Keywords

3.1 Radiopharmaceuticals Overview

Nuclear medicine is noninvasive imaging at the cellular and molecular levels based on pathological processes. Radiopharmaceuticals (radiotracers) are given to the patient for imaging purposes. Radiopharmaceuticals are chemical compounds or biological moieties that contain radioactive element within their structures for either diagnosis or treatment. They are considered radioactive drugs. Trace amounts is administered to the patient, and the mass is extremely small. This amount is not enough to produce a pharmacologic effect and chemical toxicity is not as great of a concern as with standard pharmaceuticals.

All radionuclides used in radiopharmaceutical preparations are artificially produced. In general, the production involves a stable nuclide (target) that is bombarded with high-energy particles (neutrons or positively charged particles) to yield the radioactive nuclide of interest. The nuclear medicine radionuclides are usually obtained from a reactor, cyclotron, or generator [1]. The nuclear reactor utilizes the fast neutrons emitted from fission reaction of enriched Uranium-235 (235U) [1, 2]. Enriched 235U in uranium fuel rods are placed into a tank of heavy water (D2O), which is used as a moderator for controlling fission released neutron energy. The fast neutrons are slowed down to thermal energy by their interaction with D2O. The thermal neutrons are easily captured by other Uranium atoms to initiate additional fission reactions and this chain reaction maintains the flux of neutrons. The rate of nuclear fission is controlled by the position of control rods. The control rods are made of Cadmium or Boron that have a high cross-section for absorbing neutrons. The production of neutron activated radionuclide is achieved by introducing the target material (stable nuclides) through ports into the neutron flux in the D2O tank. To insure the safety of the surroundings, the whole reactor is shielded with concrete [1, 2].

As for cyclotrons, they are a type of particle accelerator invented by Ernest O. Lawrence in 1932 in which charged particles accelerate outward from the center along a spiral path. The particles (protons) are held in a spiral path by a static magnetic field and accelerated by a rapidly varying electric field. Stable, nonradioactive isotopes are placed inside the cyclotron, then the accelerated charged particles (protons) bombard the stable isotopes creating radioactive isotopes for nuclear medicine and other purposes [1, 3].

On the other hand, generators are more convenient method of obtaining medical radionuclides with short half-lives [1, 4]. They consist of a longer lived parent radionuclide that is loaded onto a column that decays to the daughter radionuclide. The parent and daughter radionuclides are chemically different that the daughter is removed from the column by elution with a solvent while the parent stays absorbed to the column. Upon elution of the daughter radionuclide, the parent radionuclide decays to build up more daughter radionuclide until the parent activity is depleted [1, 4]. The column is shielded in lead. To date are useful medical radionuclide generators which are: 99Mo/99mTc, 68Ge/68Ga, 82Sr/82Rb,90Sr/90Y and 188W/188Re.

Radionuclide can be introduced into a drug in different approaches. Isotopic labeling involves the substitution of a stable nuclide with its radioactive isotope creating a radioactive analogue [5]. The radioactive analogue has similar chemical and biological properties to the stable parent compound. Therefore, the radioactive analogue will be a physiological tracer. However, the nonisotopic labeling approach involves incorporating a radioactive nuclide that was not previously present in the parent compound [5]. The majority of the radiopharmaceutical in clinical practice today are the nonisotopic labeled compounds. The nonisotopic labeled radiopharmaceuticals split into two different categories: essential and tagged radionuclide. The essential is also referred to as integrated radionuclide. In this labeling method, the radionuclide is an important component in the overall structure of the drug, without it the drug will not have the same biodistribution. The chelation of the radionuclide to the ligand results in the desired complex. The majority of the earlier radiopharmaceuticals were the integrated ones. More recently, the work has been toward production of target-specific radiopharmaceuticals to minimize the unnecessary radiation exposure to the body during imaging or radiotherapy. The most convenient approach is to incorporate radiometals to receptor-specific molecules such peptides, antibodies, and antigens. The radionuclide is attached to the biological molecule via a bifunctional chelate (BFC), which can hold the radiometal tightly and at the same time form a stable conjugation with the active groups of the biological molecule. In this method, the radionuclide is being tagged along till the biological molecule reaches its target.

