In drug discovery research, radiotracers have been playing a unique role. Their detection is easy. Minute quantities are adequate for investigation. The system’s chemical, pharmacological and biological equilibrium remains  undisturbed. The chemical structure of the radiotracer and its non-radioactive counterpart is the same. Modern biology’s two pillars are instrumentation and radiotracers. In drug discovery, these are used in basic science, ADME studies, bioavailability, tissue distribution, target evaluation, mass balance, pre-clinical studies, metabolic study in humans and during clinical trials. Radiotracers are non-invasive and sensitive research tools. Nuclear imaging of small animals for drug evaluation is popular for pre-clinical study of drugs.

George Hevesy was the first scientist to study the distribution of radioactivity labeled phosphate in animals to follow the course of a metabolite in living organism. This was in the 1930s. Kamen and his co-workers used carbon isotopes 11C and 14 C in the 1940s as radiotracers.

A radiotracer is a molecule in which one of the atoms is replaced by its radioisotope. It has the same chemical and biological properties as that of its non-radioactive counterpart. Radiotracers are easily detected by radiation detectors due to their alpha, beta, r, lamda or positron radiations.

Radiotracers are used in two ways.

1. Based on chemical reactions the radiotracer undergoes, there are chemical products generated. These are chemical products, generated. These contain the radio active label. The analysis of radioactivity indicates the mechanism of such chemical reactions. In vitro biological investigations thus can be carried out.
2. A radiotracer is administered into a living organism and imaging is taken. We can trace the distribution of the compound and its reaction products. This is called scintigraphy.

In vivo radiotracer techniques are applied in ADME or adsorption, metabolism, excretion studies. Autoradiography enables us to assess drug safety. Micro-dosing points out one of the major causes of failure of drug-leads during development. Maybe, there is unsatisfactory pharmacokinetic ( PK ) or ADME parameters. The entity can be evaluated prior to Phase I Clinical Trials. Nuclear imaging enables us to assess to the drug efficacy. It also plays a role in clinical trials to evaluate whether a drug has reached the target organ and its possible effect.

In vitro, radiotracers are used in target evaluation, drug-binding to receptors, bio-markers, drug analysis, delivery evaluation. Radio-receptor assays are used to identify and characterize enzymes.