UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

(Rule 13d-102)

Information Statement Pursuant to Rules 13d-1 and 13d-2

Under the Securities Exchange Act of 1934

(Amendment No. 1)*

Sutro Biopharma Inc.

(Name of Issuer)

Common stock, par value $0.001 per share

(Title of Class of Securities)

869367102

(CUSIP Number)

December 31, 2018

(Date of Event Which Requires Filing of the Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

¨ Rule 13d-1(b)

x Rule 13d-1(c)

¨ Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 869367102 13G/A Page 2 of 10 Pages

1.	NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Citadel Advisors LLC
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)       ¨ (b)       ¨
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5.	SOLE VOTING POWER 0
	6.	SHARED VOTING POWER  1,728,345 shares
	7.	SOLE DISPOSITIVE POWER 0
	8.	SHARED DISPOSITIVE POWER See Row 6 above

9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON See Row 6 above
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES   ¨
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 7.6%1
12.	TYPE OF REPORTING PERSON IA; OO; HC

¹ The percentages reported in this Schedule 13G/A are based upon 22,848,184 shares of common stock outstanding as of November 9, 2018 (according to the Form 10-Q filed by the issuer with the Securities and Exchange Commission on November 14, 2018).

CUSIP No. 869367102 13G/A Page 3 of 10 Pages

1.	NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Citadel Advisors Holdings LP
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)       ¨ (b)       ¨
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5.	SOLE VOTING POWER 0
	6.	SHARED VOTING POWER  1,728,345 shares
	7.	SOLE DISPOSITIVE POWER 0
	8.	SHARED DISPOSITIVE POWER See Row 6 above

9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON See Row 6 above
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES   ¨
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 7.6%
12.	TYPE OF REPORTING PERSON PN; HC

CUSIP No. 869367102 13G/A Page 4 of 10 Pages

1.	NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Citadel GP LLC
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)       ¨ (b)       ¨
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5.	SOLE VOTING POWER 0
	6.	SHARED VOTING POWER  1,728,345 shares
	7.	SOLE DISPOSITIVE POWER 0
	8.	SHARED DISPOSITIVE POWER See Row 6 above.

9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON See Row 6 above.
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES   ¨
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 7.6%
12.	TYPE OF REPORTING PERSON OO; HC

CUSIP No. 869367102 13G/A Page 5 of 10 Pages

1.	NAME OF REPORTING PERSON S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON Kenneth Griffin
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)       ¨ (b)       ¨
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION U.S. Citizen

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH	5.	SOLE VOTING POWER 0
	6.	SHARED VOTING POWER  1,729,043 shares
	7.	SOLE DISPOSITIVE POWER 0
	8.	SHARED DISPOSITIVE POWER See Row 6 above

9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON See Row 6 above
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES   ¨
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9) 7.6%
12.	TYPE OF REPORTING PERSON IN; HC

CUSIP No. 869367102 13G/A Page 6 of 10 Pages

Item 1(a) Name of Issuer

Sutro Biopharma Inc.

Item 1(b) Address of Issuer’s Principal Executive Offices

310 Utah Avenue, Suite 150, South San Francisco, California 94080

Item 2(a) Name of Person Filing

This Schedule 13G/A is being jointly filed by Citadel Advisors LLC (“Citadel Advisors”), Citadel Advisors Holdings LP (“CAH”), Citadel GP LLC (“CGP”) and Mr. Kenneth Griffin (collectively with Citadel Advisors, CAH and CGP, the “Reporting Persons”) with respect to shares of common stock of the above-named issuer owned by Citadel Multi-Strategy Equities Master Fund Ltd., a Cayman Islands limited company (“CM”), and Citadel Securities LLC, a Delaware limited liability company (“Citadel Securities”).

Citadel Advisors is the portfolio manager for CM. CAH is the sole member of Citadel Advisors. CGP is the general partner of CAH. CALC III LP, a Delaware limited partnership (“CALC3”), is the non-member manager of Citadel Securities. Citadel Securities GP LLC, a Delaware limited liability company (“CSGP”), is the general partner of CALC3. Mr. Griffin is the President and Chief Executive Officer of CGP, and owns a controlling interest in CGP and CSGP.

The filing of this statement shall not be construed as an admission that any of the Reporting Persons is the beneficial owner of any securities covered by the statement other than the securities actually owned by such person (if any).

Item 2(b) Address of Principal Business Office

The address of the principal business office of each of the Reporting Persons is 131 S. Dearborn Street, 32nd Floor, Chicago, Illinois 60603.

Item 2(c) Citizenship

Each of Citadel Advisors and CGP is organized as a limited liability company under the laws of the State of Delaware. CAH is organized as a limited partnership under the laws of the State of Delaware. Mr. Griffin is a U.S. citizen.

