What happened

Alnylam Pharmaceuticals (NASDAQ:ALNY) is up more than 40% at 11:53 a.m. EDT after the company released positive phase 3 trial data for the Apollo trial testing its RNAi drug patisiran in patients with hereditary ATTR amyloidosis with polyneuropathy. Ionis Pharmaceuticals (NASDAQ:IONS) is down 9.6% because it has a competing drug, inotersen, for the same rare genetic disease.

Ironically, when Ionis released its phase 3 data in May, the same thing happened -- Alnylam went up and Ionis went down -- because Ionis' drug had some side effects and investors were anticipating Alnylam's success.

A medical professional with a hand on a patient's sholder.

Image source: Getty Images.

So what

It's impossible to know whether Alnylam or Ionis is the winner on efficacy because Alnylam didn't release much in the way of data. The biotech disclosed that patisiran met the primary endpoint of improving -- lower is better -- the modified neuropathy impairment score (mNIS+7) at 18 months. Alnylam said the mNIS+7 was "significantly lower" in patients taking patisiran compared to those taking a placebo, but didn't put numbers on the changes from baseline. The drug also improved the Norfolk Quality of Life Questionnaire-Diabetic Neuropathy measurement (Norfolk QOL-DN), although again, Alnlyam didn't put any numbers on the relative improvement compared to a placebo.

For the record, Ionis did the same thing, announcing that inotersen improved mNIS+7 and Norfolk QOL-DN scores relative to a placebo without giving any further information.

The reason investors believe Alnylam's patisiran is better than Ionis' inotersen has more to do with safety -- patisiran posted a relatively clean side effect profile. Serious adverse events were actually seen in fewer patients taking patisiran compared to placebo. The only major concerns were a higher level of peripheral edema -- accumulation of fluid causing swelling, usually in the lower limbs -- and an increase in infusion-related reactions in patients taking patisiran compared to those taking a placebo, but most of those were mild-to-moderate in severity.

Ionis' inotersen caused thrombocytopenia -- low blood platelet levels -- including one patient who died due to an intracranial hemorrhage. And the drug also caused some renal side effects. The side effects appear to be manageable by catching them early with laboratory tests, but, obviously, doctors would rather prescribe a drug without side effects.

That assumes the drugs have equivalent efficacy, which we don't know about at the moment. Also on the competition front, patisiran has to be infused while inotersen can be delivered via a weekly injection, making it more convenient.

Now what

Investors won't have to wait too much longer to see the full data set; Alnylam plans to present the data at the first European ATTR Amyloidosis meeting in November.

The head-to-head competition for patients will take a bit longer to hash out. Ionis plans to submit marketing applications in the U.S. and EU "this year," while Alnylam said it'll file in the U.S. in "late 2017" and in the EU "shortly thereafter." It looks like Ionis has a slight lead, but it probably isn't enough to matter much with both companies likely to gain approvals for their drugs next year.

Brian Orelli has no position in any of the stocks mentioned. The Motley Fool owns shares of and recommends Alnylam Pharmaceuticals and Ionis Pharmaceuticals. The Motley Fool has a disclosure policy.