Physico-chemical and formulation-induced veterinary drug-product bioinequivalencies

Non-cell-autonomous OTX2 transcription factor regulates anxiety-related behavior in the mouse

The OTX2 homeoprotein transcription factor is expressed in the dopaminergic neurons of the ventral tegmental area, which projects to limbic structures controlling complex behaviors. OTX2 is also produced in choroid plexus epithelium, from which it is secreted into cerebrospinal fluid and transferred to limbic structure parvalbumin interneurons. Previously, adult male mice subjected to early-life stress were found susceptible to anxiety-like behaviors, with accompanying OTX2 expression changes in ventral tegmental area or choroid plexus. Here, we investigated the consequences of reduced OTX2 levels in Otx2 heterozygote mice, as well as in Otx2+/AA and scFvOtx2tg/0 mouse models for decreasing OTX2 transfer from choroid plexus to parvalbumin interneurons. Both male and female adult mice show anxiolysis-like phenotypes in all three models. In Otx2 heterozygote mice, we observed no changes in dopaminergic neuron numbers and morphology in ventral tegmental area, nor in their metabolic output and projections to target structures. However, we found reduced expression of parvalbumin in medial prefrontal cortex, which could be rescued in part by adult overexpression of Otx2 specifically in choroid plexus, resulting in increased anxiety-like behavior. Taken together, OTX2 synthesis by the choroid plexus followed by its secretion into the cerebrospinal fluid is an important regulator of anxiety-related phenotypes in the mouse.

Feasibility of interspecies extrapolation in determining the bioequivalence of animal products intended for intramuscular administration

To examine the validity of extrapolating parenteral product bioequivalence determinations across target animal species, the relative bioavailability of two injectable formulations of ampicillin trihydrate (PolyflexR, a water-based suspension, and Ampi-kel 10R, an oil-based suspension) was examined in calves, sheep and swine. Employing products recognized to be bioinequivalent provided an opportunity to explore potential species-by-formulation interactions. As compared with PolyflexR, Ampi-kel 10R exhibited lower area under the curve (AUC) estimates but higher peak concentrations in all target animal species. Nevertheless, marked interspecies differences were noted in the width and bounds of the confidence intervals about the differences in treatment means. Potential physiological and physico-chemical reasons for these findings are discussed.

Bioavailability and bioequivalence of veterinary drug dosage forms, with particular reference to horses: an overview

The route of administration and formulation of the dosage form affect the bioavailability (rate and extent of absorption) of a drug and may thereby influence the intensity and duration of the pharmacological effect. Location of injection site may affect the plasma concentration profile of drugs administered as aqueous suspensions or sustained release parenteral preparations (procaine penicillin G). When absorption influences the rate of elimination ('flip-flop' phenomenon), the apparent half-life of the drug will be increased (cefazolin sodium, i.m.; meclofenamic acid, p.o.). Absorption generally approximates a first-order process and either the absorption half-life or the mean absorption time (statistical moment term) will provide an estimate of the rate of absorption. The method of corresponding areas is the usual technique employed in estimating the extent of absorption (systemic availability). Inherent in this technique is the assumption that clearance of the drug remains unchanged. In horses, the time of feeding relative to oral dosing has been shown to affect systemic availability (rifampin, trimethoprim) and pattern of absorption (phenylbutazone). Oral paste formulations (trimethoprim-sulphadiazine, ivermectin) are convenient to administer, allow precision in dosage compared with powders or granules added to feed, and could provide sustained release. Assessment of bioequivalence is based on relative bioavailability, using a reference dosage form, together with a measure of the uncertainty (variance) of the estimate. Bioequivalence relies on the concept that preparations of a drug which provide essentially equivalent plasma concentration profiles should produce the same therapeutic effect.

Bioavailability and bioinequivalence of drug formulations in small animals

Differences in bioavailability of many drugs from their various dosage forms have been shown to be relatively common in human medicine. Although comparable bioavailability ('bioequivalence') is though to ensure comparable clinical effectiveness and safety ('therapeutic equivalence'), the relationship between bioinequivalence and therapeutic inequivalence is less clear. Thus the prevalence of clinically important differences in bioavailability is unknown. While similar concerns have arisen about drug products used in small animal practice, there have been few investigations and some earlier reports are incomplete. However, there are indications of bioinequivalence with enteral formulations of ampicillin, aspirin, chloramphenicol, digoxin, mitotane, oxytetracycline, penicillin V and theophylline. Other studies have suggested bioequivalence with enteral formulations of chloramphenicol, digoxin, phenytoin, oxytetracycline and thyroxine. Limited data for injectable preparations showed bioinequivalence with chloramphenicol and possibly oxytetracycline. There is no reason to expect formulation-related bioinequivalence to be less prevalent in veterinary than in human medicine. Indeed, it may be more common in veterinary practice because other potential influences on bioavailability (food, diseases, other drugs, etc.) are frequently ignored, and cheaper generic products are often favoured for economic reasons.