RSM01, a Novel Respiratory Syncytial Virus Monoclonal Antibody: Preclinical Characterization and Results of a First-in-Human, Randomised Clinical Trial

doi:10.21203/rs.3.rs-4583526/v1

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Research Article

RSM01, a Novel Respiratory Syncytial Virus Monoclonal Antibody: Preclinical Characterization and Results of a First-in-Human, Randomised Clinical Trial

https://doi.org/10.21203/rs.3.rs-4583526/v1

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Background

Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract disease among infants and young children worldwide, especially in low- and middle-income countries (LMICs). RSM01 is a novel, highly potent, half-life-extended anti-RSV monoclonal antibody (mAb) candidate primarily being developed for LMICs. Here we present the preclinical characterisation and results of a phase 1 trial of RSM01.

Methods

Preclinical characterisation of RSM01 was conducted using in-vitro neutralization assays and cotton rat models.

In the first-in-human, double-blind, phase 1 trial, 56 healthy adults were randomised 6:1 within dose cohorts to receive a single dose of RSM01 (n=48) or placebo (n=8): 300 mg intravenously (IV), 300 mg intramuscularly (IM) or 1000 mg IV (parallel cohorts), 3000 mg IV, and an expansion cohort of 600 mg IM. Systemic solicited adverse events (AEs) were assessed through day 7; unsolicited AEs were collected through day 151. Pharmacokinetics and anti-drug antibodies (ADA) to RSM01 were assessed using immunoassays. A population pharmacokinetics model predicted paediatric pharmacokinetics parameters using allometric scaling and age-specific population weight statistics of North American and African infants.

Results

RSM01 exhibited highly potent neutralizing activity in the single ng/mL range (0.7-6.4) against diverse RSV-A and RSV-B isolates in vitro. RSM01 also demonstrated prophylactic efficacy in cotton rat models with both RSV subtypes. In the phase 1 clinical trial, the most common unsolicited AEs were COVID-19 (2/48), headache (2/48), and nausea (2/48), all in RSM01-treated participants. The only systemic solicited AEs reported were headache (5/48) and tiredness (2/48) in participants receiving RSM01. No serious AEs or deaths were reported. The half-life of RSM01 was 78 days with dose-proportional increases in T_max and AUC_last after IV administration. Among RSM01-treated participants, 2/48 were ADA positive at baseline, and 1/48 seroconverted to ADA-positive post-baseline.

Conclusions

RSM01 is a highly potent, half-life-extended, RSV-neutralising mAb candidate that was shown to be well tolerated in healthy adults. The rate of ADA to RSM01 was low. The long half-life of RSM01 and pharmacokinetics profile support further development of RSM01 as a potential single dose per season prophylaxis to prevent RSV disease in infants.

Trial registration: Clinicaltrials.gov NCT05118386, Nov 12, 2021

RSV

lower respiratory tract infection

monoclonal antibody

LMIC

infants

children

pharmacokinetics

neutralising antibodies

Respiratory Syncytial Virus (RSV) is an Orthopneumovirus belonging to the Pneumoviridae family of viruses and is the most common cause of acute lower respiratory tract infection (LRTI) in children ≤ 5 years of age [1, 2]. Most children who get infected with RSV have their first infection by the time they are 2 years old, presenting with a mild, cold-like illness within 4 to 6 days after infection [1, 3, 4]. However, in some children the infection leads to a more severe illness such as bronchiolitis or pneumonia and may also increase the risk of developing subsequent asthma and/or recurrent wheezing episodes in early childhood [4–6].

The greatest burden of childhood RSV disease occurs in low- and middle-income countries (LMICs) and during a child’s first year of life [1, 2, 7]. This represents a significant unmet medical need for an affordable and effective RSV prevention strategy in LMICs [2, 7]. Palivizumab (Synagis^®, Swedish Orphan Biovitrum) was the first humanised monoclonal antibody approved for the prevention of serious RSV-LRTI in infants at high risk of RSV disease [8]. It has a half-life of about 20 days and requires monthly injections [8, 9]. Even though this first-in-class antibody has been approved for > 2 decades, palivizumab has limited use in infants in LMICs due to the high cost along with multiple doses needed per RSV season [2, 4, 7, 8].

In July 2023, the U.S. Food and Drug Administration approved a next generation monoclonal antibody nirsevimab (Beyfortus^®, AstraZeneca/Sanofi) for the prevention of RSV-associated LRTI for neonates and infants, and children at increased risk for RSV (up to 24 months of age) [10–12]. Nirsevimab has a longer half-life than palivizumab, 71 days in infants, thus potentially providing protection for an entire season with a single injection [11, 12]. However, the affordability and accessibility to nirsevimab are limited in LMICs [13].

Recently, an RSV vaccine (Abrysvo^®, Pfizer) which is approved for individuals ≥ 60 years of age for the prevention of LRTI caused by RSV, has also been approved for use in pregnant women to prevent LRTI in infants from birth through 6 months [14, 15]. Abrysvo is the first and only maternal vaccine approved to help protect infants through active immunisation of pregnant individuals [14, 15]. While the vaccine is potentially useful in LMICs, there are challenges associated with its use. The vaccination window is 32 to 36 weeks of pregnancy, and infants who are born prematurely may not be protected by the maternal vaccine [3, 15]. Furthermore, currently no vaccine exists for active immunisation of infants and children ≤ 5 years of age. Therefore, passive immunisation with a monoclonal antibody could protect young infants who may have insufficient maternal antibodies or older infants who continue to be at high risk for severe RSV disease. Thus, there remains a significant unmet medical need for a prophylactic antibody for paediatric population in LMICs.

RSM01 is a fully human IgG1 monoclonal antibody targeting antigenic site Ø of the pre-fusion conformation of the RSV-F glycoprotein [16–18]. The parental antibody (ADI-15618) was selected from a panel of > 200 monoclonal antibodies that were initially identified using Adimab’s (Lebanon, NH) B-cell technology in a high-throughput profiling study of the RSV-F antibody repertoire from healthy adults [19]. The preclinical screening process, lead selection, and the further antibody engineering into the clinical development candidate (RSM01) were performed at Arsanis Biosciences (Vienna, Austria) [18].

Following selection, ADI-15618 underwent sequence optimisation of the variable region to decrease its immunogenicity (removal of T cell epitopes) and improve its manufacturability. The YTE mutation was engineered in the Fc portion to extend the half-life of the antibody. The final molecule, RSM01 (ADI-15618-IVNS-YTE-RAS/TQ1), was selected based on thermal stability, viscosity measurements, stability under serum-like conditions, and affinity measurements, as well as in vitro and in vivo potency.

RSM01 is being developed to potentially provide an effective and affordable prevention strategy for RSV disease in LMICs. Here we present the preclinical characterisation, as well as safety, immunogenicity, pharmacokinetics (PK) and population PK results from the first-in-human, phase 1 trial of RSM01 (Gates MRI RSM01-101; NCT05118386).

Preclinical Characterisation

RSM01 epitope mapping

Binding of antibodies to peptides covering different antigenic sites on the RSV-F A2 protein was evaluated by Bio-layer interferometry (BLI; FortèBio Octet Red96 instrument, Pall Life Sciences). Peptides were designed to cover most of the surface exposed F-trimer residues (covering antigenic sites Ø, III, IV, V) and were synthesized in biotinylated and non-biotinylated forms. The sequence of biotinylated F-P1 peptide was:

Biot-GGSNIKENKCNGTDAKVKLIKQELDKYKNAVTELQL and the sequence of biotinylated F-P2 was: Biot-GGKNYIDKQLLPILNKQSCSISNIETVIEFQQK. Details of the BLI assay are provided in the supplement.

