A gene therapy approach using a so-called antibody-drug conjugate (ADC) conditioning regimen led to safe and sustained production of factor VIII (FVIII) in platelets, and prevented joint bleeding in a mouse model of hemophilia A, according to new research.

The study, “Nongenotoxic antibody-drug conjugate conditioning enables safe and effective platelet gene therapy of hemophilia A mice,” appeared in the journal Blood Advances.

Prior work in mice showed that stem cell-based gene therapy specifically targeting platelets leads to the production of FVIII — the clotting protein missing or defective in people with hemophilia A — and induces immune tolerance (no development of exacerbated immune response).

However, the procedure required a conditioning regimen with chemotherapy or total body irradiation (TBI) that may be toxic to genetic material (or genotoxic), so patients may not be agreeable to using the protocol.

Safer approaches may come from ADCs, which use antibodies against cell surface proteins for more specific targeting of cell populations.

A team of researchers from the Blood Research Institute, in Milwaukee, tested ADC-based conditioning with stem cell-based F8 gene therapy that targets platelets in a mouse model of hemophilia A. This ADC consists of saporin — a plant-derived toxin that halts protein production — bound to antibodies specific for the CD45.2 and CD117 cell surface proteins.

The scientists found that, three weeks after hematopoietic stem cell transplant (HSCT), peripheral blood counts were higher with ADC conditioning than with TBI. This suggested a lower risk of cytopenia (reduced number of mature blood cells), the investigators said.

Then they observed that the new conditioning regimen led to effective engraftment, the process through which transplanted stem cells establish themselves in the bone marrow and start producing new blood cells. At 20 weeks after transplant, all mice receiving the ADC regimen had more than 15% of donor-derived white blood cells.

Sixteen weeks after HSCT, the number of copies of therapeutic genes was similar in the mice receiving ADC and the controls on TBI. Two-thirds of the mice receiving the viral-delivered gene therapy under ADC conditioning showed sustained increases in FVIII levels in platelets.

The ADC regimen also led to increasing reconstitution of platelets and white blood cells, meaning a higher percentage of cells derived from donors.

Subsequent experiments showed that ADC also targeting the CD8 protein led to significantly higher white blood cell reconstitution, frequency of regulatory T-cells (which dampen excessive immune responses), and FVIII levels in platelets than the regimen specific for CD117 and CD45.2 only.

These effects were sustained long-term, as found when transplanting bone marrow cells from animals that had undergone HSCT to new mice under the same conditioning regimen.

Blood clotting was normalized with this gene therapy approach, and hemoglobin levels were higher than in mice with no FVIII. Joint bleeding and limping were effectively prevented in a knee joint injury model. Also, no animal with sustained FVIII expression in platelets developed anti-FVIII inhibitors.

“The central finding of this report is that platelet-directed HSC-based FVIII gene therapy is safe and effective for eliminating [hemophilia A] using nongenotoxic hematopoietic-targeted ADC conditioning,” the researchers wrote.

“This safe and effective treatment strategy could be especially meaningful for [hemophilia A] patients who are especially wary of standard preconditioning,” they added.