Exhibit 4.5

THIS WARRANT AND THE SHARES ISSUABLE HEREUNDER HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE ACT), OR THE SECURITIES LAWS OF ANY STATE AND, EXCEPT AND PURSUANT TO THE PROVISIONS OF ARTICLE 5 BELOW,
MAY NOT BE OFFERED, SOLD OR OTHERWISE TRANSFERRED, PLEDGED OR HYPOTHECATED UNLESS AND UNTIL REGISTERED UNDER SAID ACT AND APPLICABLE STATE SECURITIES LAW OR, IN THE OPINION OF LEGAL COUNSEL IN FORM AND SUBSTANCE SATISFACTORY TO THE ISSUER OF THESE
SECURITIES, SUCH OFFER, SALE OR TRANSFER, PLEDGE OR HYPOTHECATION IS EXEMPT FROM REGISTRATION.

WARRANT TO PURCHASE STOCK



|                  |  |                                                                                                                                                                                   
|----------------- |  | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|Company:          |  | ARCA biopharma, Inc., a Delaware corporation                                                                                                                                      
|Number of Shares: |  | 24,592                                                                                                                                                                            
|Class of Stock:   |  | Series B-2 Preferred                                                                                                                                                              
|Warrant Price:    |  | $3.253 per share                                                                                                                                                                  
|Issue Date:       |  | August 19, 2008                                                                                                                                                                   
|Expiration Date:  |  | The 10th anniversary after the Issue Date                                                                                                                                         
|Credit Facility:  |  | This Warrant is issued in connection with the Growth Capital facilities referenced in the Loan and Security Agreement between Company and Silicon Valley Bank dated July 17,
2007.

THIS WARRANT CERTIFIES THAT, for good and valuable consideration, SILICON VALLEY BANK (Silicon
Valley Bank, together with any registered holder from time to time of this Warrant or any holder of the shares issuable or issued upon exercise of this Warrant as permitted by Article 5.4 herein, Holder) is entitled to purchase the
number of fully paid and nonassessable shares of the class of securities (the Shares) of the Company at the Warrant Price, all as set forth above and as adjusted pursuant to Article 2 of this Warrant, subject to the provisions and upon
the terms and conditions set forth in this Warrant.

ARTICLE 1. EXERCISE.

1.1 Method of Exercise. Holder may exercise this Warrant at any time prior to the Expiration Date (and subject to early termination pursuant to
Article 1.6) by delivering a duly executed Notice of Exercise in substantially the form attached as Appendix 1 to the principal office of the Company. Unless Holder is exercising the conversion right set forth in Article 1.2, Holder shall also
deliver to the Company a check, wire transfer (to an account designated by the Company), or other form of payment acceptable to the Company for the aggregate Warrant Price for the Shares being purchased.

1.2 Conversion Right. In lieu of exercising this Warrant as specified in Article 1.1, Holder may from time to time at any time prior to the
Expiration Date (and subject to early termination pursuant to Article 1.6) convert this Warrant, in whole or in part, into a number of Shares determined by dividing (a) the aggregate fair market value of the Shares or other securities otherwise
issuable upon exercise of this Warrant minus the aggregate Warrant Price of such Shares by (b) the fair market value of one Share. The fair market value of the Shares shall be determined pursuant to Article 1.3.



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1.3 Fair Market Value. If the Companys common stock is traded in a public market and the
Shares are common stock, the fair market value of each Share shall be the closing price of a Share reported for the business day immediately before Holder delivers its Notice of Exercise to the Company (or in the instance where the Warrant is
exercised immediately prior to the effectiveness of the Companys initial public offering, the price to public per share price specified in the final prospectus relating to such offering). If the Companys common stock is
traded in a public market and the Shares are preferred stock, the fair market value of a Share shall be the closing price of a share of the Companys common stock reported for the business day immediately before Holder delivers its Notice of
Exercise to the Company (or, in the instance where the Warrant is exercised immediately prior to the effectiveness of the Companys initial public offering, the initial price to public per share price specified in the final
prospectus relating to such offering), in both cases, multiplied by the number of shares of the Companys common stock into which a Share is convertible. If the Companys common stock is not traded in a public market, the Board of
Directors of the Company shall determine fair market value in its reasonable good faith judgment.

1.4 Delivery of Certificate and New
Warrant. Promptly after Holder exercises or converts this Warrant and, if applicable, the Company receives payment of the aggregate Warrant Price, the Company shall deliver to Holder certificates for the Shares acquired and, if this Warrant has
not been fully exercised or converted and has not expired, a new Warrant representing the Shares not so acquired.