In vitro microneutralisation assay

Neutralising activity of RSM01 and clinically relevant comparator monoclonal antibodies (including MEDI8897 [now nirsevimab], and palivizumab) were tested in ELISA based virus microneutralisation assays in 96-well format. In addition to common laboratory strains (RSV-A2, RSV-A-Long, RSV-B1, RSV-B9320 and RSV-B Wash/18537), neutralisation potency was also assessed against a panel of clinical isolates, which had been collected during the 2002–2017 RSV seasons. Effective concentration leading to 50% viral neutralisation (EC50) was calculated based on 11-point dilution series using a non-linear curve fitting in GraphPad Prism 6.0 software (GraphPad Software, La Jolla, CA, USA). The assay details are described in the supplementary information.

Epitope conservation

RSV F Protein Sequences from 2024 were retrieved from National Center for Biotechnology Information Protein database (https://www.ncbi.nlm.nih.gov/protein) by searching for F protein sequences within the RSV-A and RSV-B taxonomic groups. Clustal Omega (version 1.2.4) was used for sequence alignment [20]. Sequence alignment details are described in the supplement.

Generation of monoclonal antibody resistant mutants (MARMs)

To select for antibody resistant mutants against RSM01 (and parental monoclonal antibody ADI-15618), viruses were serially passaged in vitro on HEp-2 cells (American Type Culture Collection [ATCC] CCL-23™) in presence of fixed antibody concentrations. Several experiments were carried out with laboratory strains (A2, B9320, B-Wash/18537) and clinical isolates (“consensus strains” RSV-A 13-005275 and RSV-B-07 000431, B-13-013576 and B-02000467), side by side with comparator antibodies palivizumab, nirsevimab and MEDI8897* (the non YTE version of nirsevimab). Procedures for passaging and virus sequencing are described in the supplement.

Human Fc gamma receptor (FcγR) binding assay

FcγR binding assay is described in the supplement.

Cotton rat model

Two independent animal studies were conducted at Sigmovir (Rockville, MD). Sigmovir received the blinded investigational product samples from Arsanis Biosciences. Male cotton rats (4 per group; 6 to 8 weeks of age) were weighed and administered with RSM01 or nirsevimab, or palivizumab prophylactically in the upper hind leg. Antibodies were administered 24 hours prior to intranasal challenge (10⁵ PFUs in 0.1 mL) with RSV-A2 or RSV-B Wash. RSM01 and nirsevimab were dosed at 0.3 and 1.0 mg/kg while palivizumab was dosed at 0.3, 1.0 and 3.0 mg/kg. Four days post-challenge, the lung and nasal tissues were harvested, weighed and subjected to viral titre analysis as described in the supplement. Eye bleeds (300 µL) for serum collection were performed prior to dosing with antibodies, prior to viral challenge and on day 4 post-challenge.

Animal and ethics statement

All animal experiments were performed under approval of Sigmovir's Institutional Animal Care and Use Committee.

RSV strains and clinical isolates

Prototype laboratory strains were obtained from ATCC (RSV-A2: ATCC VR-1540P; RSV-A-Long: ATCC-VR-26, RSV-B9320: ATCC VR-955; RSV-B-Wash/18537: ATCC VR1580) and BEI Resources (RSV-B1: NR4052). Clinical RSV-A and RSV-B isolates were provided by University Medical Center Utrecht (Utrecht, The Netherlands). All clinical isolates were collected during the 2002 and 2017 RSV seasons. The first two numbers in the isolate name refer to the year of isolation

Clinical Trial

Trial Design and Objectives

This was a phase 1, randomised, double-blind, placebo-controlled trial of RSM01 in healthy adults. The trial was conducted in 2 parts: a dose escalation phase with 4 dosing cohorts, followed by an expansion phase with a single cohort (Supplement Fig. 1). The dose escalation phase had parallel enrolment to cohorts 2 and 3. The first participant in each of the 4 dose-escalation cohorts received the RSM01 dose level appropriate to the assigned cohort to serve as a sentinel exposure participant. The remaining participants were randomised 5:1 to receive either RSM01 or placebo (a total of 6 RSM01 participants and 1 placebo participant in each cohort). In the dose expansion cohort, participants were randomised 6:1 to receive RSM01 (n = 24) or placebo (n = 4).

On day 1 of the trial, participants received RSM01 in single intravenous (IV) or intramuscular (IM) doses as follows:

Cohort 1: 7 participants received RSM01 300 mg IV (n = 6 total including sentinel participant) or Placebo (n = 1)
Cohort 2: 7 participants received RSM01 300 mg IM (n = 6 total including sentinel participant) or Placebo (n = 1)
Cohort 3: 7 participants received RSM01 1000 mg IV (n = 6 total including sentinel participant) or Placebo (n = 1)
Cohort 4: 7 participants received RSM01 3000 mg IV (n = 6 total including sentinel participant) or Placebo (n = 1).

Enrolment into each subsequent cohort proceeded after a safety review team reviewed data through day 15 and determined that no pausing criteria had been met. In the dose expansion phase, 28 participants received RSM01 600 mg IM (n = 24) or placebo (n = 4).

Participants were required to attend the screening visit between 30 days and 2 days before the planned day 1 visit, and the day − 1 visit within 24 hours before planned dosing to begin confinement. Participants in the dose escalation phase were confined at the trial site from day 1 until after the completion of the day 3 assessment (a total of 3 nights). Participants in the dose expansion phase also began confinement on day − 1 and were confined for at least 4 hours post-dose.

Randomization in each cohort was based on a randomly generated sequence of participant identification numbers generated using a validated Interactive Voice/Web Response System (IXRS). Randomization schedule was prepared by a statistician who was not involved in the trial in order to maintain the blind of the trial team. The sentinel participant in each dose escalation cohort was assigned to the cohort treatment in a single blinded manner wherein only the participant was blinded to the treatment, but the site was not. The remaining participants were randomized in a double-blind manner wherein the site personnel were also blinded to the treatment.

The primary objective was to evaluate the safety and tolerability of a single dose of RSM01. The secondary objectives included characterisation of the PK and anti-drug antibodies (ADAs) in capillary blood. Exploratory objectives included characterisation of PK, ADA, and RSV-neutralising antibodies in serum.

An interim analysis occurred after all participants in dose escalation cohorts (cohorts 1–4) completed day 91. The purpose of interim analysis was a strategic review to inform the broader program and future development plans, including the determination of the potential dose of RSM01 in infants. No decision directly related to this trial was made from the results of interim analysis. The primary analysis occurred after all participants in all 5 cohorts completed Day 151.

Trial Conduct

The trial took place in the United States (US) between Nov 2021 and Dec 2022, and the protocol was approved by the Institutional Review Board (IRB) of Advarra^™ (Columbia, MD, US). The trial was conducted in accordance with the principles of the Declaration of Helsinki, and Good Clinical Practice guidelines of the International Council for Harmonisation (ICH), and all applicable sections of the United States Code of Federal Regulations (CFR), 21 CFR Parts 50, 56, and 312. All participants provided written informed consent. This trial was registered with Clinical Trials.gov: NCT05118386.

