Many blood and immune disorders could be cured by transplanting healthy blood stem cells from a matched donor. But first the patients need a pretreatment procedure to eliminate their own blood stem cells, making room in the bone marrow for the donor cells to take their place.
The problem is that the standard pretreatments—chemotherapy or radiation—are very toxic. Doctors don’t want to give them to vulnerable children, such as those with a rare genetic disorder called severe combined immunodeficiency (SCID).
Infants with SCID have compromised immune systems that struggle to fight off even common infections caused by viruses and fungi. These babies have many chronic and life-threatening problems, including frequent lung infections, chronic diarrhea, and recurrent sinus infections.
“Without treatment, SCID infants usually die from infections within the first two years of life. Blood stem cell transplants are the only definitive cure for this disease,” says Judith Shizuru, MD, PhD, professor of blood and marrow transplantation and cellular therapy and of pediatrics. “But transplants usually involve chemotherapy, and we don’t want to give these agents to these children because they’re particularly susceptible to the damaging short-term and long-term effects—including growth defects, neurological problems, and increased risk of cancers. This is especially true for certain subtypes of SCID.”
Instead, SCID patients are often given a blood stem cell transplant without pretreating with chemotherapy to create space in their bone marrow. But then the donors’ self-renewing blood stem cells may not fully engraft, so the kids can’t robustly regenerate their immune systems. These children have to rely on regular intravenous immunoglobulin infusions to boost their immune response, and the effectiveness of donor immune cells can wane over time.
The great need for a less toxic pretreatment for blood stem cell transplants inspired Shizuru to initiate a Stanford study testing a novel antibody pretreatment in SCID patients—in collaboration with Rajni Agarwal-Hashmi, MD, associate professor of pediatrics, and other stem cell transplantation and regenerative medicine specialists at Stanford and UC-San Francisco.
Targeting Blood Stem Cells
The novel pretreatment uses the JSP191 antibody to target a protein called CD117, found on the surface of blood stem cells. The antibody binds to this protein, which then blocks CD117 from binding to a stem cell factor critical for keeping blood stem cells alive. When the interaction between CD117 and the essential stem cell factor is interrupted, the patient’s blood stem cells are depleted—making space for the donor’s healthy cells to engraft.
“It’s not like chemotherapy or radiation,” says Shizuru. “It’s a targeted way to deplete the blood stem cells without damaging normal healthy cells.”
The Stanford team chose SCID patients for their first human JSP191 clinical trial in part because these children have a unique biology—they lack lymphocytes, so they are less likely to immunologically reject the blood stem cells from a donor. Since immune suppressive medications aren’t necessary, the researchers can more easily see if the antibody therapy clears space in the bone marrow and the transplant works.
Initially, the clinical trial studied older children and adults with SCID whose first blood stem cell transplant had failed, so that they could evaluate whether JSP191 therapy was safe and well tolerated. The participants ranged in age from 3 years old to mid-30s, but most were between 11 and 13 years old. According to Shizuru, many of these kids had chronic infections and also wanted to be liberated from having intravenous immunoglobulin infusions.
The results are very promising, as Shizuru reported in 2019 at the American Society of Hematology annual conference. The antibody safely created room in the patients’ bone marrow, allowing healthy donor stem cell engraftment without common side effects like transfusion reactions, treatment-related toxicities, or bone marrow suppression.
“The wonderful thing about the antibody JSP191 is it’s super-safe. This conditioning agent doesn’t affect the DNA or any other organ, as far as we can tell,” explains Shizuru. “We give it as a onetime, really low dose. And it’s not showing any side effects. It’s an amazing drug.”
The study’s clinicians even remarked that the re-transplant kids looked bored in the hospital because the expected complications didn’t happen, says Shizuru. “The patients’ counts didn’t drop. They didn’t have increased infections. They didn’t need blood transfusions,” she says. “So, we decided to give the antibody as an inpatient treatment and then do everything else as outpatient after 48 hours.”
The results were promising from the start. The first participant pretreated with JSP191 was a 3-year-old girl with chronic diarrhea and infections. After about a year, she no longer had diarrhea and started going to school for the first time. In fact, her family was infected with COVID-19 and she did fine, as Shizuru learned during a public discussion.
Expanding the Clinical Trial
Based on the safety and success of the first phase, the JSP191 trial expanded to include infants newly diagnosed with SCID. Two infants have received the antibody pretreatment followed by a blood stem cell transplant.
The first infant did really well, demonstrating signs that his donor cells may fully restore his immune function. The second infant’s response was more complicated; the researchers determined that she had some immune function that may have rejected the maternal stem cells. She subsequently underwent another transplant without the antibody agent, using a mix of chemotherapies.
After their initial success, Shizuru’s team expanded the use of JSP191 to include other vulnerable populations—older adults with acute myeloid leukemia (AML) or myelodysplastic syndromes (MDS). AML is a type of leukemia in which DNA mutations cause the rapid growth of abnormal cells that build up in the bone marrow. Although it starts in the bone marrow, AML often quickly moves to the blood and sometimes spreads to other parts of the body. MDS are a group of diverse bone marrow disorders in which the bone marrow does not produce enough healthy blood cells. Both AML and MDS primarily occur in people over 65 years old.
This adult study is based on the preclinical work of Wendy Pang, MD, PhD, who was a postdoctoral fellow in the Shizuru laboratory. She showed that the disease-causing MDS and AML stem cells express CD117, so they can be targeted by JSP191. Further, the team observed synergistic eradication of stem cells when these anti-CD117 antibodies were combined with low-dose radiation.
The ongoing clinical trial utilizing JSP191 combined with low-dose radiation is led by Lori Muffly, MD, assistant professor of blood and marrow transplantation and cellular therapies. The preliminary results are encouraging based on the first six participants, who were older adults (64–74 years old) with AML or MDS. The researchers observed no side effects associated with JSP191, and the patients’ blood stem cell transplants were successful.
“We transplanted our first SCID babies and then opened the trial up to older patients with AML and MDS. So, now we’re covering the full spectrum for this targeted therapy: from a 3-month-old infant with SCID to a 74-year-old with AML,” Shizuru says.
The JSP191 project has now moved to a biotechnology company, Jasper Therapeutics. Shizuru expects that in the future, the studies will expand to include sickle cell disease, a group of inherited red blood cell disorders, where the JSP191 antibody can help to engraft the donor cells.
“In terms of pretreatment, there’s been no innovation on transplant agents in decades. People have been innovating on transplant by simply reducing the dose of chemotherapies, but we haven’t seen a successful new agent,” explains Shizuru. “The development of JSP191 leverages our understanding of the biology of blood stem cells by targeting a critically important molecule. JSP191 antibody is now the platform agent.”
This is a reposting of my feature article in the recent Stanford Medicine Annual Report.