Weekly Mosnodenvir Held Dengue Infection in a Human Challenge Model
Dengue still lacks a proven antiviral prophylaxis, and that gap remains clinically important in regions where repeated mosquito exposure drives infection and reinfection. In this phase 2a, double-blind, randomized, placebo-controlled human challenge study, mosnodenvir was tested against a controlled DENV-3 inoculation, and the antiviral signal remained visible even after the regimen was stretched from daily dosing to once-weekly administration. The cohort was small, but the biologic readout was hard to ignore: viral replication and serologic evidence of infection were suppressed in most treated participants, while every placebo recipient developed signs of infection.
The design moved beyond the earlier daily-dosing cohort into a more practical schedule. Six participants were assigned to each of four groups: once-daily mosnodenvir, once-weekly high-dose mosnodenvir, once-weekly low-dose mosnodenvir, or placebo. All participants received twice-daily loading doses for 2 days beginning on day −2, then maintenance dosing for 21 days starting on day 1. The daily-dose group received 800 mg during loading and 250 mg as maintenance. The weekly high-dose group received 450 mg during loading and 1200 mg weekly thereafter. The weekly low-dose group received 250 mg during loading and 500 mg weekly thereafter. On day 1, all participants were inoculated subcutaneously with the underattenuated recombinant dengue virus serotype 3 strain rDEN3Δ30, and follow-up extended for up to 120 days.
Viral Suppression Persisted Across Dosing Schedules
Protection against measurable infection was similar in the once-daily groups across cohorts and became even more striking in the weekly arms. In the earlier dose-finding cohort, 6 of 10 participants in the once-daily high-dose group had no signs of DENV-3 infection, defined as undetectable RNA and no anti-DENV IgM, anti-DENV IgG, or neutralizing antibodies. In this newer cohort, 4 of 6 participants in the once-daily group had no signs of infection. The weekly regimens went further: 5 of 6 participants in both the high-dose and low-dose weekly groups had no signs of DENV-3 infection, compared with 0 of 6 participants in the placebo group.
A 1-sentence result captures the clinical signal plainly: every placebo recipient became infected by the study definition, and most mosnodenvir recipients did not.
For dengue, that matters because early suppression of viral replication may blunt the entire downstream cascade of illness. Dengue pathogenesis is shaped not only by viral burden but also by the host response that follows viremic expansion, so an antiviral that limits or aborts early replication could plausibly interrupt the sequence leading to fever, systemic symptoms, and immune activation. This study was not designed to measure clinical disease prevention in natural infection, but the challenge model offers a controlled window into whether the drug can prevent the virologic events that seed illness.
NS4B Changes Emerged in Breakthrough Infections
Sequencing data were available only for participants with breakthrough infections, and emerging amino acid variations in NS4B appeared in all four mosnodenvir recipients who had breakthrough infection. The substitutions were A19L in 1 participant, V91G in 2, L94F in 1, and Y99H in 1. Among placebo recipients with breakthrough infection, NS4B variations were detected in 2 of 6 participants, including S60I in 1 and I201V in 1.
NS4B is a biologically plausible resistance locus because the protein is part of the dengue replication machinery and a direct target of antiviral pressure. The pattern here does not establish resistance the way a larger clinical resistance program would, but it does fit the expected evolutionary logic of a virus exposed to a drug that constrains replication. Breakthrough infection in treated participants, paired with emerging NS4B variation, suggests selective pressure rather than random sequence drift.
Pharmacokinetics Supported the Weekly Regimens
Mosnodenvir plasma concentrations rose rapidly after the loading doses and were maintained through the maintenance phase. Concentration-time profiles out to day 85 and day 29 showed sustained exposure, with values below the lower limit of quantification not displayed. The practical value of that pharmacokinetic profile is straightforward: short loading, durable exposure, and a schedule that no longer depends on daily dosing.
The investigators also reduced the loading period from 5 days in the earlier cohort to 2 days in this one. That shift is not just a convenience issue. In antiviral prophylaxis, the ability to reach effective concentrations before viral challenge and then hold them steady over the risk window determines whether the drug can match the biology of exposure. A weekly regimen is far easier to deploy in prevention programs than a schedule that demands daily adherence, especially in travel medicine, outbreak response, or other settings where short-term prophylaxis could be considered.
Safety Signals Remained Quiet in a Small Cohort
No clinically significant safety concerns were identified. Within the narrow bounds of this cohort, the regimen appeared tolerable across daily and weekly schedules, which is essential if the drug is ever meant to move from controlled challenge studies into broader preventive use. Small studies cannot uncover rare toxicity, and they cannot substitute for larger exposure data, but they can rule in or rule out obvious early harm. Here, no such signal emerged.
The study does leave important questions open. The sample size was small, the challenge model used a single dengue serotype, and the participants were healthy volunteers rather than patients at real-world risk in endemic settings. Protection against a controlled DENV-3 inoculation does not automatically translate into field effectiveness against multiple serotypes, repeated mosquito exposures, or heterogeneity in host immunity. Even so, the data now extend beyond daily administration and show that once-weekly dosing can preserve antiviral activity in a human infection model.
The next step is larger and more complex than this challenge study. Real-world prophylaxis will need to show whether the pharmacologic signal holds against natural dengue transmission, whether the resistance pattern stays manageable, and whether a preventive strategy built around mosnodenvir can be deployed without sacrificing safety or adherence over the longer term.