Trial Population

The trial included healthy adult males and females (of nonchildbearing potential) between 18 and 49 years of age. Eligibility was determined during the screening period based on physical examination, vital signs, medical history, laboratory assessments, and human immunodeficiency virus (HIV) test results. Participants with a BMI between 18 and 29.9 kg/m² were included. Participants were asked to stay in contact with the trial staff throughout the duration of the trial and with no plans to relocate outside of the trial area. Key exclusion criteria included acute illness and/or elevated body temperature on day of randomisation, clinically significant medical conditions, history of immune disease or deficiency, medications or therapies that may have impacted the immune system, vaccinations within 15 days before randomisation, immunosuppressive agents within 90 days of randomisation, any investigational drug within 30 days or 5 half-lives before randomisation, abnormal safety laboratory tests and/or urinalysis, abnormal electrocardiogram (ECG), reactive HIV test, chronic hepatitis infection, history of allergies, and positive urine drug screen.

Assessments

Safety

Safety evaluations included physical examination, vital signs, 12-lead ECG, laboratory assessments (haematology, serum chemistry, urinalysis), and serum pregnancy test (for females). Local injection site solicited adverse events (AEs) (including pain, redness, and swelling) were assessed after dosing and through day 7 in participants who received an IM dose. Systemic solicited AEs (including fever, headache, fatigue, muscle aches, nausea, vomiting, diarrhea, and joint pain) were assessed in all participants after dosing and through day 7. Site staff were responsible for documenting AEs during the confinement period. Participants were required to use a diary card to record solicited AEs from after completion of the confinement period through day 7, and a memory aid to collect unsolicited AEs from after completion of the confinement period through day 151. All serious adverse events (SAEs) and AEs of special interest (AESIs) were assessed through day 151. Events collected and reported as AESIs were anaphylaxis or hypersensitivity reactions, and/or infusion reactions resulting in permanent discontinuation of study intervention infusion during IV administration.

RSM01 PK, ADA, and RSV-Neutralising Antibodies

Samples for PK and ADA assessments for cohorts 1–4 (dose escalation) and cohort 5 (dose expansion) were collected on days 1 through day 151 as shown in supplement table 1. Samples for measuring RSV- neutralising antibodies were collected on days 1, 91 and 151 for all cohorts (Supplement Table 1).

RSM01 serum concentration was measured using a validated electrochemiluminescence assay using anti-idiotype capture and anti-human-IgG detection as described by White et al. (companion manuscript submitted to BMC ID). Results below the lower limit of quantitation were reported as < 300 ng/mL RSM01.

ADA in serum were detected using a validated bridging electrochemiluminescence assay as described by White et al. (companion manuscript submitted to BMC ID). The assay was conducted in 3 tiers: screening, confirmation of signal decrease in presence of excess drug, and titration until signal falls below statistically determined threshold.

RSV-neutralising antibodies were detected with a qualified mKate-RSV-A2 viral neutralisation assay described previously [17]. The viral neutralisation activity was reported as the 50% inhibitory dilution (ID50) of samples [21].

Statistical Analyses

Safety

The trial was designed to be descriptive and was not based on evaluation of formal null hypotheses. Therefore, it was not powered to detect any differences in potential safety observations between the treatment groups.

The safety population included all participants who received trial intervention. A treatment-emergent AE (TEAE) was defined as any event that was absent before exposure to trial intervention or any event already present that worsened in intensity or frequency after exposure. All TEAEs were coded using Medical Dictionary for Regulatory Activities (MedDRA) Version 25.1 and were graded according to the U.S. Department of Health and Human Services, Common Terminology Criteria for Adverse Events, Version 5.0.

PK analysis

The PK analysis included all participants who received RSM01 and had baseline and at least one post-baseline PK result. PK parameters including area under the concentration curve (AUC) from time 0 to day 151 (AUC_0-D151), initial serum concentration of RSM01 (C₀), C_D151, time to maximum concentration (T_max), were determined using noncompartmental analysis (NCA) of concentration-time data. Due to limited data from later timepoints (> 150 days), parameters such as AUC_0-inf, and t_1/2, could not be calculated accurately with NCA and were re-evaluated using population PK methodology.

Dose proportionality for RSM01 PK parameters in serum was assessed using a power regression model for AUC_0-inf, AUC_0-D91, AUC_0-D151, and C_max, expressed as: ln [PKparameter] = β0 + β1 ln [Dose]. Dose-proportionality was established if the 90% confidence interval (CI) of the slope β1 was within [1 + ln (0.5)/ln(r), 1 + ln (2.0)/ln(r)], where r is the ratio of the highest to the lowest dose.

Bioavailability of RSM01 following IM injection was determined by comparing the ratio of geometric mean AUC_0-inf and AUC_0-D151 between IM and IV doses using an ANOVA model with treatment group as a factor. Results were exponentiated to obtain ratios of geometric means and 90% CIs.

Population PK modeling

RSM01 concentration data were used to develop a population model to: describe the individual time concentration profiles of RSM01, assess potential influencing factors which contribute to PK variability across participants (i.e., covariates), support the dose or doses to be administered in subsequent phase 2 studies in both full-term and preterm infants, and inform PK sampling strategy for future trials.

For this analysis, evaluable participants were those who received RSM01 and had at least 1 measurable RSM01 concentration observation with associated sampling time and dosing information. Source data included dosing information, PK sampling information, and relevant covariate information collected in the clinical trial. Demographics and clinical laboratory values were also provided.

Simulation of RSM01 PK in infant populations

The population parameters and inter-individual variability from the final adult model were used to extrapolate the PK of RSM01 in North American and African infant populations across three age groups: 0 to 3 months, 3 to 6 months, and 6 to 12 months. Detailed methods are described in the supplement.

Immunogenicity and RSV neutralising activity

Immunogenicity incidence was tabulated by treatment administered for both the baseline status and post-baseline status of ADA. Participants were denoted as either positive or negative for each. RSV neutralising activity was baseline corrected by dividing the post-baseline ID50 by the baseline ID50 to correct for the expected baseline RSV neutralising activity from natural infection in this adult population. Summary statistics were performed for each time point.

Preclinical characterisation

Determination of RSM01 epitope

Competition analysis by BLI with known anti-RSV monoclonal antibodies including MEDI8897* (non YTE version of nirsevimab, site Ø), motavizumab/palivizumab (site II), 101F/RB1 (site IV), and REGN222 (site V) indicated that RSM01 binds to antigenic site Ø [18]. RSM01 parental antibody ADI-15618 bound F-P2 (linear epitope aa 196–226) in a dose-dependent manner confirming site Ø specificity (Table 1). No binding to the linear peptides covering the other antigenic sites including site Ø F-P1 (aa 62–95) was observed. MEDI8897 bound neither to F-P2 or F-P1 indicating a distinct epitope from RSM01 (Table 1).

