On the Trail of Ebola
One of the rare bits of good news emanating from the horrific Ebola outbreak in West Africa has been the experimental drug given to a US doctor and nurse who contracted the virus while treating infected patients at a hospital in Liberia. Updates from US Centers for Disease Control and Prevention suggest both Dr. Ken Brantly and Nurse Nancy Writebol are improving. The drug known as ZMapp—initially identified as a “secret serum” by public health authorities—uses monoclonal antibodies that were generated by exposing mice to one of Ebola’s seven proteins, and then harvesting the antibodies that the mice induced in response to the protein, a strategy known as passive immunization. The antibody cocktail has, almost overnight, elevated the work of a trio of unknown biotechs in the US and Canada. But it has also showcased the work of a newly-formed consortium established by the US National Institute of Allergy and Infectious Diseases to find antibody drug cocktails to treat Ebola. The global collaboration includes the primary developer of ZMAPP—Mapp Pharmaceuticals—and is led by Dr. Erica Ollmann Saphire, a professor at The Scripps Research Institute (TSRI) in La Jolla, Calif. Dr. Saphire is an expert in filoviruses like Ebola and Marburg, as well as Lassa fever, another viral hemorrhagic fever from the arenavirus family. Public fascination with viral hemorrhagic fevers has always been high, but with the Ebola outbreak playing out night after night on television screens, and cases and deaths mounting, interest in finding effective treatments and a vaccine has become an urgent public health priority. Eureka reached out to Dr. Saphire this week to learn more about the research behind the Ebola drug and the work of the consortium. Here are her emailed responses.
How did you come to head up a consortium seeking a treatment for Ebola?
We had been working in this area for 10 years. My lab's expertise is structural molecular biology: determining the molecular structures of the parts of Ebola and other viruses in order to understand how they infect cells, why they are so pathogenic, and most important, how we can defeat them. The molecular structures become the roadmap for developing these treatments. In other words, the images we make of the virus show us the chinks in its armor. We are the only lab in the world so far that has been able to solve structures of the viral surface molecule called GP (glycoprotein) that the virus uses to infect cells. We have also solved the structures of different antibodies attached to it to learn how they work. So, armed with these three-dimensional roadmaps, we could see where antibodies should be targeted on the GP, and which should be combined for synergistic effect. I gathered the consortium, they were all willing and eager to work together to make these treatments, we wrote the grant, and were grateful to receive strong support from NIAID for the project. The global scale of this collaboration is unprecedented.
What is it about filoviruses that make them difficult to design drugs (or vaccines) for?
They change the shape of their molecules during infection. The surface protein GP sheds much of its structure during entry. Internal proteins rearrange to achieve different functions. So, the virus is a moving target. This is a recent piece of work from our lab.
I understand you used X-ray crystallography to study the Ebola virus. Which protein or proteins were you able to obtain crystal structures of and what did you learn about them that informed the drug discovery?
GP, VP40, VP35, VP30, VP24 and we are working on the remaining few. The GP structure gave us the roadmap to understanding how antibodies in the ZMapp serum work and how an even better serum can be made going forward. In the image shown at right you can see an example of our work. The GP is the light blue structure at the center shaped like a tulip or a tooth. The three antibodies are blue, green, and yellow. This is electron microscopy imaging done by Daniel Murin and Andrew Ward at TSRI. Andrew is an Assistant Professor and Daniel is a graduate student we co-mentor.
In AIDS research, scientists have been using reverse vaccinology to find so-called broadly neutralizing antibodies in the serum of HIV-infected individuals. But that wasn’t exactly the same approach used in these experiments. Can you describe a bit more the process that was used to isolate the three antibodies eventually used in the Ebola drug cocktail? Do they function like antibody-drug conjugates?
In these experiments, mice were immunized with the GP, antibodies were elicited and isolated and analyzed in test tubes to see which were effective. The best ones were humanized, combined into cocktail treatments and tested in mice and non-human primates. The combination of three in a "cocktail" helps prevent virus escape by mutation. It also offers a synergistic effect: they neutralize the virus and recruit the immune system in different ways.