Sometimes rare safety issues with one or two drugs can sink the fortunes of other drugs in the same class of therapies. Yesterday, the FDA issued a communication that it was looking into possible safety issues with some of the best-selling biopharmaceuticals on the market.

The FDA's statement announced an investigation into the possible connection between a class of drugs called TNF inhibitors and a range of cancers in children.

TNF-inhibitor-based drugs on the market include Amgen (NASDAQ:AMGN) and Wyeth's (NYSE:WYE) more-than-$3 billion-a-year Enbrel, Johnson & Johnson (NYSE:JNJ) and Schering Plough's (NYSE:SGP) more-than-$3 billion-a-year Remicade, and Abbott Labs' (NYSE:ABT) Humira, which is expected to bring in around $4 billion in sales this year. Needless to say, any reluctance by doctors to prescribe the drugs, or a tightening of FDA restrictions on these compounds' use, could have a very negative impact on some of big pharma's top lines.

TNF inhibitors are used to treat a range of autoimmune disorders, including rheumatoid arthritis, chronic psoriasis and Crohn's disease. Some studies have already found a possible link between TNF blockers and some types of cancer in rheumatoid arthritis patients. The FDA's emphasis on these drugs' use in pediatric patients is just a more specific facet of the agency's longer-standing, more general cancer concerns.

The FDA's communication calls for TNF-blocker developers to submit all information that they have about children who developed cancer while taking these drugs. The agency will announce its next step regarding these compounds in six months; for right now, it's not suggesting that doctors alter their prescribing patterns.

Investors concerned about a repeat of the disasters that befell GlaxoSmithKline's (NYSE:GSK) diabetes treatment Avandia or Merck's (NYSE:MRK) Vioxx, after both compounds revealed new possible adverse events, have nothing to worry about yet.

At worst, the FDA will likely strengthen the warning labels for these drugs, and eventually further restrict their use in some children. The agency is requiring Belgian drugmaker UCB, developer of the recently approved TNF inhibitor Cimzia, to run a 10-year safety study with its drug. Since this study hasn't started yet, and won't be wrapped up for years (despite likely periodic interim peeks at the data), even a tightening of these compounds' labels could be a long time in coming.