Pyridoxamine

Pyridoxamine is a vitamer in the vitamin B6 family, which includes pyridoxal and pyridoxine. Pyridoxamine is converted to the biologically active form of vitamin B6, pyridoxal 5-phosphate, via the vitamin B6 salvage pathway. Vitamin B6 acts as an enzyme cofactor in a variety of metabolic processes. In food, pyridoxamine is commonly found as a 5’-phosphate derivative, which is hydrolyzed by intestinal phosphatases to pyridoxamine and absorbed in the jejunum. Absorbed pyridoxamine is converted to pyridoxamine 5’-phosphate by pyridoxal kinase, which is further converted to the active pyridoxal 5-phosphate by pyridoxamine-phosphate transaminase or pyridoxine 5’-phosphate oxidase.

Pyridoxamine is based on a pyridine ring structure, with hydroxyl, methyl, aminomethyl, and hydroxymethyl substituents. It differs from pyridoxine by the substituent at the 4-position. The phenol at position 3 and aminomethyl group at position 4 of its ring endow pyridoxamine with a variety of chemical properties, including the scavenging of free radical species and carbonyl species formed in sugar and lipid degradation and chelation of metal ions that catalyze Amadori reactions.

Research
Pyridoxamine can form fairly weak complexes with a number of transition metal ions, with a preference for Cu2+ and Fe3+. The 3'-hydroxyl group of pyridoxamine allows for efficient hydroxyl radical scavenging.

Pyridoxamine inhibits the Maillard reaction and can block the formation of advanced glycation endproducts, which are associated with medical complications of diabetes. Pyridoxamine is hypothesized to trap intermediates in the formation of Amadori products released from glycated proteins, possibly preventing the breakdown of glycated proteins by disrupting the catalysis of this process through disruptive interactions with the metal ions crucial to the redox reaction. One research study found that pyridoxamine specifically reacts with the carbonyl group in Amadori products, but inhibition of post-Amadori reactions (that can lead to advanced glycation endproducts) is due in much greater part to the metal chelation effects of pyridoxamine.

A variety of preclinical studies in animal models of diabetes indicated that pyridoxamine improved kidney histology comparable or superior to aminoguanidine. Because of these results, pyridoxamine has been investigated for clinical utility in the treatment of diabetic nephropathy.

Pyridoxamine also inhibits the formation of advanced lipoxidation endproducts during lipid peroxidation reactions by reaction with dicarbonyl intermediates. In other preclinical research, pyridoxamine may be efficacious in treating diabetic neuropathy and retinopathy associated with diabetes and kidney stone disease. In one study, pyridoxamine was more effective at protecting from ionizing radiation-induced gastrointestinal epithelial apoptosis than amifostine (the only radioprotector currently Food and Drug Administration (FDA)-approved) due to pyridoxamine reactive oxygen species and reactive carbonyl species scavanging profile.

Patent controversy
Pyridoxamine is marketed as a dietary supplement, often as the hydrochloride salt, pyridoxamine dihydrochloride. However, in the United States, the FDA ruled in January 2009 that pyridoxamine must be regulated as a pharmaceutical drug because it is the active ingredient in Pyridorin, a drug designed by Biostratum, Inc., to prevent the progression of diabetic nephropathy.

A patent for Pyridorin, from an application by Biostratum, was published in 2004. Pyridorin had success in early clinical trials, found to be effective in slowing the progression of diabetic neuropathy in a phase II trial on 224 patients. However, the clinical trial progression of Pyridorin was stalled when it was determined that the active ingredient (pyridoxamine) was commonly available for purchase on the Internet. As the subject of an Investigational New Drug Application with the FDA, Biostratum submitted a Citizen Petition to the FDA on July 29, 2005, seeking to disallow sales of pyridoxamine-containing supplements. This petition was opposed by the Council for Responsible Nutrition, a trade association of the dietary supplement industry. On January 12, 2009, the FDA ruled that products containing pyridoxamine are excluded from the definition of dietary supplements as defined by the Dietary Supplement Health and Education Act of 1994. The FDA stated that the status of Pyridorin as an investigational new drug, as a result of an application filed by BioStratum in July 1999 and effective on September 1, 1999, meant that "the marketing of pyridoxamine in a dietary supplement is essentially equivalent to the marketing of an investigational new drug as a dietary supplement" because there was an "absence of independent, verifiable evidence that the substance was marketed as a food or a dietary supplement prior to its authorization for investigation as a new drug."