Item 2(d) Title of Class of Securities

Common stock, $0.001 par value

Item 2(e) CUSIP Number

869367102

CUSIP No. 869367102 13G/A Page 7 of 10 Pages

Item 3 If this statement is filed pursuant to Rules 13d-1(b), or 13d-2(b) or (c), check whether the person filing is a:

(a) ¨    Broker or dealer registered under Section 15 of the Exchange Act;

(b) ¨    Bank as defined in Section 3(a)(6) of the Exchange Act;

(c) ¨    Insurance company as defined in Section 3(a)(19) of the Exchange Act;

(d) ¨    Investment company registered under Section 8 of the Investment Company Act;

(e) ¨    An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);

(f) ¨    An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);

(g) ¨    A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);

(h) ¨    A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act;

(i) ¨    A church plan that is excluded from the definition of an investment company under Section 3(c)(14) of the Investment Company Act;

(j) ¨ A non-U.S. institution in accordance with Rule 13d-1(b)(1)(ii)(J);

(k) ¨    Group, in accordance with Rule 13d-1(b)(1)(ii)(K).

If filing as a non-U.S. institution in accordance with Rule 13d-1(b)(1)(ii)(J), please specify the type of institution: ____________.

CUSIP No. 869367102 13G/A Page 8 of 10 Pages

Item 4 Ownership

A. Citadel Advisors LLC

(a) Citadel Advisors may be deemed to beneficially own 1,728,345 shares of common stock.

(b) The number of shares that Citadel Advisors may be deemed to beneficially own constitutes approximately 7.6% of the common stock outstanding.

(c) Number of shares as to which such person has:

(i) sole power to vote or to direct the vote: 0

(ii) shared power to vote or to direct the vote: 1,728,345

(iii) sole power to dispose or to direct the disposition of: 0

(iv) shared power to dispose or to direct the disposition of: 1,728,345

B. Citadel Advisors Holdings LP and Citadel GP LLC

(a) Each of CAH and CGP may be deemed to beneficially own 1,728,345 shares of common stock.

(b) The number of shares that each of CAH and CGP may be deemed to beneficially own constitutes approximately 7.6% of the common stock outstanding.

(c) Number of shares as to which such person has:

(i) sole power to vote or to direct the vote: 0

(ii) shared power to vote or to direct the vote: 1,728,345

(iii) sole power to dispose or to direct the disposition of: 0

(iv) shared power to dispose or to direct the disposition of: 1,728,345

CUSIP No. 869367102 13G/A Page 9 of 10 Pages

C. Kenneth Griffin

(a) Mr. Griffin may be deemed to beneficially own 1,729,043 shares of common stock.

(b) The number of shares that Mr. Griffin may be deemed to beneficially own constitutes approximately 7.6% of the common stock outstanding.

(c) Number of shares as to which such person has:

(i) sole power to vote or to direct the vote: 0

(ii) shared power to vote or to direct the vote: 1,729,043

(iii) sole power to dispose or to direct the disposition of: 0

(iv) shared power to dispose or to direct the disposition of: 1,729,043

Item 5 Ownership of Five Percent or Less of a Class
If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than 5 percent of the class of securities, check the following ¨.

Item 6 Ownership of More than Five Percent on Behalf of Another Person

Not Applicable

Item 7 Identification and Classification of the Subsidiary which Acquired the Security Being Reported on by the Parent Holding Company

See Item 2 above

Item 8 Identification and Classification of Members of the Group

Not Applicable

Item 9 Notice of Dissolution of Group

Not Applicable

Item 10 Certification
By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

CUSIP No. 869367102 13G/A Page 10 of 10 Pages

After reasonable inquiry and to the best of its knowledge and belief, the undersigned certify that the information set forth in this statement is true, complete and correct.

Dated this 14^th day of February, 2019.

CITADEL ADVISORS LLC		CITADEL ADVISORS HOLDINGS LP
By:	/s/ David Glockner	By:	/s/ David Glockner
	David Glockner, Authorized Signatory		David Glockner, Authorized Signatory
CITADEL GP LLC		KENNETH GRIFFIN
By:	/s/ David Glockner	By:	/s/ David Glockner
	David Glockner, Authorized Signatory		David Glockner, attorney-in-fact*

^* David Glockner is signing on behalf of Kenneth Griffin as attorney-in-fact pursuant to a power of attorney previously filed with the Securities and Exchange Commission, and hereby incorporated by reference herein. The power of attorney was filed as an attachment to a filing by Citadel Advisors LLC on Schedule 13G for Roku, Inc. on January 17, 2018.