What: Shares of Isis Pharmaceuticals (NASDAQ:IONS), a developer of drugs using its proprietary antisense drug development platform, gained 11.4% in Tuesday's trading session following news yesterday that it had reached a development milestone for ISIS-GSK4-Lrx, and after announcing that it would be providing a phase 3 development update next week.

So what: According to yesterday's early morning press release, Isis Pharmaceuticals earned a $5 million development milestone from collaborative partner GlaxoSmithKline (NYSE:GSK) for the initiation of phase 1 studies of ISIS-GSK4-Lrx, which is designed to treat an undisclosed ocular disease. Phase 1 patients will be healthy volunteers, and determining the experimental therapy's safety will be the No. 1 priority of the study.

Additionally, Isis Pharmaceuticals today announced that it will be reviewing ISIS-TTRrx data (a therapy also partnered with GlaxoSmithKline) at the first annual European Congress of Hereditary ATTR Amyloidosis on Tuesday, Nov. 3,and that it planned to provide a phase 3 development update during the conference. It's plausible that investors are excited about getting an update on Isis' lead late-stage products.

Now what: Another point you may not want to rule out considering today's strong move higher is Isis Pharmaceuticals' growing short interest. Short-sellers are betting a stock goes down, and they lose money when it goes up. On a month-over-month basis, Isis' shares held by short-sellers rose from 14.7 million to 16 million, or 13.4% of its float. Skittish short-sellers may possibly be exiting their positions and pushing Isis Pharmaceuticals higher.

The only certainty here is that Isis is going to be delivering important data that you want to monitor next Tuesday, Nov. 3, 2015. A phase 3 development update should give investors some idea of what catalysts are next in line, so stay tuned.

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.