A new, promising treatment for advanced prostate cancer is being tested, and it hails from a very unlikely source: the liver of a human fetus.

Early testing has shown how the natural fetal estrogen esterol, also known as E4, lowers testosterone levels. Testosterone stimulates prostate cancer tumor growth, which is why androgen deprivation therapy is one of the first lines of attack against prostate cancer.

"E4, a steroid produced by the human fetal liver during pregnancy only, is a potential candidate for the treatment of advanced prostate cancer, both as a single entity and for combination treatment with hormone therapy," said Ellen Dutman, MSc, clinical research associate at Pantarhei Oncology BV in Zeist, the Netherlands.

To determine whether oral E4 lowers testosterone levels, Dutman and her team conducted a double-blind, randomized, placebo-controlled study in 45 healthy male volunteers between the ages of 40 and 70.

The test subjects were separated into 3 groups of 15 men. Ten men in each group received the E4, and 5 received a placebo, for 28 days. The 10 men in the first group received one daily dose of 20 milligrams of E4, and once that dose was found to be safe, the 10 men in the second group received 40 milligrams of the fetal estrogen. When that dose was proven unproblematic, a third group of 10 men received a dose of 60 milligrams of the drug. At all three levels of dosage, both total and free testosterone decreased.

"E4 for the treatment of prostate cancer would offer a new and affordable option compared to current standard and new therapies. An important advantage of E4 is expected to be the avoidance of the hypoestrogenic side effects that occur with other types of testosterone-suppressing hormone therapy, including hot flushes and sweating, arthralgia, mood, sleep and cognition disturbances, and bone loss and fractures," Dutman said. "Furthermore, E4 treatment may not be as expensive as recently developed new prostate cancer therapies.”

"We expect that in the future, patients with advanced prostate cancer will have the opportunity to choose to be treated with E4, especially in combination with their current therapy," Dutman said. "The addition of E4 will further improve the efficacy of their current therapy and have a positive impact on the quality of life of the patients."

The research was presented at ENDO 2017, the annual scientific meeting of the Endocrine Society.