TRENTON, N.J. — U.S. regulators have approved a new drug that harnesses the immune system to treat bladder cancer that has spread after chemotherapy or surgery.

The Food and Drug Administration on Monday approved Imfinzi for advanced bladder cancer, along with a companion diagnostic test for identifying which patients are most likely to benefit from it.

Imfinzi, also known as durvalumab, is part of a new generation of drugs that stimulate the immune system to help fight cancer.

British drugmaker AstraZeneca PLC, which developed the drug, also is testing it for several other cancers.

The average monthly list price for Imfinzi is roughly $15,000, but it varies with the patient’s weight, according to AstraZeneca. It’s given by IV, usually every two weeks.

Imfinizi works by binding to a protein, found in varying levels on tumor cells, that blocks immune system cells from attacking the tumor.

In a company-funded study, 26 percent of patients with high levels of the protein responded to Imfinzi; their tumors shrank or stopped growing, some for 20 months or more, according to the FDA. Only 4 percent of patients with low levels of the protein responded to the drug. The diagnostic test, made by Ventana Medical Systems, distinguishes between those groups.

The FDA gave the drug accelerated approval and is requiring AstraZeneca to complete ongoing testing in more patients to confirm the drug’s benefits and safety.

Common side effects include fatigue, bone and muscle pain, nausea, swelling in hands and feet, and urinary tract infections. Infections elsewhere in the body were infrequent. However, two patients died, one from a lung infection and the other from a liver infection.

Bladder cancer is the sixth most common type of cancer in the U.S., with about 79,000 new cases and nearly 17,000 deaths expected this year, according to the National Cancer Institute. When bladder cancer has spread, only about 5 percent of patients survive beyond five years, government figures show.

Until recently, treatment options were limited to chemotherapy, radiation or surgery.