By Regina McEnery

Two New HIV Vaccine Trials Launched in Recent Weeks

Two preventive Phase I AIDS vaccine trials were launched recently, testing two different DNA-based AIDS vaccine candidates. In one trial that began in December, investigators from the UK began enrolling 36 women ages 18-45 at low risk of HIV infection in a randomized controlled trial comparing the safety and immunogenicity of a DNA-based vaccine candidate containing fragments of HIV’s spiky outer-surface protein that were isolated from a clade C virus, the most dominant strain circulating in sub-Saharan Africa and the one responsible for infecting half of the world’s 34 million people living with HIV.

The trial, known as MUCOVAC2, will be examining three different routes of vaccination. The first group of twenty women will receive a high or low dose of the candidate by intramuscular injection, administered along with the adjuvant glucopyranosyl lipid adjuvant (GLA), which was developed by the Seattle-based non-profit Infectious Disease Research Institute (IDRI), a product-development partnership that is working to develop new technologies that target diseases in developing countries. GLA appears to have the ability to boost both antibody and cellular immune responses.

Another six women will receive the vaccine candidate intranasally in the form of drops, administered together with the adjuvant chitosan, which is derived from the outer skeleton of shellfish and insects and has been found to improve the immunogenicity of other vaccines that are administered mucosally.

Another group of 10 women will receive an intramuscular injection of the vaccine candidate in conjunction with vaginal application of the candidate formulated into a gel. The vaginal gel will be applied nine times in a one-month cycle. This gel version of the vaccine candidate, which has been tested previously in clinical trials on its own, does not contain an adjuvant. Catherine Cosgrove, honorary consultant in infectious disease and general medicine at St. George’s University of London, who is leading the study, says the combination of an intramuscular injection with vaginal gel application aims to induce a more focused mucosal immune response.

“This is the first time the [candidate] is being used intranasally or intramuscularly,” adds Cosgrove. Studies in mice, rabbits, and rhesus macaques showed the vaccine candidate was safe and immunogenic. A consortium that includes St. George’s, Imperial College, Hull York Medical School, the Medical Research Council Clinical Trials Unit, and IDRI contributed to developing the vaccine candidate. The trial is being funded by the Wellcome Trust.

In another Phase I trial that began enrollment in December, investigators will evaluate the safety and immune responses induced by a DNA-based HIV vaccine candidate, developed by Profectus BioSciences, in a prime-boost regimen. The DNA candidate encodes multiple HIV proteins and is being co-delivered with the adjuvant interleukin-12 (IL-12)—a protein secreted by immune cells in response to viruses or bacteria—to help boost the immune response. The DNA candidate is being followed by vaccination with a viral vector-based candidate that uses an inactivated strain of the cold virus (adenovirus serotype 35; Ad35) to deliver HIV fragments.

Investigators plan to recruit 75 volunteers ages 18-50 from Rwanda, Kenya, and Uganda in this study, known as B004. The trial, which is being sponsored by IAVI, employs a novel technique called electroporation to deliver the DNA vaccine candidate (see It's Electric, below). The goal of electroporation, which delivers the vaccine candidate intramuscularly through a series of electric pulses, is to get more of the vaccine into cells.

Enrollment in B004 began in Rwanda in December, with vaccinations slated to begin in Kenya and Uganda in early 2012, pending regulatory approvals. By using Ad35, researchers are hoping to circumvent issues with pre-existing immunity to the viral vector. In the STEP trial, which showed that an Ad5-based vaccine candidate failed to prevent transmission or slow disease progression in vaccinated volunteers, data suggested that male volunteers who received the vaccine had a higher risk of acquiring HIV if they were uncircumcised and had pre-existing antibodies against the Ad5 vector (see VAX Oct.-Nov. 2007 Spotlight article A Step Back). Ad35 is less prevalent worldwide than Ad5, and therefore there should be less pre-existing immunity to the vector.

What are some of the challenges in developing viral vector-based vaccine candidates and how are scientists trying to overcome these challenges?

Many of the current AIDS vaccine candidates that are being tested in clinical trials utilize viral vectors to shuttle HIV fragments into the body that can stimulate an immune response (see VAX Sep. 2004 Primer on Understanding Viral Vectors). Some of the viral vector-based candidates are being tested in prime-boost combinations with other approaches (see Global News, this issue). Several different viruses have been used to develop vectors for HIV vaccine candidates, including adenovirus, a common cold virus, and pox viruses, such as modified vaccinia Ankara virus and canarypox, among others.

The viruses used as viral vectors are attenuated so that they cannot cause disease. They are also manipulated, so that in addition to containing their own genes they can carry HIV genes, referred to as antigens, which cannot cause an HIV infection.

The HIV genes that are placed into the viral vector are referred to as the vaccine insert. Once the vaccine candidate is injected into the body, HIV’s genetic material is taken up by cells and converted into protein that hopefully will trigger the immune system to respond to HIV. This sounds simple enough, but viral vector-based vaccine candidates present some unique design and manufacturing challenges. Because the manufacturing process is so complicated, additional effort is required to avoid delays in the clinical testing of viral vector-based vaccine candidates.

Not harmonious

To develop viral vector-based candidates, scientists first need to design and generate the vector with the HIV insert in cells that can support virus growth. The virus vector is then amplified multiple times to produce hundreds of virus particles carrying the HIV genes, which are then subjected to extensive testing.

One of the major challenges in making viral vector-based vaccine candidates comes down to chemistry, or rather, a lack of chemistry between the vector and the insert. Sometimes the vector and the insert are simply incompatible. For instance, if the length of the insert is too long or its configuration is not suitable to the viral vector, the vector may simply reject the insert. In other cases, the virus acting as the vector may introduce mutations into the HIV genes that may prevent production of the complete protein once inside the body. These changes can ultimately impact the generation of a good immune response following vaccination. Sometimes the vector can even clip the insert, rendering it completely useless.

In some cases the vector will tolerate the insert for a while, at other times it will reject it outright. In either case it represents a setback in the production of the vaccine candidate. It is therefore important to analyze the stability of these vectors during the early stages of vaccine development. This is accomplished by subjecting these vector particles to a series of stress tests that assess whether they are stable enough to be tested in a clinical trial. The stress test evaluates the ability of these vectors to express the HIV proteins in cells and in small animals. Even after this vetting process is complete, the vector may still reject the inserts, so it is not uncommon for researchers to have to repeat this cycle several times prior to obtaining a stable vector that expresses the HIV protein and can be advanced into clinical trials.

Mosaic antigens

Most of the inserts used in current viral vector vaccine candidates contain HIV gene sequences from a single virus found in a certain region of the world, or a consensus sequence generated from different viruses that are circulating in that region of the world. Because HIV replicates so rapidly, there is tremendous variation among viruses circulating in a population, and even within a single infected individual. To address this, scientists are also attempting to design HIV inserts known as mosaic antigens that are designed to address the overwhelming diversity of HIV (see VAX March 2009 Primer on Understanding How Inserts for Vaccine Candidates are Designed). Mosaic antigens are based on optimized genetic sequences of the many circulating strains of HIV globally and are meant to induce immune responses to a broad range of circulating strains.

To make mosaic antigens, scientists must stitch together different genetic sequences from multiple strains. These gene sequences do not exist in nature but rather are computer-generated. This makes designing and developing novel viral vector-based candidates that can carry these mosaic antigens even more challenging. Before advancing them into clinical trials, researchers will have to ensure that viral vectors carrying mosaic antigens can be obtained and manufactured at large scale, while maintaining their stability. Researchers are hoping to begin clinical trials with these vaccine candidates containing mosaic antigens soon.