Radiopharmaceuticals have been used in diagnostic and radiotherapeutic agents. There are two types of diagnostic radiopharmaceutical. Single photon emission computed tomography (SPECT) radiopharmaceuticals that contain gamma emitting radioisotopes. Positron emission tomography (PET) radiopharmaceuticals that contain positron emitting radioisotope. While therapeutic radiopharmaceuticals contain auger electrons, β− or α particles that are known to be highly ionizing as they are non-penetrating radiation, so they deliver cytotoxic doses to diseased sites, resulting in the death of the cells.

3.2 Mechanisms of Radiopharmaceuticals Localization

The uptake and retention of radiopharmaceuticals by different tissues and organs involve many different mechanisms, as summarized in Table 3.1. The pharmacokinetics, biodistribution, and metabolism of the radiopharmaceutical are very important to understanding the mechanisms of radiopharmaceutical localization in the organ or tissue of interest. Injury to a cell or tissue significantly alters the morphology and molecular biology compared with that of normal tissue or organs.

Table 3.1 Mechanisms of radiopharmaceuticals’ localization
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3.2.1 Compartmentalized Localization

Compartmentalized localization is when molecules of interest are spread in an enclosed volume or space.

  • Uniform distribution within a compartment.

    The model example of uniform dispersion within a compartment is the blood pool. The quantitative determination of blood volume can be done using the tracer dilution method. Radioiodinated human serum albumin with I-125 (I-125 HSA) is used to determine plasma volume because it is a radiopharmaceutical that diffuses in the plasma [6, 7]. Red blood cells (RBC) radiolabeled with Cr-51 is a radiopharmaceutical that diffuses within the cellular content of blood, so it is used to determine red cell volume/mass [7, 8]. Tc-99m labeled RBCs are dispersed in the blood and used in gated blood pool imaging of left ventricular wall motion and determination of left ventricular ejection fraction [9].

  • Nonuniformities within a compartment.

    In certain incidents, radiopharmaceuticals may have nonuniform distribution within the compartment due to pathological condition. For example, a localized area of increased 99mTc-RBCs activity can be caused by increased blood volume in a hemangioma [10, 11].

    In other situations, areas of decreased radiopharmaceutical concentration are usually the result of an obstruction in a compartmental space. In a complete obstruction in the lung airways demonstrated by Xe-133 ventilation, then there will be absence of Xe-133 in the area beyond the site of airway obstruction [12, 13]. If partial obstruction (common in COPD), then there will be absence of Xe-133 in the affected area upon initial inhalation and breath-hold, but Xe-133 gas will pass through the site(s) of partial obstruction over time during equilibrium rebreathing [14].

    As for In-111-pentetate (DTPA), it can diffuse freely in the extracellular fluid and can accumulate in lesions with defects in Blood Brain Barrier (BBB). Obstructions can also occur in the CSF space. Both 99mTc-DTPA andIn-111-DTPAcan be used for the assessment of BBB disruption as it localizes in areas within the cranium that had been disrupted by infection, neoplasms, trauma, or stroke [15, 16]. 99mTc-DTPA is a nondiffusible tracer for evaluation of BBB permeability, similar to 99mTc-pertechnetate and 99mTc glucoheptonate. 99mTc-DTPA brain scintigraphy has been used in the past to detect brain infarcts as well as brain metastases [15, 16].

    An obstruction in the cystic duct of the biliary tract will be visualized due to decrease of radiopharmaceutical in the gallbladder, and if the common bile duct is obstructed, there will be decrease of radiopharmaceutical in the small intestine. Tc-99m hepatobiliary radiopharmaceuticals, disofenin (DISIDA), and mebrofenin (BRIDA), are excreted from the liver into the bile and flow through the biliary tract with normal flow into the gallbladder and into the intestine [17].

  • Leakage from the compartment.

    In some pathological conditions there could be an abnormal leakage from the compartment, and radiopharmaceuticals can detect and identify the location of this leakage. For example, gastrointestinal hemorrhage (GI bleeding), blood leaks from the vasculature and accumulates in the GI tract and Tc-99m RBCs can be used to detect the site of the GI bleeding [18, 19].

  • Movement/flow within a compartment.

    In some pathological conditions, there may be a change in the direction, rate and extent of flow within a compartment. Tc-99m sulfur colloid is the preferred radiopharmaceutical for determining the rate of emptying of gastric contents into the intestine because it is not absorbed by the GI tract. Tc-99m sulfur colloid bound in scrambled eggs can be used to evaluate gastric empting of food solids while Tc-99m sulfur colloid mixed in water or other liquid such as juice that can be used to evaluate gastric emptying of liquids [20, 21]. Individual patient gastric emptying is compared to normal values.

