Document:

2017 Call Option Plan and forms of award agreements

 EXHIBIT 10.3 

SUTRO BIOPHARMA, INC. 

2017 CALL OPTION PLAN 

 SUTRO BIOPHARMA, INC. 

2017 CALL OPTION PLAN 
 1.
Definitions. As used herein, the following definitions shall apply: 
 (a) “Administrator” means the Board or his,
her or its successor, as appointed by the Board. 
 (b) “Board” means the Board of Directors of Sutro Biopharma. 

(c) “Call Option” means a right to Subject Shares, subject to vesting and granted pursuant to the Plan as described in
Section 5 hereof. 
 (d) “Cause” means Termination because of (a) Participant’s unauthorized misuse of Sutro
Biopharma’s trade secrets or proprietary information, (b) Participant’s conviction of or plea of nolo contendere to a felony or a crime involving moral turpitude, (c) Participant’s committing an act of fraud against Sutro
Biopharma or (d) Participant’s gross negligence or willful misconduct in the performance of his or her duties that has had or will have a material adverse effect on the Sutro Biopharma’s reputation or business. 

(e) “Code” means the Internal Revenue Code of 1986, as amended, or any successor statute or statutes thereto, including any
regulations and other guidance promulgated under any such statute. 
 (f) “Consultant” means a member of the Board or any
consultant or advisor if the consultant or adviser renders bona fide services to Sutro Biopharma. 
 (g) “Disability” means
that the Participant is unable to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment that can be expected to result in death or can be expected to last for a continuous period of not less
than 12 months. 
 (h) “Employee” means any person who is an employee (as defined under applicable law) of Sutro Biopharma.

 (i) “Exercise Price” means the exercise price per share of a Call Option as determined by the Administrator, which in no
case shall be less than 50% of the Fair Market Value of the SV Stock as determined by the Administrator, and documented in the corresponding Notice. 

(j) “Fair Market Value” means the fair market value per share of SV Stock, as determined by the Administrator. 

(k) “Notice” means the Notice of Call Option Grant which, together with this Plan, shall constitute a written agreement among
Participant and Sutro Biopharma, evidencing the terms and conditions of the grant of a Call Option. 
 (l) “Participant”
means an Employee or Consultant of Sutro Biopharma who is granted and continues to hold a Call Option. 

  
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 (m) “Plan” means this Sutro Biopharma 2017 Call Option Plan. 

(n) “SB Change in Control” means a change in ownership, change in effective control, or a change in the ownership of a
substantial portion of the assets of Sutro Biopharma as described in Treasury Regulation Section 1.409A-3(i)(5)(v), 1.409A-3(i)(5)(vi) and 1.409A-3(i)(5)(vii). 
 (o) “Subject Shares” means the shares of SV Stock issuable under
this Plan or otherwise underlying Call Options. 
 (p) “Sutro Biopharma” means Sutro Biopharma, Inc., a Delaware
Corporation and minority owner of SutroVax. 
 (q) “SutroVax” means SutroVax, Inc., a Delaware corporation partly-owned by
Sutro Biopharma. 
 (r) “SV Change in Control” means a change in ownership, change in effective control, or a change in the
ownership of a substantial portion of the assets of SutroVax as described in Treasury Regulation Section 1.409A-3(i)(5)(v), 1.409A-3(i)(5)(vi) and 1.409A-3(i)(5)(vii), but in all cases without regard as to whether SutroVax is a “relevant corporation” under 1.409A-3(i)(5)(ii). 

(s) “SV Stock” means the common stock of SutroVax, $0.001 par value per share. 

(t) “Termination” means, for purposes of this Plan with respect to a Participant, that the Participant has for any reason
ceased to provide services as an Employee or Consultant to Sutro Biopharma (ceased “Continuous Service Status”). A Participant will not be deemed to have ceased Continuous Service Status while the Participant is on a bona fide leave of
absence, if such leave was approved by the Sutro Biopharma in writing. In the case of an approved leave of absence, the Administrator may make such provisions respecting crediting of service, including suspension of vesting of the Call Option
(including pursuant to a formal policy adopted from time to time by Sutro Biopharma) it may deem appropriate, except (i) as required by applicable law and (ii) in no event may a Call Option be exercised after the expiration of the term set
forth in Section 5(c)(i) hereof. The Administrator will have sole discretion to determine whether a Participant has ceased Continuous Service Status and the effective date on which the Participant ceased Continuous Service Status (the
“Termination Date”). 
 2. Stock Subject to the Plan. Sutro Biopharma has reserved 450,000 Subject Shares, subject
to the terms and conditions of this Plan and the prior consent of the Board. The Subject Shares shall underlie the Call Options awarded to Participants pursuant to the Plan and shall be held for distribution to Participants pursuant to the exercise
of the Call Options in accordance with the terms of this Plan. Any unissued Subject Shares shall be retained by Sutro Biopharma. Subject to Section 6 hereof, Subject Shares that are cancelled, forfeited, settled in cash, used to pay withholding
obligations or pay the exercise price of a Call Option or that expire by their terms at any time will again be available for grant and issuance in connection with other Call Options. In the event that Subject Shares previously issued under the Plan
are reacquired by Sutro Biopharma pursuant to a forfeiture provision, right of first refusal, or repurchase by Sutro Biopharma, such Subject Shares shall be added to the number of Subject Shares then available for issuance under the Plan. 

  
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 3. Administration of the Plan. 

(a) Administrator. The Board shall have the right to appoint or replace the Administrator of the Plan. All decisions,
determinations and interpretations of the Administrator shall be final and binding on all Participants. 
 (b) Powers of the
Administrator. Subject to the provisions of the Plan, the Administrator shall have the broadest authority permitted under applicable laws in his, her or its sole discretion, to administer the Plan including but not limited to the following: 

(i) to determine the Fair Market Value for purposes of determining the applicable amount of tax on the Call Options (if any) or Subject
Shares distributed pursuant to the exercise of the Call Options; 
 (ii) to select the Employees and/or Consultants to whom Call
Options may be granted hereunder; 
 (iii) to determine the number of Subject Shares to be covered by each Call Option granted
hereunder and the vesting schedules thereof; 
 (iv) to approve forms of Notice for use under the Plan; 

(v) to determine the terms and conditions of any Call Option granted hereunder, including its Exercise Price; 

(vi) to prescribe, amend and rescind rules and regulations relating to the Plan; 

(vii) to make arrangements with Participants to satisfy withholding tax obligations under such conditions as the Administrator may
deem necessary or advisable; 
 (viii) to amend the Plan or Call Options granted under the Plan as provided in part by
Section 7 hereof; 
 (ix) to provide for such special terms as it may consider necessary or appropriate to accommodate
differences in local law, tax policy, or custom including approving supplements or amendments to the Plan; and 
 (x) to construe
and interpret the terms of the Plan and awards granted pursuant to the Plan and to exercise such powers and perform such acts as the Administrator deems necessary or desirable. 

4. Limitations. Call Options shall not confer upon a Participant any right with respect to continuing the Participant’s
employment, directorship or consulting relationship with Sutro Biopharma, nor shall they interfere in any way with the Participant’s right or the right of Sutro Biopharma to terminate such employment, directorship or consulting relationship at
any time, with or without cause. 

  
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 5. Call Options. 

(a) Grant of Call Options. 

(i) Call Options may be granted to Employees and/or Consultants. 

(ii) The Administrator may select in his, her or its discretion, the Employees and/or Consultants to whom Call Options may be granted
hereunder. Employees and/or Consultants receiving grants of Call Options shall be provided a Notice evidencing his or her Call Option. 

(iii) A Participant receiving a Call Option under a Notice shall be entitled to receive the number of Subject Shares underlying the
Call Option set forth in the Notice, subject to the terms and conditions of the Notice and the Plan. 
 (b) Vesting of Call
Options. The Administrator shall determine the vesting schedule of each Call Option, which shall be specified in the corresponding Notice. 

(c) Exercise and Distribution of Call Options. 

(i) Exercise of Call Options. Notwithstanding anything to the contrary in this Plan, each Call Option shall be exercisable as
to its then-vested portion until the earliest of (a) the end of the calendar year in which such vested portion vests, (b) such earlier time as required by Section 5(c)(iii) hereof or (c) the end of a period of such calendar year
designated for exercise of Call Options as determined by the Administrator and specified in the Notice (the “Exercise Window”). In all cases, the vested portion of a Call Option shall cease to be exercisable at 5:00 pm
(Pacific Standard Time) on December 31 of the calendar year in which such portion vests, or if December 31 is not a business day, on the last business day immediately prior to December 31. If any vested portion of a Call Option is not
timely exercised by the Participant, that unexercised portion of the Call Option shall expire and be forfeited without consideration. A Participant exercising a Call Option shall do so pursuant to the form of Call Option Exercise Notice and
Agreement attached hereto as Exhibit A. 
 (1) In the case of expiration and forfeiture of a portion of a Call Option,
subsequent vesting and exercisability of the remaining portions of such Call Option shall not be affected, provided that the Participant is in Continuous Service Status on the subsequent vesting dates in accordance the Call Option vesting schedule
as specified in the Notice. 
 (ii) Payment for Exercises. Payment for Subject Shares acquired pursuant to this Plan may be
made in cash (by check), wire transfer or where expressly approved for the Participant by the Administrator and where permitted by law: 

(1) by cancellation of indebtedness of Sutro Biopharma owed to the Participant; 

(2) by tender of a full recourse promissory note having such terms as may be approved by the Administrator and bearing interest at a
rate sufficient to avoid imputation of income under Sections 483 and 1274 of the Code; provided, however, that Participants who are not Employees or Consultants will not be entitled to purchase Subject Shares with a
promissory note unless the note is adequately secured by collateral other than the Subject Shares; 

  
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 (3) by waiver of compensation due or accrued to the Participant from Sutro Biopharma for
services rendered; 
 (4) by participating in a net exercise program implemented by the Administrator in connection with the Plan;

 (5) by any combination of the foregoing or any other method of payment approved by the Administrator. 

