Document:

EX-10.4

EXHIBIT 10.4

Mast Therapeutics, Inc. 2013 Omnibus Incentive Plan

Senior Executive Incentive Stock Option Grant Agreement

THIS SENIOR EXECUTIVE INCENTIVE STOCK OPTION GRANT AGREEMENT (this “Agreement”), effective as of
[•] (the “Grant Date”), is entered into by and between Mast Therapeutics, Inc., a Delaware
corporation (the “Company”), and [•] (the “Grantee”).

1. Grant of Option. The Company hereby grants to the Grantee a stock option (the
“Option”) to purchase [•] shares of common stock of the Company, par value $0.001 per share (the
“Shares”), at the exercise price of $[•] per Share (the “Exercise Price”) pursuant to the Mast
Therapeutics, Inc. 2013 Omnibus Incentive Plan (the “Plan”). The Option is intended to qualify as
an Incentive Stock Option under Section 422 of the Code to the fullest extent permitted by the
Code, however, the Company does not represent or warrant that this Option qualifies as such.
Grantee should consult with Grantee’s own tax advisor regarding the tax effects of this Option and
the requirements necessary to obtain favorable income tax treatment under Section 422 of the Code,
including, but not limited to, holding period requirements. (NOTE TO GRANTEE: If the Option is
exercised more than three (3) months after the date on which Grantee ceases to be an Employee
(other than by reason of Grantee’s death or permanent and total disability as defined in Section
22(e)(3) of the Code), the Option will be treated as a non-statutory stock option and not as an
Incentive Stock Option to the extent required by Section 422 of the Code.)

In addition, to the extent that the Option (together with all Incentive Stock Options granted
to Grantee under all stock option plans of the Company, including the Plan) becomes exercisable for
the first time during any calendar year for shares having a Fair Market Value greater than One
Hundred Thousand Dollars ($100,000), the portion of such options which exceeds such amount will be
treated as non-statutory stock options. Options designated as Incentive Stock Options are taken
into account in the order in which they were granted, and the Fair Market Value of stock is
determined as of the time the option with respect to such stock is granted. If the Code is amended
to provide for a different limitation from that set forth in this Section, such different
limitation shall be deemed incorporated herein effective as of the date required or permitted by
such amendment to the Code. If the Option is treated as an Incentive Stock Option in part and as a
non-statutory stock option in part by reason of the limitation set forth in this Section, Grantee
may designate which portion of such Option Grantee is exercising. In the absence of such
designation, Grantee shall be deemed to have exercised the Incentive Stock Option portion of the
Option first. To the extent that certificates are issued for Shares, separate certificates
representing each such portion shall be issued upon the exercise of the Option.

2. Subject to the Plan. This Agreement is subject to the provisions of the Plan, and,
unless the context requires otherwise, terms used herein shall have the same meaning as in the
Plan. In the event of a conflict between the provisions of the Plan and this Agreement, the Plan
shall control. All questions of interpretation concerning this Agreement and the Plan shall be
determined by the Committee or its designee. All such determinations shall be final or conclusive,
and binding upon all persons having an interest in the Option as provided in the Plan.

3. Term of Option. Unless the Option terminates earlier pursuant to the provisions of
this Agreement, the Option shall expire ten years from the Grant Date, except as provided in
paragraph (g) of Section 6.

4. Vesting. The Option shall become vested with respect to 1/48th of the
Shares on each monthly anniversary of [•]; provided, however, that the Grantee is then
providing Services.

5. Exercise of Option

(a) Manner of Exercise. To the extent vested, the Option may be exercised, in whole or
in part, by delivering notice to the Company in accordance with paragraph (g) of Section 8 in such
form as the Company may require from time to time (the “Exercise Notice”). Such Exercise Notice
shall specify the number of Shares subject to the Option as to which the Option is being exercised,
and shall be accompanied by full payment of the Exercise Price of such Shares in a manner permitted
under the terms of Section 5.5 of the Plan, except that payment with previously acquired Shares may
only be made with the consent of the Committee. The Option may be exercised only in multiples of
whole Shares and no fractional Shares shall be issued.

(b) Issuance of Shares. Upon exercise of the Option and payment of the Exercise Price
for the Shares as to which the Option is exercised, the Company shall issue to the Grantee the
applicable number of Shares in the form of fully paid and non-assessable Shares.

(c) Capitalization Adjustments. The number of Shares subject to the Option and the
Exercise Price shall be equitably and appropriately adjusted, if applicable, as provided in Section
12.2 of the Plan.

(d) Withholding. No Shares will be issued on exercise of the Option unless and until
the Grantee pays to the Company, or makes satisfactory arrangements with the Company for payment
of, any federal, state or local taxes required by law to be withheld in respect of the exercise of
the Option. The Grantee hereby agrees that the Company may withhold from the Optionee’s wages or
other remuneration the applicable taxes. At the discretion of the Company, the applicable taxes
may be withheld in kind from the Shares otherwise deliverable to the Grantee on exercise of the
Option, up to the Grantee’s minimum required withholding rate or such other rate that will not
trigger a negative accounting impact.

