tracerkinetics

Tracer kinetics, or tracerkinetics, is the study of the time-dependent distribution and fate of labeled molecules—tracers—in biological systems. Tracers can be radioisotopes, stable isotopes, or labeled compounds with fluorescent or other detectable tags. The aim is to reveal transport processes, metabolic transformations, and compartmental distribution without perturbing the system significantly.

Measurements are typically dynamic and may involve imaging, such as positron emission tomography (PET) or single-photon emission computed tomography (SPECT), or serial sampling of blood, tissue, or excreta. In vitro, cells or tissue slices may be studied with radiolabeled or stable-isotope tracers. Data yield time-activity or concentration curves used in modeling.

Analysis relies on compartmental or physiologically based models that describe tracer exchange between compartments with rate constants. Key concepts include the input function (the tracer's concentration in arterial blood or inflow), distribution volumes, and metabolic or binding rates. Graphical methods such as Patlak plots for irreversible uptake or Logan plots for reversible binding are common tools.

Applications span pharmacokinetics, drug development, and physiology. Tracer kinetics informs about organ blood flow, membrane transport, receptor–ligand interactions, and metabolic fluxes. In clinical imaging, tracers like fluorodeoxyglucose enable visualization and quantitation of metabolic activity, while research tracers probe neurochemistry, cardiology, and oncology.

Limitations include model identifiability, data noise, and biases from partial volume effects or non-steady-state conditions. Safety and ethics govern use of radioactive tracers, and stable isotopes or non-radioactive labels are chosen when appropriate. Accurate interpretation requires careful experimental design and validation against independent measurements.