Background:  Animal models to study the evolution of resistance to antiretroviral drugs used clinically are severely limited by the inactivity of non-nucleoside reverse transcriptase inhibitors (NNRTI) against simian immunodeficiency virus (SIV) RT. We therefore developed a pigtail macaque (Macaca nemestrina) SHIV model to examine NNRTI therapy and resistance.

Methods:  SIV_mne was modified by replacement of the RT coding region with that of HIV-1 to yield an RT-SHIV_mne. In contrast to RT-SHIV isolates derived from SIV_mac239, RT-SHIV_mne has an optimal tRNA-Lys3 PBS and encodes envelope tropic for monocytes/macrophages as well as CD4⁺ T cells. Viral replication studies and selection of drug-resistant variants were conducted in CEMx174 cells. Drug susceptibility studies were conducted in human and macaque reporter cell lines. RT-SHIV_mne was inoculated intravenously into 4 pigtail macaques. In addition, the virus was serially passaged intravenously from 1 animal into 2 other macaques. Plasma RT-SHIV_mne RNA was measured using a quantitative real-time RT-PCR assay. Nevirapine (NVP) pharmacokinetics were monitored in macaques after a single oral dose of 100 to 200 mg.

Results:  RT-SHIV_mne grew to high titers in vitro but had 10-fold reduced fitness relative to wild type SIV. Although the HIV-1 RT was properly processed in RT-SHIV_mne particles, the RT-SHIV_mne virions had lower levels of RT per viral genomic RNA than HIV-1. RT activity was also reduced in RT-SHIV_mne versus SIV particles. Unlike wild type SIV, RT-SHIV_mne was susceptible to the several NNRTI in both human and macaque cells. In fact, the susceptibility of RT-SHIV_mne to NNRTI was increased approximately 2-fold relative to HIV-1, probably due to reduced levels of RT in RT-SHIV_mne particles. Classic HIV-1 NNRTI resistance mutations, including K103N and Y181C, were identified in RT-SHIV_mne after in vitro selection with NNRTIs. RT-SHIV_mne RNA was detectable (> 10⁴ copies/mL) in the plasma of macaques up to 30 weeks after inoculation, with an average peak plasma viral load of > 7´10⁶ copies/mL. Inoculation of 2 animals with in vivo-passaged RT-SHIV_mne resulted in a plasma RNA setpoint above 10⁵ copies/mL in 1 animal. Low levels of NVP were detected in plasma after a single, oral NVP dose of 100 or 200 mg. Higher dose pharmacokinetic studies are underway.

Conclusions:  This macaque RT-SHIV_mne model should be useful for studies of NNRTI-based therapies and NNRTI resistance, including the selection, relative fitness, and persistence of resistant variants.

Keywords: macaque; resistance; model