Bioavailability and bioinequivalence of drug formulations in small animals

Pharmacokinetics and oral bioavailability of ampicillin and its prodrug bacampicillin in chickens and turkeys

IMPORTANCE

Reports comparing the pharmacokinetics of ampicillin (AMP) and its prodrug bacampicillin (BAC) in poultry are scarce.

OBJECTIVE

This study evaluated the pharmacokinetic characteristics of AMP and BAC in chickens and turkeys after intravenous (i.v.) or oral administrations at a single dosage of 25 mg/kg body weight.

METHODS

Thirty chickens and 30 turkeys were divided into three groups (n = 10). Blood samples (0.5-0.7 mL) were collected from the subcutenea ulnar vein at pre-assigned times up to 12 h. The AMP and BAC plasma concentrations were analyzed using high-performance liquid chromatography with a fluorescence detector. The data were analyzed using noncompartmental methods.

RESULTS

The area under the curve from 0 to 12 h, apparent total systemic, the apparent volume of distribution, and mean residence time differed significantly according to the administration routes. The terminal elimination half-life values for the i.v. AMP, oral AMP, and oral BAC routes were 1.81, 3.64, and 5.39 h (p = 0.0249, p < 0.05), respectively, in chickens, and 2.44, 2.53, and 5.5 h (p < 0.0001), respectively, in turkeys. The bioavailability for the oral AMP and oral BAC groups in chickens was 25.9% and 44.6%, respectively. Similarly, in turkeys, the bioavailability was 19.1% and 44.2% for the oral AMP and oral BAC groups, respectively.

CONCLUSIONS AND RELEVANCE

The oral route of BAC with good bioavailability can be preferred for treating various conditions. On the other hand, further studies will be needed to determine the clinical efficacy of BAC during bacterial infections and the pharmacokinetics and safety of repeated administration in poultry.

Bioequivalence of Two Intravenous Artesunate Products with Its Active Metabolite Following Single and Multiple Injections

In animal species and humans, artesunate (AS) undergoes extensive and complex biotransformation to an active metabolite, dihydroartemisinin (DHA). The bioequivalence of two intravenous AS pharmaceutical products with 5% NaHCO₃ (China Formulation) or 0.3 M PBS (WRAIR Formulation) was determined in rats in a two-formulation, two-period, and two-sequence crossover experimental design. Following single and multiple intravenous administrations, a series of blood samples was collected by using an automated blood sampler and drug concentrations were analyzed by LC-MS/MS. The 90% CI of the difference between the two intravenous formulations was contained within 80–125% of the geometric mean of pharmacokinetic parameters for AS and DHA in all animals dosed. Hematological effects were studied on days 1 and 3 after the final dosing, and a rapidly reversible hematological toxicity (significant reductions in reticulocyte levels) was seen in the peripheral blood of the rats treated with each formulation. The results showed that bioequivalence with the parent compound and active metabolite was fulfilled in the 82.3–117.7% ranges of all parameters (AUC_0−t, C_max, concentration average and degree of fluctuation) in the two-period and two-sequence crossover studies following single and repeated intravenous injections. For the metabolite, the equivalence was satisfied in most pharmacokinetic parameters tested due to the variability in the hydrolysis rate of AS to DHA. The WRAIR formulation of AS was considered to be bioequivalent to the Chinese formulation at steady-state according to the total drug exposure, in terms of both parent drug and active metabolite, rapidly reversal in reticulocyte decline, and extension of single and multiple administrations. Therefore, the parent drug and active metabolites should play similar important roles in the determination of efficacy and safety of the drug.

