EXHIBIT A

JOINT FILING AGREEMENT

The undersigned hereby agree that the statement on Schedule 13G with respect to the Common Stock, par value $0.001 per share, of Catalyst Biosciences, Inc., dated as of June 11, 2018 is, and any amendments thereto signed by each of the undersigned shall be, filed on behalf of each of us pursuant to and in accordance with the provisions of Rule 13d-1(k) under the Securities Exchange Act of 1934, as amended.

Date:June 11, 2018	NEXTHERA CAPITAL LP By: Nexthera Capital GP LLC, its general partner        By:  /s/ Daniel Malek            Name:  Daniel Malek            Title:    Managing Member
Date:June 11, 2018	NEXTHERA CAPITAL LP By: Nexthera Capital GP LLC, its general partner        By:  /s/ Daniel Malek            Name:  Daniel Malek            Title:    Managing Member
Date:June 11, 2018	DANIEL MALEK By:  /s/ Daniel Malek         Name:  Daniel Malek
Date:June 11, 2018	ORI HERSHKOVITZ By:  /s/ Ori Hershkovitz         Name:  Ori Hershkovitz