A potent Lassa virus antiviral targets an arenavirus virulence determinant.

Arenaviruses are a significant cause of hemorrhagic fever, an often-fatal disease for which there is no approved antiviral therapy. Lassa fever in particular generates high morbidity and mortality in West Africa, where the disease is endemic, and a recent outbreak in Nigeria was larger and more geographically diverse than usual. We are developing LHF-535, a small-molecule viral entry inhibitor that targets the arenavirus envelope glycoprotein, as a therapeutic candidate for Lassa fever and other hemorrhagic fevers of arenavirus origin. Using a lentiviral pseudotype infectivity assay, we determined that LHF-535 had sub-nanomolar potency against the viral envelope glycoproteins from all Lassa virus lineages, with the exception of the glycoprotein from the LP strain from lineage I, which was 100-fold less sensitive than that of other strains. This reduced sensitivity was mediated by a unique amino acid substitution, V434I, in the transmembrane domain of the envelope glycoprotein GP2 subunit. This position corresponds to the attenuation determinant of Candid#1, a live-attenuated Junín virus vaccine strain used to prevent Argentine hemorrhagic fever. Using a virus-yield reduction assay, we determined that LHF-535 potently inhibited Junín virus, but not Candid#1, and the Candid#1 attenuation determinant, F427I, regulated this difference in sensitivity. We also demonstrated that a daily oral dose of LHF-535 at 10 mg/kg protected mice from a lethal dose of Tacaribe virus. Serial passage of Tacaribe virus in LHF-535-treated Vero cells yielded viruses that were resistant to LHF-535, and the majority of drug-resistant viruses exhibited attenuated pathogenesis. These findings provide a framework for the clinical development of LHF-535 as a broad-spectrum inhibitor of arenavirus entry and provide an important context for monitoring the emergence of drug-resistant viruses.

Pharmacokinetic and pharmacodynamic modeling to determine the dose of ST-246 to protect against smallpox in humans.

Although smallpox has been eradicated, the United States government considers it a "material threat" and has funded the discovery and development of potential therapeutic compounds. As reported here, the human efficacious dose for one of these compounds, ST-246, was determined using efficacy studies in nonhuman primates (NHPs), together with pharmacokinetic and pharmacodynamic analysis that predicted the appropriate dose and exposure levels to provide therapeutic benefit in humans. The efficacy analysis combined the data from studies conducted at three separate facilities that evaluated treatment following infection with a closely related virus, monkeypox virus (MPXV), in a total of 96 NHPs. The effect of infection on ST-246 pharmacokinetics in NHPs was applied to humans using population pharmacokinetic models. Exposure at the selected human dose of 600 mg is more than 4-fold higher than the lowest efficacious dose in NHPs and is predicted to provide protection to more than 95% of the population.

Pharmacokinetic comparison of a single oral dose of polymorph form i versus form V capsules of the antiorthopoxvirus compound ST-246 in human volunteers.

ST-246, a novel compound that inhibits egress of orthopoxvirus from mammalian cells, is being tested as a treatment for pathogenic orthopoxvirus infections in humans. This phase I, double-blind, randomized, crossover, exploratory study was conducted to compare the pharmacokinetics (PK) of a single daily 400-mg oral dose of ST-246 polymorph form I versus polymorph form V administered to fed, healthy human volunteers. Both forms appeared to be well tolerated, with no serious adverse events. The order of administration of the two forms had no effect on the results of the PK analyses. Form I and form V both exhibited comparable plasma concentration versus time profiles, but complete bioequivalence between the two forms was not found. Maximum drug concentration (C(max)) met the bioequivalence criteria, as the 90% confidence interval (CI) was 80.6 to 96.9%. However, the area under the concentration-time curve from time zero to time t (AUC(0-t)) and AUC(0-∞) did not meet the bioequivalence criteria (CIs of 67.8 to 91.0% and 73.9 to 104.7%, respectively). The extent of absorption of form I, as defined by AUC(0-∞), was 11.7% lower than that of form V. Since ST-246 form I is more thermostable than form V, form I was selected for further development and use in all future studies.

Safety and pharmacokinetics of the anti-orthopoxvirus compound ST-246 following a single daily oral dose for 14 days in human volunteers.

ST-246 is being evaluated as a treatment for pathogenic orthopoxvirus infections in humans. To this end, a phase 2, double-blind, 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(τ)) 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.

SAR analysis of a series of acylthiourea derivatives possessing broad-spectrum antiviral activity.

A series of acylthiourea derivatives were designed, synthesized, and evaluated for broad-spectrum antiviral activity with selected viruses from Poxviridae (vaccinia virus) and two different genera of the family Bunyaviridae (Rift Valley fever and La Crosse viruses). A compound selected from a library screen, compound 1, displayed submicromolar antiviral activity against both vaccinia virus (EC(50)=0.25 µM) and La Crosse virus (EC(50)=0.27 µM) in cytopathic effect (CPE) assays. SAR analysis was performed to further improve antiviral potency and to optimize drug-like properties of the initial hits. During our analysis, we identified 26, which was found to be nearly fourfold more potent than 1 against both vaccinia and La Crosse viruses. Selected compounds were further tested to more fully characterize the spectrum of antiviral activity. Many of these possessed single digit micromolar and sub-micromolar antiviral activity against a diverse array of targets, including influenza virus (Orthomyxoviridae), Tacaribe virus (Arenaviridae), and dengue virus (Flaviviridae).