(e) Notice of Disposition. Grantee agrees to notify the Company in writing within
fifteen (15) days after the date of any disposition of any of the Shares issued upon exercise of
the Option that occurs within the later of two (2) years after the Grant Date or within one (1)
year after such Shares are transferred to the Grantee.

6. Termination of Option

(a) Termination of Employment or Service Relationship Other Than Due to Retirement, Death,
Disability, Involuntary Termination or Cause. Unless the Option has earlier terminated, the
Option shall terminate in its entirety, regardless of whether the Option is vested, ninety (90)
days after the date the Grantee ceases to provide Services for any reason other than, as
applicable, the Grantee’s Retirement, death, Disability, Involuntary Termination or termination for
Cause. Except as provided in paragraphs (b), (c), (d) or (e) of this Section, any portion of the
Option that is not vested at the time the Grantee ceases to provide Services shall immediately
terminate.

(b) Retirement. Upon the Retirement of the Grantee, unless the Option has earlier
terminated, the Option shall continue in effect (and, for purposes of vesting pursuant to Section
4, the Grantee shall be deemed to continue to be providing Services) until the earlier of (i) two
(2) years after the Grantee’s Retirement (or, if later, the fifth anniversary of the Grant Date),
and (ii) the expiration of the Option’s term pursuant to Section 3. For purposes of this
Agreement, “Retirement” shall mean termination of the Grantee’s employment with the Company and its
Affiliates, or a successor company (or a subsidiary or parent thereof) and their respective
subsidiaries, other than for Cause (a) if (i) the Grantee is then at least age 60 and (ii) the sum
of the Grantee’s age and years of continuous Service with the Company and its Affiliates is then
equal to at least 70 or (b) if the Committee characterizes such termination as a “Retirement” for
purposes of this Agreement. For clarity, this Section 6(b) shall apply only to Grantees who are
Employees at the time of termination.

(c) Death. Upon the Grantee’s death, unless the Option has earlier terminated, the
Grantee’s executor or personal representative, the person to whom the Option shall have been
transferred by will or the laws of descent and distribution, or such other permitted transferee, as
the case may be, may exercise the Option in accordance with paragraph (a) of Section 5, to the
extent vested, provided such exercise occurs within twelve (12) months after the date of
the Grantee’s death or by the end of the term of the Option pursuant to Section 3, whichever is
earlier.

(d) Disability. In the event that the Grantee ceases to provide Services by reason of
Disability, unless the Option has earlier terminated, the Option may be exercised, in accordance
with paragraph (a) of Section 5, to the extent vested, provided such exercise occurs within
six (6) months after the date of Disability or by the end of the term of the Option pursuant to
Section 3, whichever is earlier. For purposes of this Agreement, “Disability” shall mean the
Grantee’s becoming disabled within the meaning of Section 22(e)(3) of the Code, or as otherwise
determined by the Committee in its discretion. The Committee may require such proof of Disability
as the Committee in its sole and absolute discretion deems appropriate and the Committee’s
determination as to whether the Grantee has incurred a Disability shall be final and binding on all
parties concerned.

(e) Involuntary Termination. In the event that the Grantee ceases to provide Services
as an Employee by reason of an Involuntary Termination, unless the Option has earlier terminated,
the Option shall, immediately prior to such Involuntary Termination, vest and become exercisable
with respect to 25% of the total number of Shares subject to this Option (or [•] Shares (subject to
adjustment pursuant to paragraph (c) of Section 5)), and the Option may be exercised, in accordance
with paragraph (a) of Section 5, to the extent vested as of such Involuntary Termination (for
clarity, after taking into account the foregoing acceleration provision of this paragraph (e)),
provided such exercise occurs by the close of business on the last calendar day of the
12th full calendar month following the date of such Involuntary Termination.
Notwithstanding the increased vesting provided for in the preceding sentence, in no event shall the
Option become vested and exercisable for more than the number of Shares set forth in Section 1
(subject to adjustment as provided in paragraph (c) of Section 5). For purposes of this Agreement,
“Involuntary Termination” shall mean: (i) without the Grantee’s express written consent, an action
by the Company’s Board of Directors or external events causing or immediately portending a material
reduction or alteration of the Grantee’s duties, position or responsibilities relative to the
Grantee’s duties, position or responsibilities in effect immediately prior to such reduction or
alteration, or the removal of the Grantee from such position, duties or responsibilities; (ii)
without the Grantee’s express written consent, a material reduction by the Company (or, if
applicable, the Affiliate for whom Grantee provides Services) of the Grantee’s base salary as in
effect immediately prior to such reduction; (iii) without the Grantee’s express written consent,
the relocation of the Grantee’s principal place of employment with the Company (or, if applicable,
the Affiliate for whom Grantee provides Services) by more than fifty (50) miles; or (iv) any
termination of the Grantee by the Company without Cause or as a result of the Retirement of the
Grantee.

