Leukemia drug shows promise in treating breast cancer after it has spread

A repurposed leukemia drug could stop the most common type of breast cancer that causes tumors elsewhere in the body, according to a mouse study, although the risk of side effects may be too great to use in humans.

Frances Turrell of the Institute for Cancer Research in the UK and her colleagues induced estrogen receptor-positive (ER+) breast cancer in mice. It is the most common type of breast cancer in humans, accounting for up to 80 percent of cases, and usually occurs in people over 50 years of age.

ER+ breast cancer often recurs years after seemingly successful treatment. This is because the cells can travel to other areas of the body prior to treatment and may not be detected until therapy begins, Terrell says. “These cells essentially stay dormant and then something can wake them up.”

To find out more, the researchers gave young mice aged 8 to 10 weeks and older mice aged 9 to 18 months ER+ positive breast cancer. Mice were not treated for these tumors. Between two and five weeks later, almost all mice developed secondary cancer cells.

However, in young mice, these secondary site cells did not divide, while in older mice, the cells were more likely to develop into tumors, which mostly grew in the lungs of the animals.

The researchers then found that tumor growth in the mice was associated with the activation of a growth factor called PDGF-C in the lungs, which was less in younger mice.

In both humans and mice, lung levels of PDGF-C increase with age, Terrell says, which could trigger an environment that encourages secondary cancer cells to divide.

According to Claire Isake of the Cancer Research Institute, higher levels of PDGF-C may be associated with a weakened immune system.

In another part of the experiment, the researchers partially blocked tumor growth in aged mice by inhibiting PDGF-C with the drug imatinib, which is widely used in people with chronic myeloid leukemia.

Imatinib suppressed the growth rate of secondary tumors in mice, but some lesions did form, says Terrell.

While this may have potential for use in humans, there’s no way to predict who’s ER+ breast cancer might later recur elsewhere in their body, he says.

Imatinib has many side effects, such as stomach pain and fatigue, Isake said. “You don’t want to treat someone who won’t actually relapse.”

The researchers now plan to replicate this experiment in mice using a drug that has a more targeted effect on PDGF-C levels.

Suzanne Wardell of Duke University, North Carolina, says the study highlights the need to better understand the environmental factors in the body that cause secondary tumor growth. “Further work is needed to determine the general applicability of the findings to the treatment of human cancer,” she says.