In Vivo Evaluation Using Gamma Scintigraphy

To enhance patient compliance and treatment efficiency, several investigators have focused on developing novel routes of drug delivery or reducing the multiple dosing regimens to once-daily products in the form of controlled-release formulations (1). In vitro release studies using conventional and modified dissolution methods can provide insight into the performance of drug-delivery systems, and radionuclides incorporated into the dosage form provide information on the in vivo behavior of dosage forms. Gamma scintigraphy is a well-established radionuclide imaging technique (2). This technique is valuable for evaluating various dosage forms. It is noninvasive and provides reliable information on the transit time of dosage forms in different regions of the gastrointestinal (GI) tract and various other body organs. Gamma scintigraphy can analyze the time taken for disintegration of the drug product and the site where disintegration occurs. The effect of different conditions such as the presence of food, diseased state, and dosage size also can be explored. In current experimental protocols, it is common to evaluate the in vivo performance of drug-delivery systems in healthy volunteers or patients using this imaging technique (3). The process is significantly different from traditional techniques such as diagnostic X-ray methods where external radiation is passed through the body to form an image (4). Contrary to this approach, the gamma scintigraphic technique relies on the detection of radiation emitted from radionuclides tagged with dosage forms that are administered intravenously or orally. Release of the tagged tracer is monitored rigorously in vitro. Moreover, this technique should be performed in a protected environment (1).

Gamma scintigraphy

In gamma scintigraphy, nuclear imaging is generally carried out with planar or single photon emission computed tomography (SPECT) cameras capable of detecting the incorporated radionuclides that emit gamma radiation with energies between 100 and 250 KeV (5). Emitted radiations are further captured by external detectors such as gamma cameras. The following are advantages of gamma scintigraphy:

• Very little radiation exposure to the participating subjects compared with roentgenography (i.e., X-ray methods)

• Qualitative, as well as quantitative, observations that can be recorded that are not feasible with other techniques

• Totally noninvasive

• In vivo evaluation of dosage forms is possible under normal physiological conditions.

Radiolabeling of dosage forms

Before imaging by this technique, the dosage form should be radiolabeled. Radiopharmaceuticals labeled with ^99mTc are most commonly used; however, other sources of radionuclides that can be used in traditional gamma scintigraphy are ^81mKr, ¹¹¹In, ¹²³I, and ¹³¹I (6, 7). Table I represents half-life and the types of emitted radiation of these radionuclides. A suitable radionuclide for scintigraphic studies is selected by considering the following factors (8):

• Radiation energy of gamma rays should be within the detection range of the gamma camera

• Emitted radiation well-suited for in vivo applications

• The half-life of the radionuclide must be adapted to the period of testing

• The tracer should not alter the performance of dosage forms being investigated

• Cost and availability.

Table I: Properties of commonly used radionuclides.

The radionuclide is incorporated into the formulation using an appropriate radiolabeling technique, so it can act as a marker for a particular event. Usually, dosage forms are assessed to determine the release of a drug, but in some cases, the radionuclide is required to be retained in the formulation to investigate the ultimate fate of the dosage form in terms of site, rate, and extent of drug absorption. The observed transit of the dosage form also can be correlated with the rate and extent of drug absorption (9).

Radiolabeling techniques

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. 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. 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. 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.

Neutron activation. 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.

Scintillation camera

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.

Applications

Gamma scintigraphy can be applied in drug-delivery technologies and advanced pharmacokinetic studies. It also is also useful for evaluating new drugs in the developmental phase, for characterizing new formulations and delivery systems, for establishing bioequivalence of generic products, for monitoring the therapeutic benefits and outcomes of a drug, and for assessing site and organ targeting studies (1). Imaging, pharmacoscintigraphy, and biodistribution are other important applications.

Imaging. Imaging is commonly used to monitor the performance of a drug-delivery systems under normal physiological conditions in a noninvasive manner. The relevance of this process in oral drug delivery includes the assessment of buccal drug delivery, oesophageal transit studies, analysis of gastroretentive dosage forms, gastric-emptying studies, and GI-transit evaluation. Food effects, intra- and inter-subject variability, along with the site of delivery such as the investigation of formulations designed to target the colon, also can be explored with this study. Other possible routes that can be imaged include parenteral, rectal, nasal, pulmonary, and ophthalmic (13–15).

Pharmacoscintigraphy. Pharmacoscintigraphy integrates gamma scintigraphy and pharmacokinetic data to assess the behavior of a dosage form in subjects under investigation. Instead of relying on pharmacokinetic findings alone, it is better to unite these parameters with the technique of gamma scintigraphy to investigate the performance of the dosage form in humans (16). In these studies, the radiolabeled-dosage form is administered to volunteers or patients. Images are acquired using a gamma camera, permitting visualization of the dosage form in the body in a noninvasive manner. A radionuclide tagged with drugs, formulations and devices provides vital information about the rate and extent of drug absorption (17). This technology has the potential to play a role in evaluating various modified-release formulations, for optimizing drug bioavailability, and for understanding the causes of poor absorption (1, 5). Such investigational studies provide ways for evaluating formulations and the drug-delivery system in preclinical and clinical development. The performance of the formulation, which includes the ability of a delivery system to target a specific location, the rate of erosion in comparison with in vitro dissolution data, and the effect of the absorption window on bioavailability, also can be studied using pharmacoscintigraphy (18). Combining the imaging information with the pharmacokinetic data provides functional and valuable knowledge about the release and absorption mechanism of a drug. The imaging techniques can be used to correlate the pharmacology of various molecules, explore pharmacokinetic parameters, and develop proof of concept for drug-delivery systems (1, 2).

Biodistribution. The gamma scintigraphic technique also has been used for biodistribution studies of several drugs radiolabeled with ^99mTc. The biodistribution pattern was established for several drugs, including ciprofloxacin, sparfloxacin, and isoniazid (17). Biodistribution also is used in brain targeting, tumor imaging, gene therapy, and bone-targeting delivery systems with the help of SPECT (1, 19–21).

Radiation safety

Although exposure to radioactivity in a large dose can be harmful, the extent of radioactivity from radiopharmaceuticals and related concerns of safety are determined by a nuclear medicine physician. The International Commission on Radiological Protection has established the limits to be followed for use of radiological products (1, 22). These limits are considered to be safe for individuals. The level of radioactivity used in gamma scintigraphy is very low. The dose of radiation administered to participating subjects is well below the maximum permissible dose (5).

Conclusion

Gamma scintigraphy has been successfully used in various scientific fields such as nuclear medicine, pharmaceutical technology, and gastroenterology and can be used for in vivo tracking of drug-delivery systems. Vital information regarding the extent, rate, site, and mode of drug release, along with morphology of drug-delivery systems, in subjects under ethical norms can be obtained using this technique. The authors believe that gamma scintigraphy will continue to be a useful tool in tracking and evaluating drug-delivery systems.

Acknowledgments

The authors would like to acknowledge Kanchan Kohli, associate professor, Department of Pharmaceutics, Hamdard University, New Delhi, India, for providing valuable input for this article.

Rakesh Pahwa* is a faculty member, Himanshu Dutt is a research scholar, and Vipin Kumar and Prabodh Chander Sharma are faculty members at the Institute of Pharmaceutical Sciences, Kurukshetra University, Kurukshetra, India, tel. + 91 9896250793, rakesh_pahwa2407@yahoo.co.in.

*To whom all correspondence should be addressed.

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