Antiviral Res. 2017 Jul;143:278-286. doi: 10.1016/j.antiviral.2017.04.002.

Efficacy of Delayed Brincidofovir Treatment Against a Lethal Rabbitpox Virus Challenge in New Zealand White Rabbits

Irma M. Grossia, Scott A. Fostera, Melicia R. Gaineyb, Robert Krileb, John A. Dunna, Thomas Brundagea, and Jody M. Khouria

a Chimerix, Durham, NC USA

b Battelle, West Jefferson, OH USA



In the event of a bioterror attack with variola virus (smallpox), a potential exposure may not be identified until onset of fever and other clinical signs and symptoms.  To determine if antiviral therapy with brincidofovir (BCV; CMX001) initiated at, or following, onset of fever could prevent severe illness and death, a lethal rabbitpox model was used. BCV is in advanced development as an antiviral for the treatment of smallpox under the US Food and Drug Administration’s ‘Animal Rule.’  This phase 3 study assessed the efficacy of immediate versus delayed treatment with oral BCV following onset of symptomatic disease in New Zealand White rabbits intradermally inoculated with a lethal rabbitpox virus (RPXV), strain Utrecht.  Rabbits with confirmed fever and infection were randomized to blinded treatment with placebo, BCV, or BCV delayed by 24, 48, or 72 h.  The primary objective evaluated the survival benefit with BCV treatment.  The assessment of reduction in the severity and progression of clinical events associated with RPXV were secondary objectives.  Clinically and statistically significant reductions in mortality were observed when BCV was initiated up to 48 h following the onset of fever: survival rates were 100%, 93%, and 93% in the immediate treatment, 24-h, and 48-h delayed treatment groups, respectively, versus 48% in the placebo group (p < 0.05 for each vs. placebo).  Significant improvements in vital signs (respiratory rate, fever, etc.) and virology parameters (viral load) were also observed.  These findings provide a scientific rationale for therapeutic intervention with BCV in the event of a smallpox outbreak when vaccination is contraindicated or when diagnosis follows the appearance of clinical signs and symptoms.

KEYWORDS: Antiviral; Brincidofovir; CMX001; Orthopoxvirus; Smallpox

PMID: 28392420



Brincidofovir (BCV, CMX001) is a broad-spectrum antiviral being developed for the treatment of smallpox under the FDA Animal Rule.  BCV is a lipid conjugate of cidofovir, an acyclic nucleotide analog antiviral that inhibits viral DNA polymerase.  Its mechanism as a viral DNA polymerase inhibitor provides potent antiviral activity with a high barrier to drug resistance across multiple families of dsDNA viruses including the herpesviruses, adenoviruses and orthopoxviruses, such as the smallpox (variola) virus.

Prior to being declared globally eradicated by the World Health Organization in 1980 (Fenner 1988), the worldwide death toll from smallpox during the 20th century was to have been around 300-500 million (Lawson 2015).  Further, due to the global ubiquity of the virus prior to its eradication, the likely existence of undeclared viral stocks held outside of the two WHO-designated repository laboratories in the US and Russia (Hansen 2012), and the potential for modern synthetic re-creation of the virus (Noyce 2018), smallpox remains a significant threat as a potential biological weapon (Gates 2017; Bossi 2006).

The success of global eradication led to the end of universal vaccination programs, leaving a majority of the current population unprotected against the disease.  As a consequence, in the event of an outbreak, the rates of mortality and significant morbidity are predicted to be higher than previously documented (Henderson 1999).

To date, there are no approved therapies for the treatment of smallpox.  Vaccination will be the primary method of controlling an outbreak, but it is understood that antivirals will be needed for populations that are not well served by vaccines including individuals who are infected and showing symptoms of smallpox, those contraindicated for vaccination, and those who were vaccinated but still present with smallpox.  As human clinical trials are neither feasible nor ethical to assess the effectiveness of BCV against VARV, the efficacy of BCV for smallpox is determined in appropriate animal models of orthopoxvirus infection.