(f) Termination for Cause. Upon termination by the Company or an Affiliate or a
successor company (or a subsidiary or parent thereof) of the Grantee’s Service for Cause, unless
the Option has earlier terminated, the Option shall immediately terminate in its entirety and shall
thereafter not be exercisable to any extent whatsoever. For purposes of this Agreement, “Cause”
shall mean (i) any act of personal dishonesty taken by the Grantee in connection with his or her
responsibilities as an employee which is intended to result in substantial personal enrichment of
the Grantee; (ii) Grantee’s conviction of a felony that the Company’s Board of Directors reasonably
believes has had or will have a material detrimental effect on the Company’s or an Affiliate’s
reputation or business; (iii) a willful act by the Grantee that constitutes misconduct and is
materially injurious to the Company or any Affiliate; or (iv) continued willful violations by the
Grantee of the Grantee’s obligations to the Company or its Affiliate(s) after there has been
delivered to the Grantee a written demand for performance from the Company that describes the basis
for the Company’s belief that the Grantee has not substantially performed his or her duties.

(g) Automatic Extension of Exercise Period. Notwithstanding any provisions of Section
3 or paragraphs (a), (b), (c), (d) or (e) of this Section to the contrary, if on the last business
day of the term of the Option under Section 3 or if following termination of Service and during any
part of the time period set forth in the applicable paragraph of this Section (i) exercise of the
Option is prohibited by applicable law or (ii) the Grantee may not purchase or sell Shares due to a
“black-out period” of a Company insider trading policy, the term of the Option under Section 3 or
the time period to exercise the Option under Section 3 or paragraphs (a), (b), (c), (d) or (e) of
this Section, as applicable, shall be extended until the later of (x) thirty (30) days after the
end of the applicable legal prohibition or black-out period or (y) the end of the time period set
forth in the applicable paragraph of this Section.

7. Change in Control.

(a) Effect on Option. In the event of a Change in Control, unless the Option has
earlier terminated, the Option shall vest and become exercisable with respect to fifty percent
(50%) of the then unvested Shares on the day prior to the date of the Change in Control and shall
vest and become exercisable with respect to the remaining fifty percent (50%) of the then unvested
Shares on the one (1) year anniversary of the Change in Control; provided, however, that
the Grantee is then providing Services.

(b) Assumption or Substitution. Subject to paragraph (a) of this Section 7, in the
event of a Change in Control, to the extent the successor company (or a subsidiary or parent
thereof) does not assume or substitute for the Option on substantially the same terms and
conditions (which may include settlement in the common stock of the successor company (or a
subsidiary or parent thereof)), the Option (i) shall become fully vested and exercisable on the day
prior to the date of the Change in Control if the Grantee (A) is then providing Services or (B) was
the subject of an Involuntary Termination in connection with, related to or in contemplation of the
Change in Control and (ii) may be exercised, in accordance with paragraph (a) of Section 5,
provided such exercise occurs by the close of business on the last calendar day of the
24th full calendar month following the date of such Involuntary Termination.

(c) Tail. Subject to paragraph (a) of this Section 7, in the event of a Change in
Control, to the extent the successor company (or a subsidiary or parent thereof) assumes or
substitutes for the Option on substantially the same terms and conditions (which may include
providing for settlement in the common stock of the successor company (or a subsidiary or parent
thereof)), and in the event of an Involuntary Termination of the Grantee within 12 months following
the date of the Change in Control, the Option shall become fully vested and exercisable, and may be
exercised by the Grantee at any time until the close of business on the last calendar day of the
24th full calendar month following the date of such Involuntary Termination.

8. Miscellaneous.

(a) No Rights of Stockholder. The Grantee shall not have any of the rights of a
stockholder with respect to the Shares subject to this Option until such Shares have been issued
upon the due exercise of the Option.

(b) No Registration Rights; No Right to Settle in Cash. The Company has no obligation
to register with any governmental body or organization (including, without limitation, the U.S.
Securities and Exchange Commission (“SEC”)) any of (a) the offer or issuance of any Award
(including this Option), (b) any Shares issuable upon the exercise of this Option, or (c) the sale
of any Shares issued upon exercise of this Option, regardless of whether the Company in fact
undertakes to register any of the foregoing. In particular, in the event that any of (x) any offer
or issuance of this Option, (y) any Shares issuable upon exercise of this Option, or (z) the sale
of any Shares issued upon exercise of this Option are not registered with any governmental body or
organization (including, without limitation, the SEC), the Company will not under any circumstance
be required to settle its obligations, if any, under this Plan in cash.

(c) Nontransferability of Option. The Option shall be nontransferable otherwise than
by will or the laws of descent and distribution, and during the lifetime of the Grantee, the Option
may be exercised only by the Grantee or, during the period the Grantee is under a legal disability,
by the Grantee’s guardian or legal representative. Notwithstanding the foregoing, the Grantee may,
by delivering written notice to the Company, in a form provided by or otherwise satisfactory to the
Company, designate a third party who, in the event of the Grantee’s death, shall thereafter be
entitled to exercise the Option.

