What: Alnylam Pharmaceuticals (NASDAQ:ALNY) is up 11% at 3:15 p.m. EDT after Alnylam's competitor Ionis Pharmaceuticals (NASDAQ:IONS) disclosed that its partner GlaxoSmithKline (NYSE:GSK) won't start a phase 3 clinical trial for IONIS-TTRRx in patients with transthyretin (TTR)-related amyloid cardiomyopathy.

So what: Alnylam Pharmaceuticals also has a drug, revusiran, for TTR-amyloid cardiomyopathy. The drug is currently in phase 3 development that should finish enrolling in late 2016, putting a potential readout in mid-2018. GlaxoSmithKline plans to wait for data from a phase 3 trial Ionis Pharmaceuticals is running in a related disease called TTR-familial amyloid polyneuropathy that's scheduled to read out in the first half of next year, so Alnylam will have a substantial head start.

But it gets even better for Alnylam Pharmaceuticals. One of the reasons GlaxoSmithKline wants to wait is that Ionis Pharmaceuticals has observed thrombocytopenia, or low platelet levels, in patients with TTR-familial amyloid polyneuropathy treated with IONIS-TTRRx. Alnylam also has a drug, patisiran, to treat the polyneuropathy version of the disease.

We'll get phase 3 data from trials in the polyneuropathy version for both IONIS-TTRRx and patisiran next year. Assuming both drugs pass their pivotal trials, efficacy will be the main driver for sales, but if they produce similar results in patients, not having the risk of thrombocytopenia will give Alnylam an advantage. Low platelet levels is a manageable condition, but it's an added step for doctors to have to monitor, which they'd rather avoid if there's an equally good treatment that doesn't require it.

Now what: IONIS-TTRRx's side effect and related delay are clearly good news for Alnylam in terms of potential competition once revusiran and patisiran make it to market. However, the drugs still have to pass their phase 3 trials to get approved by regulators. Investors will get more information about the likelihood of success when Alnylam presents data from the phase 2 open-label extension trials for both drugs in July at the International Symposium on Amyloidosis.

Brian Orelli has no position in any stocks mentioned. The Motley Fool owns shares of and recommends Alnylam Pharmaceuticals and Ionis Pharmaceuticals. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.