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And then again, on the flip side, when they have autoimmune complications or immune dysregulatory complications, they're put on steroids, other immunosuppressants or a class of drugs called mTOR inhibitors to try to modulate their immune system. None of these therapies, of course, are FDA approved for the specific treatment of APDS. And again, the worst condition, worst possible outcome for these patients is that they need a stem cell transplant. And unfortunately, even stem cell transplants, although potentially curative have significant morbidity and mortality associated with that.
A consequence then of that should be that we can actually develop the immune system properly and then, again, impact all the other things that are the downstream effects of that abnormal immune system development. And that's -- and when we've gone on to study that together with Novartis, and you can see in the next slide, the overall clinical development plan, which includes a number of studies, dose finding studies co-controlled study and at the bottom, a long-term extension study. There are patients now in that long-term intention study that has been treated for a number of years, many patients -- several patients over five years, one patient who's been in the study now for seven years. So we have extensive data on the use of leniolisib both in a long-term perspective, but also in a placebo-controlled fashion.
And in the next slide, we can see some of those results. We see that the study met -- the randomized controlled study met both primary outcomes, which was, number one, to increase the number of naive T cells. Again, B cells that we're not developing properly as a result of that underlying hyperactive pathway. We were also able to achieve decreased lymphadenopathy. Again, this was a primary manifestation of APDS.
On top of that, when we look over in the randomized study as well as over a longer period of time, these patients spleen size shrinks, we see improvements in those autoimmune complications. We see in general that the drug was also well-tolerated in the data package that we submitted to FDA. For example, we have a median exposure of two years for the patient population.
On top of that, when we start looking at the longer term outcomes, we see that these patients are getting less infections, and they're using less immunoglobulin replacement therapy. So despite using less of the therapies that needed to control infections that are actually getting less infections, so it's actually very nice to see how impacting that pathway can impact the immune system and then can actually have an impact on all of these clinically relevant endpoints in terms of infections and also reduction of the immunoglobulin replacement therapy.
On the next slide, we can see where we are now in terms of safety. And what we see when we look at the randomized controlled trial data is a comparison of leniolisib on the left with placebo on the right, and you see a very similar profile in terms of the grade of adverse events that were experienced by these patients. And that mimics what we see in the long-term extension data. So in general, leniolisib has been well-tolerated. And again, as I mentioned earlier, we have some patients who on the therapy in the studies for several years now.
Sushila Hernandez
Yes. Thank you. And just one more question. When can we expect to see additional assets entering your pipeline via internal projects or in-licensing acquisitions next to a second indication for leniolisib or your gene therapy candidate?
Sijmen de Vries
Yes. So basically, the secondary indications leniolisib in the second half of this year will be informing the market about that. And with regards to in-licensing and acquisitions, we're very active. We have a small, but very efficient business development group, turning over a lot of incoming assets that we get offered/that we find ourselves.
Now, as we've been out there talking about the condition, obviously, we're finding more and more patients, including in places like France and across Europe as well as in the U.S. So that number we expect to continue to increase. I think a big driver also of diagnosis though is the availability of a potential therapy, a targeted therapy.
So I think as -- and then hopefully, we can have a therapy available for these patients soon. But once such a therapy is available, I think that will drive even further diagnosis. I think we're fairly on conversion right now with the estimates that we're providing in terms of the prevalence. But certainly, I don't think it would surprise any of us if the numbers were significantly higher.