By Ludwig Burger

FRANKFURT (Reuters) – Bayer is banking on the mild side effects of its experimental prostate cancer drug darolutamide, as it prepares to take on established rival products by Pfizer and Johnson & Johnson.

The drug, tested on early stage prostate cancer that does not respond to hormonal therapy, was shown to be safe to use with only fatigue as the most serious side effect, Bayer said on Thursday, citing a phase-three study that could be decisive for regulatory approval.

Given the cancer is not yet spreading and patients are still relatively unburdened by the disease, a drug’s tolerability is a key concern for the generally elderly men taking it, said Robert LaCaze, Bayer’s head of oncology.

“Patients may be on this drug for two to three, maybe four years … it really does fit this patient population, we feel, much better than maybe some of the alternatives available today,” he said.

Some initial results of the phase three trial were released earlier this week and sparked disappointment. Shares in Finland’s Orion, which sold certain rights to the drug in a collaboration deal with Bayer in 2014, fell as a result.

The details showed darolutamide held off metastases for nearly two years when compared to a placebo but analysts doubted this would give darolutamide an edge over approved drugs Xtandi by Astellas and Pfizer as well as J&J’s Erleada.

Bayer is in discussions with regulators about a request for marketing approval and estimates annual peak sales of at least 1 billion euros ($1.13 billion).

Analysts at UBS and Jefferies said the side effect profile was favorable but questioned whether that would suffice to set the drug apart from its established rivals because efficacy data was roughly on par.

Full details on the darolutamide study are being presented at the American Society of Clinical Oncology Genitourinary Cancers Symposium on Thursday.

An ongoing trial comparing Xtandi directly with darolutamide, with patients first taking one and then the other, may provide more clarity in about three years.

Among the 160,000 new cases of prostate cancer diagnosed per year in the United States roughly 15,000 fall under the category that was tested in the trial, LaCaze said.

Darolutamide is also being tested on advanced prostate cancer with metastases, a potentially bigger market.

Bayer is under pressure to strengthen its drug development pipeline as its pharmaceuticals division faces falling sales from 2024 amid likely competition for its two bestselling drugs.

Shares in the German company, which acquired seed maker Monsanto, have fallen amid U.S. lawsuits claiming its Roundup weed-killer played a role in the plaintiffs’ cancer.

(Editing by Alexandra Hudson)