Scanning EM of a coronary artery thrombus taken from a patient who had a heart attack. Fibrin fibers are brown; platelet aggregates are gray; red blood cells in red; leukocytes in green. John Weisel
What could be better than a natural, compatible, long-lived cell for delivering drugs to other cells in the body? Red blood cells fit this description and could be ideal at carrying drugs to specific targets. They play well with cells of the immune system, circulate for hundreds of days in the body; and can be loaded with all manner of cargo: peptides, modified full proteins, or small organic molecules.
In the 1980s, during the early stages of development of this field, HIV-related safety issues associated with blood transfusion forced many investigators to focus on synthetic carriers instead of red blood cells (RBCs). Nevertheless, several groups in the US and Europe are now working to take advantage of the unique properties of RBCs, which are much more compatible with the body than any synthetic carrier.
A number of conventional and new drugs can be encapsulated into human RBCs by several procedures. Once re-infused into the same cell donor, the circulating RBCs behave as a prolonged drug delivery system. Two European companies are conducting clinical trials with RBCs loaded with enzyme replacement therapies and other drugs including anti-inflammatory and anti-tumor agents.
An alternative approach for using RBCs as drug carriers is to, instead of loading drugs into RBCs and infusing the modified RBCs back into a patient, modify the drugs in a way so that they bind to circulating RBCs after injection.
Vladimir R. Muzykantov, MD, PhD, professor of Pharmacology and Medicine, at the Perelman School of Medicine, is devising a recombinant version of thrombomodulin -- a natural anticoagulant present in human blood vessels -- that can be injected in the bloodstream of animals, where it binds to RBCs and circulates for a long time as a prodrug that gets activated at sites of thrombosis, preventing closing up of blood vessels.
“This is a paradigm-shifting approach to immediate, short-term thromboprophylaxis that promises to improve treatment for conditions associated with acute blood clotting such as myocardial infarction and stroke,” notes Muzykantov.
Blood clots are a three-dimensional network of fibers, made up primarily of the blood protein fibrinogen, which is converted by the enzyme thrombin to the stiffer fibrin during clotting. Thrombomodulin is the protein on the surface of cells facing the inside of vessels and is the receptor for thrombin.
“The miracle is, once bound to thrombomodulin, thrombin no longer activates coagulation, but rather inhibits this pathological process, and also suppresses inflammation,” explains Muzykantov. “The problem is thrombomodulin’s activity is suppressed in certain conditions including ischemia, sepsis and inflammation, which all can predispose a person to thrombosis. “
But, a complicating factor is that intervening by injecting soluble recombinant thrombomodulin is short-lived, since it is rapidly cleared in the blood stream.
“To overcome this problem, we fused thrombomodulin with a single-chain fragment of an antibody targeted to red blood cells,” explains Muzykantov. This addition to thrombomodulin binds it to circulating RBCs without damage. This prolongs its time in circulation. Now, thrombomodulin is durable, anti-thrombotic, and anti-inflammatory, and paradoxically, mediated by pro-thrombotic thrombin.
“So, we have converted a foe into a friend, at least at the molecular level.”
In animal models in which the large and small blood vessels are severely constricted, a single dose of the modified thrombomodulin prevented clotting. It also served as a pro-drug that inhibited the activation of platelets and further thrombosis.
Having such a decisive proof-of-concept study in animals, the researchers are currently making similar recombinant prodrugs towards the next step of binding and testing human RBCs.
Zaitsev S, Kowalska MA, Neyman M, Carnemolla R, Tliba S, Ding BS, Stonestrom A, Spitzer D, Atkinson JP, Poncz M, Cines DB, Esmon CT, & Muzykantov VR (2012). Targeting recombinant thrombomodulin fusion protein to red blood cells provides multifaceted thromboprophylaxis. Blood, 119 (20), 4779-85 PMID: 22493296