ICAAC: Dapivirine safe and tolerable as potential vaginal microbicide; displays potential in combination with other agents

The NNRTI dapivirine has demonstrated clinical safety and tolerability when tested in HIV-negative women as a potential vaginal microbicide; dapivirine, tenofovir and another investigational product (L-860,167) have shown in-vitro anti-HIV activity in combination and may be candidates for a combination microbicide. The results were presented as posters at the 47^th Interscience Conference on Antimicrobial Agents and Chemotherapy.

Various antiretroviral agents are being investigated as potential microbicides which may protect against HIV infection when applied vaginally. Two posters at the 47th Interscience Conference on Antimicrobial Agents and Chemotherapy reported on several products currently in development.

Dapivirine: clinical safety and tolerability

Dapivirine (formerly known as TMC120) is a non-nucleoside reverse transcriptase inhibitor (NNRTI), an oral formulation of which has been studied in eleven clinical trials to date. A gel formulation of dapivirine is now being investigated as a vaginal microbicide. The active drug is formulated into a predominately Carbopol®-based gel at a concentration of 0.02%. The gel is then packaged in pre-filled applicators, which deliver 2.5 ml of the 0.02% gel (500 μg drug/dose).

Investigators designed a randomised, double-blinded, 42-day, phase I/II, placebo controlled trial to determine the safety and tolerability of the dapivirine microbicide. The study was conducted at a single site in Belgium. A total of 36 women were included in the study, with 24 randomised to receive the active gel and twelve to a placebo gel; both administered twice daily for 42 days. All the women were HIV-negative and in good health.

Safety and tolerability were assessed by monitoring adverse events, by using laboratory tests to detect clinical abnormalities, by colposcopy, physical examinations, and the monitoring of vital signs. Concentrations of dapivirine in plasma were assessed on days 7, 28 and 42, with a follow-up on day 56.

In women using dapivirine gel, mean plasma dapivirine concentrations were 0.44, 0.47 and 0.42 ng/ml on days 7, 28, and 42 respectively, suggesting that steady-state concentrations had been reached by or before day 7.

The safety and tolerability findings were highly encouraging. Some drugs investigated as potential microbicides have caused damage to tissue or the mucosa in sexual organs, meaning that their use could actually leave women more vulnerable to infection with HIV.

With dapivirine gel, there were no clinically significant laboratory abnormalities or any indications of tissue damage to the vulval or vaginal epithelium or cervical mucosa on a microscopic level. Colposcopic examinations did not find any difference between the treatment and placebo arms of the study, and there was difference in the frequency of genital infections between women who used dapivirine and the placebo.

Twenty (83%) of the women in the dapivirine arm and nine (75%) in the placebo arm had at least one adverse event after beginning the study, mostly genital, skin, and urinary reactions. These were mostly mild in severity. Only one severe reaction was reported, in the dapivirine arm.

At each study visit, the participants were asked, “if this gel is proven effective for HIV prevention, and you wanted to use this gel for HIV protection, how willing would you be to use this gel in the future?” On the final treatment day, results were nearly identical between women using dapivirine and those using placebo.

In the dapivirine arm, 63% of the women were “willing” or “somewhat willing”; 34% were “not willing” or “absolutely not willing.” In the placebo arm, 67% were “willing” or “somewhat willing” and 33% were “not willing” or “absolutely not willing.”

The investigators concluded that “dapivirine gel administered twice-daily for 42 consecutive days in healthy HIV-negative women was safe and well tolerated, suggesting that continued development is warranted.”

Dapivirine, tenofovir, and L-860,167: in vitro studies

In a second poster, investigators presented data on the test-tube activity of dapivirine and two other anti-HIV drugs as potential microbicides: tenofovir (Viread) and L-860,167. The activity of the drugs against CCR5 and X4 strains of HIV in peripheral blood mononuclear cells was assessed, as well as activity against X4 strains in H9 T-cells. The drugs were assessed alone and in combination.

To measure antiviral activity, cells were incubated with single drugs or with drug combinations for 60 minutes, then infected with HIV. At 2 hours post-infection the virus was removed and the cultures were incubated for 7 days; the level of cellular infection was then assessed by measuring p24 viral antigen.

The infectivity of CCR5 strains of HIV was inhibited by all three drugs. Dapivirine and tenofovir were comparably effective against X4-tropic virus, but L-860,167 was inactive against X4 strains of HIV. Dual combinations of all drugs showed enhanced effectiveness against HIV in X4-tropic H-9 T-cells, and a triple combination had enhanced efficacy against CCR5 strains in peripheral blood mononuclear cells.

Used alone, dapivirine showed inhibitory activity at EC₅₀ levels between 0.30 and 0.45 nM, depending on virus and cell type. The EC₅₀ for tenofovir ranged from 12.94 nM (for CCR5 virus) to 144.93 and 179.15 nM (for X4 virus). In combinations, EC₅₀ levels were comparable or smaller; levels for L-860,167 were in the range of 0.03 to 2.89, depending on the combination, virus type, and cell type.

None of the drugs showed evidence of cellular toxicity (as evaluated using an XTT cell killing assay) when dosed at 1000 times the effective concentration.

“In vitro assays in human peripheral blood mononuclear cells and T cell lines demonstrated enhanced inhibitory activity of combinations of these drugs versus each drug alone”, comment the investigators, “suggest[ing] there is benefit in developing these drugs as combination microbicides.”

However, limitations in methodology require caution in interpreting combined effects. Efficacy when used in combination “appeared to be dependent on the type of cell line and the virus used in the inhibition assay.” Further studies are in progress using alternative methods that may provide further insight into the potential of these combinations as microbicides.