**6. Drug delivery and Nuclear Medicine**

Drugs are usually administered as pharmaceutical dosage forms. In the development of these dosage forms, it is fundamental to ensure they will perform correctly. Before the submission to the regulatory authority for approval of a new medicine, detailed testing is required. To improve therapeutic effects and minimize toxicity, it is crucial to deliver the therapeutic drugs to the right target, in the desired time, and in the precise concentration—a "magic bullet" [7, 9].

The controlled and targeted delivery of therapeutic drugs improves their bioavailability (either by preventing premature degradation or by improving uptake), maintains their concentrations within the therapeutic window (by adjusting the release rate), and reduces side effects (by targeting disease site/cells). The ability to deliver therapeutic drugs to the target, in a minimally invasive approach, has advanced considerably with the growth of molecular‐imaging techni‐ ques [9].

When molecular‐imaging radiotracers are part of the drug‐delivery system, they enable monitoring of its *in vivo* behaviour: pharmacokinetics, distribution, release at the target site, and excretion. Some of these evaluations (such as *in vivo* fate or delivery efficiency) are not possible with a nonimage‐guided approach. Another issue with nonimaging techniques is that much testing (bioavailability, therapeutic efficacy, and dose response) must be done in separate experiments, which makes for a rather expensive and stressful process [9].

The use of Nuclear Medicine imaging in the study of a drug‐delivery process dates back to the late 1970s and is well established in pharmaceutical development. Understanding drug action and providing key information about the drug‐delivery process through a variety of routes, Nuclear Medicine can be used in the various stages of studies [6, 9, 13]. Without radiotracers, the comprehension of numerous biochemical processes would have been tremendously difficult and, in some cases, maybe impossible. Its strength lies in the quantitative nature of the images. Only Nuclear Medicine techniques can determine the precise location of tablet disintegration in the gastrointestinal (GI) tract, the depth of penetration of a nebulized solution in the lungs, or for how long does the formulation stay in the cornea [6, 9, 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

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

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

Drugs are usually administered as pharmaceutical dosage forms. In the development of these dosage forms, it is fundamental to ensure they will perform correctly. Before the submission to the regulatory authority for approval of a new medicine, detailed testing is required. To improve therapeutic effects and minimize toxicity, it is crucial to deliver the therapeutic drugs to the right target, in the desired time, and in the precise concentration—a "magic bullet" [7, 9]. The controlled and targeted delivery of therapeutic drugs improves their bioavailability (either by preventing premature degradation or by improving uptake), maintains their concentrations within the therapeutic window (by adjusting the release rate), and reduces side effects (by targeting disease site/cells). The ability to deliver therapeutic drugs to the target, in a minimally invasive approach, has advanced considerably with the growth of molecular‐imaging techni‐

When molecular‐imaging radiotracers are part of the drug‐delivery system, they enable monitoring of its *in vivo* behaviour: pharmacokinetics, distribution, release at the target site, and excretion. Some of these evaluations (such as *in vivo* fate or delivery efficiency) are not possible with a nonimage‐guided approach. Another issue with nonimaging techniques is that much testing (bioavailability, therapeutic efficacy, and dose response) must be done in separate

The use of Nuclear Medicine imaging in the study of a drug‐delivery process dates back to the late 1970s and is well established in pharmaceutical development. Understanding drug action and providing key information about the drug‐delivery process through a variety of routes, Nuclear Medicine can be used in the various stages of studies [6, 9, 13]. Without radiotracers, the comprehension of numerous biochemical processes would have been tremendously

experiments, which makes for a rather expensive and stressful process [9].

animals and humans.

ques [9].

naturally occurring stable isotope [6].

164 Advanced Technology for Delivering Therapeutics

assessment process) used radiotracers [14].

**6. Drug delivery and Nuclear Medicine**

In image‐guided drug delivery assessment, the image techniques are used for determining disease location, drug‐targeting levels and localization, and release kinetics (before and during treatment). The type and stability of the labeling will depend upon whether the study is meant to examine release, deposition, retention/dispersion, or is being used to monitor the effects of physiological process [13, 15].

The majority of studies have evaluated gastrointestinal, pulmonary, and nasal drug delivery, but ophthalmic, buccal, rectal, vaginal, and parenteral routes have also been subject of research [6].

