MYOBLOC
Clinical safety rating: caution
Comprehensive clinical and safety monograph for MYOBLOC (MYOBLOC).
Botulinum toxin type B binds to presynaptic cholinergic nerve terminals and inhibits the release of acetylcholine, thereby blocking neuromuscular transmission.
| Metabolism | Metabolized by proteolytic enzymes; no involvement of cytochrome P450 system. |
| Excretion | Renal (minimal, <1% unchanged); hepatic metabolism via proteolysis; fecal elimination of inactive metabolites (major route). |
| Half-life | Immunologic half-life of botulinum toxin type B: approximately 3-4 hours in serum; terminal half-life of neuromuscular effect: 10-14 days (recovery depends on axonal sprouting and synaptic remodeling). |
| Protein binding | Negligible (<3% bound); botulinum toxin type B is a large protein complex with minimal plasma protein interaction. |
| Volume of Distribution | Vd not established for systemic distribution due to local intramuscular administration; if systemic absorption occurs, Vd is estimated <0.3 L/kg (limited distribution). |
| Bioavailability | Not applicable for systemic administration; intramuscular injection results in >100% local bioavailability at injection site (no first-pass metabolism). |
| Onset of Action | Intramuscular: 1-3 days for detectable muscle relaxation; peak effect at 2-3 weeks. |
| Duration of Action | Intramuscular: 3-4 months (clinical effect may persist longer in some patients; based on controlled trials for cervical dystonia). |
The typical adult dose of MYOBLOC (rimabotulinumtoxinB) for cervical dystonia is 2500 to 5000 Units administered intramuscularly into affected muscles, divided among the muscles. The dose and injection sites should be individualized. The total dose should be divided among affected muscles; maximum total dose is 10,000 Units per session. Repeat treatments should occur no more frequently than every 12 weeks.
| Dosage form | VIAL |
| Renal impairment | No specific dose adjustment guidelines are provided for renal impairment. Use with caution in patients with renal impairment due to lack of data. In severe renal impairment, consider starting at the low end of the dosing range and monitor for systemic effects. |
| Liver impairment | No specific dose adjustment guidelines are provided for hepatic impairment. Use with caution in patients with hepatic impairment due to lack of data. In severe hepatic impairment (Child-Pugh class C), consider starting at the low end of the dosing range and monitor for systemic effects. |
| Pediatric use | Safety and effectiveness in pediatric patients have not been established. MYOBLOC is not recommended for use in patients <18 years of age. |
| Geriatric use | Elderly patients (≥65 years) may be more sensitive to adverse effects (e.g., dysphagia, aspiration). Dosing should be conservative, starting at the lower end of the dosing range (e.g., 2500 Units) and titrated carefully. Monitor for increased risk of falls due to muscle weakness. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for MYOBLOC (MYOBLOC).
| Breastfeeding | Unknown if rimabotulinumtoxinB is excreted in human milk. No M/P ratio available. Due to high molecular weight and local injection, minimal systemic absorption is expected, but caution is advised. Consider benefit of breastfeeding vs. risk of infant exposure. |
| Teratogenic Risk | Pregnancy Category C: In animal studies, rimabotulinumtoxinB caused embryonic lethality, reduced fetal body weight, and skeletal malformations at doses comparable to or less than the maximum recommended human dose (MRHD). No adequate and well-controlled studies in pregnant women. Risk cannot be ruled out; use only if potential benefit justifies potential risk to the fetus. First trimester: Unknown effects; second and third trimesters: Potential for fetal anomalies and growth restriction with systemic exposure. |
■ FDA Black Box Warning
Distant spread of toxin effect: Symptoms consistent with botulism may occur hours to weeks after injection, including swallowing and breathing difficulties, which can be life-threatening.
| Serious Effects |
["Hypersensitivity to botulinum toxin type B or any ingredient","Infection at injection site"]
| Precautions | ["Risk of anaphylaxis and serious allergic reactions","Potential for spread of toxin effect to distant sites","Risk of dysphagia and aspiration","Use with caution in patients with neuromuscular disorders (e.g., myasthenia gravis, ALS)","Do not exceed recommended doses and frequency"] |
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| Fetal Monitoring | Monitor for signs of distant spread of toxin effects (dysphagia, dysphonia, respiratory compromise) in the mother. Fetal monitoring: Standard prenatal care; no specific fetal monitoring required unless maternal systemic toxicity occurs. Assess for preterm labor or fetal distress if there is systemic involvement. |
| Fertility Effects | Animal studies showed impaired fertility in male rats (decreased sperm motility and increased abnormal sperm) and female rats (irregular estrus cycles) at doses similar to MRHD. Human data insufficient; potential for reversible decreases in fertility. |