3.2.2 Passive Diffusion

Passive diffusion is the movement of molecules from high to low concentration to achieve even concentration (chemical equilibrium). In biological systems, passive diffusion usually involves the ability of molecules to cross the phospholipid membrane so these molecules should be highly lipid soluble and be in a neutral state not charged under physiological pH conditions. Also, their molecular size should be small enough to pass through the small pores of the membrane. The molecule movement across the membrane is simply a molecular motion and does not require additional energy, transporters, carriers, or receptors. The passive diffusion is nonselective, noncompetitive, and not subjected to saturation.

A classic example of passive diffusion in nuclear medicine is 99mTc-DTPA brain imaging. 99mTc-DTPA cannot normally penetrate the blood–brain barrier (BBB) that is made of endothelial cells of the cerebral vessels form a continuous layer without gap junctions preventing the diffusion of hydrophilic (water-soluble) molecules. So normally, 99mTc-DTPA remains in the blood pool until cleared by the kidneys. In conditions that result in disruption of the BBB, such as tumor, stroke, and infection, the 99mTc-DTPA can diffuse across the disrupted BBB and accumulate in that affected area of the brain [15, 16].

Intact BBB allows the transport of small molecules across the plasma membrane of the neuron by facilitated diffusion. However, diffusion is not a unidirectional process, and there is a need for accumulation and retention of radiopharmaceuticals at the site of interest in order to take a meaningful image. The localization of the cerebral perfusion radiopharmaceuticals 99mTc-exametazime (HMPAO) and 99mTc-bicisate (ECD) which are lipophilic radiotracers, involve the delivery via cerebral arterial blood flow, diffusion into the brain and retention in the brain due to conversion to a more stable hydrophilic molecule and enzymatic metabolism, respectively [22, 23] (Fig. 3.1).

Fig. 3.1
figure 1

99mTc-ECD is a neutral, lipophilic complex that can cross the BBB via passive diffusion. It is retained in the brain after undergoing enzymatic hydrolysis of one of the ester functions to become an ionized metabolite by esterase enzyme

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99mTc-myocardial perfusion agents involve the delivery by blood flow through the coronary arteries, diffusion into myocardial cells and retention in those cells [24, 25]. Both 99mTc-sestamibi and 99mTc-tetrofosmin cross the cell membranes by lipophilic diffusion and then are retained by electrostatic binding to negative electrical charges on the mitochondrial membranes in normal cells when Ca2+ are significantly low [26]. In cases of irreversible ischemia when extracellular levels of Ca2+ enters the cell and binds to the mitochondria, Tc-myocardial perfusion agents are blocked from binding to the mitochondria.

[13N]NH3, as a nonionic form, is freely permeable to all cell membranes. It diffuses across the myocardial cell capillary membrane, then is converted to N-13 glutamine by glutamine synthetase, and subsequently is trapped within tissues by incorporation in the cellular pool of amino acids [27, 28]. Myocardial uptake is proportional to coronary blood flow. The linear relationship between distribution of [13N]NH3 and the regional blood perfusion allows for the imaging and measurement of cerebral and myocardial blood flows [27, 28].

67Ga-citrate is known to localize in tumors and inflammatory lesions. It is speculated based on in vivo studies, that free and unbound 67Ga-citrate diffuse into the cells and once within the cells, 67Ga binds to iron-binding proteins such as lactoferrin and ferritin preventing back-diffusion of free 67Ga [29, 30].

[18F]Fluorodopa ([18F]FDOPA) is neutral and capable of crossing the BBB. Up on crossing the BBB, [18F]FDOPA is decarboxylated by cellular aromatic amino acid decarboxylase (AAAD) to form fluorodopamine (FDA) which remains within the neuron. [18F]FDOPA selectively localize within the basal ganglia of the brain in the area that controls movement. [18F]FDOPA targets the presynaptic dopaminergic function in the brain. In degenerative diseases such as Parkinson’s disease, there is loss of dopaminergic neurons, so there will be less accumulation of [18F]FDOPA in the basal ganglia than a healthy, age-matched control. [18F]FDOPA has other applications, it can accumulate in vivo within tumors and in evaluation of pheochromocytoma and thyroid carcinoma due to the increase utilization of amino acid by the cancerous lesions [31,32,33].

