Managed Care - February 2009 - (Page 47) TOMORROW’S MEDICINE Making the diagnosis is difficult and typically requires more than one imaging modality, but suffice it to say that nuclear imaging is one of the mainstays. More than three decades ago, researchers discovered that these tumors had a tendency to secrete hormones related to neurotransmitters. properties than other pharmaceutical products in that they expire in a very short time after manufacture because of the relatively short half-life of the radioisotopes used in human diagnostic procedures. Iobenguane If an analogue of NE were tagged with a radioactive isotope, the analogue would be taken up by the tumor. This allows the tumor to “light up” with the released radiation from the radioisotope. Iobenguane was the analogue chosen, as it is a rather inert compound similar in structure to the neurotransmitter NE. Iobenguane is subject to the same uptake and accumulation pathways as NE, accumulating in adrenergic nerve terminals, presynaptic storage vesicles, and all adrenergically innervated tissue. By labeling iobenguane with a radioactive isotope of iodine, the uptake of iobenguane allows scintigraphic images to be taken of the organs and tissues in which the radiopharmaceutical accumulates. Creating radiopharmaceuticals This process of labeling iobenguane with radioactive iodine has been a mainstay of imaging neuroblastomas for decades. But recently a development has occurred that has significantly improved the process by which this radiopharmaceutical is made available to physicians. To understand this, a little background is in order. Because of the radioactive nature of radiopharmaceuticals, nuclear medicine isotopes are highly regulated, not just because of the safety concerns for the patients and medical personnel who use these drugs, but also because of heightened security issues since 9/11. They can be created and used only in licensed facilities. Transporting these compounds is controlled by a variety of federal and state laws and regulations. These compounds also have very different Unstable Most people reading this column understand that some radioactive compounds are by their very nature unstable and highly dangerous because they emit high-energy particles that can damage human cells, can remain in the environment for years and sometimes millennia, and have been associated with both acute and chronic human disease. But radioactive agents vary considerably in both their energy release and their half-life. Some radioactive agents last a fraction of a second and others last for thousands of years. The holy grail of nuclear medicine has been to find radiopharmaceuticals that balance the biology of the agent with the physical decay of the radioisotope. An ideal radiopharmaceutical used for imaging would emit radiation for a very short time at the proper “strength” of energy, exhibit rapid and specific uptake into the desired organ, and demonstrate rapid elimination from the body to minimize the radiation exposure to the patient. It must also accurately and precisely depict the target organ. Using an agent with just the right amount of energy allows for a better picture of the target organ. Labeling iobenguane with iodine has been done for decades, but the most convenient isotope, I131, was an agent with a relatively long half-life (eight days) that released a relatively high energy level of 364 kiloelectron volts (keV). A better iodine isotope is I123, which has a shorter half-life of just 13.2 hours and has an energy release of just 159 keV. The lower energy released by I123 requires less lead in the nuclear camera and produces better images. The crux of the problem is that local radiopharmaceutical producers do not create their own radioactive isotopes and must rely on outside sources. FEBRUARY 2009 / MANAGED CARE 47
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