Background: Preventing disease progression for HIV-1 infected patients requires regimens that maximally suppress virus replication. A comparative measure of antiviral activity under clinically relevant conditions would guide drug development and regimen selection but is currently lacking. We hypothesized that the slope parameter (or Hill coefficient), which is neglected in current measures of antiviral activity, might have a dramatic effect on antiviral activity of anti-HIV-1 drugs. To account for the effect of slope, we developed a new index, instantaneous inhibitory potential (IIP), as a new in vitro measure of antiviral activity.

Methods:  A single round infectivity assay with wide dynamic range in primary CD4⁺ lymphoblasts was used to obtain dose response curves for anti-HIV-1 drugs. IC₅₀ and slope parameters were determined by fitting the curves into the median effect model with least square regression analysis. IIP equals the number of logs by which single round infectivity is reduced at clinically relevant drug concentrations and was calculated based on the median effect equation.

Results:  We show that current measures of antiviral activity, IC₅₀ and inhibitory quotient, neglect a critical dimension—the dose response curve slope, which has a dramatic effect on antiviral activity. Strikingly, slope values are class-specific for antiviral drugs and define intrinsic limitations on antiviral activity for some drug classes. NRTI and integrase inhibitors have slopes of ~1, characteristic of non-cooperative reactions, while NNRTI, protease inhibitors, and fusion inhibitors unexpectedly show slopes >1. IIP is strongly influenced by slope, varies by >8 logs for current anti-HIV drugs and provides a more accurate in vitro pharmacodynamic measure of antiviral activity than traditional IC₅₀ or inhibitory quotient measures.

Conclusions:  Antiretroviral drugs acting through different mechanisms have different value for the slope parameter which has crucial influence on antiviral activity under clinically relevant conditions, as reflected in IIP. Only agents with slopes >1 achieve high level inhibition of single round infectivity, a finding with profound implications for drug and vaccine development.