Updated with new analysis and stock price information. CAMBRIDGE, Mass. ( TheStreet) -- An experimental drug from Aveo Pharmaceuticals ( AVEO) delayed tumor growth in patients with advanced kidney cancer more effectively than Onyx Pharmaceuticals' ( ONXX) currently approved drug Nexavar, according to results from a late-stage study released Tuesday. Aveo shares dropped in early trading, however, on concerns that the clinical benefit of tivozanib may not be as robust as expected. Aveo shares are down 13% to $14.95 in Tuesday pre-market trading. The stock closed 2011 at $17.20. Aveo said it plans to seek U.S. approval for its kidney cancer drug tivozanib this year. The roughly $2 billion worldwide kidney cancer drug market is crowded, but Aveo is hoping that tivozanib can grab valuable share from Onyx and even market leader Pfizer ( PFE) based on the positive results from the phase III study. Tuesday's phase III study, dubbed TIVO-1, was somewhat unusual in that it pitted the experimental tivozanib head-to-head against Onyx's Nexavar. Testing an experimental drug against an active comparator rather than a placebo can be a risky proposition, but for Aveo, the bet paid off. Treatment with tivozanib delayed the progression of cancer by a median of 11.9 months compared to a median 9.1 months for Nexavar. Both numerically and statistically, the results from TIVO-1 demonstrated that tivozanib was superior to Nexavar, Aveo said. The study enrolled 517 patients with advanced kidney cancer. In the pre-specified subgroup of kidney cancer patients previously untreated with drugs, treatment with tivozanib delayed tumor progression by a median 12.7 months compared to 9.1 months for Nexavar. While the TIVO-1 study met its primary endpoint, the roughly three-month difference in clinical benefit between tivozanib and Nexavar was relatively small and shorter than expected, largely because Nexavar performed better than it has in previous clinical studies. Tivozanib, Nexavar and Pfizer's Sutent are all oral drugs that turn off the same signaling pathway cancers use to grow. Aveo developed tivozanib to be more selective in its ability to target and turn off this key cancer growth pathway. Aveo chose not to test tivozanib directly against Sutent but the 11.9-month delay in tumor progression observed in TIVO-1 tops the 11 months demonstrated by Sutent in its separate pivotal study. Sutent is currently the most prescribed therapy for advanced kidney cancer patients. Whether or not doctors reach for tivozanib over Sutent may depend largely on the relative safety and tolerability of the two drugs. Previous studies have shown tivozanib to have a more benign side effect profile than Sutent. Last February, Aveo licensed global marketing rights to tivozanib to Japanese drugmaker Astellas. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein. >To submit a news tip, send an email to: email@example.com. Follow TheStreet on Twitter and become a fan on Facebook.