By JENNIFER CORBETT DOOREN

Cancer-drug shortages in the U.S. have caused hundreds of clinical trials to be stopped or delayed, threatening progress on new treatments,  a top health official told Congress Friday.

Howard Koh, assistant secretary of health for the Department of Health and Human Services, said drug shortages are "dramatically affecting clinical trials." Dr. Koh testified Friday before the House Energy and Commerce Subcommittee on Health during a hearing on drug shortages.

"The inability to obtain adequate supplies of these cancer drugs for research has resulted in promising clinical trials being suspended indefinitely and patient enrollment being abruptly halted," Dr. Koh said in written testimony. More than 300 clinical studies being paid for by the National Cancer Institute involve a drug that is in short supply, according to the testimony.

While the drug shortages continue to be a problem for patients, doctors are concerned the delay in cancer clinical trials will affect new and future treatments.

"The next generation of cancer therapy is driven by today's clinical trials," said Robert DiPaola, director of the Cancer Institute of New Jersey, in testimony prepared for Friday's hearing.

He explained that experimental cancer drugs are often added to existing cancer treatments in clinical studies in order to test the new drug.

"During a clinical trial, a shortage of only a few weeks in an existing drug might mean delays of years for the development of new drugs," Dr. DiPaola said.

The FDA reported a record 178 drug shortages in 2010, and Dr. Koh said there is "an even greater" number of shortages this year.

Most of the shortages involve older drugs administered by injection or intravenously. Along with cancer drugs, they also include antibiotics to treat infections and nutritional drugs for patients who can't eat. They are mainly generic, not highly profitable and are now made by only one or two companies. Teva Pharmaceutical Industries Ltd. and Hospira Inc. are two of the bigger producers of generic drugs.

The shortages are growing more severe, in part because of industry consolidation and manufacturing problems in the past year. When one company runs into a manufacturing problem with a product or decides to quit making a drug, competing companies can't quickly fill the void. In April, Teva reopened a California plant that it had shut down voluntarily for about a year, in part to retool to meet Food and Drug Administration manufacturing guidelines.

The cancer-drug shortage involves chemotherapy drugs that were developed decades ago. They are still the backbone of cancer treatment as newer drugs are typically added to chemotherapy. Some of the drugs that have been in short supply include doxorubicin, often used to treat breast cancer, and cytarabine, a leukemia drug that has no substitute.

The shortages of key cancer and other critical-care drugs used in emergency rooms and intensive-care units have caused the majority of hospitals to ration drugs this year, according to doctors and industry surveys. Hospitals and clinics have reported delaying cancer treatment or switching to an alternative drug that might not be as effective.