**5.3. Drug and formulation development and drug‐regulatory affairs**

Nuclear Medicine studies on drug delivery have been accepted by regulatory authorities as supporting evidence in product registration dossiers such as Investigational New Drug Applications or New Drug Applications [13].

Nuclear Medicine stimulates and supports drug development in a noninvasive way. With the radiolabeling of drug molecules, it is possible to monitor distribution, release, and kinetics, through the observation of its *in vivo* distribution and allowing the visualization of their metabolism in both target and nontarget sites [1, 13]. These studies can be performed in both animals and humans.

In drug approval, most studies are performed with new chemical entities (NCEs), because information on their metabolic outcome is required. Studies of biopharmaceuticals metabolism using radiotracers are less frequent, because it can be difficult to substitute a radiotracer for a naturally occurring stable isotope [6].

Regulatory drug‐testing programs that employ radiotracers are generally classified into two groups: explorative and standard studies. Standard studies are habitually done for the majority of NCEs, with varying characteristics, depending on the drug class and specific circumstances. Explorative studies, although very important (even mandatory sometimes), are not usually required. More than 80% of all drug‐safety‐testing assessment programs (by the US safety assessment process) used radiotracers [14].
