Laboratory Purists and Renegade Empiricists Duke It Out Over How to Proceed with Vaccine Development Efforts
Current Products ‘Certain to Fail’
Although significant scientific hurdles continue to impede progress toward the development of an ideal HIV vaccine candidate, a number of HIV vaccine trials have recently been initiated in the U.S. (see also table). And with increasing attention focused on the role of private industry in HIV vaccine research and development, the announcement last year of key changes at two pharmaceutical firms with well-regarded vaccine programs may serve to further energize HIV vaccine development efforts. Building on the work of IAVI's David Gold and Sam Averitt (and an 11th hour interview with Aaron Diamond's John Moore), Mike Barr attempts to bring us up-to-date on the state of HIV vaccine research on the eve of 1998-some 14 years after HHS's Margaret Heckler's now notorious miscalculation.
Early last year, Merck disclosed that Emilio Emini, head of the company's protease inhibitor program, would oversee all vaccine research at the company. At the same time, a number of reports suggest that the pharmaceutical giant may have decided to devote additional corporate resources and attention to its HIV vaccine program. Just months later, in September 1997, Chiron announced that it had hired Margaret Liu (former chief of Merck's DNA vaccine program) to lead its vaccine research program.
DNA vaccines are created by inserting one or more genes from the targeted pathogen (in this case, HIV) into a piece of DNA which acts as the "vector." The genetic material can then be injected directly into muscle tissue, although other types of administration are also being studied. Merck's DNA effort is based on a licensing agreement with Vical, a San Diego based biotech company. In an experiment with 3 chimpanzees at the University of Pennsylvania last year, a type of dual plasmid HIV DNA-based vaccine seems to have induced both neutralizing antibodies and cytotoxic T-lymphocyte (CTL) responses which were protective against heterologous challenge. Prior to the U. Penn experiment, DNA vaccines had never been shown to induce protective responses in the chimpanzee model. [N.B. The chimps were immunized eight times before being challenged, though, and the challenge virus was of the MN laboratory strain-which is quite closely related to the SF2 lab strain used to produce the vaccine.]
In general, genetic immunization with "naked DNA" seems capable of inducing decent CTL responses, but the antibody responses generated by this method are relatively weak. Because of this, one strategy being pursued by a number of research teams, including Merck's, is to vaccinate with DNA and then boost with an HIV envelope protein which can induce high levels of neutralizing antibodies, although some vaccinologists question the effectiveness of the antibodies generated by recombinant envelope proteins.
Prime Boost Results in Monkeys
In recent PNAS paper (Letvin et al.), researchers described a pilot study in which two macaque monkeys were given multiple immunizations of a DNA vaccine encoding HIV envelope followed by an HIV envelope protein (gp160) boost. In total, the animals were given 9 injections of 1 µg and 3 injections of 2 µg of the DNA vaccine. Other macaques were given only the gp160 or blank DNA. All of the monkeys were then challenged intravenously. At 28 weeks post-challenge both monkeys given the DNA vaccine plus the gp160 boost were completely protected and had no detectable virus. By comparison, all the control animals (who received the blank DNA or only the gp160 injection) became infected. Merck reported that the protection demonstrated suggests that "DNA immunization warrants active investigation." On the other hand, other vaccine regimens, including the prime-boost combination of canarypox and gp120, have also demonstrated protection against a chimeric SHIV challenge in monkeys, but few people expect this particular prime-boost combination to perform well in humans.
Avipox and Vaccinia Based Vaccines
Like the DNA-based vaccines, the avipox vaccine vectors (such as canarypox) are reasonably successful at generating good cellular immunity against HIV but are rather poor immunogens for the stimulation of an effective antibody response. Duke University's Kent Weinhold, however, notes that cytotoxic T-lymphocyte (CTL) activity is stimulated in only about 50% of those receiving the canarypox (ALVAC vCP205)/gp120 prime-boost vaccine combination-and that fewer than 12% maintain this CTL activity out to one year. On the brighter side, the CTL responses that were generated were capable of neutralizing cells infected with many different HIV subtypes. Weinhold suspects that the techniques currently used to expand and measure CTLs in vaccine recipients may not be detecting all CTLs that are generated. A 420-patient Phase II trial of the Mérieux vCP205 with a gp120 boost has just begun.
In addition to using the canarypox virus as a vector, vaccinia (cowpox) viruses have also been used. One of the drawbacks of using a vaccinia vector, however, is that people who have received childhood immunizations against related pox-type infections are likely to already have immunological memory against vaccinia. At the University of Washington in Seattle, researchers report that 6 monkeys were protected from intravenous challenge with SHIV after being immunized with an HIV env-expressing vaccinia vector followed by a gp120 boost. Swedish researchers working with a similar vaccinia-based vaccine, however, reported little protection against mucosal challenge, which more accurately reflects the predominant route of transmission worldwide.
