Quantification of Loperamide by Gas Chromatography Mass Spectrometry

Due to reported pharmacological activity similar to classical opioids at supratherapeutic concentrations, abuse of the anti-diarrheal medication loperamide (Imodium AD™) has become a target in the opioid epidemic. While this phenomenon is not new, published quantitative analytical methods use liquid chromatography tandem mass spectrometry. Described here is an 11 min method for quantification of loperamide in postmortem whole blood by gas chromatography mass spectrometry. Validation studies performed followed SWGTOX guidelines and included: accuracy, specificity, limit of detection (LOD), regression model analysis, stability, and matrix recovery enhancement and/or suppression. The accuracy study consisted of inter-day, intra-day, reproducibility and dilution integrity experiments. Inter-day and intra-day accuracy, precision and coefficient of variation (CV) were measured; normalized results were 1.05 ± 0.09 with 8.87% CV (n = 36) and 1.03 ± 0.09 with 8.53% CV (n = 27), respectively. Reproducibility was evaluated through standard addition with an observed CV of 10.84% (n = 10). Dilution integrity (2× and 4×) resulted in 0.94 ± 0.13 with a CV of 13.9% (n = 5). No interference was observed through analyses of the internal standard (loperamide-d6), endogenous compounds (10 blank matrices) or 60 commonly encountered analytes. The LOD/decision point was 100 ng/mL (CV 8.40%). A linear calibration model was established from 100 to 1,000 ng/mL. Stability was examined; observed analyte-to-internal standard response resulted in 6.59% CV. Recovery was determined for loperamide and loperamide-d6 (31% and 36%, respectively). Neither matrix suppression nor enhancement was observed with loperamide at 750 ng/mL and loperamide-d6 at 300 ng/mL (-6.5% and -4.2%); however, some suppression was exhibited at lower concentrations (-39.8%). The designed method was determined to be sufficient for the analysis of loperamide-related death cases in Alabama (n = 8) and offers postmortem toxicology laboratories an alternative approach that is both highly selective and specific.