Gatenby arrives at his corner office at Moffitt most days by 7 am. He’s 67 now, and his hair is gray at the temples, but his eyebrows are still brown. His children—the ones who were napping in that hotel room when he jotted down his Darwinian inspiration—now have children of their own, and he has the “I ♥ Grandpa” coffee mug to prove it. A hospital lanyard around his neck, he rolls up his crisp shirtsleeves and settles down at his desk. Outside his office, roughly 30 scientists and PhD students spend their days researching patterns of cancer growth using equations like those describing population dynamics.

To Gatenby's knowledge, no one had endeavored to exploit evolution against cancer in a clinical trial until he developed his prostate cancer experiment. He picked prostate cancer to test this approach partly because, unlike other cancers, a routine blood draw for a molecule called prostate-specific antigen (PSA) can offer an immediate proxy for the cancer’s progression.

To design a clinical trial, Gatenby and his Moffitt collaborators first needed to account for their idea that tumor cells vie against each other for resources. They turned to game theory to plot this dynamic and plugged the numbers into the Lotka-­Volterra equations. The computer simulations they ran with these equations estimated how quickly drug-resistant cells would outcompete other tumor cells when exposed to the continuous dosage of Zytiga typically given to advanced-stage prostate cancer patients.

In the simulations, the typical administration of the drug led to drug-resistant cancer cells rapidly running rampant. The treatment would ultimately fail each time. That bleak outcome matched up with the results seen in hospital records. In contrast, the computer simulations suggested that if Zytiga were administered only when the tumor seemed to be growing, then the drug-resistant cells would take much longer to gain enough advantage to overrun the cancer.

In 2014 the Moffitt team managed to get the first small study to test this adaptive therapy approach off the ground, recruiting Robert Butler and a small group of other men with advanced prostate cancer. Butler’s oncologist explained to him how it would work. He would remain on the Lupron he’d taken for years, and each month he would go to the hospital to get his PSA level tested, to judge whether his prostate tumor was growing. Every three months, he would get a CT scan and a full-body bone scan to watch for disease spread. Whenever his PSA level edged above where it stood when he entered the trial, he would start taking the more powerful Zytiga. But when his PSA level fell to under half of the baseline, he could go without Zytiga. This is appealing because Zytiga and drugs like it can cause side effects like hot flashes, muscle pain, and hypertension.

The Moffitt approach also promised to be far cheaper than taking Zytiga continuously. When purchased wholesale, a one-month supply costs almost $11,000. Butler had health insurance, but even so, his first month’s supply each year would set him back $2,700 in out-of-pocket copayments, and $400 a month thereafter. Going off the drug whenever his PSA level was low would translate to huge cost savings.

“Conceptually it’s a beautifully simple approach. He’s turning cancer into a chronic disease.”

Butler was participating in a so-called pilot trial, which was less rigorous than a large clinical trial, because it didn’t randomly assign patients to receive the experimental or standard treatments. Rather, the study relied on a group of patients treated outside the trial as well as results from a 2013 paper on Zytiga to come up with a benchmark for how patients typically fare when receiving continuous dosing of the drug.

When the early results of their new trial trickled in, the Moffitt scientists were gratified and relieved. Ahead of the trial, “we were, to be honest, terrified,” Gatenby says. The benefit of adaptive therapy appeared to be huge. Of the 11 men in the study, one left the trial after his disease spread, but most were living longer than expected without their cancer progressing. Men getting continuous dosing of Zytiga go a median of 16.5 months before the cancer becomes resistant to the drug and spreads. In comparison, the median time to progression for the men receiving adaptive therapy was at least 27 months. Moreover, they were on average using less than half of the standard amount of Zytiga. Joel Brown, an evolutionary ecologist and one of Gatenby's collaborators, said the team felt a moral obligation to get the word out: “The effect was so big that it would be unethical not to report it immediately,” he says.