Table 1

Response units for RSV monoclonal antibodies binding to biotinylated peptides from antigenic site Ø
Antibody	Concentration (nM)	Biot-F-P2	Biot-F-P1
Motavizumab (site II)	100	-0.0358	0.0917
REGN2222 (site V)	100	-0.0332	-0.0194
D25 (site Ø)	100	-0.0388	-0.0212
ADI-15618 (site Ø)	100	1.6265	-0.0186
REGN2222 (site V)	5000	-0.0271	-0.0029
MEDI8897 (site Ø)	5000	0.0035	-0.0382
ADI-15618 (site Ø)	5000	4.7153	0.0179
Binding of peptide was analysed by BLI. Two biotinylated peptides from antigenic site Ø were F-P2 (aa 62–95) and F-P1 (aa 196–226). Biot biotinylated; Biot biotinylated; BLI bio-layer interferometry; RSV Respiratory syncytial virus; aa amino acid

X-ray crystallography was employed for fine epitope mapping of RSM01; the structure of the antibody-antigen complex (ADI-15618 Fab bound to RSV pre-F ecto A2 DS-Cav1 trimer) was solved at 3.9Å resolution (unpublished observations). Contact residues in the primary interface between RSM01 CDRs (complementary-determining regions) and F-protein are all located in site Ø with the majority in F-P2 region in agreement with binding and peptide mapping data. The main contact residues in RSM01 F protein interface were identified as: K68, N70, G71, T72, K201, Q202, L203, P205, I206, K209, Q210, S211, C212, S213, I214, S215, N216, T219. The primary interface was used to select clinical isolates with variations in these residues to assess potential resistance to RSM01 (Supplement Table 2).

In-vitro neutralisation activity

To determine the breadth of RSM01 neutralising activity, a panel of RSV-A and B viruses was assembled. The panel included common laboratory strains A2, A-Long, B9320, B1, B-Wash/18537, and 19 clinical strains isolated during the 2002–2017 RSV seasons. The clinical isolates were selected to cover all available sequence polymorphisms in antigenic site Ø (Supplement Table 2) allowing assessment of the impact of existing variations on antibody activity thereby increasing the probability to identify strains with lower susceptibility to RSM01.

RSM01 exhibited highly potent neutralising activity against all tested RSV strains (Fig. 1, Supplement Table 3). It was 2-3-fold more potent than nirsevimab and approximately 51-196-fold more potent than palivizumab for RSV-A strains (Supplement Table 4). For RSV-B, RSM01 was 1–10 fold more potent than nirsevimab and 3–90 fold more potent than palivizumab (Supplement Table 4). Potency was in the single ng/mL range (0.7–6.4) for all RSV-A subtypes and most of RSV-B subtypes (Fig. 1, Supplement Table 3). Interestingly, a RSV-B Wash/18537 isolate that was initially used at Sigmovir (Rockville, MD) for cotton rat studies (“RSV-B Wash-Military”), was found to be completely resistant to nirsevimab (Fig. 1, Table 3). This phenotype was confirmed in vitro and could be linked to the additional L203I sequence variation in the RSV-F-protein which was not present in the original RSV-B Wash/18537 isolate from ATCC (Supplement Table 2). Although L203I is located within the RSM01 epitope it did not abolish neutralising activity of RSM01 (EC50 = 21.5 ng/mL; Figure. 1, Supplement Table 3). All later cotton rat studies were performed with the original strain not harbouring this additional mutation, restoring the in vivo efficacy of nirsevimab.

In 2017, the rise of an RSV-B mutant strain was responsible for the failure of the REGN2222 binding to antigenic site V of RSV-F [20]. As expected, due to the different binding site, an RSV-B virus isolated during the 2017 season (17–000478) that was resistant to REGN2222 was fully susceptible to RSM01 (Supplement Table 3).

Epitope conservation

To understand the epitope conservation, an RSV sequence alignment was performed in 2018 using available RSV-A (n = 1632) and B (n = 668) sequences from GenBank. Since laboratory strains are heavily passaged and do not represent a consensus sequence, a first alignment of the GenBank sequences was done, and the consensus sequence of F protein was derived. Two clinical isolates represented this consensus sequence of the F protein: RSV-A strain 13-005275 and RSV-B strain 07–00043. Of note, both clinical isolates were fully neutralised by RSM01 (Fig. 1, Supplement Table 3).

Based on these initial results the RSM01 epitope was found to be highly conserved. Only 2.5% of the published RSV-A genomes and 12.15% of RSV-B strains showed sequence polymorphisms in the binding site of RSM01 (Supplement Table 5). For RSV-A, S213R was the most frequent variation and the only polymorphism that occurred with a relative frequency of > 1% (1.84%). Several strains (e.g. RSV-A-Long) carrying this mutation were included in the panel for in vitro testing and were confirmed to be neutralised by RSM01 (Fig. 1, Supplement Table 3).

A similar trend was observed for RSV-B with most variations occurring at very low frequency of < 1% (Supplement Table 5). Only one variant, Q209K, was found to be present in 7.49% of the available RSV-B genomes. This polymorphism was tested with a clinical isolate (RSV-B 13-013576) and RSM01 binding remained unaffected (EC50 = 1.6ng/ml) (Fig. 1, Supplement Table 3). The remaining variations accounted for only 0.15%-0.75% of the published sequences (Supplement Table 5), and 3 of these variants (Q202R, L203I, Q209R) were covered in the RSV-isolate panel tested in vitro (Supplement Tables 2 and 3).

A second sequence alignment of RSV F proteins was performed in 2024 (Supplement Table 6). The number of RSV sequences in GenBank substantially increased since 2018 to ~ 6,000 for each RSV subtype. To assess which amino acids were predominant, the most dominant animo acids were identified. Amino acids variation for RSV-A in antigenic site Ø remained very low with < 0.1% variation and no major differences from the 2018 assessment in the RSM01 primary interface (Supplement Table 6). For RSV-B, more variations were seen in the primary interface though the majority remained at low levels.

The most dominant amino acid polymorphisms were found in positions 206, 209 and 211. In the 2018 analysis, the variant I206M was a rare amino acid substitution reported only in 0.15% of RSV-B strains. In the 2024 analysis, M was the dominant amino acid at position 206 (68% of isolates) with I (now designated M206I) found only in 32% of strains (Supplement Table 6). Similarly, while Q209R was rare (0.3%) based on the original alignment in 2018, R now became the dominant residue (69% of isolates) according to the updated analysis; Q (R209Q) was found in only 31% (Supplement Table 6). Of note, RSV-B-09-017069, an RSV-B isolate harboring the Q209R mutation was included in the in vitro testing and was neutralized by RSM01 (EC50 4ng/ml (Supplement Table 3). Overall, we found that about 30% of all sequences analysed had variations in 206 or 209 positions (i.e. amino acids other than M206 or R209) and of these 48% had variations in both positions. The M206I and R209Q variations were co-present in most of these viruses.

Another polymorphism that increased in frequency was found to be in position 211 (S211N). While less than 0.5% of all available sequences showed variation in this position in 2018, this increased to 11.89% in the updated analysis (Supplement Table 6). Binding and neutralisation for this variant need to be confirmed in future studies.

Generation of MARMs

The use of monoclonal antibodies against viruses raises potential concern about the emergence of MARMs. Such escape variants have been described earlier for the benchmark monoclonal antobodies used in this study, palivizumab and nirsevimab [22–27]. A total of 11 experiments using 8 different RSV strains and antibody concentrations ranging from 60 ng/mL to 50 µg/mL were performed during the lead selection/characterisation phase of RSM01 [18]. No escape mutants emerged against RSM01 (or parental antibody ADI-15618), independent of the antibody concentration tested. However, MARMs were readily obtained with palivizumab (at 2.5 µg/mL and 50 µg/mL) and MEDI8897*/nirsevimab (at 60 ng/mL or 50 µg/mL). The mutations associated with the breakthrough viruses under palivizumab or nirsevimab/MEDI8897* were detected as early as passage 2 (P2). Observed mutations in the antibody binding sites are shown in the supplement (Supplement Table 7). Of note, passaging of the clinical isolate RSV-B 07 000431 (representing the consensus RSV-F sequence) in the presence of nirsevimab resulted in the rapid emergence of a resistant mutant carrying the L203I mutation in the F protein. This mutation was previously found in the nirsevimab resistant RSV-B-Wash/18537 strain, suggesting that L203I is indeed responsible for the resistance phenotype.

