Scientific experts agree with NTP about cobalt and cancer
By Robin Mackar and Kira Bradford
A panel of scientific experts agreed with the National Toxicology Program (NTP) draft recommendation to list a class of cobalt and cobalt compounds as reasonably anticipated to be human carcinogens. The class includes cobalt and cobalt compounds that release cobalt ions in vivo.
Once the draft is finalized and approved, the listing will become part of the congressionally mandated Report on Carcinogens (RoC), explained Ruth Lunn Dr.P.H., director of the Office of the RoC, during her opening remarks at the July 22 peer review panel meeting.
Cobalt is a naturally occurring metallic element that can be present in different forms. It is mixed with other metals to make alloys that can be used to make durable industrial products, such as military and industrial equipment. NTP representatives said cobalt is also used to make rechargeable batteries, pigments, and some medical devices.
Sources of exposures
People can be exposed to cobalt compounds through occupational or other sources. Individuals who work in hard metal production producing cobalt powder, working with diamond cutting wheels, or polishing diamonds are all at potentially high risk for exposure.
People may also be exposed to low levels of cobalt though food, cigarette smoke, and the environment.
Another major source of exposure is from hip implants. These medical devices often include metal components that contain cobalt. Patients with hip implants that failed due to excessive wear or corrosion by body fluids may have been exposed to higher levels of cobalt than the general public.
All panel members agreed that there was significant exposure to cobalt and cobalt compounds in the United States. Cobalt in the form of vitamin B12 is not subject to listing because it does not release cobalt ions in the body.
Human cancer studies
NTP walked the panel through the occupational health studies that were evaluated and included in the draft. Both NTP and the peer reviewers determined there was inadequate evidence in these studies to evaluate the relationship between human cancer and cobalt exposure.
Cancer and toxicity studies on hip replacements were not included in the RoC evaluation. “The complexity of the mixtures of chemicals and metals that may be used in some medical devices makes it difficult to home in on cobalt specific metals and how they may impact cancer,” said Lunn.
The panel suggested that NTP look at the available literature to be sure nothing was missed by excluding these studies. NTP Associate Director John Bucher, Ph.D., said the NTP would review the joint replacement patient literature.
Animal and mechanistic data support reasonably anticipated listing
The evidence to support this preliminary listing decision is based on sufficient evidence in animal studies, and on supporting data from studies of mechanisms, or biological changes, that contribute to the development of cobalt-related cancers.
After hearing the presentations and reading the materials, the 10-member panel, chaired by Melissa McDiarmid, M.D., from the University of Maryland, unanimously voted in favor of NTP listing these compounds as reasonably anticipated to be a human carcinogen.
NTP will consider the panel’s comments, as well as public comments received, as it makes revisions to the draft. The updated monograph will be presented to the NTP Board of Scientific Counselors at an upcoming meeting.
(Robin Mackar is news director in the NIEHS Office of Communications and Public Liaison, and a frequent contributor to the Environmental Factor. Kira Bradford, a Ph.D. candidate at the University of North Carolina at Chapel Hill, contributed to the story as part of the Immersion Program to Advance Career Training, or ImPACT, internship program.)