Dtxcid5024346

**General effects** Cyanocobalamin corrects vitamin B12 deficiency and improves the symptoms and laboratory abnormalities associated with pernicious anemia (megaloblastic indices, gastrointestinal lesions, and neurologic damage). This drug aids in growth, cell reproduction, hematopoiesis, nucleoprotein, and myelin synthesis. It also plays an important role in fat metabolism, carbohydrate metabolism, as well as protein synthesis. Cells that undergo rapid division (for example, epithelial cells, bone marrow, and myeloid cells) have a high demand for vitamin B12. **Parenteral cyanocobalamin effects** The parenteral administration of vitamin B12 rapidly and completely reverses the megaloblastic anemia and gastrointestinal symptoms of vitamin B12 deficiency. Rapid parenteral administration of vitamin B12 in deficiency related neurological damage prevents the progression of this condition. **Nasal spray effects** In 24 vitamin B12 deficient patients who were already stabilized on intramuscular (IM) vitamin B12 therapy, single daily doses of intranasal cyanocobalamin for 8 weeks lead to serum vitamin B12 concentrations that were within the target therapeutic range (>200 ng/L).

Vitamin B12 serves as a cofactor for _methionine synthase_ and _L-methylmalonyl-CoA mutase_ enzymes. Methionine synthase is essential for the synthesis of purines and pyrimidines that form DNA. L-methylmalonyl-CoA mutase converts L-methylmalonyl-CoA to _succinyl-CoA_ in the degradation of propionate, an important reaction required for both fat and protein metabolism. It is a lack of vitamin B12 cofactor in the above reaction and the resulting accumulation of methylmalonyl CoA that is believed to be responsible for the neurological manifestations of B12 deficiency. Succinyl-CoA is also necessary for the synthesis of hemoglobin. In tissues, vitamin B12 is required for the synthesis of _methionine_ from homocysteine. Methionine is required for the formation of S-adenosylmethionine, a methyl donor for nearly 100 substrates, comprised of DNA, RNA, hormones, proteins, as well as lipids. Without vitamin B12, tetrahydrofolate cannot be regenerated from 5-methyltetrahydrofolate, and this can lead to functional folate deficiency,. This reaction is dependent on methylcobalamin (vitamin B12) as a co-factor and is also dependent on folate, in which the methyl group of methyltetrahydrofolate is transferred to homocysteine to form _methionine_ and _tetrahydrofolate_. Vitamin B12 incorporates into circulating folic acid into growing red blood cells; retaining the folate in these cells. A deficiency of vitamin B12 and the interruption of this reaction leads to the development of megaloblastic anemia.

Vitamin B12 or cyanocobalamin obtained from food is initially bound by _haptocorrin_, a protein found in the saliva with high affinity for B12. This forms a _haptocorrin-B12_ complex. Cyanocobalamin passes through the stomach and is protected from acid degradation due to its binding to haptocorrin. In the duodenum, pancreatic _proteases_ release cobalamin from the _haptocorrin-B12 complex_ and from other proteins containing protein-bound B12 that have been ingested. Following this, the binding of cobalamin to a second glycoprotein, _intrinsic factor_, promotes its uptake by terminal ileum mucosal cells by a process called _cubilin_/AMN receptor-mediated endocytosis. After absorption into enterocytes, intrinsic factor is broken down in the lysosome, and cobalamin is then released into the bloodstream. The transporter ABCC1, found in the basolateral membrane of intestinal epithelial and other cells, exports cobalamin bound to transcobalamin out of the cell. Cyanocobalamin then passes through the portal vein in the liver, and then reaches the systemic circulation. The active forms of cyanocobalamin are _methylcobalamin_ and _adenosylcobalamin_,.

Vitamin B12 is quickly absorbed from intramuscular (IM) and subcutaneous (SC) sites of injection; with peak plasma concentrations achieved about 1 hour after IM injection. Orally administered vitamin B12 binds to intrinsic factor (IF) during its transport through the stomach. The separation of Vitamin B12 and IF occurs in the terminal ileum when calcium is present, and vitamin B12 is then absorbed into the gastrointestinal mucosal cells. It is then transported by transcobalamin binding proteins. Passive diffusion through the intestinal wall can occur, however, high doses of vitamin B12 are required in this case (i.e. >1 mg). After the administration of oral doses less than 3 mcg, peak plasma concentrations are not reached for 8 to 12 hours, because the vitamin is temporarily retained in the wall of the lower ileum.

This drug is partially excreted in the urine. According to a clinical study, approximately 3-8 mcg of vitamin B12 is secreted into the gastrointestinal tract daily via the bile. In patients with adequate levels of intrinsic factor, all except approximately 1 mcg is reabsorbed. When vitamin B12 is administered in higher doses that saturate the binding capacity of plasma proteins and the liver, the unbound vitamin B12 is eliminated rapidly in the urine. The body storage of vitamin B12 is dose-dependent.

Cobalamin is distributed to tissues and stored mainly in the liver and bone marrow.

During vitamin loading, the kidney accumulates large amounts of unbound vitamin B12. This drug is cleared partially by the kidney, however, multiligand receptor _megalin_ promotes the reuptake and reabsorption of vitamin B12 into the body,.