(iii) Termination. Subject to earlier termination pursuant to Sections 5(c)(i) and 7 hereof, exercise of a Call Option
will always be subject to the following terms and conditions. 
 (1) Other than Death or Disability or for Cause. If the
Participant ceases Continuous Service Status for any reason other than death, Disability or for Cause, then the Participant may exercise such Participant’s Call Options only to the extent that such Call Options have vested upon the Termination
Date or as otherwise determined by the Administrator. Such Call Options must be exercised by the Participant, if at all, as to all or some of the their vested portions calculated as of the Termination Date or such other date determined by the
Administrator, within three (3) months after the Termination Date (or within such shorter time period, not less than thirty (30) days, or within such longer time period after the Termination Date as may be determined by the Administrator),
but in any event no later than the expiration date of the Call Options as per Sections 5(c)(i)(a) and 5(c)(i)(c) hereof. 
 (2)
Death or Disability. If the Participant ceases Continuous Service Status because of Participant’s death or Disability (or the Participant dies within three (3) months after a Termination other than for Cause), then
Participant’s Call Options may be exercised only to the extent that such Call Options have vested by Participant on the Termination Date or as otherwise determined by the Administrator. Such Call Options must be exercised by Participant (or
Participant’s legal representative or authorized assignee), if at all, as to all or some of their vested portions calculated as of the Termination Date or such other date determined by the Administrator, within six (6) months after the
Termination Date, but in any event no later than the expiration date of the Call Options as per Sections 5(c)(i)(a) and 5(c)(i)(c) hereof. 

(3) For Cause. If the Participant ceases Continuous Service Status due to termination for Cause, the Participant may exercise
such Participant’s Call Options, but not to an extent greater than such Call Options have vested upon the Termination Date and Participant’s Call Options shall expire on such Participant’s Termination Date, or at such later time and
on such conditions as are determined by the Administrator, but in any event no later than the expiration date of the Call Options as per Sections 5(c)(i)(a) and 5(c)(i)(c) hereof. 

(iv) No Fractional Rights. No fractional Subject Shares or rights for fractional shares shall be issued under the Plan. 

(d) Acceleration of vesting of Call Options. Notwithstanding Section 5(b) above, upon either a SB Change in Control or an
SV Change in Control, 100% of each Call Option shall accelerate, and shall become exercisable as to the entirety of the Call Option, only on such terms and conditions as the Administrator and Sutro Biopharma may require each in its sole discretion.
The exercisability of Call Options accelerated pursuant to this Section shall expire upon the closing of the SB Change in Control or SV Change in Control, as applicable, or such earlier time as may be determined by the Administrator upon written
notice to the Participant. 

  
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 6. Adjustments upon Changes in Capitalization. In the event that any dividend or
other distribution (whether in the form of cash, SV Stock, other securities, or other property), recapitalization, reclassification, stock split, reverse stock split, reorganization, merger, consolidation, split-up,
spin-off, combination, repurchase, liquidation, dissolution, or sale, transfer, exchange or other disposition of all or substantially all of the assets of SutroVax, or exchange or other disposition of SV Stock
or other securities of SutroVax, issuance of warrants or other rights to purchase SV Stock or other securities of SutroVax, or other similar corporate transaction without consideration, the Administrator shall make adjustments to any Call Option to
the extent required by applicable law or may make adjustments as the Administrator otherwise determines, including without limitation, adjustment to any or all of (i) the number and kind of Subject Shares (or other securities or property) with
respect to which Call Options may be granted or awarded and (ii) the number and kind of Subject Shares (or other securities or property) subject to outstanding Call Options. 

7. Amendment and Termination of the Plan. The Administrator may at any time wholly or partially amend, alter, suspend or
terminate the Plan. Termination of the Plan shall not affect the Administrator’s ability to exercise the powers granted to it hereunder with respect to Call Options granted or awarded under the Plan prior to the date of such termination. Unless
earlier terminated as provided herein, this Plan will automatically terminate ten (10) years after the later of (i) the effective date of the Plan. 

8. Participant Agreements and Representations. 

(a) As a condition to the grant of a Call Option or delivery of Subject Shares pursuant to the exercise of a Call Option, Participant
agrees that (i) Participant accepts the grant of the Call Option subject to all of the terms set forth in the Notice and the Plan which will supersede in their entirety all prior undertakings, understandings and agreements of Sutro Biopharma
and Participant with respect to the subject matter hereof, (ii) the grant of the Call Option is made in lieu of and replaces in its entirety any promise, whether express or implied, or any agreement by Sutro Biopharma to grant Participant an
interest in SutroVax, (iii) that SutroVax is an intended third party beneficiary of Section 11 hereof, (iv) notwithstanding anything to the contrary in this Plan or the Notice, the Administrator and Sutro Biopharma are not required to
design, establish or administer the Plan in a manner which will result in a beneficial tax treatment to the Participant, (v) the Administrator and Sutro Biopharma shall not distribute the Subject Shares underlying a Call Option unless and until
the Participant makes arrangements acceptable to the Administrator and Sutro Biopharma for the payment of applicable taxes related to the transactions under this Plan, and (vi) Participant has been advised to consult his or her own tax advisor
regarding the tax consequences to him or her of the grant of the Call Option and is not relying on any statements or representations made by the Administrator or Sutro Biopharma or its employees or agents regarding the tax consequences to him or her
of the grant of the Call Option or distribution of Subject Shares pursuant to the exercise of the Call Option. 
 (b) The
Administrator or Sutro Biopharma may require a Participant, as a condition to the grant of a Call Option or distribution of Subject Shares pursuant to the exercise of a Call Option, to give such written representations as the Administrator or Sutro
Biopharma may require. 

  
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 9. Code Section 409A. The Call Options are intended to be exempt
from Section 409A of the Code as short-term deferral payments under U.S. Treasury Regulation Section 1.409A-1(b)(4). To the extent that any provision of this Plan is ambiguous as to its exemption or
compliance with Section 409A, the provision will be read in such a manner so that all payments hereunder are exempt from Section 409A to the maximum permissible extent, and for any payments where such construction is not tenable, that
those payments comply with Section 409A to the maximum permissible extent. Payments pursuant to this letter agreement (or referenced in this letter agreement) are intended to constitute separate payments for purposes of Section 1.409A-2(b)(2) of the regulations under Section 409A. 
 10. Tax Withholding and
Reporting. By accepting a Call Option, a Participant agrees to the reporting of taxable income as taxable compensation and the withholding of income and employment taxes by Sutro Biopharma, and to make arrangements, as necessary for the payment of
such taxes, as required by applicable local, state and federal tax laws, and the foreign tax laws applicable to the Participant. 

11. Transfer Restrictions. Except as permitted by the Administrator, Call Options granted under this Plan, and any interest
therein, will not be transferable or assignable by Participant, other than by will or by the laws of descent and distribution or by instrument to an inter vivos or testamentary trust in which the Call Options are to be passed to beneficiaries upon
the death of the trustor (settlor), or by gift to a “family member” as that term is defined in Rule 701 et seq. promulgated by the Securities and Exchange Commission under the Securities Act of 1933, as amended, and may not be made
subject to execution, attachment or similar process. For the avoidance of doubt, the prohibition against assignment and transfer applies to a Call Option and, prior to exercise, the shares to be issued on exercise of a Call Option, and pursuant to
the foregoing sentence shall be understood to include, without limitation, a prohibition against any pledge, hypothecation, or other transfer, including any short position, any “put equivalent position” or any “call equivalent
position” (in each case, as defined in Rule 16a-1 promulgated under the Securities Exchange Act of 1934, as amended). Unless a Call Option is transferred pursuant to the terms of this Section, during the
lifetime of the Participant, a Call Option will be exercisable only by the Participant or the Participant’s legal representative, and any elections with respect to a Call Option may be made only by the Participant or the Participant’s
legal representative. The terms of a Call Option shall be binding upon the executor, administrator, successors and assigns of the Participant who is a party thereto. 

12. Applicable Laws. The validity and enforceability of this Plan shall be governed by and construed in accordance with the laws
of the State of California without regard to otherwise governing principles of conflicts of law. Reference to any section of a foreign, federal or state regulation, statute or law herein shall include any successor section. 

