In the 1890s, a New York surgeon named William Coley found that he could destroy tumors by injecting patients with live streptococcal bacteria. Coley didn't know how the bacteria produced this effect, but that didn't stop him from producing bacterial extracts for cancer treatment, which became widely known as “Coley's toxins.” More than 1,000 patients were treated with the toxins, but clinical results proved inconsistent and sometimes dangerous. Coley was ridiculed as a quack.

A measure of vindication came decades later when, in 1998, Bruce Buetler, M.D., and colleagues at the Scripps Institute in La Jolla, Calif., showed that lipopolysaccaride—a bacterial toxin—activates immune system molecules called toll-like receptors (TLRs). In turn, these receptors activate inflammatory compounds that can kill cancer cells and tumors. Beutler's discovery prompted renewed interest in Coley, who is now recognized as a pioneer in cancer immunotherapy, and launched a broad exploration of TLR pathways for treating cancer and other diseases.

Now TLRs are hot targets for drug development. The industry has spent hundreds of millions of dollars developing new substances that they hope will kill cancer by TLR activation. Farthest along are Coley Pharmaceuticals, a biotech firm named after the Victorian-era surgeon, and its partner, Pfizer. They have developed a candidate TLR drug for non–small-cell lung cancer that is in phase III clinical trials. Recent unpublished preliminary data, presented by Coley and University of Heidelberg investigators at a European cancer meeting last year, show that this drug, known as PF-3512676, produced a 17% increase in the number of patients surviving 1 year after diagnosis when combined with standard therapy. Specifically, 50% of patients who take the Coley drug along with standard therapy live for up to 1 year, as opposed to 33% who take standard therapy alone. TLR-based drugs for other cancers, including non-Hodgkin lymphoma and renal cell carcinoma, are also under development.

“TLRs represent an entirely new direction for immunotherapy,” says Alberto Mantovani, M.D., scientific director of the Instituto Clinico Humanitas in Milan, Italy. “We've used microbial products for immunotherapy in the past, and now we know they activate TLR. So our challenge is to choose our TLR targets in a more intelligent way.”

What Are TLRs?

TLRs are proteins located on a variety of immune cells that traverse cell membranes. They work by binding with specific molecules on foreign pathogens during the immune system's attack. Of particular interest are those TLRs located on innate immune cells, the first responders to foreign pathogens, such as dendritic cells and macrophages. As these cells engulf invading pathogens, their TLRs trigger the release of inflammatory cytokines that attract additional defensive cells from the blood. These secondary cells, namely B cells and T cells, constitute the forces of active immunity. In a key distinction, the first responding cells attack foreign pathogens indiscriminately, whereas the active immune system responds just to entities that it knows from prior exposure. Thus, TLRs link innate and active immunity in a newly recognized role.

Of the 10 TLRs discovered, those deemed potentially most useful in cancer treatment thus far include TLR7 and TLR9. Indeed, the only TLR cancer drug now on the market—a treatment for superficial basal cell carcinoma made by the pharmaceutical company 3M—targets TLR7 in a way discovered only after its commercial release. And Coley's drug, along with a drug candidate for renal cell carcinoma made by Idera Pharmaceuticals, targets TLR9. Both TLR7 and TLR9 trigger an active immune response called Th1, characterized by an influx of cancer-killing T cells, interferons, and other inflammatory mediators that enhance T-cell activity and inhibit tumor growth. These active immune components can recognize a tumor's characteristics, which immunizes patients against the spread of the disease, according to Sudhir Agrawal, Ph.D., Idera's chief scientific officer.

“So while the innate immune system fights the tumor, it triggers adaptive responses that learn to memorize cancer antigens,” he explains. Agrawal and his colleagues have shown that surviving mice treated with TLR agonists to counter implanted tumor cells remain immunized against the effects of future cancer cell exposures. “The memory response is still there,” he explains.

Inflammation: A Gray Area

The view among most researchers is that Th1-induced inflammation inhibits cancer growth. Thus, Coley and other companies have developed TLR agonists that trigger Th1 to achieve clinical benefits.

“In general, for fighting cancer, what you want is the Th1 response,” emphasizes Coley's chief science officer, Art Krieg , M.D.

