Microbicides 2008: the second generation is on its way

The next generation of microbicides designed to protect women against HIV infection will rely on maintaining a high level of an antiretroviral drug in the genital fluids and tissue rather than using a barrier gel, Sharon Hillier, Principal Investigator of the Microbicides Trials Network, told the Microbicides 2008 conference this week in Delhi.

She described the second generation of microbicides about to start major trials, as ‘topical PrEP’ – pre-exposure prophylaxis with antiretroviral drugs in the vaginal tissues and fluids. Although this approach may be potent it could have drawbacks too – leakage of the drug into the bloodstream, with a potential risk of drug resistance if the user becomes infected, and toxicity.

She also reported preliminary findings from the two remaining studies of ‘first generation’ microbicides, the HPTN 035 study of 0.5% PRO-2000 or Buffergel, and the MDP 301 study of 0.5% PRO-2000, in which the arm using 2% PRO-2000 was stopped recently because of futility (meaning it would produce no meaningful results). (These findings will be reported separately.)

The trial progammes

There are three major programmes of trials which will lead up to phase IIb/III efficacy trials of antiretroviral-containing microbicides for vaginal use:

• First to report - if all goes well - will be the CAPRISA 004 (Centre for the AIDS Programme of Research of South Africa) study of 1% tenofovir gel in 980 women at two sites in KwaZulu Natal, South Africa, a collaboration between CONRAD and Family Health International. This is the only study looking at ‘coitally-dependent’ use, in which women are told only to use the gel when they think they will have sex. CAPRISA 004 already has 566 women enrolled, and may report by April 2010.
• The Microbicides Trials Network (MTN) is co-ordinating studies leading up to the VOICE (Vaginal and Oral Interventions to Control the Epidemic) study, an inventive trial which, as its name suggests, will directly compare a tenofovir gel with oral tenofovir pre-exposure prophylaxis in 4,200 women at 10 sites in South Africa, Malawi, Uganda, Zambia and Zimbabwe. This is due to start in late 2008 and may report by 2011.
• The International Partnership for Microbicides (IPM) is co-ordinating a series of studies largely in Africa but also in the USA and Belgium, using the NNRTI dapivirine (TMC120) both as a gel and as drug infused into a silicone vaginal ring which can be left in place for a month: the phase III gel study (IPM 009) is at the planning stage, while the timing of the vaginal ring studies depends on preliminary safety and formulation studies. Study 009 may report by 2012/13.
• In addition to these vaginal studies, there is the U-19 programme using the NNRTI UC-781 as a rectal microbicide: see this report for more.

Further into the future, IPM has now negotiated licensing agreements with drug companies to develop CCR5 inhibitors as microbicides: with Pfizer for its drug maraviroc, and – just announced at the Microbicides Conference - with Merck for a CCR5 inhibitor called MRK 167.

There were a number of preliminary results and presentations from the preparatory safety and drug-monitoring studies for these large trials at the Microbicides Conference.

Safety and acceptability: tenofovir gel

Sharon Hillier herself reported on safety and acceptability findings for a preliminary study on tenofovir gel in women that had both a coitally-dependent arm (as in the CAPRISA study) and a regularly-dosed arm (as envisaged in the VOICE study).

This, the HPTN 059 Study, gave tenofovir gel to 200 women aged 18 to 50, randomised to use tenofovir or placebo either daily, or only when sex was anticipated, for six months. The study results were widely reported in the Indian press, as one of the sites was at Pune in southern India (100 women), the others being at Birmingham, Alabama (52 women), and in New York City (48 women) in the USA. Safety and acceptability assessments were done at one, three and six months.

Retention of trial subjects was good, with 96% returning for their final visit at six months. There was only one pregnancy in the study. This was also good, as trial subjects have to drop out when pregnant and this has led to low numbers and meaningless results in certain African studies where pregnancy rates as high as 30 per cent have been recorded. Most future studies will provide women with contraception for this reason.

There were interesting differences between the Indian and the American women. They were about the same average age (33 and 31 respectively). However, in a country where marriage is the highest single risk factor for HIV in women), all but one of the Indian participants was married, compared to 28% of the US women. Sixty-three per cent of the US women had had over twelve years of education, whereas only 21% of the Indian women had had over ten years. And there was a stark reminder of economic inequality in that the average income of the US women was $1503 a month, compared with $55 a month in the Indian women.

There was one new case of herpes infection in the study and three of chlamydia, with no other STIs.

Self-reported adherence to the daily gel was good, and didn’t vary much between sites, with 82% of the Pune women reporting use of the gel for at least six of the last seven days and 75% of the New York women (though see this report for the unreliability of self-reported adherence in microbicide trials). The most common reason for not using the gel daily was because the woman was having her period.

Adherence to the intercourse-dependent dosing was more variable, with the Pune women reporting use 88% of the time and the Birmingham women only 58% of the time.