18F-Fluoromisonidazole ([18F]FMISO) diffuses freely into all cells. However, it accumulates in viable hypoxic cells, diffuses out of normoxic cells and is not retained in necrotic cells. Upon entering a viable cell, 18F-MISO is in an environment where electron transport is taking place, the NO2 substituent (which has a high electron affinity) takes on an electron to form the radical anion reduction product. If O2 is also present, that electron is rapidly transferred to oxygen and 18F-MISO changes back to its original structure and can leave the cell [34, 35]. However, in the absence of O2 in hypoxic cell, a second electron reacts with the nitroimidazole (radical anion reduction product) to form a 2-electron reduction product then the reduced [18F]FMISO reacts nondiscriminately with peptides and RNA within the cell and gets trapped. Therefore, the retention of [18F]FMISO is inversely related to the intracellular partial pressure of O2 [34, 35].

3.2.3 Facilitated Diffusion

Facilitated diffusion requires a carrier to transport a molecule across the membrane. Carriers are selective and only specific molecules fit into them. Therefore, there is competition with similar molecules that fit into this carrier and due to limited number of carriers, it is possible to reach saturation. However, facilitated diffusion utilizes carriers that are passive, so it does not require external energy but needs a concentration gradient to operate. This mechanism allows the transport of molecules in either direction through the membrane based on the concentration of gradient. The most commonly used PET radiotracer, F-18 flurodeoxyglucose ([18F]FDG), is transferred into the cell through facilitated diffusion mechanism. [18F]FDG, is a radiolabeled analogue of d-glucose (Fig. 3.2), so it enters the cell via transmembrane protein transporters [GLUT] similar to glucose. Cellular uptake of [18F]FDG reflects the glucose metabolism so glucose and [18F]FDG are competing for the same GLUT transporters, and elevated blood levels of glucose will decrease cellular uptake of [18F]FDG. Once inside the cell, both glucose and [18F]FDG are phosphorylated by hexokinase resulting in Glucose-6-phosphate and [18F]FDG-6-phosphate, respectively (Fig. 3.3). Glucose-6-phosphate enters the glycolytic pathway but [18F]FDG-6-phosphate is blocked and is retained in the cell as it does not fit in the GLUT to diffuse out, and this is referred to as metabolic trapping. [18F]FDG-6-phosphate may be converted back to [18F]FDG, however the enzyme responsible for such a conversion is present in very low concentration or not present all in cancer tissue allowing for better images of oncological patients. [18F]FDG accumulated in granulomatous tissue and macrophages so it has been used to image infection and inflammation as well [36,37,38,39].

Fig. 3.2
figure 2

[18F]FDG is an analogue of d-glucose. The hydroxyl group at the second position in d-glucose is replaced by F-18 in [18F]FDG

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Fig. 3.3
figure 3

Both of glucose and [18F]FDG are transported into the cell via Glut transporters. Upon entering the cell, both undergo phosphorylation at the 6 position by hexokinase. Phosphorylated-glucose will undergo additional enzymatic steps, however, phosphorylated-[18F]FDG at the 6 position does not and becomes trapped inside the cell.

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On the other hand, the hepatocytes in the liver extract substances from the blood and secrete them into the bile. 99mTc labeled tracers like disofenin and mebrofenin diffuse through pores in the endothelial lining of the sinusoids, bind to the anionic membrane-bound carriers on the hepatocyte and secreted into the bile similar to bilirubin [40].

At alkaline pH (pH 8–9), 99mTc forms a pentavalent complex with DMSA (99mTc (V) DMSA). This complex mimics phosphate ion and is rapidly excreted in the urine. It localizes in a number of tumors such as medullary thyroid carcinoma (MTC), bone metastases and other bone lesions. Its uptake is dependent on extracellular Na+ concentration, indicating the importance of sodium-dependent transporter in 99mTc (V) DMSA uptake [15].

3.2.4 Active Transport

Active transport utilizes carriers to transport molecules across membranes but unlike facilitated diffusion it requires energy such as ATP. By using energy to transport molecules across the membrane, molecules can be transported against a concentration gradient. Since a carrier is used, it is selective and only specific molecules that fit the carrier will be transported across the membrane. Therefore, there is competition with similar molecules that fit into this carrier and due to limited number of carriers, it is possible to reach saturation.

