In this study, PRO 140 demonstrated potent, rapid, and prolonged antiretroviral activity when it was administered as single 5-mg/kg or 10-mg/kg intravenous infusions to individuals infected with CCR5-tropic HIV-1. The mean maximum decrease in viral load was 1.8 log10
units at each dose level, and this value compares favorably to the reductions observed in prior studies of PRO 140 (6
) and with small-molecule CCR5 antagonists (4
). Overall, single doses of 5 mg/kg or 10 mg/kg were generally well tolerated when they were administered as short-term monotherapy. Notably, we observed that 10 mg/kg, the highest dose tested to date, did not demonstrate any dose-dependent pattern of adverse events relative to placebo or to the 5-mg/kg dose. The present study adds to our understanding of the pharmacologic, pharmacokinetic, and safety profiles of this agent.
There was a striking consistency in the antiviral effects observed in the present study and a prior study of intravenous PRO 140 (6
). Remarkably, the mean maximum reduction in the HIV-1 RNA level was 1.8 log10
units for doses of 5 mg/kg or higher in each study. The consistency of outcomes underscores the robustness of the single-dose activity observed for intravenous PRO 140. The median reduction in viral load was slightly higher in the 10-mg/kg group, and more subjects in the 10-mg/kg group achieved a ≥2-log10
-unit reduction in HIV-1 RNA levels. In addition, there was a trend toward more prolonged antiviral effects with the 10-mg/kg dose. Overall, the findings indicate that doubling the dose from 5 mg/kg to 10 mg/kg resulted in modestly greater single-dose antiviral effects.
The original Trofile assay was used to determine coreceptor tropism for enrollment into the study. The original assay was validated to have 100% sensitivity in detecting CXCR4-using viruses when they were present at 10% or more of a virus population (23
). The original Trofile assay has since been replaced with an enhanced-sensitivity assay that was validated to have 100% sensitivity in detecting 0.3% CXCR4-using viruses in a virus population (17
). The enhanced-sensitivity assay is the method currently used in clinical practice. One subject enrolled into the 10-mg/kg group based on R5 tropism in the original Trofile assay was found to have dual/mixed virus 2 weeks posttreatment. This subject was later reclassified using the enhanced-sensitivity assay as having dual/mixed virus at screening, and the data for that subject were censored from the efficacy analysis. Similar approaches have been adopted in efficacy analyses of other studies of CCR5 coreceptor antagonists (18
), consistent with the view that the enhanced-sensitivity Trofile assay provides an improved method of identifying candidates for therapy with CCR5 coreceptor antagonists. Similar censoring of subjects treated in the MERIT study of maraviroc (18
) was performed for efficacy analyses that supported FDA approval of this agent's use in antiretroviral treatment-naïve patients.
Compared with the 5-mg/kg dose, the 10-mg/kg dose of PRO 140 resulted in proportionally higher peak (Cmax
) and overall (AUC0-∞
) exposures to PRO 140. The higher drug exposures attained with 10 mg/kg were not associated with any obvious toxicity or pattern of toxicity. The maximum tolerated dose of i.v. PRO 140 has not been determined. The 10-mg/kg i.v. dose resulted in peak serum concentrations that are 15-fold higher, on average, than those observed following s.c. dosing (7
), suggesting a sizeable margin of safety for s.c. PRO 140.
All pretreatment viruses were susceptible to inhibition by PRO 140 in vitro. The concentrations required for 50% inhibition varied by <5-fold across the panel of 31 viruses, and all viruses were efficiently inhibited (98 to 100%) at higher concentrations. With the exception of the one subject in the 10-mg/kg group who was reclassified as having dual/mixed virus at screening, there was no change in coreceptor tropism or emergence of PRO 140-resistant virus during the course of this study. The results support the view that PRO 140 broadly inhibits R5 HIV-1 with a high barrier to resistance.
High levels of receptor occupancy were observed following treatment with either 5 mg/kg or 10 mg/kg PRO 140. Statistically significant levels of receptor occupancy preceded significant reductions in viral load by at least 2 days. This result is concordant with the dynamics of inhibiting HIV-1 entry and with the half-life of virus-producing T cells (12
). Receptor occupancy values also appeared to rebound later than viral loads. This apparent discordance could reflect issues related to assay sensitivity and sampling. Given the modest numbers of CCR5+
lymphocytes at the baseline (~20 cells/μl, on average), the assay had a limited ability to determine mean receptor occupancy levels above 90%. Therefore, the times of maximum receptor occupancy and of initial rebound in receptor occupancy levels could not be determined precisely. In addition, receptor occupancy is measured on cells in the periphery, whereas HIV replication occurs primarily within tissues (20
). PRO 140 concentrations and levels of receptor occupancy may differ at local sites of HIV-1 replication. As with viral load reductions, the duration of receptor occupancy was modestly greater with the 10-mg/kg dose relative to that with the 5-mg/kg dose, consistent with the higher serum concentrations of drug achieved at the higher dose.
To date, 84 HIV-infected individuals have been treated with i.v. or s.c. forms of PRO 140 in three short-term monotherapy studies (6
). In each study, 1.5- to 2.0-log10
-unit mean reductions in HIV-1 RNA levels were observed with the higher dose levels. The viral load reductions were long-lived and highly statistically significant. No dose-limiting toxicity or pattern of toxicity was identified in these studies. In addition, no emergence of R5 viral resistance was observed, even though >1-log10
-unit reductions in viral loads were observed for up to 6 weeks in some subjects.
In the present study, the duration of antiviral activity increased somewhat as the i.v. dose was increased from 5 mg/kg to 10 mg/kg. However, neither i.v. dose would appear to support highly infrequent (e.g., monthly) administration, and Emax
analysis indicated that further increases in i.v. dose would result in incremental increases in antiviral effects. In a study of s.c. PRO 140, significant antiviral effects were observed when the drug was administered weekly or every other week, and virologic suppression was maintained between successive doses (7
). While both i.v. and s.c. dosage forms have demonstrated favorable antiviral and tolerability profiles, s.c. PRO 140 was selected for further development on the basis of its potential to be self-administered by patients. Self-administration may offer greater convenience for many patients. Nevertheless, the s.c. dosage form is undergoing clinical study, and i.v. administration may be preferred in certain treatment settings.
In summary, single intravenous infusions of 5 mg/kg and 10 mg/kg PRO 140 demonstrated potent, long-lived antiretroviral activity and a favorable tolerability profile in this study. The findings provide new insights into the safety and virological properties of this agent, which represents a novel and long-acting approach to treating R5 HIV-1 infection.