Promising Microbicide Approach Published in Nature

Published today in Nature (advance online papers) is an encouraging animal study of a novel combination of entry inhibitors used as microbicides. The following article from TAG (forthcoming in the next issue of TAGline) reports on this same study as it was presented at the July IAS conference on HIV Pathogenesis & Treatment in Rio.

UPDATE: Washinton Post story: AIDS Gel on a Faster Track.

Combination Microbicides: Moore Tries More

Amidst the largely unspectacular data presentations at the 3^rd IAS Conference on HIV Pathogenesis & Treatment, held this past July in Rio de Janeiro, one highlight was a talk on novel combination microbicide approaches delivered by John Moore from Cornell University. Moore unveiled results from studies exploring the utility of various attachment and entry inhibitors as vaginal microbicides in rhesus macaques. The studies were conducted in collaboration with Melissa Pope (Population Council), Robin Shattock (St.George’s Hospital, London) and Ron Veazey and Preston Marx (Tulane Regional Primate Research Center).

Moore began by outlining the rationale for developing microbicides, which could potentially offer a topical means of preventing sexual transmission of HIV in the continued absence of an effective vaccine. A successful microbicide might conceivably become part of a package of overlapping prevention approaches in the future, perhaps complementing a partially effective vaccine and/or pre-exposure prophylaxis using antiretroviral drugs.

Moore also described the four key properties for any candidate microbicide:

• Safe - no localized toxicity, including no damage to the vaginal epithelium during sustained, repetitive use, and no localized inflammatory responses.
• Effective - must have a significant degree of efficacy in routine use.
• Cheap - priced comparably to a condom
• User-friendly - must be compatible with use during sex, from both female and male perpectives

Moore then described the mechanism of action of the drugs that his team was interested in evaluating. There are several steps in the process by which HIV enters target cells that can now be targeted by antiretrovirals. The first is the binding of HIV to the CD4 molecule, a step that is targeted by the experimental drugs PRO542, BMS-378806 and TNX-355. The second step involves viral engagement of the co-receptor (CCR5 or CXCR4) which can be blocked by several experimental compounds including vicriviroc, maraviroc, PRO140, and AMD070. The final step involves formation of a complex called the 6-helix bundle and fusion of the viral and cellular membranes, inhibited most famously by the approved drug Fuzeon (T-20) and its discontinued sister compound T-1249.

For their experiments, Moore’s team chose drugs targeting each of these steps and then studied their efficacy as microbicides using a macaque challenge model.

• BMS-378806, a small molecule attachment inhibitor developed by Bristol-Myers Squibb
• CMPD 167, an investigational CCR5 inhibitor developed (but discontinued) by Merck
• C52L, a peptide fusion inhibitor similar to Fuzeon discovered at Cornell

The animals used were all adult female rhesus macaques that were treated with progesterone for 30-33 days in order to thin the vaginal epithelium and increase susceptibility to the challenge virus (a CCR5-tropic SIV/HIV hybrid called SHIV162p3). The drugs were administered in 5ml of a benign 2.5% HMC gel and introduced into the vaginal vault for 30 minutes prior to challenge; control animals were given gel alone. A high dose of virus (300 TCID₅₀) was used in all experiments.

Moore explained that a pilot study (published two years ago, see J Exp Med 198, 1551-1562, 2003) had tested the efficacy of CMPD 167, but the results were disappointing: only 2/11 treated animals were protected. However, there was a statistically significant diminution in post-infection viral load compared to controls and this encouraging finding led Moore and colleagues to explore the possibility of enhancing the delivery of the drug and combining it with other compounds.

A new series of studies was undertaken beginning with CMPD 167 in a modified HMC gel (the ph was altered thereby increasing the concentration of CMPD 167 5-fold). The results were dramatically improved, with 8/10 animals completely protected from infection. The other drugs were also evaluated singly prior to embarking on combination studies. C52L was somewhat protective alone, preventing infection in 3/5 macaques. Similarly, BMS-378806 was protective in 6/8 challenged animals.

Several dual combinations were then explored: CMPD 167 + BMS-378806, CMPD 167 + C52L and C52L + BMS-378806. The doses of CMPD 167 and BMS-378806 were also reduced slightly. The combinations protected 3/6, 5/6 and 6/6 macaques, respectively (see table for the complete data summary).

Drug	Concentration	Macaques protected	p-value
Control (HMC gel only)		0/9
CMPD 167	5 mM	8/10	0.00060
BMS-378806	5.5 mM	6/8	0.0023
C52L peptide	1.5 mM	3/5	0.027
CMPD 167 + BMS-378806	1 mM, 2mM	3/6	0.044
CMPD 167 + C52L	1 mM, 1.5 mM	5/6	0.0020
C52L + BMS-378806	1.5 mM, 2mM	6/6	0.00020

While the results are encouraging, particularly for the combination of C52L and BMS-378806, Moore noted that the experimental conditions are highly artificial compared to the human situation where, as he put it, “time is the enemy.” The reasons include the inevitable decline in concentration that occurs with a topically applied product and the possibility that HIV might penetrate into tissues and proliferate when local drug levels decline. A very small study was undertaken to take a preliminary look at this question: CMPD 167 was administered as a microbicide but viral challenge was delayed by 30 minutes, two hours or six hours.

Delay before challenge	Macaques protected
30 min	8/10
2 hours	2/3
6 hours	2/3

While the numbers are far too small to draw firm conclusions, Moore argued that the results suggest that sustained protection is possible. He also detailed some of the theoretical advantages of the combination microbicide approach:

• There are a number of possible pathways by which HIV establishes infection (e.g. direct infection of resting CD4 T cells, capture by dendritic cells followed by transfer to CD4 T cells), and it remains uncertain which is most important. Combinations may be able to simultaneously block more than one possible transmission pathway.
• Synergy between some inhibitors may reduce the concentrations needed for efficacy.
• An increased breadth of coverage against circulating HIV-1 strains may be provided by combinations.
• Combinations may reduce the probability of transmission of variants that are resistant to any one inhibitor.
• Targeting the virus as well as cellular co-receptors may reduce the infectivity of HIV-1 in vaginal fluids and hence also reduce female-to-male transmission.

The task now for Moore’s group is to translate their work into practical products suitable for human testing. Currently they are in the process of identifying compounds with even greater potency and also working on creating collaborations between the companies that own the drugs, non-governmental microbicide organizations and NIH to develop specific products for further development. The next steps will involve the creation of formulations for human use and the initiation of safety studies in women. It’s clear that after a period of gloom precipitated by the dismal failure of N-9 as a microbicide, the work of Moore’s team and others like them is opening up new and far more promising vistas for microbicide research. The field’s biannual conference is slated for April 23-26, 2006 in Cape Town, see http://www.microbicides2006.org/ for more information.