Stability of cocaine in formalin solution and fixed tissues

Abstract 

Embalming and formalin fixation are common, and yet they can create problems for the forensic scientist if a drug has been the cause of death and if the only available specimens to be analyzed are formalin-fixed tissues. Previous studies have demonstrated that during fixation xenobiotics are extracted into formalin according to tissue and fixing solution characteristics. In some cases formalin can react with the analyte resulting in the production of new chemical entities. Regarding cocaine and its metabolites, Cingolani et al. have reported that formalin-fixation extracts benzoylecgonine (BE) from tissues and that BE is stable in the fixing solution. However, the stability and kinetic properties of cocaine remain so far unexplored. Our data show that in buffered formalin (pH 7.4) cocaine is hydrolyzed to BE in agreement with a pseudo first-order reaction kinetic (half-life time ∼7 days), whereas in unbuffered formalin (pH∼3.5) it is relatively stable over a period of 30 days. The analysis of brain and liver samples at different fixation times indicates that during fixation an extraction process occurs for both analytes and that the extraction is more efficient in the liver than in the brain, probably because of a greater lipophilicity of the brain tissue.

In conclusion, our study demonstrates that formalin-fixed tissues and their fixing solutions can be used for cocaine analysis only if a short time period has passed since the fixation beginning. The rapid extraction process of cocaine into formalin and the concomitant hydrolysis to BE occurring in buffered formalin may prevent the identification of cocaine in both tissues and formalin solution already at 15–30 days after fixation. Moreover, the unpredictable extraction rate of both analytes, along with the hydrolysis of cocaine into BE significantly affects tissue concentrations, thus complicating the interpretation of quantitative results.

Keywords: Forensic toxicology, Cocaine, Benzoylecgonine, Formalin, Fixation process