(d) Severability. If any provision of this Agreement shall be held unlawful or
otherwise invalid or unenforceable in whole or in part by a court of competent jurisdiction, such
provision shall (i) be deemed limited to the extent that such court of competent jurisdiction deems
it lawful, valid and/or enforceable and as so limited shall remain in full force and effect, and
(ii) not affect any other provision of this Agreement or part thereof, each of which shall remain
in full force and effect.

(e) Governing Law. This Agreement shall be governed by, and interpreted in accordance
with, the laws of the State of California, other than its conflict of laws principles.

(f) Headings. The headings in this Agreement are for reference purposes only and shall
not affect the meaning or interpretation of this Agreement.

(g) Notices. Any document relating to participation in the Plan or any notice
required or permitted hereunder shall be given in writing and shall be deemed effectively given
(except to the extent that this Agreement provides for effectiveness only upon actual receipt of
such notice) upon personal delivery, electronic delivery at the e-mail address, if any, provided
for Grantee by the Company (or, if applicable the Affiliate for whom Grantee provides Services), or
upon deposit in the U.S. Post Office or foreign postal service, by registered or certified mail, or
with a nationally recognized overnight courier service, with postage and fees prepaid, addressed to
the other party at the address of such party set forth or at such other address as such party may
designate in writing from time to time to the other party. Notice by mail shall be deemed
delivered on the date on which it is postmarked.

Mailed notices to the Company should be addressed to:

Mast Therapeutics, Inc.

12390 El Camino Real, Suite 150

San Diego, CA 92130

Attention: Legal Department

Mailed notices to the Grantee should be addressed to the Grantee at the Grantee’s address as
it appears on the records of the Company or the Affiliate for whom Grantee is providing Services or
a successor company (or a subsidiary or parent thereof). The Company or the Grantee may by writing
to the other party, designate a different address for notices.

The Plan documents, which may include but do not necessarily include: the Plan, this
Agreement, the Plan Prospectus, and any reports of the Company provided generally to the Company’s
stockholders, may be delivered to Grantee electronically. In addition, if permitted by the
Company, Grantee may deliver electronically the Exercise Notice called for by paragraph (a) of
Section 5 to the Company or to such third party involved in administering the Plan as the Company
may designate from time to time. Such means of electronic delivery may include but do not
necessarily include the delivery of a link to a Company intranet or the internet site of a third
party involved in administering the Plan, the delivery of the document via e-mail or such other
means of electronic delivery specified by the Company.

Grantee acknowledges that Grantee has read this paragraph (g) of Section 8 and consents to the
electronic delivery of the Plan documents and, if permitted by the Company, the delivery of the
Exercise Notice, as described above. Grantee acknowledges that he or she may receive from the
Company a paper copy of any documents delivered electronically at no cost to Grantee by contacting
the Company by telephone or in writing. Grantee further acknowledges that the Grantee will be
provided with a paper copy of any documents if the attempted electronic delivery of such documents
fails. Similarly, Grantee understands that Grantee must provide the Company or any designated
third party administrator with a paper copy of any documents if the attempted electronic delivery
of such documents fails. Grantee may revoke his or her consent to the electronic delivery of
documents described above or may change the electronic mail address to which such documents are to
be delivered (if Grantee has provided an electronic mail address) at any time by notifying the
Company of such revoked consent or revised e-mail address by telephone, postal service or
electronic mail. Finally, Grantee understands that he or she is not required to consent to
electronic delivery of documents.

(h) Agreement Not a Contract.  This Agreement (and the grant of the Option) is not an
employment or service contract, and nothing in the Option shall be deemed to create in any way
whatsoever any obligation on Grantee’s part to continue as an employee or director of or consultant
to the Company or any Affiliate or a successor company (or a subsidiary or parent thereof), or of
the Company or any Affiliate or a successor company (or a subsidiary or parent thereof) to continue
Grantee’s Service as such an employee, director or consultant.

(i) Entire Agreement; Modification. This Agreement and the Plan contain the entire
agreement between the parties with respect to the subject matter contained herein and may not be
modified, except as provided in the Plan or in a written document signed by each of the parties
hereto, and may be rescinded only by a written agreement signed by both parties.

IN WITNESS WHEREOF, the parties have executed this Agreement effective as of the Grant Date.

MAST THERAPEUTICS, INC.

	 
	By:

	 
	GranteeEX-10.5

EXHIBIT 10.5

Mast Therapeutics, Inc. 2013 Omnibus Incentive Plan

CMO Incentive Stock Option Grant Agreement

THIS CMO INCENTIVE STOCK OPTION GRANT AGREEMENT (this “Agreement”), effective as of [•] (the
“Grant Date”), is entered into by and between Mast Therapeutics, Inc., a Delaware corporation (the
“Company”), and [•] (the “Grantee”).

