Review: Insights on Current FDA-Approved Monoclonal Antibodies Against Ebola Virus Infection

Abstract

The unprecedented 2013-2016 West Africa Ebola outbreak accelerated several medical countermeasures (MCMs) against Ebola virus disease (EVD). Several investigational products (IPs) were used throughout the outbreak but were not conclusive for efficacy results. Only the Randomized Controlled Trial (RCT) on ZMapp was promising but inconclusive. More recently, during the second-largest Ebola outbreak in North Kivu and Ituri provinces, Democratic Republic of the Congo (DRC), four IPs, including one small molecule (Remdesivir), two monoclonal antibody (mAb) cocktails (ZMapp and REGN-EB3) and a single mAb (mAb114), were evaluated in an RCT, the Pamoja Tulinde Maisha (PALM) study. Two products (REGN-EB3 and mAb114) demonstrated efficacy as compared to the control arm, ZMapp. There were remarkably few side effects recorded in the trial. The FDA approved both medications in this scientifically sound study, marking a watershed moment in the field of EVD therapy. These products can be produced relatively inexpensively and can be stockpiled. The administration of mAbs in EVD patients appears to be safe and effective, while several critical knowledge gaps remain; the impact of early administration of Ebola-specific mAbs on developing a robust immune response for future Ebola virus exposure is unknown. The viral mutation escape, leading to resistance, presents a potential limitation for single mAb therapy; further improvements need to be explored. Understanding the contribution of Fc-mediated antibody functions such as antibody-dependent cellular cytotoxicity (ADCC) of those approved mAbs is still critical. The potential merit of combination therapy and post-exposure prophylaxis (PEP) need to be demonstrated. Furthermore, the PALM trial has accounted for 30% of mortality despite the administration of specific treatments. The putative role of EBOV soluble Glycoprotein (sGP) as a decoy to the immune system, the virus persistence, and relapses might be investigated for treatment failure. The development of pan-filovirus or pan-species mAbs remains essential for protection. The interaction between FDA-approved mAbs and vaccines remains unclear and needs to be investigated. In this review, we summarize the efficacy and safety results of the PALM study and review current research questions for the further development of mAbs in pre-exposure or emergency post-exposure use.

Keywords: Ebola virus; antibodies; filovirus; monoclonal; therapeutics.

Figure 1

mAb114 development technique. Blood collected from EVD survivor. PBMC isolation followed by identification of the mAb114 memory B cell clone. Heavy and light chain sequences obtained by PCR amplification and DNA sequencing. Heavy and light chain sequences cloned into a standard expression vector. Expression vector used to generate a stably transfected CHO DG44 cell line for use in manufacture.

Figure 2

Binding sites of FDA-approved mAbs therapeutics on EBOV GP. (A) Crystal structure of GP Muc interacting with Fabs of mAb114. Fabs are shown in pink (heavy chain) and white (light chain). GP Muc promoters are in green and beige for molecular surfaces while the third is represented by ribbon strings. (B) Relative binding sites of REGN-EB3 on EBOV GP. adapted from ref (20). REGN3471, REGN3470, and REGN3479 combined reconstructions on a single EBOV GPΔTM, demonstrating the relative locations of the epitopes on GP from the three competition groups. Only one Fab is shown per antibody.

Figure 3

Ebola virus uptake, entry model, and mAb114 mode of action [(adapted from ref (21). Viral particles bind to various surface factors and trigger macropinocytosis, leading to trafficking to endosomes. Acid-dependent proteases (cathepsin B and cathepsin L) remove the MLD and glycan cap in the low-pH endosome environment, revealing the RBD in the GP1 core, previously blocked by these domains. The host cell receptor, NPC1(red), is then engaged by virions with exposed RBDs, resulting in the host and viral cell membranes fusion and the release of the viral genome into the target cell cytoplasm. In late endosomes, mAb114 binding prevents the GP from interacting with the host cell receptor protein NPC1. Brown shading indicates the cell membrane and cytoplasm. The endosomal membrane is green with a brown border, while the endosomal lumen is light gray. The nucleus is dark gray, with a blue-black membrane around it.

Figure 4

REGN-EB3 development technique. To generate an immune response, transgenic mice (VelocImmune) are immunized with a DNA vaccine encoding EBOV GP and/or recombinant EBOV GP. Splenocytes are harvested and fused with myeloma cells to create hybridoma cells that produce antibodies continuously. Selected leads are utilized to generate chimeric or humanized antibodies after they have been screened.

Figure 5

Illustrative mechanisms of FDA-approved mAbs therapeutics. (A) Neutralization Illustrative representation of mAb114 (lavender color) and two REGN-EB3 components [REGN3470 (blue color) and REGN3479 (red color)] neutralization activity. (B) ADCC. Illustrative representation of mAb114 (lavender color) and two REGN-EB3 components [REGN3470 (blue color) and REGN3471 (black color)] ADCC activity.

Figure 6

Linear schematic representation of the Ebola virus GP monomer with GP1,2 subunits and sGP [adapted from ref (21)]. The GP1 core (blue, residues 33–189), the cathepsin cleavage loop (yellow, residues 190–210), the glycan cap (yellow, residues 211–308), and the highly glycosylated mucin-like domain (white, residues 309–501) are the four parts of GP1. An internal fusion loop (IFL) region (red, residues 502–553), an hepta repeat (HR)/membrane-proximal external region (MPER) (orange, residues 554–650), and a transmembrane domain/cytoplasmic tail region (green residues 651–676) constitute GP2. The co-transcriptional editing of the GP gene-editing site, which results in the transcript of the sGP product and the post-transcriptional cleavage of the GP precursor by a host furin, are also shown. The proteolytic cleavages of cathepsin B (CatB) and cathepsin L (CatL) are also depicted. Cathepsin B cleaves at position 190, while cathepsin L cleaves at position 201. Branched lines indicate the approximate positions of potential glycosylation sites.

Figure 7

Antigen and antibodies kinetic during EBOV infection. Dotted lines indicate maximal range, and solid lines indicate mean range. After the period of incubation estimated from 2 to 21 days, EVD patients become symptomatic. Virus antigen becomes detectable early after the beginning of symptoms and can persist until day 6-16. Antibody production kinetic during EVD is described as an early occurrence between day 2 and 9 for IgM and between day 6 and 19 for IgG. IgM quickly reach the plateau and fall off around day 29 or longer (day 168); while IgG increase progressively to reach the plateau and last for years (11-40 years).