Julie A. Parsons, MDHaberfield Endowed Chair in Pediatric Neuromuscular DisordersProfessor of Clinical Pediatrics and NeurologyUniversity of Colorado School of Medicine, Children's Hospital ColoradoAurora, CO, USAThe gene transfer trials for musculoskeletal disorders, if we look at musculoskeletal and neurologic disorders, we really do have the highest success rate in terms of treatment, but we also carry the highest incidence of treatment-emergent severe adverse events. And why is that true? Yesterday, when we were hearing about Donovan as well, we looked and said, When the first gene transfer therapies were started, he had a single muscle that was injected.When we look at Luxturna, we injected the retina. Now, what is happening with these disorders is that we're giving these huge, massive doses of viral vector to patients. There haven't been a lot of gene transfer therapies that have reached the market. But you saw yesterday, so many gene transfer therapies being worked on, but there are very few that have actually come to market. There are a couple of reasons for that.One is with the indications that we have, we know that the musculoskeletal disorders are most likely to achieve benefit, but there are the high risk of severe adverse events. Route of Administration, IV, for most of our disorders is the way we're going. We may end up having some Intrathecal therapies as well that are coming on board, but right now it's IV, and that means, a huge dose of this viral vector and antigenic risk that is being administered.In the vector design now, we actually have more specific vectors as well as promoters that are being utilized to really target specific tissues, so that we're able to focus in a little bit more on the tissues that we want to have affected. And then the dose has gone from these little tiny local injections to really systemic, much broader. And now our patients, are larger. So we're giving a viral genome per kilo dose that is just massive as we look at that.Then there really are challenges in terms of the translation of clinical trials to commercial treatment with these agents. And we don't always know, we're not always great when we do tests in clinical trials in small populations, about when that's broadened to the commercial availability and we hit larger heterogeneous populations.There are safety issues arising from these therapies, and I think that we have some experience now, certainly with the three diseases that I mentioned at the beginning, in terms of collecting some data and information to have a little bit more of an idea what to expect. Although to me, the recurring esteem is always, expect the unexpected. Because we still are learning about this. Hepatotoxicity. We know that transaminitis is something that we see in almost every gene transfer therapy that has been delivered, and we have to watch really, really closely and follow our patients closely for this. We also have to select patients that we don't think have risk for additional liver injury or underlying liver pathology, because as we found out in the XLMTM boys, we missed that. Thrombotic Microangiopathy. We look at this disorder. We've had deaths in SMA from TMA. We have Duchenne patients that have had TMA.This is scary because as many of us as clinicians who have treated patients, you know that we end up getting thrombocytopenia. So is that it this time, or are they going to be fine, or the platelet is going to go back to normal? This is another one that we have to watch really, really closely for. Cardiac Toxicity. We have had cardio myositis. We've had deaths from cardiac toxicity.Something really, really important for us to think about. In little kids, vomiting could be a sign of cardiac myositis. And for most of us who've treated patients with gene transfer therapy, what's one of the first issues that you get?You get nausea of vomiting, they don't feel good. So is that myocarditis or is it just a standard side effect that we're seeing with treatment? Importantly, as we discovered, there actually can be an immune response to the transgene. It's not just the viral vector capsid, it's actually the transgene as well. That was discovered in patients who were treated for Duchenne. So that's a really important thing in terms of looking now at what's our patient's selection and how do we pick the right patients.Next part, Dr. Parsons will discuss understanding and preparing risk factors associated with AAV gene therapies.