A hormone used in the contraceptive Depo-Provera may hamper the spread of breast cancer cells.

However, that hasn't been shown in people. Human breast cancer cells have been studied but only in labs and in mice, not patients. The results appear in the Journal of the National Cancer Institute's May 4 issue.

The report "provides tantalizing clues for a new prevention strategy based on work in laboratory mice," a journal editorial notes. However, "the trick is to get the right agent at the right place at the right time," which may be difficult, it continues.

The study was conducted by researchers including Diane Palmieri, a research fellow at the National Cancer Institute. The editorial came from V. Craig Jordan, PhD, DSc, of Philadelphia's Fox Chase Cancer Center. Both emphasize the need for further testing.

Hormone Used in Depo-Provera

The hormone, called medroxyprogesterone acetate (MPA), is a progestin. It's used in Depo-Provera, an injectable birth control method for women.

The FDA approved Depo-Provera as a contraceptive in 1992. Last November, the FDA ordered a "black box" warning to be added to Depo-Provera stating that prolonged use of the drug may result in the loss of bone density.

The warning did not question Depo-Provera's effectiveness as a contraceptive, focusing only on possible bone density loss with long-term use.

The warning says that "bone loss is greater with increasing duration of use and may not be completely reversible" and that Depo-Provera "should be used as a long-term birth control method (e.g., longer than two years) only if other birth control methods are inadequate."

Depo-Provera is made by Pfizer, a WebMD sponsor.

Hormone Halts Breast Cancer Spread

Palmieri's study tested MPA against human cancer cells in lab tests and in mice. The researchers say the results show that MPA appeared to boost production of a gene (Nm23-H1) that suppresses cancer's ability to spread (metastasis).

A total of 180 mice were injected with human breast cancer cells. Previous studies had shown that the human breast cancer cells could be used in mouse experiments.

Four weeks later, the mice were assigned to receive MPA or not. Several MPA doses were tested that resulted in levels achievable in humans.

The MPA groups got the drug for four weeks in biweekly or daily injections. For the next eight weeks, they got one injection every four weeks as maintenance.

Less Breast Cancer With MPA

Treatment with MPA resulted in reduced cancer growth and formation after injection with human breast cancer cells.

Cancer spread developed in all of the mice not treated with MPA. In one experiment, cancer spread in 73 percent of the mice that got 2 milligrams of MPA; in another test, the percentage was 64 percent with the same MPA dose.

On average the numbers of cancers that spread per mouse were lower in the MPA-treated group.

Nm23-H1 was produced at high levels in more of the MPA-treated mice (43 percent of cancers that spread to the lung compared with only 13 percent of cancer spread in untreated mice), according to the study.

Side Effects

"The reported human side effects of MPA include weight gain and decreased bone density," write Palmieri and colleagues.

Mice receiving at least 1 milligram doses of MPA gained more weight than those not given MPA. However, no bone density changes were seen in the MPA-treated mice, say researchers.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Palmieri, D. Journal of the National Cancer Institute, May 4, 2005; vol 97: pp 632-642. Jordan, V. Journal of the National Cancer Institute, May 4, 2005; vol 97: pp 619-621. News release, Journal of the National Cancer Institute. WebMD Medical Reference provided in collaboration with The Cleveland Clinic: "Your Guide to Birth Control: Depo-Provera." News release, FDA. WebMD Medical News: "New Warning for Depo-Provera Users." Pfizer.