The required quantity of radiotracer in a drug formulation is very small and does not commit with the performance of the delivery system. Radiolabeling must reveal high *in vitro* and *in vivo* stability and can be performed in two different ways. The radiolabeled compound can be directly incorporated into the preparation, or a dosage form that contains a nonradioactive tracer can be neutron‐activated. The latter is advantageous in extensive or complicated delivery systems, and also has the advantage of the dosage formulation being manufactured and produced under normal conditions [7, 9]. Still, Nuclear Medicine imaging of drug‐delivery processes involves several challenges and is affected by numerous factors such as target expression, *in vivo* availability of the receptor, type of drug, enhanced permeability and retention effect, tracer protein dose, and timing of imaging [9].

Once administered, the radiolabeled compound will be monitored *in vivo* over a period of time (depending on its half‐life), using appropriate image equipment: gamma‐scintigraphy or PET. It is then crucial to choose the most suitable radioisotopes, with fitting half‐lives to pair with the pharmacokinetics of their drug carriers, and the most adequate imaging system [6, 9, 13].

In the last decades, most studies used gamma‐scintigraphy and Tc‐99m‐labeled radiophar‐ maceuticals containing chelating agents (such as diethylene‐triamine‐pentaacetic acid (DTPA)), colloids (such as sulfur colloid), diphosphonates (such as hydroxymethane di‐ phosphonate (HDP)), cells and blood elements, and cellulose macromolecules [7, 13].

More recently, the use of PET is increasing, including in drug‐delivery development studies. An important attribute of PET studies is that some of the radioactive atoms available include radioisotopes of carbon, nitrogen, and fluorine. This makes it possible to synthesize PET radiotracers with the same chemical structure as the unlabeled molecules, without altering their biological function [9, 13].

Unlike radiological imaging, Nuclear Medicine allows serial images to be obtained without submitting the subjects to higher radiation burdens, and radioactive concerns are neglected. Besides the reliability and reproducibility of Nuclear Medicine studies on the nature and characteristics of products, they may also be performed in the groups of patients intended to receive the dosage forms therapeutically, which is important since the presence of pathology can have significant impact on physiology [13].

It is well established that diseases alter physiological processes, and subsequently biodistri‐ bution of drug formulations. Thus, studies in humans are the most relevant and Nuclear Medicine imaging probably represents the only technique available that allows the quantifi‐ cation of drug release and pharmacokinetics in specific groups, namely patients with diseases [13]. This know‐how has the potential for patient selection for targeted therapy and for the monitoring of therapeutic response after the drug is delivered [9].

Targeted therapy involves the integration of multidisciplinary fields such as cell and molecular biology, chemistry, physics, and so on. Significant advances in the field have been attributed to the progress in nanotechnology, with the development of nanosized, multifunctional drug‐ delivery platforms. For example, a single platform can be used to detect, treat, and monitor treatment response in tumors. These systems present several advantages, including minimal clearance by the immune system, prolonged circulating times, attachment to suitable vectors (peptides, proteins, antibodies, etc.) and targeting of specific receptors, and improved treat‐ ment effects by shielding entrapped drugs from degradation [9].

For example, stimulus‐responsive polymeric nanomaterials can be synthesized to mimic the behavior of biological molecules, minimizing side effects and maximizing predictability. Another example, the liposomal carriers (the first and most extensive studied drug‐delivery carriers), is used in the delivery of anticancer drugs, antineoplastic agents, antimicrobial compounds, immunomodulators, anti‐inflammatory agents, cardiovascular drugs, and so on. Some other nanosized drug‐delivery systems that have also been developed for molecular‐ imaging purposes are metallic nanoparticles, oxide nanoparticles, polymeric nanoparticles, and carbon nanostructures [9].

The major goal of molecular imaging is to maximize therapy effect in diseased tissues, and reduce systemic effects and toxicity as much as possible [9].

## **6.1. Gastrointestinal tract studies**

Despite the vast diversity of new drug‐delivery systems, oral administration is still preferred.

Most commonly, oral‐dosage forms immediately release in the stomach, but the use of more sophisticated, modified release systems is increasing and requires new methods of evaluation. Furthermore, regulatory authorities require evaluation of the *in vivo* performance of new oral formulations.

Nuclear Medicine imaging is one of the most popular methods to investigate those GI release systems, because pharmacokinetic measurements are often unreliable. Its combination with pharmacokinetic studies provides accurate data about transit, absorption, and release per‐ formance of oral‐dosage formulations [7].

Nuclear Medicine imaging provides information, from both control and patient groups, on swallowing dynamics of tablet and capsule formulations, disintegration of immediate release formulations, gastric emptying and gastroesophageal reflux, release of enteric‐coated formu‐ lations, GI transit times, the effect of formulation size on GI delivery, visualization of targeted release or delivery to the colon, the effects of time of dosing on delivery, local permeability within the colon, disintegration characteristics likely to influence drug absorption, and residence time of the material within the colon [7, 9].

