On April 18, 2019, David P. Southwell notified Spero Therapeutics, Inc. of his decision to step down from the Company's Board of Directors effective as of April 25, 2019. Mr. Southwell is stepping down from the Board as a result of contractual obligations he has taken on in conjunction with his appointment as CEO of another operating company. Mr. Southwell's decision to resign is not the result of any disagreement with the Company on any matter relating to the Company's operations, policies or practices.