On the basis of its efficacy in two animal models and pharmacokinetic and safety data in humans, tecovirimat is being advanced as a therapy for smallpox in accordance with the FDA Animal Rule. (Funded by the National Institutes of Health and the Biomedical Advanced Research and Development Authority; ClinicalTrials.gov number, NCT02474589 .).

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The therapeutic efficacies of smallpox vaccine ACAM2000 and antiviral tecovirimat given alone or in combination starting on day 3 postinfection were compared in a cynomolgus macaque model of lethal monkeypox virus infection. Postexposure administration of ACAM2000 alone did not provide any protection against severe monkeypox disease or mortality. In contrast, postexposure treatment with tecovirimat alone or in combination with ACAM2000 provided full protection. Additionally, tecovirimat treatment delayed until day 4, 5, or 6 postinfection was 83% (days 4 and 5) or 50% (day 6) effective.

Background: Proteins associated with the late endosome (LE) appear to play a central role in the envelopment of a number of taxonomically diverse viruses. How viral proteins interact with LEassociated proteins to facilitate envelopment is not well understood. LE-derived transport vesicles form through the interaction of Rab9 GTPase with cargo proteins, and TIP47, a Rab9-specific effector protein. Vaccinia virus (VV) induces a wrapping complex derived from intracellular host membranes to envelope intracellular mature virus particles producing egress-competent forms of virus.

The Tyr-X-X-Leu (YxxL) motif of the vaccinia virus F13L protein was examined for late (L) domain activity. The ability of an F13L deletion virus to form plaques was restored by PCR products containing single alanine substitutions within the motif and a YAAL construct but not by constructs lacking both the Y and L residues. Recombinant viruses possessing alanine substitutions in place of the tyrosine or the leucine residue in the YxxL motif demonstrated small, asymmetrical plaques. RNA interference-dependent depletion of Alix and TSG101 (host proteins involved in L domain-dependent protein trafficking) diminished extracellular enveloped virion production to various degrees, suggesting that the YxxL motif is a genuine L domain.

e ST-246 is being evaluated as a treatment for pathogenic orthopoxvirus infections in humans. To this end, a phase 2, doubleblind, randomized, placebo-controlled, multicenter trial was conducted to assess the safety, tolerability, and pharmacokinetics (PK) of ST-246 when administered as a single daily oral dose (400 mg or 600 mg) for 14 days in fed adult volunteers. ST-246 was safe and well tolerated, with no deaths or serious adverse events reported during the study. There was a low incidence of treatment-emergent adverse events (TEAEs), the most common of which were mild nausea and headache. There were no clinically significant results from laboratory assessments, vital sign measurements, physical examinations, or electrocardiograms. The PK and dose proportionality of ST-246 were determined. The PK analysis showed that steady state was achieved by day 5 for the ST-246 400-mg treatment group and by day 6 for the 600-mg group. The dose proportionality analysis showed that the 400-and 600-mg ratio of dose-normalized peak drug concentration in plasma (C max ) and relative exposure for each dosing interval (AUC ) ranged from 80% to 85%. However, the 90% confidence intervals did not include 1.0, so dose proportionality could not be concluded. Overall, ST-246 was shown to be safe, and the PK was predictable. These results support further testing of ST-246 in a multicenter pivotal clinical safety study for licensure application.

BackgroundST-246® is an antiviral, orally bioavailable small molecule in clinical development for treatment of orthopoxvirus infections. An intravenous (IV) formulation may be required for some hospitalized patients who are unable to take oral medication. An IV formulation has been evaluated in three species previously used in evaluation of both efficacy and toxicology of the oral formulation.Methodology/Principal FindingsThe pharmacokinetics of ST-246 after IV infusions in mice, rabbits and nonhuman primates (NHP) were compared to those obtained after oral administration. Ten minute IV infusions of ST-246 at doses of 3, 10, 30, and 75 mg/kg in mice produced peak plasma concentrations ranging from 16.9 to 238 µg/mL. Elimination appeared predominately first-order and exposure dose-proportional up to 30 mg/kg. Short IV infusions (5 to 15 minutes) in rabbits resulted in rapid distribution followed by slower elimination. Intravenous infusions in NHP were conducted at doses of 1 to 30 mg/kg. The length of single infusions in NHP ranged from 4 to 6 hours. The pharmacokinetics and tolerability for the two highest doses were evaluated when administered as two equivalent 4 hour infusions initiated 12 hours apart. Terminal elimination half-lives in all species for oral and IV infusions were similar. Dose-limiting central nervous system effects were identified in all three species and appeared related to high Cmax plasma concentrations. These effects were eliminated using slower IV infusions.Conclusions/SignificancePharmacokinetic profiles after IV infusion compared to those observed after oral administration demonstrated the necessity of longer IV infusions to (1) mimic the plasma exposure observed after oral administration and (2) avoid Cmax associated toxicity. Shorter infusions at higher doses in NHP resulted in decreased clearance, suggesting saturated distribution or elimination. Elimination half-lives in all species were similar between oral and IV administration. The administration of ST-246 was well tolerated as a slow IV infusion.