Pharmacokinetics of erythromycin after intravenous, intramuscular and oral administration to cats

The aim of this study was to characterise the pharmacokinetic properties of different formulations of erythromycin in cats. Erythromycin was administered as lactobionate (4 mg/kg intravenously (IV)), base (10mg/kg, intramuscularly (IM)) and ethylsuccinate tablets or suspension (15 mg/kg orally (PO)). After IV administration, the major pharmacokinetic parameters were (mean ± SD): area under the curve (AUC)((0-∞)) 2.61 ± 1.52 microgh/mL; volume of distribution (V(z)) 2.34 ± 1.76L/kg; total body clearance (Cl(t)) 2.1 0 ± 1.37 L/hkg; elimination half-life (t(½)(λ)) 0.75 ± 0.09 h and mean residence time (MRT) 0.88 ± 0.13 h. After IM administration, the principal pharmacokinetic parameters were (mean ± DS): peak concentration (C(max)), 3.54 ± 2.16 microg/mL; time of peak (T(max)), 1.22 ± 0.67 h; t(½)(λ), 1.94 ± 0.21 h and MRT, 3.50 ± 0.82 h. The administration of erythromycin ethylsuccinate (tablets and suspension) did not result in measurable serum concentrations. After IM and IV administrations, erythromycin serum concentrations were above minimum inhibitory concentration (MIC)(90)=0.5 microg/mL for 7 and 1.5h, respectively. However, these results should be interpreted cautiously since tissue erythromycin concentrations have not been measured and can reach much higher concentrations than in blood, which may be associated with enhanced clinical efficacy.

Companion animal physiology and dosage form performance

Among the most critical parameters for any drug candidate are tolerability, dose, solubility and permeability. For controlled release formulations, gastrointestinal transit is an added hurdle. While we might assume that intestinal transit is independent of the drug candidate, the relative importance of gastrointestinal transit time (GITT) depends directly on the other parameters. For example, a formulation of a drug with low solubility (LS) and/or low permeability (LP) characteristics might provide the required systemic concentrations when administered with food, but not if administered on an empty stomach. In the LS case, the drug may require the solubilizing effects of increased fluid and bile salts that accompany the meal. Likewise, a controlled release formulation of a drug with a region of preferred absorption may empty from the fasted stomach and move beyond the region before drug release is complete. Companion animals (e.g. cats and dogs) differ from humans and each other with respect to GITT, food effects, eating habit influences, breed and size variability, gastric pH, intestinal enzymes, GI permeability and absorption regions. This review examines how the anatomy and physiology of companion animals relates to the performance of orally administered immediate and controlled release formulations. Examples are presented of techniques used to predict the dose and acceptable solubility of drug candidates, and the performance of formulations in companion animals.

Pharmacological adrenalectomy with mitotane

The potential of mitotane (ortho, para'-DDD, commonly used to treat adrenal carcinomas in humans and dogs) was investigated as an alternative to surgical adrenalectomy in birds, salamanders, and lizards. House sparrows (Passer domesticus) were injected twice daily with vehicle or one of two doses of mitotane (225 or 450 mg/kg), and basal and stress-induced levels of corticosterone (CORT) were measured 3 and 5 days after injections. Mitotane reduced basal CORT levels to nondetectable and abolished stress-induced CORT increases by the 3rd day of treatment. In another study, a single injection of mitotane was effective in lowering endogenous CORT levels 36 h later, but levels had apparently recovered by 10 days after the injection. Mitotane did not effect testicular weights and had no detectable effect on testosterone levels. In contrast to its effects on house sparrows, mitotane did not lower endogenous CORT levels in either tiger salamanders (Ambystoma tigrinum) or tree lizards (Urosaurus ornatus), even at doses much higher than those used in house sparrows.

Effect of ingesta and of tablets of different strengths on the systemic availability of digoxin in normal dogs

The plasma concentrations of digoxin were measured in eight normal dogs given digoxin on four occasions, using three different feeding regimens and tablets of two strengths. Although ingesta tended to slow the absorption of digoxin, the systemic availability of the drug, based on measurements of Cmax, tmax and AUC did not differ when digoxin tablets were given with canned food, with dry food, or without food. However, some of the pharmacokinetic characteristics and smaller individual variations with the dry food regimen would be considered advantageous for maintenance therapy. Tablets containing 62.5 micrograms or 250 micrograms of digoxin had a similar relative bioavailability. The peak plasma digoxin concentrations were higher in female dogs, and the trends in other data also suggested that the systemic availability of digoxin was better in female dogs.