An intracellular concentration assay is an experimental method used to quantify the amount of a compound, e.g. a drug, that accumulates within cells, whether human or bacterial. Such assays are essential for understanding pharmacokinetics at the cellular level, including drug uptake, distribution, efflux, and intracellular retention.
Techniques commonly used include liquid chromatography coupled with mass spectrometry (LC–MS/MS), which enables medium to low-throughput sensitive and specific measurement of intracellular drug concentrations. Other approaches include fluorescence, bioluminescence- or radiolabel-based methods support high-throughput testing despite their generally lower precision as they rely on indirect signals affected by cells variability and background noise.
Such assays are fundamental in antibiotic development. In infections caused by intracellular pathogens, they provide data on drug accumulation within host cells, which correlates with antimicrobial efficacy, toxicology, pharmacology, and microbiological behaviour. Additionally, they allow to assess compound uptake in bacterial cells, particularly Gram-negative bacteria, where complex outer membranes and efflux systems limit drug penetration. By identifying compounds with favourable intracellular exposure, intracellular concentration assays contribute to the design of drugs with improved potency and therapeutic profiles.
See also: intracellular concentration