1. Grant of Option. The Company hereby grants to the Grantee a stock option (the
“Option”) to purchase [•] shares of common stock of the Company, par value $0.001 per share (the
“Shares”), at the exercise price of $[•] per Share (the “Exercise Price”) pursuant to the Mast
Therapeutics, Inc. 2013 Omnibus Incentive Plan (the “Plan”). The Option is intended to qualify as
an Incentive Stock Option under Section 422 of the Code to the fullest extent permitted by the
Code, however, the Company does not represent or warrant that this Option qualifies as such.
Grantee should consult with Grantee’s own tax advisor regarding the tax effects of this Option and
the requirements necessary to obtain favorable income tax treatment under Section 422 of the Code,
including, but not limited to, holding period requirements. (NOTE TO GRANTEE: If the Option is
exercised more than three (3) months after the date on which Grantee ceases to be an Employee
(other than by reason of Grantee’s death or permanent and total disability as defined in Section
22(e)(3) of the Code), the Option will be treated as a non-statutory stock option and not as an
Incentive Stock Option to the extent required by Section 422 of the Code.)

In addition, to the extent that the Option (together with all Incentive Stock Options granted
to Grantee under all stock option plans of the Company, including the Plan) becomes exercisable for
the first time during any calendar year for shares having a Fair Market Value greater than One
Hundred Thousand Dollars ($100,000), the portion of such options which exceeds such amount will be
treated as non-statutory stock options. Options designated as Incentive Stock Options are taken
into account in the order in which they were granted, and the Fair Market Value of stock is
determined as of the time the option with respect to such stock is granted. If the Code is amended
to provide for a different limitation from that set forth in this Section, such different
limitation shall be deemed incorporated herein effective as of the date required or permitted by
such amendment to the Code. If the Option is treated as an Incentive Stock Option in part and as a
non-statutory stock option in part by reason of the limitation set forth in this Section, Grantee
may designate which portion of such Option Grantee is exercising. In the absence of such
designation, Grantee shall be deemed to have exercised the Incentive Stock Option portion of the
Option first. To the extent that certificates are issued for Shares, separate certificates
representing each such portion shall be issued upon the exercise of the Option.

2. Subject to the Plan/Administration. This Agreement is subject to the provisions of
the Plan, and, unless the context requires otherwise, terms used herein shall have the same meaning
as in the Plan. In the event of a conflict between the provisions of the Plan and this Agreement,
the Plan shall control. All questions of interpretation concerning this Agreement and the Plan
shall be determined by the Committee or its designee. All such determinations shall be final or
conclusive, and binding upon all persons having an interest in the Option as provided in the Plan.

3. Term of Option. Unless the Option terminates earlier pursuant to the provisions of
this Agreement, the Option shall expire ten years from the Grant Date, except as provided in
paragraph (g) of Section 6.

4. Vesting. The Option shall become vested with respect to 1/48th of the
Shares on each monthly anniversary of [•]; provided, however, that the Grantee is then
providing Services.

5. Exercise of Option

(a) Manner of Exercise. To the extent vested, the Option may be exercised, in whole or
in part, by delivering notice to the Company in accordance with paragraph (g) of Section 8 in such
form as the Company may require from time to time (the “Exercise Notice”). Such Exercise Notice
shall specify the number of Shares subject to the Option as to which the Option is being exercised,
and shall be accompanied by full payment of the Exercise Price of such Shares in a manner permitted
under the terms of Section 5.5 of the Plan, except that payment with previously acquired Shares may
only be made with the consent of the Committee. The Option may be exercised only in multiples of
whole Shares and no fractional Shares shall be issued.

(b) Issuance of Shares. Upon exercise of the Option and payment of the Exercise Price
for the Shares as to which the Option is exercised, the Company shall issue to the Grantee the
applicable number of Shares in the form of fully paid and non-assessable Shares.

(c) Capitalization Adjustments. The number of Shares subject to the Option and the
Exercise Price shall be equitably and appropriately adjusted, if applicable, as provided in Section
12.2 of the Plan.

(d) Withholding. No Shares will be issued on exercise of the Option unless and until
the Grantee pays to the Company, or makes satisfactory arrangements with the Company for payment
of, any federal, state or local taxes required by law to be withheld in respect of the exercise of
the Option. The Grantee hereby agrees that the Company may withhold from the Optionee’s wages or
other remuneration the applicable taxes. At the discretion of the Company, the applicable taxes
may be withheld in kind from the Shares otherwise deliverable to the Grantee on exercise of the
Option, up to the Grantee’s minimum required withholding rate or such other rate that will not
trigger a negative accounting impact.

(e) Notice of Disposition. Grantee agrees to notify the Company in writing within
fifteen (15) days after the date of any disposition of any of the Shares issued upon exercise of
the Option that occurs within the later of two (2) years after the Grant Date or within one (1)
year after such Shares are transferred to the Grantee.