13. Severability. If any provision of this Plan shall be held to be illegal, invalid or unenforceable under any applicable law,
then such contravention or invalidity shall not invalidate the entire Plan and the remainder of the provisions shall remain in full force and effect and in no way shall be affected, impaired or invalidated. Such defective provision shall be deemed
to be modified to the extent necessary to render it legal, valid and enforceable, and if no such modification shall render it legal, valid and enforceable, then this Plan shall be construed as if not containing the provision held to be invalid. 

  
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 EXHIBIT A 

FORM OF CALL OPTION EXERCISE NOTICE AND AGREEMENT 

  
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 CALL OPTION EXERCISE NOTICE AND AGREEMENT 

SUTRO BIOPHARMA, INC. 

2017 CALL OPTION PLAN 

*NOTE: You must sign this Notice on Page 3 before submitting it to Sutro Biopharma, Inc. (the “Company”) AND you
must also sign (a) the then-current signature pages to the SutroVax Co-Sale Agreement and SutroVax Voting Agreement (as those terms are defined in the Notice of Call Option Grant governing the Call
Option) and (b) if applicable, (i) an executed Consent of Spouse in the form of Exhibit D to the SutroVax Co-Sale Agreement and (ii) an executed Consent of Spouse in the form of Exhibit C to the
SutroVax Voting Agreement, before submitting this Notice to the Company. 
 OPTIONEE INFORMATION: Please
provide the following information about yourself (“Optionee”): 
  

					
	 Name:
	 	 «Optionee»
	  	
Social Security Number:             
                                    

			
	 Address: 
	 	 	  	 Employee Number:
                                         
               

			
		 	 	  	
Email Address:                
                                         
       

 CALL OPTION INFORMATION: Please provide this information on the call
option being exercised (the “Call Option”): 
 Grant No. «No» 

Date of Grant: «Grant_Date» 
 Call Option Price per
Share: $____ 
 Total number of shares of Common Stock of SutroVax, Inc. 

subject to the Call Option: «Total_Number_of_Options» 

EXERCISE INFORMATION: 

Number of shares of Common Stock of SutroVax, Inc. for which the Call Option is now being exercised [________________]. (These shares are referred to below as
the “Purchased Shares.”) 
 Total Exercise Price Being Paid for the Purchased Shares: $____________ 

Form of payment enclosed [check all that apply]: 
  

	☐	Check for $____________, payable to “SUTRO BIOPHARMA, INC.” 

  

	☐	Wire transfer for $____________ 

  

	☐	Certificate(s) for ________________ shares of Common Stock of SutroVax, Inc. These shares will be valued as of the date this notice is received by the Company. [Requires Company consent.]

 AGREEMENTS, REPRESENTATIONS AND ACKNOWLEDGMENTS OF
OPTIONEE: By signing this Call Option Exercise Notice and Agreement, Optionee hereby agrees with, and represents to, the Company as follows: 
  

	14.	Terms Governing. I acknowledge and agree with the Company that I am acquiring the Purchased Shares by exercise of this Call Option subject to all other terms and conditions of the Notice of Call Option Grant and
the Company’s 2017 Call Option Plan, as it may be amended (the “Plan”). 

	15.	Investment Intent; Securities Law Restrictions. I represent and warrant to the Company that I am acquiring and will hold the Purchased Shares for investment for my account only, and not with a view to, or for
resale in connection with, any “distribution” of the Purchased Shares within the meaning of the Securities Act of 1933, as amended (the “Securities Act”). I understand that the Purchased Shares have not been
registered under the Securities Act by reason of a specific exemption from such registration requirement and that the Purchased Shares must be held by me indefinitely, unless they are subsequently registered under the Securities Act or I obtain an
opinion of counsel (in form and substance satisfactory to the Company and its counsel) that registration is not required. I acknowledge that the Company is under no obligation to register the Purchased Shares under the Securities Act or under any
other securities law. 

  

	16.	Accredited Investor. I hereby represent and warrant to the Company that I am an “accredited investor” under state and federal securities laws and the regulations under those laws, and I have completed
the Accredited Investor Questionnaire attached hereto as Exhibit A. 

  

	17.	Restrictions on Transfer. 

 (a) Rule 144. I will not sell, transfer or
otherwise dispose of the Purchased Shares in violation of the Securities Act, the Securities Exchange Act of 1934, or the rules promulgated thereunder (including Rule 144 under the Securities Act described below “Rule
144”)) or of any other applicable securities laws. I am aware of Rule 144, which permits limited public resales of securities acquired in a non-public offering, subject to satisfaction of
certain conditions, which include (without limitation) that: (a) certain current public information about the Company is available for a specified period of time; (b) the resale occurs only after the holding period required by
Rule 144 has been met; (c) the sale occurs through an unsolicited “broker’s transaction”; and (d) the amount of securities being sold during any three-month period does not exceed specified limitations. I understand
that the conditions for resale set forth in Rule 144 have not been satisfied and that the Company has no plans to satisfy these conditions in the foreseeable future. 

(b) Agreement to Enter into Co-Sale and Voting Agreements. I agree to enter into
and execute (i) the SutroVax Co-Sale Agreement and the SutroVax Voting Agreement and (ii) a Consent of Spouse in the form of Exhibit D to the SutroVax Co-Sale
Agreement and a Consent of Spouse in the form of Exhibit C to the SutroVax Voting Agreement, in each case concurrently with my exercise of the Call Option. I acknowledge that by entering into the SutroVax
Co-Sale Agreement, I will be subjecting the Purchased Shares to the rights of first refusal, co-sale rights and all the other provisions of the SutroVax Co-Sale Agreement, and that by entering into the Voting Agreement, I will be subjected to voting and other obligations and covenants regarding all SutroVax, Inc. shares I own and all other provisions of the SutroVax
Voting Agreement. 
  

	18.	Access to Information; Understanding of Risk in Investment. I acknowledge that I have received and had access to such information as I consider necessary or appropriate for deciding whether to invest in the
Purchased Shares and that I had an opportunity to ask questions and receive answers from the Company regarding the terms and conditions of the issuance of the Purchased Shares. I am aware that my investment in the Company is a speculative investment
that has limited liquidity and is subject to the risk of complete loss. I am able, without impairing my financial condition, to hold the Purchased Shares for an indefinite period and to suffer a complete loss of my investment in the Purchased
Shares. 

  

	19.	Other Restrictions. I acknowledge and consent to all restrictions, including transfer restrictions, in Section 11 of the Plan. 

 

	20.	Form of Ownership. I acknowledge that the Company has encouraged me to consult my own adviser to determine the form of ownership of the Purchased Shares that is appropriate for me. In the event that I choose to
transfer my Purchased Shares to a trust, I agree to sign a Stock Transfer Agreement. 

  
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	21.	Investigation of Tax Consequences. I acknowledge that the Company has encouraged me to consult my own adviser to determine the tax consequences of acquiring the Purchased Shares at this time. 

 

	22.	Other Tax Matters. I agree that the Company does not have a duty to design or administer the Plan or its other compensation programs in a manner that minimizes my tax liabilities. I will not make any claim
against the Company or its Board, officers or employees related to tax liabilities arising from my options or my other compensation. 

  

	23.	Spouse Consent. I agree to seek the consent of my spouse to the extent required by the Company to enforce the foregoing. 

  

	24.	Tax Withholding. As a condition of exercising this Call Option, I agree to make adequate provision for foreign, federal, state or other tax withholding obligations, if any, which arise upon the grant,
vesting or exercise of this Call Option, or disposition of the Purchased Shares, whether by withholding, direct payment to the Company, or otherwise. 

The undersigned hereby executes and delivers this Call Option Exercise Notice and Agreement and agrees to be bound by its terms. 

 

							
	SIGNATURE:	 		 	DATE:
	«OPTIONEE»	 		 	
				
	 	 		 		 	 

 ACCEPTED BY: 

Sutro Biopharma, Inc. 
  

	
	
	 
	By
	
	 
	Title
	
	DATE:
	TIME:

 [Signature Page to Sutro Biopharma, Inc. Call Option Exercise Notice and Agreement] 

  
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 EXHIBIT A 

ACCREDITED INVESTOR QUESTIONNAIRE 

The purpose of this Questionnaire is to determine whether you are an “accredited investor” under state and federal securities laws
and the regulations under those laws with respect to the issuance of equity shares of SutroVax, Inc. by Sutro Biopharma, Inc. (the latter, the “Company”). Your answers will be kept confidential at all times. However, by
signing this Questionnaire, you agree that the Company may present this Questionnaire to such parties as it deems appropriate to establish the availability of exemptions from registration or qualification requirements under federal and state
securities laws. 
  

	 	1.	CONTACT INFORMATION 

 Full Legal Name of Investor:
                                        

 Address (including zip code):
                                        

 Phone Number: (_____) ______-__________                Fax Number: (_____)
______-___________ 
 E-mail Address: ______________________________________     

 

	 	2.	DEFINITION OF “ACCREDITED INVESTOR” 

 The definition of “accredited
investor” within the meaning of the Securities and Exchange Commission Rule 501 of Regulations D, as presently in effect, includes, for example, (i) any person whose individual net worth, or joint net worth with that person’s spouse,
at the time of purchase of securities exceeds $1,000,000, excluding the value of that person’s primary residence and (ii) any person who had an individual income in excess of $200,000 in each of the two most recent years or joint income
with that person’s spouse in excess of $300,000 in each of those years and has a reasonable expectation of reaching the same income level in the current year. If any of the below apply to you please initial here_________. Please indicate the
number(s) of the basis for your status as an “accredited investor” as stated below. 
  

