Absolute quantitation of propranolol from 200‐μm regions of mouse brain and liver thin tissues using laser ablation‐dropletProbe‐mass spectrometry

Rationale The ability to quantify drugs and metabolites in tissue with sub‐mm resolution is a challenging but much needed capability in pharmaceutical research. To fill this void, a novel surface sampling approach combining laser ablation with the commercial dropletProbe automated liquid surface sampling system (LA‐dropletProbe) was developed and is presented here. Methods Parylene C‐coated 200 × 200 μm tissue regions of mouse brain and kidney thin tissue sections were analyzed for propranolol by laser ablation of tissue directly into a preformed liquid junction. Propranolol was detected by high‐performance liquid chromatography/tandem mass spectrometry (HPLC/MS/MS) in positive electrospray ionization mode. Quantitation was achieved via application of a stable‐isotope‐labeled internal standard and an external calibration curve. Results The absolute concentrations of propranolol determined from 200 × 200 μm tissue regions were compared with the propranolol concentrations obtained from 2.3‐mm‐diameter tissue punches of adjacent, non‐coated sections using standard bulk tissue extraction protocols followed by regular HPLC/MS/MS analysis. The average concentration of propranolol in both organs determined by the two employed methods agreed to within ±12%. Furthermore, the relative abundances of phase II hydroxypropranolol glucuronide metabolites were recorded and found to be consistent with previous results. Conclusions This work illustrates that depositing a thin layer of parylene C onto thin tissue prior to analysis, which seals the surface and prevents direct liquid extraction of the drug from the tissue, coupled to the novel LA‐dropletProbe surface sampling system is a viable approach for sub‐mm resolution quantitative drug distribution analysis.