In the studies, two-thirds of the men took one of the androgen receptor inhibitors, while a third took a placebo. They all continued to receive androgen deprivation therapy.

In the study of men receiving apalutamide, it took, on average, 40.5 months for cancer to spread to the point where it could be detected by conventional scans. For men receiving the placebo, the cancer spread in 16.2 months, on average. In the enzalutamide study, metastasis took 36.6 months on average in men receiving that drug compared to 14.7 months with placebo.

“Delaying median time to metastases by over two years is a big deal,” said Dr. Scott Eggener, a urologic oncologist and professor of surgery at University of Chicago, who was not involved in the studies. He said the studies were also important scientifically because they show that “maximally decreasing testosterone production and its ability to bind or enter cancer cells leads to meaningful clinical improvement for these men.”

Still, he said, while the studies both show preliminary indications that the drugs might extend patients’ survival, researchers will have to follow the patients longer to know.

Both studies were funded by the companies that make the drugs. Janssen Pharmaceutical Companies of Johnson & Johnson, the maker of apalutamide, has applied for approval from the Food and Drug Administration, which has put it under priority review, Dr. Smith said.

The developers of enzalutamide, Pfizer and Astellas Pharma, have applied to the F.D.A. for approval to expand the use of the drug, marketed as Xtandi, to patients in this category, said Dr. Maha Hussain, deputy director of the Robert H. Lurie Comprehensive Cancer Center at Northwestern University’s Feinberg School of Medicine. She co-led that study with Dr. Cora Sternberg, chief of medical oncology at San Camillo and Forlanini Hospitals in Rome.

Both drugs appear to be safe with relatively few serious side effects, experts said. Negative effects for some patients included fatigue, hypertension, rashes, fractures, falls, nausea, and mild cognitive and memory slippage.