But finding the cure for one disease is personal.
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Brennan's father Joseph died from complications of Alzheimer's Disease 11 years ago.
"He was there for every game and every event I was involved in," recalled Brennan. "It was very painful to see his decline from a sharp, vibrant person to someone who was really disabled at the end."
Turning grief into action, he turned to Penn's Institute on Aging and its director John Trojanowski.
"The impetus for it was the frustration all families feel as they go through this, the helplessness," he said.
Brennan and Dr. Trojanski started a "5k for IOA" to raise money for Alzheimer's research. In the past decade, they've raised more than $400,000 so far.
However, this is the first year Dr. Brennan is organizing the race without Trojanowski.
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The famed doctor passed away earlier this year, which is why this race means even more for Dr. Brennan.
It's always been for his dad, but this year, he'll honor his friend too.
"He was my inspiration. I wanted to do something when my dad passed. I don't know what I would've done if John were not here," he said.
The race is on Sunday, October 2. It will start at Franklin Field and go through Penn Park.
Dr. Brennan is hoping for the biggest turnout ever so his team can get the funds they need to find a cure for Alzheimer's Disease.