Prophylactic efficacy of RSM01 in cotton rat model

At 1 mg/kg, RSM01 reduced viral load in the lungs by 3.0 and 2.8 logs, nirsevimab reduced viral load by 3.1 and 1.6 logs and palivizumab by 1.0 and 1.1 logs in RSV-A and RSV-B infected animals, respectively (Fig. 2). At 1 mg/kg, RSM01 reduced nasal viral load by 3.0 and 2.4 logs while nirsevimab reduced viral load by 3.3 and 0.2 logs and palivizumab did not reduce viral titers in RSV-A and RSV-B infected animals, respectively (Supplement Table 8). Overall RSM01 was as potent as nirsevimab in reducing viral load in lungs or nasal tissue for RSV-A and more potent than nirsevimab for RSV-B. Compared with palivizumab, RSM01 was more potent in both compartments and with both RSV subtypes. The EC90 in cotton rats for nirsevimab was reported as 6.8 µg/mL [28, 29]. Based on efficacy analysis in cotton rat challenge model, it appears EC90 of RSM01 may to be comparable to nirsevimab.

Human FcγR binding

In-vitro binding studies demonstrated that RSM01 can bind to all human FcγRs, including Fcγ receptor I (CD64), IIA167His (CD32A167His), IIA167Arg (CD32A167Arg), IIB (CD32B), IIIA176Val (CD16A176Val), IIIA176Phe (CD16A176Phe), and IIIB (CD16B) (Supplement Table 9).

RSM01 Phase 1 Clinical Trial

Participants’ baseline characteristics and disposition

The trial period was 16 Nov 2021 to 07 Dec 2022 and all participants were followed through day 151. Of the 56 randomised participants, 48 received RSM01, 8 received placebo, and 53 completed the trial (Fig. 3). Two participants, both from the RSM01 3000 mg IV cohort, were lost to follow-up, and 1 participant, from the RSM01 600 mg IM cohort, discontinued from the trial due to a sponsor decision (participant unavailable for final PK assessment). Overall, demographics and baseline characteristics were comparable between the RSM01 and placebo groups (Table 2).

Table 2

Demographics and Baseline Characteristics
Characteristic	RSM01 Cohorts						Total Placebo N = 8
Characteristic	300 mg IV, N = 6	300 mg IM, N = 6	1000 mg, IV N = 6	3000 mg, IV N = 6	600 mg IM, N = 24	Total RSM01 N = 48	Total Placebo N = 8
Median age, years (range)	28 (24–36)	32 (19–45)	32 (20–41)	25 (20–45)	33 (19–48)	30 (19–48)	30 (25–39)
Sex, n (%) Male	5 (83)	3 (50)	1 (17)	4 (67)	13 (54)	26 (54)	3 (38)
Race, n (%) White	4 (67)	4 (67)	3 (50)	5 (83)	16 (67)	32 (67)	5 (63)
Black	1 (17)	2 (33)	2 (33)	0	6 (25)	11 (23)	3 (38)
Native Hawaiian or Other Pacific Islander	0	0	0	1 (16.7)	0	1 (2.1)	0
Multiple	1 (17)	0	1 (17)	0	2 (8)	4 (8)	0
BMI, kg/m² Mean (SD)	23.3 (2.0)	25.4 (3.0)	24.8 (2.8)	24.5 (4.5)	24.7 (3.2)	24.6 (3.1)	22.9 (2.1)
BMI body mass index; IM intramuscular; IV intravenous; SD standard deviation

Safety

Overall, 12 (25%) participants in the RSM01 group and 2 (25%) in the placebo group, experienced unsolicited TEAEs (Table 3). COVID-19, headache, and nausea were the only unsolicited TEAEs in ≥ 2 participants in any of the RSM01 cohorts. There was no pattern in the AEs by route or dose of RSM01. Most AEs were mild or moderate in severity. One (2.1%) participant in the RSM01 3000 mg IV cohort experienced a severe AE of increased blood pressure which was not considered related to RSM01. The participant’s blood pressure increased from 135/90 mmHg predose to a maximum systolic blood pressure of 156 mmHg and a maximum diastolic blood pressure of 103 mmHg post dose. The event was resolved the next day.

Table 3

Safety Summary
AEs category	RSM01 Cohort, n (%)						Total Placebo N = 8
AEs category	300 mg IV N= 6	300 mg IM N = 6	1000 mg IV N = 6	3000 mg IV N = 6	600 mg IM N = 24	Total RSM01 N = 48	Total Placebo N = 8
Any TEAE	1 (17)	2 (33)	3 (50)	3 (50)	3 (13)	12 (25)	2 (25)
Treatment-related TEAEs	0	1 (17)	2 (33)	0	1 (4)	4 (8)	0
Severe of life threatening TEAEs	0	0	0	1 (17)	0	1 (2)	0
TEAEs leading to dose interruption/withdrawal (IV participants only)	0	NA	1 (17)	0	NA	1 (2)	0
Any SAE	0	0	0	0	0	0	0
Any AESI	0	0	0	0	0	0	0
Unsolicited TEAEs in ≥ 2 Participants
COVID-19	0	0	0	1 (17)	1 (4)	2 (4)	0
Headache	0	0	2 (33)	0	0	2 (4)	1 (13)
Nausea	0	0	2 (33)	0	0	2 (4)	0
Deaths	0	0	0	0	0	0	0
Events are sorted by frequency of TEAEs in the total RSM01 group. Participants with multiple adverse events within a MedDRA Preferred Term were counted only once. AESI adverse event of special interest; IM intramuscular; IV intravenous; NA not applicable; SAE serious adverse event; TEAE treatment emergent adverse event.

Treatment-related unsolicited AEs were reported in 4 (8.3%) participants (Table 3). One participant in the 300 mg IM cohort had injection site pruritus and one in the 600mg IM cohort had pruritus. One participant in the 1000 mg IV cohort had nausea and one had nausea, infusion site pain, and headache. All treatment related AEs were mild to moderate in severity. The infusion site pain led to dose interruption that lasted for 11 minutes, after which the dosing was completed. No SAEs, AEs leading to discontinuation of trial intervention, or death, or AESIs were reported in the trial.

Solicited systemic and local AEs were reported more frequently with RSM01 than with placebo (Supplement Table 10). Five (10.4%) participants in the RSM01 group reported headache and 2 (4.2%) reported tiredness. None of the participants reported fever, joint pain, muscle pain, nausea, vomiting, or diarrhea within 7 days after RSM01 administration. All systemic solicited AEs were mild or moderate in severity and the majority had durations of 1 or 2 days. Of the 11 (45.8%) participants in the 600 mg IM cohort who received two injections in one thigh, 1 reported a mild solicited systemic AE of tiredness on day 1. In the placebo group, 1 (12.5%) participant reported a systemic solicited AE of fever (duration of 3 days; mild in severity).

Laboratory assessments, vital signs measurements and ECG parameters measured at baseline and throughout the trial were generally comparable between the RSM01 and placebo groups. There were no individual clinically significant abnormalities and no clinically meaningful difference in toxicity grade shifts between the RSM01 and placebo groups during the trial.

