Image source: Ionis Pharmaceuticals.

What: Ionis Pharmaceuticals (NASDAQ:IONS) was down as much as 13% today, although it's pulled back a bit in midday trading after announcing that the Food and Drug Administration put a clinical hold on a clinical trial, dubbed CARDIO-TTR, being run by its partner, GlaxoSmithKline (NYSE:GSK).

So what: Clinical holds are usually really bad news because the FDA usually issues them when it's worried about the safety of a drug.

In this case, though, the phase 3 clinical trial testing IONIS-TTRRX in patients with transthyretin (TTR) amyloid cardiomyopathy hadn't started yet, so the clinical hold appears to just be precautionary while the FDA waits for GlaxoSmithKline to answer its questions.

Ionis is currently running another phase 3 trial, NEURO-TTR, testing the drug in patients with a related disease, TTR familial amyloid polyneuropathy, which affects the nervous system rather than the heart. The FDA hasn't put that trial on a hold, so the issue appears to be disease specific -- patients with the cardiomyopathy version are sicker than those with the polyneuropathy version.

Now what: Worst-case scenario, the FDA won't let Ionis and GlaxoSmithKline test IONIS-TTRRX for the cardiomyopathy version. Best-case scenario, the FDA just wants more monitoring of patients, and GlaxoSmithKline only needs to adjust the clinical trial protocol to convince the FDA that patients will be well monitored in case issues arise.

It may be a few months before investors know whether IONIS-TTRRX is alive or dead as a treatment for TTR amyloid cardiomyopathy. Unfortunately, the delay allows the duo's competitor, Alnylam Pharmaceuticals, to get further ahead with its already-started phase 3 clinical trial testing revusiran in TTR amyloid cardiomyopathy -- that one is scheduled to read out in early 2019.

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