Ionis Pharmaceuticals touted a clinical win for its investigational drug, inotersen, which just wrapped up Phase 3 trials. But safety concerns make it unclear if the drug will ever reach the market.

Inotersen is meant to treat a rare and crippling nerve condition called familial amyloid polneuropathy (FAP), which has no FDA-approved treatments in the U.S. Ionis tested its drug in 172 patients for 15 months and saw “statistically significant” improvement on two tests that measure their symptoms compared to placebo.

But the company also discovered some patient reactions to the drug that altered how the company monitors its patients. Three patients in the trial had dangerously low platelet levels while on the drug. Two patients recovered, but the other died from a brain hemorrhage. A fourth patient (whose platelet count dropped while on the drug) stopped treatment.

Although the efficacy data was promising, the safety concerns might give regulators pause.

The FDA once put a clinical hold on inotersen, causing Ionis’s partner on the drug, GlaxoSmithKline (GSK), to edge away from its collaboration on the program. GSK still has an option to snag marketing rights on the drug.

On an investor call, Ionis senior vice president of drug discovery Brett Monia said the safety concerns can be closely watched, are “manageable” and that reactions can be reversed if caught early enough.

Monia said the data suggests the drug may deliver more benefit than risk. The FDA might agree, given the lack of available alternatives for the drug.

On the call, CEO Stanley Crooke said Ionis “assumes” GSK will exercise its option to acquire marketing rights for the drug, but “will be delighted” to commercialize inotersen on its own if GSK isn’t interested.