(Reuters) - Cancer drug applications at the U.S. Food and Drug Administration are rising, with 20 submissions expected this year, as a better understanding of the molecular makeup of the disease leads to new treatments.

Some of the novel techniques that are proving to be successful include targeting specific gene mutations in tumors and harnessing the body's own immune system to seek out and kill cancer cells.

"There are a large number of drugs being developed in oncology," said Dr. Richard Pazdur, head of the FDA's office of oncology products. "There is greater understanding of some of the disease processes."

Last year, 10 out of 30 new drugs approved by the FDA were for treatment of cancer.

"This year we expect over 20 oncology applications will be filed," Pazdur said in an interview at a meeting of the American Society of Clinical Oncology in Chicago.

In terms of approvals, he said some will have decision deadlines that fall into next year or could take longer than expected to review due to issues such as manufacturing, while some may not be approved at all.

So far this year, cancer drugs approved by the FDA include Roche's Erivedge for basal cell carcinoma; Pfizer's Inlyta for kidney cancer; GlaxoSmithKline's Votrient for soft tissue sarcoma; and Leo Pharma's Picato for actinic keratosis.

The agency is slated to decide by Friday on Roche's application for pertuzumab, an antibody designed for use in the 25 percent or so of breast cancer patients whose tumors generate a protein called HER-2, which can fuel cancer growth.

Over the past year, the FDA has cleared several cancer drugs ahead of its legislated deadline.

"It is much easier to approve drugs that have greater efficacy," Pazdur said. "Our staff is interested getting the drugs out earlier ... it has to be drug that we really think is important."

BETTER SURVIVAL

Oncology drugs are unique in that the regulatory emphasis is on effectiveness rather than safety because patients are often so gravely ill.

"In oncology, the greatest difficulty has been the efficacy issue," Pazdur said. "We are starting to see new drugs with important survival advantages."

That does not mean, however, that every cancer drug to go before the FDA is a slam-dunk if a clinical trial demonstrates a statistically significant benefit for patients.

The issue is particularly sensitive for drug trials designed to show an improvement in "progression-free survival," defined as the length of time a patient lives without their cancer getting worse. Cancer drug trials with PFS as their goal are usually much shorter than studies looking at the rate of overall survival for patients treated with an experimental drug.

"It isn't just statistical significance. That gets you in the door," Pazdur said. "You could overpower the trial -- and we have seen it -- to have a trivial improvement in PFS."

The FDA last year revoked its conditional approval of Roche's Avastin as a treatment for breast cancer because, although there was evidence that it slowed progression of the disease, there was no conclusive data showing that it extended the lives of breast cancer patients. The drug is still approved for glioblastoma, colorectal, lung and kidney cancers.

Pazdur said the FDA is chiefly focused on the magnitude of improvement demonstrated in a clinical trial.

"There is a big difference between a 10 percent improvement in PFS and a 60 percent improvement," Pazdur said.

He also said there is now more emphasis at the FDA on meeting with drug developers earlier and more frequently.

"When we are seeing drugs with a high degree of efficacy -- so-called 'breakthrough' drugs -- it requires a different interaction with the companies," Pazdur said. "We may change the registration strategy or the size of the trial."