PK

PK parameters for RSM01 in serum are shown in Table 4. In the 300 mg and 600 mg IM cohorts, peak concentrations in serum (T_max) were reached between approximately 6 days and 8 days post dose (Fig. 4). RSM01 concentration and AUC increased dose-proportionally following IV and IM administration (Supplement Fig. 2). RSM01 was eliminated gradually, in a monophasic manner following IM administration and in a biphasic manner following IV administration (Supplement Fig. 3). While comparing serum PK profiles, the IM route showed higher individual variability in the clearance (CL), than IV route. The observed between-participant variability was in line with the anticipated variability for a monoclonal antibody following IM administration.

Table 4

Summary of RSM01 serum PK in healthy adults from phase 1 trial
PK Parameter	300 mg IV N = 6	300 mg IM N = 6	600 mg IM N = 24	1000 mg IV N = 6	3000 mg IV N = 6
T_max [day]	0.05 (0.05–0.05)	5.97 (5.06–27.9)	7.22 (6.9–29.1)	0.24 (0.07–1.07)	0.26 (0.09–0.73)
C_max or C₀ [µg/mL]	98.2 (15.4)	39.8 (12.4)	90.7 (21.6)	314 (34.1)	1050 (23.5)
AUC_last [day*µg/mL]	4095 (43.4)	2691(56.0)	6958 (40.7)	14833(26.6)	450000 (23.9)
All data are presented as geometric mean (geometric %CV) except for T_max which are presented as median (min, max). AUC_last area under the curve from dosing to last measurable concentration; C₀ initial concentration; C_max maximum concentration; CV Coefficient of variance; IM intramuscular; IV intravenous; PK Pharmacokinetics; T_max time to maximum concentration.

Population PK modeling and simulation

The PK analysis dataset comprised 420 samples, of which 372 samples from 48 participants contained measurable PK observations. Forty-eight serum observations, representing 11% of the total, fell below the lower limit of quantitation (BLQ) for the assay. These BLQ samples were considered missing data in the pharmacokinetics modeling analysis.

The PK of RSM01 following IV and IM administration was well characterised by a 2-compartment model with a zero-order absorption (for IM) and first-order elimination. The model estimated the CL, central volume of distribution (Vc), peripheral volume of distribution (Vp) and inter compartment clearance (Q) at 0.002 L/hr, 3.2 L/hr, 2.18 L/hr, and 0.029 L/hr, respectively. The model-derived half-life in adults after a 300 IM dose was 78 days (95% CI 31–190). The bioavailability of RSM01 was estimated to be approximately 80%. Differences in PK parameters based on body size were accounted for by using fixed-exponent allometric relationships. No covariates were selected in the covariate analysis.

The population PK model was used to extrapolate and simulate dynamics within virtual populations representative of infants in North America and Africa. Predicted PK parameters for the RSM01 50-mg intramuscular (IM) dose given to the North American and African infant populations are summarised in Table 5. The highest C₁₅₀ and C_max were observed for the 0 to < 3 months old infants for both the North American and African infant populations. Among the three infant age subgroups, the median C₁₅₀ was 31 µg/mL and 52 µg/mL for 0 to < 3 months old, 21 µg/mL and 25 µg/mL for 3 to < 6 months old, and 16 µg/mL and 20 µg/mL for 6 to < 12 months old, for the North American and African infant populations, respectively. Similar trend was observed for C_max for both the North American and African infant populations.

Table 5

Predicted PK parameters of RSM01 exposure in virtual North American and African infant populations following a 50 mg IM dose
	0 to < 3 Months (N = 500)	3 to < 6 Months (N = 500)	6 to < 12 Months (N = 500)
North American Infants
Weight (kg)	5.9 (4.7, 7.5)	7.6 (5.9, 9.3)	9.4 (7.1, 11)
C_max (µg/mL)	83 (60, 120)	65 (46, 94)	52 (38, 74)
C₁₅₀ (µg/mL)	31 (12, 53)	21 (6.3, 40)	16 (3.6, 30)
AUC (mg•day/mL)	6.8 (4.4, 10)	5.1 (3.2, 7.9)	4.1 (2.4, 6.0)
Fraction of infants with C₁₅₀ > 6.8 µg/mL (%)	98.2	93.6	88.8
African Infants
Weight (kg)	3.6 (2.4, 5.1)	5.6 (4.4, 6.9)	7.3 (5.4, 9.2)
C_max (µg/mL)	130 (83, 220)	84 (60, 120)	66 (48, 94)
C₁₅₀ (µg/mL)	52 (23, 91)	25 (8.1, 49)	20 (5.5, 38)
AUC (mg•day/mL)	11 (6.5, 18)	6.4 (3.9, 9.8)	5.2 (3.0, 8.4)
Fraction of infants with C₁₅₀ > 6.8 µg /mL (%)	99.6	96.0	92.8
Data are presented as median (5th percentile, 95th percentile). AUC area under the RSM01 concentration-time curve; C₁₅₀ trough RSM01 concentration at day 150; C_max maximum RSM01 concentration; N number of subjects with available information; PK pharmacokinetics

The population PK model was used to predict the proportion of infants who would have RSM01 concentration levels above the EC90 threshold of 6.8 µg/mL 150 days post dose, to determine the potential rate of protection per RSV season (Table 4 and Fig. 5). According to the simulations, in African infants, following an RSM01 dose of 50 mg IM, 99.6%, 96.0%, and 92.8% of infants in the age groups of 0 to < 3 months, 3 to < 6 months, and 6 to < 12 months, respectively, were predicted to maintain RSM01 concentrations above the EC90 threshold of 6.8 µg/ml. For North American infants, following an RSM01 dose of 50 mg IM, 98.2%, 93.6%, and 88.8% of infants for the same respective age groups were predicted to maintain RSM01 concentrations above 6.8 µg/ml. These simulation results indicated that a single dose of RSM01 50 mg could potentially provide protection to infants (0 to < 12 months old) for the entire RSV season.

Immunogenicity

The baseline ADA-positive rate was 2/48 (4.2%) in RSM01 group. The two baseline ADA positive participants were in the RSM01 600 mg IM cohort and one remained ADA-positive after RSM01 administration. One baseline ADA-negative participant in the RSM01 1000 mg IV cohort was categorized as postbaseline ADA-positive, for a treatment-emergent ADA-positive rate of 1/48 (2.1%) in the RSM01 group. All participants in the placebo group were ADA‑negative at baseline and throughout the trial.

None of the participants who were either baseline ADA-positive or treatment emergent ADA-positive experienced any unsolicited or solicited AEs after receiving RSM01. There was no difference in pharmacokinetics parameters in serum between the participants who were either baseline ADA-positive or treatment-emergent ADA-positive and the other participants in their respective cohorts (Supplement Fig. 4).

RSV Neutralising Activity of RSM01 in participants’ serum

RSV neutralising activity was measurable in all serum samples including prior to the first dose, with no samples BLQ. Analysis of baseline corrected RSV neutralising activity showed that neutralising activity correlated with the RSM01 dose (Fig. 6).

Here we present the preclinical characterisation and first-in-human clinical trial results for RSM01, a novel, fully human, half-life extended anti-RSV monoclonal antibody candidate. Based on the results from our preclinical experiments, RSM01 demonstrated high potency with in vitro potency comparable to nirsevimab and higher than palivizumab. In the cotton rat model of RSV infection, RSM01 was shown to markedly reduce both lung and nose viral load of both RSV-A and -B subtypes and was more potent against RSV-B compared with nirsevimab. Based on efficacy analysis in cotton rats, it appears the EC90 for RSM01 appears may be similar to nirsevimab.

