Effects of Treatment Delay on Efficacy of Tecovirimat Following Lethal Aerosol Monkeypox Virus Challenge in Cynomolgus Macaques

Abstract

Background: Tecovirimat (ST-246) is being developed as an antiviral therapeutic for smallpox for use in the event of an accidental or intentional release. The last reported case of smallpox was 1978 but the potential for use of variola virus for biowarfare has renewed interest in smallpox antiviral therapeutics.

Methods: Cynomolgus macaques were challenged with a lethal dose of monkeypox virus (MPXV) by aerosol as a model for human smallpox and treated orally with 10 mg/kg tecovirimat once daily starting up to 8 days following challenge. Monkeys were monitored for survival, lesions, and clinical signs of disease. Samples were collected for measurement of viremia by quantitative real-time polymerase chain reaction, and for white blood cell counts.

Results: Survival in animals initiating treatment up to 5 days postchallenge was 100%. In animals treated starting 6, 7, or 8 days following challenge, survival was 67%, 100%, and 50%, respectively. Treatment initiation up to 4 days following challenge reduced severity of clinical manifestations of infection.

Conclusions: Tecovirimat treatment initiated up to 8 days following a lethal aerosol MPXV challenge improves survival and, when initiated earlier than 5 days after challenge, provides protection from clinical effects of disease, supporting the conclusion that it is a promising smallpox antiviral therapeutic candidate.

Figure 1.

Kaplan–Meier survival analysis of monkeypox virus (MPXV) aerosol challenge by delay of treatment initiation following challenge. Cynomolgus macaques were challenged with MPXV Zaire 79 by the aerosol route and then treated with tecovirimat once daily by the oral route. The day treatment was initiated following challenge is indicated in the key. The effect of tecovirimat treatment on survival was statistically significant (log-rank test, P < .05) for study groups initiating tecovirimat from 1 to 7 days following lethal aerosol challenge with MPXV.

Figure 2.

Group mean daily percentage weight change by interval between monkeypox virus challenge and initiation of treatment. Number columns below the x-axis indicate statistically significant differences (determined using analysis of variance; P < .05) in the fractional weight loss between the untreated placebo group and the groups initiating treatment on the indicated day following challenge (treatment days 1–8). If a number is present, then there was a significant difference between the group mean body weight change of the indicated treatment group and the placebo group. Numbers are color-matched to the treatment groups (treatment days 1 and 5, red; treatment days 2 and 6, blue; treatment days 3 and 7, green; treatment days 4 and 8, orange).

Figure 3.

Mean daily clinical observation scores following lethal aerosol challenge with monkeypox virus (MPXV). Detailed clinical observations including temperature, respiratory characteristics, neurological signs, appetite, body weight, appearance, natural cage behavior, provoked (chair) behavior, and gastrointestinal/urogenital characteristics were monitored and documented. Recorded observations were assigned numerical values as indicated below, were compiled into quantitative scores, and the group mean score plotted on each study day. Observations fell into 4 severity categories. Category 1 observations were assigned a value of 1 and included dry cough, mild lethargy, reduced grooming, and thin appearance. Category 2 observations were assigned a value of 2 and included rapid respiration, ataxia, minimal urine output, and sustained piloerection. Category 3 observations were assigned a value of 3 and included dyspnea and extended anorexia, among others. Category 4 observations were assigned a value of 4 and included gasping, hunched or prostrate posture, and unresponsiveness. Results for placebo subjects (solid black line) from day 13 onward represent 2 survivors. Number columns below the x-axis indicate statistically significant differences (determined using analysis of variance; P < .05) in the group mean clinical score between the untreated placebo group and the groups initiating treatment on the indicated day following MPXV challenge (treatment day 1–8). If a number is present, then there was a significant difference between the indicated treatment group and the placebo group. Numbers are color-matched to the treatment groups (treatment days 1 and 5, red; treatment days 2 and 6, blue; treatment days 3 and 7, green; treatment days 4 and 8, orange).

Figure 4.

Quantitative real-time polymerase chain reaction results from blood, following monkeypox virus (MPXV) aerosol challenge, for each treatment group. In subjects initiating treatment with tecovirimat starting 1–3 days following challenge, samples were collected prior to day 0 and on days 3, 5, 6, 7, 8, 9, 10, 14, 17, 24, 35, and 45. In subjects initiating treatment with tecovirimat 5–8 days following challenge, samples were collected prior to day zero and on days 4, 5, 6, 7, 8, 9, 10, 12, 14, 16, 18, 20, 22, 30, 35, and 44. The placebo group consists of 4 subjects from each of the sample collection schedules described (8 subjects total), and the group that initiated tecovirimat treatment on day 4 consists of 8 subjects starting sample collection on day 3 and 4 subjects starting sample collection on day 4 (12 subjects total). Number columns below the x-axis indicate statistically significant differences (determined using Kruskal–Wallis test; P < .05) in group mean hemagglutinin (HA) gene copies between the untreated placebo group and the groups initiating treatment on the indicated day following MPXV challenge (treatment day 1–8). If a number is present, then there was a significant difference between the indicated treatment group and the placebo group. Numbers are color-matched to the treatment groups (treatment days 1 and 5, red; treatment days 2 and 6, blue; treatment days 3 and 7, green; treatment days 4 and 8, orange).

Figure 5.

Pox lesion enumeration. Average number of lesions per animal by treatment group following monkeypox virus (MPXV) challenge. The spike in group mean lesion counts in the placebo group is due to results from a single subject in which lesion development appeared delayed relative to the other survivor in the group. Lesions were counted on the same schedule as described for quantitative real-time polymerase chain reaction blood sample collection. Number columns below the x-axis indicate statistically significant differences (determined using Kruskal–Wallis test; P < .05) in group mean lesion counts between the untreated placebo group and the groups initiating treatment on the indicated day following MPXV challenge (treatment day 1–8). If a number is present, then there was a significant difference between the indicated treatment group and the placebo group. Numbers are color-matched to the treatment groups (treatment days 1 and 5, red; treatment days 2 and 6, blue; treatment days 3 and 7, green; treatment days 4 and 8, orange).

Figure 6.

Average cell counts by interval between monkeypox virus (MPXV) challenge and initiation of treatment with tecovirimat. With respect to normal ranges, 2 sources of data were used: mean values compiled from the literature [27–32] and average results compiled from prior studies performed at Lovelace Respiratory Research Institute. Normal ranges are indicated in the figures by horizontal dashed lines. Number columns below the x-axis indicate statistically significant differences (determined using analysis of variance and Kruskal–Wallis test; P < .05) in group mean white blood cell (WBC) counts between the untreated placebo group and the groups initiating treatment on the indicated day following MPXV challenge (treatment day 1–8). If a number is present, then there was a significant difference between the indicated treatment group and the placebo group. Numbers are color-matched to the treatment groups (treatment days 1 and 5, red; treatment days 2 and 6, blue; treatment days 3 and 7, green; treatment days 4 and 8, orange).