Scientists are developing a experimental cancer drug that shows early promise for advanced cases of melanoma skin cancer, one of the most aggressive cancers. 

How does the drug work? It activates the immune system, strengthening it to fight off cancer cells. 

Researchers identified this correlation during an early stage trial of just 31 patients. As the results did show promise, experts are cautiously optimistic. The drug's side effects were manageable, and four patients saw their tumors shrink, but more studies need to be done. 

The trial studies a small number of people but a trial like this is largely aimed at seeing whether a drug is safe and finding a tolerable dose. "Any time you see some responders in an early study, it's encouraging," said Dr. Jeffrey Weber, a melanoma expert at the Moffitt Cancer Center in Florida, who was not involved in the research.

In the end, there were four patients who showed a "partial response" to the drug, which meant their tumors shrunk. One patient continued to see a regression with further treatment, and is still stable after more than 10 months, according to Middleton.

The most common side effects were rash, fever and "tumor flare" -- swelling and tenderness at the site of a tumor. Two of four patients who got the highest drug dose did have an immediate drop in blood pressure, so the researchers have since set the maximum dose below that.

Much more remains to be learned about the drug -- known for now as IMCgp100. It is good enough to show some activity in an early stage trial. It's definitely worth pursuing, researchers noted.

Immunotherapies for Treating Cancer

Immunotherapy is an up and coming solution for treating cancer. Unlike chemotherapy and radiation which kills good cells along with cancer cells, immunotherapy boosts the immune system and "smartens" it to identify the cancer cells from good T cells. 

IMCgp100 is just one of several "immunotherapies" under study for advanced melanoma. The general idea behind immunotherapy is to help the body's immune system do a better job of recognizing and killing off cancer cells. IMCgp100 targets melanoma in two ways: It attaches to a specific, tiny protein found on some melanoma cells, and it activates nearby T-cells to attack the tumor. The catch is that a patient's cancer must be positive for that protein, called HLA A2 -- which is only in 45% of patients with melanoma.

Melanoma: A Deadly Cancer

Melanoma is the rarest and deadliest form of skin cancer. If caught early, it's curable with surgery but once it spreads to distant lymph nodes or other organs (Stage 4 Cancer) the disease becomes incredibly difficult to treat. 

In the past few years, researchers have made a lot progress towards finding treatments for this aggressive cancer. In 2011, the United States and Europe approved an immunotherapy drug called ipilimumab (Yervoy) -- the first treatment shown to prolong the lives of some patients with advanced melanoma. Still, only a minority of patients respond to the drug, and it can cause severe side effects -- including life-threatening inflammation of the liver or digestive tract.

The 31 patients in this study were all HLA A2-positive, and all but one had stage 4 melanoma. They were split into eight groups and given different doses of IMCgp100. Patients who were able to tolerate the first infusion received six more weekly treatments.

In the United States, about 76,000 people will be diagnosed with melanoma this year, and 9,700 will die of the disease, according to the American Cancer Society. While melanoma is relatively uncommon, its incidence has been rising for the past few decades, the cancer society notes.