Efficacy Studies in Animals

Figure 1. Figure 1. Efficacy and Pharmacokinetic Profiles of Tecovirimat in Animal Models. On day 0, nonhuman primates (Panels A and B) and rabbits (Panels C and D) were infected with a lethal dose of monkeypox virus or rabbitpox virus, respectively. Survival was monitored for 18 to 42 days after infection, indicated on the horizontal axis labels. Tecovirimat was administered by oral gavage at doses ranging from 0.3 to 20 mg per kilogram of body weight in nonhuman primates and from 20 to 120 mg per kilogram in rabbits. Tecovirimat was administered for 14 consecutive daily doses starting on day 4 after infection, after the onset of clinical signs (pock lesions in nonhuman primates and fever and viremia in rabbits) in each study. Comparison of the exposures required for efficacy in rabbits and nonhuman primates showed that the nonhuman primate was the more conservative model, with higher exposures required for full effectiveness. Therefore, the exposure profiles of tecovirimat in plasma in nonhuman primates and humans were compared after the first dose (Panel E) and after the last dose at steady state (Panel F) to evaluate whether exposures in humans exceeded those in nonhuman primates, providing a reasonable expectation of efficacy in humans.

Figure 2. Figure 2. Differences in Survival Rates with Tecovirimat as Compared with Placebo. Shown is a forest-plot summary comparing differences in survival rates between tecovirimat and placebo in each study. The exact 95% confidence intervals (horizontal bars in the forest plot) are based on the score statistic of the difference in survival rates. The P value is from a one-sided Fisher’s exact test for the comparison of tecovirimat with placebo. Data from the study of dose exploration and pharmacokinetics in rabbits are not shown on the forest plot, since the study did not include a placebo control.

In aggregate, 1 of 20 nonhuman primates that received placebo survived infection with monkeypox virus and no rabbits that received placebo survived rabbitpox virus infection (Figure 1 and Figure 2, and Tables S7 and S8 in the Supplementary Appendix). Additional results for viral load and lesion counts are provided in Figures S4, S5, and S6 in the Supplementary Appendix. On the basis of the results of the first two studies in nonhuman primates, the minimum effective dose was determined to be 3 to 10 mg per kilogram (Fig. S3 in the Supplementary Appendix), which provided nearly full protection from death (i.e., a survival rate of approximately 95%, as compared with 5% in the placebo group) and reduced viral loads and lesion counts. Subsequent studies were conducted with the dose of 10 mg per kilogram, because this dose reduced viral load and lesion counts to a greater extent than the dose of 3 mg per kilogram.

Table 1. Table 1. Geometric Mean of Noncompartmental Exposures in Nonhuman Primates, Rabbits, and Humans — Identification of the Most Conservative Animal Model and Comparison with Human Exposures.

As previously reported by Berhanu et al.,26 in the treatment-delay study, a dose of 10 mg per kilogram initiated at 4 or 5 days after exposure resulted in a survival rate of 83%, whereas treatment initiation 6 days after exposure resulted in a survival rate of 50%. In the treatment-duration study, 3 daily doses at 10 mg per kilogram initiated 4 days after exposure resulted in a survival rate of 50%, whereas 5 and 7 daily treatments resulted in a survival rate of 100%. Ten daily treatments resulted in 80% survival. The pharmacokinetic study in nonhuman primates showed that after the 14th dose (steady state) of 10 mg per kilogram, the mean values in plasma that were associated with efficacy were a maximum concentration (C max ) of 1444 ng per milliliter, a minimum concentration (C min ) of 169 ng per milliliter, an average (mean) concentration (C avg ) of 598 ng per milliliter, and an area under the concentration–time curve over 24 hours (AUC 0-24hr ) of 14,352 ng×hours per milliliter (Table 1).

