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MECHANISMS OF LOCALIZATION

The design of radiopharmaceuticals is based solely upon physiological function of the target organ. The mechanism of localization of a radiopharmaceutical in a particular target organ depends upon processes as varied as antigen-antibody reactions, physical trapping of particles, receptor site binding, removal of intentionally damaged cells from circulation, and transport of a chemical species across a cell membrane and into the cell by a normally operative metabolic process. Radiochemistry plays a significant part in the development of these compounds and methods of performing quality control to insure radiochemical purity.


1. Active Transport: involves use of a normally operative metabolic pathway in the body for moving a radiopharmaceutical across a cell membrane and into the cell. Example: I- 131 NaI for thyroid imaging.Active Transport involves use of a normally operative energy-dependent metabolic pathway in the body to move a radiopharmaceutical across a cell membrane and into the cell. For example, thyroid uptake of radioiodide is by active transport. The first step involves trapping of the iodide; it then undergoes intermediate syntheses involving a thyroglobulin intermediate and is ultimately converted into T3 and T4 by the process of organification. Initial localization following IV injection is in the thyroid, stomach, parotids, and choroid plexus; ultimately, the iodide is stored in the thyroid as thyroxines with a tbiol of approximately 3 weeks or cleared through the kidneys.
Myocardial perfusion imaging is routinely performed with Tl-201 in the form of thallous ion (Tl1+ ). This involves utilization of the normally operative metabolic pathway for handling potassium since Tl1+ is a potassium analog and is therefore handled efficiently by the well-documented ATPase-driven Na/K pump mechanism. Initial localization of Tl1+ following IV injection is in the heart, liver, and muscle; ultimately it is recycled so very little is cleared through the kidneys.The whole body tbiol is approximately 10 days. This use of Tl1+ is also an excellent example of active transport.
Renal imaging with I-131 o-iodohippurate or Tc-99m MAG3 for tubular secretion studies is also an example of active transport. These compounds are processed predominantly by tubular secretory function. Approximately 80+% of both hippuran and MAG3 is removed from the blood stream by tubular secretion; the remainder is by GFR. Imaging is typically begun immediately post injection and acquisition is divided into frames, permitting generation of renogram curves.
Uptake by brain localizing radiopharmaceuticals such as Tc-99m HMPAO, Tc-99m ECD, I-123 IMP or I-123 HIPDM, probably also falls under the category of Active Transport. While the mechanism of cerebral uptake has not been completely elucidated, it appears to be related to "pH Shift"; that is, intracerebrocellular pH is lower than blood pH and these agents, which have the unique ability to penetrate an intact blood-brain barrier, are immobilized in brain cells due to this small change in pH of the compound. Their uptake may also be receptor-related. These agents basically take a "snapshot" of cerebral blood flow at the time of injection since brain uptake is very rapid and irreversible. Initial localization in the brain is in the range of 4-9%; often there is significant localization in the lungs, requiring shielding for performance of SPECT studies. Brain uptake remains essentially constant for the duration of the study.
Imaging tumors of neuroendocrine origin also probably falls under the category of active transport although metabolic incorporation is perhaps a better name for the mechanism. The I-123 or I-131 m-iodobenzylguanidine (mIBG) injected is so similar structurally to guanethidine, the precursor of epinephrine, that these tumors, which include pheochromocytomas, neuroblastomas, paragangliomas, carcinoid type tumors, and medullary hyperplasia, attempt to use it as a substrate for synthesis of hormones. Because of this attempt at chemistry by the tumors, this material accumulates within them. Since conversion of the mIBG to epinephrine doesn't take place, however, the accumulated tracer activity simply increases in the tumor as a function of time. By 24-48 hr, several % of the injected dose localizes in the tumors; a small amount accumulates in the liver; and parotids and normal adrenals are usually visualized. The remainder is excreted by the kidneys. Depending upon the radioisotope used, initial imaging is typically performed 24-48 hr post injection. In selected patients, imaging may be performed at 72 hr with the I-131 compound.

