Drug delivery by red blood cells: vascular carriers designed by mother nature

Abstract

Importance of the field: Vascular delivery of several classes of therapeutic agents may benefit from carriage by red blood cells (RBC), for example, drugs that require delivery into phagocytic cells and those that must act within the vascular lumen. The fact that several protocols of infusion of RBC-encapsulated drugs are now being explored in patients illustrates a high biomedical importance for the field. AREAS COVERED BY THIS REVIEW: Two strategies for RBC drug delivery are discussed: encapsulation into isolated RBC ex vivo followed by infusion in compatible recipients and coupling therapeutics to the surface of RBC. Studies of pharmacokinetics and effects in animal models and in human studies of diverse therapeutic enzymes, antibiotics and other drugs encapsulated in RBC are described and critically analyzed. Coupling to RBC surface of compounds regulating immune response and complement, affinity ligands, polyethylene glycol alleviating immune response to donor RBC and fibrinolytic plasminogen activators are described. Also described is a new, translation-prone approach for RBC drug delivery by injection of therapeutics conjugated with fragments of antibodies providing safe anchoring of cargoes to circulating RBC, without need for ex vivo modification and infusion of RBC.

What the reader will gain: Readers will gain historical perspective, current status, challenges and perspectives of medical applications of RBC for drug delivery.

Take home message: RBC represent naturally designed carriers for intravascular drug delivery, characterized by unique longevity in the bloodstream, biocompatibility and safe physiological mechanisms for metabolism. New approaches for encapsulating drugs into RBC and coupling to RBC surface provide promising avenues for safe and widely useful improvement of drug delivery in the vascular system.

Figure 1. Examples of therapeutic sites and targets accessible for intravascular drug delivery using RBC carriers

Large, long-circulating RBC have an access and can deliver their cargo to targets in blood, including other blood cells (lymphocytes, white blood cells WBC), and pathogenic agents such as bacteria and toxins, to sites of vascular injury and endothelial cells lining vascular lumen (EC), and to diverse components of reticuloendothelial system (RES), such as macrophages in liver and spleen.

Figure 2. Strategies for coupling therapeutic agents to RBC surface

This schema uses plasminogen activators (PA, see section 3.5) to illustrate methods for coupling drugs to RBC, yet these principles are applicable to a wide variety of therapeutics. A. Direct coupling of drugs (PA) to RBC using covalent or non-covalent cross-linking agents is a prototype paradigm that involves modification of isolated or donor RBC followed by infusion in a patient. This paradigm is especially attractive for pursuing PEG-stealth RBC approach (Section 3.2). B. Non-covalent binding of a drug to its receptor conjugated to RBC may provide a more physiological anchorage some biotherapeutics and provide an additional modality for functional regulation of their activity (see an example of coupling urokinase uPA to its receptor suPAR coupled to RBC in Section 3.5.4). C. Conjugation of a drug with fragments of antibodies binding to RBC avoids ex vivo manipulations with RBC, eases administration and dosing, enhancing clinical feasibility (Sections 3.4 and 3.5.4).

Figure 3. A concept of prophylactic fibrinolysis by scFv/tPA targeting to RBC

(A). Plasminogen activators (dots) are relatively ineffective, in part due to rapid uptake by liver, and unsafe due to bleeding (indiscriminate lysis of hemostatic mural clots), vascular side effects (e.g. activation of receptors on endothelial cells, EC) and injurious effects of tPA diffusing into the CNS. (B) Coupling to RBC will dramatically prolong the longevity of scFv/tPA variant. RBC will restrain scFv/tPA binding to cellular receptors, and restrict its access into mural hemostatic clots and the CNS. Propulsion of RBC towards the mainstream will further offset interactions of the pro-drug with hemostatic clots and vascular walls. RBC-bound scFv/tPA will have virtually unlimited access to the interior of nascent pathological thrombi and thereby will dissolve pathological intravascular clots and prevent vascular occlusion.