Decisions about drug safety and efficacy are far from easy. Tysabri, a multiple sclerosis (MS) drug that was voluntarily withdrawn from the market last year after the appearance of a previously unknown side effect, illustrates some of the conundrums.
In advance of the publication of three critical new studies on Tysabri in this week's issue of the New England Journal of Medicine, a major news organization recently asked me, as a physician and former FDA official, whether I knew of examples of prescription drugs that have "efficacy but [also] serious safety issues." That is, I responded, the rule rather than the exception.
Obvious examples include the antimetabolites used for traditional chemotherapy. Because these drugs are no more than poisons administered in a way intended to be more toxic to cancer cells than normal ones, it is not surprising that their side effects are often serious and even life-threatening. When I was a medical resident three decades ago, hospital gallows humor included referring to BCNU, an experimental cancer drug, as "Be seein' you." Approved in 1977, it is still widely used.
A more recent example is aldesleukin, a drug that has offered new hope to victims of kidney cancer and malignant melanoma. It is highly effective in a small proportion of patients but exhibits significant toxicity. The patient information booklet warns that those taking the drug "frequently experience serious, life-threatening or fatal adverse events," including dangerously low blood pressure and reduced organ perfusion, impaired function of infection-fighting white blood cells, disseminated infection and autoimmune disease.
Antibiotics are another class of wonder drugs that sometimes manifest significant toxicity. Chloramphenicol, a drug that is effective against a wide spectrum of bacterial infections, causes rare cases of fatal aplastic anemia, so it is used only sparingly. The potent antibiotic gentamicin is often lifesaving but can cause damage to the kidneys, nerves and ears. And significant numbers of patients are allergic to other important antibiotics, including the penicillins and cephalosporins.
But let us return to Tysabri, only the sixth medication approved -- and the first in several years -- for the treatment of MS, a common and debilitating autoimmune disease that affects the central nervous system. The impressive results of the drug's testing in clinical trials -- the frequency of clinical relapses reduced by more than half -- induced the FDA to grant accelerated approval in 2004. By the time that several thousand patients were being treated with Tysabri, however, three had contracted progressive multifocal leukoencephalopathy (PML), a rare neurological disorder caused by a virus. (Because the drug suppresses certain components of the immune response, regulators, clinicians and the product's developers had from the beginning been sensitive to the possibility of infections as a side effect.) Immediately -- some would say prematurely -- the manufacturers of the drug voluntarily halted production and distribution and withdrew Tysabri from the market. MS patients and many neurologists were bitterly disappointed.