	 	1	Your individual net worth, or your joint net worth with your spouse, exceeds $1,000,000, excluding the value of your primary residence. 

 

	 	2	You personally have had an individual income in excess of $200,000 in each of the two (2) most recent years and you reasonably expect an income in excess of $200,000 in the current year. 

 

	 	3	Your joint income with your spouse is in excess of $300,000 in each of the two (2) most recent years and you reasonably expect a joint income in excess of $300,000 in the current year. 

The information provided in this Questionnaire is true and complete as of the date provided below in all material respects and the undersigned
recognizes that the Company is relying on the truth and accuracy of such information. 
  

							
				
	 	 		 	 Date:
	 	 
	(Signature)	 		 		 	
	
Name:                  
                                         
                           
	 		 		 	
	 (Please Print or Type)
	 		 		 	

 SUTRO BIOPHARMA, INC. 

2017 CALL OPTION PLAN 

NOTICE OF CALL OPTION GRANT 
 The
individual listed below (“Participant”) has been granted the Call Option set forth below on the terms and conditions set forth in this Notice of Call Option Grant (this “Notice”) and the Sutro
Biopharma, Inc. 2017 Call Option Plan (the “Plan”) by Sutro Biopharma Inc., a Delaware corporation. The terms defined in the Plan shall have the same defined meanings in this Notice. 

 

			
	Grant Number:	  	
		
	Date of Grant:	  	
		
	Participant Name:	  	
		
	Number of Shares of Common Stock of SutroVax Subject to Call Option (the “Subject Shares”):	  	
		
	Exercise Price:	  	
		
	Vesting Start Date:	  	January 1, 2017
		
	Vesting Schedule:	  	Except as otherwise determined by the Administrator, and provided that a Participant is in Continuous Service Status on each of the following dates, 25% of the Call Option shall vest on each of the following dates: January 1,
2017; January 1, 2018; January 1, 2019; and January 1, 2020.
		
	Call Option Expiration Date:	  	As provided in Section 5(c)(i) of the Plan.
		
	Call Option Exercise Window:	  	The period commencing on October 1 and ending on December 31 of each calendar year.
		
	Restrictions on Transfer of Subject Shares:	  	As a material inducement and consideration for Sutro Biopharma to enter into this Notice, Participant hereby delivers a duly authorized and executed copy of the Joinder Agreement attached hereto as Exhibit A, pursuant
to which Participant (a) agrees to enter into and become a party to (i) the Amended and Restated Right of First Refusal and Co-Sale Agreement, dated as of March 3, 2017, by and among SutroVax
and certain stockholders and other investors in SutroVax, as such may be amended and/or restated from time to time, or any other agreement that is a successor to or replacement of such agreement (collectively, the “SutroVax Co-Sale Agreement”) (and to subject the Subject Shares to the rights of first refusal held by SutroVax and other SutroVax investors thereunder and the co-sale
rights of other investors thereunder), and (ii) the Amended and Restated Voting Agreement, dated as of March 3, 2017, by and

			
		  	among SutroVax and certain stockholders and other investors in SutroVax, as such may be amended and/or restated from time to time, or any other agreement that is a successor to or replacement of such agreement (collectively, the
“SutroVax Voting Agreement’) (pursuant to which Participant would agree to vote all Subject Shares held by Participant for the election of directors and in favor of certain material transactions (such as mergers or sales of
SutroVax)) and deliver to SutroVax signature pages thereto, and (b) if applicable, agrees to deliver to SutroVax (i) an executed Consent of Spouse in the form of Exhibit D to the SutroVax Co-Sale
Agreement and (ii) an executed Consent of Spouse in the form of Exhibit C to the SutroVax Voting Agreement, in each case at the time of exercising this Call Option and as a condition to such exercise.

 This Notice shall not be effective unless and until this Notice is signed by Participant and representatives of Sutro
Biopharma and returned to Sutro Biopharma. A copy of this Notice and the Plan shall be provided to Participant. 
 Participant hereby acknowledges receipt
of a copy of this Notice and a copy of the Plan and agrees that Participant accepts the grant of the Call Option subject to all of the terms set forth in this Notice and the Plan (which is incorporated herein by this reference), which together
constitute the entire agreement of the parties and supersede in their entirety all prior undertakings, understandings and agreements of Sutro Biopharma and Participant with respect to the subject matter hereof. 

[Signature page follows] 

 This Notice has been executed and agreed to by the parties as of the date or dates set forth below. 

 

									
	PARTICIPANT	 		 	      SUTRO BIOPHARMA, INC.

									
					
	 	 	 	 		 	By:	 	 
	Signature	 		 		 	
	 	 	 	 		 	Title:	 	 
	Print Name	 		 		 	
		 		 		 		 	

									
	Date:	 	 	 		 	Date:	 	 

 [Signature page to Sutro Biopharma, Inc. Notice of Call Option Grant] 

 EXHIBIT A 

FORM OF JOINDER AGREEMENT 

 JOINDER AGREEMENT 

This Joinder Agreement (this “Agreement”) is made and entered into as of
[            ], 2017 (the “Effective Date”) by and among [            ]
(“Participant”), Sutro Biopharma, Inc., a Delaware corporation (“Sutro Biopharma”), and SutroVax, Inc., a Delaware corporation (“SutroVax”). 

WHEREAS, Sutro Biopharma previously purchased Three Million (3,000,000) shares of Common Stock of SutroVax (the “Purchased
Shares”) pursuant to that certain Common Stock Purchase Agreement (the “Purchase Agreement”) dated as of December 12, 2013 by and between Sutro Biopharma and SutroVax. 

WHEREAS, Sutro Biopharma has granted to Participant the option (the “Call Option”) to purchase
[            ] of the Purchased Shares (the “Call Option Shares”) pursuant to the terms of (i) the Sutro Biopharma, Inc. 2017 Call Option
Plan and (ii) that certain Notice of Call Option Grant dated on or about the date hereof by and between Participant and Sutro Biopharma. 

WHEREAS, in connection with the grant of the Call Option to Participant, (i) SutroVax desires to consent to and waive any rights of first
refusal with respect to the grant of the Call Option to Participant and, upon the exercise of the Call Option by Participant, the purchase of the Call Option Shares by Participant, and (ii) Participant has agreed to, upon exercise of the Call
Option by Participant, (a) be bound by and subject to all of the provisions of the Purchase Agreement and (b) become party to that certain Amended and Restated Right of First Refusal and Co-Sale
Agreement, dated as of March 3, 2017, by and among SutroVax and certain stockholders and other investors in SutroVax, as such may be amended and/or restated from time to time, or any other agreement that is a successor to or replacement of such
agreement (collectively, the “SutroVax Co-Sale Agreement”) and that certain Amended and Restated Voting Agreement, dated as of March 3, 2017, by and among SutroVax and certain
stockholders and other investors in SutroVax, as such may be amended and/or restated from time to time, or any other agreement that is a successor to or replacement of such agreement (collectively, the “SutroVax Voting
Agreement,” and together with the SutroVax Co-Sale Agreement, the “Stockholder Agreements”). 

NOW THEREFORE, in consideration of the mutual promises contained herein, the parties agree as follows: 

1. Consent to Grant of Call Option. SutroVax consents to and waives any rights of first refusal, whether under the
Purchase Agreement, the Stockholder Agreements or otherwise, with respect to the grant of the Call Option to Participant and, upon the exercise of the Call Option by Participant, to the purchase of the Call Option Shares by Participant. 

2. Joinder. 

2.1 Participant agrees to, upon exercise of the Call Option by Participant, be bound by and subject to all of the provisions of the Purchase
Agreement, including, without limitation, the right of first refusal and lock-up agreement set forth therein, and hereby makes the investment representations listed on Exhibit A to the Purchase Agreement to
the Company as of the date of this Agreement. 
 2.2 Participant agrees to, upon exercise of the Call Option by Participant, be subject to
all the rights and obligations of a “Key Common Holder” under the SutroVax Co-Sale Agreement and to be bound by and subject to all of the provisions thereof. 

 2.3 Participant agrees to, upon exercise of the Call Option by Participant, be subject to all the
rights and obligations of a “Key Common Holder” under the SutroVax Voting Agreement and to be bound by and subject to all of the provisions thereof. 

2.4 Participant hereby agrees to deliver to SutroVax, upon exercise of the Call Option by Participant (i) a duly executed counterpart
signature page to each Stockholder Agreement (attached as Exhibit A) and (ii) if Participant is married at such time, (a) an executed Consent of Spouse in the form of Exhibit D to the SutroVax Co-Sale Agreement and (b) an executed Consent of Spouse in the form of Exhibit C to the SutroVax Voting Agreement. 

2.5 Participant hereby acknowledges receipt of true and complete copies of the Purchase Agreement and each of the Stockholder Agreements. 

3. General Provisions. 

3.1 Governing Law. This Agreement and all acts and transactions pursuant hereto and the rights and obligations of the parties hereto
shall be governed, construed and interpreted in accordance with the laws of the State of California, without giving effect to principles of conflicts of law. 

