Treatment for Rare Cancer May Help Cushing’s Patients

The cancer medicine bexarotene may hold promise for treating Cushing’s disease, a study suggests.

The study, “Targeting the TR4 nuclear receptor with antagonist bexarotene can suppress the proopiomelanocortin signalling in AtT‐20 cells,” was published in the Journal of Cellular and Molecular Medicine.

Cushing’s disease is caused by a tumor on the pituitary gland, leading this gland to produce too much adrenocorticotropic hormone (ACTH). Excess ACTH causes the adrenal glands to release too much of the stress hormone cortisol; abnormally high cortisol levels are primarily responsible for the symptoms of Cushing’s.

Typically, first-line treatment is surgical removal of the pituitary tumor. But surgery, while effective in the majority of cases, does not help all. Additional treatment with medications or radiation therapy (radiotherapy) works for some, but not others, and these treatments often have substantial side effects.

“Thus, the development of new drugs for CD [Cushing’s disease] treatment is extremely urgent especially for patients who have low tolerance for surgery and radiotherapy,” the researchers wrote.

Recent research has shown that a protein called testicular receptor 4 (TR4) helps to drive ACTH production in pituitary cancers. Thus, blocking the activity of TR4 could be therapeutic in Cushing’s disease.

Researchers conducted computer simulations to screen for compounds that could block TR4. This revealed bexarotene as a potential inhibitor. Further biochemical tests confirmed that bexarotene could bind to, and block the activity of, TR4.

Bexarotene is a type of medication called a retinoid. It is approved to treat cutaneous T-cell lymphoma, a rare cancer that affects the skin, and available under the brand name Targretin.

When pituitary cancer cells in dishes were treated with bexarotene, the cells’ growth was impaired, and apoptosis (a type of programmed cell death) was triggered. Bexarotene treatment also reduced the secretion of ACTH from these cells.

In mice with ACTH-secreting pituitary tumors, bexarotene’s use significantly reduced tumor size, and lowered levels of ACTH and cortisol. Cushing’s-like symptoms also eased; for example, bexarotene treatment reduced the accumulation of fat around the abdomen in these mice.

Additional cellular experiments suggested that bexarotene specifically works on TR4 by changing the location of the protein. Normally, TR4 is present in the nucleus — the cellular compartment that houses DNA — where it helps to control the production of ACTH.

But with bexarotene treatment, TR4 tended to go outside of the nucleus, leading to lower ACTH production. The researchers noted that other mechanisms may also be involved in the observed effects of bexarotene.

“In summary, our work demonstrates that bexarotene is a potential inhibitor for TR4. Importantly, bexarotene may represent a new drug candidate to treat CD,” the researchers concluded.