It is well established that diseases alter physiological processes, and subsequently biodistri‐ bution of drug formulations. Thus, studies in humans are the most relevant and Nuclear Medicine imaging probably represents the only technique available that allows the quantifi‐ cation of drug release and pharmacokinetics in specific groups, namely patients with diseases [13]. This know‐how has the potential for patient selection for targeted therapy and for the

Targeted therapy involves the integration of multidisciplinary fields such as cell and molecular biology, chemistry, physics, and so on. Significant advances in the field have been attributed to the progress in nanotechnology, with the development of nanosized, multifunctional drug‐ delivery platforms. For example, a single platform can be used to detect, treat, and monitor treatment response in tumors. These systems present several advantages, including minimal clearance by the immune system, prolonged circulating times, attachment to suitable vectors (peptides, proteins, antibodies, etc.) and targeting of specific receptors, and improved treat‐

For example, stimulus‐responsive polymeric nanomaterials can be synthesized to mimic the behavior of biological molecules, minimizing side effects and maximizing predictability. Another example, the liposomal carriers (the first and most extensive studied drug‐delivery carriers), is used in the delivery of anticancer drugs, antineoplastic agents, antimicrobial compounds, immunomodulators, anti‐inflammatory agents, cardiovascular drugs, and so on. Some other nanosized drug‐delivery systems that have also been developed for molecular‐ imaging purposes are metallic nanoparticles, oxide nanoparticles, polymeric nanoparticles,

The major goal of molecular imaging is to maximize therapy effect in diseased tissues, and

Despite the vast diversity of new drug‐delivery systems, oral administration is still preferred.

Most commonly, oral‐dosage forms immediately release in the stomach, but the use of more sophisticated, modified release systems is increasing and requires new methods of evaluation. Furthermore, regulatory authorities require evaluation of the *in vivo* performance of new oral

Nuclear Medicine imaging is one of the most popular methods to investigate those GI release systems, because pharmacokinetic measurements are often unreliable. Its combination with pharmacokinetic studies provides accurate data about transit, absorption, and release per‐

Nuclear Medicine imaging provides information, from both control and patient groups, on swallowing dynamics of tablet and capsule formulations, disintegration of immediate release formulations, gastric emptying and gastroesophageal reflux, release of enteric‐coated formu‐ lations, GI transit times, the effect of formulation size on GI delivery, visualization of targeted release or delivery to the colon, the effects of time of dosing on delivery, local permeability

monitoring of therapeutic response after the drug is delivered [9].

ment effects by shielding entrapped drugs from degradation [9].

reduce systemic effects and toxicity as much as possible [9].

and carbon nanostructures [9].

166 Advanced Technology for Delivering Therapeutics

**6.1. Gastrointestinal tract studies**

formance of oral‐dosage formulations [7].

formulations.

GI nuclear studies often require both solid‐ and liquid‐phase markers, there being two conventional approaches for the labeling of the oral‐dosage forms. One method involves incorporating a nonabsorbable chelate of the radioactive isotope (e.g., DTPA‐Tc‐99m). The other incorporating a radiolabeled ion‐exchange resin, which has the advantage of giving information about the *in vivo* position of the radiolabeled drug, because the resin remains within the device [7].

**Figure 1.** *In vitro* radiolabeling and evaluation of the drug‐delivery system.

In two different studies aimed at evaluating the gastroretentive behavior of pharmaceutical dosage forms, Barata et al. used Tc‐99m‐HDP to label HMPC tablets supplemented with calcium phosphate. The radiolabeling process consisted in soaking the tablet in a Tc‐99m‐HDP and NaCl 0.9% solution, with activity counts of 400–600 millicuries. The addition of calcium to the systems did not affect drug release, but significantly increased the binding of the radiopharmaceutical to the dosage form. In fact, the dosage form remained integer and well radiolabeled for a period of over 4 h, which was sufficient to study the gastroretentive pattern of the produced tablets. This proved to be an easy and reproducible method of extemporane‐ ously radiolabeling tablets produced outside Nuclear Medicine Department facilities, solving radiation‐related transportation problems (see **Figure 1**).

