Cell-based Gene Therapy May Offer Long-term Treatment for Hemophilia A, Mouse Study Suggests

Cell therapy with endothelial progenitor cells and stem cells genetically engineered to produce a functional clotting factor VIII (FVIII) may provide a stable and long-term treatment for hemophilia A, a mouse study has found.

The study shows that the transplant of cells from human placenta and umbilical cord blood, engineered to express FVIII, was able to relieve symptoms, namely a shorter bleeding time, in mice with a hemophilia A-like disorder.

An important finding was that the transplant of both cell types at the same time enabled a stable and long-lasting engraftment of cells in mice.

This may have important implications for people with hemophilia A, as cell-based therapies hold promise for this bleeding disorder but have been hampered by the fact that transplanted cells do not remain inside the human body for a long enough period of time.

The study, “Potential long-term treatment of hemophilia A by neonatal co-transplantation of cord blood-derived endothelial colony-forming cells and placental mesenchymal stromal cells,” was published in the journal Stem Cell Research & Therapy.

The current standard of care for hemophilia A is protein replacement therapy, which consists of frequent infusions of FVIII protein concentrates, derived either from human plasma or produced in the lab by DNA technology, known as recombinant FVIII (e.g., Bayer’s Kogenate and Kovaltry).

Despite the benefits to life expectancy and quality of life brought by these therapies, their high cost, need for lifelong infusions, and inability to maintain stable of clotting factor levels make them far from ideal.

New treatments are needed to overcome these limitations, and gene therapy has been considered a promising alternative, given that hemophilia A stems from single-gene defects, namely in the F8 gene.

A number of candidates, such as SB-525 by Sangamo Therapeutics and Pfizer; AMT-180 by uniQure; and BioMarin’s valoctocogene roxaparvovec, or BMN 270, are under development and clinical testing. All of them are based on a viral carrier called an adeno-associated virus (AAV) vector, meant to deliver a functional version of the F8 gene to the patient’s cells.

But the long-term effectiveness of AAV-based gene therapies are still unknown, and about one-third of patients are not suitable due to prior contact with AAV viruses, meaning they have the potential to develop an immune response against the carrier.

One other possibility is to use cell-based gene therapies where a functional F8 gene is first introduced in target cells outside the patient’s body, and then those engineered cells are transplanted into the patient.

Two cell types, called placenta-derived mesenchymal stromal cells (PMSCs) and endothelial colony-forming cells (ECFCs), have been considered candidates of choice for this type of therapy.

ECFCs are cells that give rise to endothelial cells, the ones that line blood vessels, the primary source of the FVIII factor in our body. Once isolated in the lab, ECFCs can quickly replicate and acquire the features of endothelial cells.

PMSCs are a type of mesenchymal stem cells, a group of cells “well established as a stem cell therapy product for a wide variety of diseases and conditions,” according to the researchers

One of the difficulties delaying the success of this treatment is that the transplanted cells produce the clotting factor inside the patient’s body for too short a period of time.

In this study, the researchers tested a new double transplant protocol to see if it would improve the long-term engraftment of clotting factor-producing cells for the treatment of hemophilia A.

First, they collected PMSCs from discarded human term placenta and ECFCs from umbilical cord blood. These cells were then genetically engineered with a version of F8 to express a functional FVIII, using lentiviral-based gene therapy.

The genetically modified cells were transplanted alone or in combination into immune-deficient mice to check which one of the regimens allowed cells to last longer inside the body.

The results showed that the co-transplant of PMSCs and ECFCs resulted in the best and longer transplants, stably producing a functional FVIII inside the mice’s bodies over at least six months. In addition, the transplant was more successful if performed a few days after birth, compared with adults.

Based on that observation, the researchers tested the combined PMSC-ECFC transplant in a mouse model of hemophilia A, which had less than 1% of normal FVIII activity. They treated post-natal mice who were 2 weeks old.

Importantly, the co-transplant was able to reduce by more than two-thirds the volume of blood lost by mice after a cut, showing it made their blood clot faster.

“This work demonstrated that co-transplantation of ECFCs with PMSCs at the neonatal age is a potential strategy to achieve stable, long-term engraftment, and thus holds great promise for cell-based treatment of [hemophilia A],” the researchers concluded.