# EDGAR Filing Document

**Accession Number:** 0001701108
**File Stem:** 0001193125-26-182256
**Filing Date:** 2026-4
**Character Count:** 5060
**Document Hash:** 64691fc2942d37dafb771e628ceb0f47
**Contains OCR:** False
**Source Format:** 

## Filing Content

## Filing Summary
**0001193125-26-182256.hdr.sgml**: 20260427

**ACCESSION NUMBER**: 0001193125-26-182256

**CONFORMED SUBMISSION TYPE**: DEFA14A

**PUBLIC DOCUMENT COUNT**: 4

**FILED AS OF DATE**: 20260427

**DATE AS OF CHANGE**: 20260427

**FILER**: 

**COMPANY DATA:**
- **COMPANY CONFORMED NAME:** Spero Therapeutics, Inc.
- **CENTRAL INDEX KEY:** 0001701108
- **STANDARD INDUSTRIAL CLASSIFICATION:** PHARMACEUTICAL PREPARATIONS [2834]
- **ORGANIZATION NAME:** 03 Life Sciences
- **EIN:** 000000000
- **STATE OF INCORPORATION:** DE
- **FISCAL YEAR END:** 1231

**FILING VALUES:**
- **FORM TYPE:** DEFA14A
- **SEC ACT:** 1934 Act
- **SEC FILE NUMBER:** 001-38266
- **FILM NUMBER:** 26901630

**BUSINESS ADDRESS:**
- **STREET 1:** 675 MASSACHUSETTS AVENUE
- **STREET 2:** 14TH FLOOR
- **CITY:** CAMBRIDGE
- **STATE:** MA
- **ZIP:** 02139
- **BUSINESS PHONE:** 857-242-1600

**MAIL ADDRESS:**
- **STREET 1:** 675 MASSACHUSETTS AVENUE
- **STREET 2:** 14TH FLOOR
- **CITY:** CAMBRIDGE
- **STATE:** MA
- **ZIP:** 02139

**FORMER COMPANY:**
- **FORMER CONFORMED NAME:** Spero Therapeutics, LLC
- **DATE OF NAME CHANGE:** 20170316

**UNITED STATES** 

**SECURITIES AND EXCHANGE COMMISSION** 

**WASHINGTON, D.C. 20549** 

**SCHEDULE 14A** 

**Proxy Statement Pursuant to Section 14(a) of the** 

**Securities Exchange Act of 1934** 

**(Amendment No.)** 

Filed by the Registrant ☒ Filed by a Party other than the Registrant ☐

Check the appropriate box:

☐ Preliminary Proxy Statement

☐ **Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))**

☐ Definitive Proxy Statement

☒ Definitive Additional Materials

☐ Soliciting Material Under §240.14a-12

## Spero Therapeutics, Inc.
**(Name of Registrant as Specified in its Charter)** 

**(Name of Person(s) Filing Proxy Statement, if other than the Registrant)** 

Payment of Filing Fee (Check the appropriate box all boxes that apply):

☒ No fee required.

☐ Fee paid previously with preliminary materials.

☐ Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-1.

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![LOGO](g120525dsp01.jpg)

You invested in SPERO THERAPEUTICS, INC. and it's time to vote!

You have the right to vote on proposals being presented at the Annual Meeting. **This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on June 23, 2026.**

Get informed before you vote

View the Notice and Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to June 9, 2026. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy.

![LOGO](g120525dsp01a.jpg)

\*Please check the meeting materials for any special requirements for meeting attendance.

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**Vote at www.ProxyVote.com** 

**THIS IS NOT A VOTABLE BALLOT** 

This is an overview of the proposals being presented at the

upcoming stockholder meeting. Please follow the instructions on

the reverse side to vote these important matters.

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| | | | |
|:---|:---|:---|:---|
| **Voting Items** | **Voting Items** | **Board**<br> **Recommends** | **Board**<br> **Recommends** |
| 1. | To elect two Class III directors to serve until the Company's 2029 Annual Meeting of Stockholders or until their respective successors are elected and qualified. |  |  |
|  | **Nominees:** |  |  |
| 1a. | Milind Deshpande, Ph.D. | ![LOGO](g120525dsp02.jpg) | **For** |
| 1b. | Kathleen Tregoning | ![LOGO](g120525dsp02.jpg) | **For** |
| 2. | To ratify the appointment of PricewaterhouseCoopers LLP as the Company's independent registered public accounting firm for the fiscal year ending December 31, 2026. | ![LOGO](g120525dsp02.jpg) | **For** |
| 3. | To approve by an advisory vote the compensation of the Company's named executive officers, as disclosed in the proxy statement. | ![LOGO](g120525dsp02.jpg) | **For** |
| 4. | To approve an amendment to the Company's Amended and Restated Certificate of Incorporation to increase the total number of shares of common stock authorized for issuance thereunder from 120,000,000 shares to 240,000,000 shares. | ![LOGO](g120525dsp02.jpg) | **For** |
| 5. | To approve adoption of the 2026 Spero Therapeutics, Inc. Stock Incentive Plan. | ![LOGO](g120525dsp02.jpg) | **For** |
| &nbsp;&nbsp; **NOTE:** To transact such other business that is properly presented at the annual meeting and any adjournments or postponements thereof. | &nbsp;&nbsp; **NOTE:** To transact such other business that is properly presented at the annual meeting and any adjournments or postponements thereof. |  |  |

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