Clesrovimab

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Clesrovimab
Monoclonal antibody
TypeWhole antibody
SourceHuman
TargetRespiratory syncytial virus F protein
Clinical data
Trade namesEnflonsia
Other namesMK-1654, clesrovimab-cfor
AHFS/Drugs.comMonograph
License data
Routes of
administration		Intramuscular
Drug classAntiviral
ATC code
  • None
Legal status
Legal status
Identifiers
CAS Number
DrugBank
UNII
KEGG
Chemical and physical data
FormulaC6526H10118N1732O2039S40
Molar mass146747.22 g·mol−1

Clesrovimab, sold under the brand name Enflonsia, is a fully human immunoglobulin G1 kappa monoclonal antibody designed to prevent respiratory syncytial virus (RSV) infection.[1] It is a respiratory syncytial virus (RSV) F protein-directed fusion inhibitor.[1] It was developed by Merck,[2] and was approved for medical use in the United States in June 2025.[3]

Clesrovimab is a fully human immunoglobulin G1 kappa (IgG1κ) neutralizing monoclonal antibody with a triple amino acid substitution in the Fc region which increases binding to the neonatal Fc receptor leading to an extended serum half-life.[4] Clesrovimab provides passive immunity by targeting the RSV outer membrane fusion protein to prevent viral entry into cells.[4]

Medical uses

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Clesrovimab 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.[1]

History

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The safety and efficacy of clesrovimab were supported by two clinical trials (trials 004 and 007).[3] The key measure of efficacy was the incidence of RSV-associated medically-attended lower respiratory infection characterized as cough or difficulty breathing and requiring ≥1 indicator of lower respiratory infection (wheezing, rales/crackles) or severity (chest wall in-drawing/retractions, hypoxemia, tachypnea, dehydration due to respiratory symptoms) through 150 days after dosing.[3] Medically attended includes all healthcare provider visits in settings such as outpatient clinic, clinical study site, emergency department, urgent care center, and/or hospital.[3]

Trials 004 and 007 were randomized, multicenter clinical trials to evaluate the safety, pharmacokinetics and efficacy of clesrovimab in preventing RSV-associated medically-attended lower respiratory infection.[3] Trial 004 was a double-blind, placebo-controlled trial in which 3,614 infants (born at ≥29 to <35 weeks GA or at ≥35 weeks GA) were randomized and received at least one dose of either clesrovimab or placebo.[3] Trial 007 was a partially-blind, palivizumab-controlled trial in which 896 infants who were at increased risk for severe RSV disease (born at ≥29 to ≤35 weeks GA or with chronic lung disease of prematurity or congenital heart disease of any GA) were randomized and received at least one dose of clesrovimab or palivizumab.[3] Trial 004 was a randomized, double-blind placebo-controlled, multi-site trial conducted in 22 countries from the Northern and Southern Hemispheres to evaluate the efficacy of clesrovimab in early and moderate preterm infants (≥29 to <35 weeks gestational age) and late preterm and full-term infants (≥35 weeks gestational age).[3] Among 3,614 participants who received clesrovimab or saline placebo, the median age of infants was 3.1 months (range: 0 to 12 months); 80% were younger than 6 months, 16% were between 6 and 9 months, and 4% were 9 months of age and older; and 51% were male.[3] Of these participants, 18% were gestational age between 29 weeks and 35 weeks, and 82% were gestational age 35 weeks and older.[3] Trial 007 was a randomized, partially-blind, palivizumab-controlled, multi-site trial conducted in 27 countries from the Northern and Southern Hemispheres to evaluate the efficacy of clesrovimab in early (<29 weeks gestational age) or moderate preterm infants (≥29 to ≤35 weeks gestational age), and infants with chronic lung disease of prematurity or congenital heart disease of any gestational age, who are at increased risk for severe RSV disease.[3] Among 896 participants who received clesrovimab or palivizumab, the median age of infants was 2.5 months (range: 0 to 12 months); 89% were younger than 6 months, 9% were between 6 and 9 months, and 2% were 9 months of age and older; and 50% were male.[3] Of these participants, 28% had chronic lung disease, 11% had congenital heart disease, 6% were gestational age younger than 29 weeks with neither chronic lung disease nor congenital heart disease and 55% were gestational age 29 weeks and older with neither chronic lung disease nor congenital heart disease.[3]

Society and culture

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Clesrovimab was approved for medical use in the United States in June 2025.[2][5]

In September 2025, the Committee for Medicinal Products for Human Use of the European Medicines Agency adopted a positive opinion, recommending the granting of a marketing authorization for the medicinal product Enflonsia, intended for the prevention of respiratory syncytial virus (RSV) lower respiratory tract disease in neonates and infants during their first RSV season.[4] The applicant for this medicinal product is Merck Sharp & Dohme B.V.[4]

Names

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Clesrovimab is the international nonproprietary name.[6]

Clesrovimab is sold under the brand name Enflonsia.[1]

References

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  1. ^ a b c d e "Enflonsia- clesrovimab injection, solution". DailyMed. 20 June 2025. Retrieved 6 July 2025.
  2. ^ a b "U.S. FDA Approves Merck's Enflonsia (clesrovimab-cfor) for Prevention of Respiratory Syncytial Virus (RSV) Lower Respiratory Tract Disease in Infants Born During or Entering Their First RSV Season" (Press release). Merck. 9 June 2025. Archived from the original on 14 June 2025. Retrieved 14 June 2025 – via Business Wire.
  3. ^ a b c d e f g h i j k l m "Drug Trials Snapshots: Enflonsia". U.S. Food and Drug Administration (FDA). 9 June 2025. Retrieved 24 August 2025. Public Domain This article incorporates text from this source, which is in the public domain.
  4. ^ a b c d "Enflonsia EPAR". European Medicines Agency (EMA). 19 September 2025. Retrieved 27 September 2025. Text was copied from this source which is copyright European Medicines Agency. Reproduction is authorized provided the source is acknowledged.
  5. ^ "Clesrovimab Receives FDA Approval for RSV Lower Respiratory Tract Prevention in Infants". 9 June 2025. Archived from the original on 9 June 2025. Retrieved 12 June 2025.
  6. ^ World Health Organization (2022). "International nonproprietary names for pharmaceutical substances (INN): recommended INN: list 88". WHO Drug Information. 36 (3). hdl:10665/363551.

Further reading

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  • Clinical trial number NCT04767373 for "Efficacy and Safety of Clesrovimab (MK-1654) in Infants (MK-1654-004) (CLEVER)" at ClinicalTrials.gov
  • Clinical trial number NCT04938830 for "Clesrovimab (MK-1654) in Infants and Children at Increased Risk for Severe Respiratory Syncytial Virus (RSV) Disease (MK-1654-007) (SMART)" at ClinicalTrials.gov