Long-acting fusion inhibitor albuvirtide looks promising in early studies

A new fusion inhibitor that prevents HIV entry into cells showed potent antiviral activity in early clinical studies and has a long half-life, suggesting it may be suitable for once-weekly dosing, Chinese researchers reported yesterday at the 52^nd Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) in San Francisco.

Multidrug-resistant HIV remains a concern, especially for highly treatment-experienced people who have used most types of antiretroviral drugs. Fusion inhibitors – which prevent HIV from fusing with T-cell membranes to allow cell entry – may remain active against virus that has developed resistance to commonly used drug classes.

The sole approved fusion inhibitor, enfuvirtide (Fuzeon, formerly known as T-20), must be administered by subcutaneous injection twice daily, leaving much to be desired in terms of convenience and ease of use.

Albuvirtide (FB006M), a synthetic peptide being developed by Chongqing Frontier Biotechnologies, is chemically related to enfuvirtide, and also works by binding to the HIV gp41 envelope protein. The new drug binds strongly to the human blood protein albumin, however, which enables it to last longer in the body but makes it unable to penetrate into the brain or testicles.

In preclinical in vitro and in vivo studies, albuvirtide demonstrated potent activity against a broad spectrum of HIV isolates and had a favourable safety profile. 

Researchers at ICAAC presented results from two clinical trials of albuvirtide monotherapy in previously untreated people with HIV in Beijing.

The first was a phase I proof-of-concept single-dose escalation trial. A total of 55 participants were randomly assigned to receive single doses of albuvirtide ranging from 20mg to 640mg, or placebo. The drug was administered by intravenous infusion and participants were hospitalised for observation. All but one were treated and analysed, and two discontinued treatment prematurely.

About 70% of participants were men and the median age was 38 years. The median HIV viral load was approximately 20,000 copies/mL, the median CD4 cell count at study entry was about 400 cells/mm³ and all had at least 250 cells/mm³.

Albuvirtide was generally safe and well tolerated at all doses tested. No serious adverse events or injection site reactions were reported.

The drug had a good pharmacokinetic profile. The plasma half-life was eleven days, with a linear elimination pattern. Antiviral activity continued for six to ten days after a single dose.

The second study was a phase IIa open-label trial of single and multiple doses of albuvirtide. This study included twelve treatment-naive participants assigned to receive either 160mg or 320mg albuvirtide. They received the drug once daily for the first three days, followed by two once-weekly administrations at days 8 and 15.

All participants in this study were gay men. There were some baseline differences between the dose arms. In the 160mg arm the average age was 36 years, the median HIV RNA level was about 30,000 copies/mL and the median CD4 count was 484 cells/mm³. In the 320mg arm, the average age was 27 years, the median viral load was about 8,000 copies/mL and the median CD4 count was 406 cells/mm³.

Again, albuvirtide was found to be safe and well tolerated with no serious adverse events, drug-related side-effects or injection-site reactions. Participants did not develop detectable antibodies against the drug after multiple injections.

In the 160mg arm viral load declined by 0.68 log₁₀ copies/mL on average and HIV RNA fell to 0.5 log₁₀ or less in 83% of participants. In the 320mg arm, the mean viral load decrease was 1.05 log₁₀ copies/mL and 100% reached 0.5 log₁₀ or less.

There was a significant dose-response relationship between albuvirtide plasma concentration and viral load suppression. While sustained viral suppression was seen after the first three days of dosing, HIV RNA rebounded more rapidly after the later once-weekly doses, a possible indicator of drug resistance.

The researchers concluded that albuvirtide produced good viral suppression with a clear relationship between dose and antiviral efficacy. The eleven-day half-life supports once-weekly dosing, they added. Further studies are planned using albuvirtide in combination antiretroviral regimens.

Wu H et al. Albuvirtide, the first long-acting HIV fusion inhibitor, suppressed viral replication in HIV-infected adults. 52^nd Interscience Conference on Antimicrobial Agents and Chemotherapy, San Francisco, abstract H-554, 2012. View the abstract on the conference website.