Radioimmunoassay for the measurement of S9788 in serum and microdialysis samples

Levels of vancomycin in the cerebral interstitial fluid after severe head injury

OBJECTIVE

To evaluate the concentrations of vancomycin in the cerebral interstitial fluid after intravenous administration by multiple boli.

DESIGN

Prospective non randomized study

SETTING

University hospital general ICU.

PATIENTS

Four patients undergone to craniotomy for evacuation of cerebral posttraumatic hemorrhage, who developed pneumonia 4-7 days from the injury

METHODS

Two microdialysis catheters were placed in each patient: one in the edematous brain surrounding the focal lesion and one in the subcutaneous tissue of abdomen. Levels of vancomycin were measured in serum and in the microdialysates samples.

RESULTS

Plasmatic concentration of 10-15 microg/ml in the trough level was obtained after four administrations of vancomycin. Levels of vancomycin in the subcutaneous tissue was above minimum inhibitory concentrations in all patients after the second administration. Mean serum/brain ratio was 8%. Cerebral interstitial concentration of vancomycin was never above minimum inhibitory concentrations; its maximum value was 1.2 microg/ml.

CONCLUSIONS

In edematous brain close to a posttraumatic hemorrhage the levels of vancomycin do not differ from that in healthy subjects. At these plasmatic concentrations cerebral interstitial levels of vancomycin were insufficient while subcutaneous interstitial levels were effective for clinical use.

Assaying protein unbound drugs using microdialysis techniques

Compared with traditional sampling methods, microdialysis is a technique for protein unbound drug sampling without withdrawal of biological fluids and involving minimal disturbance of physiological function. Conventional total drug sample consists of unbound drugs and protein bound drugs, which are loosely bound to plasma proteins such as albumin and alpha-1 acid glycoprotein, forming an equilibrium ratio between bound and unbound drugs. However, only the unbound fraction of drug is available for absorption, distribution, metabolism and elimination, and delivery to the target sites for pharmacodynamic actions. Although several techniques have been used to determine protein unbound drugs from biological fluids, including ultrafiltration, equilibrium dialysis and microdialysis, only microdialysis allows simultaneous sampling of protein unbound chemicals from plasma, tissues and body fluids such as the bile juice and cerebral spinal fluid for pharmacokinetic and pharmacodynamic studies. This review article describes the technique of microdialysis and its application in pharmacokinetic studies. Furthermore, the advantages and limitations of microdialysis are discussed, including the detailed surgical techniques in animal experiments from rat blood, brain, liver, bile duct and in vitro cell culture for unbound drug analysis.

Analytical considerations for microdialysis sampling

Adaptations in microdialysis probe designs have made it possible to obtain samples from the extracellular fluid of a variety of tissues with high temporal resolution. The resulting small volume samples, often with low concentration of the analyte(s) of interest, present a particular challenge to the analytical system. Rapid separations can be coupled on-line with microdialysis to provide near real-time data. By combining microdialysis sampling with a liquid chromatographic or capillary electrophoretic separation and a highly sensitive detection method, a separation-based sensor can be developed. Such sensors have been applied to the investigation of drug entities as well as to study endogenous analytes.