1.5 Replacement of
Warrants. On receipt of evidence reasonably satisfactory to the Company of the loss, theft, destruction or mutilation of this Warrant and, in the case of loss, theft or destruction, on delivery of an indemnity agreement reasonably satisfactory
in form and amount to the Company or, in the case of mutilation on surrender and cancellation of this Warrant, the Company shall execute and deliver, in lieu of this Warrant, a new warrant of like tenor.

1.6 Treatment of Warrant Upon Acquisition of Company.

1.6.1 Acquisition. For the purpose of this Warrant, Acquisition means any sale, exclusive license, or other disposition of all or substantially all of the assets of the Company, or any
reorganization, consolidation, or merger of the Company where the holders of the Companys securities before the transaction beneficially own less than 50% of the outstanding voting securities of the surviving entity after the transaction.

1.6.2 Treatment of Warrant at Acquisition.

(a) Upon the written request of the Company, Holder agrees that, in the event of an Acquisition that is not an asset sale and in which the sole consideration is cash, either (i) Holder shall exercise its
conversion or purchase right under this Warrant and such exercise will be deemed effective immediately prior to the consummation of such Acquisition or (ii) if Holder elects not to exercise the Warrant, this Warrant will expire upon the
consummation of such Acquisition. The Company shall provide Holder with written notice of its request relating to the foregoing (together with such reasonable information as Holder may request in connection with such contemplated Acquisition giving
rise to such notice), which is to be delivered to Holder not less than ten (10) days prior to the closing of the proposed Acquisition.



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(b) Upon the written request of the Company, Holder agrees that, in the event of an Acquisition that is
an arms length sale of all or substantially all of the Companys assets (and only its assets) to a third party that is not an Affiliate (as defined below) of the Company (a True Asset Sale), either (i) Holder shall
exercise its conversion or purchase right under this Warrant and such exercise will be deemed effective immediately prior to the consummation of such Acquisition or (ii) if Holder elects not to exercise the Warrant, this Warrant will continue
until the Expiration Date if the Company continues as a going concern following the closing of any such True Asset Sale. The Company shall provide Holder with written notice of its request relating to the foregoing (together with such reasonable
information as Holder may request in connection with such contemplated Acquisition giving rise to such notice), which is to be delivered to Holder not less than ten (10) days prior to the closing of the proposed Acquisition.

(c) Holder agrees that in the event of an Acquisition of the Company in which the consideration consists of capital stock of a publicly traded acquirer
(or a combination of cash and capital stock of a publicly traded acquirer), if, on the record date for the Acquisition, the fair market value of the Shares (or other securities issuable upon exercise of this Warrant) is equal to or greater than two
(2) times the Warrant Price, the Company may require the Warrant to be deemed automatically exercised and Holder shall participate in the Acquisition as a holder of the Shares (or other securities issuable upon exercise of this Warrant) on the
same terms as the other holders of the same class of securities of the Company.

(d) Upon the closing of any Acquisition other than those
particularly described in subsections (a), (b) and (c) above, the successor entity shall assume the obligations of this Warrant, and this Warrant shall be exercisable for the same securities, cash, and property as would be payable for the
Shares issuable upon exercise of the unexercised portion of this Warrant as if such Shares were outstanding on the record date for the Acquisition and subsequent closing. The Warrant Price and/or number of Shares shall be adjusted accordingly.

As used herein Affiliate shall mean any person or entity that owns or controls directly or indirectly ten (10) percent or more of
the stock of Company, any person or entity that controls or is controlled by or is under common control with such persons or entities, and each of such persons or entitys officers, directors, joint venturers or partners, as applicable.

ARTICLE 2. ADJUSTMENTS TO THE SHARES.

2.1 Stock Dividends, Splits, Etc. if the Company declares or pays a dividend on the Shares payable in common stock, or other securities, then upon exercise of this Warrant, for each Share acquired, Holder shall receive, without cost
to Holder, the total number and kind of securities to which Holder would have been entitled had Holder owned the Shares of record as of the date the dividend occurred. If the Company subdivides the Shares by reclassification or otherwise into a
greater number of shares or takes any other action which increase the amount of stock into which the Shares are convertible, the number of shares purchasable hereunder shall be proportionately increased and the Warrant Price shall be proportionately
decreased. If the outstanding shares are combined or consolidated, by reclassification or otherwise, into a lesser number of shares, the Warrant Price shall be proportionately increased and the number of Shares shall be proportionately decreased.

