Antibody therapy for Ebola

Abstract

Ebola viruses can cause severe hemorrhagic fever in humans and nonhuman primates with fatality rates up to 90%, and are identified as biosafety level 4 pathogens and CDC Category A Agents of Bioterrorism. To date, there are no approved therapies and vaccines available to treat these infections. Antibody therapy was estimated to be an effective and powerful treatment strategy against infectious pathogens in the late 19th, early 20th centuries but has fallen short to meet expectations to widely combat infectious diseases. Passive immunization for Ebola virus was successful in 2012, after over 15 years of failed attempts leading to skepticism that the approach would ever be of potential benefit. Currently, monoclonal antibody (mAbs)-based therapies are the most efficient at reversing the progression of a lethal Ebola virus infection in nonhuman primates, which recapitulate the human disease with the highest similarity. Novel combinations of mAbs can even fully cure lethally infected animals after clinical symptoms and circulating virus have been detected, days into the infection. These new developments have reopened the door for using antibody-based therapies for filovirus infections. Furthermore, they are reigniting hope that these strategies will contribute to better control the spread of other infectious agents and provide new tools against infectious diseases.

The initial use of antibodies (in the form of serum containing polyclonal antibodies) to treat infectious diseases can be dated back to the late 19th century. After Roux and Yersin had discovered the diphtheria toxin,¹ Behring and Kitasato found that administration of specific antitoxin raised in horses could protect susceptible individuals.² Prior to the availability of antibiotics and most vaccines, administration of immune serum containing high titered polycolonal antibodies was the only and most effective way to prevent a variety of viral infections including measles, hepatitis A, rabies, and smallpox, some toxin based bacterial diseases including diptheria, tetanus, and botulism as well as pneumococcal and Hemophilus influenzae B infections.^3,4 Passive immunization was also used to treat patients with diphtheria, rabies, pneumococcal infection, and certain complications of vaccination. However, these applications were restricted by availability, batch to batch variation, limited potency, and side effects, especially allergic reactions and serum sickness. Antibody therapy was essentially sidelined in the late 1930s with the discovery of antibiotics.

The introduction of hybridoma technology by Kohler and Milstein in 1975 made possible the generation of one antibody or monoclonal antibody (mAb) produced from one B-cell clone, specific for one antigenic epitope.^5,6 The technique was initially developed in mice and consequently generated murine mAbs. Following further technical advancements, these mAbs were partially or fully humanized^7,8 to minimize or eliminate the potentially immunogenic mouse components that could be reactogenic in humans. This has led to mAbs having marked successes in the clinic^9,10. Indeed, there are currently around 350 mAbs in the clinical pipeline,¹¹ even though most of them are still in early developmental stages. The majority (~90%) of these mAbs target antigens relevant either to cancer or inflammatory or immunological disorders.¹¹ Antibody therapy in the field of infectious diseases, in contrast, has been largely hampered over the last several decades owing to the current availability of antimicrobial drugs, small markets, high costs of production, and microbial antigenic variation. To date, there is only one commonly used licensed mAb product for the prevention of respiratory syncytial virus (RSV) infection in premature babies¹²; another was recently approved by the FDA for inhalational anthrax disease, and a handful of mAb products are undergoing clinical evaluation for infectious disease indications, including methicillin-resistant Staphylococcus aureus and Clostridium difficile.¹³ However, there are signs that the use of mAb therapies in infectious diseases is being reinvestigated due to a number of reasons: (1) increases in antibiotic drug resistance; (2) the emergence of new pathogenic microbes for which no therapy is available; (3) large numbers of immunocompromised people in whom antimicrobial therapy is not as effective as in hosts with intact immunity; (4) a better understanding of importance of the normal microbiome; (5) increased availability of diagnostic tests and improvements in environmental detection; (6) advancement in mAb technology and manufacturing; and (7) development of mAb cocktail formulations which will bring down the cost and potential side effects. All of the above is reflected by approval rates, from clinical trials evaluation, of around 20% for licensure of mAb products, compared with 5% for new chemical entities.^9,14 Only a few antibiotics have been approved by the Food and Drug Administration (FDA) between 2003 and 2007 and just 2 from 2008–2011.¹⁵

The family Filoviridae consist of enveloped, negative-sense RNA viruses with a long, filamentous virion. Filoviruses can be divided into 2 major genera, Ebolavirus and Marburgvirus, as well as a new genus, Cuevavirus.¹⁶ The Cuevavirus only contains one virus named Lloviu which was recently identified among bats in Spain,¹⁷ but is not known to cause disease in humans. However, the ebolaviruses and marburgviruses cause highly lethal hemorrhagic fever in humans and non-human primates. The genus Ebolavirus includes 5 species, each named after the location of the outbreak in which they were first identified. These include Ebola virus (Zaire ebolavirus; now abbreviated EBOV), Sudan virus (Sudan ebolavirus; SUDV), Reston virus (RESTV), Taï Forest virus (formerly known as Cote d’Ivoire ebolavirus, now abbreviated TAFV), and Bundibugyo virus (BDBV). Among the 5 species, Ebola and Sudan are the 2 most lethal and common ebolaviruses. While Ebola virus is the most lethal with fatality rates up to 90%, Sudan virus caused the largest outbreak of Ebola Hemorrhagic Fever with 425 human cases, in the Gulu district of Sudan in 2000. Outbreaks localized to sub-Saharan Africa have occurred sporadically since the first Ebola outbreak in 1976, with over 2300 confirmed infections to date and over 1500 fatalities.¹⁸ Due to high morbidity and mortality rates in natural outbreaks, lack of prophylactic and treatment options, aerosol transmission potential, and their highly virulent nature, Ebola viruses have been identified as both NIAID Category A Priority pathogens and CDC Category A Agents of Bioterrorism. There are no approved vaccine or treatments available for human use, and the current protocol for patients with suspect or confirmed EBOV diagnosis is quarantine and primarily supportive management and palliative care.¹⁹

