A STEP back?

Additional data released from the STEP trial raises many questions

By Andreas von Bubnoff

Clinical trials are always complex, but according to Mark Feinberg of Merck, the recent STEP trial may be an extraordinary case in this regard. "I've never seen more complicated data emerge from a study in any field that I've witnessed."

The public got a taste of this complexity at an open session of the HIV Vaccine Trials Network (HVTN) meeting on November 7 in Seattle. There, Merck, along with several representatives from the HVTN and the National Institute of Allergy and Infectious Diseases (NIAID), released mounds of additional data from the STEP trial. This Phase IIb test-of-concept trial evaluated the safety and efficacy of Merck's AIDS vaccine candidate, known as MRKAd5. This candidate uses a common cold virus (adenovirus serotype 5 or Ad5) as a vector to deliver fragments of HIV to the immune system, hopefully triggering an immune response against HIV. Since immunizations were stopped in this trial on September 21, investigators have spent many sleepless nights analyzing data and interpreting this pivotal study.

And the results, based on data from all 3,000 volunteers, show that even though the vaccine candidate induced immune responses against HIV, these were not effective at either preventing HIV infection or in reducing levels of the virus in individuals who became HIV infected through exposure to the virus despite vaccination. In the STEP trial, the study's sponsors revealed in Seattle, there were 49 HIV infections overall in the vaccine group and 33 among those who received placebo as of October 17 (see Primer).

Moreover, researchers reported a worrisome trend towards a higher number of HIV infections among some sub-groups of individuals who received the vaccine candidate, compared to those who received injections of inactive placebo. The vaccine candidate itself did not cause HIV infection but in individuals with higher levels of pre-existing immunity to the Ad5 vector, there tended to be more volunteers who received the vaccine and later were infected with HIV through exposure to the virus (see Table 1). Pre-existing immunity to the Ad5 vector occurs because individuals are exposed naturally to this commonly-circulating cold virus and generate antibodies against it. The levels of antibodies against Ad5 vary greatly between individuals. In individuals with what is considered a high level of Ad5 antibody (≥200), there were 21 infections in vaccinees compared to 9 in placebo recipients. "This difference is clinically important for at least one subgroup," says Keith Gottesdiener of Merck. "I don't really need any statistics to make a declaration that it's an important factor to take into consideration."

The explanation for this difference is still not clear. Steve Self, a biostatistician with the HVTN and the Fred Hutchinson Cancer Research Center who analyzed this data says, "There is great uncertainty about some of these trends." Regardless, researchers are taking it seriously. "When looking at potential harm we have to pay close attention," says Susan Buchbinder of the University of California in San Francisco and principal investigator of the STEP trial.

For many, this was an unanticipated outcome. "It was a surprise to us that there were actually more infections in vaccinees than in placebo recipients," says Mike Robertson of Merck. "We didn't expect that," says Peggy Johnston of the Division of AIDS at NIAID.

There are several possible factors that could contribute to this trend, including geographical region, age, and circumcision status of the volunteers. At this stage of the analysis, the trend towards increasing rates of HIV infection among vaccinees persists even after factoring in all of these potential differences, says Self. But it is possible that there is a yet unidentified difference between the groups that received the vaccine candidate or the placebo.

"There are going to be a lot of different hypotheses that need to be tested to try and understand what went wrong; why this wasn't efficacious and why there was a trend toward more infections with vaccine than the placebo," says Bruce Walker of Harvard Medical School in Boston, who is leading a team of scientists who will analyze the data from the STEP trial. But the devil is in the details and until the full analysis of this trial is complete, and maybe even after that, there will be many unanswerable questions. "We were entering into this thinking that we will find an answer, but even that's not absolutely guaranteed," Walker adds.

Searching high and low

The STEP trial—also known as HVTN 502 and Merck V520-023—was co-sponsored by Merck and NIAID. It was a Phase IIb test-of-concept trial of MRKAd5, a candidate that induces cellular immune responses (CD4+ and CD8+ T cells) rather than antibodies against the virus (see VAX March 2004 Primer on Understanding the Immune System, Part II). Antibody responses are how most, if not all, licensed vaccines provide protection. This study involved 3,000 healthy volunteers at high risk of HIV infection at HVTN sites in North and South America, the Caribbean, and Australia. All volunteers were scheduled to receive three shots of placebo or vaccine, which contains a mix of Ad5 vectors carrying different fragments of HIV known as immunogens. A companion study, known as Phambili, with the same vaccine candidate was also conducted in South Africa (see Removing the blindfold in Global News).

