The day after Christmas in 2019, Annette Harlow received what would be the final infusion of her lung cancer therapy. Just weeks later, in a doctor’s office in Lake Charles, La., she heard the news every cancer patient dreads: the treatment had stopped working. “Is that it?” she asked. “I’m afraid so,” she recalled her doctor saying. “Right now, there’s nothing that we have for you.” The very next day, she received a message through her patient portal: If her tumor carried a mutation in a gene called KRAS, she might qualify for a clinical trial run by Dr. David Hong in Houston. “You could see my spirit just flying in the room. There was something for me,” she said, her voice lifting at the memory.

The test on her tumor came back positive. Soon, she was making the two-and-a-half-hour drive to the University of Texas M. D.

Anderson Cancer Center, having enrolled in Dr. Hong’s Phase 1 clinical trial. The experimental drug she received there, sotorasib, blocks a mutant version of the KRAS gene, which for decades scientists had considered resistant to drug therapy. Ms. Harlow was among the first people in the world to take it. More than six years later, she is an energetic and optimistic woman in her late 80s, and her cancer is under control. “I have had many blessings in life,” she wrote in an email, “and for sure the two most remarkable ones have been Dr. Hong and sotorasib.” A Phase 1 clinical trial is the initial point at which a potential treatment leaves the laboratory and meets human reality.

Every drug that reaches approval for the general public must pass through this gate. We owe the whole of modern medicine to countless trials like it. And for patients with advanced cancer, they offer what is often a last hope.

As the deputy chair of M. D. Anderson’s Phase 1 program, Dr. Hong is among the country’s leading academic oncologists.

He has spent his career translating the latest discoveries from the laboratory into the clinic, offering second chances to patients who have run out of all other options. Not all his patients are as lucky as Ms. Harlow — but he has seen enough patients recover from supposedly fatal diagnoses to appreciate what new, innovative drugs can do. In August, the Food and Drug Administration approved daraxonrasib, a therapy that almost doubles survival rates in pancreatic cancer, one of biology’s most ruthless killers. Dr. Hong led the Phase 1 trial that first showed the drug could work in humans.

If anyone has reason to be triumphant about American drug development, he does. But when we spoke, Dr. Hong was not triumphant. He was worried.

He told me that the system that gave Ms. Harlow her life-extending treatment is under serious strain. The cost of running a clinical trial in the United States has climbed steeply in the past decades. For Phase 1 trials specifically, the cost for generating the evidence needed just to open one has doubled, on average, since 2017, and for some complex therapies the cost can reach several million dollars per patient.

In response, drug companies are opening fewer trials than they otherwise might have, and they are increasingly taking the ones they do conduct abroad, to countries such as Australia and China. That leaves American patients with fewer chances to enroll in a clinical trial. The timing makes this especially bitter: A.

I. and the genomics revolution are turning long-promised ideas into potentially curative therapies. It is no longer the science, Dr. Hong believes, that is holding back potential cures, but our capacity to try new medicines in clinical trials. His fear, echoed by the many oncologists and medical experts I interviewed, is that the most important science may happen elsewhere or not at all. “For the first time, patients in America will no longer be among the first to benefit from the latest biomedical discoveries,” he told me.

The damage is hard to assess: the trials that were never run, the breakthroughs that never happened.