The Surface Area to Volume Ratio Changes the Pharmacokinetic and Pharmacodynamic Parameters in the Subcutaneous Tissue Cage Model: As Illustrated by Carprofen in Sheep

Abstract

IntroductionPharmacokinetic and pharmacodynamic models can be powerful tools for predicting outcomes. Many models are based on repetitive sampling of the vascular space, due to the simplicity of obtaining samples. As many drugs do not exert their effect in the vasculature, models have been developed to sample tissues outside the bloodstream. Tissue cages are hollow devices implanted subcutaneously, or elsewhere, that are filled with fluid allowing repetitive sampling to occur. The physical dimensions of the cage, namely, the diffusible surface area to volume ratio, would be expected to change the rate of drug movement into and out of tissue cages.MethodsSeven sheep were implanted with five pairs of tissue cages, subcutaneously. Each pair of cages had a different length but a fixed diffusible surface area, so the surface area to volume ratio differed. Carrageenan was injected into half of the cages in each animal during one sampling period in a cross-over design. Samples from each cage and the bloodstream were obtained at 14-time points during two sampling periods. The concentration of carprofen was measured using LC–MS/MS and the results were modeled using nonlinear mixed-effects techniques. Prostaglandin metabolites were also measured and the change over time was analyzed using linear mixed effect modeling.ResultsThe presence of carrageenan within an animal changed the systemic pharmacokinetics of carprofen. The rate of drug movement into and out of the tissue cages varied with the surface area to volume ratio. The concentration time curve for prostaglandin metabolites changed with cage size.ConclusionThe surface area volume ratio of tissue cages will influence the calculated pharmacokinetic parameters and may affect calculated pharmacodynamics, thus, it is an important factor to consider when using tissue cage data for dosing regimes.

Moxidectin is a candidate for use as an in vivo internal standard in pharmacokinetic studies, as demonstrated with use in simultaneous tissue cage and ultrafiltration fluid collection

“…Animals were anaesthetised and two tissue cages (6 cm and 10 cm length) were inserted under the skin on one side of the neck three weeks prior to the experiment, as previously described ( 16 , 18 ), with the addition of a local anaesthetic field block being placed around the surgical site with 0.75% ropivacaine. The side of neck into which tissue cages were implanted was alternated so that 4 sheep had the left side implanted and 4 the right side.…”

“…Samples were subjected to separate carprofen and moxidectin LCMS analysis. Carprofen analysis was as previously described ( 16 ) with the following refinements; sample preparation was simplified to 100 μL of sample in addition to 400 μL of Meclofenamic Acid Internal Standard Working Solution being added to Ostro Pass-through plates (Waters Australia Rydalmere, NSW). Acetonitrile (ACN) was replaced with 50: 50 methanol: ACN as mobile phase B.…”

“…The samples were reconstituted with 200 μL of ACN before transferring to 96-well plates for analysis. A 20 μL injection was made by the autosampler into a Shimadzu LCMS/MS system fitted with a C18 column as previously described ( 16 ), the column oven was held at 50°C. The liquid chromatography program began at 30% organic mobile phase (Isopropyl Alcohol:ACN, 75:25) rising to 90% organic at 5 min, the mobile phase was rapidly switched to 95% aqueous (MilliQ water) before returning to the starting conditions for 2 min.…”

“…Pharmacokinetic analysis was performed in Monolix (2023R1, SimulationsPlus) using a custom model as previously described ( 16 ), the tissue cage concentrations were driven by the central compartment with the length of the cage in centimeters used as the regressor value. The model was run on both raw carprofen concentrations and on the corrected carprofen concentration.…”

“…In our previous work in sheep ( 16 ) simultaneous samples were collected, from five sizes of tissue cages and from blood, following carprofen dosing. The surface area to volume ratio differed between the tissue cage sizes and a model was constructed to simultaneously fit the pharmacokinetics for plasma and all tissue cages.…”