Cocrystal Pharma (COCP) said Tuesday it plans to extend enrollment in a phase 2a influenza challenge study due to low influenza infection among study participants, making virology results uninterpretable.

The study evaluating Cocrystal's investigational inhibitor CC-42344 has shown no serious adverse events so far, the company said.

Cocrystal Pharma shares were down 32% in recent premarket trading.