Syntara (ASX:SNT) said the US Food and Drug Administration (FDA) provided guidance that a phase 2 trial with a control arm be undertaken for its amsulostat drug candidate for the potential treatment of myelofibrosis to acquire additional safety and efficacy data, focusing on improvements in symptoms and spleen volume reductions, according to a Monday Australian bourse filing.

This data will be used to optimize the design and efficiency of a subsequent pivotal phase 3 trial, the filing said.

The regulator reviewed data from the ongoing open-label trial of amsulostat in combination with ruxolitinib, as well as a proposal for a pivotal registrational study.

The firm's shares plunged 47% in early trading on Monday.