2. Phagocytosis: physical entrapment of colloidal particles by Kupffer cells in the RE System. Example: Tc-99m sulfur colloid for liver/spleen imaging.Phagocytosis involves the physical entrapment of colloidal particles by Kupffer cells in the reticulothelial system following an intravenous injection. Colloidal suspensions contain particles in the range of approximately 0.05 to 4 μm (see Figure 5) and may include things as diverse as Tc-SC and cigarette smoke in air. The most commonly used phagocytic agents, Tc-sulfur colloid and Tc-microaggregated albumin, typically have particle sizes ranging from approximately 0.1-2.0 μm. The smaller the particles, the greater the bone marrow uptake; larger particles tend to localize in the liver and spleen. Due to the small size of the colloid compared to the diameter of the average capillary, which is 7 μm, capillary blockade does not occur. Distribution in the RES is typically 85% in the liver, 10% in the spleen, and 5% in marrow. In severely diseased livers, the ratio may change significantly with increased uptake in the spleen. The tbiol of Tc-sulfur colloid in the liver is infinitely long; by comparison, tbiol of microaggregated albumin is 6-12 hr. The t1/2 of clearance from the blood for these agents is approximately 2.5 min, so in 10 min only approximately 6% remains in the blood stream. Imaging may therefore begin as early as 5-10 min post injection.

3. Capillary blockade: intentional microembolization of a capillary bed with particles. Example: Tc-99m MAA for pulmonary perfusion imaging.Capillary blockade involves the intentional microembolization of a capillary bed with particles, permitting external visualization of the perfusion of this capillary bed. This is achieved by the IV injection of a radiolabeled, precipitated, biodegradable macroaggregate of human serum albumin commonly known as Tc-99m MAA 34. Compared to the 7 μm diameter of the average capillary, at least 90% of the MAA particles are between 10-90 μm in size; none are >150 μm in their longest aspect.These are the legal particle size requirements as listed in the manufacturers' package inserts and in the current USP.
For an adult without known pulmonary hypertension, the ideal number of particles to be injected is 350,000 with a suggested range of 200,000-700,000. Even though these appear to be very large numbers, there is a very large margin of safety since fewer than 1/1,000 capillaries are blocked by the typical injection. For a patient with known pulmonary hypertension, the number of particles should be limited to 150,000. The tbiol of Tc-MAA is 5-12 hr, depending upon the manufacturer.

4. Cell Sequestration: Injection of damaged RBC's to produce a spleen scan with no visualization of the liver. Example: heat damaged autologous Tc-99m RBC's.Cell Sequestration involves radiolabeling and then heat damaging a small volume of the patient's red cells (usually 10 ml) to take advantage of the spleen's normal function, i.e., removal of damaged red cells. If the cells are radiolabeled properly, this procedure permits visualization of the spleen with minimal visualization of the liver. Rarely performed, it nevertheless is considered one of the classical mechanisms of localization of radiopharmaceuticals. The labeled damaged red cells have a moderately long tbiol, probably in the range of 10-20 hr.

5. Simple/exchange diffusion: a mechanism whereby a radiotracer diffuses across cell membranes and then binds/attaches to a cell component. Example: F-18 NaF for bone imaging.Simple diffusion describes a mechanism whereby a radiotracer diffuses across cell membranes and then redistributes itself elsewhere in the body. The perfect example is the ability of Xe-133 gas to diffuse across membranes in the lungs and to circulate in the blood stream. Exchange diffusion involves the diffusion of a radiotracer into a cell where a chemical exchange takes place. For example, one of the earliest bone imaging agents, the F-18 fluoride ion (F-), was capable of exchanging with the hydroxide ion (OH-) on the hydroxyapatite structure of bone tissue to form F-18 fluorapatite, a very stable molecule. This permitted external visualization by collecting the 511 keV annihilation photons produced during the decay by positron emission of this isotope.

