AstraZeneca Plc (AZN) just expanded its reach in the rare disease space. On Thursday, the FDA approved Koselugo (selumetinib) for adult patients with neurofibromatosis type 1 (NF1) who have symptomatic, inoperable plexiform neurofibromas. It's a meaningful win for a patient population that hasn't had many treatment options.

So what exactly is NF1? It's a rare, progressive genetic condition triggered by either a spontaneous or inherited mutation in the NF1 gene. The disease brings a variety of symptoms, including soft lumps on and under the skin known as cutaneous neurofibromas. But here's where it gets more serious: up to 50% of patients develop tumors called plexiform neurofibromas (PN) that grow along nerve sheaths.

These PN aren't just cosmetic concerns. They can cause disfigurement, motor dysfunction, pain, airway dysfunction, visual impairment, and bladder or bowel dysfunction. In other words, they can seriously impact quality of life.

The FDA's approval was backed by positive results from the KOMET Phase 3 trial. In the primary analysis, Koselugo delivered a statistically significant and clinically meaningful overall response rate (ORR) of 20% compared to just 5% with placebo by cycle 16. Even better, 86% of patients on Koselugo maintained their response for at least six months.

The trial design had an interesting twist. After 12 cycles, patients on placebo were switched to Koselugo, while those already on the drug continued for an additional 12 cycles. The safety profile remained consistent with what's already known from pediatric use.

This approval is part of a broader global rollout. Koselugo was recently approved in the EU, Japan, and other countries for the same indication, all based on the KOMET Phase 3 data. Additional regulatory reviews are ongoing elsewhere. In the U.S., Koselugo granules were recently approved for pediatric patients aged one year and older with NF1 PN, so this adult approval rounds out the patient population.

Price Action: AZN stock was up 0.66% at $89.58 on Thursday.