October 11, 2021
Over a year ago, Cure Rare Disease catalyzed the community’s focus on and understanding of neutralizing antibodies as they relate to the use of AAV for delivering gene therapies for neuromuscular diseases such as Duchenne and Limb-girdle muscular dystrophies. For patients and patient-families, the diagnosis of a rare disease is life-altering. Through the advancement of gene replacement and gene editing technologies, we are becoming increasingly capable of fixing genetic mutations involved with DMD or LGMD. However, these gene therapies are currently being delivered with viruses known as AAV (adeno associated virus). Patients can have innate antibodies to the AAV type (known as serotype) including the more commonly used serotypes, AAV9 and AAVrh74 as it relates to gene therapies for muscular dystrophy.
Many patients and patient-families desire to know where they or their loved one stands regarding seropositivity to AAV (having too many antibodies to AAV). While there is not yet a uniformly agreed upon threshold or test for measuring antibodies, companies have developed their own test (known as an assay) to measure these antibodies. Typically, patients may learn their seropositivity status via screening for inclusion in a clinical trial. More rarely, patients can elect to have antibodies measured through research grade tests like the program that Cure Rare Disease offers or can pay out of pocket to have a clinical grade test conducted through companies like Athena Diagnostics.
It’s important to understand the difference between research-grade and clinical-grade assays. Research-grade assays are not intended for use in making medical decisions since they are not validated. This means that results can vary and potentially provide an over- or under-estimation vs. the actual value. Clinical grade tests are validated and can be used to make medical decisions (ex: enrolling in a gene therapy clinical trial).
Below, we share insights from our year-long campaign of screening patients for AAV NAbs. The results Cure Rare Disease presents were done using a research grade neutralizing antibody assay in collaboration with the University of Massachusetts. Patients were enrolled in our IRB-approved research study and blood was drawn to measure the AAV NAb levels of two commonly used serotypes in muscular dystrophy gene therapies: AAV9 and AAVrh74. Results from these tests are presented as a ratio – the lower the number, the less AAV antibodies there are, the safer it generally is, ceteris paribus, to deliver AAV-based therapeutics. For instance, a patient with a titer of 1:20 to AAV9 has significantly less antibodies to AAV9 than a patient with a titer of 1:200 to AAV9. The threshold that companies choose to use for inclusion or exclusion criteria in a clinical trial vary from company to company. Some companies do not disclose the threshold and there is no standardization yet given the nascency of the field. For instance, based on publicly available information, Sarepta Therapeutics utilized a titer of >1:400 as an exclusion criteria in their trial SRP-9001 from several years ago which included 4 patients between the ages of 4 years to 7 years.
Here are the key takeaways from the Cure Rare Disease AAV Seropositivity Study after one year:
It should be noted that given the variability in NAb assays, it is difficult to compare across studies. Moreover, it is oversimplistic to look at NAbs in isolation – other relevant factors impact the ability for a gene therapy product to successfully transduce muscle fibers (in the case of muscular dystrophy). Manufacturing factors such as empty vs. full viral capsids are known to impact the immune response to a gene therapy. Nonetheless, even one patient with NAbs that result in an inability to receive a potentially life-saving gene therapy is too many. The data suggests that it is critical for industry and academia to share results and data in the quest to find appropriate go/no-go dosing thresholds. Moreover, it is critical that we act to enable patients who currently are seropositive to get access to gene therapy trials and eventually products. We must not forget these patients and patient-families. If we take the AAV9 seropositive percentage of patients and apply it to the DMD population, roughly 5,000 out of the 15,000 patients may be unable to receive an AAV9-based gene therapy. A recently reported number from Sarepta Therapeutic’s study cited an AAVrh74 seropositivity rate of 17% based on a titer threshold of 1:400. Applied to the DMD population, this is roughly 2,550 patients who may be unable to get an AAVrh74-based gene therapy. These findings show the critical need for a solution to antibody suppression to enable the successful delivery of a gene therapy product. One seropositive patient is simply too many.
For patients who have already been dosed with first-generation gene therapies, it is becoming increasingly clear, that re-dosing will be necessary sooner rather than later. Technologies such as plasmapheresis (filtering the blood) and other medicines working in coordination can help to suppress antibodies to create a temporary “window” for dosing. With an N=1 strategy (N being the number of patients), we have a greater degree of flexibility than approaching treatment for an entire population. As we reach our first clinical infusion, Cure Rare Disease will be deploying several of these technologies to help set the precedent for their use in the clinic and ultimately for their application to patients with high levels of NAbs.
If patients or patient families would like to learn more or enroll in Cure Rare Disease’s Seropositivity study, please find more information here.