Shares of Ionis Pharmaceuticals (NASDAQ:IONS) ended the day down 10.6% after an earnings report from partner Biogen (NASDAQ:BIIB) started the day on a sour note. Then shares dropped even further midday after data was released for IONIS-HTTRx, a treatment for Huntington's disease. Biogen managed to end the day in the green, up 1.1%.

IONS Price Chart

IONS Price data by YCharts.

So what

Biogen reported $364 million in sales of its spinal muscular atrophy drug Spinraza, which was a whopping 647% higher than the year-ago quarter, when the drug first launched, but the sales were essentially flat compared to the $363 million in the fourth quarter of last year.

The good news is that the number of patients on Spinraza continues to increase, up 16% in the U.S. and 56% elsewhere compared to the fourth quarter of 2017. Sales didn't track with new patient starts because patients who started in previous quarters are getting fewer injections due to the dosing schedule. As that bolus of patients works through the system, sales should start to increase again.

Foot examination of an infant

Image source: Getty Images.

On the IONIS-HTTRx front, Ionis reported data from a phase 1/2 study at the American Academy of Neurology meeting. We already knew the drug was able to knock down mutant huntingtin protein levels, but the new data from a post-hoc analysis shows that reductions in the protein level improves the total motor score and symbol digit modalities score, two measures doctors use to rate the severity of Huntington's disease. Unfortunately two other scores -- the Stroop test and the total functional capacity -- didn't improve, although the total functional capacity is trending in the right direction.

The functional tests are what IONIS-HTTRx will eventually be judged on, but investors should remember that this trial wasn't really set up to measure clinical efficacy, so it's hard to draw a firm conclusion about the lack of a signal on the Stroop test.

Now what

While Spinraza royalties and the potential eventual regulatory approval of IONIS-HTTRx are important for Ionis, the company has an extensive pipeline of drugs, including two under FDA review right now, so investors should evaluate today's news events -- neither of which seem earth-shattering -- in the context of Ionis' overall potential.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.