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Kevin Williamson has coached every one of his five children’s basketball teams, but in recent years, stage 4 bladder cancer has forced him to sit on the sidelines. Now, he’s back on the court— coaching his only daughter’s team— thanks in large part to an immunotherapy drug that has shrunk his tumors and stopped them from spreading further.

“I’m a walking, breathing stage 4 cancer patient,” Williamson, 47, the president of a mortgage company, told FoxNews.com.

Bladder cancer is the fifth most common cancer in the United States, but the Food and Drug Administration (FDA) has not approved a new drug for advanced stages of the disease in the past 30 years. However, in mid-May, the agency granted accelerated approval of the drug that’s sustaining Williamson’s life, TECENTRIQ, which works by re-gearing the immune system to eradicate cancer cells. Part of an emerging class of cancer drugs, TECENTRIQ is an immunotherapy, which generally cause fewer side effects than chemotherapy because they target only diseased cells. An ongoing clinical trial suggests TECENTRIQ has reduced about 15 percent of 311 patients’ tumors by at least 30 percent.

Williamson, who lives in Wheaton, Illinois, about 30 miles west of Chicago, was diagnosed with bladder cancer in March 2013 after experiencing unbearable gas. He didn’t have a family history of bladder cancer and doesn’t smoke, which doctors say is the biggest risk factor for the disease.

In April 2013, Williamson underwent surgery to remove an 18-inch cyst and then began a few months of chemotherapy to treat the cancer, which was deemed inoperable. When Williamson didn’t respond to chemotherapy, doctors surgically removed his bladder, which led him to use an external bag and an ileal conduit to empty his bladder and store his urine.

“At 44 years old, I wasn’t excited about that and neither was my wife,” he said. “She did a lot of research, and we found out that several doctors in the U.S. do what’s called a neobladder,” where doctors make a bladder out of the small intestine. Williamson underwent the procedure in October 2013 at the University of Chicago, where he still receives treatment.

Despite those surgeries— which each required two months to recover— Williamson is still living with cancer because inoperable disease remains near his hip and kidney.

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A subsequent genetic test at the University of Chicago revealed that he was eligible to enroll in a clinical trial for afantinib, a lung cancer drug being tested on bladder cancer. Over six months, Williamson’s tumors shrank by 40 percent, but then the drug stopped working and his tumors began spreading again.

In July 2014, he enrolled in the trial for TECENTRIQ (chemical name atezolizumab), which re-shrank his tumors by 40 percent, and they have remained stable in size ever since.

“I have cancer, I get tired at times, I feel much older than I am. But I am capable of doing anything that I want,” Williamson said.

Dr. Arjun Balar, co-author of the clinical trial that compelled the FDA to fast-track approval of TECENTRIQ, has been leading a separate trial to see whether the immunotherapy could work as a first-line treatment for advanced bladder cancer. His team will present their findings in early June at the American Society of Clinical Oncology (ASCO) Annual Meeting.

Williamson and the other participants from the previous TECENTRIQ trial qualified because the current first-line treatment for advanced bladder cancer, platinum-based chemotherapy, proved unsuccessful for them. All of the participants also had locally advanced-stage bladder cancer, which means their disease spread to parts of the body beyond the bladder region.

The study findings, published in the March edition of The Lancet suggested that TECENTRIQ successfully blocks a protein on tumor cells that deactivates disease-fighting T cells. The drug effectively switches on a subset of T cells, CD8-positive cells, to ward off cancer.

“TECENTRIQ blocks that ‘off’ switch … so the lights stay on, so to speak, so the immune system can recognize cancer and do what it needs to do,” Balar, assistant medicine professor at NYU Langone Medical Center, and co-leader at genitourinary cancers at NYU Perlmutter Cancer Center, told FoxNews.com.

Balar attributed the lack of new treatments for advanced-stage bladder cancer to previous shortfalls in understanding the disease’s biology. Now, it appears that immunotherapy works well not just for skin and lung cancer, but also bladder cancer, as the diseases share similar mutational burdens.

“It turns out that bladder cancer, lung cancer and melanoma have some of the highest mutations in degrees itself, and what that reflects is the body’s immune system’s ability to recognize those mutations as foreign,” Balar said. “The immune system is already trying to fight these cancers— they just need a little help, and that’s how these treatments work.”

Results suggest about 15 percent of patients experience side effects on TECENTRIQ mostly due to an overactive immune system, Balar said. Those side effects include fever, constipation, fatigue and a decreased appetite, but they can be moderated with steroids.

Williamson manages diarrhea, one of the drug’s known side effects, with Immodium. He also began testosterone therapy in December after levels of the hormone dipped in summer 2015 and he developed gynecomastia, which is marked by imbalanced hormones, and swelling and pain in the chest.

“My energy has come back,” Williamson said. “The growths have gone away, and the pain has gone away.”

Patients like Williamson had an average life expectancy of six months after first-line chemotherapy failed, but immunotherapy like TECENTRIQ— as well as a handful of other drugs being tested on the cancer— offer them hope.

Williamson said his faith has helped him stay grounded, and he described his approach to cancer treatment as practical and logical. His next challenge is coaching that girls’ basketball team, for his daughter Ainsley, 11— a first for him after coaching his four sons’ teams.

“I’m looking forward to doing that for a few more years,” he said. “I think I learned the lay of the land this year; you have to change your style a bit.”

If TECENTRIQ stops working, he said he’ll simply sign up for another clinical trial.

“You’re either part of the problem or part of the solution,” Williamson said. “I mean, God’s got a plan and whatever happens, happens. I don’t think that means you just gotta sit back and do nothing, so we will try to find things, and hopefully this continues to work and we don’t have to.”