The aim of the present study was to investigate the use of vitreous humor as an alternative biological matrix in Forensic Toxicology for the determination of quetiapine and its metabolites, 7-hydroxy-quetiapine and nor-quetiapine. The distribution of these substances in vitreous humor was studied, by correlating their concentrations in vitreous humor with the respective in blood samples. Only few studies concerning the determination of quetiapine in the vitreous humor exist, while no study has been performed for the distribution of the main metabolites of quetiapine in this material.

During this study, a method for the determination of these substances was developed and validated. This method includes the isolation of the analytes from vitreous humor and blood samples using solid phase extraction, with Bond Elut LRC C18 columns and the derivatization with BSTFA with 1% TMCS prior to GC/MS analysis. The developed method is characterized by a dynamic range of 10.0–1000.0ng/mL (R2≥0.991) for the three substances, with a limit of detection and quantification of 3.0 and 10.0ng/mL, respectively. Its accuracy and repeatability are below 8.09% and 8.99%, respectively, for both biological materials. To our knowledge, this is the only GC-MS method that simultaneously identifies quetiapine with its metabolites in vitreous humor. The method was applied to vitreous humor and blood samples of 16 forensic cases, where quetiapine was involved.

According to the results of the analysis, quetiapine, 7-hydroxy-quetiapine and nor-quetiapine are easily distributed in vitreous humor. It was observed that when the substances are detected in blood, they are always found in vitreous humor, even when blood concentrations are very low. The mean value concentrations for quetiapine, 7-hydroxy-quetiapine and norquetiapine in postmortem blood samples were 130.10ng/mL, 28.60ng/mL and 90.76ng/mL respectively, while in vitreous humor the corresponding values were 65.2ng/mL, 38.00ng/mL and 70.00ng/mL. The ratio of concentrations in vitreous humor to those in blood was calculated from 0.02 to 2.54, with a mean of 0.89 for quetiapine, from 0.36 to 5.24, with a mean of 1.80 for 7-hydroxy-quetiapine and from 0.10 to 12.55, with a mean of 1.20 for norquetiapine. Two cases of toxic concentration of quetiapine in blood were observed. From the analysis of the results it was concluded that the simultaneous determination of both quetiapine and 7-hydroxy-quetiapine in vitreous humor at higher than average levels is likely to support the intake of large amounts of quetiapine and its possible association with the cause of death. In addition, the presence of quetiapine and/or 7-hydroxy-quetiapine in vitreous humor at levels close to or below the mean values is likely to be unrelated to the cause of death, even in cases where quetiapine is at toxic levels in postmortem blood.

This is the first study to investigate the distribution of both quetiapine and its metabolites in vitreous humor. The results of the study indicate the usefulness of vitreous humor in toxicological analysis for the determination of these substances when the traditional biological materials are not available. In addition, the levels of both quetiapine and 7-hydroxy-quetiapine in vitreous humor can provide important information for a more accurate investigation of forensic cases. To determine therapeutic and toxic concentrations of quetiapine and its metabolites in this matrix, the study of more cases with a clear history of quetiapine overdose is necessary in order vitreous humor to be established as an important alternative material in Forensic Toxicology.