Editor’s Note: Jacqueline Hunter, PhD, a postdoctoral fellow in the Wolfe Laboratory, wrote this article as part of the Advanced Career Exploration (ACE) Fellowship at Children’s Hospital of Philadelphia Research Institute. The ACE program gives fellows the opportunity to pursue projects beyond their main research focus. We’re especially excited to share Dr. Hunter’s work during National Postdoc Appreciation Week.
Normally, the body responds to a bleeding event by forming a clot, which is a complicated process involving multiple proteins in an elegantly orchestrated cascade. When specific proteins of this cascade are absent, one of several debilitating disorders can occur that result in recurrent spontaneous bleeding into the joints and muscles. Researchers at Children’s Hospital of Philadelphia are using clever maneuvering to figure out new therapeutic options for patients with hemophilia A and B.
Developing a ‘FIX’ for Hemophilia B
Hemophilia B is a X-linked bleeding disorder that occurs when factor IX (FIX), a clotting protein, is missing or defective. It’s a condition that occurs predominantly in males, at a rate of one in 25,000 to 30,000 births worldwide. In the current standard of care, patients are treated depending on the severity of disease by intravenous infusion of concentrated FIX product one to three times a week, or as needed, in response to bleeds. For patients with severe or moderate disease, these treatments do not completely prevent the medical complications that are associated with the hemophilia B. They experience decreased mobility as they age, and they often become wheelchair bound.
Over the last two decades, CHOP researchers have been developing a gene therapy to allow the patient’s body to produce its own FIX. This approach utilizes a recombinant viral vector, such as adeno-associated virus (AAV), to deliver the missing FIX gene, giving the body the blueprint it needs to produce the protein.
While some success was seen with early AAV-mediated gene transfer studies, early clinical trials resulted in either low expression of FIX or short-lived expression due to removal of virus by the patient’s immune system. Continued endeavors sought to develop a method of producing long-lasting expression of FIX. The serendipitous discovery in a family in Padua, Italy, of factor IX-R338L (FIX-Padua), a variant of FIX with eight to 12 times higher activity than the normal protein, was critical to the therapeutic success of the current gene therapy trials. Valder Arruda, MD, PhD, an associate professor in the Division of Hematology at CHOP, was a key member of the team of investigators that developed FIX-Padua to be used in gene therapy studies for hemophilia B.
“What brought me to the field was the desire to find a therapeutic approach that could be simple, done a few times, and then follow the patient,” Dr. Arruda said. “So over the years, we developed a series of discoveries, and now we are very happy to say that we can get levels of factor IX that change the phenotype of severe hemophilia to a mild one.”
The higher intrinsic activity of FIX-Padua allowed investigators to drastically reduce the amount of vector to a level that would hopefully not prompt an immune response in patients. Preclinical successes led this therapy, now named SPK-9001, to be further developed by Spark Therapeutics in collaboration with Pfizer, as part of their SPK-FIX program. The U.S. Food and Drug Administration (FDA) awarded SPK-9001 with “orphan product” and “breakthrough therapy” designations in 2015 and 2016, respectively, allowing for expedited drug development.
Therapeutic Success of SPK-9001 in Phase I/IIa Studies
Researchers have completed a preliminary multicenter phase I/IIa study to assess the safety of SPK-9001. Overseeing this study is Lindsey George, MD, attending physician in the Division of Hematology at CHOP. For this study, 10 men ages 18 to 53 years were enrolled across four institutions. This group represented a population of patients with varied disease severity, varying degrees of joint disease, and some individuals with liver damage due to hepatitis C virus.
“Hemophilia is a disease that’s actually really nice to study clinically,” Dr. George said, “because you’re missing a protein, and we can measure that protein by a simple blood draw, and we know what levels of expression of that protein mean clinically.”
The patients enrolled in the study had severe (<1 percent of normal activity) or moderate (1 to ≤2 percent of normal activity) disease. The therapeutic goal was to increase factor IX activity to at least 5 percent of normal activity. The average level of factor IX observed in patients in the trial was around 35 percent. At this level of expression, incidents of breakthrough bleeding were drastically reduced, allowing these patients to stop their prior treatments. In addition, the study team did not observe serious adverse events.
“It’s 10 patients, but they’re actually pretty heterogeneous in their clinical backgrounds, and pretty homogeneous in their response to the therapy,” Dr. George said. “I think it’s highly likely that what we’ve seen in the phase I/II trial is going to translate to phase III.”
Dr. George presented these results at the 58th American Society of Hematology (ASH) Annual Meeting, and they appeared in the New England Journal of Medicine in December 2017. SPK-9001 is now moving into phase III clinical trials, bringing this novel therapeutic to a larger population of hemophilia B patients. If SPK-9001 continues to be safe and effective in a larger population, this therapy could be a game-changer for patients with hemophilia B. Patients would have the option of a one-time treatment, instead of repeated infusions of FIX, to either prevent or control bleeding episodes, which will hopefully prevent or reduce the joint damage caused by bleeds as the patients age.
“What’s neat about these patients on the trial is we get to know them really well because the follow-up is very intense,” Dr. George said. “And they all come back, and they tell me these really neat stories of things they’re doing that they weren’t doing before … It really keeps you in perspective.”