3.2 Assignment; Binding Upon Successors and Assigns. No party may assign any of its respective rights or obligations under this
Agreement without the written consent of the other parties to such assignment. This Agreement, and the rights and obligations of the parties hereunder, will be binding upon and inure to the benefit of the parties hereto and their respective
permitted successors, assigns, heirs, executors, administrators and legal representatives. 
 3.3 Severability. If one or more
provisions of this Agreement are held to be unenforceable under applicable law, such provision shall be excluded from this Agreement and the balance of the Agreement shall be interpreted as though such provision were so excluded and shall be
enforceable in accordance with its terms. 
 3.4 Expenses. Each party will bear its respective legal, auditors’, investment
bankers’ and financial advisors’ fees and other expenses incurred with respect to this Agreement and the transactions contemplated hereby. 

3.5 Notices. Any notice required or permitted by this Agreement shall be in writing and shall be deemed sufficient upon delivery, when
delivered personally or by overnight courier or sent by fax (upon customary confirmation of receipt), or 48 hours after being deposited in the U.S. mail, as certified or registered mail, with postage prepaid, addressed to the party to be notified at
such party’s address as set forth on the signature page hereto, or as subsequently modified by written notice, and if to Sutro Biopharma, with a copy to Fenwick & West LLP, Attention: Effie Toshav, 1191 Second Avenue, 10th Floor, Seattle, WA 98101. 
 3.6 Amendment and Waivers. This Agreement may be
amended only by a written agreement executed by each of the parties hereto. Any amendment effected in accordance with this section will be binding upon all parties hereto and each of their respective successors and assigns. 

3.7 Attorneys’ Fees. If any action at law or in equity (including arbitration) is necessary to enforce or interpret the terms of
this Agreement, the prevailing party shall be entitled to reasonable attorney’s fees, costs and necessary disbursements in addition to any other relief to which such party may be entitled. 

  
 4 

 3.8 Counterparts: Facsimile Signatures. This Agreement may be executed in any number of
counterparts, each of which when so executed and delivered will be deemed an original, and all of which together shall constitute one and the same agreement. This Agreement may be executed and delivered by facsimile and upon such delivery the
facsimile signature will be deemed to have the same effect as if the original signature had been delivered to the other party. 
 3.9
Further Assurances. The parties agree to execute such further documents and instruments and to take such further actions as may be reasonably necessary to carry out the purposes and intent of this Agreement. 

3.10 Entire Agreement. This Agreement, the Purchase Agreement, the Stockholder Agreements and all other documents referred to herein
constitute the entire agreement and understanding of the parties with respect to the subject matter of this Agreement, and supersede all prior understandings and agreements, whether oral or written, between or among the parties hereto with respect
to the specific subject matter hereof. 
 [Signature page follows] 

  
 5 

 IN WITNESS WHEREOF, each of the parties has executed this Agreement as of the Effective
Date. 
  

							
	SUTRO BIOPHARMA, INC.	 		 	PARTICIPANT

							
				
	By:	 	 	 		 	 
	Name:	 	 	 		 	
	Title:	 	 	 		 	
	Address:	 		 		 	Address:
	 	 	 	 		 	 
	 	 	 	 		 	 

							
			
	SUTROVAX, INC.	 		 	

							
				
	By:	 	 	 		 	
	Name:	 	 	 		 	
	Title:	 	 	 		 	
	Address:	 		 		 	
	 	 	 	 		 	
	 	 	 	 		 	

 LIST OF EXHIBITS 

Exhibit A:    Signature Pages to Stockholder Agreements 

  
 [SIGNATURE PAGE TO
JOINDER AGREEMENT] 

 EXHIBIT A 

Signature Pages to Stockholder Agreements 

 
			
	KEY COMMON HOLDER
		
	Name:	 	 
	
	 
		 	 (Signature)

  
 (Signature page to the
Amended and Restated Right of First Refusal and Co-Sale Agreement) 

 
			
	KEY COMMON HOLDER
		
	Name:	 	 
	
	 
		 	 (Signature)

  
 (Signature page to the
Amended and Restated Right of First Refusal and Co-Sale Agreement)William Newell Offer Letter

 EXHIBIT 10.6 
  

 
 December 29, 2008 

William Newell 
 [PRIVATE ADDRESS] 

Dear Bill, 
 On behalf of the Search Committee, we are pleased to
offer you the position of Chief Executive Officer with Sutro Biopharma, Inc. (the “Company”). We are delighted that you have agreed to join us! As Chief Executive Officer, you will receive an annual salary of $325,000, which will be paid
semi-monthly in accordance with the Company’s normal payroll procedures. In addition, you will be eligible for a bonus of up to 30% of your annual salary based upon your performance and the Company’s performance. The bonus payout will be
determined annually, typically in March, by the Compensation Committee. You will first be eligible for bonus consideration in March, 2010. As an employee, you will also be eligible to receive certain employee benefits including health insurance,
life insurance and disability insurance, with reasonable and customary coverage and deductibles or co-payments. Please refer to the benefits summary for specific details. You should note that the Company may modify job titles, salaries and benefits
from time to time as it deems necessary. 
 In addition, if you decide to join the Company, it will be recommended at the first meeting of the
Company’s Board of directors following your start date that the Company grant you an option to purchase 1,428,452 shares which is approximately 4% of the outstanding post 2nd tranche shares
of the Company’s Common Stock at a price per share equal to the fair market value per share of the Common Stock on the date of grant, as determined by the Company’s Board of Directors. Twenty-five percent (25%) of the shares subject
to the option shall vest 12 months after the date your vesting begins subject to your continuing employment with the Company, and no shares shall vest before such date. The remaining shares shall vest monthly over the next 36 months in equal monthly
amounts subject to you continuing employment with the company. 

 
This option grant shall be subject to the terms and conditions of the company’s Stock Option Plan and Stock Option Agreement, including vesting requirements. No right to any stock is earned
or accrued until such time that vesting occurs, nor does the grant confer any right to continue vesting or employment. 
 The Company is excited about your
joining and looks forward to a beneficial and productive relationship. Nevertheless, you should be aware that your employment with the Company is for no specified period and constitutes at-will employment. As a result, you are free to resign at any
time, for any reason or for no reason. Similarly, the Company is free to conclude its employment relationship with you at any time, with or without cause, and with or without notice. We request that, in the event of resignation, you give the Company
at least two weeks notice. 
 The Company reserves the right to conduct background investigations and/or reference checks on all of its potential employees.
Your job offer, therefore, is contingent upon a clearance of such a background investigation and/or reference check, if any. 
 For purposes of federal
immigration law, you will be required to provide to the Company documentary evidence of your identity and eligibility for employment in the United States. Such documentation must be provided to us within three (3) business days of your date of
hire, or our employment relationship with you may be terminated. 
 We also ask that, if you have not already done so, you disclose to the Company any and
all agreements relating to your prior employment that may affect your eligibility to be employed by the Company or limit the manner in which you may be employed. It is the Company’s understanding that any such agreements will not prevent you
from performing the duties of your position and you represent that such is the case. Moreover, you agree that, during the term of your employment with the company, you will not engage in any other employment, occupation, consulting or other business
activity directly related to the business in which the Company is now involved or becomes involved during the term of your employment, nor will you engage in any other activities that conflict with your obligations to the Company. Similarly, you
agree not to bring any third party confidential information to the Company, including that of your former employer, and that in performing your duties for the Company you will not in any way utilize any such information. 

As a Company employee, you will be expected to abide by the company’s rules and standards. Specifically, you will be required to sign an acknowledgment
that you have read and that you understand the Company’s rules of conduct, which are included in the Company Handbook. 

 As a condition of your employment, you are also required to sign and comply with an At-Will Employment,
Confidential Information, Invention Assignment and Arbitration Agreement which requires, among other provisions, the assignment of patent rights to any invention made during your employment at the Company, and non-disclosure of company proprietary
information. In the event of any dispute or claim relating to or arising out of our employment relationship, you and the Company agree that (i) any and all disputes between you and the Company shall be fully and finally resolved by binding
arbitration, (ii) you are waiving any and all rights to a jury trial but all court remedies will be available in arbitration, (iii) all disputes shall be resolved by a neutral arbitrator who shall issue a written opinion, (iv) the
arbitration shall provide for adequate discovery, and (v) the Company shall pay all but the first $125 of the arbitration fees. Please note that we must receive your signed Agreement before your first day of employment. 

To accept the Company’s offer, please sign and date this letter in the space provided below. A duplicate original is enclosed for your records. If you
accept our offer, your first day of employment will be January 7, 2009. This letter, along with any agreements relating to proprietary rights between you and the Company, set forth the terms of your employment with the Company and supersede any
prior representations or agreements including, but not limited to, any representations made during your recruitment, interviews or pre-employment negotiations, whether written or oral. This letter, including, but not limited to, its at-will
employment provision, may not be modified or amended except by a written agreement signed by Sutro’s Board of Directors on behalf of the Company and you. This offer of employment will terminate if it is not accepted, signed and returned 72
hours following your receipt of a signed signature page. 
 We look forward to working with you at Sutro Biopharma and together, building a successful
Company! 
 [signature page follows] 

 

 
 To indicate your acceptance of the Company’s offer, please sign and date this letter in the space provided below and
return it to me. A duplicate original is enclosed for your records. 
 Sincerely, 

/s/ Daniel Gold 

	Name	 Daniel Gold 

	Title	 CEO 

Agreed to and accepted: 
  

			
		
	Signature:  	 	  /s/ William J. Newell

			
		
	Printed Name:	 	William J. Newell

			
		
	Date: 	 	Dec. 29, 2008

 Enclosures: 
 Duplicate Original
Letter 
 At-Will Employment, Confidential Information, Invention Assignment and Arbitration Agreement (to be provided as soon as practical) 

TriNet Benefits Summary 

 SUTRO BIOPHARMA, INC. 

