Aveo Pharmaceuticals Inc released data that their new investigational drug, tivozanib, showed improved rates of  progression-free survival (PFS) in patients with advanced renal cell carcinoma (RCC) than the already-approved sorafenib (Nexavar), yet stocks fell as the outlook for the new treatment failed to meet investor expectations.

The TIVO-1 study, an international, randomized phase III clinical trial, compared tivozanib with sorafenib in patients with advanced RCC. Five-hundred and seventeen patients with RCC who had undergone a prior nephrectomy and had not been previously treated with either VEGF or mTOR therapy were included in the study. Patients who received tivozanib experienced a median PFS of 11.9 months versus 9.1 months for patients in the sorafenib group.

Patients who had received no prior anticancer therapy experienced a median PFS of 12.7 months versus 9.1 months in the sorafenib group. Aveo reported that this subgroup represented approximately 70% of the total study population. The study also showed that tivozanib has a well-tolerated safety profile; hypertension was the most common side effect.

“We are very pleased by these results, especially the PFS benefit demonstrated in the treatment-naïve population, which represents the most significant market opportunity for tivozanib,” said Tuan Ha-Ngoc, president and chief executive officer of Aveo, in a statement released on Tuesday.

Despite performing better than sorafenib, stocks for Aveo fell on Tuesday morning after the results of TIVO-1 were announced. According to various reports, analysts were hoping that tivozanib would show PFS in the 12 to14 months range. The observed PFS in the TIVO-1 study fell just short of the lower end of expectations.

However, Ha-Ngoc said in a press conference on Tuesday morning that Aveo was moving forward with plans to hire more employees–including increasing its sales force in light of these results–and plans to seek approval of the drug in the United States.

According to the National Cancer Institute, RCC is the most common type of kidney cancer in adults. There were 60,920 new cases of kidney cancer diagnosed in the US in 2011, and 13,120 kidney cancer-related deaths were reported last year.