6. Compartmental Localization: placement of a radiotracer in a fluid space and imaging of that fluid space. Example: Tc-99m HSA for MUGA's, In-111 DTPA for cisternograms, Xe-133 gas for pulmonary ventilation. Compartmental localization is defined as the placement of a radiopharmaceutical in a fluid space and maintaining it there long enough to image that fluid space. Since a fluid is defined as a liquid or a gas, the airways of the lungs qualify as a fluid space. The use of Xe-133 gas, Xe-127 gas, or Kr-81m gas as a ventilation agent is therefore a good example of this mechanism. Immediate distribution is to the lungs; since Xe-133 is lipophilic and can cross cell membranes, the gas passively diffuses into pulmonary capillaries and the activity is circulated through the blood stream, permitting cerebral blood flow studies. The tbiol of all these gases in the lungs is <0.5 min in most patients. Ultimately the activity is cleared from the body through the lungs.
Another example of compartmental localization is blood pool imaging using autologous Tc-99m labeled red cells or Tc-99m Human Serum Albumin within the blood pool. The immediate distribution is within the blood pool; ultimately the Tc-99m dissociates from these compounds and is cleared through the kidneys. The tbiol of Tc-99m HSA in the blood pool is approximately 1-2 hr; for Tc-99m RBC's, The tbiol is approximately 20 hr.
One can perform a cisternogram following injection of In-111 DTPA directly into the cerebrospinal fluid (CSF). This use of compartmental localization involves early and delayed views, permitting tracing of the kinetics of CSF. Immediate distribution is entirely within CSF; ultimately the activity bathes the brain and brain stem and indicates the presence of CSF leakage into the nasopharynx. The tbiol of In-DTPA is approximately 20 hr; radioactivity is eventually cleared through the kidneys.
Even an "artificial" localization such as the infusion of a dilute solution of Tc-99m pertechnetate or a suspension of Tc-99m sulfur colloid into the urinary bladder in a voiding cystogram qualifies as compartmental localization. The immediate localization is in the bladder; the activity is rapidly emptied via catheter with a tbiol measured in minutes. This study provides significant clinical information while conferring a minimal radiation dose to the patient due to the short retention time of the radiopharmaceutical in the bladder.

7. Chemisorption: surface binding of radiopharmaceutical to a solid structure, e.g., In- 111 platelets bound to surface of an active thrombus.Another important mechanism is known as physicochemical adsorption or chemisorption. The phosphate or phosphonate groups on currently used bone agents bind instantaneously, avidly, and essentially irreversibly to the hydroxyapatite structure of bone tissue. In addition, by the same mechanism, they localize in lesions metastatic to bone. Tc-99m MDP, Tc-99m HDP, and Tc-99m PYP all bind to bone tissue by this mechanism. Typically, 40-50% of the injected dose localizes in bone; the remainder is excreted through the kidneys. Since bone uptake is relatively slow, especially in adults, it is common practice to begin imaging 3 hr post injection.
A closely related example is the imaging of acute myocardial infarctions with Tc-99m PYP 3. When myocardial cells become necrotic following an acute myocardial infarction, there is an influx of calcium ions into the cells. The Ca2+ ions react with circulating phosphate ions to form Ca3(PO4)2 crystals, known as hydroxyapatite. Tc-99m pyrophosphate binds avidly and
irreversibly to these crystals at the periphery of the infarct where some perfusion is maintained (none localizes in the central region of the infarct). Images are routinely taken approximately 2 hr post injection. Optimal imaging time post-infarct is 1-3 days; after 6 days an infarct is considered "old" and the rate of false negative studies increases significantly.


8. Antigen/antibody reaction:
uptake at tumor site due to specific binding of radiolabeled antibody to surface antigens on tumors. Example: In-111 ProstaScint for localization of recurrent prostate cancer.

One of the newer mechanisms to consider is the antigen/antibody reaction. In this case, highly purified radiolabeled monoclonal antibodies with high specificity for a particular antigen are injected intravenously and imaged at a later point in time, often 1-3 days post injection. For example, In-111 ProstaScint, a monoclonal antibody specific for PSMA (Prostate Specific Membrane Antigen), has been very useful in evaluating recurrence of prostate cancer. A variety of these radiolabeled antibodies is in clinical trials for imaging a wide variety of diseases such as lung carcinoma, prostate and breast cancer. In addition, two commercially available radiopharmaceuticals, I-131 Bexxar and Y-90 Zevalin, are designed for therapy of non-Hodgkin's Lymphoma. They are both radiolabeled antibodies targeted to the CD-20 receptors on B-cell lymphoma.


9. Receptor-binding:
binding of radiopharmaceutical to high-affinity receptor sites. Example: In-111 octreotide for localization of neuroendocrine and other tumors based on binding of a somatostatin analog to receptor sites in tumors.

An increasingly important mechanism to consider is use of receptor binding radiopharmaceuticals designed specifically to trace receptor sites; for example, I-123 IQNB localizes at the site of muscarinic receptors, I-123 iomazenil localizes in benzodiazepine receptors, and I-123 mIBG permits visualization of adrenergic receptors in the myocardium. Critical features of drug development in this case include special attention to the specific activity and radiochemical purity of the starting material; otherwise, the specific activity of the final product may be unsuitable for localizing receptor sites due to saturation with cold, nonradioactive material.

 
 

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March 10, 2013