2.2 Reclassification, Exchange, Combinations or Substitution. Upon any reclassification, exchange, substitution, or other event
that results in a change of the number



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and/or class of the securities issuable upon exercise or conversion of this Warrant, Holder shall be entitled to receive, upon exercise or conversion of this
Warrant, the number and kind of securities and property that Holder would have received for the Shares if this Warrant had been exercised immediately before such reclassification, exchange, substitution, or other event. Such an event shall include
any automatic conversion of the outstanding or issuable securities of the Company of the same class or series as the Shares to common stock pursuant to the terms of the Companys Certificate of Incorporation upon the closing of a registered
public offering of the Companys common stock. The Company or its successor shall promptly issue to Holder an amendment to this Warrant setting forth the number and kind of such new securities or other property issuable upon exercise or
conversion of this Warrant as a result of such reclassification, exchange, substitution or other event that results in a change of the number and/or class of securities issuable upon exercise or conversion of this Warrant. The amendment to this
Warrant shall provide for adjustments which shall be as nearly equivalent as may be practicable to the adjustments provided for in this Article 2 including, without limitation, adjustments to the Warrant Price and to the number of securities or
property issuable upon exercise of the new Warrant. The provisions of this Article 2.2 shall similarly apply to successive reclassifications, exchanges, substitutions, or other events.

2.3 Adjustments for Diluting Issuances. The Warrant Price and the number of Shares issuable upon exercise of this Warrant or, if the Shares are
preferred stock, the number of shares of common stock issuable upon conversion of the Shares, shall be subject to adjustment, from time to time in the manner set forth in the Companys Certificate of Incorporation as if the Shares were issued
and outstanding on and as of the date of any such required adjustment. The provisions set forth for the Shares in the Companys Certificate of Incorporation relating to the above in effect as of the Issue Date may not be amended, modified or
waived, without the prior written consent of Holder unless such amendment, modification or waiver affects the rights associated with the Shares in the same manner as such amendment, modification or waiver affects the rights associated with all other
shares of the same series and class as the Shares granted to Holder.

2.4 No Impairment. Except and to the extent waived by Holder,
the Company shall not, by amendment of its Certificate of Incorporation or through a reorganization, transfer of assets, consolidation, merger, dissolution, issue, or sale of securities or any other voluntary action, avoid or seek to avoid the
observance or performance of any of the terms to be observed or performed under this Warrant by the Company, but shall at all times in good faith assist in carrying out of all the provisions of this Article 2 and in taking all such action as may be
necessary or appropriate to protect Holders rights under this Article against impairment.

2.5 Fractional Shares. No
fractional Shares shall be issuable upon exercise or conversion of this Warrant and the number of Shares to be issued shall be rounded down to the nearest whole Share. If a fractional share interest arises upon any exercise or conversion of the
Warrant, the Company shall eliminate such fractional share interest by paying Holder the amount computed by multiplying the fractional interest by the fair market value of a full Share.

2.6 Certificate as to Adjustments. Upon each adjustment of the Warrant Price, the Company shall promptly notify Holder in writing, and, at the
Companys expense, promptly compute such adjustment, and furnish Holder with a certificate of its Chief Financial Officer setting forth such adjustment and the facts upon which such adjustment is based. The Company shall, upon written request,
furnish Holder a certificate setting forth the Warrant Price in effect upon the date thereof and the series of adjustments leading to such Warrant Price.



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ARTICLE 3. REPRESENTATIONS AND COVENANTS OF THE COMPANY.

3.1 Representations and Warranties. The Company represents and warrants to Holder as follows:

(a) The initial Warrant Price referenced on the first page of this Warrant is not greater than (i) the price per share at which the Shares were fast
issued in an arms-length transaction in which at least $500,000 of the Shares were sold and (ii) the fair market value of the Shares as of the date of this Warrant.

(b) All Shares which may be issued upon the exercise of the purchase right represented by this Warrant, and all securities, if any, issuable upon conversion of the Shares, shall, upon issuance, be duly authorized,
validly issued, fully paid and nonassessable, and free of any liens and encumbrances except for restrictions on transfer provided for herein or under applicable federal and state securities laws.

(c) The Companys capitalization table attached hereto as Schedule 1is true and complete as of the Issue Date.