MANAGEMENT CONTINUITY AGREEMENT 

This Management Continuity Agreement (the “Agreement”) is made and entered into by and between William Newell (the
“Employee”) and Sutro Biopharma, Inc. (the “Company”), effective as of the last date signed below (the “Effective Date”). 

1.      Term of Agreement. This Agreement shall terminate upon the date that all obligations of the
parties hereto with respect to this Agreement have been satisfied. 
 2.      At-Will Employment. The
Company and the Employee acknowledge that the Employee’s employment is and shall continue to be at-will, as defined under applicable law. If the Employee’s employment terminates for any reason, the Employee shall not be entitled to any
payments, benefits, damages, awards or compensation other than as provided by this Agreement, or as may otherwise be available in accordance with the Company’s established employee plans and practices. 

3.      Certain Terminations Prior to or More than One Year After a Change of Control. If, prior to a
Change of Control, as defined herein (a “Change of Control”) or more than one year after a Change of Control, Employee’s employment with the Company is involuntarily terminated other than for Cause, as defined herein
(“Cause”) then, subject to Employee complying with Section 6 hereof, Employee shall receive (a) continued payment of Employee’s base salary (less applicable tax withholdings) for nine (9) months following such
termination, such amounts to be paid in accordance with the Company’s normal payroll policies; (b) nine months of accelerated vesting on all outstanding Company stock options; and (c) 100% Company-paid premiums paid for continued
health benefits for Employee (and any eligible dependents) under the Company’s group health, dental and vision plans until the earlier of (A) nine (9) months or (B) the date upon which Employee and his eligible dependents become
covered under similar plans of another employer; subject, however, to Employee validly electing to continue coverage under the Consolidated Omnibus Budget Reconciliation Act of 1985, as amended (“COBRA”). 

4.      Certain Terminations Within One Year After a Change of Control. If, on or within one year
following a Change of Control, Employee’s employment with the Company is either (i) involuntarily terminated other than for Cause, or (ii) voluntarily terminated by Employee for Good Reason, as defined herein (“Good
Reason”), then, subject to Employee complying with Section 6 hereof, Employee shall receive (a) continued payment of Employee’s base salary (less applicable tax withholdings) for fifteen (15) months following such
termination, such amounts to be paid in accordance with the Company’s normal payroll policies; (b) 100% accelerated vesting on all outstanding Company stock options; and (c) 100% Company-paid premiums paid for continued health
benefits for Employee (and any eligible dependents) under the Company’s group health, dental and vision plans until the earlier of (A) twelve (12) months or (B) the date upon which Employee and his eligible dependents become
covered under similar plans of another employer; subject, however, to Employee validly electing to continue coverage under COBRA. 

 5.      Other Employment Terminations. 

(a)      Certain Voluntary Resignations; Termination For Cause. If the Employee’s employment
terminates by reason of the Employee’s voluntary resignation (other than for Good Reason on or within one year following a Change of Control), or if the Employee is terminated for Cause, then the Employee shall not be entitled to receive
severance or other benefits except for those (if any) as may then be established under the Company’s then existing severance and benefits plans and practices or pursuant to other agreements with the Company. 

(b)      Death or Disability. If the Company terminates the Employee’s employment as a result of the
Employee’s death or Disability, then the Employee shall not be entitled to receive severance or other benefits except for those (if any) as may then be established under the Company’s then existing severance and benefits plans and
practices or pursuant to other agreements with the Company. 
 6.      Release of Claims. The receipt
of any benefits pursuant to the sections 3 or 4 hereof will be subject to Employee signing and not revoking a separation agreement and release of claims substantially in the form attached to this agreement as Exhibit A (the
“Release”), provided that such release becomes effective no later than sixty (60) days following Employee’s termination date or such earlier date required by the Release (such deadline, the “Release Deadline”). If the
Release does not become effective by the Release Deadline, Employee will forfeit any rights to severance or benefits under this letter, and in no event will severance payments or benefits be paid or provided until the Release actually becomes
effective. In the event Employee’s termination occurs at a time during the calendar year where the Release could become effective in the calendar year following the calendar year in which Employee’s termination occurs, then any severance
payments or benefits under this letter that would be considered Deferred Compensation Separation Benefits (as defined on Exhibit B hereto) will be paid on the first payroll date to occur during the calendar year following the calendar year in
which such termination occurs, or, if later, (i) the Release Deadline, (ii) such time as required by the payment schedule applicable to each payment or benefit as set forth in Section 7, or (iii) such time as required by this
paragraph. 
 7.      Internal Revenue Code Section 409A. 

(a)      Notwithstanding anything to the contrary herein, no Deferred Compensation Separation Benefits (as
defined below) will become payable hereunder unless and until Employee has a “separation from service” within the meaning of Section 409A of the Internal Revenue Code of 1986, as amended (the “Code”), and any proposed or
final regulations and guidance promulgated thereunder (“Section 409”). Further, if Employee is a “specified employee” within the meaning of Section 409A at the time of Employee’s termination (other than due to death),
and the severance or other benefits payable to Employee, if any, hereunder, when considered together with any other severance payments or separation benefits, are considered deferred compensation under Section 409A (together, the “Deferred
Compensation Separation Benefits”), such Deferred Compensation Separation Payments that are otherwise payable within the first six (6) months following Employee’s termination of employment will become payable on the first payroll date
that occurs on or after the date six (6) months and one (1) day following the date of Employee’s termination of employment (or such later date as is 

  
 2 

 
required to avoid the imposition of additional tax under Section 409A). All subsequent Deferred Compensation Separation Benefits, if any, will be payable in accordance with the payment
schedule applicable to each payment or benefit. Notwithstanding anything herein to the contrary, if Employee dies following his termination but prior to the six (6) month anniversary of his termination (or any later delay date), then any
payments delayed in accordance with this paragraph will be payable in a lump sum as soon as administratively practicable after the date of Employee’s death and all other Deferred Compensation Separation Benefits will be payable in accordance
with the payment schedule applicable to each payment or benefit. Each payment and benefit payable hereunder is intended to constitute a separate payment for purposes of Section 1.409A-2(b)(2) of the Treasury Regulations. 

(b)      Any amount paid hereunder that satisfies the requirements of the “short-term deferral” rule
set forth in Section 1.409A-1(b)(4) of the Treasury Regulations shall not constitute Deferred Compensation Separation Benefits for purposes of section (a) above. 

(c)      Any amount paid hereunder that qualifies as a payment made as a result of an involuntary separation
from service pursuant to Section 1.409A-1(b)(9)(iii) of the Treasury Regulations that does not exceed the Section 409A Limit shall not constitute Deferred Compensation Separation Benefits for purposes of section (a) above. For
purposes of this section (c), “Section 409A Limit” means the lesser of 2 times: (i) Employee’s annualized compensation based upon the annual rate of pay paid to Employee during Employee’s taxable year preceding the
Employee’s taxable year in which his employment terminated as determined under Treasury Regulation Section 1.409A-1(b)(9)(iii)(A)(1) and any Internal Revenue Service guidance issued with respect thereto; or (ii) the maximum amount
that may be taken into account under a qualified plan pursuant to Section 401(a)(17) of the Internal Revenue Code for the year in which Employee’s employment is terminated. 

(d)      The foregoing provisions are intended to comply with the requirements of Section 409A so that none
of the severance payments and benefits to be provided under the letter will be subject to the additional tax imposed under Section 409A, and any ambiguities herein will be interpreted to so comply. Employee and the Company agree to work
together in good faith to consider amendments to the letter and to take such reasonable actions which are necessary, appropriate or desirable to avoid imposition of any additional tax or income recognition prior to actual payment to Employee under
Section 409A. 
 8.      Definition of Terms. The following terms referred to in this Agreement
shall have the following meanings: 
 (a)      Cause.  “Cause” means (i) an
unauthorized use or disclosure of the Company’s confidential information or trade secrets, which use or disclosure causes material harm to the Company; (ii) a deliberate material failure in the performance of Employee’s duties as
Chief Executive Officer or any other duties as pertaining to employees of the Company generally; provided however that in such event Employee may only be terminated for Cause following receipt of notice from the Board of Directors specifying the
alleged act or acts of deliberate material failure and an opportunity for the Employee to respond to the Board of Directors in person, with his attorney present; (iii) conviction of, or pleas of “guilty” or “no contest” to,
a felony under the laws of the United States or any state thereof; (iv) gross misconduct; or (v) a continued failure to perform assigned duties customarily performed by a Chief Executive Officer of a corporation of similar size, after
receiving written notification of such failure from the Board of Directors. 

