Image source: Ionis Pharmaceuticals.

What: Shares of Ionis Pharmaceuticals (IONS -0.82%), a drug developer focusing on a broad spectrum of therapeutic ailments using its propriety antisense technology platform, were pummeled and lost 45% of their value in May, based on data from S&P Global Market Intelligence. Weakness in Ionis can be traced to two particular events.

So what: By far, the biggest issue for Ionis came late in the month, when it provided an update on its IONIS-TTRrx program, which was being developed in collaboration with GlaxoSmithKline (GSK 0.22%). As one of Ionis' most advanced programs, hopes were high that GlaxoSmithKline was ready to move IONIS-TTRrx into a phase 3 study known as CARDIO-TTR for patients with TTR amyloid cardiomyopathy. As the press release on May 26 from Ionis noted, GSK declined (at least for the time being) to move CARDIO-TTR forward into a pivotal phase 3 study. Glaxo instead chose to wait for data from the ongoing NEURO-TTR phase 3 study for patients with transthyretin (TTR) familial amyloid polyneuropathy, which should yield clinical results sometime next year.

What's the hold-up? It all relates to instances of thrombocytopenia, or low platelet levels, in some patients with TTR familial amyloid polyneuropathy. GSK holding off on commencing phase 3 studies has raised concerns that low platelet counts could be troublesome in more than just IONIS-TTRrx, thus causing the majority of Ionis' May swan dive. For what it's worth, Ionis doesn't believe this is a class effect issue, but only clinical data will prove its suspicions true or false.

The other issue for Ionis was its first-quarter earnings report released in early May that featured a wider-than-expected first-quarter loss. Despite bringing in $36.9 million in revenue, the majority of which came from milestone payments and the amortization of upfront fees and manufacturing services from its partners, Ionis' net loss more than tripled to $62.9 million, or $0.52 per share, from $16.7 million, or $0.14 per share, from the year earlier. Wall Street had been looking for a narrower loss of just $0.50 a share.

Image source: GlaxoSmithKline via Flickr.

Now what: It was a miserable month for shareholders, but the interesting thing is that there's still plenty of promise for Ionis.

Even excluding IONIS-TTRrx and volanesorsen, which also had reports of patients with low platelet levels, Ionis is working on more than two dozen ongoing clinical studies and has nearly a dozen brand-name development partners. The company's antisense program allows it to crank out high-quality clinical candidates faster than many of its peers, which ensures its pipeline remains rich with possible blockbusters.

You'll also not want to overlook that Ionis Pharmaceuticals finished the first quarter with $703.8 million in cash and cash equivalents. In spite of having just one Food and Drug Administration approved product, its multiple partnerships and licensing deals supply it with a fairly steady stream of upfront cash and development milestone payments. Funding isn't an issue for Ionis, which means investors can focus on its extremely deep pipeline.

Following its pummeling in May, I'd encourage risk-tolerant and biotech-savvy investors to give Ionis Pharmaceuticals a closer look.