AT ASH 2016
SAN DIEGO (FRONTLINE MEDICAL NEWS) – Novel microcapsules loaded with factor VIII outperformed systemic factor VIII infusions in a model of hemophilia A with inhibitory antibodies, Caroline E. Hansen reported at the annual meeting of the American Society of Hematology.
“This is a completely new paradigm that uses platelet biomechanics to target and deliver a drug,” Ms. Hansen said at a press briefing.
Hemophilia A patients with inhibitors are difficult to treat because antibodies neutralize systemically delivered factor VIII, she noted. Alternative therapies are problematic – immune tolerance induction therapy is costly and only marginally effective, and bypassing agents can cause thrombosis. Therefore, Ms. Hansen and her associates at the Georgia Institute of Technology in Atlanta developed microcapsules loaded with factor VIII that have fibrinogen on their exterior so they can adhere to platelets. These platelet-hybridized microcapsules are designed to circulate inertly until they reach a site of vascular injury, where platelet activation and contraction causes them to rupture and release factor VIII into the growing clot before inhibitory antibodies can target it.
To create the microcapsules, the investigators deposited alternatingly charged layers of polyelectrolytes, poly-L-lysine, and poly-L-glutamic acid onto a calcium carbonate core covered with factor VIII and dextran. They added fibrinogen to the final polyelectrolyte layer and then chelated out the innermost core, leaving the dextran layer as a shield between factor VIII and the outside of the microcapsule. Initial in vitro experiments showed that the microcapsules adhered to platelets and were incorporated into fibrin networks when platelets were activated, Ms. Hansen reported. Because the microcapsules only ruptured upon platelet contraction, factor VIII was only delivered to actively forming clots as intended, she added.
As a next step, the researchers perfused recalcified whole blood and platelet-poor plasma into a collagen and tissue factor patch designed to mimic vascular injury, and then measured fibrin fluorescence on the patch. Microcapsules lacking dextran, fibrinogen, or loaded factor VIII did not work – a treated sample and a phosphate-buffered saline (PBS) control yielded statistically similar fibrin production. However, complete microcapsules loaded with 0.01 U/mL factor VIII produced four times more fibrin than systemic infusion of 0.05 U/mL factor VIII.
“These were really promising results but we want to take a step back and see if a clot would form in the presence of inhibitory antibodies,” Ms. Hansen said. Accordingly, they added factor VIII inhibitory antibody 2-76 into blood samples from healthy donors. The microcapsules triggered 2.7 times more fibrin production in this setting than systemic treatment did (P less than .05). “This increased efficacy is likely due to the microcapsule shielding effect on factor VIII, preventing exposure to inhibitory antibodies,” Ms. Hansen and her associates concluded in their abstract .
The investigators are now studying the extent to which the microcapsules induce thrombin production, and how agents such as blebbistatin, ROCK, and myosin affect platelet contraction force and the efficiency of the microcapsule.
Ms. Hansen had no disclosures.