Robert Bazell, NBC News

A cancer drug is turning out not to be the miraculous treatment for Alzheimer’s that many had hoped. Two papers out Wednesday in the New England Journal of Medicine warn families of Alzheimer’s victims not to seek treatment with Targretin (generic name: bexarotene). 

Last February a study from Case Western Reserve University reported that the drug rapidly cleared the clumps of protein known as beta-amyloid, the hallmark of Alzheimer’s, from the brains of mice with a version of the disease. Since the drug was already on the market, approved as a treatment for lymphoma, doctors could immediately prescribe it in so-called off-label use for Alzheimer’s. And thousands of families understandably asked.

But one mouse study does not prove that a drug is effective in humans. The drug is expensive – about $14,000 a year - and off-label use is often not covered by insurance. The drug can also bring on severe side-effects. In one paper in the Journal, Justin Lowenthal, Sara Hull and Steven Pearson of the National Institutes of Health and Massachusetts General Hospital conclude that for this drug “even if the patients are willing to take the risks for the potential benefit, the physician's answer should be no.” In the second paper Frank LaFerla of the University of California, Irvine observes “the field has been down this road before, as successes in preclinical models have thus far not translated well into the clinic.”

But the reasons for disappointment with Targretin appear to be stronger than these theoretical concerns.  Word among Alzheimer’s scientists is that no one yet has independently replicated the Case Western results in mice. In science, a single study has little value until it is repeated. It could be just a fluke or a mistake. No funding agency would support a clinical trial in humans of a potential drug unless the mouse study were confirmed.

These are not good times for potential Alzheimer’s treatments. On Monday, Pfizer announced it was abandoning its efforts to market bapineuzumab, a drug that definitely cleared amyloid plaques in people, after a second, large clinical trial found it did not make the patients noticeably better. 

The reason for this failure -- and other recent similar ones -- could be new evidence that the brain changes leading to Alzheimer’s begin decades before symptoms appear. Treatment may have to start long before there is perceptible loss of memory or other mental function. "The impending tsunami of new cases and the time required for introducing a drug to the market increase the urgency of identifying new therapeutic targets and strategies," LaFerla writes. That presents a daunting challenge, but science cannot give up trying to prevent the misery and expense from Alzheimer’s that is beginning to overwhelm us and will only get worse as the population ages.

Robert Bazell is NBC's chief science and medical correspondent. Follow him on Facebook and on Twitter @RobertBazellNBC

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