 2400, when the animals were active, resulted in a 50% decrease in mean residence time (MRT) and a 20% increase in systemic clearance (Cltotal). The increase in Cltotal was reflected by increases in biliary (Clbile) and renal (Clrenal) clearance at 2400. In addition, the steady-state volume of distribution (Vss) at 2400 was also decreased by about 50% as compared to dosing at 1200. Interestingly, no difference in the pharmacokinetic parameters were observed in fasting animals at 1200 and 2400. Since rats consume very little food during their sleep cycle, restriction of food intake did not have any affect on the pharmacokinetics of LEA at 1200. However, fasting rats had an approximately 36% decrease in Cltotal as compared to nonfasting rats at 2400. This decrease in systemic clearance was paralleled by a 60% and 24% decrease in Clbile and Clrenal, respectively. These variations could be attributed to changes in bile composition and/or lipoprotein concentrations in the plasma as a result of "forced" fasting at 2400 when the animals are generally more active and food intake is high.