Researchers race to create ultra short-lived medical isotopes
Scientists from the University of Alabama Birmingham (UAB), the University of Wisconsin, and Argonne National Laboratory are working on a new process to produce a pair of radioisotopes of the element scandium (Sc) for the diagnosis and treatment of cancer. The tricky bit is that the isotopes are so unstable that one must be used within four hours of creation, so the process must be extremely fast and efficient.
The use of radioactive isotopes in medicine isn't new. The practice dates back to the discovery of radium and since the atomic age began scientists have greatly advanced and refined the technique. However, it's also a field that presents its own peculiar challenges.
One particularly promising line of research is the use of radioactive isotopes of scandium – specifically, 43Sc and 47Sc. These isotopes emit high levels of radiation in a very short time and doctors are interested in using them in what is called a “theranostic” pair. That is, two radioactive isotopes where one is used for diagnosing an illness and the other is used to treat it at the same time. Hence the neologism for therapeutic and diagnostic.
In the case of scandium, 43Sc emits positrons that produce gamma rays when they hit other atoms, which can be detected and measured by PET scanners. Meanwhile, 47Sc emits beta rays that destroy solid cancer tumors. By attaching these isotopes to a targeting peptide that seeks out and bonds with the tumor, they could be a powerful team in the diagnosis and treatment of cancer.
The problem is that neither of these isotopes occur in nature. In fact, there are no radioactive isotopes of scandium outside of laboratories precisely because they are such powerful radio emitters. This is because emission strength is a function of half-life or how long it takes for half the atoms of a sample of a radioactive element to split and change into another element.
For example, uranium 235, which is used in nuclear reactors and weapons, has a half-life of 703.8 million years, which means that it's so low a radio emitter that you could use it as a paperweight. Iodine 131, on the other hand, has a half-life of only eight days, so you don't want to be anywhere near it.
The half-life of 47Sc is only a little over three days, so it has to be used within that time before most of it is gone. It's even worse for 43Sc. It has a half-life of only 3.9 hours, in which time it must be produced in quantity, refined, prepared for use, and then given to the patient before its effectiveness drops off radically.
Working under a grant from the US Department of Energy, the team led by Suzanne Lapi, director of the UAB Cyclotron Facility, professor in the UAB Department of Radiology, is working on ways to speed up the production of scandium isotopes. Using the UAB 24 MeV cyclotron, they found that by firing protons at targets made of titanium oxide, the metal atoms would transmute into scandium isotopes. These could then be separated out from the oxide by dissolving the targets in acid and ammonium bifluoride, then pouring the solution through an ion exchange column.
So far so good, but natural titanium is a mixture of five stable isotopes that react differently to proton bombardment, so the experiments couldn't produce pure 43Sc and 47Sc. Therefore, the next step will be to make the targets using pure samples of a single titanium isotope. Meanwhile, the University of Wisconsin will concentrate on irradiating calcium oxide targets with deuteron particles and Argonne will irradiate titanium targets with gamma rays.