Emily Pearson looked like everyone else in her T-shirt and shorts last fall when she crossed the finish line of a sun-drenched Twin Cities Marathon.

Which made her extraordinary.

Four-plus hours in daylight used to cause “insufferable” pain because of a rare genetic skin disorder, forcing her to do training runs in darkness at 3 a.m. or covered head to toe in protective clothes. That all largely went away last summer after the 25-year-old started taking an experimental medication for the condition, which is known mainly by its acronym, EPP.

“It completely transformed my life in general, but especially the marathon training,” Pearson said. “I could run at 2 or 3 in the afternoon. I could run with friends. My sleep schedule wasn’t out of whack.”

Whether she gets to stay on the daily pill depends on the U.S. Food and Drug Administration, which unexpectedly rejected the medication in February after the agency’s commissioner selected it last fall for accelerated consideration. The drug, bitopertin, was the first of 18 prioritized by a new FDA voucher program in President Donald Trump’s administration to hasten development of pharmaceuticals against diseases that are rare or have few other treatments.

Manufacturer Disc Medicine presented regulators with data showing bitopertin was safe and reduced the toxic buildup of protoporphyrin, an organic compound that causes extreme light sensitivity. But the FDA wanted more proof that patients actually got better as a result of this molecular change.

The decision shocked Kristen Wheeden, an advocate for patients who suffer extreme daylight sensitivity as a result of EPP, or erythropoietic protoporphyria. An accelerated review allows the FDA, per its own policies, to grant interim approval for a drug based on positive laboratory data “that is reasonably likely to predict clinical benefit.” Wheeden said the research on bitopertin appeared to meet that threshold.

Reconsideration could take another year or two, leaving EPP patients bunkered in their homes this spring and summer because of their extreme sensitivity to daylight, said Wheeden, president of the United Porphyrias Association, which advocates for patients with EPP and related conditions.

Many have to “shadow jump between cars” when they go to stores and race to the closest shade or indoor entrances, she said. “What they want is to just be normal, to walk out of their home and not be afraid, to walk the dog, to go to the mailbox.”

Protoporphyrin normally plays a healthy role in the body by helping to convey oxygen through the bloodstream. But excessive amounts in skin cells cause extreme sensitivity to daylight, and can’t be prevented by traditional sunscreens that protect against ultraviolent (UV) light. About 4,000 people in the U.S. have been diagnosed.

Some people with EPP suffer extreme reactions after a minute of daylight exposure, while others can last an hour or longer. The pain can be like “being boiled alive,” Wheeden said.

“Going on a boat is like a little piece of hell for people, because there’s no escape,” she said.

Pearson wept at age 17 when she was diagnosed at Mayo Clinic in Rochester, because she had been living for 10 years with unexplainable skin pain. Sometimes daylight exposure would cause swelling, but often the pain was invisible and people wouldn’t believe her when she complained.

Her case was mild compared to others, because she could tolerate about an hour of daylight before the pain hit. As a teenager in Duluth, she said she was “reckless” and would go to the beach and then suffer later.

“There’s no way to make it better other than time,” she said. “It will eventually go away within a few days.”

A doctor prescribed her a drug off-label that is normally used to reduce stomach acid, but Pearson said it didn’t help much. Then late last summer a specialist in Boston referred her to a clinical trial of bitopertin.

As an environmental scientist, she was often fully covered when doing field work such as collecting stormwater samples to test for contamination, and was similarly conspicuous last year at the Minnesota State Fair.

Two months later, she was running without protection in the Twin Cities Marathon, on a day when race organizers declared “yellow flag” conditions because of excessive heat.

Pearson gets to stay on the medication for now, despite the FDA rejection, because she is part of an ongoing clinical trial that could result in the federal agency reversing its decision. But it remains unavailable for prescription while researchers try to build a stronger case that it works.

Without the drug, people with EPP have one FDA-approved alternative, Scenesse, which is implanted just below the skin every few months to produce a tan that acts as a barrier to light.

The FDA’s Feb. 13 rejection letter said there is “significant uncertainty that bitopertin will have the effect it purports” without more scientific proof. Disc Medicine’s chief executive replied in a written statement four days later that “we have confidence” in the ongoing clinical trial and the drug’s eventual chances for FDA approval.

The FDA has approved five of the first 18 drugs selected for the new expedited review process, rejecting only bitopertin. Some financial analysts were surprised, but one former FDA official said the decision is proof that the new process is fast but not careless.

“Accepting a drug for a swifter review does not mean that the drug is going to be accepted,” said Peter Pitts, the former FDA official who co-founded the Center for Medicine in the Public Interest. The New York advocacy group generally argues against more government regulation of the pharmaceutical industry.

Pearson said she hopes the trial will prove the drug’s benefit so that others can gain relief and no longer have to hide from daylight. Windows aren’t a sufficient filter, so many people have to keep shades drawn and avoid daylight when indoors as well.

Next up for Pearson is Grandma’s Marathon in June. She has worked up to 10-plus-mile training runs in preparation.

“I can’t wait to experience more normal things in life,” she said.

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