Developing a therapy for an ultra-rare condition has its challenges, including finding enough patients for clinical enrollment and convincing regulatory authorities that limited data prove the candidate is safe and effective. For that reason, Stealth Biotherapeutics Inc. has faced numerous roadblocks getting its mitochondria-targeting elamipretide across the finish line for Barth syndrome, a condition that affects about 230 to 250 males worldwide, including fewer than 150 in the U.S.