In spite of the effort invested into the development of post-exposure treatment strategies for Ebola infection through small molecular drugs and vaccines, only limited protection has been achieved, with treatments that require initiation within one hour after infection in non-human primates (NHPs), the animals that most accurately recapitulate human disease.²⁰ Antibody therapy against Ebola infection was first suspected to be a potentially beneficial strategy after employing crude blood transfusions from convalescent patients during the 1995 Kikwit outbreak where 7 of 8 treated patients survived.²¹ The results of antibody therapy have been controversial in follow-up studies in NHPs^22,23; especially since the potent neutralizing monoclonal antibody (mAb) KZ52 failed to protect NHPs against EBOV challenge and did not reduce viral replication following infection, which was not explained by neutralization escape mutants.²⁴ As a result, the enthusiasm for investigating passive immunotherapy as an option in the treatment of Ebola infections had been curbed, and the predominant opinion about the protective mechanism to Ebola infection changed from the humoral to the cellular immune response. As a result, antibody therapy has been neglected in the field over the last decade. This changed in early 2012 with a study that showed successful protection of naïve NHPs using passive transfer of species-matched antibodies from vaccinated rhesus macaques that had survived EBOV infection.²⁵ This reopened the door for using antibody-based therapies for filovirus infections. The results in NHPs from 3 independent studies published in mid-2012 demonstrated that protection against EBOV disease can be achieved using mAb therapy with the treatment window as late as 24–48 h post challenge.^26-28 These findings were further supported by additional experimental evidence published in 2012 demonstrating that the humoral immune response correlates with survival, and by inference, plays an important role in protection.^29,30 Learning from the unsuccessful KZ52 study, all 3 most recent studies instead used mAb cocktails comprising of either 2 (ch133+ch266)²⁶ or 3 (MB-003 or ZMAb)^27,28 mAbs. The mAb cocktail approach could emerge as a solution for the treatment of pathogens which are larger in size, highly pathogenic, or mutating rapidly, which may include Ebola or HIV virus. The strategy to target multiple epitopes at once makes it difficult for the pathogen to evade the treatment even for rapidly changing and adapting microorganisms. Recent advances have shown that mAbs therapy is a promising option against HIV infection.^31,32 Similarly, some types of bacteria, which may require the neutralization of toxins as well as some other determinants of pathogenicity, could be sensitive targets to a multi-prong attack by mAbs.

Encouraged by the developments of mAb cocktails against Ebola, further work was performed to investigate the efficacy of these treatments after certain milestones in the course of disease had elapsed. One research team demonstrated 43% (3 of 7 NHPs) protection in the rhesus macaque animal model when their specific mAb cocktail was only administrated after the documentation of “two triggers,” a positive RT-qPCR result, and a fever, which happened 4–5 d after EBOV challenge.³³ The other research team increased their cocktail efficacy by combining ZMAb with Ad5-IFNα, a recombinant human adenovirus serotype 5 virus expressing consensus human IFNα. This combination therapy extended the treatment window up to 4 d in NHPs with 100% survival achieved at 3 d post-exposure in rhesus macaques.³⁴ Currently the 2 research groups are collaborating with the goal of identifying the most efficacious mAb cocktail between 6 candidate mAbs, which could improve the protection limit and extend the treatment window even further. This goal is based on the promising individual results of the 2 mAb cocktails.

The successful treatment of lethally infected NHPs with mAb cocktails after the manifestation of clinical disease is a significant breakthrough and constitutes a practical option that could potentially be used in future outbreaks and other exposures to cure Ebola virus infections. However, it is a strategy with its own limitations since mutations of epitopes targeted by mAbs would likely abolish treatment efficacy and necessitate the development of more than one mAb cocktail. Along the same lines, outbreak viruses should be closely monitored for escape mutations, an important consideration driving the rationale behind using oligoclonal antibodies targeting several epitopes. Furthermore, mAb cocktails against other lethal filoviruses such as Sudan, Bundibugyo, and Marburg need to be developed. As mAb cocktails currently offer the highest level of protection against EBOV infection at the latest time points in NHPs and because of the safety profile as a biological product and known development path to licensure based on a multitude of other mAb-based treatments, they have been identified by several Biosafety Level 4 (BSL-4) programs as the preferred clinical option in case of an accidental laboratory exposure to EBOV.³⁵ Efforts to further develop these cocktails for clinical applications and facilitate authorized access for BSL-4 researchers are underway. With the formation of the International Filovirus Immunotherapeutic Consortium in 2012, scientists in the field of EBOV research will be able to collaborate more closely and share new developments efficiently. This initiative will help accelerate the development of mAbs against filovirus infections and their availability in different stockpiles. Moreover, the FDA has allowed mAb cocktails to be tested and regulated as single products, which will significantly reduce regulatory costs.¹³ We have reason to believe that the tide of antibody therapy is turning around, and hope that within the next decade, a mAb-based post-exposure treatment for filoviruses will become commercially available, and therefore infection with the Ebola viruses will no longer be a near-certain death sentence. Overall, after decades of sputtering successes, the old idea of using mAbs to fight infectious diseases, which was jump-started in 2012 for the high profile Ebola virus and is now expanding to HIV and others, may prove itself as valuable as the antibiotics in the 20th Century.

10.4161/hv.27813