You may not be able to teach old dogs new tricks, but drug companies can certainly try to get old drugs approved for new indications.
Gammagard Liquid is essentially antibodies purified from human plasma. It has been used to treat immunodeficiency conditions for almost 30 years.
Baxter hopes that the antibodies in Gammagard Liquid will attack and clear out beta-amyloid plaque in the brain, which many researchers believe is the cause of Alzheimer's disease. Elan
Unfortunately, human plasma is in short supply. Even if other makers of intravenous immunoglobulin (IVIG), such as Omrix Biopharmaceuticals
The best hope for patients is that Gammagard is found to be effective at reversing the disease, and that researchers are subsequently able to determine which antibodies in Gammagard are responsible for the positive effects. Those antibodies could then be formulated into a lab-made drug.
In the meantime, I'm not sure that Baxter will benefit that much from a positive clinical trial result. The increased demand might allow it to command a higher price tag for Gammagard for a while, but ultimately, a drug specifically for Alzheimer's disease will replace Gammagard. The antibodies that replace Gammagard could be purified from almost any IVIG product, so a FDA approval doesn't give Baxter researchers much of an advantage. Baxter seems to realize that its potential benefit is rather small, since it's convinced the National Institutes of Health to pick up part of the tab for the phase 3 trial.