Biomechanical and Functional Features of the Carrier Erythrocytes Prolonging Circulation Time of Biotherapeutic Targeted to Glycophorin A

Red blood cells (RBCs) serve as natural transporters and can be modified to enhance the pharmacokinetics and pharmacodynamics of a protein cargo. Affinity targeting of Factor IX (FIX) to the RBC membrane is a promising approach to improve the (pro)enzyme's pharmacokinetics. For RBC targeting, purified human FIX was conjugated to the anti-mouse glycophorin A monoclonal antibody Ter119. The goal of this study was to characterize the activity of the FIX-Ter119 conjugate and efficacy of its loading on RBCs, as well as to investigate the biodistribution, pharmacokinetics, and various biological properties of the loaded RBCs. Mouse RBCs were incubated with the Ter119-FIX conjugate, where adding 10,000 molecules per RBC resulted in 37% binding (4K/RBC), and 50,000 molecules per RBC resulted in 34% binding (17K/RBC). The pharmacokinetics (PK) profile showed that more than 90% of the Ter119-FIX conjugate was associated with RBCs and circulated stably bound to the RBCs for 24 h, increasing the area under the PK curve 7.6 times vs free FIX. Ter119-FIX loaded RBCs have specific procoagulant FIXa activity, including promotion of thrombin generation and acceleration of clotting in FIX-deficient plasma. Morphological characterization shows that Ter119-FIX-loaded RBCs undergo a shape change, with an increased fraction of echinocytes and spheroidal RBCs. Ektacytometry and electron microscopy assessment of RBC compressibility reveal a dose-dependent reduction in the deformability of RBCs loaded with Ter119-FIX. In conclusion, RBCs loaded with Ter119-FIX have the potential to serve as prohemostatic agents, but their reduced deformability warrants further engineering of Ter119-FIX to improve the safety profile.