PLATCOV is an ongoing phase 2 open label, randomised, controlled adaptive platform trial (ClinicalTrials.gov: NCT05041907) (35). It provides a standardised quantitative comparative method for in vivo assessment of potential antiviral treatments in low-risk adults with early symptomatic COVID-19. Daily oropharyngeal viral densities are estimated by serial qPCR. The primary outcome measure in PLATCOV is the viral clearance rate derived from the slope of the log10 oropharyngeal viral clearance curve over the first 7 days following randomisation, estimated under a linear model (36). The treatment effect is defined as the multiplicative change in viral clearance rate relative to the contemporaneous no study drug arm (detailed below). The trial was conducted in the Faculty of Tropical Medicine (FTM), Mahidol University, Bangkok; Bangplee hospital, Samut Prakarn; and Vajira hospital, Navamindradhiraj University, Bangkok, all in Thailand and in Belo Horizonte, Minas Gerais, Brazil (see Supplementary materials). All patients provided fully informed written consent and the study had full Ethics and Regulatory approvals (see supplementary materials). The PLATCOV trial was coordinated and monitored by the Mahidol Oxford Tropical Medicine Research Unit (MORU) in Bangkok, and overseen by a trial steering committee (TSC). Interim results were reviewed regularly by a data and safety monitoring board (DSMB). The funders had no role in the design, conduct, analysis or interpretation of the trial.
Participants and procedures
Previously healthy adults aged between 18 and 50 years were eligible for the trial if they had early mild symptomatic COVID-19 (i.e. reported symptoms for <4 days), oxygen saturation ≥96%, were unimpeded in activities of daily living, and gave fully informed consent to study participation. SARS-CoV-2 positivity was defined either as a nasal lateral flow antigen test which became positive within two minutes (STANDARD® Q COVID-19 Ag Test, SD Biosensor, Suwon-si, Korea) or a positive PCR test within the previous 24hrs with a cycle threshold value (Ct) <25 (all viral gene targets), both of which suggest high pharyngeal viral densities. Exclusion criteria included taking any potential antivirals or pre-existing concomitant medications, chronic illness or significant comorbidity, haematological or biochemical abnormalities, pregnancy (a urinary pregnancy test was performed in females), breastfeeding, or contraindication or known hypersensitivity to any of the study drugs (35).
Enrolled patients were either admitted to the study ward (in Thailand), consistent with National recommendations at the time, or followed as outpatients at home (in Brazil). After randomisation and baseline procedures (see Supplementary materials) oropharyngeal swabs (two swabs from each tonsil) were taken as follows. Each flocked swab (Thermo Fisher MicroTest® and later COPAN FLOQSwabs®) was rotated against the tonsil through 360o four times and placed in Thermo Fisher M4RT viral transport medium (3mL). Swabs were transferred at 4-8oC, aliquoted, and then frozen at -80oC within 48hrs. Separate swabs from each tonsil were taken once daily from day 0 to day 7, and again on day 14. Each swab was processed and tested separately. Vital signs were recorded three times daily and symptoms and any adverse effects were recorded daily (35).
Patients allocated to favipiravir received 1800mg on an empty stomach, (nine 200mg tablets) Favir®, Government Pharmaceutical Organization in Thailand, n=100; or Avigan®, FUJIFILM Toyama Chemical Co., Ltd. in Brazil n=16, at the start of treatment followed 12 hours later by a further 1800mg. Thereafter the patients took 800mg twice daily for a further 6 days totalling 13.2g over 7 days.
The TaqCheck® SARS-CoV-2 Fast PCR Assay (Applied Biosystems, Thermo Fisher Scientific, Waltham, Massachusetts) quantitated viral densities (SARS-CoV-2 RNA copies per mL). This multiplexed real-time PCR method detects the SARS-CoV-2 N and S genes, and human RNase P in a single reaction. RNase P was used to correct for variation in human cell content in samples. Viral densities were quantified against ATCC heat-inactivated SARS-CoV-2 (VR-1986HK strain 2019-nCoV/USA-WA1/2020) standards. Viral variants were identified using Whole Genome Sequencing (see Supplementary materials).
Outcome measures
The primary outcome measure was the rate of viral clearance, expressed as a slope coefficient (36), and estimated under a Bayesian hierarchical linear model (mixed-effects model) fitted to the daily log10 viral density measurements between days 0 and 7 (18 measurements per patient). Before model fitting, Ct values were transformed to RNA copies per mL using a random effects linear model fit to the ATCC controls (random slope and intercept for each plate with additional fixed effects for each laboratory). Viral load measurements below the limit of quantification (Ct values≥40) were treated as left-censored under the model. A non-linear model (allowing an initial log-linear increase in viral loads followed by a log-linear decrease in some patients) was also fitted to the data as a sensitivity analysis. All models included slope and intercept covariate effects for the virus variant, expressed as the major sub-lineages). Additional models included slope and intercept covariate effects for age, vaccination status, and days since symptom onset. The estimated individual viral clearance rates (i.e. slope coefficients from the model fit) can be expressed as clearance half-lives (t1/2 = log10 0.5/slope). The treatment effect was defined as the multiplicative change (%) in the mean viral clearance rate relative to the no study drug arm (i.e. how much the test treatment accelerates on average the viral clearance) (36). Thus, a 50% increase in clearance rate equals a 33% reduction in clearance half-life. All-cause hospitalisation for clinical deterioration (until day 28) was a secondary endpoint. For each studied intervention the sample size was adaptive based on prespecified futility and success stopping rules. Initially the futility stopping rule was set as a probability >0.9 that the acceleration in viral clearance was <5%, but at the prespecified open first interim analysis performed after 50 patients had been enrolled, the futility threshold was increased to 12.5%.
Adverse events were graded according to the Common Terminology Criteria for Adverse Events v.5.0 (CTCAE). Summaries were generated if the adverse event was ≥grade 3 and was new or had increased in intensity. Serious adverse events were recorded separately and reported to the DSMB.
Statistical analysis
All analyses were done in a prespecified modified intention-to-treat (mITT) population, comprising patients who had >3 days follow-up data. A series of linear and non-linear Bayesian hierarchical models were fitted to the viral quantitative PCR (qPCR) data (Supplementary materials). Model fits were compared using approximate leave-one-out comparison as implemented in the package loo. All data analysis was done in R version 4.0.2. Model fitting was done in stan via the rstan interface. All code and data are openly accessible via GitHub: https://github.com/jwatowatson/PLATCOV-Favipiravir .