Cancer vaccines are a dead end, or at least that's what many analysts were thinking until recently. Amgen (AMGN 0.04%) just released some results that suggest the cancer vaccine field isn't finished, but maybe it just needs a slight twist.

Last fall, GlaxoSmithKline (NYSE: GSK) announced that its cancer vaccine had failed to extend disease-free survival compared to placebo in Stage 3 melanoma patients. What made the failure particularly depressing was that the trial involved MAGE-A3 positive patients, for whom the vaccine was specifically designed.

To be fair, Glaxo is no slouch when it comes to personalized medicine or melanoma. Earlier this year the company won an approval for a combination therapy specifically aimed at patients with a BRAF mutation. Despite the early failure of the MAGE-A3 vaccine, Glaxo will continue the melanoma trial in hopes that an even more specific set of patients will respond.

Something different
Glaxo's MAGE-A3 cancer immunotherapeutic works like most vaccines. It stimulates the immune system to attack tumor cells with a specific protein on their surface, in this case MAGE-A3.

Amgen's talimogene laherparepvec, or T-Vec, goes a step further. It's actually an active virus that infects tumor cells specifically. Once inside, it causes tumor cells to secrete a protein that starts a local immune response. Then it makes so many copies of itself that the cancer cell bursts. Those copies circulate about looking for more cancer cells and repeat the process.

It's working
What's great about T-Vec is that, like any virus, it should spread and be difficult to combat – and so far that is precisely what Amgen’s data indicates. Investigators measured thousands of melanoma lesions on stage 3B/C and stage 4 melanoma patients injected with T-Vec.

What they found was pretty amazing. Nearly two-thirds of lesions injected with T-Vec shrank by 50% or more. The virus also spread, and about one-third of tumors that weren’t injected also reduced by half or more. The side effects were exactly what you would expect from the immune system chewing up lots of infected cells -- inflammation and fever.

A perfect match?
T-Vec seems like it does a pretty good job on its own. Now imagine pairing it with a therapy that releases the brakes tumors use to shut down local immune responses.

Merck (MRK 0.09%) and Amgen intend to find out. Last month the two companies announced an agreement to test a combination of T-Vec and MK-3475, Merck's superstar immunotherapy candidate. Its late stage results have been so promising that investors have looked past declining earnings figures and pumped Merck's shares up nearly 30% this year.

What can go wrong
The pair should complement each other nicely, at least in theory. But If there's one thing to know about this industry, it’s that clinical results are full of surprises. Cancer treatments that are overly effective at bursting tumor cells can cause life-threatening conditions. Amgen noted that T-Vec alone caused skin inflammation and fever at a "serious" level. Cutting the brake-line with MK-3475 might leave the immune response speeding off a cliff. A safety and tolerability study of the combination is slated for this fall, and investors should keep an eye open for the results. 

Final take
The excitement surrounding anti-PD-1 immunotherapies like MK-3475 has taken a lot of attention from oncolytic immunotherapies like T-Vec. Amgen's latest results might lead laboratories across the industry to dust off similar programs, but for now, Amgen's virus is leading the field. If it iseffective in combination with other immunotherapies, late stage cancer patients will have some exciting new treatment options, and Amgen may have a healthy new revenue stream for the long term.