AMSTERDAM ( TheStreet) -- With promising early-stage study results presented this weekend, AstraZeneca's ( AZN) experimental lung cancer drug AZD9291 looks to be a potent tumor-shrinking competitor for Clovis Oncology's ( CLVS) CO-1686. The AstraZeneca and Clovis drugs are both designed to be effective against non-small cell lung cancer that no longer responds to currently approved therapies because of a genetic mutation known as T790M. There are no drugs currently approved for T790M-positive lung cancer, which affects approximately 15,500 people in the U.S. The mutation is more prevalent in Asian lung cancer patients, according to Stifel analyst Brian Klein. Preliminary results from an ongoing phase I study of AstraZeneca's AZD9291 demonstrated partial tumor shrinkage in 12 of 26 patients, or a response rate of 46%. Twelve of these patients had tumors that carried the T790M mutation and seven reported partial tumor shrinkage -- a response rate of 58% in T790M-positive tumors. A mild, grade 1 rash was the most commonly reported side effect, but did not lead to any treatment discontinuations. AstraZeneca presented the AZD9291 study results at the European Cancer Congress (ECCO) 2013. The study is enrolling additional patients and higher doses of AZD9291 are being tested. "If the partial responses seen for AZD9291 in NSCLC patients with T790M are shown to be durable, and the data can be reproduced in a multi-center setting then I would not be surprised if AZD9291 obtained breakthrough status from the FDA," said Pieter Droppert of the Biotech Strategy Blog, who is in Amsterdam covering ECCO 2013 with collaborator and noted cancer drug analyst Sally Church. Last June, Clovis shares doubled to $75 on the presentation of early-stage data on CO-1686 in which three of four patients with T790M-positive lung cancer reported partial tumor shrinkage. The CO-1686 phase I study continues with updated results expected to be presented at a lung cancer meeting at the end of October. More recently, Clovis shares have fallen to $60.59 on unconfirmed reports that the company put itself up for sale but could not find a buyer. Still, Clovis shares have more than tripled in value this year. The company is also developing a second drug targeted against breast and ovarian cancer. -- Reported by Adam Feuerstein in Boston. Follow Adam Feuerstein on Twitter.