HYPERION THERAPEUTICS, INC. WARRANT TO PURCHASE COMMON STOCK

Exhibit 4.3

THIS WARRANT AND THE UNDERLYING SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”). THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT AS TO SUCH SECURITIES UNDER THE ACT OR AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED.

HYPERION THERAPEUTICS, INC.

WARRANT TO PURCHASE COMMON STOCK

VOID AFTER DECEMBER 13, 2012

THIS CERTIFIES THAT, for value received, KEELIN REEDS PARTNERS, with its principal office at 800 Menlo Ave., Suite 210, Menlo Park, California 94025, or assigns (the “Holder”), is entitled to subscribe for and purchase from HYPERION THERAPEUTICS, INC., a Delaware corporation, with its principal office at 601 Gateway Blvd., Suite 200, South San Francisco, CA 94080 (the “Company”) the Exercise Shares (as defined below) at the Exercise Price (as defined below). This Warrant is being issued pursuant to the terms of a Letter Agreement, dated March 6, 2007 by and among the Company and the Holder therewith (the “Letter Agreement”).

1. DEFINITIONS. Capitalized terms used but not defined herein shall have the meanings set forth in the Letter Agreement. As used herein, the following terms shall have the following respective meanings:

(a) “Exercise Period” shall mean the period commencing with the date hereof and ending five (5) years later, unless sooner terminated as provided below.

(b) “Exercise Price” shall mean $0.30 per Exercise Share subject to adjustment pursuant to Section 4.

(c) “Exercise Shares” shall mean 25,000 shares of the Company’s Common Stock issuable upon exercise of this Warrant, subject to adjustment pursuant to Section 4.

2. EXERCISE OF WARRANT. The rights represented by this Warrant may be exercised in whole or in part at the option of the Holder at any time during the Exercise Period, by delivery of the following to the Company at its address set forth above (or at such other address as it may designate by notice in writing to the Holder):

(a) An executed Notice of Exercise in the form attached hereto;

(b) Payment of the Exercise Price either (i) in cash or by check, or (ii) by

cancellation of indebtedness; and

(c) This Warrant.

Upon the exercise of the rights represented by this Warrant, a certificate or certificates for the Exercise Shares so purchased, registered in the name of the Holder or persons affiliated with the Holder, if the Holder so designates, shall be issued and delivered to the Holder within a reasonable time after the rights represented by this Warrant shall have been so exercised. In the event that this Warrant is being exercised for less than all of the then-current number of Exercise Shares purchasable hereunder, the Company shall, concurrently with the issuance by the Company of the number of Exercise Shares for which this Warrant is then being exercised, issue a new Warrant exercisable for the remaining number of Exercise Shares purchasable hereunder.

A Holder may condition any exercise of this Warrant upon any event, happening or condition (or the absence thereof) either within a stated period of time or otherwise, and no exercise hereof shall be deemed to have occurred except subject to and in conformance with such conditions. Any such conditions shall be in writing and delivered to the Company together with such Holder’s notice of exercise.

The person in whose name any certificate or certificates for Exercise Shares are to be issued upon exercise of this Warrant shall be deemed to have become the holder of record of such shares on the date on which this Warrant was surrendered and payment of the Exercise Price was made, irrespective of the date of delivery of such certificate or certificates, except that, if the date of such surrender and payment is a date when the stock transfer books of the Company are closed, such person shall be deemed to have become the holder of such shares at the close of business on the next succeeding date on which the stock transfer books are open.

2.1 Net Exercise. Notwithstanding any provisions herein to the contrary, if the fair market value of one Exercise Share is greater than the Exercise Price (at the date of calculation as set forth below), in lieu of exercising this Warrant by payment of cash, the Holder may elect to receive shares equal to the value (as determined below) of this Warrant (or the portion thereof being canceled) by surrender of this Warrant at the principal office of the Company together with the properly endorsed Notice of Exercise in which event the Company shall issue to the Holder a number of Exercise Shares computed using the following formula:

For purposes of the above calculation, the fair market value of one Exercise Share shall be determined by the Company’s Board of Directors in good faith; provided, however, that in the event that this Warrant is exercised pursuant to this Section 2.1 in connection with the Company’s initial public offering of its Common Stock, the fair market value per share shall be the product of (i) the per share offering price to the public of the Company’s initial public offering, and (ii) the number of shares of Common Stock_, into which each Exercise Share is convertible at the time of such exercise; provided, further, if this Warrant is exercised after, and not in connection with, the Company’s initial public offering, and if the Company’s Common Stock is traded on a securities exchange or The Nasdaq Stock Market or actively traded over-the-counter, the fair market value per share shall be deemed to be the product of (x) the closing bid quotation for a share of the Company’s Common Stock as of the close of the last trading day occurring prior to the day on which this Warrant is being exercised and (y) the number of shares of Common Stock into which each Exercise Share is convertible on such date.