The epitope of RSM01 was determined to be present in the site Ø of RSV-F protein. Although the F protein is a highly conserved protein, antigenic site Ø accounts for up to 25% of variability in F protein sequences [30]. Alignment of F proteins available in GenBank Database showed that the RSM01 epitope is highly conserved. However, amino acids 209 and 211 showed high variability between analyses conducted in 2018 and 2024. This modulation was perhaps the result of altered RSV circulation pattern during the COVID-19 pandemic. A similar sequence analysis by Wilkins et al. using sequences from three surveillance studies from 17 countries (OUTSMART-RSV, INFORM-RSV, South Africa Pilot) that covered the 2015–2021 seasons reported > 30% variation frequency for I206M and Q209R [31]. Results from our recent sequence alignment in 2024 also showed reversions to the older predominant amino acids.

The EC50 for variant viruses were tested using a panel of RSV clinical isolates. Our results showed very similar EC50 values in single digit ng/mL range (0.7, 6.4) for all RSV-A viruses and most of RSV-B viruses in the panel. The only differences were observed in laboratory strains that have been extensively passaged in vitro. An updated panel with circulating RSV strains from LMICs is currently being assembled to further examine the breadth of RSM01 activity. RSM01 neutralization of the additional new variants that we identified need to be confirmed in future studies.

RSM01 exhibited a high barrier to resistance with no escape mutants identified during serial passage experiments. Further, RSM01 remained active against an L203I mutation selected during passage with nirsevimab. Importantly, one of the clinical RSV-B isolates with the amino acid substitution Q209K that is reported in about 7.5% of all circulating RSV-B strains remained highly susceptible to RSM01. A high barrier to resistance could be a valuable characteristic of an RSV monoclonal antibody candidate.

The mechanism of action of approved RSV monoclonal antibodies is neutralisation [27]. However, other mechanisms such as Fc-mediated effector functions could potentially contribute to efficacy. RSM01 was shown to bind to all human FcγRs though it was less efficient than the control IgG1, consistent with the reports that addition of YTE can reduce FcγR binding and functionality [32]. The relevance of effector function in antibody protection against RSV, particularly in a prophylactic setting where neutralisation precludes virus entry, has not been established. Overall, the preclinical characteristics including high potency in vitro and in vivo, epitope conservation, and a high barrier to resistance led progression into the first-in-human clinical trial.

The first-in-human randomised, double-blind, placebo-controlled phase 1 trial of RSM01 monoclonal antibody candidate evaluated the safety, tolerability, and pharmacokinetics of single ascending doses of RSM01 in healthy adults. In this trial, a total of 56 healthy adult participants were enrolled and dosed (48 received RSM01 and 8 received placebo), in a dose escalation/dose expansion design, and 53 completed the trial. The single doses of RSM01 administered by the IV or IM route across a range of doses from 300 mg to 3000 mg were generally well tolerated in this trial. This is consistent with the anticipated profile of a fully human monoclonal antibody and with data from similar antibodies with extended half-life such as nirsevimab [33–35]. Most TEAEs in the trial were mild or moderate in severity and only one severe event was reported. Solicited systemic and local AEs were more common in the RSM01 group than placebo, however all events were mild or moderate in severity and lasted for only 1 or 2 days. There were no deaths, SAEs, or AESIs reported during the trial.

To characterise the initial PK of RSM01 in humans, both IV and IM routes were evaluated in this trial. In the IM cohorts, the T_max was between 6 and 8 days. Increased dosing of RSM01 within IV and IM cohorts suggested PK dose proportionality. The observed variability in RSM01 concentrations between participants was in line with the anticipated variability for a mAb following IM administration. The gradual elimination of RSM01 with a half-life of 78 days is consistent with data available for other fully human monoclonal antibodies with an extended half-life [36]. Overall, the PK profile of RSM01 was similar to that of nirsevimab in the healthy adult population [36].

The PK of RSM01 was well characterised by a 2-compartment model with zero-order absorption for the IM route of administration and proportional error model. The estimated bioavailability was roughly 80%. Body size-based differences in PK parameters were accounted for by using fixed-exponent allometric relationships. No covariates were selected in the covariate analysis. The population PK model was used to extrapolate and simulate RSM01 PK within virtual populations of North American and African infants. Qualitatively, RSM01 exposure decreased with an increase in body weight because of the inclusion of weight-dependent allometric scaling on CL. Since the North American infant population has generally higher body weight than the African infant population, the simulations showed about 30% lower RSM01 exposure in North American infants relative to the exposure in African infants for the same age subgroups. Among the three infant age subgroups, the median C₁₅₀ was highest for the 0 to < 3 months old age subgroup, followed by 3 to < 6 months old, and 6 to < 12 months old, for both the North American and African infant populations.

Overall, > 88% of infants were predicted to maintain RSM01 concentrations above the presumed EC90 threshold of 6.8 µg/ml after 150 days following a 50 mg RSM01 IM dose. These results suggest that a single dose of 50 mg RSM01 may have the potential to help protect infants up to 12 months old from RSV infection for the entire RSV season.

Neutralising activity was detected in all samples across all participants, confirming the presence of natural immunity in the adult population. The increase over baseline exhibited a dose-response curve by cohort following administration of RSM01. A 10- to 100-fold-increase was observed through day 151 for all participants dosed with RSM01, including those with RSM01 below the immunoassay limit of quantitation (0.5 µg/mL). This is suggestive that ex vivo RSV neutralising activity is maintained below 0.5 µg/mL RSM01.

The incidence of ADA at baseline and the occurrence of treatment emergent ADA were relatively low and generally consistent with the expected incidence for a fully human monoclonal antibody including nirsevimab and clesrovimab (MK-1654) [36, 37]. There was no observed impact on either safety or PK in the participants with detectable ADA. The potential to generate a memory response in ADA-positive participants is not applicable for RSM01, since the intended use case is a single administration in the first year of life.

The first-in-human trial has some limitations which are common for phase 1 studies. The sample size was small in both treatment groups. The trial was designed to be descriptive and was not powered to detect any differences in potential safety data between treatment groups. The trial population was healthy adults from high-income countries. Further trials on PK, safety, and efficacy of RSM01 are planned in the intended target population of infants from LMICs.

In conclusion, the preclinical characteristics, clinical safety, immunogenicity and PK profile of RSM01 observed thus far support further clinical evaluation in infants as a single dose injection to help prevent RSV disease through an entire RSV season.