  
 3 

 (b)      Change of Control.  “Change of
Control” means the occurrence of any of the following events: (i) the closing of a consolidation or merger of the Company with or into any other corporation in which the holders of the Company’s outstanding shares immediately before
such consolidation or merger do not, immediately after such consolidation or merger, retain stock representing a majority of the voting power of the surviving corporation of such consolidation or merger; or (ii) a sale of all or substantially
all of the assets of the Company. Notwithstanding the foregoing, in no event shall (A) an initial public offering of Common Stock pursuant to a registration statement filed with the Securities and Exchange Commission; (B) any equity
financing (including the issuance of convertible debt) of the Company in a single transaction or a series of transactions; or (C) a transaction whose primary purpose is to change the state of the Company’s incorporation and/or to create a
holding company that will be owned in substantially the same proportions by the persons who held the Company’s securities before such transaction constitute a Change of Control for purposes of this Agreement. 

(c)      Disability.  “Disability” shall mean that the Employee has been unable to
perform his Company duties as the result of his incapacity due to physical or mental illness, and such inability, at least 26 weeks after its commencement, is determined to be total and permanent by a physician selected by the Company or its
insurers and acceptable to the Employee or the Employee’s legal representative (such Agreement as to acceptability not to be unreasonably withheld). 

(d)      Good Reason.  “Good Reason” shall mean (a) a reduction in
Employee’s title, or any material reduction in Employee’s level of responsibility and/or scope of authority; or (b) a material reduction in Employee’s base salary (other than a reduction generally applicable to executive officers
of the Company implemented for expense management purposes); or (c) a requirement for Employee to relocate to an office that is more than fifty (50) miles from the location of the Company’s principal offices at the time of such
relocation; provided, however, that the Employee must provide written notice to the Company of the condition that could constitute a “Good Reason” event within ninety (90) days of the initial existence of such condition and such
condition must not have been remedied by the Company within thirty (30) days of such written notice. For the avoidance of doubt, on and after a Change of Control (so long as the Company’s equity securities are not publicly traded on an
established U.S. national securities market immediately prior to such Change of Control), a reduction in Employee’s title, or a material reduction in Employee’s level of responsibility and/or scope of authority shall not be deemed to occur
if Employee retains principal management authority and responsibility for the business unit or division containing substantially all of the Company’s business acquired in the Change of Control. 

  
 4 

 9.      Successors 

(a)      Company’s Successors.  Any successor to the Company (whether direct or indirect
and whether by purchase, merger, consolidation, liquidation or otherwise) to all or substantially all of the Company’s business and/or assets shall assume the obligations under this Agreement. 

(b)      Employee’s Successors.  The terms of this Agreement and all rights of the
Employee hereunder shall inure to the benefit of, and be enforceable by, the Employee’s personal or legal representatives, executors, administrators, successors, heirs, distributees, devisees and legatees. 

10.    Miscellaneous Provisions 

(a)      No Duty to Mitigate.  The Employee shall not be required to mitigate the amount of any
benefit contemplated by this Agreement, nor shall any such benefit be reduced by any earnings that the Employee may receive from any other source. 

(b)      Waiver.  No provision of this Agreement shall be modified, waived or discharged unless
the modification, waiver or discharge is agreed to in writing and signed by the Employee and by an authorized officer of the Company (other than the Employee). No waiver by either party of any breach of, or of compliance with, any condition or
provision of this Agreement by the other party shall be considered a waiver of any other condition or provision or of the same condition or provision at another time. 

(c)      Whole Agreement.  No agreements, representations or understandings (whether oral or
written and whether express or implied) which are not expressly set forth in this Agreement have been made or entered into by either party with respect to severance. This Agreement shall take precedence over any other documents that may conflict
with this Agreement. This Agreement, the offer letter by and between Employee and the Company dated December 29, 2008, any agreements relating to proprietary rights between Employee and the Company, Employee’s stock option agreements and
the At-Will Employment, Confidential Information, Invention Assignment and Arbitration Agreement set forth the terms of Employee’s employment with the Company and supersede any prior representations or agreements including, but not limited to,
any representations made during Employee’s recruitment, interviews or pre-employment negotiations, whether written or oral. This Agreement, including, but not limited to, its at-will employment provision, may not be modified or amended except
by a written agreement signed by a duly authorized member of the Company’s Board of Directors on behalf of the Company and Employee. 

(d)      Choice of Law.  The validity, interpretation, construction and performance of this
Agreement shall be governed by the laws of the State of California. 

(e)      Severability.  The invalidity or unenforceability of any provision or provisions of
this Agreement shall not affect the validity or enforceability of any other provision hereof, which shall remain in full force and effect. 

  
 5 

 (f)      Counterparts.  This Agreement may be
executed in counterparts, each of which shall be deemed an original, but all of which together will constitute one and the same instrument. 
 IN WITNESS
WHEREOF, each of the parties has executed this Agreement, in the case of the Company by its duly authorized officer, on the dates set forth below. 
  

									
	SUTRO BIOPHARMA, INC.	 		 	EMPLOYEE
				
	By:	 	/s/ Daniel Gold	 		 	 /s/ William J. Newell

		 		 		 		 	(Signature)
				
	Date:              Jan 28            , 2009	 		 	Date:	 	            Jan. 28            , 2009

  
 6 

 EXHIBIT A 

GENERAL RELEASE OF ALL CLAIMS 
 In
consideration of the severance benefits to be provided to William Newell by Sutro Biopharma, Inc. (the “Company”), pursuant to the terms of the Management Continuity Agreement you entered into with the Company dated as of
[                    ], 2008 (the “Agreement”), you, on your own behalf and on behalf of your heirs, executors, administrators, and
assigns, hereby fully and forever release and discharge the Company and its directors, officers, employees, agents, successors, predecessors, subsidiaries, parent, stockholders, employee benefit plans and assigns (together called the
“Releasees”), from all known and unknown claims and causes of action including, without limitation, any claims or causes of action arising out of or relating in any way to your employment with the Company, including the termination of that
employment. 
 Eight days after you sign (and do not revoke) this General Release of All Claims (“Release”), provided that it is not signed
earlier than your cessation of employment, you will be entitled to the severance benefits or change of control benefits set forth in the Agreement, subject to any other requirements set forth therein that are conditioned on this Release, including
any payment delay required by Sections 6 or 7 of the Agreement. 
 You understand and agree that this Release is a full and complete waiver of all
claims, including (without limitation) claims to attorneys’ fees or costs, claims of wrongful discharge, constructive discharge, breach of contract, breach of the covenant of good faith and fair dealing, harassment, retaliation, discrimination,
violation of public policy, defamation, invasion of privacy, interference with a leave of absence, personal injury, fraud or emotional distress and any claims of discrimination or harassment based on sex, age, race, national origin, disability or
any other basis under Title VII of the Civil Rights Act of 1964, the Fair Labor Standards Act, the Equal Pay Act of 1963, the Americans With Disabilities Act, the Age Discrimination in Employment Act of 1967 (ADEA), the California Labor Code,
the California Fair Employment and Housing Act, the California Family Rights Act, the Family Medical Leave Act or any other federal or state law or regulation relating to employment or employment discrimination. You further understand and agree that
this waiver includes all claims, known and unknown, to the greatest extent permitted by applicable law. 
 You also hereby agree that nothing
contained in this Release shall constitute or be treated as an admission of liability or wrongdoing by the Releasees or you. 
 In addition, you
hereby expressly waive any and all rights and benefits conferred upon you by the provisions of Section 1542 of the Civil Code of the State of California, which states as follows: 

A general release does not extend to claims which the creditor does not know or suspect to exist in his or her favor at the time of executing
the release, which if known by him or her must have materially affected his or her settlement with the debtor. 

 If any provision of this Release is found to be unenforceable, it shall not affect the enforceability of the
remaining provisions and the court shall enforce all remaining provisions to the full extent permitted by law. 
 You agree to provide, at the
Company’s expense, including reimbursement of your time and/or the reasonable fees and expenses of your counsel, reasonable cooperation and complete and accurate information to the Company (voluntarily, without requiring a subpoena or other
compulsion of law) in the event of litigation against the Company and/or its officers or directors. You also agree that you will not assist any person in bringing or pursuing any claim or action of any kind against the Company, unless pursuant to
subpoena or other compulsion of law. 
 This Release constitutes the entire agreement between you and Releasees with regard to the subject matter of this
Release. It supersedes any other agreements, representations or understandings, whether oral or written and whether express or implied, which relate to the subject matter of this Release except as otherwise set forth in the Agreement. However, this
Release covers only those claims that arose prior to the execution of this Release. Execution of this Release does not bar any claim that arises hereafter, including (without limitation) a claim for breach of the Agreement. 

You understand that you have the right to consult with an attorney before signing this Release. You have 21 days after receipt of this Release to review and
consider this Release, discuss it with an attorney of your own choosing, and decide to execute it or not execute it. You also understand that you may revoke this Release during a period of seven days after you sign it and that this Release will not
become effective for seven days after you sign it (and then only if you do not revoke it). In any event, this Release is not to be signed, and will not become effective, prior to your cessation of employment. In order to revoke this Release, within
seven days after you execute this Release you must deliver to                          at the Company a letter stating
that you are revoking it. 
 You understand that if you choose to revoke this Release within seven days after you sign it, you will not receive the
severance benefits set forth in the Agreement that are conditioned on this Release and the Release will have no effect. 
 You agree not to disclose to
others the terms of this Release, except that you may disclose such information to your spouse, your domestic partner and to your attorney or accountant in order for such attorney or accountant to render services to you related to this Release. 

