Pharmacokinetics and Intrinsic Factor
For successful absorption of cyanocobalamin from the gastrointestinal tract, a specific glycoprotein—intrinsic factor—is critical. It is secreted by parietal cells of the stomach. Upon entering the duodenum, the vitamin binds to this factor to form a complex, which is then successfully absorbed into the bloodstream.
In the systemic circulation, the drug is transported by specialized carrier proteins—transcobalamins—which deliver it directly to bone marrow cells.
An interesting feature of $B_{12}$ pharmacokinetics is its massive body stores. Vitamin reserves in a healthy individual reach 2–5 mg, whereas daily losses are only 2–5 mcg. Because of this disproportion, a clinically apparent deficiency develops very slowly—several years after complete cessation of its intake.
Role in Erythropoiesis and Anemia Pathogenesis
In the body, the drug is transformed into an active coenzyme—methylcobalamin. This form triggers a vital metabolic cascade:
- Ensures the synthesis of tetrahydrofolic acid (the active form of vitamin $B_9$).
- Tetrahydrofolic acid, in turn, participates in the formation of deoxythymidine—a key component of DNA.
When vitamin $B_{12}$ or folic acid is deficient, DNA synthesis is disrupted, which primarily affects rapidly dividing bone marrow cells. Erythroblasts continue to grow but lose the ability to undergo normal cell division. As a result, giant immature cells—megaloblasts—appear in the blood, along with large erythrocytes over-saturated with hemoglobin.
The hematological picture is quite specific: the decrease in erythrocyte count predominates over the drop in hemoglobin levels, and the color index rises above one (> 1). Hyperchromic megaloblastic anemia develops.
Neurological Disorders and Pernicious Anemia
In addition to erythropoiesis, cyanocobalamin is responsible for the synthesis of myelin—the protective sheath of nerve fibers. This is why $B_{12}$-deficiency anemia is always accompanied by damage to the nervous system.
A specific form of pathology is pernicious anemia. It occurs not due to a dietary vitamin deficiency, but as a result of impaired absorption in the absence of intrinsic factor (e.g., due to atrophic gastritis, tumors, or gastric resection).
Important clinical nuance: Only cyanocobalamin can resolve neurological symptoms. Administering folic acid in this case will be completely ineffective for restoring nerve tissue.
Clinical Application
The drug is the drug of choice for treating $B_{12}$-deficiency states.
- Routes of administration: subcutaneously or intramuscularly. Intravenous administration is acceptable in severe pernicious anemia.
- Dosing regimen: the standard protocol involves administering 0.1–0.2 mg of the drug every other day (once every 2 days).
- Side effects: allergic reactions, nervous excitation, and cardiac manifestations (tachycardia, chest pain) are possible.
- Contraindications: polycythemia vera (erythrocytosis) and acute thromboembolism.
It should be remembered that iron preparations (e.g., ferrous sulfate) or erythropoietin are not used for hyperchromic megaloblastic anemias—they are intended for the treatment of hypochromic anemias and anemia of chronic disease, respectively.
Iatrogenic Megaloblastic Anemia
Certain medications can disrupt folate metabolism and DNA synthesis, provoking the development of megaloblastic anemia as an adverse effect (drug-induced toxicity). Such inducing drugs include:
- Folic acid antagonists: the immunosuppressive and antineoplastic agent Methotrexate, the antiprotozoal drug Pyrimethamine, and the potassium-sparing diuretic Triamterene.
- Antiepileptic drugs: e.g., Ethosuximide.