3. REPRESENTATIONS OF HOLDER.

3.1 Acquisition of Warrant for Personal Account. The Holder represents and warrants that it is acquiring the Warrant and the Exercise Shares solely for its account for investment and not with a view to or for sale or distribution of said Warrant or Exercise Shares or any part thereof. The Holder also represents that the entire legal and beneficial interests of the Warrant and Exercise Shares the Holder is acquiring is being acquired for, and will be held for, its account only.

3.2 Information and Sophistication. The Holder hereby: (i) acknowledges that it has received all the information it has requested from the Company and it considers necessary or appropriate for deciding whether to acquire the Warrant and Exercise Shares, (ii) represents that it has had an opportunity to ask questions and receive answers from the Company regarding the terms and conditions of the offering of the Warrant and Exercise Shares and to obtain any additional information necessary to verify the accuracy of the information given the Holder and (iii) further represents that it has such knowledge and experience in financial and business matters that it is capable of evaluating the merits and risk of this investment.

3.3 Ability to Bear Economic Risk. Holder acknowledges that investment in the Warrants and Exercise Shares involves a high degree of risk, and represents that it is able, without materially impairing its financial condition, to hold the Warrant and Exercise Shares for an indefinite period of time and to suffer a complete loss of its investment.

3.4 Securities Are Not Registered.

(a) The Holder understands that the Warrant and the Exercise Shares

have not been registered under the Securities Act of 1933, as amended (the “Act”) on the basis that no distribution or public offering of the stock of the Company is to be effected. The Holder realizes that the basis for the exemption may not be present if, notwithstanding its representations, the Holder has a present intention of acquiring the securities for a fixed or determinable period in the future, selling (in connection with a distribution or otherwise), granting any participation in, or otherwise distributing the securities. The Holder has no such present intention.

(b) The Holder recognizes that the Warrant and the Exercise Shares must be held indefinitely unless they are subsequently registered under the Act or an exemption from such registration is available. The Holder recognizes that the Company has no obligation to register the Warrant or the Exercise Shares of the Company, or to comply with any exemption from such registration.

(c) The Holder is aware that neither the Warrant nor the Exercise Shares may be sold pursuant to Rule 144 adopted under the Act unless certain conditions are met, including, among other things, the existence of a public market for the shares, the availability of certain current public information about the Company, the resale following the required holding period under Rule 144 and the number of shares being sold during any three month period not exceeding specified limitations. Holder is aware that the conditions for resale set forth in Rule 144 have not been satisfied and that the Company presently has no plans to satisfy these conditions in the foreseeable future.

3.5 Disposition of Warrant and Exercise Shares.

(a) The Holder further agrees not to make any disposition of all or any part of the Warrant or Exercise Shares in any event unless and until:

(i) The Company shall have received a letter secured by the Holder from the Securities and Exchange Commission stating that no action will be recommended to the Commission with respect to the proposed disposition;

(ii) There is then in effect a registration statement under the Act covering such proposed disposition and such disposition is made in accordance with said registration statement; or

(iii) The Holder shall have notified the Company of the proposed disposition and shall have furnished the Company with a detailed statement of the circumstances surrounding the proposed disposition, and if reasonably requested by the Company, the Holder shall have furnished the Company with an opinion of counsel, reasonably satisfactory to the Company, for the Holder to the effect that such disposition will not require registration of such Warrant or Exercise Shares under the Act or any applicable state securities laws. The Company agrees that it will not require an opinion of counsel with respect to transactions under Rule 144 of the Securities Act of 1933, as amended, except in unusual

circumstances.

(iv) Notwithstanding the provisions of subsections (i), (ii) and (iii) above and subject to Section 9 hereof, no such registration statement or opinion of counsel shall be necessary for a transfer by such Holder to a partner (or retired partner) or member (or retired member) of such Holder in accordance with partnership or limited liability company interests, or transfers by gift, will or intestate succession to any spouse or lineal descendants or ancestors, if all transferees agree in writing to be subject to the terms hereof to the same extent as if they were Holders hereunder.

