Opportunistic infections aren't an opportunity for Roche and Biogen Idec (Nasdaq: BIIB). In fact, they have ocrelizumab, the follow-up to the companies' near-blockbuster Rituxan, on the ropes and headed for a TKO.

Yesterday, the companies said they're suspending development of ocrelizumab for rheumatoid arthritis because patients were coming down with opportunistic infections. These infections don't normally happen in healthy people, and some of them were fatal. The stoppage isn't a major surprise; Roche and Biogen had previously halted a trial in early stage rheumatoid arthritis, and another in lupus.

That's the problem with treating autoimmune diseases like rheumatoid arthritis and lupus, where the body is attacking itself. Drugs have to dial back the immune system enough to treat the disease, but not so much that the immune system can't fight infections like it's supposed to.

Ocrelizumab is also in development for multiple sclerosis. That line of research hasn't been terminated. Yet.

Multiple sclerosis is more severe than rheumatoid arthritis, so a higher level of side effects is often tolerated by regulators and doctors. For instance, Biogen and Elan's (NYSE: ELN) Tysabri is associated with a potentially fatal brain infection, Teva Pharmaceutical's (Nasdaq: TEVA) Copaxone has a warning about modifying the immune response, and both Rebif from EMD Serono and Pfizer (NYSE: PFE) and Novartis' (NYSE: NVS) Extavia are associated with liver toxicity or signals for it.

The side effects might be a reasonable risk for multiple sclerosis; it's hard to tell because the companies didn't quantify the rate of opportunistic infections. But lupus is also a pretty serious disease with limited treatment options. Besides Human Genome Sciences (Nasdaq: HGSI) and GlaxoSmithKline's (NYSE: GSK) Benlysta, there hasn't been a drug developed specifically for the disease in decades. If ocrelizumab's side effects are too severe for lupus, I'm guessing multiple sclerosis will be the next to see the ax.

That's just the way of life for drug development. Sometimes drugs bring opportunity; other times, opportunistic infections.

Elan is a Motley Fool Rule Breakers selection. The newsletter is always on the hunt for hot drug stocks and other cutting-edge picks. Click here to see all of our latest discoveries with a free 30-day trial subscription. 

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is an Inside Value choice. Novartis is a Global Gains pick. The Fool owns shares of GlaxoSmithKline and has a disclosure policy.