X0306 4 2017-11-06 0 0001701108 Spero Therapeutics, Inc. SPRO 0001630850 Breum Casper C/O LUNDBECKFOND INVEST A/S SCHERFIGSVEJ 7 COPENHAGEN G7 DK-2100 DENMARK 1 0 0 0 Common Stock 2017-11-06 4 C 0 289102 A 289102 I Lundbeckfond Invest A/S Common Stock 2017-11-06 4 C 0 179450 A 468552 I Lundbeckfond Invest A/S Common Stock 2017-11-06 4 C 0 408937 A 877489 I Lundbeckfond Invest A/S Common Stock 2017-11-06 4 P 0 214285 14.00 A 1091774 I Lundbeckfond Invest A/S Series A Preferred Stock 2017-11-06 4 C 0 1153846 0.00 D Common Stock 289102 0 I Lundbeckfond Invest A/S Series B Preferred Stock 2017-11-06 4 C 0 681818 0.00 D Common Stock 179450 0 I Lundbeckfond Invest A/S Series C Preferred Stock 2017-11-06 4 C 0 2485272 0.00 D Common Stock 408937 0 I Lundbeckfond Invest A/S Reflects shares of the Issuer's common stock that were purchased in connection with the Issuer's initial public offering. Amount of underlying securities reflects the application of the applicable conversion ratio as set forth in the Issuer's Amended and Restated Certificate of Incorporation in effect immediately prior to the closing of the initial public offering (as adjusted by the 1-for-6.0774 reverse stock split of the Issuer's common stock effected on October 20, 2017). The preferred stock had no expiration date. Shares held by Lundbeckfond Invest A/S. The Reporting Person shares voting and investment power with respect to the shares held by Lundbeckfond Invest A/S and disclaims beneficial ownership of such shares except to the extent of his pecuniary interest therein. /s/ Matthew Tikonoff, Attorney-in-Fact for Casper Breum 2017-11-07