UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

(Amendment No.    )

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☐	Preliminary Proxy Statement
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☐	Definitive Proxy Statement
☒	Definitive Additional Materials
☐	Soliciting Material Pursuant to § 240.14a-12

CALITHERA BIOSCIENCES, INC.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement if Other Than the Registrant)

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Important Notice of Availability of Proxy Materials for the Stockholder Meeting of CALITHERA BIOSCIENCES, INC. To Be Held On: June 11, 2021 at 9:00 am Pacific Time via the Internet please visit https://web.lumiagm.com/282387153 (password: cala2021) COMPANY NUMBER ACCOUNT NUMBER CONTROL NUMBER This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. We encourage you to access and review all of the important information contained in the proxy materials before voting. If you want to receive a paper or e-mail copy of the proxy materials you must request one. There is no charge to you for requesting acopy. To facilitate timely delivery please make the request as instructed below before May 28, 2021. Please visit http://ir.calithera.com/financial-information/annual-reports, where the following materials are available for view: Proxy Statement Form of Electronic Proxy Card Annual Report on Form 10-K TO REQUEST MATERIAL: TELEPHONE: 888-Proxy-NA (888-776-9962) 718-921-8562 (for international callers) E-MAIL: info@astfinancial.com WEBSITE: https://us.astfinancial.com/OnlineProxyVoting/ProxyVoting/RequestMaterials TO VOTE: ONLINE: To access your online proxy card, please visit www.voteproxy.com and follow the on-screen instructions or scan the QR code with your smartphone. You may enter your voting instructions at www.voteproxy.com up until 11:59 PM Eastern Time the day before the cut-off or meeting date. VIRTUALLY AT THE MEETING: The company will be hosting the meeting live via the Internet this year. To attend the meeting via the Internet please visit https://web.lumiagm.com/282387153 (password: cala2021) and be sure to have available the control number. MAIL: You may request a card by following the instructions above. 1. To elect the Board’s three nominees for director to serve until the 2024 Annual Meeting of stockholders or until their successors are selected. NOMINEES: Sunil Agarwal, M.D. Scott Garland Jean M. George Please note that you cannot use this notice to vote by mail. 2. To ratify the selection by the Audit Committee of the Board (the “Audit Committee”) of Ernst & Young LLP as the independent registered public accounting firm of the Company for the year ending December 31, 2021. 3. To approve, on an advisory basis, the compensation of the Company’s named executive officers, as disclosed in the proxy statement accompanying this notice. Note: To conduct any other business properly brought before the meeting. These items of business are more fully described in the Proxy Statement accompanying this Notice. The record date for the Annual Meeting is April 12, 2021. Only stockholders of record at the close of business on that date may vote at the meeting or any adjournment thereof. THE BOARD OF DIRECTORS RECOMMENDS A VOTE “FOR” THE ELECTION OF DIRECTORS AND “FOR” PROPOSALS 2 AND 3.