Presented During:

Monday, Mar 8, 2021: 11:20 AM  - 11:30 AM 
Virtual CROI 2021  

Abstract Number:


Abstract Type:

General Abstract  


Munjal Patel1, Xiaowei Zang1, Youfang Cao1, Randolph P. Matthews1, Rebeca M. Plank1, Peter Sklar1, Jay A. Grobler1, Michael N. Robertson1, Ryan Vargo1


1Merck & Co, Inc, Kenilworth, NJ, USA

Presenting Author:

Dr Munjal Patel, PhD  


Innovations in HIV-1 pre-exposure prophylaxis (PrEP) are needed to address the global HIV epidemic and meet the diverse needs of individuals at risk of acquiring HIV-1. Islatravir (ISL) is the first nucleoside reverse transcriptase translocation inhibitor (NRTTI) in development for the treatment and prevention of HIV-1. In this PK/PD work, we present the data that defined the exposure threshold for ISL for PrEP and the corresponding oral once monthly (QM) dose for the phase 3 clinical development program.


The lower efficacious exposure threshold for ISL PrEP efficacy was based on (1) the estimated protective ISL-triphosphate (ISL-TP) EC90 from a study of male rhesus macaque undergoing SHIV intra-rectal challenge using once weekly oral ISL, (2) the minimum efficacious ISL-TP concentration in a Phase 1b proof-of-concept trial (NCT02217904) in treatment-naïve adults with HIV-1, and (3) relevant data from the literature regarding the protective concentrations of FTC/TDF. Population PK simulations were conducted using data from the Phase 1 and 2 studies of ISL to determine the phase 3 oral QM dose.


Totality of the data from the rhesus macaque study, the Phase 1b trial, and benchmarking PK exposures from TDF-DP resulting from both pre-clinical and clinical studies suggest that full efficacy for HIV-1 prevention is achieved with approximate inhibitory quotient (IQ) of ~1-5 (Table). Based on these observations, the efficacious exposure threshold for ISL PrEP was set at 0.05 pmol/106 cells in PBMCs, which is ~5-fold above the in vitro IC50 (0.00974 pmol/106 cells) of ISL-TP against wild type HIV-1. In an ongoing Phase 2 trial (NCT04003103), the observed mean ISL-TP exposure 4 weeks after a 60-mg dose was ~26 fold above the PK threshold. Additionally, population PK simulations suggest that oral ISL 60-mg QM will achieve ISL-TP concentrations well above the PK threshold for all participants following the first monthly dose. All participants at this dose are predicted to be above the PK threshold with the lower 2.5th prediction interval having an ~IQ of 17.


To provide efficacious exposures for protection against HIV-1, a 60-mg oral QM dose of ISL was selected for the Phase 3 clinical program. This dose is expected to maintain ISL-TP concentrations above the conservative PK threshold of 0.05 pmol/106 cells for all participants with sustained exposures in the event of a delayed or missed monthly dose.


(G) Clinical Pharmacology in Adults


Oral PrEP
Phase 3

Supporting Image: CROI_2021_Munjal.jpg

Does this abstract include any aspects of research on SARS-CoV-2 or COVID-19?


Prior Presentation or Publication: In general, CROI does not accept work that has been previously published or publicly presented. Abstract text that is under copyright by a publication or another conference should not be submitted verbatim to CROI. Consideration may be given to a previously presented submission if meaningful newer data or different analyses are included or if the prior presentation was to a conference not focused on HIV- or SARS-CoV-2-related topics. Have your study data or abstract information been published, submitted for publication where publication is anticipated on or before the start of the CROI where you will present, or presented at any other major national or international scientific or medical HIV-related conferences (ie, generally 400 or more attendees)?