Crunch time for microbicides, says top researcher

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Professor Robin Shattock, director of the microbicide research programme at St George’s, University of London, warned delegates in an opening plenary at the 2010 International Microbicides Conference in Pittsburgh yesterday that “we stand at a critical timepoint in microbicide development”.

“There is a recognised need to prioritise and accelerate efficacy testing in clinical trials,” he said, before funders lost interest in the approach.

The failure of the first-generation of non-ARV-based candidates had left only three efficacy microbicide studies in humans ongoing or planned, he pointed out.

Glossary

microbicide

A product (such as a gel or cream) that is being tested in HIV prevention research. It could be applied topically to genital surfaces to prevent or reduce the transmission of HIV during sexual intercourse. Microbicides might also take other forms, including films, suppositories, and slow-releasing sponges or vaginal rings.

concentration (of a drug)

The level of a drug in the blood or other body fluid or tissue.

broadly neutralising antibodies (bNAbs)

A neutralising antibody (NAb) is an antibody that fully defends its target cell from an antigen. A broadly neutralising antibody (bNAb) is a neutralising antibody that has this effect against a wide range of antigens. A number of broadly neutralising antibodies have been isolated from persons living with HIV. Some of them are being studied and, in some cases, used in clinical trials, to defend humans against HIV infection, treat HIV infection, and kill HIV-infected CD4+ T cells in latent reservoirs.

neutralising antibody

An antibody that neutralises (renders harmless) an infectious microorganism.

phase II

The second stage in the clinical evaluation of a new drug or intervention, in which preliminary data on effectiveness and additional information about safety is collected among a few hundred people with the disease or condition.

The first is CAPRISA in South Africa, a phase 2b trial of tenofovir gel which will present its findings at the Vienna International AIDS Conference in July 2010.

VOICE, a phase 3 trial testing tenofovir gel and tenofovir and tenofovir/FTC PrEP, is recruiting now and will report in 2011.

The long-awaited IPM009 trial of dapivirine in a vaginal ring and/or gel is not due to start till 2012.

From having been a favoured option as new prevention technologies, microbicides were now second to pre-exposure prophylaxis in terms of the number of efficacy trials ongoing or planned, though some trials such as VOICE are studying both concepts.

Shattock said that the concept of a microbicide was amply biologically plausible, unlike any current vaccine.

More than this, no fewer than 13 different compounds in more than 20 studies, ranging from drugs as familiar as tenofovir to ones as exotic as nucleocaspid protein inhibitors and broadly neutralising antibodies, had demonstrated efficacy in blocking HIV infection in monkeys.

“Biological plausibility for microbicides has never been stronger,” he said. “But how can this knowledge bridge to better human trials?”

One problem was to do with ensuring a drug got to the place it needed to, in a state which was effective, and in the concentration needed. Better assays were being developed to better assess activity in human tissue.

An even bigger problem, perhaps, was to do with the difficulty of ensuring adherence. An analysis of adherence in the Carraguard trial which failed two years ago showed that subjects simply found it impossible to reapply the microbicide if they had sex more than once a night.

Developing new formulations which maximised adherence and new ways of monitoring adherence would be crucial to the success of future studies, he said.

Developing combination products containing two or three drugs was also increasing in importance. Dosage formulations such as vaginal rings that supplied a constant level of drugs would overcome this problem.

The challenge now, Shattock later told a press conference, was not to develop more candidates – there were plenty of those – but to raise the amount of money needed to take the best candidates into efficacy trials.

He pointed out that an effective ARV-based microbicide would be much cheaper per efficacious dose than the same drug used orally as PrEP or treatment: a single maraviroc tablet could provide enough microbicide to deliver 16 to 20 microbicide doses.

“But,” he added, “without success or even an indication of success soon it will be impossible to sustain the funding for microbicides.”

Nono Eland of the South African Treatment Action Campaign, following Shattock, commented that “if we fail on prevention, we will lose the progress we’ve made on treatment,” because funders will be unwilling to provide ever-expanding funding for an uncontainable disease.

References

Shattock R State of the ART for microbicides. Opening plenary, 2010 International Microbicides Conference, Pittsburgh, presentation #1, 2010.

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