BEYFORTUS
Clinical safety rating: caution
Comprehensive clinical and safety monograph for BEYFORTUS (BEYFORTUS).
BEYFORTUS (nirsevimab) is a recombinant human monoclonal antibody that binds to the prefusion conformation of the respiratory syncytial virus (RSV) F protein, inhibiting viral entry into host cells by blocking the fusion of the viral envelope with the host cell membrane.
| Metabolism | Nirsevimab is degraded via catabolic pathways into small peptides and amino acids. |
| Excretion | Beyfortus (nirsevimab) is eliminated primarily via catabolism to small peptides and amino acids. No specific data on renal or biliary excretion; expected to undergo proteolytic degradation with minimal renal or fecal elimination of intact drug. |
| Half-life | Terminal elimination half-life is approximately 26.8 days in infants, supporting season-long protection after a single dose. |
| Protein binding | Protein binding is approximately 99.5%, primarily to albumin. |
| Volume of Distribution | Volume of distribution is approximately 4.5 L in infants (mean Vd ≈ 0.3 L/kg), indicating distribution primarily in plasma and interstitial fluid. |
| Bioavailability | Bioavailability after intramuscular injection is approximately 70-80% (absolute bioavailability not established; relative to IV data). |
| Onset of Action | Subcutaneous administration: onset of protection is within hours to days; peak serum concentrations achieved by day 6 post-dose. |
| Duration of Action | Duration of protection is at least 5 months (one RSV season) after a single intramuscular dose based on pharmacokinetic modeling. |
Not applicable; BEYFORTUS (nirsevimab) is indicated for prevention of respiratory syncytial virus lower respiratory tract disease in neonates and infants. No adult dose exists.
| Dosage form | INJECTABLE |
| Renal impairment | No dosage adjustment required for renal impairment; nirsevimab is a monoclonal antibody not renally cleared. |
| Liver impairment | No dosage adjustment required for hepatic impairment; nirsevimab is a monoclonal antibody not hepatically metabolized. |
| Pediatric use | Neonates and infants weighing <5 kg: 50 mg intramuscular (IM) single dose; infants weighing ≥5 kg: 100 mg IM single dose. Administer during RSV season. |
| Geriatric use | Not indicated for geriatric population; no dosing recommendations available. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for BEYFORTUS (BEYFORTUS).
| Breastfeeding | There are no data on the presence of nirsevimab in human milk, effects on the breastfed infant, or effects on milk production. Nirsevimab is a human monoclonal antibody (IgG1) and is expected to be excreted into human milk in small amounts due to the high molecular weight and limited transfer via the neonatal Fc receptor. The M/P ratio has not been determined. The developmental and health benefits of breastfeeding should be considered along with the mother's clinical need for BEYFORTUS and any potential adverse effects on the breastfed infant from the drug or underlying condition. |
| Teratogenic Risk | BEYFORTUS (nirsevimab) is a human monoclonal antibody against respiratory syncytial virus. There are no adequate and well-controlled studies in pregnant women. In animal reproduction studies, no adverse developmental effects were observed in pregnant rabbits or cynomolgus monkeys at doses up to 10 times the human clinical exposure. However, because monoclonal antibodies are transported across the placenta in increasing amounts as pregnancy progresses (especially in the third trimester), potential fetal exposure may occur. Based on limited data, the risk of major birth defects and miscarriage is unknown but expected to be low due to the IgG1 nature and lack of known teratogenic signal. |
■ FDA Black Box Warning
No black box warning.
| Serious Effects |
["History of serious hypersensitivity reaction to nirsevimab or any component of the formulation."]
| Precautions | ["Hypersensitivity reactions including anaphylaxis have been reported.","Use caution in patients with thrombocytopenia or any coagulation disorder due to risk of bleeding from intramuscular injection."] |
| Food/Dietary | No known food interactions. BEYFORTUS is administered by intramuscular injection and does not interact with dietary components. |
| Clinical Pearls |
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| Fetal Monitoring | No specific maternal or fetal monitoring is required beyond standard prenatal care. As a prophylactic agent given as a single intramuscular injection before or during RSV season, no ongoing monitoring for adverse effects is necessary. In the event of anaphylaxis or severe hypersensitivity reactions, appropriate medical support should be available. There is no need for fetal monitoring or dose adjustments during pregnancy based on pharmacokinetic changes. |
| Fertility Effects | There are no data on the effects of nirsevimab on human fertility. Animal studies in cynomolgus monkeys and rabbits showed no adverse effects on male or female fertility parameters at doses up to 10 times the human clinical exposure. Based on the mechanism of action (monoclonal antibody targeting RSV fusion protein), no direct effect on reproductive function is expected. |
| BEYFORTUS (nirsevimab) is a recombinant human monoclonal antibody for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants. It is administered as a single intramuscular injection, typically 50 mg for infants <5 kg and 100 mg for infants ≥5 kg. It is not a treatment for active RSV infection. It does not interfere with live attenuated vaccines; however, administration with other injectable vaccines at different sites is acceptable. Do not administer to infants with a history of severe hypersensitivity to nirsevimab or any excipients. Efficacy has not been established in infants with a history of RSV infection. |
| Patient Advice | This vaccine is given as a single shot to prevent serious RSV disease in your infant. · It is not a treatment for active RSV infection; if your infant has RSV symptoms, inform the healthcare provider. · Common side effects include injection site reactions, rash, and fever. Contact your provider if these persist or worsen. · Inform the healthcare provider of any allergic reactions or bleeding disorders before administration. · Your infant can still receive other vaccines as scheduled. |