(b) The Holder understands and agrees that all certificates evidencing the Exercise Shares to be issued to the Holder may bear the following legend:

THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE “ACT”). THEY MAY NOT BE SOLD, OFFERED FOR SALE, PLEDGED OR HYPOTHECATED IN THE ABSENCE OF AN EFFECTIVE REGISTRATION STATEMENT AS TO THE SECURITIES UNDER THE ACT, AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT REQUIRED OR A NO-ACTION LETTER FROM THE SECURITIES AND EXCHANGE COMMISSION.

3.6 Accredited Investor Status. The Holder is an “accredited investor” as defined in Regulation D promulgated under the Act.

3.7 Further Assurances. The Holder agrees and covenants that at any time and from time to time it will promptly execute and deliver to the Company such further instruments and documents and take such further action as the Company may reasonably require in order to carry out the full intent and purpose of this Agreement and to comply with state or federal securities laws or other regulatory approvals.

4. ADJUSTMENT OF EXERCISE PRICE AND NUMBER OF EXERCISE SHARES. In the event of changes in the Common Stock comprising the Exercise Shares by reason of stock dividends, splits, recapitalizations, reclassifications, combinations or exchanges of shares, separations, reorganizations, liquidations, or the like, the number and class of Exercise Shares available under the Warrant in the aggregate and the Exercise Price shall be correspondingly adjusted to give the Holder of the Warrant, on exercise for the same aggregate Exercise Price, the total number, class, and kind of shares as the Holder would have owned had the Warrant been exercised prior to the event and had the Holder continued to hold such shares until after the event requiring adjustment. The form of this Warrant need not be changed because of any adjustment in the number of Exercise Shares subject to this Warrant.

When any adjustment is required to be made in the Exercise Shares or the Exercise Price pursuant to this Section 4, the Company shall promptly mail to the Holder a certificate setting

forth (i) a brief statement of the facts requiring such adjustment, (ii) the Exercise Price after such adjustment and (iii) the kind and amount of stock or other securities or property into which this Warrant shall be exercisable after such adjustment.

5. FRACTIONAL SHARES. No fractional shares shall be issued upon the exercise of this Warrant as a consequence of any adjustment pursuant hereto. All Exercise Shares (including fractions) to be issued upon exercise of this Warrant shall be aggregated for purposes of determining whether the exercise would result in the issuance of any fractional share. If, after aggregation, the exercise would result in the issuance of a fractional share, the Company shall, in lieu of issuance of any fractional share, pay the Holder otherwise entitled to such fraction a sum in cash equal to the product resulting from multiplying the then current fair market value of one Exercise Share by such fraction.

6. EARLY TERMINATION. In the event of, at any time during the Exercise Period, an Acquisition or Asset Transfer (each as defined in the Company’s Amended and Restated Certificate of Incorporation), the Company shall provide to the Holder twenty (20) days advance written notice of such Acquisition or Asset Transfer, and this Warrant shall be deemed exercised pursuant to Section 2.1 immediately prior to the closing of such Acquisition or Asset Transfer; provided, however, that if the fair market value of the Exercise Shares as determined in accordance with Section 2.1 hereof is less than the Exercise Price, this Warrant shall terminate unless exercised immediately prior to the closing of such Acquisition or Asset Transfer.

7. MARKET STAND-OFF AGREEMENT. Holder shall not sell, dispose of, transfer, make any short sale of, grant any option for the purchase of, or enter into any hedging or similar transaction with the same economic effect as a sale, any Common Stock (or Exercise Shares or other securities) of the Company held by Holder, for a period of time specified by the managing underwriter(s) (not to exceed one hundred eighty (180) days) following the effective date of a registration statement of the Company filed under the Act in connection with the Company’s initial public offering. Holder agrees to execute and deliver such other agreements as may be reasonably requested by the Company and/or the managing underwriter(s) which are consistent with the foregoing or which are necessary to give further effect thereto. In order to enforce the foregoing covenant, the Company may impose stop-transfer instructions with respect to such Common Stock (or other securities) until the end of such period. The underwriters of the Company’s stock are intended third party beneficiaries of this Section 7 and shall have the right, power and authority to enforce the provisions hereof as though they were a party hereto.

8. NO STOCKHOLDER RIGHTS. This Warrant in and of itself shall not entitle the Holder to any voting rights or other rights as a stockholder of the Company.

9. TRANSFER OF WARRANT. Subject to applicable laws and the restriction on transfer set forth on the first page of this Warrant, this Warrant and all rights hereunder are transferable, by the Holder in person or by duly authorized attorney, upon delivery of this Warrant and the form of assignment attached hereto to any transferee designated by Holder. The

transferee shall sign an investment letter in form and substance satisfactory to the Company.