3.2 Notice of Certain Events. If the Company proposes at any time (a) to declare any dividend or distribution upon any of its stock, whether in cash, property, stock, or other securities and whether or not a
regular cash dividend; (b) to offer for sale any shares of the Companys capital stock (or other securities convertible into such capital stock), other than (i) pursuant to convertible debt instruments, (ii) pursuant to the
Companys stock option or other compensatory plans, (iii) in connection with commercial credit arrangements or equipment financings, or (iv) in connection with strategic transactions for purposes other than capital raising;
(c) to effect any reclassification or recapitalization of any of its stock; (d) to merge or consolidate with or into any other corporation, or sell, lease, license, or convey all or substantially all of its assets, or to liquidate,
dissolve or wind up; or (e) offer holders of registration rights the opportunity to participate in an underwritten public offering of the Companys securities for cash, then, in connection with each such event, the Company shall give
Holder: (1) at least 10 days prior written notice of the date on which a record will be taken for such dividend, distribution, or subscription rights (and specifying the date on which the holders of common stock will be entitled thereto) or for
determining rights to vote, if any, in respect of the matters referred to in (a) and (b) above; (2) in the case of the matters referred to in (c) and (d) above at least 10 days prior written notice of the date when the same
will take place (and specifying the date on which the holders of common stock will be entitled to exchange their common stock for securities or other property deliverable upon the occurrence of such event); and (3) in the case of the matter
referred to in (e) above, the same notice as is given to the holders of such registration rights. Company will also provide information reasonably requested by Holder reasonably necessary to enable Holder to comply with Holders accounting
or reporting requirements.

3.3 Registration Under Securities Act of 1933, as amended. The Company shall add the Holder as a party
to that certain Amended and Restated Investor Rights Agreement, dated as of May 31, 2007, as amended from time to time, for purposes of Sections 1, 2.1, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 2.10, 2.11, 2.12, 2.13 and 5 thereof.

3.4 No Shareholder Rights. Except as provided in this Warrant, Holder will not have any rights as a shareholder of the Company until the exercise
of this Warrant.



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ARTICLE 4. REPRESENTATIONS, WARRANTIES OF HOLDER. Holder represents and warrants to the Company as follows:

4.1 Purchase for Own Account. This Warrant and the securities to be acquired upon exercise of this Warrant by Holder will be
acquired for investment for Holders account, not as a nominee or agent, and not with a view to the public resale or distribution within the meaning of the Act. Holder also represents that Holder has not been formed for the specific purpose of
acquiring this Warrant or the Shares.

4.2 Disclosure of Information. Holder has received or has had full access to all the
information it considers necessary or appropriate to make an informed investment decision with respect to the acquisition of this Warrant and its underlying securities. Holder further has had an opportunity to ask questions and receive answers from
the Company regarding the terms and conditions of the offering of this Warrant and its underlying securities and to obtain additional information (to the extent the Company possessed such information or could acquire it without unreasonable effort
or expense) necessary to verify any information furnished to Holder or to which Holder has access.

4.3 Investment Experience.
Holder understands that the purchase of this Warrant and its underlying securities involves substantial risk. Holder has experience as an investor in securities of companies in the development stage and acknowledges that Holder can bear the economic
risk of such Holders investment in this Warrant and its underlying securities and has such knowledge and experience in financial or business matters that Holder is capable of evaluating the merits and risks of its investment in this Warrant
and its underlying securities and/or has a preexisting personal or business relationship with the Company and certain of its officers, directors or controlling persons of a nature and duration that enables Holder to be aware of the character,
business acumen and financial circumstances of such persons.

4.4 Accredited Investor Status. Holder is an accredited
investor within the meaning of Regulation D promulgated under the Act.

4.5 The Act. Holder understands that this Warrant and
the Shares issuable upon exercise or conversion hereof have not been registered under the Act in reliance upon a specific exemption therefrom, which exemption depends upon, among other things, the bona fide nature of Holders investment intent
as expressed herein. Holder understands that this Warrant and the Shares issued upon any exercise or conversion hereof must be held indefinitely unless subsequently registered under the Act and qualified under applicable state securities laws, or
unless exemption from such registration and qualification are otherwise available.

ARTICLE 5. MISCELLANEOUS.

5.1 Term. Subject to early termination in accordance with Article 1.6, this Warrant is exercisable in whole or in part at any time and from time to
time on or before the Expiration Date.

5.2 Legends. This Warrant and the Shares (and the securities issuable, directly or
indirectly, upon conversion of the Shares, if any) shall be imprinted with a legend in substantially the following form:

THIS WARRANT AND
THE SHARES 1SSUABLE HEREUNDER HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933, AS AMENDED (THE ACT), OR THE SECURITIES LAWS OF ANY STATE AND, EXCEPT AND PURSUANT TO THE PROVISIONS OF ARTICLE 5 BELOW, MAY NOT BE OFFERED, SOLD OR
OTHERWISE TRANSFERRED, PLEDGED OR HYPOTHECATED UNLESS AND UNTIL REGISTERED UNDER SAID ACT AND APPLICABLE STATE SECURITIES LAW OR, IN THE OPINION OF LEGAL COUNSEL IN FORM AND SUBSTANCE SATISFACTORY TO THE ISSUER OF THESE SECURITIES, SUCH OFFER, SALE
OR TRANSFER, PLEDGE OR HYPOTHECATION IS EXEMPT FROM REGISTRATION.



