VI-TWEL
Clinical safety rating: caution
Comprehensive clinical and safety monograph for VI-TWEL (VI-TWEL).
Hydroxocobalamin is converted to methylcobalamin and adenosylcobalamin, which act as cofactors for methionine synthase and methylmalonyl-CoA mutase, essential for DNA synthesis, myelin formation, and hematopoiesis. It is also a nitric oxide (NO) scavenger, reducing NO-mediated vasodilation in cyanide toxicity.
| Metabolism | Hydroxocobalamin undergoes intracellular conversion to methylcobalamin and adenosylcobalamin. It is not metabolized by CYP enzymes. Elimination is primarily via bile and urine. |
| Excretion | Primarily renal: 50-98% of a dose excreted unchanged in urine; biliary/fecal elimination accounts for <2%. |
| Half-life | Terminal elimination half-life is approximately 6 days (range 4-10 days) in patients with normal renal function; prolonged in renal impairment (up to 40 days in anuria). |
| Protein binding | Highly bound to transcobalamin II (approximately 90% bound) and other binding proteins; total plasma protein binding ~90%. |
| Volume of Distribution | 0.82-1.2 L/kg (total body water); extensive distribution into tissues, particularly liver, kidney, and bone marrow. |
| Bioavailability | Intramuscular: 100% bioavailability. Oral: 1-2% bioavailability due to limited intrinsic factor-mediated absorption; large oral doses (1 mg) allow ~1% absorption via passive diffusion. |
| Onset of Action | Parenteral (IM/SC): Reticulocyte response begins within 3-5 days; hematologic improvement noted within 7-10 days. Oral: Onset delayed and variable; not reliable for acute treatment. |
| Duration of Action | After a single IM dose, effects on erythropoiesis last 2-3 weeks; for maintenance therapy, repeat dosing every 1-3 months. Oral dosing requires daily administration. |
1000 mcg intramuscularly once daily for 5-7 days, then 1000 mcg intramuscularly once weekly for 4 weeks, followed by 1000 mcg intramuscularly once monthly.
| Dosage form | INJECTABLE |
| Renal impairment | No dose adjustment required for renal impairment; not dialyzable. |
| Liver impairment | No specific dose adjustment guidelines; use with caution in severe hepatic impairment. |
| Pediatric use | 30-50 mcg intramuscularly once daily for 5-10 days, then 100-250 mcg intramuscularly every 2-4 weeks. |
| Geriatric use | Same as adult dosing; monitor for hypokalemia and fluid retention. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for VI-TWEL (VI-TWEL).
| Breastfeeding | Vitamin B12 is excreted into human milk; M/P ratio not determined but concentrations correspond to maternal intake. Considered compatible with breastfeeding at recommended doses. Avoid excessive doses. |
| Teratogenic Risk | First trimester: No evidence of teratogenicity in animal studies; limited human data but no increased risk of major malformations reported. Second and third trimesters: No known adverse fetal effects. Vitamin B12 is essential for fetal development and deficiency may be harmful. |
| Fetal Monitoring |
■ FDA Black Box Warning
None.
| Serious Effects |
Hypersensitivity to hydroxocobalamin, cyanocobalamin, or any component of the formulation. Leber's disease (relative contraindication for cyanocobalamin; hydroxocobalamin may be used with caution).
| Precautions | Hypersensitivity reactions including anaphylaxis. Transient hypertension, headache, and injection site reactions. May cause reddish discoloration of urine, skin, and mucous membranes. Use caution in patients with Leber's disease (hereditary optic nerve atrophy) as cyanocobalamin may cause optic atrophy. Monitor potassium levels during treatment of megaloblastic anemia (risk of hypokalemia). |
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| Monitor maternal serum B12 levels, complete blood count, and neurological status. No specific fetal monitoring required. |
| Fertility Effects | No known adverse effects on fertility. Adequate B12 levels are necessary for normal reproductive function. |