Biological Cells as Therapeutic Delivery Vehicles

Abstract

One of the significant challenges remaining in the field of drug delivery is insufficient targeting of diseased tissues or cells. While efforts to perform targeted drug delivery by engineered nanoparticles have shown some success, there are underlying targeting, toxicity, and immunogenicity challenges. By contrast, live cells usually have innate targeting mechanisms, and can be used as drug-delivery vehicles to increase the efficiency with which a drug accumulates to act on the intended tissue. In some cases, when no native cell types exhibit the desired therapeutic phenotype, preferred outcomes can be achieved by genetically modifying and reprogramming cells with gene circuits. This review highlights recent advances in the use of cells to deliver therapeutics. Specifically, we discuss how red blood cells (RBCs), platelets, neutrophils, mesenchymal stem cells (MSCs), and bacteria have been utilized to advance drug delivery.

Keywords: drug delivery; synthetic biology; therapeutic cells.

Figure 1:. Biological cells as therapeutic delivery vehicles.

Various cell types can be used for therapeutic delivery. The choice of cell type is important for different applications. Anucleate cells (red) include red blood cells and platelets. These cells have no nucleus and a wide biodistribution (pro), however, they are non-dividing cells which makes them incapable of expanding in vitro without manipulation (con). Mesenchymal stem cells (MSCs) (blue) are innately regenerative, immunomodulatory, and can target injured tissue (pros), however, when MSCs are injected into the body, they are rapidly cleared and cancer cells have been shown to hijack MSCs for their own benefit (cons). Bacteria (green) are easy to manipulate, they double every 45 minutes, and they can penetrate tumors (pros), however, they can also disrupt the native microbiome and elicit immune responses (cons). Neutrophils (purple) are natural responders to sites of inflammation and are usually the first cells on site for repair (pros), however neutrophils are short-lived and they have poor survival after isolation, preventing their expansion in vitro (cons).

Figure 2:. Anucleate Cells as Delivery Systems.

Engineered RBCs are surface modified (upper) by expressing an enzyme recognition site (ERS) on the RBC surface, enabling the covalent tethering of an array of functional molecules to the RBC surface. This endows RBCs with various therapeutic molecules, denoted by the yellow diamond attached to an RBC receptor. Drug molecules can also be conjugated to antibodies, antibody fragments, and peptides, respectively, to native RBC surface molecules. Intracellular encapsulation of therapeutics in RBCs is achieved through hypotonic loading of pro-drugs into the RBC. Once inside, the native RBC machinery converts prodrugs into active forms that freely diffuses out of the membrane. Enzymatic proteins like L- Asparaginase are also loaded into RBCs using this method to reduce aberrant levels of substrate throughout the body. Additionally, non-membrane associated enzymatic proteins like phenylalanine ammonia lyase are expressed in RBCs where they act in concert as a miniature bioreactor to breakdown toxic substrates in circulation. Biomimetic design of RBCs (lower left) includes using native RBC membranes to cloak nanoparticle (NP) cores (left) and synthetic RBCs that mimic native RBC shape, size, and deformability used to transport oxygen or drug molecules (right). Engineered platelets are surface modified (upper) to express TRAIL and PD-1 as anti-cancer therapeutics. Intracellular encapsulation (middle) of therapeutics in platelets is achieved via diffusion of free doxorubicin (DOX) into the intracellular platelet compartment. Additionally, platelets have been engineered to express intracellular platelet factor VIII (pFVIII) that gets released upon platelet activation. Biomimetic design of platelets (lower) includes coating NP cores with native platelet membrane (left) and use of heteromultivalent, surface-decorated liposomes marketed as SynthoPlate (right). VBP = von Willebrand binding peptide; CBP = collagen binding peptide; FMP = fibrinogen mimetic peptide.

Figure 3:. Harnessing the innate properties of mesenchymal stem cells for drug delivery.

Mesenchymal stem cells (MSCs) are multipotent adult stem cells capable of self-renewal and differentiation into myocytes, osteocytes, adipocytes, chondrocytes, and fibroblasts. The paracrine function of MSCs them their primary therapeutic functions that can be leveraged for the delivery of therapeutics. Using paracrine cues, MSCs are able to modulate the immune system and generally favor anti-inflammatory phenotypes. This innate property of MSCs is beneficial during wound healing and treating cancer.

Box 1, Figure I:. Hematopoiesis.

Hematopoietic stem cells have the potential to become all of the cells of the blood system. They can become all of the cells of the immune system, or anucleate cells red blood cells and platelets (black box).