Clesrovimab

ENFLONSIA is a monoclonal antibody with anti-RSV activity [see Microbiology (12.4) ].

• ENFLONSIA is indicated for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season. ENFLONSIA is a respiratory syncytial virus (RSV) F protein-directed fusion inhibitor indicated for the prevention of RSV lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.

• ENFLONSIA is contraindicated in infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of ENFLONSIA [see Warnings and Precautions (5.1) and Description (11) ]. ENFLONSIA is contraindicated in infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of ENFLONSIA.contraindicated

Recommended dosage: 105 mg administered as a single intramuscular (IM) injection. ( 2.1 ) 2.1 Recommended Dosage The recommended dose for neonates and infants born during or entering their first RSV season is 105 mg administered as a single intramuscular (IM) injection. For neonates and infants born during the RSV season, administer ENFLONSIA once starting from birth. For infants born outside the RSV season, administer ENFLONSIA once prior to the start of their first RSV season considering the duration of protection provided by ENFLONSIA [see Clinical Pharmacology (12.2) ]. Infants Undergoing Cardiac Surgery with Cardiopulmonary Bypass For infants undergoing cardiac surgery with cardiopulmonary bypass during or entering their first RSV season, an additional 105 mg dose administered as an IM injection is recommended as soon as the infant is stable after surgery to ensure adequate clesrovimab-cfor serum levels. 2.2 Administration Instructions ENFLONSIA must be administered by a healthcare provider. Before injection, remove ENFLONSIA from the refrigerator and allow the prefilled syringe to come to room temperature for approximately 15 minutes. Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit. ENFLONSIA is a clear to slightly opalescent, colorless to slightly yellow solution. This product should not be used if particulate matter or discoloration is found.

Hypersensitivity Including Anaphylaxis: Serious hypersensitivity reactions, including anaphylaxis, have been observed with other human immunoglobulin G1 (IgG1) monoclonal antibodies. Initiate appropriate medications and/or supportive care if such reactions occur. ( 5.1 ) 5.1 Hypersensitivity Including Anaphylaxis Serious hypersensitivity reactions, including anaphylaxis, have been observed with other human immunoglobulin G1 (IgG1) monoclonal antibodies. If signs or symptoms of a clinically significant hypersensitivity reaction or anaphylaxis occur, initiate appropriate medications and/or supportive therapy. 5.2 RSV Diagnostic Test Interference Clesrovimab-cfor may interfere with some immunologically-based RSV diagnostic assays (i.e., rapid antigen tests) as observed in laboratory studies. Confirmation using a reverse transcriptase polymerase chain reaction (RT-PCR) assay is recommended when rapid antigen assay results are negative and clinical observations are consistent with RSV infection [see Drug Interactions (7.1) ].

Most common adverse reactions were injection-site erythema (3.8%), injection-site swelling (2.7%) and rash (2.3%). ( 6.1 ) To report SUSPECTED ADVERSE REACTIONS, contact Merck Sharp & Dohme LLC at 1-877-888-4231 or FDA at 1-800-FDA-1088 or www.fda.gov/medwatch . 6.1 Clinical Trials Experience Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in practice. The safety of ENFLONSIA was evaluated in 2,858 infants who received ENFLONSIA in Phase 2b/3 and Phase 3 clinical trials (Trial 004 and Trial 007). Neonates and Infants Entering Their First RSV Season (Trial 004) Trial 004 was a Phase 2b/3, randomized, double-blind placebo-controlled, multisite trial conducted in early and moderate preterm infants (≥29 to <35 weeks gestational age (GA)) and late preterm and full-term infants (≥35 weeks GA). Participants were randomized 2:1 and received a single 105 mg dose of ENFLONSIA (N=2,412, including 422 early and moderate preterm infants) or saline placebo (N=1,202, including 209 early and moderate preterm infants) by IM injection. Participants were monitored for 30 minutes post-dose. Safety was assessed using an electronic diary device from Days 1 through 42 post-dose. Participants were monitored for serious adverse events (SAEs) through the duration of their participation for up to 365 days post-dose. A subset of participants was monitored for SAEs for up to 515 days post-dose.

