What is endotoxin?
In gram-negative bacteria, there are two membranes: the inner membrane (IM) and the outer membrane (OM). The inner membrane is very similar to the plasma membrane of eukaryotic cells that is composed of a phospholipid bilayer. The outer membrane serves as a protective structure to the bacteria and is made up of asymmetrical lipid structures including lipopolysaccharides (LPS). When talking of endotoxins, people are referencing the biological activity of LPS once it is removed from the OM.
Mechanism of Endotoxin Disease
The LPS itself has 2 major components: hydrophobic lipid A and hydrophilic polysaccharide (O-antigen). The lipid A structure is the membrane-anchoring region of the LPS and is highly conserved in bacteria. The O-antigen refers to strands of oligosaccharide subunits that are attached to the core polysaccharide. Oligosaccharide subunits are made of 3-5 sugars and ranges to 40 repeating units (usually longer than the core polysaccharide) and have a major antibody-combining site that cecropins interact with. A fun fact about these oligosaccharides is that the sugars that comprise them are characteristically dideoxyhexoses, which only naturally occur in gram-negative bacteria.
In terms of toxicity, both lipid A and polysaccharide side chains contribute to virulence. Lipid A is the toxic section of the LPS while the polysaccharides are nontoxic and immunogenic. LPS binds to lipid binding proteins which eventually causes inflammation which activates the complement and coagulation cascades. When this biological activity is at a high enough rate, it can cause septic shock.
Global Burden of Endotoxin-related Disease
10-15% of sepsis patients have septic shock from high activity of endotoxins (endotoxin septic shock; ESS), adding up to 5-7 million cases globally each year. Mortality is correlated to the level of endotoxins within the blood (endotoxemia). ESS patients can be treated with anti-endotoxin therapy via antibiotics like polymyxin B hemadsorption that bind and inhibit endotoxins. Research is continuing to be done to develop more effective therapies to treat gram-negative bacterial infections.
Why is it important to ensure injectable and inhaled drugs and certain medical devices are endotoxin-free?
Endotoxin causes septic shock, a life-threatening condition. If an injectable or inhaled drug contains high levels of endotoxin, it can cause major disease and even death in patients.
How Do Pharmaceutical and Medical Device Companies Ensure Products are Endotoxin-Free?
There are several valid test methods to detect endotoxin in pharmaceuticals and medical devices. They include gel-clot testing, the turbidimetric method, and the chromogenic method. All methods are described in <USP> 85. The methods vary in terms of sensitivity and cost, but all are a reliable means of ensuring medical devices and pharmaceuticals are endotoxin-free.
Sources:
https://www.sciencedirect.com/science/article/pii/S2452316X17300406
https://link.springer.com/article/10.1186/s13054-023-04690-5