Preliminary data from the first trial of a Zika virus (ZIKV) vaccine in human subjects, ZIKA-001, show that three doses of the synthetic GLS-5700 DNA vaccine led to binding antibody production in all subjects and neutralizing antibodies in more than half of samples tested, researchers reported in an article published online October 4 in the New England Journal of Medicine.

Further, in vitro studies showed that the serum antibodies blocked viral infection in neuronal cell assays, and serum from vaccinated participants protected nearly all mice challenged with a lethal dose of ZIKV.

“In this phase 1, open-label clinical trial, a DNA vaccine elicited anti-ZIKV immune responses. Further studies are needed to better evaluate the safety and efficacy of the vaccine,” Pablo Tebas, MD, professor of medicine, Division of Infectious Diseases, Perelman School of Medicine, University of Pennsylvania, Philadelphia, and colleagues conclude.

The phase 1, dose-ranging, open-label study was sponsored by GeneOne Life Science and codeveloped with Inovio Pharmaceuticals and the investigators.

The GLS-5700 DNA vaccine encodes ZIKV premembrane and envelope proteins.

The researchers enrolled 40 healthy adult participants who tested negative for dengue virus infection. Participants were randomly assigned to receive three 0.1-mL intradermal injections of vaccine containing either 1 or 2 mg DNA. Each injection was followed by electroporation to increase immunogenicity. This involves the use of small electric pulses to introduce the DNA sequence into cells. Injections were given at baseline, 4 weeks, and 12 weeks. Patients will be monitored for 60 weeks. This article reports interim data analysis at 14 weeks.

The safety analysis showed that the most common adverse events were local reactions at the injection site, such as pain, redness, swelling, and itching, which occurred in about half of participants. There were no serious adverse events.

The researchers collected blood samples at baseline and at weeks 1, 4, 8, 12, 14, 20, 36, and 60 to measure binding antibody responses to the recombinant vaccine-matched ZIKV envelope protein.

None of the subjects had measurable ZIKV antibody responses at baseline. At week 4, 41% of subjects (25% in the 1-mg group and 60% in the 2-mg group) had developed binding antibody responses. By week 14, 100% of subjects had ZIKV-specific antibodies, with geometric mean titers of 1642 in the 1-mg group and 2871 in the 2-mg group. Mean titers were higher in the 2-mg group than in the 1-mg group at all times, but the difference was only statistically significant at week 6.

The idea of DNA vaccines has been around since 1993, Peter Hotez, MD, PhD, dean of the National School of Tropical Medicine, professor of pediatrics and molecular virology and microbiology, and head of pediatric tropical medicine at the Baylor College of Medicine, Houston, Texas, told Medscape Medical News.

“The problem has been, while it’s been possible to achieve high levels of protective antibody and immunity in mice, so far, that has not translated to similar levels of protection in humans for reasons that are unclear. As a result, 25 years later we still have no licensed human DNA vaccine. Possibly the GeneOne Life Sciences group has solved this problem, although it’s not clear to me if the level of antibody titers and neutralizing antibodies they achieved are comparable to, say, a recombinant protein vaccine.” Dr Hotez was not involved in the GLS-5700 study.

Dr Tebas and colleagues report that a neural cell culture assay showed that by week 14, serum from 95% of participants could reduce ZIKV infection of glioblastoma cells by 50%, and serum from 70% of participants could reduce ZIKV infection by 90%. However, titers of neutralizing antibodies did not correlate with vaccine dose. Furthermore, there was no correlation between titers of binding antibodies and neutralizing antibodies.

To further test the efficacy of the vaccine response, the researchers used a strain of mice that lacks an interferon receptor (IFNAR knockout mice) and cannot clear Zika virus infection. They pretreated the animals either with serum taken from study participants at baseline or at week 14 or with buffered saline, followed by intraperitoneal injection of a Puerto Rico ZIKV clinical strain of the virus.

All animals treated with phosphate-buffered saline or baseline serum died within 7 to 9 days, but 92% of mice pretreated with week 14 serum survived. However, protection in this model did not correlate with neutralizing antibody titer, and mice injected with serum from study subjects who had binding antibody responses but no neutralizing antibody responses still had a 92% survival rate.

Dr Tebas told Medscape Medical News, “The biggest surprise for me from our phase 1 study was that the neutralizing antibody levels did not correlate with protection in the mice. Similar effects have been seen in some monkey studies, and this challenges the assumption that neutralizing antibody titers are the most important consideration in deciding to move a vaccine forward in clinical trials. It may be that with Zika or other flaviviruses a vaccine might protect but not neutralize the virus. Or, it might be that our ways of measuring neutralization are not sensitive enough to fully explain what is happening.”

Dr Tebas said that the Zika-001 study will continue until all patients have been monitored for 60 weeks to test the durability of the vaccine response, and that he expects to have final results available in the next 2 to 3 months.

Ongoing Phase 1/2 Study

Meanwhile, Dr Tebas said the Zika-002 phase 1/2 study, which is testing the vaccine in 160 dengue-seropositive adults, is continuing in Puerto Rico, despite being somewhat affected by delayed clinic visits resulting from the damage from Hurricane Maria.

Dr Tebas said, “All of the patients had already received all vaccinations, but some of the data analysis may be delayed,” he said. That study includes 80 adults randomly assigned to receive 3 doses of GLS-5700 at 2 mg DNA/dose with electroporation and 80 adults randomly assigned to receive saline injections.

A key advantage of the DNA vaccine approach is speed. Dr Tebas described it as “an order of magnitude” faster than live vaccine methods in moving a potential vaccine from conception to human clinical trials. That is because vaccines can be quickly redesigned using a common platform to express antigens from an emerging pathogen. GLS-5700 was moved into phase 1 in 7 months.

“We need better ways to do phase 1 and phase 2 vaccine development quickly,” Dr Tebas said. “Currently, as with Ebola, we are too often chasing the tail of an epidemic because vaccine development takes so long. Especially with large urban outbreaks, we just cannot wait 10 or 12 years to have a vaccine.”

On September 1, 2017, Sanofi Pasteur, which had been developing a whole-virus Zika vaccine in collaboration with researchers from Walter Reed Army Medical Center, halted Zika vaccine research.

Dr Hotez added, “There are several different (and competing) Zika vaccines now in phase 1 trials, so I anticipate that additional reports will be forthcoming in the biomedical literature. In some ways, the ability to develop a Zika vaccine will not be as complicated as a dengue vaccine, which requires neutralizing antibody to four different viruses.

“However, the complicating factors around Zika will be a) showing safety in women who are pregnant or might soon become pregnant, which is a high hurdle in terms of regulatory approval by [the US Food and Drug Administration], and b) the fact that Zika transmission is way down this year in the Western Hemisphere, and which could throw cold water on investor interest.”

He says investors should not be misled by the lull, however. “I believe it’s possible that Zika could return in subsequent years, possibly similar to what we see for West Nile virus infection.”

Dr Tebas agreed that more safety data are needed, but said that DNA vaccines (because they do not involve live virus) are likely to be safe for use during pregnancy, and that possible transfer of infection to the fetus should not be an issue, as one might worry with a viral based vaccine.

The study was funded by GeneOne Life Science, which is developing GLS-5700 in collaboration with Inovio Pharmaceuticals. Five coauthors are employees of GeneOne Life Science. Four coauthors are employees of Inovio Pharmaceuticals. Dr Hotez has disclosed no relevant financial relationships.

N Engl J Med. Published online October 4, 2017. Abstract