What happened

Shares of Ionis Pharmaceuticals (NASDAQ:IONS) jumped nearly 13% last month, according to data provided by S&P Global Market Intelligence. The move can be characterized as a recovery from the sudden and surprising news that Ionis and Akcea Therapeutics (NASDAQ:AKCA) announced at the end of August, in which the U.S. Food and Drug Administration issued a complete response letter rejecting the application for the jointly developed rare-disease drug Waylivra.

The recovery didn't last long. In the first 10 days of October, shares of Ionis Pharmaceuticals gave up all of their gains from September and then a little bit more, settling near $44 per share. And that's despite receiving Food and Drug Administration marketing approval for a new rare disease drug. Should investors be concerned?

Hands holding up stacks of coins and a green arrow showing growth.

Image source: Getty Images.

So what

While the development duo didn't provide details explaining what issues the FDA had with Waylivra, investors can guess it has to do with safety. That's because some patients taking the drug during clinical trials saw an unsafe drop in blood platelet levels -- which may not be easy or possible to remedy, although Ionis Pharmaceuticals is better positioned to withstand the blow than Akcea Therapeutics.

That had investors worried about the upcoming FDA decision for Tegsedi, a treatment for polyneuropathy of hereditary transthyretin-mediated amyloidosis (hATTR), which also came with a drop in blood platelet levels in some patients. However, the FDA approved the drug in early October, alleviating concerns.

Now what

Unfortunately for shareholders, removing the source of uncertainty for Tegsedi didn't help Ionis Pharmaceuticals stock escape the receding tide of the broader stock market. While the recent slide isn't influenced by factors within the company's control, the current $6 billion market cap may still represent a hefty premium for the business at the moment.