Rabbitpox is a well characterized model of orthopoxvirus infection in which New Zealand White (NZW) rabbits are infected via the intradermal route with the rabbitpox virus (RPXV) (Adams 2007).  RPXV is closely related to VARV, causing pneumonia and fatal systemic infection in rabbits. The disease produced in this model is severe, resulting in a high mortality rate (Adams 2007; Chapman 2010).  Characteristic disease-related signs include fever, subcutaneous edema, nasal discharge and a majority of rabbits present with secondary skin lesions distant to the initial intradermal infection site.  The 8-12 day course of disease in RPXV infection, inclusive of disease-related signs, mirrors the approximate 28 day course of human smallpox infection [Figure 1].

The overall objective of this study was to determine the treatment effect of immediate or delayed oral administration of BCV in the intradermal NZW rabbit RPXV model.  The study was designed to inform the potential utility of BCV for treatment of smallpox following confirmed infection via the appearance of clinical signs and symptoms.  Rabbits inoculated with a lethal challenge of RPXV were randomized by sex into one of five blinded treatment groups following detection of fever, defined as a ≥ 1.5°F increase from mean baseline body temperature, and confirmed by a second measurement approximately 1 hour (±10 minutes) later.  The first dose of each blinded treatment (BCV or placebo) was administered within 4 hours following confirmation of fever, i.e., “randomization” at Day 0 (RD0).  Subsequent doses of BCV or placebo were administered at approximately 24 (± 2) hours intervals, resulting in one placebo treatment group, one immediate BCV treatment group (0 hour), and three delayed BCV treatment groups (initiated at 24, 48, and 72 hours post-randomization).  Following randomization, all rabbits, regardless of group assignment, received a daily dose (either BCV or placebo) for 8 consecutive days.  All BCV groups received three BCV doses, separated by a 48 hour interval [Table 1].

Clinical evaluations and virology assessments were performed on all surviving animals until their scheduled termination on post-randomization (RD42).  Continuous endpoints were compared using standard parametric methods.  Survival data were summarized with Kaplan–Meier methods.



Study Conclusions:

  • BCV demonstrated a clinically and statically significant treatment effect (i.e., survival) in this lethal orthopoxvirus infection model when administered within 48 hours following the onset of fever, approximately the midpoint of disease progression in this model [Figure 2].
  • BCV treatment initiated within 48 hours following the onset of fever was associated with a more rapid recovery in body temperature, compared with placebo.
  • Compared with placebo, viral load as measured by qPCR and plaque assay, was significantly reduced in animals that received BCV within 48 hours following the onset of fever [Figure 3].
  • The reduction in viral load among BCV-treated animals may reflect reduced infectivity in these animals, and potential additional public health benefits in the event of a smallpox bioterror event.
  • All surviving animals developed neutralizing antibody as measured by PRNT assay. BCV did not prevent the generation of an immune response to infection, or the development of protective immunity.
  • The demonstration of improved survival in rabbits treated with BCV within 48 hours of fever (midpoint of disease progression) provides a scientific rationale for therapeutic intervention in humans exposed to smallpox even after the appearance of clinical signs and symptoms, and at which point vaccination is no longer an option.







  • Adams, M.M., Rice, A.D., Moyer, R.W., (2007). Rabbitpox virus and vaccinia virus infection of rabbits as a model for human smallpox. J. Virol. 81 (20), 11084– 11095.
  • Bossi P, Garin D, Guihot A, Gay F, Crance JM, Debord T, Autran B, and Bricaire F., (2006) Bioterrorism: Management of Major Biological Agents.  Cellular and Molecular Life Sciences 63: 2196-2212
  • Chapman, J.L., Nichols, D.K., Martinez, M.J., Raymond, J.W. (2010) Animal models of orthopoxvirus infection. Vet. Pathol. 47, 852-870.
  • Fenner F. Henderson DA, Arita I, Jezek Z, Ladnyi ID (1988) Smallpox and its Eradication, World Health Organization, Geneva
  • Gates, Bill . (2017)
  • Henderson DA. (1999) Smallpox: Clinical and epidemiologic features. Emerg Infect Dis 5:537-539
  • Hansen JC (2012) Smallpox: New Perspectives Regarding Risk Assessment & Management. J Bioterr Biodef S4:002. doi:10.4172/2157- 2526.S4-002
  • Lawson S (2015) Theories of International Relations: Contending Approaches to World Politics.  In John Wiley & Sons PT 19. ISBN 9780745695136.
  • Noyce Ryan S, Lederman Seth, and Evans David (2018) Construction of an Infectious Horsepox Virus vaccine from Chemically Synthesized DNA Fragments.  PLOS ONE