In the late 1700s, physician Edward Jenner injected a young boy with cowpox to test his hypothesis that those who were infected with that disease never contracted the vastly more serious smallpox. A few weeks later, Jenner gave the boy a dose of smallpox. Fortunately for the boy, it worked, and Jenner is known as a hero and a pioneer in vaccination, not a homicidal kook.

While this experiment seems barbaric by today's standards, that's how science was done in an era before the FDA and Institutional Review Boards. And that ushered in the age of vaccination, where people are injected with dead or crippled pathogens to proactively stave off much more serious infections.

It is the immune system's job to eradicate foreign invaders, such as viruses, but to be effective, the immune system must be trained to recognize what is foreign. This is why the first time a person's body is infected with a pathogen it doesn't recognize, the person gets sick, and it takes time for the immune system to fight back. But the next time the pathogen comes along, the immune system is ready and wipes it out quickly.

Because vaccination is so effective at harnessing the power of the immune system against infectious agents, there have been extensive efforts to direct the immune system to wipe out cancerous cells. Unfortunately, despite many years of research and truckloads of money invested, cancer vaccines have yet to emerge as a viable drug development strategy. With their added technology risk, I have cautioned against investing in these companies.

Yet at the same time, I find the approach inherently interesting. While small molecules -- and, more recently, monoclonal antibodies -- have been the most successful drugs in the cancer market, their effectiveness can be underwhelming. And that leaves the door wide open for a truly innovative approach to come along and dominate a market.

Seasoned biotech investors might scoff at optimism for cancer vaccines, but there has been enough recent clinical data to suggest that the field may be finally turning the corner. Today, I'll present three biotech companies working on cancer vaccines where there is, at the very least, clinical data suggesting that the agent is active against cancer.

Dendreon (NASDAQ:DNDN) has reported positive results from a controlled phase 3 trial using its drug Provenge in the treatment of prostate cancer. In men with metastatic prostate cancer, there was a statistically significant improvement in patient survival for men treated with the drug, compared with those who received a placebo. Statistically significant survival benefits obtained in controlled clinical trials are the gold standard against which all cancer drugs are judged.

The company is currently enrolling patients in another phase 3 study, and the company has indicated that it hopes to have the drug approved by 2006. If that timing holds up, Dendreon will be the first mover in the cancer vaccine field.

Cell Genesys (NASDAQ:CEGE) is right on Dendreon's heels with its GVAX prostate cancer vaccine. A phase 3 trial began in the second half of 2004, and a second phase 3 trial in prostate cancer patients will start soon. Though Cell Genesys is behind Dendreon in this regard, it does have results from a phase 2 trial that suggest the drug has some benefit in treating prostate cancer.

In a phase 2 trial, the company reported that patients receiving GVAX had a median survival of 26.2 months. There was no control arm for comparison purposes, but the historical average for patients using the standard of care is about 18 months. While there appears to be a meaningful benefit, it still needs to be confirmed in phase 3 trials. It is important to remember that comparing results from an uncontrolled trial to historical figures can be misleading and we shouldn't assume that a drug works from this type of data. In my view, it is simply suggestive of a therapeutic effect.

Cell Genesys also has a deep pipeline with other active cancer vaccine programs in lung, leukemia, and pancreatic cancers. Some of these other vaccines also have preliminary indicators of efficacy in clinical trials, so anyone interested in the cancer vaccine space should take a look.

Also of interest is a small Canadian biotech company called Biomira (NASDAQ:BIOM). The company has had a rocky year with the failure of its breast cancer vaccine, Theratope, and the subsequent termination of a partnership for this drug by its partner Merck KGaA.

Despite that setback, not all is doom and gloom for Biomira, since the company recently reported interesting results from a controlled phase 2 study of its non-small-cell lung cancer (NSCLC) vaccine BLP25. This study enrolled patients with stage IIIB/IV lung cancer, a late-stage lung cancer where the prognosis is poor.

The company reported that patients receiving the BLP25 vaccine had a median survival 4.4 months longer than patients receiving only supportive care, although this was not a statistically significant result because of its small sample size. Overall, results are mixed, but there is reason for optimism for patients with stage IIIb NSCLC: 60% of these patients were still living two years after treatment, a statistically significant improvement over supportive care. However, it is crucial to note that this is not a definitive result because you cannot pull a subgroup out of a trial where the overall population did not achieve statistical significance. The FDA does not like that at all and would never approve a drug on this data alone.

That caveat out of the way, I think the data is interesting, is potentially clinically meaningful, and makes BLP25 worthy of further study. Biomira and Merck KGaA will conduct a confirmatory phase 3 study in patients with stage IIIB NSCLC. That trial is set to start later this year.

Final thoughts
Biotech companies developing cancer vaccines have been in investors' doghouses for a long time. These companies tend to have valuations much lower than companies developing small-molecule or monoclonal antibody drugs for cancer. Much of the skepticism toward these companies is warranted given the lack of a successful cancer vaccine.

But with the clinical data from the three companies above, it is possible that the trend is turning and perhaps we will see a cancer vaccine on the market in the coming years. I certainly think there is a lot left to prove and the ongoing controlled phase 3 trials will go a long way toward providing definitive answers. For now though, there are enough positives here to warrant the market's attention, though not necessarily the investment dollar.

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Motley Fool Rule Breakers biotech analyst Charly Travers owns shares of Cell Genesys. The Motley Fool has a disclosure policy.