Seattle Genetics Inc.'s urothelial cancer drug alone reduced tumors in a mid-stage study in almost half of patients, all of whom had seen their disease return after receiving chemotherapy and immunotherapy.

The phase 2 single-arm study, also funded by Tokyo-based Astellas Pharma Inc., showed that 44% of the total 125 participants experienced shrinkage of their tumors, or no growth, while 12% of those treated had no signs of cancer after treatment. The median time participants survived was 11.7 months, the companies said June 3.

Patients had locally advanced or metastatic urothelial cancer and had received a median of three previous treatments. The study participants were 70% male, and the median age was 69.

Seattle Genetics' drug candidate, called enfortumab vedotin, or EV, targets a protein called Nectin-4, which is found in 97% of urothelial cancers. The drug is an antibody-drug conjugate, a type of drug that specifically targets cancer cells.

Among patients who had not responded to a checkpoint inhibitor, a cancer therapy that targets the immune system, 41% demonstrated a response to EV, while 38% of patients whose disease had spread to their liver responded to treatment with EV.

"These phase 2 results replicate the phase 1 results very closely," said lead study author Daniel Petrylak, a professor of medicine and urology at Yale Cancer Center.

According to Bothell, Wash.-based Seattle Genetics, EV was well-tolerated among patients. Common side effects included fatigue, hair loss and decreased appetite.

Urothelial cancers, which are more common in men, include cancer of the bladder, urethra, ureters, renal pelvis and other adjacent organs. About 75% to 80% of urothelial cancer patients see their disease return after treatment with an immune checkpoint inhibitor, the only treatment option available besides chemotherapy.

Approximately 80,470 people will be diagnosed with bladder cancer in 2019 and an estimated 17,670 people will die from the disease, according to the American Cancer Society.

Seattle Genetics said a phase 3 study to confirm the results is ongoing. Another trial will investigate EV in combination with Merck & Co. Inc.'s checkpoint inhibitor Keytruda, and EV with a platinum-based chemotherapy, in newly diagnosed advanced urothelial cancer patients.

EV received breakthrough-therapy designation from the U.S. Food and Drug Administration in March 2018.

The 2019 American Society of Clinical Oncology meeting is expected to bring together more than 32,000 professionals from around the world, with more than 2,400 study abstracts to be presented on site and an additional 3,200 abstracts to be published online.