One of the first active transport radiotracers in Nuclear Medicine is the concentration of iodide in the thyroid gland. Radioisotopes of iodine such as 123I− and 131I− are useful radiopharmaceuticals to evaluate thyroid function. Iodide ions are transported into thyroid cells via the Na+/I− symporter. In the thyroid, iodide is oxidized to iodine and becomes bound to tyrosine which is transformed to the thyroid hormones [41,42,43,44]. In addition, Tc-99m-pertechnetate (99mTcO4−) accumulates in the thyroid similar to iodide since it has a negative charge and similar ionic size [44]. Presence of iodide in the blood from iodine containing medications or iodine contrast agents will compete with these radiopharmaceuticals for thyroid uptake resulting in poor image quality.

Another well-known example of active transport is the Na+/K+ pump, especially of importance in the heart muscle. Thallous chloride (201Tl+) has been used for myocardial perfusion scans. 201Tl+ is a radiometal so it is not an analogue of K+ but it has a similar ionic radius, a single positive charge and fits in the Na+/K+ pump [45,46,47]. Uptake in heart muscle demonstrates viability. The delivery to the myocardial cells is by blood flow through the coronary arteries so the heart muscle uptake reflects coronary perfusion. It has been also used for tumors such as brain tumors, osteosarcomas low-grade lymphomas, Kaposi sarcomas, and parathyroid tumors [45, 46]. This accumulation is a function of blood flow and active transport system of Na+/K+ pump within cell membrane. The uptake can be inhibited by blocking the Na+/K+ pump with ouabain, digitalis and furosemide [48, 49]. A PET myocardial perfusion radiotracer is rubidium chloride (82Rb+). 82Rb+ is a chemical analog of potassium as it falls immediately below it on the periodic table. 82Rb+ fits in the Na+/K+ pump and its uptake is similar to 201Tl+ [48].

I-123-metaiodobenzylguanidine (123I-MIBG) is an analog of noradrenaline. It fits in the prenorepinephrine transporter (adrenergic presynaptic neurons) in adrenergic nerve terminals. Norepinephrine transporter is a transmembrane carrier that transports monoamine neurotransmitters into neurons where they are accumulated in storage vesicles. These transporters are over expressed on certain neoplasms such as neuroblastoma, pheochromocytoma, medullary thyroid carcinoma, retinoblastoma, melanoma and bronchial carcinoma [49, 50].

At acidic pH (pH 2–3), 99mTc forms a trivalent complex with DMSA (99mTc (III) DMSA). 99mTc-DMSA accumulates in proximal tubular cells of kidneys and thereby used for renal cortical imaging. 99mTc-DMSA is filtered as bound to α1-microglobulin and accumulates in the kidneys by megalin/cubilin-mediated endocytosis of the 99mTc-DMSA protein complex. Renal accumulation of 99mTc-DMSA is dependent on megalin/cubilin receptor function and therefore is a marker of proximal tubule endocytic activity [51,52,53].

Fluciclovine, anti-1-amino-3-18F-fluorocyclobutane-1-carboxylic acid ([18F]FACBC). [18F]FACBC is for men with suspected prostate cancer recurrence based on their elevated prostate specific antigen (PSA) levels. [18F]FACBC takes advantage of the increased amino acid transport in prostate cancer cells, and it is taken up by the l-amino acid transporter and alanine-serine-cysteine transporter systems. These transporters are unregulated in prostate cancer and are associated with more aggressive disease. Once inside the cell, [18F]FACBC does not undergo metabolism and the amino acid transporters mediate influx and efflux of amino acids, so [18F]FACBC washout occurs over time [54,55,56,57]. Therefore, early imaging is recommended to maximize lesion uptake.

18F-Fluorothymidine ([18F]FLT) is utilized to measure cellular proliferation as the concentration of [18F]FLT in cells is proportional to cellular proliferation. It is transported from the blood into cells by active transport. Once in the cell, [18F]FLT is a substrate for thymidine kinase I (TK1) and is phosphorylated but not incorporated into DNA (Fig. 3.4). Phosphorylated FLT cannot exit the cell. One advantage of [18F]FLT is that it is only a substrate for TK1 and not for mitochondrial TK2 and so it is a more specific tracer compared with other fluorinated tracers for cellular proliferation [58, 59].

Fig. 3.4
figure 4

Upon entering the cell via an active transport mechanism, [18F]FLT undergoes phosphorylation by TK1. Phosphorylated-[18F]FLT does not exit the cells and its concentration in cells is proportional to cellular proliferation

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18F-Fluoro-Ethyl-Tyrosine ([18F]FET), is an amino acid analog and it reflects the increased amino acid transport of tumor cells. It is a neuro-oncologic PET radiotracer. It is actively taken up in tumor cells via amino acid transport system L. It is neither incorporated into proteins nor readily degraded, resulting in high intracellular concentrations [60]. Radiolabelled amino acid-based agents are useful in PET brain tumor imaging because [18F]FDG is somewhat insensitive for detecting tumors and lesions in the brain due the high levels of glycolytic metabolism in the normal cortex and white matter [61].

