Reply to Pandey et al.: Understanding the efficacy of a potential antiretroviral drug candidate in humanized mouse model of HIV infection

We appreciate Pandey et al.’s (1) interest in further understanding our published work (2). Our responses are given below the excerpts from the Letter.

Our first and foremost concern is the data presented in their figure 6C. We fail to understand why there was a fall in the plasma viral loads after day 19 in the control group (i.e., the group not given any intervention). Furthermore, not only was the fall in the plasma viral load significant (to below the limit of detection; <150 copies per milliliter) but also it was more rapid (showing a steeper decline) compared with the treated groups by day 32. If the viral loads are decreasing so rapidly and significantly in the noninterventional (control group), can we draw conclusions about the efficacy of interventional arms?

The human peripheral blood lymphocyte model, generated by transplanting adult peripheral blood mononuclear cells in NSG mice that undergo xenogeneic expansion, is a commonly used humanized mouse model for the quick assessment of investigational compounds with antiretroviral properties (3, 4). The CD4⁺ T cells in this model are highly susceptible to HIV-1 infection, and a rapid depletion in CD4⁺ T cell numbers occurs within a short time postinfection. In our figure 6C, a fall in plasma viral loads (PVLs) in the control group is observed after day 19 due to depletion of all T cells in the absence of drug (also shown in figure 6 A and B). The virus has no more cells to grow in and replicate, causing PVLs to drop below the limit of detection. Compound I blocks virus replication and prevents depletion of CD4⁺ T cells in the treated mice (our figure 6 A and B) due to its antiretroviral effects and, importantly, with its preservation of T cells, as happens with antiretroviral therapy.

Second, it would be interesting to know the possible reasons for the steep increase in the serum concentrations of compound I (group 1; green circles) when dosed continuously (at a dose of 100 mg⋅kg⁻¹⋅d⁻¹ until day 32) after day 16 until day 25 as depicted in their figure 5B. Had the drug accumulation been due to saturation of elimination processes, we would have expected a further increase in concentrations even after day 25 until the day of last measurement, that is, day 32. We fail to understand the mechanism underlying the initial steep rise followed by the fall in drug concentrations despite continued dosing of free compound I at the same dose level for 32 d.

In our figure 5B, the group receiving 100 mg/kg of compound I daily as a free drug showed a slight decline in serum concentration at day 32 relative to day 25, which is not significant based on unpaired t test (P = 0.93). Additionally, this study is not focused on investigating mechanisms of metabolism and elimination of compound I but on comparative efficacy of compound I delivered as a daily dose of free drug or as a single dose in a poly(lactide-coglycolide)-based nanoparticle formulation (NP). Hence, it is beyond the scope of this report to speculate on the mechanisms that may be involved in metabolism and elimination. Previous studies have examined preliminary pharmacokinetic and absorption, distribution, metabolism, excretion, and toxicity properties of the free compound I through pharmacological profiling and demonstrated excellent therapeutic indices, comparable or better than for the current US Food and Drug Administration-approved nonnucleoside HIV drugs (2, 5). The current study confirms the antiretroviral efficacy of the daily-dose free drug and single-dose NP of compound I in an established animal model for HIV-1 infection.