CMV Viral Load Kinetics as Surrogate Endpoints for Antiviral Prophylaxis Trials

Abstract

<h3>Introduction</h3> The paradigm for preventing clinically significant cytomegalovirus (CMV) infection and CMV disease after hematopoietic cell transplantation (HCT) has shifted from preemptive treatment to antiviral prophylaxis with the approval of letermovir. Previously, our group validated the use of quantitative CMV DNA viral load as a surrogate endpoint for tissue-invasive CMV disease (ASBMT 2018) in the <i>pre-emptive treatment setting</i>. <h3>Objectives</h3> To address whether CMV viral load kinetics could serve as valid surrogate endpoints for CMV disease in the <i>prophylactic setting</i>, we performed CMV viral load testing of frozen serum samples collected during a placebo-controlled randomized trial of ganciclovir (GCV) given at engraftment (Goodrich et al. Ann Intern Med 1993). Because this study pre-dated the approval of ganciclovir, CMV disease occurred in an extremely high proportion of patients, allowing us to link viral load kinetics with CMV disease, which would not be possible in the modern treatment environment. <h3>Methods</h3> We calculated each patient's CMV viral load kinetics (mean, peak, maximum change, percentage of positive viral loads) from samples collected during the first five weeks after randomization and analyzed their value as surrogates for CMV disease by 24 weeks after randomization. First, we tested each marker for fulfillment of the Prentice criteria for valid surrogate endpoints using multivariable logistic regression, including quantifying the degree to which each marker captured the effect of ganciclovir on CMV disease. Secondly, to determine the ability of the viral load markers to predict CMV disease, we developed a ‘SuperLearner' machine learning model and quantified prediction accuracy by cross-validated area under the receiver-operator characteristic curves (cv-AUCs). <h3>Results</h3> Mean, peak, change, and percentage of positive viral loads were significantly higher in the placebo group than in the ganciclovir group. See viral loads by group in Fig 1. Percentage of positive viral load satisfied Prentice criteria as a valid surrogate endpoint for CMV disease by week 24 after randomization (Fig 2). All kinetics explained greater than 80% of ganciclovir's reduction in CMV disease (mean: 86%, peak: 83%, maximum change: 95%, percent positive: 94%). We found that a SuperLearner model that included all four viral load kinetics, CMV donor serostatus, acute graft-versus-host disease, and baseline viral load predicted CMV disease with 91% cv-AUC in the placebo group, 78% in the ganciclovir group, and 82% in the combined groups (Fig 3). <h3>Conclusion</h3> We have shown for the first time in a clinical endpoint-rich, placebo-controlled antiviral prophylaxis trial that CMV viral load kinetics fulfill the Prentice criteria as valid surrogate endpoints and have high predictive value for CMV disease after HCT.