Live Attenuated Vaccine Candidates
A flurry of activity surrounding live attenuated HIV vaccines began in last fall when the Chicago-based International Association of Physicians in AIDS Care (IAPAC) announced that more than 50 individuals had volunteered to participate in a study of a live attenuated vaccine (nicknamed "delta-4" because the vaccine contains live HIV with four genes deleted: nef, vpr, vpu and the binding site transcription factor: nuclear factor-B) of Harvard Medical School's Ron Desrosiers. Desrosiers and other research teams have shown that live attenuated SIV vaccines could provide impressive protection in monkeys.
Concerns over the safety of attenuated vaccines, however, began to mount when reports surfaced of newborn and adult monkeys who had developed simian AIDS from the vaccines. All told, at least four separate research groups have reported monkeys which show signs of immune suppression after receiving a live attenuated SIV vaccine. These groups include the Aaron Diamond AIDS Research Center, the Dana-Farber Cancer Institute, the Walter Reed Army Institute of Research and Desrosiers' own lab at the New England Regional Primate Center. The monkeys received SIV with deletions in either the nef-gene (delta-nef) or three genes, including nef (delta-3). The initial report that the delta-3 vaccine could cause AIDS in newborn monkeys was made back in 1995 by Dana-Farber's Ruth Ruprecht. These reports led some researchers, including NIAID's Anthony Fauci and Barry Bloom of the UNAIDS Vaccine Advisory Committee, to publicly state that human studies of the live attenuated vaccines would be premature.
All this has done nothing to dampen the determination of three separate groups to launch human trials of just such a vaccine construct. In addition to IAPAC, University of Massachusetts Medical School's John Sullivan has proposed a study of the delta-4 vaccine in terminal cancer patients with non-treatable solid tumors. According to Sullivan, since many terminal cancer patients have competent immune systems with normal CD4 counts, important information could be obtained from the trial. Such a trial would be "an excellent prelude to launching a small study in healthy human volunteers," Sullivan argues.
Finally, John Mills, of the Macfarlane Burnet Centre in Australia, and his Sydney research team have produced a live vaccine that mimics an apparently weakened HIV strain found in a cohort of Australian long-term non-progressors who became infected from a common blood donor. These nine individuals have a large missing segment in the nef gene (one of HIV's nonstructural genes of uncertain function) as well as rearrangements in the long terminal repeat (LTR), which is the control system that regulates the virus's ability to replicate. Mill's vaccine is to be mass-produced from infectious molecular clones by making a DNA replica of the genetic material of the Sidney cohort virus and using it as a vaccine. In contrast to the live HIV that IAPAC is proposing, Mills believes that infectious DNA will be less expensive to produce, store and administer. (A live HIV vaccine, as is being proposed by IAPAC, must be grown in laboratory cultures containing well-characterized living cells, Mills explains. The only feasible approach to growing large amounts of HIV consistently is to use "transformed" human cell lines. But in the past, the FDA has been reluctant to approve the use of these transformed T-cell lines for the production of human vaccines.) "If a live attenuated HIV vaccine strategy is going to be practical in developing countries," Mills explains, "it will have to utilize the DNA construct approach." Human trials of the Australian vaccine could begin in late 1998.
In spite of what might be described as a renewed sense of interest in HIV vaccine development, significant scientific hurdles remain. Many experts in the field will openly decry that, "The tools still are not there yet to develop an effective HIV vaccine." At the same time, as Aaron Diamond's John Moore recently explained, "it is true [historically] to say that we don't know -- in detail -- how any vaccine works." Thus as the scientific tug-of-war between the laboratory-based and the empiric approaches continues, it is perhaps telling that phase I and phase II vaccine trials move ahead in Thailand, Uganda, Brazil and, interesting enough, in Cuba, where Cuba's Centro de Ingenieria Genetica y Biotecnologia has recently begun a Phase I study of a candidate construct called TAB9, a vaccine based on recombinant proteins from different regions of the V3 loop.
In a candid acknowledgement of the competing commercial and careerist interests which too often drive development decisions, Diamond's Moore notes that current prime-boost and soluble protein regimens are certain to fail. "They didn't work in phase I," Moore notes with stinging irony, "so people say, 'let's throw them into a large phase II.'" Yet in spite of all the sophisticated molecular genetics, there are still those who argue that the best way to find a vaccine to stave off the worldwide plague might simply be to throw the best candidate into a large-scale human trial. And if the impassioned advocates of the live-attenuated approach have their way, theirs may be the first -- long-term safety risks notwithstanding.
|Selected HIV Vaccine Approaches|
|Prime-boost||canarypox205 + gp120||Mérieux/Chiron|
|vaccinia (env,gag,pol)||+ gp120||Therion/VaxGen|
|HIV DNA||+ gp160||Merck/Vical|
|Naked DNA||HIV DNA (gag,pol)||Apolon|
|Recombinant protein||HIV p24||Chiron|
|TAB9||(V3 loop)||Centro de Genética|
|Live attenuated||delta-4 (nef,vpr,vpu,nf-B)||Therion (Desrosiers)|