In vivo solid-phase microextraction for monitoring intravenous concentrations of drugs and metabolites

This protocol for in vivo solid-phase microextraction (SPME) can be used to monitor and quantify intravenous concentrations of drugs and metabolites without the need to withdraw a blood sample for analysis. The SPME probe is inserted directly into a peripheral vein of a living animal through a standard medical catheter, and extraction occurs typically over 2-5 min. After extraction, the analytes are removed from the sorbent and analyzed by, for example, liquid chromatography-tandem mass spectrometry. It has been validated in comparison with conventional blood analysis, and we describe here the in vitro experiments typically conducted during method development. The new-generation biocompatible SPME probes are designed specifically for extraction of semi-volatiles and nonvolatiles directly from aqueous samples and can be steam sterilized. Sorbents are coated on fine-gauge surgical steel wire (200-mu m diameter), which is more rugged and biocompatible than conventional fibers (100-mu m fused silica fiber). They incorporate a binding agent that resists fouling by the biological matrix and does not cause an immune response in the experimental animal. The sorbents used (coating thickness of similar to 50 mu m) are selected for their affinity for the types of small molecules of interest. The procedure is illustrated by the analysis of benzodiazepines with polypyrrole-coated wires inserted into peripheral blood vessels of beagles, although it can be adapted for use in smaller animals. The in vivo sampling can require as little as 1 min, in which case the entire procedure from sampling to instrumental analysis can take as little as 30 min.