Endotoxin

Endotoxin refers to bacterial lipopolysaccharide (LPS), the major glycolipid of the outer membrane of Gram-negative bacteria. LPS consists of a hydrophobic lipid A domain, which is responsible for most of its toxic activity, a core oligosaccharide and a distal O-antigen polysaccharide. Lipid A is recognized by Toll-like receptor 4 (TLR4) on innate immune cells, triggering inflammatory signaling. Endotoxin is released when bacteria die or divide and is the principal cause of pyrogenic (fever-inducing) reactions to contaminated parenteral products. It is detected mainly with the Limulus amebocyte lysate (LAL) test, which has largely replaced the rabbit pyrogen test.

Injectable medicines are tested for endotoxin before release because contamination can come from water, raw materials, equipment, containers and, for recombinant products, the bacteria used to make them. Killing bacteria does not remove endotoxin, so a product can be sterile and still pyrogenic. A 2024 study of semaglutide vials sold online without a prescription found endotoxin in every sample tested.

Endotoxin is one of the main reasons injectable medicines are made under strict controls. The immune system detects lipid A through TLR4, a receptor of innate immunity, and responds with inflammatory signaling. In the bloodstream this can cause fever and, at higher exposures, more serious systemic reactions. Because these effects are triggered by a molecule rather than by live infection, a product can pass a sterility test and still cause a pyrogenic reaction.

For that reason, endotoxin testing is a routine release test for parenteral products. The rabbit pyrogen test was the first standard method; it has largely been replaced by the Limulus amebocyte lysate (LAL) test, which uses a clotting reaction from horseshoe crab blood cells that is triggered by endotoxin. Manufacturers also monitor raw materials and intermediate steps, since endotoxin is difficult to remove once it has bound to a product.

Peptides sold outside the regulated supply chain are not required to meet these controls, and buyers usually have no way to check them. In a 2024 study of semaglutide products bought from illegal online pharmacies, the lyophilized samples contained no viable microorganisms when tested, but endotoxin was detected in all of them. A certificate that reports purity alone does not address endotoxin, which is measured with a separate, specific assay.

“If a product is sterile, it is free of endotoxin”

Sterility means no living microorganisms. Endotoxin is a molecule released from bacterial cell walls, and it remains after the bacteria are killed or filtered out. Sterility and endotoxin are tested separately for injectable medicines because passing one says nothing about the other.

“High HPLC purity means low endotoxin”

HPLC purity measures how much of the detected material is the target peptide. It does not detect lipopolysaccharide at the levels that matter for pyrogenic reactions. Endotoxin requires its own assay, such as the LAL test, and a purity figure cannot substitute for it.

“Endotoxin is only a concern for products made in bacteria”

Recombinant products made in Gram-negative bacteria carry an obvious risk, but endotoxin can also enter through water, excipients, equipment and container systems. Chemically synthesized peptides are therefore not automatically endotoxin-free, and testing of illegally sold semaglutide vials has found it.

What is the LAL test?

The Limulus amebocyte lysate (LAL) test uses an extract of blood cells from the horseshoe crab. Endotoxin activates a clotting cascade in the extract, which can be read as a gel clot, a color change or increased turbidity. It is the most widely used endotoxin test and has largely replaced the older rabbit pyrogen test.

Why does endotoxin cause fever?

The lipid A part of endotoxin is recognized by Toll-like receptor 4 (TLR4) on immune cells. Activation of TLR4 sets off inflammatory signaling, including the release of mediators that act on temperature regulation and produce fever. This is why endotoxin is described as a pyrogen.

Are research peptides tested for endotoxin?

Peptides sold as research chemicals are not subject to the release testing required for approved injectable medicines, and buyers usually cannot verify whether endotoxin testing was done or what it found. Independent testing of illegally sold semaglutide found endotoxin in every sample, so the absence of a test result should not be read as the absence of contamination.