September 24, 2007

Volume 85, Number 39

pp. 34-37

TB Vaccines May Offer A Needed Boost

New tuberculosis vaccine candidates advance into clinical testing

Ann Thayer

EXISTING TUBERCULOSIS control strategies based on vaccination and drug therapy have failed to contain the spread of the disease, particularly the development of drug-resistant forms. As a global health intervention, a more effective vaccine is considered one of the best hopes. While a few decades have passed since the last approved TB drug (see page 21), the wait for a new vaccine has taken nearly a century.

In 1908, Albert Calmette and Camille Guérin discovered what is now called the bacille Calmette-Guérin (BCG) vaccine, which is live but attenuated Mycobacterium bovis, the bacterium that causes TB in cattle. BCG was first used in 1921, and since 1974, when the World Health Organization included it in immunization programs, about 4 billion doses, or more than 100 million each year, have been given. Used largely in areas where the disease burden is high, BCG is the most widely administered vaccine worldwide.

This wide use of BCG continues despite protection being limited to young children and, since immunity wanes after 10–15 years, its unreliable effectiveness against pulmonary TB in adolescents and adults. Nor does the vaccine prevent latent TB infection, the noncontagious, asymptomatic form that most people develop and that today infects an estimated 2 billion people. BCG finds limited use in the U.S., where the TB infection risk is low and the vaccine would interfere with the commonly used diagnostic TB skin test (see page 39).

The Stop TB Partnership, an international network of more than 500 public and private organizations, has set the goal of having a new vaccine by 2015. Although BCG itself may eventually be replaced, many in the TB community believe the best chance for early success will come in using it as the first part of a prime-boost strategy. Inoculation with BCG, or an improved BCG, to prime an infant's immune system would be followed by a second newer vaccine to boost and prolong immunity.

The prime-boost approach makes sense because BCG is entrenched in public health services and has practical advantages. "It's already in clinics everywhere, costs less than 25 cents per dose, and has an essentially unrivaled safety record," says T. Mark Doherty, research and strategy coordinator in the TB vaccine group at Statens Serum Institut (SSI), in Copenhagen. "It's not perfect, but you'd face a lot of opposition to replacing BCG, even if we had a candidate vaccine we thought was better."

SSI is one of several organizations with programs in TB vaccine development. Its support comes from some of the top backers of TB R&D. A public enterprise integrated into Denmark's health services, SSI also generates income as a leading producer of vaccines and diagnostics.

SSI's approach centers largely on subunit vaccines consisting of antigens from Mycobacterium tuberculosis (Mtb), the bacterium that causes TB in humans, to generate the desired immune response. Peter Andersen, SSI's vice president of vaccine R&D, came up with a novel and effective way of collecting and purifying these immunologic proteins.

In the past decade, vaccine development has been helped by the sequencing of the Mtb genome and the ability to genetically manipulate the bacterium. TB researchers are nearing the point of having cloned and tested almost a quarter of the Mtb genome, Doherty says. "The goal now is not just to find an antigen that works but to find something that works better than the best candidates already in the clinical pathway."

TWO LEADING booster vaccine candidates from SSI are Hybrid-1, a fusion molecule of the Ag85B and ESAT6 antigens, and HyVac-4, combining Ag85B and TB10.4. Although Hybrid-1 shows high immunogenicity, SSI designed HyVac-4 as an alternative, Doherty says, since ESAT6 was later found to be a virulence factor for pathogen survival. This makes ESAT6 valuable for diagnostic purposes, and having it in a vaccine would interfere with diagnostic tests.

Because fusion proteins are defined molecular constructs, it is easier to do druglike stability and safety testing with them than with live vaccines, Doherty says. They also are easy to make in bulk and to cost-effectively distribute. Vaccine safety is always a concern, but subunit vaccines are expected to avoid the potential risk of causing disease in immunocompromised people. This specific concern arises because TB frequently occurs where HIV is prevalent and in people who are HIV-infected.

Aeras Global TB Vaccine Foundation is helping fund the development of HyVac-4 and other vaccines. Founded in 1998, nonprofit Aeras works through public-private partnerships. Its primary support comes from the Bill & Melinda Gates Foundation, which gave Aeras a five-year, $83 million grant in 2004. At the time, the grant was the largest ever for TB vaccine research and more than doubled the annual amount spent worldwide.