  
 8 

 You state that before signing this Release, you: 

 

	 	•	 	 Have read it, 

  

	 	•	 	 Understand it, 

  

	 	•	 	 Know that you are giving up important rights, 

 

	 	•	 	 Are aware of your right to consult an attorney before signing it, and 

 

	 	•	 	 Have signed it knowingly and voluntarily. 

 

			
		
	    Date:  	 	 
		
	    By:	 	 
		
		 	William Newell

 
 TO BE SIGNED UPON CESSATION OF EMPLOYMENT 

  
 9 

 September 27, 2016 

Sutro Biopharma, Inc. 
  

	 	Re:	 Amendments to Management Continuity Agreement 

To Whom It May Concern: 
 This letter (this
“Amendment”) serves to partially amend the original Management Continuity Agreement by and between Sutro Biopharma, Inc. (the “Company”) and William Newell effective January 28, 2009 (the
“Management Continuity Agreement”). This Amendment shall be effective as of the first date set forth above. In consideration for Mr. Newell’s service to the Company: 

1.      Section 3 of the Management Continuity Agreement is superseded and replaced in its entirety with
the following: 
 “Certain Terminations; Change of Control. Upon the earlier to occur of (i) a Change of Control, as defined
herein, occurring during the Employee’s employment with the Company and (ii) the termination by the Company of the Employee’s employment with the Company other than for Cause, or the voluntary termination by Employee of employment
with the Company for Good Reason, as defined herein (each, a “Trigger Event”), subject to Employee complying with Section 6 hereof, Employee shall receive the following: 

(a)      if the Trigger Event is a Change of Control, (1) continued payment of
Employee’s base salary (less applicable tax withholdings) for eighteen (18) months following such Change of Control, such amounts to be paid in accordance with the Company’s normal payroll policies, and (2) 100% accelerated
vesting on all outstanding Company stock options, including without limitation all stock options for which Employee has made an early exercise and that are still subject to vesting, and Company restricted stock; or 

(b)      if the Trigger Event is termination by the Company of the Employee’s employment
with the Company other than for Cause, or the voluntary termination by Employee of employment with the Company for Good Reason, (1) the continued payment of Employee’s base salary (less applicable tax withholdings) for eighteen
(18) months following such termination, such amounts to be paid in accordance with the Company’s normal payroll policies, (2) eighteen (18) months of accelerated vesting on all outstanding Company stock options, including without
limitation all stock options for which Employee has made an early exercise and that are still subject to vesting, and Company restricted stock, and (3) 100% Company-paid premiums paid for continued health benefits for Employee (and any eligible
dependents) under the Company’s group health, dental and vision plans until the earlier of (A) eighteen (18) months or (B) the date upon which Employee and his eligible dependents become covered under similar plans of another
employer; subject, however, to Employee validly electing to continue coverage under the Consolidated Omnibus Budget Reconciliation Act of 1985, as amended (“COBRA”). 

 Notwithstanding the foregoing, if the Trigger Event is a Change of Control, and if
Employee’s employment with the Company is subsequently terminated by the Company other than for Cause, or the Employee voluntarily terminates employment with the Company for Good Reason, Employee shall receive 100% Company-paid premiums paid
for continued health benefits for Employee (and any eligible dependents) under the Company’s group health, dental and vision plans until the earlier of (A) eighteen (18) months or (B) the date upon which Employee and his eligible
dependents become covered under similar plans of another employer; subject, however, to Employee validly electing to continue coverage under COBRA. For the avoidance of doubt, the benefits set forth in this paragraph are not intended to be a
duplication of the benefits set forth in Section 3(b) above. 
 Notwithstanding the foregoing, if the Company, in its sole discretion,
determines that it cannot provide the foregoing reimbursement of COBRA premiums without potentially causing the Company to incur additional expense or being subject to an excise tax under applicable law (including, without limitation,
Section 2716 of the Public Health Service Act), the Company instead shall provide Employee with a taxable monthly payment (a) in an amount equal to the monthly COBRA premium that Employee would be required to pay to continue the group
health coverage in effect on Employee’s Separation (which amount shall be based on the premium for the first month of COBRA coverage), (b) which shall be made regardless of whether Employee elects COBRA continuation coverage and
(c) which shall commence on the later of (i) the first day of the month following the month in which Employee’s Separation occurs and (ii) the effective date of the Company’s determination, and shall end on the earlier of
(x) the effective date on which Employee become covered by a health, dental or vision insurance plan of a subsequent employer, and (y) the date upon which the Company has made eighteen (18) monthly payments. Employee shall have no
right to an additional gross-up payment to account for the fact that such COBRA premium amounts are paid on an after-tax basis. 
 Any
amounts or benefits payable upon Employee’s Separation that are subject to, and not exempt from, Section 409A of the Internal Revenue Code of 1986, as amended (“Section 409A”) will be delayed if Employee is a
“specified” employee under Section 409A until six months after such separation or, if earlier, Employee’s death. Delayed payments will be paid in a lump sum at the expiration of the delay period and any payments not delayed will
be paid in accordance with their original schedule. No payment will be made upon a disability or terminal illness unless and until such condition qualifies as a “Disability” within the meaning of Section 409A or, earlier, upon a
timely established permissible payment event under Section 409A, if any. Payments under this Agreement are intended to constitute separate payments for purposes of Section 409A to the maximum extent permitted. 

  
 2 

 For purposes of this section, “Separation” means a “separation from service,”
as defined in Treasury Regulation §1.409A-1(h).” 
 2.      Section 4 of the Management
Continuity Agreement is superseded and replaced in its entirety with the following: 
 “[Reserved].” 

3.      Section 8(b) of the Management Continuity Agreement is superseded and replaced in its entirety with
the following: 
 “Change of Control. “Change of Control” means: 

(i) any “person” (as such term is used in Sections 13(d) and 14(d) of the Securities Exchange Act of 1934, as amended (the
“Exchange Act”)), other than a trustee or other fiduciary holding securities of the Company under an employee benefit plan of the Company, becomes the “beneficial owner” (as defined in Rule 13d-3 promulgated under the Exchange
Act), directly or indirectly, of securities of the Company representing 50% or more of (A) the outstanding shares of common stock of the Company or (B) the combined voting power of the Company’s then outstanding securities;

 (ii) the Company is party to a merger or consolidation which results in the voting securities of the Company outstanding immediately prior
thereto failing to continue to represent (either by remaining outstanding or by being converted into voting securities of the surviving or another entity) at least fifty (50%) percent of the combined voting power of the voting securities of the
Company or such surviving or other entity outstanding immediately after such merger or consolidation; or 
 (iii) the sale or disposition of
all or substantially all of the Company’s assets (or consummation of any transaction having similar effect). 
 Notwithstanding the
foregoing, a transaction will not be deemed a Change of Control unless the transaction qualifies as a change of control event within the meaning of Code Section 409A, as it has been and may be amended from time to time, and any proposed or
final Treasury Regulations and Internal Revenue Service guidance that has been promulgated or may be promulgated thereunder from time to time.” 

4.      Section 8(d) of the Management Continuity Agreement is superseded and replaced in its entirety with
the following: 
 “Good Reason. “Good Reason” means: 

(a) a material reduction in Employee’s level of duties, responsibility and/or scope of authority; or (b) a material reduction in
Employee’s base salary (other than a reduction generally applicable to executive officers of the Company implemented for expense management purposes); or (c) a requirement for Employee to relocate

  
 3 

 
to an office that is more than fifty (50) miles from the location of the Company’s principal offices at the time of such relocation; provided, however, that (i) the Employee must
provide written notice to the Company of the condition that could constitute a “Good Reason” event within ninety (90) days of the initial existence of such condition, (ii) the Employee must give the Company an opportunity to cure
such condition within thirty (30) days following delivery of such written notice and (iii) provided the Company has failed to cure such condition within such 30-day cure period, Employee must terminate his employment within ten
(10) days following expiration of such cure period. For the avoidance of doubt, on and after a Change of Control (so long as the Company’s equity securities are not publicly traded on an established U.S. national securities market
immediately prior to such Changes of Control), a material reduction in Employee’s level of responsibility and/or scope of authority shall not be deemed to occur if Employee retains principal management authority and responsibility for the
business unit or division containing substantially all of the Company’s business acquired in the Change of Control.” 
 All other
terms and conditions of the Management Continuity Agreement shall continue in full force and effect. 
 Please sign below to indicate
acceptance of the terms of this Amendment. 
  

			
	Sincerely,
	
	Sutro Biopharma, Inc.
	
	/s/ [illegible]

 
			
	By:	 	  

 
			
	Its:	 	  

 The undersigned has read and understood this Amendment and hereby acknowledges, accepts and agrees to the terms as set
forth above. 
  

	
	 /s/ William J. Newell

	William J. Newell

  
 4

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