[18F]Choline and [11C]Choline target the cellular membrane phospholipids. They enter the cell through choline transporters with accumulation in tumors due to malignancy-induced overexpression of choline kinase (CK) that catalyzes the phosphorylation of choline to form phosphorylcholine followed by generation of phosphatidylcholine in the tumor cell membrane [62,63,64,65]. They have been approved for recurrent prostate cancer detection.

3.2.5 Filtration

Filtration is a passing of molecules through pores or channels due to hydrostatic or osmotic pressure gradient. The molecular size vs. pore size and availability are the most important factors in filtration. In addition, filtration requires a force or pressure gradient, it does not require external energy or carriers. Filtration is not competitive so it is not subjected to saturation. Glomerular filtration by the kidney is the prime example of filtration to estimate the function of the renal tissue. Only small hydrophilic molecules (i.e., molecular weight of <5000) can pass through the glomerular pores, and only the ones that are free in plasma (not bound to proteins) are available to be filtered, and these molecules should not be able to be secreted or reabsorbed by tubule. Blood pressure is the pressure gradient for glomerular filtration. Even though many radiopharmaceuticals are excreted by glomerular filtration, 99mTc-DTPA, 99mTc-MAG3, 125I-iothalamate, and 51Cr-EDTA are the radiopharmaceuticals that are mainly used for glomerular function renal imaging studies [66,67,68,69].

3.2.6 Secretion

Secretion is active transport of substances out of glands and other tissues. For example, the secretion of hydrochloric acid (HCl) by the stomach, secretion of H+, K+, NH3, urea, creatinine or histamine by the kidney tubular cells into the urine, and secretion of bilirubin by the liver into the bile. In Meckel’s Diverticulum, a patch of ectopic stomach tissue is usually found in the intestine, so it may secrete hydrochloric acid (HCl) that erodes the intestinal wall, which leads to bleeding. Tc-99m-pertechnetate (TcO4−) is negatively charged and of similar size as chloride (Cl−), so it is secreted as pertechnic acid (H+TcO4−) by both normal stomach tissue and Meckel’s Diverticula [70, 71].

As for the kidneys, the tubular cells secrete some waste products into the urinary collecting system. 99mTc-mertiatide (99mTc-MAG3) is cleared from the blood by this mechanism, resulting in much higher urinary concentrations and better contrast compared to radiopharmaceuticals eliminated by glomerular filtration [66,67,68,69].

3.2.7 Phagocytosis

Phagocytosis is a Greek word for cell eating. It is a process of the cell engulfing a particle and internalizing it. Reticuloendothelial system (RES) cells, such as Kupffer cells in the liver and reticular cells in the spleen, capture and engulf colloidal particles such as Tc-99m sulfur colloid (99mTc-SC) of particle size range between 0.1–1.0 μm [72, 73]. Focal areas that does not have Kupffer cells, such a tumor, cyst, abscess, or hemangioma, will not have an uptake of 99mTc-sulfur colloid (cold region) [74].

Also, colloidal particles smaller than 0.1 μm show rapid clearance from the interstitial space into lymphatic vessels and significant retention in lymph nodes when injected into the interstitial fluid [75]. Cancerous nodes replaced by tumor tissues will not sequester the colloids so no uptake of radioactivity will be visualized (cold region). 99mTc-antimony sulfide colloid (0.002–0.015 μm), 99mTc-human serum albumin (0.01–0.02 μm) or 99mTc-nanocolloid are ideal for lymphoscintigraphic studies [76]. However, these radiotracers are not available in the USA, so passing 99mTc-SC through 0.2 μm filter is being used for lymphoscintigraphic [76].

3.2.8 Cell Sequestration

Cell sequestration is the process of removal of old or damaged red blood cells from circulation which is mainly associated with the spleen. RBC are labeled with Tc-99m using the commercially available kit, then they are carefully denatured by heating at 49–50 °C for 15 min. Heating RBCs changes their shape from tough biconcave disks to spherocytes with knobby projections and a fragile cell membrane. When they squeeze through the 3 μm pores in the cords of the red pulp they get lysed, releasing their radioactive contents within the spleen. Splenic removal of RBCs is a more selective process than removal by the liver and other RES tissue. This imaging procedure is especially useful for localizing and/or identifying ectopic accessory spleens [77, 78].