A complicating factor, however, is that some cancers also appear to be sustained by inflammation, and Th1 activation is also known to suppress an inflammation-related immune response called Th2. The Th2 response, which is dominated by the production of B cells, drives some of inflammation's more toxic outcomes, including sepsis, which kills more than 200,000 U.S. patients a year. In turn, Th2 activation suppresses Th1, indicating that these immune responses crossregulate each other.

Recent findings by Gaëtan Jego, Ph.D, from France's National Institute for Health and Medical Research (INSERM) and Janne Bohnhorst, Ph.D., from the Norwegian University of Science and Technology, scheduled to be published in the June issue of Leukemia , show that B cells in patients afflicted with multiple myeloma express “a vast repertoire” of TLRs that apparently promote their proliferation and survival. Responding to these findings, Mantovani—who wrote an accompanying editorial for Leukemia —acknowledges that triggering inflammation by innate immune system activation can be a “double-edged sword” when it comes to cancer therapy.

“This is the message: we know plasmocytomas (like multiple myeloma) use inflammation to grow better,” he says. “But on the other hand, we know that if we activate innate immunity [to trigger inflammation] correctly, we can [in some cases] promote anti-tumor activity. This relates to the appropriate choice of tumor for treatment. We have to learn to use the good side of the sword.”

Adds Coley's Krieg, “This is a fascinating conundrum: Inflammation is a broad term. We know there are certain types of inflammation that appear to accelerate tumor growth. But right now we don't know enough about the specific circumstances that will accelerate tumor growth or decelerate it.”

Coley and Idera's drugs, for non–small-cell lung cancer and renal cell carcinoma, respectively, both trigger inflammation to limit tumor growth by using TLR9. Drugs designed to inhibit inflammation, as might be developed for multiple myeloma, would act as TLR antagonists and block TLRs that promote disease. The drug development industry has not yet developed a TLR antagonist for cancer therapy. Most researchers have found TLR agonists easier to create because they work through logical, mainstream pathways, namely, activating immunity and host defenses, Mantovani says. “Right now we don't have effective ways of blocking TLRs,” he adds.

One approach for blocking TLRs, he suggests, might be to block targets that control the function of multiple TLRs rather than trying to selectively block just one TLR. A gene that controls the expression of several TLRs, called the myeloid differentiation response gene, could be an interesting choice, he suggests. “Block that target and you then commensurately block the upstream TLRs at the same time,” Mantovani explains.

Clinical Uses

The only TLR-based drug currently on the market—3M's Aldara, for superficial basal cell carcinoma—is given without other treatments. But scientists say that TLR drugs given in combination with other therapies pose interesting clinical opportunities. “Combination therapies are where we think the field is going to go,” Krieg says.

Mantovani concurs, suggesting that TLR-based drugs will be particularly useful once radiation, surgery, and chemotherapy reduce the bulk of a tumor. Coley's Phase 3 findings offer proof of principle for TLR drugs, Mantovani suggests, which is consistent with other forms of cancer immunotherapy like the monoclonal antibodies that target cancer cells to disrupt their activity or enhance their immune response against cancer. “You wouldn't use those alone as single agents,” he says.

An ongoing challenge when moving forward will be to control side effects from TLR-based drugs, says Douglas Golenbock , a professor at the University of Massachusetts Medical School in Worcester, citing autoimmune diseases like lupus that might emerge from an overactive Th1 response. “It's all a matter of finding the right therapeutic index,” he says.

Douglas Golenbock

Douglas Golenbock

Ultimately, TLR-based drug development faces two opposing arms of the immune system: the arm that fosters immunity against invading pathogens versus the arm that triggers unwanted autoimmune attacks against the patient. Whether this conflict poses a risk to patients is somewhat controversial, however. Mantovani downplays autoimmune disease risks, pointing out that autoimmune reactions within tissues that enclose a tumor sometimes actually benefit patients. “I don't see this risk of autoimmunity as something to worry about,” he says.

Optimistically, TLR-based drugs could give cancer immunotherapy a shot in the arm, but the field is young and many questions remain. “We just need more data,” Agrawal says. “Everyone is making good progress, and now is the time to start doing the clinical studies. That's the biggest challenge: waiting for the data.”