Several approaches for radiolabeling are available. Whole-dose radiolabeling, point radiolabeling, surrogate markers, and
neutron activation are predominantly used (6, 9–12).
. Whole-dose radiolabeling uniformly incorporates radiolabeled-carrier particles within the formulation matrix during manufacture.
This approach is particularly important when the key objective is to assess the release pattern of a drug from the dosage
form over time.
. Point radiolabeling also is known as drill and fill, by which the radiolabeled-carrier particles are inserted into a hole drilled within the surface of a tablet, which is subsequently
sealed. The radiolabel acts as a marker for location in the GI tract and also provides some information about the physical
integrity of the dosage forms.
. Surrogate markers generally are useful for several multiparticulate systems. In these studies, a second population (e.g.,
ion-exchange resin or nonpareil beads), labeled with a suitable radionuclide, is mixed with drug pellets. These radiolabeled
pellets reveal the information on the location of the drug containing pellets in the GI tract, and data can be further correlated
with a pharmacokinetic profile.
. Under neutron activation, a stable isotope is incorporated into the dosage form before its manufacture, which is followed
by neutron irradiation of the intact dosage form. Thermal neutron irradiation converts the carefully selected stable isotopes
(i.e., 152Sm or 170Er) into radioactive gamma-emitting isotopes (i.e., 153Sm or 171Er) that can be detected by external imaging devices.
Gamma camera, also called scintillation camera or Anger camera, is a device used to detect and image gamma radiation emitted
from a radionuclide. A gamma camera is provided with a scintillator that exhibits the property of luminescence and transforms
the gamma radiation into an emission of light. Monocrystals of sodium iodide, activated by thallium, is the most commonly
used scintillator. A collimator made of lead is placed immediately in front of the crystal to stop and filter a stream of
radiation arriving at an angle. A photomultiplier array and electronic circuitry are used for amplifying the light signal
produced in the crystal, quantifying the intensity of the incident gamma rays, and locating its origin. It is thus possible
to determine the distribution of the tracer on an image formed as a matrix of pixels (8). This image is subsequently computer-processed
to accurately determine the distribution and relative concentration of a radioactive tracer element in the organs and tissues
imaged, or in the so-called regions of interest.