"The task is not only making something that works and proving it works, but also making sure it can be manufactured at the required scale and at prices that will be affordable," says Jerald C. Sadoff, Aeras' chief executive officer and president. Previously, Sadoff was executive director of clinical development for vaccines at Merck.

The most advanced vaccines in the Aeras pipeline are rBCG30, a more potent recombinant BCG that overexpresses Ag85B; Aeras 402, an Ad53 adenovirus vector developed by Dutch biotech company Crucell that expresses Ag85A, Ag85B, and TB10.4; and, in collaboration with GlaxoSmithKline, Mtb72f, a fusion molecule in a proprietary adjuvant that boosts cellular, or nonantibody, immunity.

NIH's National Institute of Allergy & Infectious Diseases (NIAID) helped support early development of Mtb72f. To assist vaccine R&D more broadly, NIAID supports the TB Vaccine Testing Facility at Colorado State University, where researchers can send candidates for screening in animal models of TB. In the past nine years, NIAID says, more than 100 TB vaccine candidates have been tested. In 2005, NIH accounted for 36% of the $70 million spent on TB vaccine work by the top 40 funders of TB R&D; the Gates Foundation was the largest supporter at $29 million.

In its own labs, Aeras has created protein shells that encase double-stranded RNA engineered to encode for TB antigens. "They have been shown to be very immunogenic in animals," Sadoff says. The technology is inexpensive, allowing vaccines to be made for just pennies per dose, and the vaccines can be easily administered orally, he adds. Testing in humans is expected to begin next year.

Another Aeras candidate is a recombinant BCG (rBCG) expressing several antigens and perfringolysin, an enzyme that punches holes in infected cells that help expose the antigens to the immune system. This escape strategy has also been used by scientists at Max Planck Institute for Infection Biology, in Berlin. They have licensed VPM1002, their listeriolysin-expressing rBCG, to Vakzine Projekt Management (VPM), which has moved it into early clinical testing and developed a production process.

OPERATING AS the management center of a vaccine development and commercialization consortium, VPM was founded in 2002 by Germany's Federal Ministry for Education & Research. Last month, VPM joined the EC's TB-VAC consortium, which involves nearly 40 universities, research institutions, and a few companies from 12 European and African countries. In 2004, the EC invested about $45 million to set up TB-VAC and MUVAPRED (Mucosal Vaccines for Poverty Related Diseases).

The EC and Wellcome Trust are funding the development of a candidate from Oxford University. Known as MVA85A, it consists of a modified vaccinia Ankara virus expressing Ag85A. It has been tested in early clinical trials in different patient populations, including those who are HIV-infected, and entered Phase II trials in South Africa this summer. Its developers say the vaccine is safe and stimulates high levels of the T-cell response believed to be needed for protection against TB.

Besides the groups whose candidates are already in clinical studies, many other teams are designing vaccines and conducting preclinical work. For example, Pasteur Institute scientists have put genes lost during the attenuation process back into a BCG vaccine to increase potency. Likewise, Aeras and Vanderbilt University collaborators are investigating BCG with diminished superoxide dismutase activity. With ImmunoBiology in the U.K., Aeras is creating subunit vaccines containing heat-shock proteins. And a group at the University of Zaragoza, in Spain, has a live Mtb vaccine in which a virulence gene is deleted.

Last month, researchers at Albert Einstein College of Medicine reported on a prototype vaccine based on gene deletions. Einstein professor of microbiology and immunology William Jacobs commented at the time: "Tweaking the marginally useful BCG vaccine is the wrong strategy." Instead, the team took a virulent Mtb strain and knocked out genes to yield a live, attenuated version that still produced a strong immunological response.

The team discovered a gene called secA2 that the bacterium relies on to prevent apoptosis of the human macrophage cells in which it can hide for decades. A mutant lacking secA2 caused infected cells to undergo apoptosis and elicit a T-cell response. Vaccination of mice and guinea pigs with the mutant strain provided greater protection than did standard BCG. They will knock out additional genes to make the vaccine safer for human use, but no less protective, and they hope initial clinical trials might begin in two to three years.

