LONDON (Reuters) - Scientists have developed a potential new type of targeted therapy for prostate cancer, which is activated by a protein made by the cancerous cells themselves.

In early laboratory and animal tests, the therapy called PRX302 killed the cancerous prostate cells without harming any healthy cells or tissue. Researchers at Johns Hopkins University in Baltimore, Maryland who devised the new treatment told a medical conference in Prague it produced significant results.

"This represents a different kind of targeted therapy in that it seeks to use a protein made by the cancer to destroy itself," Professor Sam Denmeade, an associate professor of oncology at the university, said in a statement.

Nearly 680,000 men are diagnosed each year with prostate cancer, which causes 220,000 deaths. It occurs most often in older men.

When Denmeade and his team tested the molecule in monkeys, it destroyed a quarter to a half of the diseased tissue depending on the dose. Two weeks after the drug had been given to the animals, the researchers found evidence that the treatment was still working.

They developed the protoxin by modifying an inactivate molecule called proaerolysin and designed it to be activated by a protein called prostate-specific antigen (PSA), which is made in large amounts by prostate cancer.

"Our observations suggest that injections into the prostate of this engineered, PSA-activated protoxin might have potential in treating men with locally recurrent or advanced prostate cancer," Denmeade said.

He added that the treatment might also help men with an enlarged prostate gland, or benign prostatic hyperplasia (BPH), which causes urinary problems. By the age of 80, about 80 percent of men will suffer from BPH.

The treatment is given by injection into the prostate, but the researchers who reported their findings at the Molecular Target and Cancer Therapeutics Symposium, said they hope to develop it into an easier-to-administer form.

The treatment is now being tested in a phase 1 clinical trial for men whose cancer has returned after radiation therapy.