ADA	anti-drug antibody
AE	adverse event
AESI	adverse event of special interest
AUC	area under the concentration curve
AUC_0-inf	AUC from time 0 to the infinity
AUC_0-D91	AUC from time 0 to day 91
AUC_0-D151	AUC from time 0 to day 151
ATCC	American type culture collection
BLI	bio-layer interferometry
BLQ	below the lower limit of quantification
C₀	initial RSM01 concentration
C_D91	day 91 concentration of RSM01
C_D151	day 151 concentration of RSM01
C_max	maximum concentration of RSM01
CFR	Code of Federal Regulations
CI	confidence interval
CL	total body clearance
CL/F	apparent total body clearance
EC50	effective concentration leading to 50% viral neutralisation
ECG	electrocardiogram
FcγR	Fc gamma receptor
HIV	human Immunodeficiency Virus
ID50	50% inhibitory dilution
IM	intramuscular
IV	intravenous
LMICs	low- and middle-Income Countries
LRTI	lower respiratory tract infection
MARM	monoclonal antibody resistant mutant
MedDRA	medical dictionary for regulatory activities
NCA	noncompartmental analysis
PK	pharmacokinetics
Q	inter compartment clearance
RSV	respiratory syncytial virus
RSV F	respiratory syncytial virus fusion glycoprotein
SAE	serious adverse event
SD	standard deviation
t_1/2	apparent terminal half‑life
TEAE	treatment‑emergent adverse event
T_max	time to maximum capillary blood concentration of RSM01
US	United States
Vc	central volume of distribution
Vp	peripheral volume of distribution

Ethics approval and consent to participate

All animal experiments were performed under approval of Sigmovir's Institutional Animal Care and Use Committee. The clinical trial was conducted in accordance with the principles of the Declaration of Helsinki, and Good Clinical Practice guidelines of the ICH, and all applicable sections of the US CFR, 21 CFR Parts 50, 56, and 312. The trial protocol was approved by IRB of Advarra^TM (Columbia, MD, US). All participants provided written informed consent. This trial was registered with Clinical Trials.gov NCT05118386.

Consent for publication

Not Applicable

Availability of data and materials

The Bill & Melinda Gates Medical Research Institute (Gates MRI; Cambridge, Massachusetts) is committed to data sharing that advances science and medicine while protecting privacy of trial participants. All preclinical experimental data supporting the findings described in this manuscript are available within the manuscript and its supplementary Information. Summary results of clinical trial data underlying the findings described in this manuscript are available at Clinical Trials.gov (NCT05118386). Other datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Competing interests

ABo, ML, SW, ABA, JTW, MS and MWD are employees of Bill & Melinda Gates Medical Research Institute; DT is a former employee of PPD Inc and received funding to PPD Inc from Bill & Melinda Gates Medical Research Institute, DT is currently an employee of Boehringer Ingelheim and reports stocks/bonds from Sanara Medtech Inc; LMS and JA are former employees of Bill & Melinda Gates Medical Research Institute; HR and ABa are former employees of Arsanis Biosciences and report stocks/bonds from X4 Pharmaceuticals; AR is an employee of intiGROWTH LLC and received funding to intiGROWTH LLC from Bill & Melinda Gates Medical Research Institute; JT is employed by University Medical Centre Utrecht and received funding to University Medical Centre Utrecht from Bill & Melinda Gates Medical Research Institute.

Authors names/initials for reference above:

Aurelio Bonavia (ABo), Micha Levi (ML), Shayne Watson (SW), Aparna B. Anderson (ABA), Joleen T. White (JTW), Michael Shaffer (MS), Michael W. Dunne (MWD), Dale Taylor (DT), Luisa M. Stamm (LMS), Jintanat Ananworanich (JA), Harald Rouha (HR), Adriana Badarau (ABa), Jonne Terstappen (JT), Andrijana Radivojevic (AR)

Funding

This work was funded by grants from the Bill & Melinda Gates Foundation. Work performed by the Bill and Melinda Gates Medical Research Institute was supported by grants INV-008522, INV-057216. Work performed by Arsanis BioSciences was supported by grants INV-006534, INV-010446. Under the grant conditions of the Foundation, a Creative Commons Attribution 4.0 Generic License has already been assigned to the Author Accepted Manuscript version that might arise from this submission.

Authors' contributions

JA, ML, ABo and JTW contributed to study design and conceptualization. ML, ABo, LMS and JTW contributed to data analysis and interpretation. MS contributed to data analysis. ABA and AR carried out the statistical analysis. HR and ABa were involved in the preclinical discovery, characterization and final lead selection of RSM01 at Arsanis Biosciences. JT, MS, SW and JTW contributed to data acquisition. MWD provided study oversight. All authors prepared the original draft of the manuscript, reviewed the draft critically, and contributed to the interpretations of results. All authors read and approved the final submitted version.

Acknowledgements

We are grateful to the trial participants, clinical investigators and all members of the Gates MRI RSM01-101 clinical trial team. We would like to acknowledge the RSV team at Arsanis BioSciences, in particular Eszter Nagy, Lukas Stulik, Georgios Tsouchnikas, Irina Mirkina and Ivana Dolezilkova for their contributions in antibody lead selection and optimization. We are thankful to Kristina Djinovic-Carugo’s laboratory at Max Perutz Labs, Vienna, Austria for X-ray crystallography work. We thank Laura Walker from Adimab for the discovery efforts that lead to the discovery of ADI-15618 and Frank E.J. Coenjaerts from UMC Utrecht for kindly providing the RSV clinical isolates. In addition, we thank Immunologix Laboratories for their contributions to the bioanalytical assays conducted in this study. Medical writing services were provided by Madeeha Aqil, PhD, MWC, CMPP and was funded by the Gates MRI in accordance with Good Publication Practice GPP 2022 (ismpp.org) guidelines.

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Competing interest reported. ABo, ML, SW, ABA, JTW, MS, and MWD are employees of Bill & Melinda Gates Medical Research Institute; DT is a former employee of PPD Inc and received funding to PPD Inc from Bill & Melinda Gates Medical Research Institute, DT is currently an employee of Boehringer Ingelheim and reports stocks/bonds from Sanara Medtech Inc; LMS and JA are former employees of Bill & Melinda Gates Medical Research Institute; HR and ABa are former employees of Arsanis Biosciences and report stocks/bonds from X4 Pharmaceuticals; AR is an employee of intiGROWTH LLC and received funding to intiGROWTH LLC from Bill & Melinda Gates Medical Research Institute; JT is employed by University Medical Centre Utrecht and received funding to University Medical Centre Utrecht from Bill & Melinda Gates Medical Research Institute.

Authors names/initials for reference above: Aurelio Bonavia (ABo), Micha Levi (ML), Shayne Watson (SW), Aparna B. Anderson (ABA), Joleen T. White (JTW), Michael Shaffer (MS), Michael W. Dunne (MWD), Dale Taylor (DT), Luisa M. Stamm (LMS), Jintanat Ananworanich (JA), Harald Rouha (HR), Adriana Badarau (ABa), Jonne Terstappen (JT), Andrijana Radivojevic (AR)

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RSM01, a Novel Respiratory Syncytial Virus Monoclonal Antibody: Preclinical Characterization and Results of a First-in-Human, Randomised Clinical Trial

Status:

Version 1

Abstract

Figures

Introduction

Methods

Preclinical Characterisation

RSM01 epitope mapping

In vitro microneutralisation assay

Epitope conservation

Generation of monoclonal antibody resistant mutants (MARMs)

Human Fc gamma receptor (FcγR) binding assay

Cotton rat model

Animal and ethics statement

RSV strains and clinical isolates

Clinical Trial

Trial Design and Objectives

Trial Conduct

Trial Population

Assessments

Safety

RSM01 PK, ADA, and RSV-Neutralising Antibodies

Statistical Analyses

Safety

PK analysis

Population PK modeling

Simulation of RSM01 PK in infant populations

Immunogenicity and RSV neutralising activity

Results

Preclinical characterisation

Determination of RSM01 epitope

In-vitro neutralisation activity

Epitope conservation

Generation of MARMs

Prophylactic efficacy of RSM01 in cotton rat model

Human FcγR binding

RSM01 Phase 1 Clinical Trial

Participants’ baseline characteristics and disposition

Safety

PK

Population PK modeling and simulation

Immunogenicity

RSV Neutralising Activity of RSM01 in participants’ serum

Discussion

Conclusions

Abbreviations

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1