Viagra Might Someday Enhance Bone Marrow Transplants

The brand name version of sildenafil citrate and its generic version.
Photo: AP

The long story of sildenafil citrate, best known by the brand name Viagra, may have another chapter to it. Researchers at the University of California Santa Cruz think the drug can improve how we perform bone marrow transplants, as part of a combination therapy that allows doctors to harvest stem cells from patients quicker and safer than the traditional method. So far, though, it’s only been tested in mice.

Famously, Viagra didn’t start out as a drug for erectile dysfunction. Because of its effects in opening up blood vessels (what scientists call a vasodilator), it was originally studied as a way to treat high blood pressure and relieve chest pain caused by heart disease. But in human trials, it soon became apparent that sildenafil was better at boosting men’s erections, which led to its rechristening as the little blue pill.

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But according to study author Camilla Forsberg, there’s a chance that sildenafil could be recycled for something else yet again.

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There are different types of stem cells used in medicine. And the most commonly used are stem cells from our bone marrow, called hematopoietic stem cells, which can be used to rebuild a healthy supply of precursor and adult blood cells. Currently, doctors usually collect stem cells for a bone marrow transplant from the bloodstream of a donor. But there aren’t enough stem cells hanging around normally, so people have to take a drug for several days to stimulate bone marrow growth.

This method works most of the time but not always, and the drug comes with some nasty side effects that prevent some people from using it at all, such as those already ill, elderly, or who have sickle cell disease. This especially matters because doctors can also use a person’s own stem cells for a transplant, which lets them avoid the risk of rejecting a donor’s cells.

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An illustration of how the procedure works, compared to the standard mobilization treatment.
Illustration: Smith-Berdan, et al (Stem Cell Reports)

Forsberg and other researchers had earlier found evidence that you could speed up the harvesting process by using drugs that make blood vessels more porous, and that led them to their latest findings, which are published Thursday in Stem Cell Reports.

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“The rationale was to test whether known and approved vascular drugs could be repurposed to coax the hematopoietic stem and progenitor cells from the marrow to the blood—and this turned out to be right,” she told Gizmodo via email.

In experiments with mice, they used sildenafil in combination with another drug that stimulates bone marrow growth but isn’t effective enough on its own, called plerixafor. In these mice, the combination method didn’t coax quite as many stem cells as the standard treatment, but the team was able to harvest enough stem cells for successful transplants in just two hours, rather than the five days it normally takes (the timeline is the same for both mice and people).

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An image of a mouse clutching Viagra, created by the research team.
Graphic: University of California Santa Cruz

“Our two-drug strategy is much quicker and less invasive than the standard treatment,” Forsberg told Gizmodo via email, “and I think a shorter and simpler treatment may make it much easier to get healthy volunteers to donate stem cells to patients in need.”

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For people with sickle cell disease, she added, sildenafil might even help treat their symptoms while also preparing them for a self-donated transplant, which could be very useful in the near future. In recent years, scientists have experimented with harvesting the stem cells of sickle cell patients, genetically editing them to be healthy, and plugging them back in to reboot their blood cells—an effective cure that’s shown some early success. Forsberg’s method, provided it continues to show promise in larger animals and eventually human trials, could widen how many people with sickle cell would be eligible for this treatment, she said.

Forsberg’s lab doesn’t directly study people, but she said that she’s already reaching out to doctors who do to work together on trials in the near future. And because both drugs are already approved by the FDA (and in the case of sildenafil, off-patent and cheaper), this research should ideally move along faster than usual. In the meantime, she said, their work serves as an example of why it’s important for scientists to continue studying older drugs.

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“Viagra has been around for a long time now, but it was not obvious that it could be used for this indication,” she said. “So I think a lesson to be learned is that basic research with one eye looking out for the unexpected and the other eye on translation can be productive and exciting.”

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About the author

Ed Cara

Science writer at Gizmodo and pug aficionado elsewhere