Hemoglobin-based oxygen carriers, or HBOCs, are artificial solutions engineered to perform one of the primary functions of red blood cells: oxygen transport. These products are not “artificial blood,” as they do not replicate the full range of functions carried out by whole blood, such as immune response or clotting. Instead, they are molecular oxygen taxis designed to pick up oxygen in the lungs and deliver it to the body’s tissues. This technology was conceived to address situations where conventional blood transfusions are not an option or are unavailable.
The hemoglobin within HBOCs is a protein, normally housed within red blood cells, that binds to oxygen. By extracting and modifying this protein, scientists have created a product that mimics this specific action of blood. The purpose of these carriers is to provide a temporary bridge for oxygenation when a patient experiences significant blood loss or anemia.
The Rationale for Blood Substitutes
The quest to develop a viable blood substitute is driven by significant logistical and medical challenges associated with traditional blood transfusions. The primary motivators include:
- Blood supply shortages, which are a persistent issue for blood banks and are worsened by mass casualty events or the needs of patients with rare blood types.
- The limited shelf-life of donated blood, which requires refrigeration and must be discarded after about 42 days. In contrast, many HBOCs are designed for long-term stability for years at room temperature, making them ideal for military or remote use.
- The need for universal compatibility. HBOCs are cell-free and lack the surface antigens (A, B, and Rh) that determine blood type, eliminating the need for time-consuming blood typing and cross-matching in emergencies.
- Safety and patient-specific concerns. An engineered, sterilized product would bypass the small but existing risk of transmitting infections through donated blood and provide a life-saving alternative for patients who refuse transfusions for religious reasons.
How Hemoglobin Based Oxygen Carriers are Created
The production of HBOCs is a multi-step process that begins with sourcing hemoglobin and then chemically modifying it for safe use. The hemoglobin is derived from one of three main sources: expired human blood, purified bovine (cow) hemoglobin, or hemoglobin produced through recombinant DNA technology. Each source requires a rigorous purification and sterilization process to isolate the hemoglobin and ensure the final product is free of contaminants.
Injecting purified, cell-free hemoglobin directly into the bloodstream is not viable. Outside the protective environment of a red blood cell, the hemoglobin tetramer—a molecule made of four protein subunits—quickly breaks down into smaller, toxic dimers. These components are rapidly filtered out by the kidneys, leading to renal damage, and have a very short circulatory half-life, rendering them ineffective.
Scientists employ chemical modification techniques to stabilize the hemoglobin molecule. One common strategy is polymerization, where molecules are linked together using a cross-linking agent. This process creates larger molecules that are less likely to be filtered by the kidneys and can circulate in the bloodstream for a longer period.
Another technique is conjugation, which involves attaching other molecules, such as polyethylene glycol (PEG), to the hemoglobin’s surface in a process known as PEGylation. Attaching PEG chains increases the HBOC’s size, helps it retain fluid in the vascular space, and can shield the hemoglobin from interactions that cause adverse effects. These stabilization methods are fundamental to creating a functional carrier.
Mechanism of Action and Physiological Effects
The intended action of an HBOC is to mimic the oxygen transport function of a red blood cell. The modified hemoglobin molecules circulate in the plasma, bind with oxygen in the lungs, and release it to tissues where it is needed. Because HBOC particles are much smaller than red blood cells, they can potentially perfuse into tissues where larger cells cannot, such as in areas of swelling or constricted blood vessels.
Despite this straightforward goal, the use of HBOCs has been hindered by severe physiological side effects, the most significant of which is the scavenging of nitric oxide (NO). Nitric oxide is a gas molecule produced by the cells lining blood vessels that signals the surrounding muscle to relax in a process called vasodilation. This relaxation keeps blood vessels open and maintains healthy blood pressure.
Hemoglobin that is free in the plasma binds to nitric oxide with extremely high affinity, effectively removing it from the bloodstream. The direct consequence of this NO depletion is vasoconstriction, a widespread tightening of the blood vessels. This vessel constriction leads to a sharp and dangerous increase in blood pressure, or hypertension, which has been a hallmark adverse event in clinical trials. This hypertensive effect can strain the cardiovascular system and has been linked to an increased risk of heart attack and stroke.
Another major issue is oxidative stress. Unstable, cell-free hemoglobin is prone to autoxidation, a process where the iron atom within it becomes unable to bind oxygen. This reaction can also lead to the release of heme and iron, which promote the formation of highly reactive molecules known as free radicals, causing tissue damage and inflammation.
Clinical Development and Regulatory Hurdles
The journey of HBOCs from concept to clinical application has been marked by numerous high-profile trial failures. Throughout the 1990s and early 2000s, several products, such as HemAssist, PolyHeme, and Hemopure, advanced into late-stage trials. These studies showed that while the products could transport oxygen, they were associated with the significant cardiovascular side effects described earlier, including hypertension and increased rates of myocardial infarction.
As a result of these safety concerns, no HBOC is currently approved for general clinical use by the U.S. Food and Drug Administration (FDA) or in Europe. The FDA placed a hold on many HBOC trials in 2008, demanding a clearer understanding of the toxicity mechanisms before further human studies could proceed. This regulatory stance paused widespread development and prompted a shift in research toward creating safer, third-generation products.
Despite the lack of broad approval, some HBOCs have found niche applications. Hemopure, a bovine-derived HBOC, was approved for use in humans in South Africa in 2001 and later in Russia. In the United States and Europe, certain HBOCs can sometimes be accessed through compassionate use programs for life-threatening situations where no other options are available.
Current research is concentrated on designing new HBOCs that avoid the pitfalls of their predecessors. Scientists are exploring more complex molecular structures that better mimic the protective environment of the red blood cell to reduce nitric oxide scavenging and limit oxidative damage. The goal is to engineer a product that retains the oxygen-carrying benefit without triggering the harmful vascular side effects that have prevented their widespread adoption.