3.2.9 Capillary Blockade

Capillary blockade is the physical trapping of particles in capillaries and pre-capillary arterioles (microembolization). The diameter of capillaries and pre-capillary arterioles are about 10 microns and 20–30 microns, respectively, so the radiotracer particles should be a little bigger in size between 15 and 50 microns. The delivery to the capillary is through the blood flow so when the radiolabelled particles are injected intravenously, the lungs are the first capillary encountered. Tc-99m radiolabeled macroaggregated albumin (Tc-99m-MAA) have been used for perfusion lung imaging. The localization in each of the lungs is an indicator for the relative blood flow to each of the two lungs. Therefore, 99mTc-MAA perfusion lung imaging can also be used to assess blood flow through the pulmonary arteries [79, 80]. If 99mTc-MAA is injected through a catheter positioned in the hepatic artery, then the hepatic blood flow will deliver it to the capillaries in the liver to evaluate blood flow within the liver [81].

3.2.10 Ion Exchange

Ion exchange is the exchange of ionic chemical analogs. Current radiopharmaceuticals that localize by this mechanism are strontium chloride (89Sr2+), a beta-emitter used to treat painful bone metastases, and sodium fluoride (18F−), a PET agent used for bone scans [82, 83]. In the hydroxyapaptite of the bone matrix,89Sr+2 replaces Ca+2 while 18F− replaces OH−. Fluoride ions diffuse from the blood compartment and exchange with hydroxyl groups in hydroxyapatite crystal to form fluoroapatite. Uptake of fluoride ion into bone may be due to primary and metastatic tumors as well as bone turnover (metabolism) [82, 83].

3.2.11 Chemisorption

Chemisorption is the binding of phosphate-type compounds onto the surface of the bone. The strength of this binding is intermediate between chemical covalent bonding and hydrogen bonding (adsorption). Radiolabeled diphosponates compounds, 99mTc-MDP and 99mTc-HDP are used for bone imaging while 153Sm-EDTMP is used for the treatment of painful bone metastases. Localization is on the surface, therefore, uptake is proportional to the surface area [84, 85]. The larger the surface area of increased bone metabolism, the higher the uptake in that areas such as fracture, infection, and tumor. In addition to chemisorption on the surface of bone, there can also be chemisorption onto calcium phosphate crystals that precipitate in certain soft tissues as a consequence of severe hyperparathyroidism, hypercalcemia, and myocardial infarction.

3.2.12 Cellular Migration

Cellular migration is the directed movement of cells due to stimuli such as the chemotaxis of white blood cells to the site of infection and inflammation. Radiolabeled autologous leukocytes with 111In-oxine or 99mTc-exametazime (99mTc-HMPAO) can be used to localize sites of infection similar to the circulating leukocytes due to the attraction of released chemotactic factors [86, 87].

3.2.13 Receptor Binding

Receptor binding is similar to the “lock-and-key” concept. It is the binding of a biological molecule to a specific receptor such as the binding of an antibody or antibody fragment to an antigen and peptides, hormones or neurotransmitter binding to their receptors. Receptor and antigen bindings are very selective and specific; therefore, competition from similar molecules for these receptors and antigens binding is a concern with the possibility saturation. Many tumor cells express antigens or receptors that are expressed in small amounts in normal cells. But tumor cells have higher expression of these antigens and receptors. The localization of radiolabeled antibodies, proteins, hormones, and peptides depend on the blood clearance, tumor blood flow, tumor mass and tumor cell viability. The radionuclide of use should match the pharmacokinetics of the biological molecule. For example, antibodies should be labeled with long half-life radionuclides such as 111In and 131I, while peptides can be labeled with shorter lived radionuclides such as 99mTc, 18F, or 123I for imaging studies. Iodine-131 is the most used radionuclide for both diagnostic and therapeutic studies. Radioiodide can be labeled on tyrosine residue in the antibody or peptide. However, the other radionuclides are indirectly labeled in which they are coordinated to a chelate such as DTPA or DOTA that is covalently attached to the biological molecule (bifunctional chelating approach).