6. Termination of Option

(a) Termination of Employment or Service Relationship Other Than Due to Retirement, Death,
Disability, Involuntary Termination or Cause. Unless the Option has earlier terminated, the
Option shall terminate in its entirety, regardless of whether the Option is vested, ninety (90)
days after the date the Grantee ceases to provide Services for any reason other than, as
applicable, the Grantee’s Retirement, death, Disability, Involuntary Termination or termination for
Cause. Except as provided in paragraphs (b), (c), (d) or (e) of this Section, any portion of the
Option that is not vested at the time the Grantee ceases to provide Services shall immediately
terminate.

(b) Retirement. Upon the Retirement of the Grantee, unless the Option has earlier
terminated, the Option shall continue in effect (and, for purposes of vesting pursuant to Section
4, the Grantee shall be deemed to continue to be providing Services) until the earlier of (i) two
(2) years after the Grantee’s Retirement (or, if later, the fifth anniversary of the Grant Date),
and (ii) the expiration of the Option’s term pursuant to Section 3. For purposes of this
Agreement, “Retirement” shall mean termination of the Grantee’s employment with the Company and its
Affiliates, or a successor company (or a subsidiary or parent thereof) and their respective
subsidiaries, other than for Cause (a) if (i) the Grantee is then at least age 60 and (ii) the sum
of the Grantee’s age and years of continuous Service with the Company and its Affiliates is then
equal to at least 70 or (b) if the Committee characterizes such termination as a “Retirement” for
purposes of this Agreement. For clarity, this Section 6(b) shall apply only to Grantees who are
Employees at the time of termination.

(c) Death. Upon the Grantee’s death, unless the Option has earlier terminated, the
Grantee’s executor or personal representative, the person to whom the Option shall have been
transferred by will or the laws of descent and distribution, or such other permitted transferee, as
the case may be, may exercise the Option in accordance with paragraph (a) of Section 5, to the
extent vested, provided such exercise occurs within twelve (12) months after the date of
the Grantee’s death or by the end of the term of the Option pursuant to Section 3, whichever is
earlier.

(d) Disability. In the event that the Grantee ceases to provide Services by reason of
Disability, unless the Option has earlier terminated, the Option may be exercised, in accordance
with paragraph (a) of Section 5, to the extent vested, provided such exercise occurs within
six (6) months after the date of Disability or by the end of the term of the Option pursuant to
Section 3, whichever is earlier. For purposes of this Agreement, “Disability” shall mean the
Grantee’s becoming disabled within the meaning of Section 22(e)(3) of the Code, or as otherwise
determined by the Committee in its discretion. The Committee may require such proof of Disability
as the Committee in its sole and absolute discretion deems appropriate and the Committee’s
determination as to whether the Grantee has incurred a Disability shall be final and binding on all
parties concerned.

(e) Involuntary Termination. In the event that the Grantee ceases to provide Services
as an Employee by reason of an Involuntary Termination, unless the Option has earlier terminated,
the Option shall, immediately prior to such Involuntary Termination, vest and become exercisable
with respect to 18.75% of the total number of Shares subject to this Option (or [•] Shares (subject
to adjustment pursuant to paragraph (c) of Section 5)), and the Option may be exercised, in
accordance with paragraph (a) of Section 5, to the extent vested as of such Involuntary Termination
(for clarity, after taking into account the foregoing acceleration provision of this paragraph
(e)), provided such exercise occurs by the close of business on the last calendar day of the
9th full calendar month following the date of such Involuntary Termination.
Notwithstanding the increased vesting provided for in the preceding sentence, in no event shall the
Option become vested and exercisable for more than the number of Shares set forth in Section 1
(subject to adjustment as provided in paragraph (c) of Section 5). For purposes of this Agreement,
“Involuntary Termination” shall mean: (i) without Grantee’s express written consent, a material
reduction or alteration of Grantee’s duties, position or responsibilities relative to Grantee’s
duties, position or responsibilities in effect immediately prior to such reduction or alteration,
or Grantee’s removal from such position, duties or responsibilities; (ii) without Grantee’s express
written consent, a material reduction by the Company (or, if applicable, the Affiliate for whom
Grantee provides Services) of Grantee’s base salary as in effect immediately prior to such
reduction; (iii) without Grantee’s express written consent, the relocation of Grantee’s principal
place of employment with the Company (or, if applicable, the Affiliate for whom Grantee provides
Services) by more than fifty (50) miles; (iv) any termination of Grantee’s employment by the
Company without Cause. Except in the case of a termination of Grantee by the Company (or,
if applicable, the Affiliate for whom Grantee provides Services) without Cause, an “Involuntary
Termination” shall not be deemed to occur until the Company has received written notice from
Grantee of the occurrence of an Involuntary Termination and had thirty (30) days after the
Company’s receipt of such notice to cure or remedy such Involuntary Termination (the “Remedy
Period”). Any such notice provided by Grantee shall indicate the specific termination provision
relied upon, shall set forth in reasonable detail the facts and circumstances claimed to provide a
basis for termination under the provision so indicated, and shall specify the effective date of
your “separation from service” within the meaning of Section 409A of the Code. In order to be
effective, a resignation for Involuntary Termination must occur within ten (10) business days after
the end of the Remedy Period in which the Company (or, if applicable, the Affiliate for whom
Grantee provides Services) failed to cure or remedy the Involuntary Termination and Grantee must
have provided the foregoing written notice of the occurrence of an Involuntary Termination event to
the Company within ninety (90) days of Grantee’s awareness of the initial existence of the
applicable Involuntary Termination event.