In rabbits, the minimum effective dose was determined to be 20 to 40 mg per kilogram, which provided protection from death and illness equivalent to that of the higher doses we evaluated (80 and 120 mg per kilogram) (Figure 1 and Figure 2, and Fig. S3 in the Supplementary Appendix). The dose of 40 mg per kilogram was selected as the standard for fully effective dosing. A population pharmacokinetic model was developed on the basis of plasma exposures after days 1, 7, and 14 of treatment. The mean values in plasma at steady state that were associated with efficacy after treatment with the dose of 40 mg per kilogram were a C max of 374 ng per milliliter, a C min of 25 ng per milliliter, a C avg of 138 ng per milliliter, and an AUC 0-24hr of 3318 ng×hours per milliliter (Table 1).

Although the efficacious exposure values in nonhuman primates treated with 10 mg per kilogram and in rabbits treated with 40 mg per kilogram were similar after the first dose (ratios of exposure in nonhuman primates as compared with that in rabbits ranged from 1.1 to 1.4), the exposure values in nonhuman primates were higher at steady state, with ratios (nonhuman primates:rabbits) of 3.9 for C max , 6.9 for C min , 4.3 for C avg , and 4.3 for AUC 0-24hr (Table 1). These data suggest that the nonhuman primate is the more conservative animal model, since it requires higher exposure for maximum efficacy and would result in a higher predicted dose in humans for achieving efficacy against smallpox.

Tecovirimat Clinical Trial

Table 2. Table 2. Demographic and Clinical Characteristics of the Participants at Baseline (Safety Population).

In the expanded safety trial, 851 volunteers were screened, of whom 452 underwent randomization and were assigned to receive tecovirimat at a dose of 600 mg twice daily (361 participants) or matching placebo (91 participants) for 14 consecutive days (Fig. S7 in the Supplementary Appendix). On the basis of the exposure–response relationship in animal models and previous clinical pharmacokinetic data over a range of doses,27,28 modeling and simulation studies29 predicted that a dose of 600 mg twice daily would provide exposure in excess of that provided by the efficacious doses in animals. Although studies in animals have shown that five daily doses is the minimum number that is sufficient to provide a survival advantage similar to that associated with longer dosing durations, the 14-day regimen was selected on the basis of the kinetics of the humoral immune response in smallpox,3 which is necessary to clear the virus and prevent recrudescence of disease. The demographic and baseline characteristics of the trial participants were well balanced in the trial groups (Table 2). A total of 431 participants completed the trial. The overall rate of adherence was 94.4% in the placebo group and 93.6% in the tecovirimat group; the corresponding rates among participants in the pharmacokinetic portion of the trial were 100% and 96.9%.

Table 3. Table 3. Adverse Events That Occurred or Worsened during Receipt of Tecovirimat or Placebo in the Overall Summary Safety Population.

The pharmacokinetic profiles and exposures of approximately 48 participants in a fed state are presented to compare with those in the nonhuman primate model (Figure 1E and Figure 1F and Table 1, and Table S9 and Fig. S8 in the Supplementary Appendix). There were 386 nonserious adverse events reported throughout the trial by 164 participants; 208 of these events were thought to be related to tecovirimat or placebo (Table 3, and Table S10 in the Supplementary Appendix). The adverse events related to the trial regimen that occurred or worsened during the trial period were similar in the two groups (Table S11 in the Supplementary Appendix). Adverse events of grade 3 or higher occurred or worsened during treatment at a frequency of 1.1% in both the tecovirimat group and the placebo group (Table S12 in the Supplementary Appendix) and included headache, osteoarthritis, and hidradenitis. One fatal adverse event was found to be related to a pulmonary embolism that occurred in a participant 1 week after completion of tecovirimat treatment. The participant had a history of recent recurrent deep-vein thromboses but was not being treated with anticoagulant agents. Toxicologic testing revealed no ethanol or drugs of abuse, and the death was judged by the investigators to be unrelated to tecovirimat. Eight participants (2 in the placebo group and 6 in the tecovirimat group) discontinued the trial regimen because of adverse events (Table S13 in the Supplementary Appendix).