Depyrogenation refers to the removal of pyrogens from solution, most commonly from injectable pharmaceuticals. A is defined as any substance that can cause a fever. Bacterial pyrogens include endotoxins and exotoxins, although many pyrogens are endogenous to the host. Endotoxins include lipopolysaccharide molecules found as part of the cell wall of Gram-negative bacteria, and are released upon bacterial cell lysis. Endotoxins may become pyrogenic when released into the bloodstream or other tissue where they are not usually found. Although the colon contains Gram-negative bacteria in abundance, they do not cause a pyrogenic effect as the bacteria are not undergoing gross lysis, and the immune system is not exposed to free endotoxin while the colonic wall is intact. When LPS is released upon bacterial cell lysis, the lipid A component is first bound by serum LPS-Binding Protein and then transferred to CD14. This monomerises the aggregated LPS, as the LPS receptor Toll-like Receptor 4 cannot recognise LPS while aggregated. Monomeric LPS is then transferred to MD-2 pre-complexed with TLR4 on macrophages and monocytes. This leads to release of pro-inflammatory cytokines and nitric oxide, which may lead ultimately to septic shock depending on the strength of response. Vascular endothelial cells also express TLR4 and MD-2 and so respond to LPS directly, as well as via cytokines and nitric oxide. Bronchial epithelial cells and colonic epithelial cells also express TLR4, but as they do not express MD-2 they rely on LPS precomplexed with serum MD-2 in order to signal to LPS.
Maximum acceptable endotoxin level
Because endotoxin molecular weight may vary a great deal, endotoxin levels are measured in "endotoxin units". One EU is approximately equivalent to 100 pg of E. coli lipopolysaccharide—the amount present in around 105 bacteria. Humans can develop symptoms when exposed to as little as 5 EU/kg body weight. These symptoms include, but are not limited to, fever, low blood pressure, increased heart rate, and low urine output; and even small doses of endotoxin in the blood stream are often fatal. The FDA has set the following maximum permissible endotoxin levels for drugs distributed in the United States:
Drug - 0.2 EU/kg body weight
Drug - 5 EU/kg body weight
Sterile water - 0.25-0.5 EU/ml
Pyrogen detection
Rabbit Test
Early endotoxin detection was accomplished by injecting rabbits with the sample and observing the response in their body temperature. Rabbits have similar endotoxin tolerance to humans, and were thus an ideal choice. However, this method was costly, time consuming, and prompted protests from animals rights advocates. But perhaps the biggest drawback of this test was its inability to quantify the endotoxin level.
Currently, one common method for endotoxin detection is the Limulus Amebocyte Lysate test. This test is based on Dr. Frederik Bang's observation that horseshoe crab blood forms clots when exposed to endotoxins. Amoebocyte extract from horseshoe crab blood is mixed with a sample suspected of endotoxin contamination, and a reaction is observed if endotoxins are present. The FDA has approved four variations of the LAL test: gel-clot, turbidimetric, colorimetric, and chromogenic assay. The differences in these variations refer to the characteristics of the amoebocyte/endtoxin reaction. This test is fast and highly sensitive. However, because it only detects LPS endotoxins, some pyrogenic materials can be missed. Also, certain conditions can lead to false negatives. Glucans from carbohydrate chromatography matrices can also lead to false positives. Since 2003, a synthetic substitute for the LAL test has been commercially available. This recombinant factor C test is based on Limulus clotting factor C, the LPS-sensitive part of LAL. The adoption of this test was slow, which began to change in 2016 when the European Pharmacopeia listed this test as an accepted bacterial-toxin test.
The Monocyte Activation Test uses the monocytes in human blood in vitro to detect pyrogens. It was added to the European Pharmacopeia in 2010 and accepted by the FDA in 2012.
Pyrogen removal (depyrogenation)
Pyrogens can often be difficult to remove from solution due to the high variability of their molecular weight. Pyrogens are also relatively thermally stable and insensitive to pH changes. However, several removal techniques exist. ;Ion exchange chromatography ;Ultrafiltration ;Distillation
Inactivation/destruction
Because pyrogens are often difficult to remove, inactivation or destruction of the LPS molecule can sometimes be preferable. ;Acid-base hydrolysis ;Oxidation ;Heating ;Sodium Hydroxide
Preventive methods
Because virtually all raw materials involved in a production process, including factory employees, can be potential sources of pyrogen contamination, raw material screening and depyrogenation can often go a long way to ensuring the final product is free of pyrogens and does not require costly removal or inactivation methods. Ultrafiltration of chemicals and buffer solutions, applying appropriate hygienic practices, and performing regular tests can all be helpful.
