The medication tamoxifen, best known as a treatment for breast cancer, dramatically reduces symptoms of the manic phase of bipolar disorder more quickly than many standard medications for the mental illness, a new study shows. Researchers at the National Institutes of Health’s National Institute of Mental Health (NIMH) who conducted the study also explained how: Tamoxifen blocks an enzyme called protein kinase C (PKC) that regulates activities in brain cells. The enzyme is thought to be over-active during the manic phase of bipolar disorder.

By pointing to PKC as a target for new medications, the study raises the possibility of developing faster-acting treatments for the manic phase of the illness. Current medications for the manic phase generally take more than a week to begin working, and not everyone responds to them.

Tamoxifen itself might not become a treatment of choice, though, because it also blocks estrogen – the property that makes it useful as a treatment for breast cancer – and because it may cause endometrial cancer if taken over long periods of time. Currently, tamoxifen is approved by the Food and Drug Administration for treatment of some kinds of cancer and infertility, for example. It was used experimentally in this study because it both blocks PKC and is able to enter the brain.

Results of the study were published online in the September issue of Bipolar Disorders by Husseini K. Manji, MD, Carlos A. Zarate Jr., MD, and colleagues.