Microwave-assisted extraction in toxicological screening of skeletal tissues

Abstract

The use of microwave-assisted extraction (MAE) in screening of decomposed bone tissue for model drugs of abuse is described. Rats received 50 mg/kg (i.p.) pentobarbital (n=2), 75 mg/kg (i.p.) ketamine (n=2) or 16 mg/kg (i.p.) diazepam (n=1), or remained drug-free (control). Drug-positive animals were euthanized within 20 min of drug administration. Animal remains were allowed to decompose in a secure outdoor environment to the point of complete skeletonization. Bones (tibiae, femora, vertebrae, ribs, pelvi, humeri and scapulae) were collected and pooled (according to drug) in order to minimize effects due to inter-bone differences in drug distribution. Bones were crushed and cleaned of marrow and residual soft tissue in alkaline solution or phosphate buffer with ultrasonication. Cleaned bones were then ground and underwent MAE in phosphate buffer (pH 6), methanol or a methanol:water mixture (1:1, v/v) at atmospheric pressure in a domestic microwave oven, or passive extraction in methanol. Bone extracts (control and drug-exposed) containing methanol were evaporated to dryness before reconstitution in phosphate buffer (pH 6) and subsequent analysis by ELISA, while bone extracts containing only phosphate buffer were assayed directly by the same ELISA protocol. Measured absorbance values were expressed as the decrease in absorbance, measured as a percentage, relative to the corresponding drug-free control bone extract. The semi-quantitative nature of the ELISA assay allowed examination of the effects of extraction solvent and bone sample mass on the assay response for each drug examined, and subsequent comparison to assays of extracts obtained through passive methanolic extraction of various bone tissues. Overall, the time required for maximal extraction varied with extraction solvent and bone mass for each drug investigated, with significant extraction occurring with all solvent systems examined. MAE may represent a substantially faster extraction system than passive extraction, with significant extraction recovery observed within 1 min of exposure for all samples examined. The implications of these results in the context of the available literature on drug analysis in skeletal tissues are discussed.