Continuing investigation of immune responses triggered by vaccines will be critical to developing more effective ones, researchers tell C&EN. Most of the focus has been on generating CD4⁺ and CD8⁺ T-cell responses. These T cells are sources of interferon-γ and tumor necrosis factor, two cytokines believed to be involved with controlling TB, and are antigen-specific memory cells that can respond to infection later. To learn more about vaccine-induced immune responses, the South African TB Vaccine Initiative is conducting a study in which it will analyze samples from 7,000 BCG-vaccinated infants.

Also important for TB R&D are advances in understanding the processes of infection and latency. About 90% of people exposed to TB never get sick, but a large number develop latent infections that can reactivate. A $13 million grant under the Gates Grand Challenges program has gone to an international consortium of 15 institutions led by researchers at Max Planck Institute to find biomarkers of protective immunity. They are looking for immune system differences between people exposed to TB who never get sick and those who develop serious disease.

MOST OF THE CANDIDATES are designed as preexposure vaccines to prevent TB. It's not yet known whether giving these prophylactic vaccines to people with latent infections might contain latent TB and prevent reactivation. Andersen at SSI is lead investigator on an $11 million Grand Challenges grant for evaluation of a postexposure vaccine, or one that would have therapeutic use in eliminating latent infection. In combination with drug therapy, therapeutic vaccines might also treat reactivated disease.

Earlier this year, Colorado State researchers announced what they say is a novel vaccine with the potential to work after exposure. It is a fusion-protein vaccine that triggers a response by attaching to specific immune cell receptors responsible only for recognizing Mtb. According to the researchers, human trials could start in two to three years.

Yet another possibility would be a multistage vaccine using antigens from stages of the Mtb life cycle for prophylactic and therapeutic effect. Such vaccines are still in the early stages of development, and the process for creating them can be tricky, Doherty says. One problem is avoiding immunointerference, in which strongly recognized antigens elicit a poorer response when combined.

Research milestones would include having animal models that are predictive of the prevention of infection or of the prevention of latency. Another would be having biomarkers of a protective immune response. "Having human immune responses to these vaccines correlated to a surrogate of protection would be a major event and move the field very far forward," Sadoff says. These surrogates of protection would help streamline clinical testing by providing relevant measures to assess immunogenicity and show vaccine efficacy.

But the only way to prove a valid surrogate of protection or animal model is to conduct clinical trials in humans, Sadoff adds, and it's important that these trials start soon. To this end, Aeras, NIAID, and other organizations are building clinical trial capacity in Africa, India, and East Asia. They also have been trying to engage regulators, since no clear regulatory path exists for TB vaccines. Anticipated Phase III clinical trials in a few years will be lengthy and large, involving about 14,000 infants and 25,000 adolescents, and will cost at least $120 million each, Sadoff says.

Stop TB estimates that it will take $3.6 billion in R&D support to reach the goal of having a new TB vaccine by 2015. Available funding over that time is estimated to be about $2.1 billion, which leaves a sizable gap. Government, public, and private charitable organizations have stepped up so far to cover much of the high-risk, early investment phase. Insufficient industrial investment, possibly arising from a lack of commercial incentives, concerns many in TB R&D since this support will be needed to sustain longer term development, manufacturing, and distribution of new vaccines.

On a positive note, an October 2006 analysis by BIO Ventures for Global Health attempts to make a business case for private-sector involvement. The group estimates the annual market for a BCG replacement vaccine would reach $450 million; that for a booster vaccine, nearly $800 million; and that for a new prime-boost combination, $1 billion. Some evidence that interest may be growing is that there are a few companies visibly involved in TB vaccine development. Although it hasn't said which neglected diseases will be targeted, Novartis is starting up the Novartis Vaccines Institute for Global Health in Siena, Italy.

In the meantime, Aeras, for example, is trying to ensure that new vaccines can be made affordable and accessible through its partnership agreements. In 2006, the organization opened its own manufacturing facility that can produce the bulk material for 150 million to 200 million doses per year of rBCG or similar vaccines. It has partnered with developing world manufacturers for making the finished products and for distribution. Simply put, "we're not virtual," Sadoff says. "Our goal is to have new TB vaccines licensed and available in the next seven to nine years."

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