The advantage of treating diseases like high cholesterol, diabetes, or rheumatoid arthritis is that the drugs don't actually cure the patient. Unlike antibiotics or even cancer drugs that are only needed for a period of time, chronic diseases require chronic treatments.

And chronic treatments mean big bucks. Pfizer's (NYSE: PFE) Lipitor, Merck's (NYSE: MRK) Januvia, and Abbott Labs' (NYSE: ABT) Humira are all multi-billion dollar drugs. When you hit those levels, keeping patients on treatment is just as important as capturing new patients.

So a new research study reporting 28% of patients taking Humira for rheumatoid arthritis developed antibodies against the drug should worry investors a little. Antibodies against a drug can inactivate it, causing the patient to become resistant to the therapy.

The formation of antibodies to a biologic drug isn't that surprising, but the number of patients forming antibodies is. Humira's label says just 5% of rheumatoid arthritis patients formed antibodies to the drug during clinical trials. In other diseases that Humira is approved for, antibodies formed at a higher rate, but seeing it in a quarter of patients is pretty astonishing.

Humira is extremely important to Abbot; the drug contributed to over 18% of Abbott's sales last year and likely considerably more to profits. Losing patients because they build up a resistance to the drug will hurt overall sales.

So far that doesn't seem to be happening. Despite being approved after Johnson & Johnson (NYSE: JNJ) and Merck's Remicade and Pfizer and Amgen's (Nasdaq: AMGN) Enbrel, Humira has come on strong raking in over $6.5 billion last year and growing 19% year over year.

Either the current study is overestimating the antibody-formation issue or new patient additions are making up for patients being taken off Humira because it no longer works. Investors should hope for the former because the latter will eventually catch up to the company.

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