Safety and pharmacokinetics of the antiorthopoxvirus compound ST-246 following repeat oral dosing in healthy adult subjects.

ST-246, a novel compound that inhibits egress of orthopoxvirus from infected cells, is being evaluated as a treatment for pathogenic orthopoxvirus infections in humans. This phase I, double-blind, randomized, placebo-controlled, escalating multiple-dose study was conducted to determine the safety, tolerability, and pharmacokinetics of ST-246 administered as a single daily oral dose of 250, 400, or 800 mg for 21 days to nonfasting healthy human volunteers. ST-246 appeared to be well tolerated, with no serious adverse events (AEs). Headache, for which one subject in the 800-mg group discontinued the study, was the most commonly reported AE in all treatment groups. The multiple-dose pharmacokinetics of ST-246 was well characterized. The day 21 mean elimination half-lives were calculated at 18.8, 19.8, and 20.7 h for each of the 250-, 400-, and 800-mg/day dose groups, respectively. Steady state was reached by day 6 (within 3 to 5 half-lives), saturable absorption was observed at the 800-mg dose level, and the fraction of parent drug excreted in the urine was very low. Based on these results, administration of 400 mg/day ST-246 can be expected to provide plasma concentrations above the efficacious concentration demonstrated in nonhuman primate models in earlier studies.

Single-dose safety and pharmacokinetics of ST-246, a novel orthopoxvirus egress inhibitor.

ST-246 is a novel, potent orthopoxvirus egress inhibitor that is being developed to treat pathogenic orthopoxvirus infections of humans. This phase I, double-blind, randomized, placebo-controlled single ascending dose study (first time with humans) was conducted to determine the safety, tolerability, and pharmacokinetics of ST-246 in healthy human volunteers. ST-246 was administered in single oral doses of 500, 1,000, and 2,000 mg to fasting healthy volunteers and 1,000 mg to nonfasting healthy volunteers. ST-246 was generally well tolerated with no serious adverse events, and no subject was withdrawn from the study due to ST-246. The most commonly reported drug-related adverse event was neutropenia, which was found, upon further analysis, not to be treatment related. ST-246 was readily absorbed following oral administration with mean times to maximum concentration from 2 h to 3 h. Absorption was greater in nonfasting volunteers than in fasting volunteers. Administration of ST-246 resulted in exposure levels predicted to be sufficient for inhibiting orthopoxvirus replication compared to exposure levels in nonhuman primates in which ST-246 protected animals from lethal orthopoxvirus infection.

Pharmacokinetics and interspecies allometric scaling of ST-246, an oral antiviral therapeutic for treatment of orthopoxvirus infection.

Plasma pharmacokinetics of ST-246, smallpox therapeutic, was evaluated in mice, rabbits, monkeys and dogs following repeat oral administrations by gavage. The dog showed the lowest Tmax of 0.83 h and the monkey, the highest value of 3.25 h. A 2- to 4-fold greater dose-normalized Cmax was observed for the dog compared to the other species. The mouse showed the highest dose-normalized AUC, which was 2-fold greater than that for the rabbit and monkey both of which by approximation, recorded the lowest value. The Cl/F increased across species from 0.05 L/h for mouse to 42.52 L/h for dog. The mouse showed the lowest VD/F of 0.41 L and the monkey, the highest VD/F of 392.95 L. The calculated extraction ratios were 0.104, 0.363, 0.231 and 0.591 for mouse, rabbit, monkey and dog, respectively. The dog showed the lowest terminal half-life of 3.10 h and the monkey, the highest value of 9.94 h. The simple allometric human VD/F and MLP-corrected Cl/F were 2311.51 L and 51.35 L/h, respectively, with calculated human extraction ratio of 0.153 and terminal half-life of 31.20 h. Overall, a species-specific difference was observed for Cl/F with this parameter increasing across species from mouse to dog. The human MLP-corrected Cl/F, terminal half-life, extraction ratios were in close proximity to the observed estimates. In addition, the first-in-humans (FIH) dose of 485 mg, determined from the MLP-corrected allometry Cl/F, was well within the dose range of 400 mg and 600 mg administered in healthy adult human volunteers.

Comparison of the safety and pharmacokinetics of ST-246® after i.v. infusion or oral administration in mice, rabbits and monkeys.

BACKGROUND: ST-246® is an antiviral, orally bioavailable small molecule in clinical development for treatment of orthopoxvirus infections. An intravenous (i.v.) formulation may be required for some hospitalized patients who are unable to take oral medication. An i.v. formulation has been evaluated in three species previously used in evaluation of both efficacy and toxicology of the oral formulation. METHODOLOGY/PRINCIPAL FINDINGS: The pharmacokinetics of ST-246 after i.v. infusions in mice, rabbits and nonhuman primates (NHP) were compared to those obtained after oral administration. Ten minute i.v. 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 i.v. 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 i.v. infusions were similar. Dose-limiting central nervous system effects were identified in all three species and appeared related to high C(max) plasma concentrations. These effects were eliminated using slower i.v. infusions. CONCLUSIONS/SIGNIFICANCE: Pharmacokinetic profiles after i.v. infusion compared to those observed after oral administration demonstrated the necessity of longer i.v. infusions to (1) mimic the plasma exposure observed after oral administration and (2) avoid C(max) 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 i.v. administration. The administration of ST-246 was well tolerated as a slow i.v. infusion.