Using the adherence-efficacy relationship of emtricitabine and tenofovir disoproxil fumarate to calculate background hiv incidence: a secondary analysis of a randomized, controlled trial

David V Glidden, Moupali Das, David T Dunn, Ramin Ebrahimi, Yongwu Zhao, Oliver T Stirrup, Jared M Baeten, Peter L Anderson, David V Glidden, Moupali Das, David T Dunn, Ramin Ebrahimi, Yongwu Zhao, Oliver T Stirrup, Jared M Baeten, Peter L Anderson

Abstract

Introduction: Randomized trials of new agents for HIV pre-exposure prophylaxis (PrEP) compare against emtricitabine and tenofovir disoproxil fumarate (F/TDF), without a placebo group. We used the well-characterized adherence-efficacy relationship for F/TDF to back-calculate the (non-PrEP) counterfactual background HIV incidence (bHIV) in a randomized trial of a novel PrEP agent and estimate comparative efficacy (to counterfactual bHIV).

Methods: The DISCOVER trial (ClinicalTrials.gov: NCT02842086) randomized 5387 men who have sex with men (MSM) and transgender women who have sex with men and demonstrated non-inferiority of emtricitabine and tenofovir alafenamide (F/TAF) to F/TDF (HIV incidence rate ratio [IRR] 0·47, 95% CI: 0·19 to 1.15). Tenofovir diphosphate (TFV-DP) levels in dried blood spots (DBS) were assessed for all diagnosed with HIV and in a random 10% of the cohort. We used a Bayesian model with a diffuse prior distribution, derived from established data relating tenofovir diphosphate levels to HIV prevention efficacy. This prior, combined with the F/TDF seroconversion rate and tenofovir diphosphate levels in DISCOVER, yielded Bayesian inferences on the counterfactual bHIV.

Results: There were six versus 11 postbaseline HIV infections (0.14 vs. 0.25/100 person-years [PY]) on F/TAF and F/TDF respectively. Of the 11 on F/TDF, 10 had low, none had medium and one had high tenofovir diphosphate levels; among HIV-negative controls, 5% of the person-time years had low, 9% had medium and 86% had high TFV-DP levels. A non-informative prior distribution for counterfactual bHIV, combined with the prior for TFV-DP level-efficacy relationship, yielded a posterior counterfactual bHIV of 3·4 infections/100 PY (0.80 Bayesian credible interval [CrI] 1·9 to 5·9), which suggests a median HIV efficacy of 96% (0.95 CrI [88% to 99%]) for F/TAF and 93% (0.95 CrI [87% to 96%]) for F/TDF compared to bHIV.

Conclusions: Based on the established connection of drug concentrations to PrEP prevention efficacy, a Bayesian framework can be used to estimate a synthetic non-PrEP control group in randomized, active-controlled PrEP trials that include a F/TDF-comparator group.

Keywords: Bayesian inference; PrEP; clinical trial design; counterfactual; tenofovir alafenamide; tenofovir disoproxil fumarate.

© 2021 The Authors. Journal of the International AIDS Society published by John Wiley & Sons Ltd on behalf of the International AIDS Society.

Figures

Figure 1
Figure 1
Bayesian estimates of posterior efficacy distributions for F/TAF and F/TDF. F/TAF, co‐formulated emtricitabine and tenofovir alafenamide; F/TDF, co‐formulated emtricitabine and tenofovir disoproxil fumarate. F/TAF, co‐formulated emtricitabine and tenofovir alafenamide; F/TDF, co‐formulated emtricitabine and tenofovir disoproxil fumarate.
Figure 2
Figure 2
Estimates of counterfactual bHIV by the adherence‐efficacy method under varying assumptions and corresponding efficacy estimates.
Figure 3
Figure 3
Comparing Counterfactual Estimates of Background Placebo HIV Incidence. *Based on Centers for Disease Control and Prevention (CDC) surveillance data [42]. BL, baseline; F/TAF, co‐formulated emtricitabine and tenofovir alafenamide; F/TDF, co‐formulated emtricitabine and tenofovir disoproxil fumarate.

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