There were 16 instances of cervical, vaginal or vulvar lesions seen in women using the daily dose of tenofovir gel compared with six (all cervical) using placebo, and ten in the sex-dependent arm, with none on placebo, indicating some marginal toxicity of the tenofovir-containing gel, although only one subject was withdrawn from the study due to adverse reaction to tenofovir gel.

What was encouraging was the acceptability data. Forty per cent of women said the gel was easy to use, and few found it difficult. No one said it made sex less pleasurable, and 12% said it made sex better. Two African studies also reported at the conference found that up to a third of participants said that using the gels made sex better. However six per cent of the sex-dependent users and 11% of the daily users said they’d had some opposition to the use of the gel from their partners.

Safety: dapivirine

Shanique Smyth of IPM presented data from a safety study of dapivirine gel. Dapivirine gel was packaged into pre-filled applicators delivering 2.5 millilitres of gel. Three concentrations were tested, ten micrograms per millilitre (mcg/ml), 20mcg/ml and 50 mcg/ml against a placebo gel (hydroxyethylcellulose or HEC, an inactive ‘carrier’ gel which is becoming the standard placebo in microbicide trials).

One hundred and eleven women in Rwanda, South Africa and Tanzania were instructed to use the dapivirine gel twice-daily for 42 days. There were 32 women in the 10mcg and 20mcg arms, 31 in the 50mcg arm and 16 in the placebo arm.

Safety was evaluated by adverse events (AEs), clinical laboratory tests, and colposcopy (examination of the cervix), with a follow-up visit at Day 56.

No serious adverse events thought to be related to the drug were reported. Four women had lesions to the vulva, the vaginal lining (epithelium) or the cervix: two from the 10mcg group and one each from the 20mcg and placebo groups.

Six women did develop neutropenia, a fall in white blood cells: two each in the 10mcg and 20mcg arms, and one each in the 50mcg and placebo groups. These were serious (grade 3 or 4), but thought unrelated to the drug and certainly didn’t seem dose dependent.

Pharmacokinetics: dapivirine

Systemic absorption – leakage of the drug from the vagina into the bloodstream – is a crucial issue for ARV-containing microbicides, as the biggest safety concern surrounding these compounds is whether women using them who are HIV-positive but undiagnosed, or who catch HIV despite using them, could develop drug resistance.

In a study of dapivirine drug levels, IPM’s Annalene Nel used the same three doses of dapivirine in 18 women, who used the gel for ten days. Dapivirine levels were measured in the blood after the first and last doses at six intervals during the day then daily from two to five days after the last dose.

Despite dapivirine being very insoluble, and chosen for microbicide use precisely because of its poor oral bioavailability, women did develop measurable levels of dapivirine in their blood. After the first dose peak levels were 60 picograms (trillionths of a gram) per millilitre (pg/ml) at the 20mcg dose and 80 pg/ml at the 50mcg dose. At day ten drug levels were higher, with a peak level of 500 pg/ml at the 50mcg dose twelve hours after application. Drug levels tailed off extremely slowly, with levels halving every 65-88 hours. “Increasing half-life at this level could indicate increasing accumulation of the drug in tissues,” commented Annalene Nel.

Pharmacokinetics: tenofovir

Buildup in tissues could be good or bad depending on which tissues are affected, and whether cells containing HIV are active in them. Jill Schwartz of CONRAD reported on a similar pharmacokinetic study of tenofovir gel as part of the studies leading up to the VOICE trial.

Twenty-one women used a single dose of four grams of tenofovir gel. Drug levels were taken seven times in the 24 hours after the dose. Drug levels were also measured in vaginal fluid and inter- and intracellular levels in vaginal tissues were measured by biopsy. Once this was done the women then took a single oral dose of tenofovir to compare levels.

Most tenofovir concentrations in the blood were below five nanograms (billionths of a gram) per millilitre (ng/ml), though peak values of up to19.5 ng/ml were measured in some women.

This is way below the dose seen when tenofovir is taken orally, and indeed the study then gave an oral dose to the women to compare levels, and blood levels 100 times greater were seen, at 296 ng/ml, after the oral dose.

Vaginal fluid levels were, as one would expect, a million times higher than blood levels, at 1.5 to five milligrams per millilitre at eight hours, and 0.045 to 0.47 milligrams per millilitre at 24 hours.

Levels also built up in the tissues lining the vagina, to levels a thousand times those seen in the blood, with levels at one hour after dose of 0.45 milligrams per gram of tissue and 0.015 milligrams per gram at 24 hours. The tenofovir was mainly concentrated into the gaps between cells but low but detectable levels were seen inside the cells of three-quarters of the women. As long as this is localised to vaginal tissues, this is a good thing as it should mean that tenofovir is being absorbed to a sufficient depth to defend the immune cells vulnerable to HIV against infection.