[Microdialysis for the determination of free drugs in tissues]

Ex vivo examination of the biocompatibility of biodegradable magnesium via microdialysis in the isolated perfused bovine udder model

PURPOSE

Being biodegradable, magnesium is considered a promising future implant material but very little is known about the biocompatibility for the tissues in direct contact with it. In this study, the degradation of pure magnesium implants in the skin of an isolated bovine udder was examined over a period of five hours.

METHODS

Microdialysis technique was used in order to investigate the reactions at the interface of implant and tissue. Pure titanium implants served as control. Degradation behavior and biocompatibility were evaluated via extracellular magnesium ion concentration and PGE2 and TNF alpha served as indicators of inflammation.

RESULTS

Concentrations of 5.5 mmol/l Mg2+ were detected at the beginning, which decreased to a plateau of about 3.5 mmol/l after approximately two and a half hours. PGE2 and TNF alpha concentrations indicated no major inflammatory tissue response to the degradation.

CONCLUSIONS

These results give an idea of the ion burden at the implantation site of degrading magnesium and suggest good biocompatibility even at the tissue-implant interface.

Relationship between serum and free interstitial concentrations of cefodizime and cefpirome in muscle and subcutaneous adipose tissue of healthy volunteers measured by microdialysis

Microdialysis is a suitable method to monitor unbound concentrations of antimicrobial drugs in the interstitial tissue space which is the site of many infections. The aim of this investigation was to examine whether free tissue levels of cefodizime (81% plasma protein binding) and cefpirome (10% plasma protein binding) in muscle and subcutaneous adipose tissue of healthy volunteers obtained by microdialysis are consistent with the extent of their respective plasma protein binding. Healthy volunteers were given cefodizine and cefpirome at a single intravenous 2-g dose in a randomized crossover design. Microdialysis probes were inserted into a medial vastus muscle and into the periumbilical subcutaneous layer. After calibration of the probe, samples of serum and microdialysis fluid were obtained and drug concentrations were measured using a microagar diffusion-bioassay. There was a reasonable agreement between plasma protein binding data and the tissue penetration of both cephalosporins (AUC0-infinity tissue, free/AUC0-infinity serum, total-ratios) into the interstitial fluid of the muscle tissue, but not for the subcutaneous tissue layer. Furthermore, the serum and tissue concentrations of both drugs were fitted to an open two-compartment body model. The measured free-tissue concentrations were compared with calculated unbound concentrations in the peripheral compartment. Good agreement was observed for the free muscle concentrations, but unbound concentrations in the subcutaneous tissue was somewhat higher (cefpirome) or lower (cefodizime) than predicted. This may be due to the different lipophilicities of the two compounds.