Six existing drug classes now being tested as microbicides

A number of presentations at the 2010 International Microbicides Conference in Pittsburgh concerned microbicide research using established classes of HIV drugs that have not been used as microbicides before.

Trial of microbicides and pre-exposure prophylaxis (PrEP) in humans have so far only involved two classes of drugs: the nucleoside reverse transcriptase inhibitors (NRTIs) tenofovir and FTC in large randomised controlled trials, and the non-nucleoside reverse transcriptase inhibitors (NNRTIs) dapivirine and UC781 in phase 1 safety trials.

If people taking PrEP are nonetheless infected with HIV, or are doing so with an undiagnosed infection, there is a significant possibility of the development of HIV drug resistance, and there is a theoretical risk of resistance in microbicides too.

There is therefore an urgent need to research other drugs as microbicides and PrEP agents both to minimise the risk of prevention and to provide 'second-line prevention' should HIV drug resistance become prevalent.

There are many experimental classes of drug being tested for use as microbicides, but using ones with a history as HIV treatments removes some of the need to demonstrate safety and shortens the preclinical and phase 1 phases of development: leading researcher Robin Shattock told the conference that the failure of microbicide-specific classes to show efficacy has injected new urgency into the need to demonstrate proof of concept (see separate story).

Two new classes of drug – an integrase inhibitor and the CCR5 inhibitor, maraviroc (Celsentri) – are amongst drugs that have now been used to prevent HIV infection in monkeys, and await funding to go into human trials.

L-870812 is an integrase inhibitor, and one of the compounds that was investigated as a candidate HIV treatment drug during the development of the first one eventually licensed, raltegravir (Isentress).

Investigators from the US Centers for Disease Control found that the drug protected monkeys from vaginal exposure to HIV with an efficacy similar to other classes. Two out of three macaques were protected from 14 vaginal challenges with a virulent monkey/human artificial virus while an unprotected macaque was infected after three challenges.

However, one monkey was infected after seven weeks. This animal was maintained on L-870812 microbicide for a further 15 weeks to see if it developed drug-resistant HIV, and it did not. There appeared to be no relationship between infection and drug concentration as absorbed into the bloodstream: analyses of drug concentrations in tissues are ongoing.

Fuller data was also presented on a study of the CCR5 inhibitor maraviroc, initially presented at the Conference on Retroviruses and Opportunistic Infections in February 2010.

Maraviroc also underwent further investigation as a possible rectal microbicide and for use in combination. This CCR5 inhibitor drug was licensed by Pfizer to the International Partnership for Microbicides (IPM) in 2008.

The research team, led by Robin Shattock at St George’s, University of London, has been testing maraviroc for its ability to defend living rectal, cervical and penile tissue cells (explants) against HIV infection, both from free virus and from cell-associated virus. It was also tested in combination with tenofovir, UC781 and dapivirine.

Maraviroc had previously shown only modest anti-HIV activity as a microbicide. The current studies confirmed that its potency in preventing infection of cervical and penile tissue was relatively modest, unless the cells are already in an inflammatory state. The fact that, despite this, it protected monkeys from vaginal viral challenge suggests in might be a very potent rectal microbicide.

This is partly because, since rectal cells are more easily infected by HIV, a microbicide can make the most obvious difference to their vulnerability; it offers the most protection to the most easily-infected tissue cells, but makes less difference to less easily-infected tissue.

When used as an HIV treatment, the IC₅₀, the dose needed of maraviroc to inhibit infection by 50%, is 33 nanomols (a one-in-30-million dilution in test-tube experiments).

When rectal cells were incubated with HIV, a single three-hour exposure to maraviroc required 110 nanomols in solution to inhibit infection by 50%, but when the cells were bathed overnight in a dilution of maraviroc the dilution required fell fivefold and when they were exposed to maraviroc continuously the IC₅₀ was only 2.2 nanomols, a one-in-450-million dilution.

What this all means is that maraviroc will need to be combined with other candidate microbicide drugs in order to be part of an effective compound but, if so, this compound may have good dual vaginal/rectal activity.

It has been co-formulated in a gel along with the NNRTI drug dapivirine (TMC120), which is already under investigation by IPM as a vaginal microbicide. The drugs proved to have a synergistic effect – they made each other more powerful.

When maraviroc was combined with dapivirine, the IC₅₀ of dapivirine was cut by 92% – i.e. it was more than ten times more potent – and the IC₅₀ of maraviroc by 41%.

It also however implies that maraviroc would have to be delivered in such a way as to maintain concentrations in the rectum – not an easy task given that methods like vaginal rings cannot be used.

Robin Shattock told aidsmap that, because ARV-containing microbicides do not require large volumes of gel to be present as a physical barrier, thicker formulations could be developed that would enable the drug to adhere to the colorectal mucosa for long periods, though there would have to be careful safety studies of this concept.

Protease inhibitors (PIs) have previously not been considered as microbicide candidates because they act at the post-integration stage of HIV’s lifestyle and therefore in theory might not be able to prevent an infection.

However, they could certainly inhibit the second stage of infection, when infection in mucosal cells is ‘amplified’ and locally-infected lymphocytes travel to the lymph nodes, where they seed a systemic infection.

The St George’s team evaluated the PIs saquinavir, lopinavir, ritonavir and darunavir as potential microbicides and found darunavir was the most potent by an order of magnitude.

When darunavir was combined with dapivirine, the latter drug’s efficacy in preventing infection of dendritic cells, which are probably the first cells in the genital tract to be infected by HIV, rose by 82% and darunavir’s by 44%. The team envisage this PI/NNRTI combination will be taken forward to animal studies.

Finally, large molecules which would need injecting if used as treatments are no problem in topical formulations like microbicides.

The St George’s team and Princeton University in the USA are developing fusion inhibitors similar to T20 (enfuvirtide, Fuzeon) as microbicides.

The St George’s team have taken a pre-existing fusion inhibitor molecule called C34, an HIV treatment candidate initially developed by Merck, and attached it to a cholesterol ‘tail’ that greatly enhances its ability to bind to the HIV gp41 entry protein and stop it entering cells.

L'644, the resultant compound, has exceptional potency as a microbicide: in test-tube studies it works at sub-nanomolar concentrations (less than one-in-a-billion-solution), demonstrates superior potency to the existing fusion inhibitors T20 and T1249 and significantly inhibited HIV infection of rectal explants (living samples of rectal tissue) even when applied an hour after infection.