The third generation and beyond

Some people describe ARV-based combination products as the third generation of microbicide candidates. The broad base of commitment to this approach is demonstrated by the recent formation of two new entities to advance it. As described above, the Combined Highly Active Anti-retroviral Microbicides (or CHAARM) programme and the coincidentally named Combination HIV Antiretroviral Rectal Microbicides were established in late 2009 and early 2010 specifically to pursue development of such products.

The growing body of knowledge regarding how HIV infects mucosal tissue is shaping the search for, and development of, the compounds which will eventually comprise the third generation of microbicide candidates. Basic laboratory research has demonstrated that pre-existing surface damage to vaginal and rectal mucous membrane is not required in order for HIV to infect the vulnerable cells below. Researchers at McMaster University, in Ontario, Canada, have shown that HIV can weaken the integrity of surface cells, even when they are undamaged.

Previously, researchers thought that transmission likely occurred either when the mucous membrane was damaged (via trauma or ulcers), or when many activated immune cells were present (as occurs during STI infection, for example). But the McMaster University group found that HIV causes the electrical barrier resistance of epithelial cells to decrease. They hypothesize that, given sufficient viral load and exposure time, HIV can probably disrupt any mucosal barrier in the body, although infection may not necessarily occur every time.¹

Such basic science breakthroughs provide new areas of focus for prevention research. In this case, the finding suggests that preventing attachment to the epithelial cells, themselves, could be an even more critical area for intervention than previously supposed.

The US-funded Microbicide Innovation Program (MIP) is designed to “cast a wide net to capture novel and unique activities for the advancement of microbicides”.²

One factor considered by MIP is the need for candidates that are very inexpensive to manufacture in order to be potentially well suited to broad based distribution in the developing countries. Griffithsin (GRFT) is a potent HIV-entry inhibitor being explored by researchers at the University of Louisville in Kentucky (US) in collaboration with colleagues at other institutions. GRFT can be harvested from the tobacco plant, nicotiana benthamiana, after it is infected with a tobacco mosaic virus that expresses the genes for GRFT. Nicotiana benthamiana is widely used in botanical research because it can be easily infected with this virus and engineered to express large amounts of proteins for use in therapeutics and prevention.  

The plant-produced GRFT blocks HIV transmission, as well as HSV, and has been shown in human cervical explants and rabbit vaginal irritation models to be non-cytotoxic.[ref] Developers of GRFT have calculated that an: “environmentally controlled greenhouse producing 3,000 kg of leaf tissue per acre could yield … over 2 million doses per year”.³

One barrier to pursuing investigation of such recombinant protein candidates to date has been the expense of manufacturing them. MIP-funded researchers working on another project, however, have shown that the associated costs for GRFT production as described above could potentially be reduced by growing nicotiana benthamiana hydroponically – a measure that might also improve the plant yield and allow for better control of its quality.⁴ This approach may be appropriate for future consideration in the development of GRFT.

Other 'far future' possibilities include the development of probiotics and mucosal vaccines.

Probiotics are described by their developers as “live microbicides” that “have the potential to function as a long-term, self-replicating delivery system and combat reproductive tract infections such as HIV.”⁵

Researchers at Ocel, Inc. a bacterial therapeutics company, are working to show how the naturally occurring, hydrogen peroxide-producing lactobacilli that already play a key role in maintaining vaginal health could be genetically engineered to become “a self-renewing vehicle for mucosal delivery of protein-based HIV inhibitors”.⁶ They have demonstrated that viable lacrobacilli capable of expressing cyanovirin-N (CV-N, an HIV-fusion inhibitor derived from blue-green algae) can be dried, stored in powder form and still reactivate upon rehydration in vitro. Ocell are currently investigating the conditions under which such lactobacilli can be best created, preserved and delivered into the vagina.

In parallel, they have demonstrated in a Phase 2 trial that treatment with a bacterial therapeutic called Lactobacillus crispatus CTV-05 (LACTIN-V) is well tolerated and successfully combats recurrent bacterial vaginosis (a common vaginal infection) by repopulating the normal vaginal flora.⁷ This research helps to establish the acceptability and safety of the lactobacillus vehicle, and thus may facilitate future testing of the CV-N-expressing lactobacilli.

A mucosal vaccine could, conceivably, be applied topically and work simultaneously as a microbicide and a vaccine. Researchers at St George’s, University of London, describe this as “avery long-lasting microbicide” that would incorporate proteins from the surface of HIV to elicit antibodies that would protect people at the sites of infection. The vaccine mechanism would effectively be boosted every time the microbicide is applied. They caution, however, that their research in this area is unlikely to produce any definitive results for several years.⁸

Twenty years ago, women called for “virucides which are effective, safe and acceptable to women". Twenty years later, the first product capable of meeting that demand has been identified, demonstrating that this challenge can be met. Substantial scientific progress has been made despite insufficient resources, a political barrier that can and must be removed. Progress in this field is also slowed by the indispensably deliberate pace of careful research and mandates of bioethics. Neither of these fundamental requirements can be compromised, despite the urgency of the task at hand.

The proof of concept provided by the CAPRISA 004 trial, together with the wealth of innovative candidates, strategies and approaches described above, illustrate the abundance of creative intellect in the microbicides field, as well as unflagging determination of its actors. We now have every reason to expect that several safe, effective microbicides will reach the market before another two decades pass. The goal articulated by the first advocates to raise the demand for these tools is within reach. As Lori Heise, founding director of the Global Campaign for Microbicides, said: “We have to find a way to make sex safe for our daughters, as the Pill did for us.”