Postexposure Treatment of Marburg Virus Infection

Rhesus monkeys are protected from disease when a recombinant vesicular stomatitis virus–based vaccine is administered 20–30 min after infection with Marburg virus. We protected 5/6 monkeys when this vaccine was given 24 h after challenge; 2/6 animals were protected when the vaccine was administered 48 h postinfection.

T he fi loviruses, Marburg virus (MBGV) and Ebola virus (EBOV), have been associated with sporadic episodes of hemorrhagic fever (HF) in Central Africa that produce severe disease and high mortality rates among infected patients (1). MBGV and EBOV are also considered potential biological weapons. No approved active or passive therapeutic modalities exist for fi lovirus infections. Although much progress has been made in developing preventive vaccines that can protect nonhuman primates against lethal challenge with MBGV and EBOV, advances in development of postexposure interventions against the fi loviruses have not kept pace. Some degree of success has been achieved by using strategies that mitigate the coagulation abnormalities characterizing fi loviral infection (2,3). Also, new postexposure treatment approaches, based on small interfering RNA (4) and antisense oligomers (5,6), have shown promising results in rodent models, but no reports have been published of evaluations of either strategy in the more stringent macaque models.
Recently, we showed the fi rst complete postexposure protection of nonhuman primates against a fi lovirus by administering a live-attenuated recombinant vesicular stomatitis virus (rVSV) vaccine vector expressing the MBGV glycoprotein (GP) (VSVΔG MBGV GP) shortly after a high-dose MBGV challenge (7,8). We demonstrated that an rVSV vector, expressing the Zaire EBOV (ZEBOV) GP, protected 50% of rhesus macaques when administered shortly after a high-dose ZEBOV challenge (9). We further showed that an rVSV vector expressing the Sudan EBOV GP completely protected rhesus monkeys from a lethal challenge with this virus when administered shortly after exposure (10). All animals in these 3 studies were treated once with rVSV vectors 20-30 min after fi lovirus challenge. The primary question raised from these investigations is how far out treatment can be delayed before there is no survival or benefi cial effect. Using a homologous VSVΔG MBGV GP vector, we have delineated a window of opportunity for treatment of MBGV-infected rhesus macaques.

The Study
Animal research was conducted in compliance with the Animal Welfare Act and other federal statutes and regulations relating to animals and experiments involving animals and adhered to the principles stated in the Guide for the Care and Use of Laboratory Animals, National Research Council, 1996. Fifteen healthy, fi lovirus-seronegative rhesus macaques (each weighing 4 kg-7 kg) were randomized into 2 experimental groups of 6 monkeys per group and 3 control groups of 1 animal per group. All 15 animals were challenged by intramuscular (IM) injection with 1,000 PFU of MBGV (Musoke strain). Approximately 24 h after MBGV challenge, animals in experimental group 1 received a single IM injection of VSVΔG MBGV GP (≈2 × 10 7 PFU) (8), and the animal in control group 1 received an equal dose of a VSV vector encoding a nonrelated GP (VSVΔG/LassaGPC). Approximately 48 h after MBGV challenge, animals in experimental group 2 received a single IM injection of VSVΔG MBGV GP (≈2 × 10 7 PFU), and the animal in control group 2 received an equal dose of VSVΔG/LassaGPC. The animal in control group 3 was not treated. Blood samples for viral infectivity titration, reverse transcription-PCR (RT-PCR), hematologic analysis, serum biochemical analysis, and immunoglobulin (Ig) G were collected before MBGV challenge and on days 3, 6, 10, 14, and 31-35 after MBGV challenge.
Two of the 6 animals that survived MBGV challenge (animals 1 and 6) showed no change in appearance or behavior that indicated overt illness. Changes in hematologic results and/or blood parameters were observed in 5 of the surviving animals (2, 4, 5, 7, and 10) during the course of the study (online Appendix Table). Plaque assay and RT-PCR were unable to detect any evidence of MBGV in the plasma of 6 of the 7 surviving animals (1, 2, 4-6, and 10). However, RT-PCR showed evidence of MBGV in peripheral blood mononuclear cells of 2 of these surviving animals (1 and 2) at day 10 ( Table 2). Viremia of 4.2 log 10 PFU/mL developed on day 10 in 1 surviving animal (7) treated 48 h after infection, and RT-PCR showed evidence of MBGV in peripheral blood mononuclear cells of this animal on days 6 and 10. Viremia in plasma was cleared, and the animal showed little evidence of illness by day 14. The serologic response profi le of MBGV infection after treatment was evaluated by IgG ELISA. All 7 animals that were treated with VSV∆G/MBGV GP and survived infection showed moderate to high levels of IgG by day 14 (320-1,000); humoral response against MBGV was not detectable in the treated animals that died or in the control animals ( Table 2).

Conclusions
This rhesus macaque model represents a worse-case scenario such as an accidental needle-stick exposure of a laboratory worker or fi rst responder to a high infectious dose of a fi lovirus. Accidents such as these have occurred several times over the past 5 years (11)(12)(13). Of direct relevance to our study was a recent laboratory accident in which an rVSV vector expressing the ZEBOV GP, which had been used successfully in postexposure treatment of experimentally infected nonhuman primates (9), was administered to a human ≈40 h after a ZEBOV needle-stick exposure (13). The patient received a dose of ≈5 × 10 7 PFU of the VSV ZEBOV GP vaccine, which is consistent with doses used in nonhuman primate studies (7,9,10). Fever, headache, and myalgia developed in the patient hours after injection but were successfully controlled with analgesics and antipyretics. Other adverse effects were not reported,   but whether treatment was effective or whether the patient never became infected remains uncertain. MBGV infection of humans normally progresses at a slower rate than does MBGV infection of macaques, with case-fatality rates in humans of 23%-90% (1) suggesting that the therapeutic window may be larger for humans than for infected macaques. In addition, the challenge dose that we employed in the rhesus monkey model of MBGV HF of 1,000 PFU represents >10,000 LD 50 doses (14), again showing that this is a robust challenge model. In the current study, we achieved near complete protection from death when treatment with a single-dose regimen was delayed 24 h and 33% protection when treatment was delayed 48 h postexposure. Because no approved treatments exist for exposure to infectious fi loviruses, the rVSV vectors described in the current study merit consideration for treating potential exposures and for further development for human use.