(f) Termination for Cause. Upon termination by the Company or an Affiliate or a
successor company (or a subsidiary or parent thereof) of the Grantee’s Service for Cause, unless
the Option has earlier terminated, the Option shall immediately terminate in its entirety and shall
thereafter not be exercisable to any extent whatsoever. For purposes of this Agreement, “Cause”
shall mean: (i) any act of personal dishonesty by Grantee in connection with Grantee’s hiring
process with the Company that the Company’s Board of Directors reasonably believes has had or will
have a material detrimental effect on the Company’s or an Affiliate’s reputation or business; (ii)
any act of personal dishonesty by Grantee in connection with Grantee’s responsibilities as an
employee which is intended to result in Grantee’s substantial personal enrichment; (iii) continued
willful violations by Grantee of Grantee’s obligations to the Company or its Affiliate(s) after
there has been delivered to Grantee a written demand for performance from the Company that
describes the basis for the Company’s belief that Grantee has not substantially performed Grantee’s
duties; (iv) a willful act by Grantee that constitutes misconduct and is materially injurious to
the Company or any Affiliate; and (v) Grantee’s conviction (including any plea of guilty or nolo
contendere) of any felony that the Company’s Board of Directors reasonably believes has had or will
have a material detrimental effect on the Company’s or any Affiliate’s reputation or business.

(g) Automatic Extension of Exercise Period. Notwithstanding any provisions of Section
3 or paragraphs (a), (b), (c), (d) or (e) of this Section to the contrary, if on the last business
day of the term of the Option under Section 3 or if following termination of Service and during any
part of the time period set forth in the applicable paragraph of this Section (i) exercise of the
Option is prohibited by applicable law or (ii) the Grantee may not purchase or sell Shares due to a
“black-out period” of a Company insider trading policy, the term of the Option under Section 3 or
the time period to exercise the Option under Section 3 or paragraphs (a), (b), (c), (d) or (e) of
this Section, as applicable, shall be extended until the later of (x) thirty (30) days after the
end of the applicable legal prohibition or black-out period or (y) the end of the time period set
forth in the applicable paragraph of this Section.

7. Change in Control.

(a) Assumption or Substitution. In the event of a Change in Control, unless the
Option has earlier terminated, to the extent the successor company (or a subsidiary or parent
thereof) does not assume or substitute for the Option on substantially the same terms and
conditions (which may include settlement in the common stock of the successor company (or a
subsidiary or parent thereof)), the Option (i) shall vest and become exercisable on the day prior
to the date of the Change in Control if the Grantee (A) is then providing Services or (B) was the
subject of an Involuntary Termination in connection with, related to or in contemplation of the
Change in Control and (ii) may be exercised, in accordance with paragraph (a) of Section 5,
provided such exercise occurs by the close of business on the last calendar day of the
18th full calendar month following the date of such Involuntary Termination or by the
end of the term of the Option pursuant to Section 3, whichever is earlier.

(b) Tail. In the event of a Change in Control, to the extent the successor company
(or a subsidiary or parent thereof) assumes or substitutes for the Option on substantially the same
terms and conditions (which may include providing for settlement in the common stock of the
successor company (or a subsidiary or parent thereof)) and in the event of an Involuntary
Termination of the Grantee within 24 months following the date of the Change in Control, the Option
shall become fully vested and exercisable, and may be exercised by the Grantee at any time until
the close of business on the last calendar day of the 18th full calendar month following
the date of such Involuntary Termination or by the end of the term of the Option pursuant to
Section 3, whichever is earlier.

8. Miscellaneous.

(a) No Rights of Stockholder. The Grantee shall not have any of the rights of a
stockholder with respect to the Shares subject to this Option until such Shares have been issued
upon the due exercise of the Option.

(b) No Registration Rights; No Right to Settle in Cash. The Company has no obligation
to register with any governmental body or organization (including, without limitation, the U.S.
Securities and Exchange Commission (“SEC”)) any of (a) the offer or issuance of any Award
(including this Option), (b) any Shares issuable upon the exercise of this Option, or (c) the sale
of any Shares issued upon exercise of this Option, regardless of whether the Company in fact
undertakes to register any of the foregoing. In particular, in the event that any of (x) any offer
or issuance of this Option, (y) any Shares issuable upon exercise of this Option, or (z) the sale
of any Shares issued upon exercise of this Option are not registered with any governmental body or
organization (including, without limitation, the SEC), the Company will not under any circumstance
be required to settle its obligations, if any, under this Plan in cash.