10. LOST, STOLEN, MUTILATED OR DESTROYED WARRANT. If this Warrant is lost, stolen, mutilated or destroyed, the Company may, on such terms as to indemnity or otherwise as it may reasonably impose (which shall, in the case of a mutilated Warrant, include the surrender thereof), issue a new Warrant of like denomination and tenor as the Warrant so lost, stolen, mutilated or destroyed. Any such new Warrant shall constitute an original contractual obligation of the Company, whether or not the allegedly lost, stolen, mutilated or destroyed Warrant shall be at any time enforceable by anyone.

11. AMENDMENT. Any term of this Warrant may be amended or waived with the written consent of the Company and the Holder of the Warrant issued pursuant to the Letter Agreement.

12. NOTICES, ETC. All notices required or permitted hereunder shall be in writing and shall be deemed effectively given: (a) upon personal delivery to the party to be notified, (b) when sent by confirmed telex or facsimile if sent during normal business hours of the recipient, if not, then on the next business day, (c) five (5) days after having been sent by registered or certified mail, return receipt requested, postage prepaid, or (d) one (1) day after deposit with a nationally recognized overnight courier, specifying next day delivery, with written verification of receipt. All communications shall be sent to the Company at the address listed on the signature page and to Holder at the address listed in the Letter Agreement or at such other address as the Company or Holder may designate by ten (10) days advance written notice to the other parties hereto.

13. ACCEPTANCE. Receipt of this Warrant by the Holder shall constitute acceptance of and agreement to all of the terms and conditions contained herein.

14. GOVERNING LAW. This Warrant and all rights, obligations and liabilities hereunder shall be governed by and construed under the laws of the State of Delaware.

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IN WITNESS WHEREOF, the Company has caused this Warrant to be executed by its duly authorized officer as of the day and year first above written.

NOTICE OF EXERCISE

TO: HYPERION THERAPEUTICS, INC.

(1) ¨ The undersigned hereby elects to purchase              shares of                      (the “Exercise Shares”) of Hyperion Therapeutics, Inc. (the “Company”) pursuant to the terms of the attached Warrant, and tenders herewith payment of the exercise price in full, together with all applicable transfer taxes, if any.

¨ The undersigned hereby elects to purchase              shares of                      (the “Exercise Shares”) of Hyperion Therapeutics, Inc. (the “Company”) pursuant to the terms of the net exercise provisions set forth in Section 2.1 of the attached Warrant, and shall tender payment of all applicable transfer taxes, if any.

(2) Please issue a certificate or certificates representing said Exercise Shares in the name of the undersigned or in such other name as is specified below;

(Name)

(Address)

(3) The undersigned represents that (i) the aforesaid Exercise Shares are being acquired for the account of the undersigned for investment and not with a view to, or for resale in connection with, the distribution thereof and that the undersigned has no present intention of distributing or reselling such shares; (ii) the undersigned is aware of the Company’s business affairs and financial condition and has acquired sufficient information about the Company to reach an informed and knowledgeable decision regarding its investment in the Company; (iii) the undersigned is experienced in making investments of this type and has such knowledge and background in financial and business matters that the undersigned is capable of evaluating the merits and risks of this investment and protecting the undersigned’s own interests; (iv) the undersigned understands that Exercise Shares issuable upon exercise of this Warrant have not been registered under the Securities Act of 1933, as amended (the “Securities Act”), by reason of a specific exemption from the registration provisions of the Securities Act, which exemption depends upon, among other things, the bona fide nature of the investment intent as expressed herein, and, because such securities have not been registered under the Securities Act, they must be held indefinitely unless subsequently registered under the Securities Act or an exemption from such registration is available; (v) the undersigned is aware that the aforesaid Exercise Shares may not be sold pursuant to Rule 144 adopted under the Securities Act unless certain conditions are met and until the undersigned has held the shares for the number of years prescribed by Rule 144, that among the conditions for use of the Rule is the availability of current information to the public about the Company and the Company has not made such information available and has no present plans to do so; and (vi) the undersigned agrees not to

make any disposition of all or any part of the aforesaid shares of Exercise Shares unless in accordance with Section 3.5 of the Warrant.

ASSIGNMENT FORM

FOR VALUE RECEIVED, the foregoing Warrant and all rights evidenced thereby are hereby assigned to

NOTE: The signature to this Assignment Form must correspond with the name as it appears on the face of the Warrant, without alteration or enlargement or any change whatever. Officers of corporations and those acting in a fiduciary or other representative capacity should file proper evidence of authority to assign the foregoing Warrant.