The safety and effectiveness of ENFLONSIA have not been established in children older than 12 months of age. ( 8.4 ) 8.1 Pregnancy ENFLONSIA is not indicated for use in females of reproductive potential. 8.2 Lactation ENFLONSIA is not indicated for use in females of reproductive potential. 8.4 Pediatric Use The safety and effectiveness of ENFLONSIA have been established for the prevention of RSV lower respiratory tract disease in neonates and infants born during or entering their first RSV season and the information on this use is discussed throughout the labeling. The safety and effectiveness of ENFLONSIA have not been established in children older than 12 months of age.

Metabolism

The PK of clesrovimab-cfor is approximately dose-proportional following a single IM administration of doses ranging from 20 mg to 210 mg in infants. Following IM administration of the 105 mg recommended dose, the geometric mean (% geometric CV) area under the time concentration curve from day 1 to day 150 (AUC 0-150 ) is 6,470 mcg×d/mL (22.6%), peak concentration (C max ) is 120 mcg/mL (25.4%), and the concentration at day 150 (C 150 ) is 10.3 mcg/mL (36.6%).

There is limited experience of overdose with ENFLONSIA. There is no specific treatment for an overdose with ENFLONSIA. In the event of an overdose, the infant should be monitored for the occurrence of adverse reactions and provided with symptomatic treatment as appropriate.

• Jun 9, 2025BLABLA761432Merck Sharp Dohme

1. 1No Adverse Event436%
2. 2Extra Dose Administered218%
3. 3Injection Site Pain218%
4. 4Injection Site Swelling218%
5. 5Pyrexia218%
6. 6Bacterial Test Positive19.1%
7. 7Blood Culture Positive19.1%
8. 8Influenza19.1%
9. 9Injection Site Mass19.1%
10. 10Medication Error19.1%
11. 11Product Administered To Patient Of Inappropriate Age19.1%
12. 12Product Dose Omission Issue19.1%
13. 13Rash19.1%
14. 14Respiratory Syncytial Virus Infection19.1%

The 4 most recently updated of 4 ClinicalTrials.gov registrations naming Clesrovimab as an intervention. Registration is not evidence of efficacy or safety — reference crosswalk only.

• Clesrovimab (MK-1654) in Infants and Children at Increased Risk for Severe Respiratory Syncytial Virus (RSV) Disease (MK-1654-007)
  Completed · Phase 3 · Interventional · 1,003 enrolled · Merck Sharp & Dohme LLC
  NCT04938830updated 2026-05-08
• Efficacy and Safety of Clesrovimab (MK-1654) in Infants (MK-1654-004)
  Completed · Phase 2 · Phase 3 · Interventional · 3,632 enrolled · Merck Sharp & Dohme LLC
  NCT04767373updated 2026-02-11
• Safety, Tolerability, and Pharmacokinetics of Clesrovimab (MK-1654) in Infants (MK-1654-002)
  Completed · Phase 1 · Phase 2 · Interventional · 183 enrolled · Merck Sharp & Dohme LLC
  NCT03524118updated 2025-01-14
• Phase 2a Respiratory Syncytial Virus (RSV) Human Challenge Study of Clesrovimab (MK-1654) in Healthy Participants (MK-1654-005)
  Completed · Phase 2 · Interventional · 80 enrolled · Merck Sharp & Dohme LLC
  NCT04086472updated 2022-09-14

Frequently asked questions

How does Clesrovimab work?

ENFLONSIA is a monoclonal antibody with anti-RSV activity [see Microbiology (12.4) ].

What is Clesrovimab used for?

According to FDA labeling, Clesrovimab carries indications including: ENFLONSIA is indicated for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season. ENFLONSIA is a respiratory syncytial virus (RSV) F protein-directed fusion inhibitor indicated for the prevention of RSV lower respiratory tract disease in neonates and infants who are born during or entering their first RSV season.. This is a reference summary of labeled uses, not medical advice or a treatment recommendation.

What class of drug is Clesrovimab?

Clesrovimab is classified as Antiviral monoclonal antibodies, Respiratory Syncytial Virus Anti-F Protein Monoclonal Antibody, Fusion Protein Inhibitors, Increased Immunologic Activity.

What are the brand names for Clesrovimab?

Clesrovimab is marketed under brand names including Enflonsia.

What are the contraindications for Clesrovimab?

Clesrovimab labeling lists contraindications including: ENFLONSIA is contraindicated in infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of ENFLONSIA [see Warnings and Precautions (5.1) and Description (11) ]. ENFLONSIA is contraindicated in infants with a history of serious hypersensitivity reactions, including anaphylaxis, to any component of ENFLONSIA.. Always consult the full prescribing information and a clinician.