(Corrects to remove reference to Novartis' ceritinib in paragraph 7, which is not in the trial)

By Julie Steenhuysen

CHICAGO, June 1 (Reuters) - The National Cancer Institute in July will start enrolling patients in a clinical trial seeking to match the underlying genetic defect driving a person's tumor with one or more of 20 approved or experimental drugs targeting that gene.

The announcement, made at the American Society for Clinical Oncology meeting on Monday, is meant to use approved or experimental drugs to develop insights that will ultimately enable doctors to prescribe drugs based on the molecular cause of the cancer, rather than the organ in which it was originally discovered.

Overall, trial investigators plan to sequence the DNA of about 3,000 patients nationwide during the full course of the trial, known as NCI-MATCH. Of those, they plan to enroll about 1,000 patients in the various drug treatment arms in the trial.

"What we're trying to do is sequence their tumor for various cancer drivers," said Dr. Barbara Conley, associate director of the NCI's Cancer Diagnosis Program. "If they have that driver, they will be able to get the drug that was chosen to attack that driver."

Patients will be treated as long as their tumor shrinks or remains stable.

Drugs being used in NCI-MATCH include both approved and experimental agents that are being contributed by a number of companies.

Initial agents in the trial will include Pfizer Inc's lung cancer drug crizotinib, sold as Xalkori; Boehringer Ingelheim's afatinib, sold as Gilotrif, Roche's TDM1 drug Kadcyla and two investigational drugs: AstraZeneca's AZD9291, a drug being tested in non-small cell lung cancer, and Verastem's VS-6063 or defactinib, a drug being tested in mesothelioma.

Enrollment in NCI-MATCH will be available through 2,400 sites participating in the National Clinical Trials Network.

Separately, ASCO said a trial called TAPUR would help patients gain access to new drugs based on the genetic makeup of their tumors.

It said TAPUR will provide 13 approved treatments for free. Drug companies providing treatments include: AstraZeneca, Bristol-Myers Squibb, Eli Lilly and Co, Roche's Genentech and Pfizer.

Genetic profiling gives doctors a read-out of molecular targets driving a patient's tumors. Often these occur in locations other than the organ in which a drug is approved. While doctors can prescribe approved drugs off-label, insurance companies are reluctant to pay for them.

(Reporting by Julie Steenhuysen; Editing by W Simon and Christian Plumb)