(c) Nontransferability of Option. The Option shall be nontransferable otherwise than
by will or the laws of descent and distribution, and during the lifetime of the Grantee, the Option
may be exercised only by the Grantee or, during the period the Grantee is under a legal disability,
by the Grantee’s guardian or legal representative. Notwithstanding the foregoing, the Grantee may,
by delivering written notice to the Company, in a form provided by or otherwise satisfactory to the
Company, designate a third party who, in the event of the Grantee’s death, shall thereafter be
entitled to exercise the Option.

(d) Severability. If any provision of this Agreement shall be held unlawful or
otherwise invalid or unenforceable in whole or in part by a court of competent jurisdiction, such
provision shall (i) be deemed limited to the extent that such court of competent jurisdiction deems
it lawful, valid and/or enforceable and as so limited shall remain in full force and effect, and
(ii) not affect any other provision of this Agreement or part thereof, each of which shall remain
in full force and effect.

(e) Governing Law. This Agreement shall be governed by, and interpreted in accordance
with, the laws of the State of California, other than its conflict of laws principles.

(f) Headings. The headings in this Agreement are for reference purposes only and shall
not affect the meaning or interpretation of this Agreement.

(g) Notices. Any document relating to participation in the Plan or any notice
required or permitted hereunder shall be given in writing and shall be deemed effectively given
(except to the extent that this Agreement provides for effectiveness only upon actual receipt of
such notice) upon personal delivery, electronic delivery at the e-mail address, if any, provided
for Grantee by the Company (or, if applicable the Affiliate for whom Grantee provides Services), or
upon deposit in the U.S. Post Office or foreign postal service, by registered or certified mail, or
with a nationally recognized overnight courier service, with postage and fees prepaid, addressed to
the other party at the address of such party set forth or at such other address as such party may
designate in writing from time to time to the other party. Notice by mail shall be deemed
delivered on the date on which it is postmarked.

Mailed notices to the Company should be addressed to:

Mast Therapeutics, Inc.

12390 El Camino Real, Suite 150

San Diego, CA 92130

Attention: Legal Department

Mailed notices to the Grantee should be addressed to the Grantee at the Grantee’s address as
it appears on the records of the Company or the Affiliate for whom Grantee is providing Services or
a successor company (or a subsidiary or parent thereof). The Company or the Grantee may by writing
to the other party, designate a different address for notices.

The Plan documents, which may include but do not necessarily include: the Plan, this
Agreement, the Plan Prospectus, and any reports of the Company provided generally to the Company’s
stockholders, may be delivered to Grantee electronically. In addition, if permitted by the
Company, Grantee may deliver electronically the Exercise Notice called for by paragraph (a) of
Section 5 to the Company or to such third party involved in administering the Plan as the Company
may designate from time to time. Such means of electronic delivery may include but do not
necessarily include the delivery of a link to a Company intranet or the internet site of a third
party involved in administering the Plan, the delivery of the document via e-mail or such other
means of electronic delivery specified by the Company.

Grantee acknowledges that Grantee has read this paragraph (g) of Section 8 and consents to the
electronic delivery of the Plan documents and, if permitted by the Company, the delivery of the
Exercise Notice, as described above. Grantee acknowledges that he or she may receive from the
Company a paper copy of any documents delivered electronically at no cost to Grantee by contacting
the Company by telephone or in writing. Grantee further acknowledges that the Grantee will be
provided with a paper copy of any documents if the attempted electronic delivery of such documents
fails. Similarly, Grantee understands that Grantee must provide the Company or any designated
third party administrator with a paper copy of any documents if the attempted electronic delivery
of such documents fails. Grantee may revoke his or her consent to the electronic delivery of
documents described above or may change the electronic mail address to which such documents are to
be delivered (if Grantee has provided an electronic mail address) at any time by notifying the
Company of such revoked consent or revised e-mail address by telephone, postal service or
electronic mail. Finally, Grantee understands that he or she is not required to consent to
electronic delivery of documents.

(h) Agreement Not a Contract.  This Agreement (and the grant of the Option) is not an
employment or service contract, and nothing in the Option shall be deemed to create in any way
whatsoever any obligation on Grantee’s part to continue as an employee or director of or consultant
to the Company or any Affiliate or a successor company (or a subsidiary or parent thereof), or of
the Company or any Affiliate or a successor company (or a subsidiary or parent thereof) to continue
Grantee’s Service as such an employee, director or consultant.

(i) Entire Agreement; Modification. This Agreement and the Plan contain the entire
agreement between the parties with respect to the subject matter contained herein and may not be
modified, except as provided in the Plan or in a written document signed by each of the parties
hereto, and may be rescinded only by a written agreement signed by both parties.

IN WITNESS WHEREOF, the parties have executed this Agreement effective as of the Grant Date.

MAST THERAPEUTICS, INC.

	 
	By:

	 
	Grantee

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