The study team hopes to translate this success to a more recently initiated Phase I/IIa hemophilia A gene therapy trial of SPK-8011.
Just as every child is unique, with different personalities and talents, sometimes a treatment that works well for one child does not work for another. Such is the case in hemophilia.
“I always felt like you need to have more options, because if you have three, four options, if you don’t fit very well with this, you fit with that,” Dr. Arruda said. “In my clinical years, I never saw a drug that everyone responds to the same way. There’s always a rare case that does something different. So therefore you need to be ready for that.”
While there has been significant progress in advancing hemophilia treatment, for a subset of patients, one problem that remains is the formation of antibodies, called inhibitors, to the replacement clotting factor. The patient’s own immune system generates inhibitors to attack the clotting factor protein, as if it were a virus or bacteria. This causes the life-saving factor to be removed from the body before it can treat a bleed. The presence of inhibitors, which occurs in about 30 percent of hemophilia A cases and about 5 percent of hemophilia B cases, complicates treatment and increases the likelihood of developing medical problems due to disease progression.
Dr. Arruda’s lab is developing strategies to combat, or circumvent, inhibitor formation that can occur during infusion therapy. A novel approach for hemophilia A targets the immune cells responsible for the production of inhibitor by using chimeric antigen receptor (CAR) therapy, in collaboration with Michael Milone, MD, PhD, anassociate professor of Pathology and Laboratory Medicine at of the University of Pennsylvania. Part of this study, which is supported by a grant from the National Heart, Lung, and Blood Institute, is to use CAR T-cells (CAR-T) to identify and eliminate specific populations of B cells that may be responsible for the production of inhibitory antibodies. If successful, this approach may be a new way to induce immune tolerance, without the limitations of ITI.
In addition to using CAR-T to identify the B cells responsible for making inhibitor, Dr. Arruda plans to reverse the CAR concept. Instead of recognizing and eliminating an entire population of B cells regardless of whether they are responsible for producing inhibitor, T cells are genetically modified to express the specific parts of the factor VIII protein that inhibitors are known to target. This would allow the T cells to zone in on the B cells producing inhibitor, while sparing the B cells that are not producing inhibitor. Since this approach uses a chimeric alloantibody, Dr. Arruda calls the reverse CAR approach “CALLAR.”
“The interesting thing is ‘callar’, in my mother language, Portuguese, it means ‘shut up’. So I thought, that’s what we want to do. ‘Shut up’ the B cells. That’s why we used the nickname CALLAR.”
If successful, the CALLAR approach may be beneficial to not only hemophilia A patients with inhibitor, but also patients receiving similar types of therapy for other diseases. Just as in the case of hemophilia, formation of inhibitors is an ongoing challenge in several diseases that require enzyme replacement therapy, such as Pompe’s disease.
Finding a Bypass
If a patient develops antibodies to factor VIII or IX, another strategy currently used to overcome this is to trigger coagulation by using an alternative pathway in the clotting cascade. Administering a different factor entirely does this, and this factor is referred to as a bypass agent. Activated factor VII (factor VIIa) is a bypass agent that has been FDA approved since 1999 for use in hemophiliacs with inhibitors. In this case, formation of antibodies is not an issue, since the patient already produces factor VII. While this strategy is effective, how exactly factor VIIa works in this situation is still controversial.
To track down an answer to this question, Paris Margaritis, DPhil, a researcher in the Division of Hematology at CHOP, and his group are looking at a receptor that is normally involved in the anti-coagulant response. These studies, which have been funded by multiple Bayer Hemophilia Awards Program (BHAP) grants, aim to investigate the interaction of factor VIIa with the endothelial protein C receptor (EPCR), and the role this interaction plays in the clotting response. Interestingly, EPCR is part of the mechanism the body uses to prevent clotting. However, studies done by Dr. Margaritis’ group in a mouse model of hemophilia demonstrated that while infusion of factor VIIa acts as a procoagulant in response to a bleeding event, infusion of a factor VIIa variant that is unable to interact with EPCR is not as effective in controlling a bleed.
“We know this interaction is important, but we really don’t know how it actually comes about,” Dr. Margaritis said. “So if this interaction is truly facilitating this, then we may be able to use it to manipulate the actual molecule to make it work in a better way.”
The next step, and one of the goals of their current BHAP grant, is to address this question by visualizing how a clot is formed in real time. Using intravital microscopy and fluorescent reagents, the group will be able to determine what affect the factor VIIa-EPCR interaction has on clot formation. The hope is that they will be able to shed some light on some of the interesting clinical observations that have been made on prophylaxis with factor VIIa in hemophilia — that the clinical benefit of this molecule was able to persist for three months post-prophylaxis, despite having very short half-life. The factor VIIa-EPCR interaction may also play a role there.
“We believe this may be something that, apart from learning from it, we can also use the information to go back to a translational approach where we can design new molecules and maybe make the current therapy with factor VIIa more efficient,” Dr. Margaritis said. “So what really excites me is that we have our hands on both basic science and kind of a direct path to more translational aspects of the work.”