Antibodies (Ab) are also known as immunoglobulins (Ig) which are a group of glycoprotein molecules produced by B-lymphocytes in response to antigenic stimulation. Ab binds to a specific site of the antigen in which an antigen can have multiple binding sites for different Ab. Although much research has been conducted with radiolabeled antibodies, few are currently marketed. In-111 capromab pendetide (ProstaScint), a monoclonal murine IgG antibody directed to prostate specific membrane antigen (PSMA), is used for staging and follow-up of prostate cancer [88,89,90]. PSMA is a membrane protein that is expressed in prostate tissue and overexpressed on prostate carcinoma. PSMA has a unique structure consisting of three sections: internal cellular, transmembrane and extracellular portions. ProstaScint was later found to bind to the intracellular epitope of PSMA. Therefore, it was discontinued by the manufacturing company in 2018 after the FDA approved smaller urea-based molecules targeting the extracellular epitope of PSMA. 68Ga-DOTA-PSMA and [18F]PSMA, are the urea-based, low molecular weight inhibitors of PSMA that have replaced ProstaScint [88,89,90].

131I-tositumomab and 111In/90Y-ibritumomab tiuxetan, are monoclonal murine IgG that bind to CD20 receptors on B-cells and non-Hodgkin’s lymphoma tumor cells. These labeled antibodies are used for diagnostic, monitoring, and treatment of non-Hodgkin’s lymphoma [91, 92].

Many neuroendocrine tumors have an over expression of somatostatin receptors (SSTR) and there are 5 SSTR subtypes. The two naturally occurring SST peptides, 14 and 28 amino acids long, which are known to have short biological half-life due to enzymatic degradation. A number of biologically stable SST analogues were synthesized. Octreotide analogue is an 8 amino acid long analogue that has a high affinity to 2 and 5 SSTR subtypes. 111In-pentetreotide (Octreoscan), a radiolabeled form of octreotide, is used to detect, localize and evaluate such somatostatin-expressing tumors by binding to these receptors [93]. When octreotide is labeled with 90Y or 177Lu, then it is used for therapeutic purposes. Recently 68Ga-DOTA-octreotide has been approved for PET studies of neuroendocrine tumors [94, 95]. In addition, 64Cu-DOTA-tyr3-octreotate has been approved as a PET diagnostic agent for neuroendocrine tumors but Cu-64 emits β− so it can be used for therapeutic purposes as well [96].

[18F]Florbetapir (Amyvid, Eli Lilly/Avid Radiopharmaceuticals), [18F]florbetaben (Neuraceq, Piramal Imaging) and [18F]flutemetamol (GE Healthcare. VizamylTM) are radiopharmaceuticals used for patients who are being evaluated for Alzheimer’s disease (AD) and other causes of cognitive impairment in the cortical regions and hippocampus. After injection, the tracers diffuse through the blood-brain barrier and binds with high affinity and specificity to β-amyloid neuritic plaques (Aβ aggregates) in the brain of adult patients with cognitive impairment. They rapidly enter the brain and quickly washes out from the brain if not bound to β-amyloid neuritic plaques [97, 98]. These radiotracers share a common imaging target and similar imaging characteristics (Aβ tracers). They can differ in their tracer kinetics, specific binding ratios and optimal imaging parameters therefore, they will have different recommended injected doses, time to initiate imaging post-injection, and scan duration [97, 98].

[18F]Fluoroestradiol ([18F]FES) is an analog of estrogen and is used to detect estrogen receptor-positive breast cancer lesions. It has a high overall sensitivity and specificity in assessing the ER status in breast cancers. [18F]FES uptake has been approved to guide in therapy selection and to predict endocrine treatment response [99].

[123I]ioflupane (DaTscan) is a chemical derivative of cocaine. It binds to presynaptic dopamine transporters, which are primarily located in the striatum. Loss of dopamine transporter density, as occurs in Parkinson’s disease, results in reduced uptake of the radiopharmaceutical. The radiotracer localizes to the dopamine transporters in the basal ganglia [100].

3.3 Altered Biodistributions Due to Radiochemical Impurities from Improper Radiopharmaceutical Preparations

Improper preparation can lead to the presence of radiochemical impurities in the final radiopharmaceutical dose. Radiochemical impurities have different pharmacokinetics and biodistributions from the radiopharmaceutical product of interest, and hence these impurities can reduce the image quality while exposing the patients to unnecessary radioactive dose. Therefore it is very important to recognize common radiochemical impurities and their localizations. Table 3.2 lists the main possible radiochemical impurities along with localization site [101].

Table 3.2 Radiopharmaceuticals’ predominant radiochemical preparation impurity
Full size table