What happened

Shares of Athenex (NASDAQ:ATNX), a clinical-stage biopharmaceutical company, slid 21.2% in the first hour of trading on Friday. The stock recovered some of that loss as investors digested pivotal trial results that the company released this morning, but it was still down 17.2% as of 12:57 p.m. EST on Friday.

So what 

Trouble on Friday the 13th started early this morning for Athenex shareholders. The company's lead candidate, an orally available version of a frequently used chemotherapy called oral paclitaxel, may have hit a roadblock. 

Oral paclitaxel provides important benefits over the intravenous version, called IV paclitaxel. Unfortunately, there were some troubling safety signals in the data Athenex released this morning.

Frustrated trader looking at stock charts.

Image source: Getty Images.

During a pivotal study with advanced-stage breast cancer patients, oral paclitaxel shrank tumors for 35.8% of patients, compared with just 23.4% of patients treated with IV paclitaxel. As expected, there was far less risk of nerve damage in the oral paclitaxel group, but there were higher rates of neutropenia, infection, and gastrointestinal issues. The stock was beaten down because Athenex wasn't specific about how much higher the rates of those adverse events were in terms of severity or frequency.

Now what

Although the response-rate differences weren't exactly amazing, less nerve damage tilts the risk-to-benefit ratio heavily in oral paclitaxel's favor. But if it significantly increases patients' risk of white blood cell loss and opportunistic infections, the stock will tank.

Investors won't have to wait long to learn the fate of oral paclitaxel, and perhaps Athenex's entire orally available chemotherapy pipeline. The company will add some color to the announcement at 4:15 p.m. EST today in an oral presentation at the 2019 San Antonio Breast Cancer Symposium.