William Blair analyst John Sonnier views the 29% selloff Thursday in shares of Portola Pharmaceuticals as a buying opportunity. While the Phase III APEX trial evaluating betrixaban for the extended prophylaxis of venous thromboembolism prevention failed to meet the pre-specified level of statistical significance in the primary patient population, it was highly significant among all patients in the trial, Sonnier tells investors in a research note. The overall clinical benefit may support regulatory filing, the analyst contends. He says Portola remains undervalued despite the betrixaban setback based on its lead asset andexanet alfa alone. Sonnier keeps an Outperform rating on the name. Goldman Sachs this morning downgraded Portola to Neutral.