Scientists at the National Institute of Standards and Technology (NIST) have developed the first US standard for measuring the radioactivity of actinium-225, a radioactive isotope that drug companies are using to develop a new class of anticancer drugs. 

The new standard, which is tied to the International System of Units (SI), has enabled NIST to open a calibration service for drug companies and research institutions studying the cancer-fighting potential of actinium-225.  

By comparing NIST’s measurement of a sample of actinium-225 to their own measurements, the companies can ensure that human volunteers injected with actinium-225 receive the exact amount of radioactivity required for it to be effective. 

“Health care providers don’t want to overdose patients. Then they risk doing more harm than good,” said NIST Chemist, Denis Bergeron. “But they also don’t want to underdose patients. In a way, that’s even worse because a patient is exposed to potentially harmful radiation without effectively treating their cancer. This is a case where you have to get it precisely right. That’s our job at NIST. For actinium-225, that means accurately measuring the injected radioactivity.” 

This latest calibration service could facilitate FDA review of anticancer drugs based on actinium-225, potentially speeding their deployment to cancer patients. 

Radioactive anticancer missiles

More than 15 clinical trials in the US have revealed that drugs based on actinium-225 show promise for fighting several cancers, including prostate cancer, neuroendocrine tumours and acute myeloid leukaemia. 

Actinium-225 is one of several radioisotopes — radioactive versions of stable elements — that dump a massive amount of energy, in the form of alpha particles, within an extremely short distance in the human body. 

Taking advantage of this short-range blast of energy, clinicians have devised drugs that act like anticancer missiles, binding actinium-225 or another alpha-emitting radioisotope to molecules that seek out and attach to cancer cells specifically.  

Once the radioactive source arrives at a tumour, alpha particles destroy the DNA of the cancer cells while leaving healthy cells unscathed. 

When it decays, actinium-225 successively transforms into a series of smaller atoms that are also unstable and emit their own alpha particles, along with gamma rays (a form of high-energy electromagnetic radiation) and beta particles (electrons). 

To create the new standard, Bergeron and his NIST colleagues relied on an established method of measuring radioactivity known as the triple-to-double coincidence ratio (TDCR). They placed a small amount of actinium-225 in a vial filled with a liquid that emits flashes of light when struck by radioactive particles. They then converted the flashes into electrical signals. 

This allowed the researchers to accurately measure the number of decays per second of actinium-225, a unit of measure known as the becquerel that is defined by using fundamental constants of nature.

Diana Spencer, Senior Digital Content Editor, DDW