The original plans for the STEP study only included 1,500 individuals with low levels of Ad5 antibody (less than 200) because researchers thought that having pre-existing Ad5 immunity might hinder immune responses induced by the vaccine candidate to HIV. But after the trial began, data emerged from earlier trials showing that Ad5 antibody levels did not compromise immune responses to HIV as much as was initially expected. In July 2005, seven months after the STEP trial began, the protocol was amended to include a second group of 1,500 volunteers who had what is considered high Ad5 antibody levels (greater than 200).

Immunizations in the STEP trial were halted on September 21 after the trial's independent data safety monitoring board (DSMB) reviewed the for the first time data from volunteers in the sub-group of 1,500 volunteers with low Ad5 antibody levels (see VAX June 2007 Primer on Understanding Data Safety Monitoring Boards and VAX September 2007 Special Report). The DSMB concluded that based on the breakdown of infections at this time—19 in the vaccine group and 11 in placebo recipients—it was futile to continue immunizations because the vaccine was not effective.

After this, researchers at Merck and NIAID decided to proceed with analysis of the data collected up to that point, according to Robertson, who warns that all these interpretations should be taken "with a big grain of salt." When immunizations were stopped, only one HIV infection had occurred within the 1,150 women enrolled in the trial and this volunteer received placebo, not vaccine. All of the subsequent analyses, including the breakdown of infections by Ad5 antibody levels, were therefore conducted on data collected from the 1,850 male volunteers only.

Heads or tails

Despite the massive amount of data that has already been interpreted and presented on the STEP trial, there is much more work to be done. One of the leading questions researchers will set out to answer is why the vaccine was not efficacious.

The results collected so far show that the immune responses induced by the vaccine against HIV were similar or higher in the group with low Ad5 immunity to those seen in previously conducted trials. The immune responses in trial volunteers were measured by interferon (IFN)-g ELISPOT assay (see VAX August 2007 Primer on Understanding Immunogenicity). "The lack of efficacy is not explained by sub-optimal immune responses," says Robertson.

Researchers will now look more closely at the immune responses induced by the vaccine candidate. "We had evidence of IFN-g production but that doesn't tell you if the cells would kill virus-infected cells, so we will obviously be looking a little bit more at the function of the immune responses," says Walker. These results may also shed light on whether or not the IFN-g ELISPOT assay is a useful tool for assessing the relative efficacy of AIDS vaccine candidates in the future.

 Sizing it up

Another issue raised by the STEP trial is the use of Phase IIb test-of-concept trials to evaluate the efficacy of AIDS vaccine candidates (see VAX September 2005 Primer on Understanding Test of Concept Trials). The idea of using trials that are smaller and less expensive than Phase III efficacy trials, which typically involve 10,000 or more volunteers, has become fashionable in the field. These preliminary efficacy trials give researchers a quick read on whether or not a candidate is likely to protect against HIV infection, or to provide some level of partial protection that could limit disease progression in volunteers who become HIV infected through exposure to the virus despite vaccination. The STEP trial was the first to use a Phase IIb trial to evaluate an AIDS vaccine candidate—though similar trials have been used for other vaccines—and it successfully showed that this design can yield earlier results with fewer volunteers than a full Phase III trial. "The STEP study trial design was an enormous success," says Steve Self, a biostatistician with the HIV Vaccine Trials Network (HVTN).

Many people praised Merck for deciding to evaluate their candidate in a Phase IIb test-of-concept trial and for planning an early analysis by the data safety monitoring board. "It enabled us to get an answer as quickly as possible," says Peggy Johnston of the Division of AIDS, part of the National Institute of Allergy and Infectious Diseases (NIAID). "That, in hindsight, proved to be an excellent decision." Andrew McMichael of Oxford University agrees. "Maybe we should do more [such] trials rather than the full blown 10,000-person Phase III trial."

But some argue that even smaller trials, an idea known as screening-test-of-concept or STOC trials, could provide preliminary efficacy data for candidates faster yet. This novel clinical trial concept has been championed by IAVI as a way to conduct rapid, less costly trials in far fewer volunteers.

An article describing the design of STOC trials was recently published in the scientific journal AIDS. These trials would involve 500 to 1,000 volunteers in areas with high HIV incidence, compared to the 3,000 participants in the Phase IIb STEP study or the 8,500 volunteers in the original plans for the PAVE 100 trial. "We at IAVI feel that it's important to move quickly and be as efficient as possible in collecting clinical data to guide the field," says Pat Fast of IAVI.