**Figure 2.** *In vivo* analysis of the drug‐delivery system floating behavior.

For the floating‐device study, a non‐floating tablet was labeled with Tc‐99m‐HDP, while the floating pharmaceutical dosage form was labeled with Ga‐68 (gallium‐68), using the same soaking method. Despite not as effective as Tc‐99m‐HDP labeling, Ga‐68 labeling was enough to allow a clear visualization of the system. The physical properties of the two radiopharma‐ ceuticals (different energy photons) enabled the simultaneous identifying of each system over time: initially, both in the stomach (floating in the upper part of the stomach or staying in the antrum), then, the effective gastroretention of the floating device for over 4 h (see **Figure 2**).

**Figure 3.** *In vivo* analysis of the drug‐delivery system gastroretentive behavior.

In the other study, aiming to demonstrate the gastroretentive efficacy of high‐density tablets, a similar process was used to radiolabel the produced tablets. Again, it was possible to see the *in vivo* positioning of the pharmaceutical dosage form and confirm that the high‐density controlled release strategy is effective for delivering drugs with a narrow upper GI absorption window (see **Figure 3**).

These examples prove that incorporating gamma‐emitting radionuclides into a delivery device or formulation provides an appropriate, simple method of observing the transit and residence within the GI tract—of particular interest in determining the time and site of release of delayed release formulations.

## **6.2. Other system studies**

**Figure 2.** *In vivo* analysis of the drug‐delivery system floating behavior.

168 Advanced Technology for Delivering Therapeutics

**Figure 3.** *In vivo* analysis of the drug‐delivery system gastroretentive behavior.

window (see **Figure 3**).

For the floating‐device study, a non‐floating tablet was labeled with Tc‐99m‐HDP, while the floating pharmaceutical dosage form was labeled with Ga‐68 (gallium‐68), using the same soaking method. Despite not as effective as Tc‐99m‐HDP labeling, Ga‐68 labeling was enough to allow a clear visualization of the system. The physical properties of the two radiopharma‐ ceuticals (different energy photons) enabled the simultaneous identifying of each system over time: initially, both in the stomach (floating in the upper part of the stomach or staying in the antrum), then, the effective gastroretention of the floating device for over 4 h (see **Figure 2**).

In the other study, aiming to demonstrate the gastroretentive efficacy of high‐density tablets, a similar process was used to radiolabel the produced tablets. Again, it was possible to see the *in vivo* positioning of the pharmaceutical dosage form and confirm that the high‐density controlled release strategy is effective for delivering drugs with a narrow upper GI absorption

These examples prove that incorporating gamma‐emitting radionuclides into a delivery device or formulation provides an appropriate, simple method of observing the transit and residence Both SPECT and PET imaging play an increasing role in the development of new targeted drug‐delivery systems [13].

Nuclear Medicine is being used to surpass the severe systemic toxicity of anticancer drugs, which are usually more effective in high doses. Additionally, numerous conventional thera‐ peutic agents repeatedly fail to reach their target, rendering them ineffective. The ideal drug should be specific for the cancer cell and devoid of systemic effects. For these reasons, the ideal drug‐delivery system is aimed, and Nuclear Medicine, with all its already‐known advantages, plays an important role in this area [9, 16].

PET image‐guided drug delivery allows for the treatment of a variety of diseases with minimal systemic involvement (sparing normal cells), while monitoring its efficacy.

In cancer‐targeted treatment, chemotherapeutic drugs can be loaded onto multifunctional drug carriers (such as liposomes, micelles, and nanoparticles) and coupled with several targeting ligands (such as monoclonal antibodies, peptides, and antibody fragments). Carriers are multifunctional and may also carry PET radiopharmaceuticals for diagnostic purposes. These systems are important examples of theranostics. One example is the streptavidin/biotin interaction that is used for binding numerous carriers to targeting proteins and antibodies.

In another approach, therapeutic radionuclides are conjugated with targeting ligands (by means of bifunctional‐linking strategies), without any image‐enabling radionuclide. PET imaging for the diagnosis or monitoring of therapeutic response is carried out separately, by conjugation of the targeting ligands with suitable PET radioisotopes [9].

Investigation‐wise, PET allows the *in vivo* quantification and drug distribution, determining its extraction fraction and washout from different organs/systems.

PET imaging allows the study of the first‐pass liver elimination of a drug, a fundamental knowledge, since first‐pass metabolism excludes oral delivery of several drug molecules and compounds.

PET studies are also useful in the understanding of the brain and of several neurological conditions, including anxiety and depression [13]. In Parkinson's and Alzheimer's diseases, the *in vivo* study of neuroreceptors binding is fundamental for the drug‐design process (ex. enzyme/prodrug‐based delivery approach with 18F for Parkinson's disease) [9, 13].
