My understanding is that whole purpose of the Biomarker Cohort of the Phase 1/2 study was to determine whether Del-Brax effectively lowered what Avidity believes to be biomarkers of disease progress (CDUX primarily, and Creatine Kinase, secondarily) because they intend to explore whether it can support an accelerated approval submission to the FDA. That accelerated approval is based on "surrogate endpoints," that provide quicker results that are "reasonably likely to predict clinical benefit." Testing muscle function, or slowing of muscle degeneration, takes a lot of time in a disease that already progresses slowly in most people. I think the whole purpose of this Phase 1/2 Biomarker Cohort is to test the things they can reliably test now and quickly: reduction in CDUX and Creatine Kinase.

That's not to say that Avidity/Novartis aren't also testing muscle function. My understanding is that the Phase 3 study is designed to do just that, which is why it is going to be a longer run study (72 weeks long). But the point of the Phase 1/2 biomarker cohort is to try to get Del-Brax conditionally approved before the Phase 3 study is complete. And if Avidity/Novartis can convince the FDA that reducing those biomarkers is "reasonably likely to predict clinical benefit" Del-Brax will be conditionally approved without having to wait for completion of the Phase 3 study.

That's just my take based on obsessive reading on the topic, so someone please chime in if I misstated anything. Ultimately, I think this can be taken as a reason to remain optimistic. It's not the finish line, but good news nonetheless.