But the STOC trials will also provide more limited information than can be collected from larger Phase IIb studies. The current STOC design would not allow researchers to determine if a candidate protects against HIV infection. It would only allow researchers to detect a difference in viral load in volunteers who do acquire HIV, despite vaccination.

"If we think that there may be differences in acquisition of infection, then that's not the design to do," says Johnston. But many researchers think the best possible hope for AIDS vaccine candidates that induce cellular immune responses, and not antibodies against HIV, is a reduction in the quantity of virus or viral load in vaccinated individuals if they become HIV infected. Especially now, given the results of the STEP trial. Still some are cautious. "We still don't know if the basic assumption is correct," says José Esparza of the Bill & Melinda Gates Foundation. "After the current results, we need to be extra careful with our assumptions."

Ian Gust of the University of Melbourne and a member of IAVI's board of directors, says that both Phase IIb and STOC trials have validity, but he views the use of STOC trials as an attempt to move the field forward as rapidly as possible



There are also many additional studies planned. Researchers will analyze the viruses that infected some of the volunteers and see how they varied from the HIV immunogens included in the vaccine candidate. This work may help researchers determine if this candidate failed because the immunogens selected did not protect against diverse strains of HIV. There are also plans to sequence the genomes of the volunteers to identify any genetic characteristics that might have enhanced susceptibility to HIV or, conversely, provided protection to placebo recipients. "Some of those things will take months and some may take longer than that," says Walker.

Researchers are now also hard at work trying to determine any role the vaccine may have had in increasing susceptibility to HIV infection in some individuals. There is great uncertainty about this, but there are some possible biological explanations and researchers must now sort out their plausibility. Julie McElrath of the Fred Hutchinson Cancer Research Center plans to continue studying the CD4+ T cell responses induced in the volunteers that became infected with HIV to see if these provide any clues.

Broader strokes

Based on the complexity of the data generated by this trial, it may be a while until the results are fully understood. For now, most agree it is too early to close the door on vaccine candidates that induce cellular immune responses. "[The] STEP results proved that this product failed and should not be construed as indicative that all adenoviral vectors or other viral vectors will fail," says Johnston.

But until any possible association between Ad5 immunity and increased susceptibility to HIV is ironed out, most researchers are urging caution. "Any further trials of adenoviral vectors should be done very cautiously," says Johnston.

PAVE 100 was the next Phase IIb test-of-concept trial on tap with an Ad5-based candidate—it was scheduled to begin just weeks after Merck and NIAID announced that immunizations in the STEP trial were stopped. The original plans for this NIAID-sponsored 8,500-person trial were to test the safety and efficacy of a prime-boost combination of two vaccine candidates administered sequentially. One uses DNA to deliver HIV immunogens and the other uses an Ad5 vector, which is slightly different than Merck's, to deliver a different set of HIV immunogens. Both of these candidates were developed at the Vaccine Research Center (VRC), part of NIAID, and the trial was planned in collaboration with the HVTN, IAVI, and the US Military HIV Research Program (USMHRP). This same regimen was also to be tested in a Phase II trial, known as V002, conducted by IAVI in Rwanda, Kenya, Uganda, and Zambia. After immunizations in the STEP trial were halted, the opening of both of these trials was postponed.

"There are substantial differences between the Merck product and the VRC product," says Gary Nabel, director of the VRC. One difference is the prime-boost combination of two different candidates. In both preclinical and clinical studies, researchers at the VRC report that this combination induces different immune responses than when an Ad5-based candidate is administered alone.

But when the latest data from the STEP trial was released at the HVTN meeting, researchers began grappling with additional questions about how, or if, to proceed with the PAVE 100 trial. Some groups, including the AIDS Vaccine Advocacy Coalition (AVAC), continue to support ongoing efforts to analyze STEP data and also "advocate for a careful consideration of key implications before launching additional efficacy trials," according to Mitchell Warren of AVAC. But many researchers think it is still imperative to test other candidates. "I certainly feel there are ways to go forward safely, but we have to do that together," says Scott Hammer of Columbia University and chair of the PAVE 100 protocol team.

Hammer and his colleagues on the PAVE 100 team will be meeting soon to discuss possible changes to the trial design. "It has to be amended in light of the STEP trial," says Hammer. "We do not have the details of that amendment in place. The regimen won't change, but the study design might." Some possible alterations might involve the populations enrolled in the trial or the way the data is monitored while the trial is underway to ensure the safety of the volunteers.