RSM01

RSM01 is a novel, highly potent, half-life-extended anti-RSV monoclonal antibody (mAb) candidate primarily being developed for low- and middle-income countries (LMICs). It targets the RSV prefusion F protein, specifically antigenic site zero, which is considered highly neutralization sensitive[1][3][8]. ## Key Characteristics RSM01 has demonstrated highly potent neutralizing activity in the single ng/mL range (0.7-6.4) against diverse RSV-A and RSV-B isolates in vitro. It also showed prophylactic efficacy in cotton rat models with both RSV subtypes[1][2][5]. The monoclonal antibody acts by binding to and inhibiting the pre-fusion form of RSV glycoprotein F on the virus surface, blocking a critical step in the membrane fusion process. It contains a YTE mutation in the Fc region that increases its serum half-life, which was measured at 78 days in clinical trials[1][2][8]. ## Clinical Development In a first-in-human, double-blind, phase 1 trial (NCT05118386), 56 healthy adults aged 18-49 were randomized to receive a single dose of RSM01 (n=48) or placebo (n=8) in various dosing cohorts: - 300 mg intravenously (IV) - 300 mg intramuscularly (IM) - 1000 mg IV - 3000 mg IV - 600 mg IM (expansion cohort) The most common unsolicited adverse events were COVID-19 (2/48), headache (2/48), and nausea (2/48), all in RSM01-treated participants. The only systemic solicited adverse events reported were headache (5/48) and tiredness (2/48). No serious adverse events or deaths were reported[1][2][5]. The pharmacokinetics showed dose-proportional increases in Tmax and AUClast after IV administration. Among RSM01-treated participants, 2/48 were anti-drug antibody (ADA) positive at baseline, and 1/48 seroconverted to ADA-positive post-baseline, indicating a low rate of immunogenicity[1][2][6]. ## Development Status RSM01 is currently in Phase 1 development for respiratory syncytial virus infections. The trial began on November 16, 2021[3]. Its long half-life and pharmacokinetics profile support further development as a potential single-dose per season prophylaxis to prevent RSV disease in infants, particularly in low- and middle-income countries[1][2][5].