But a few weeks after taking their first dose, nearly all of them began to recover.

Lee Reyes, 30, of Fresno, Calif., who had begun using a feeding tube because of a growth pressing against his throat, bit into a cinnamon roll.

Nothing, he told his mother, had ever tasted as good.

Rita Quigley, who had been grateful just to find herself breathing each morning since learning she had the virulent skin cancer, went shopping for new clothes with her daughters at a mall in Huntsville, Ala.

Randy Williams, 46, who drove 600 miles from his home in Jonesboro, Ark., to the M.D. Anderson Cancer Center in Houston to get the experimental drug, rolled out of bed. “Something’s working,” he thought, “because nothing’s hurting.”

It was a sweet moment, in autumn 2008, for Dr. Keith Flaherty, the University of Pennsylvania oncologist leading the drug’s first clinical trial. A new kind of cancer therapy, it was tailored to a particular genetic mutation that was driving the disease, and after six years of disappointments his faith in the promise of such a “targeted” approach finally seemed borne out. His collaborators at five other major cancer centers, melanoma clinicians who had tested dozens of potential therapies for their patients with no success, were equally elated.

In a kind of “pinch me” exercise, the six doctors sent one another “before and after” CT scans of their patients.