Intracellular drug concentrations.

Many drug targets are intracellular. To access them, a drug molecule must pass through the cell membrane, a process often facilitated or impeded by transporters. Once in the cytoplasm, basic molecules may become concentrated in organelles. To predict the pharmacologic effect accurately, there must be data concerning the concentration at the target, which is difficult to measure. Techniques that combine mass spectrometry and imaging techniques (matrix-assisted laser desorption/ionization, secondary ion mass spectrometry (SIMS), and nanoSIMS) have promise in addressing this problem.