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5.3 Compliance with Securities Laws on Transfer. This Warrant and the Shares issuable upon
exercise of this Warrant (and the securities Issuable, directly or indirectly, upon conversion of the Shares, if any) may not be transferred or assigned in whole or in part without compliance with applicable federal and state securities laws by the
transferor and the transferee (including, without limitation, the delivery of investment representation letters and legal opinions reasonably satisfactory to the Company, as reasonably requested by the Company). The Company shall not require Silicon
Valley Bank (Bank) to provide an opinion of counsel if the transfer is to Banks parent company, SVB Financial Group (formerly Silicon Valley Bancshares), or any other affiliate of Bank, provided such affiliate makes representations
and warranties substantially similar to those set forth in Article 4. Additionally, the Company shall also not require an opinion of counsel if there is no material question as to the availability of current information as referenced in Rule 144(c),
Holder represents that it has complied with Rule 144(d) and (e) in reasonable detail, the selling broker represents that it has complied with Rule 144(f), and the Company is provided with a copy of Holders notice of proposed sale.

5.4 Transfer Procedure. After receipt by Bank of the executed Warrant, Bank will transfer all of this Warrant to SVB Financial
Group by execution of an Assignment substantially in the form of Appendix 2. Subject to the provisions of Article 5.3, and subject to the written consent of the Company for any transfer other than to an affiliate of Bank, and upon providing the
Company with written notice, SVB Financial Group and any subsequent Holder may transfer all or part of this Warrant or the Shares issuable upon exercise of this Warrant (or the Shares issuable directly or indirectly, upon conversion of the Shares,
if any) to any transferee, provided, however, in connection with any such transfer, SVB Financial Group or any subsequent Holder will give the Company notice of the portion of the Warrant being transferred with the name, address and taxpayer
identification number of the transferee and Holder will surrender this Warrant to the Company for reissuance to the transferee(s) (and Holder if applicable). The Company may refuse to transfer this Warrant or the Shares (a) to any person who
directly competes with the Company, unless, in either case, the stock of the Company is publicly traded, or (b) if, in the reasonable good faith determination of the Company in consultation with its legal counsel, such transfer would not be
exempt from the registration requirements of the Act.

5.5 Notices. All notices and other communications from the Company to Holder,
or vice versa, shall be deemed delivered and effective when given personally or mailed by first-class registered or certified mail, postage prepaid, at such address as may have been furnished to the Company or Holder, as the case may (or on the
first business day after transmission by facsimile) be, in writing by the Company or such Holder from time to time. Effective upon receipt of the fully executed Warrant and the initial transfer described in Article 5.4 above, all notices to Holder
shall be addressed as follows until the Company receives notice of a change of address in connection with a transfer or otherwise:

SVB
Financial Group

Attn: Treasury Department

3003 Tasman Drive, HA 200

Santa Clara, CA 95054

Telephone: 408-654-7400

Facsimile:
408-496-2405



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Notice to the Company shall be addressed as follows until Holder receives notice of a change in address:

ARCA biopharma, Inc.

Attn: Patrick Wheeler

8001 Arista Place, Suite 200

Broomfield, CO 80021

Telephone: 720-940-2107

Facsimile: 720-208-9261

5.6 Waiver. This Warrant and any term hereof may be changed, waived,
discharged or terminated only by an instrument in writing signed by the party against which enforcement of such change, waiver, discharge or termination is sought.

5.7 Attorneys Fees. In the event of any dispute between the parties concerning the terms and provisions of this Warrant, the party prevailing in such dispute shall be entitled to collect from the other
party all costs incurred in such dispute, including reasonable attorneys fees.

5.8 Automatic Conversion upon Expiration. In
the event that, upon the Expiration Date, the fair market value of one Share (or other security issuable upon the exercise hereof) as determined in accordance with Section 1.3 above is greater than the Warrant Price in effect on such date, then
this Warrant shall automatically be deemed on and as of such date to be converted pursuant to Section 1.2 above as to all Shares (or such other securities) for which it shall not previously have been exercised or converted, and the Company
shall promptly deliver a certificate representing the Shares (or such other securities) issued upon such conversion to Holder.

5.9
Counterparts. This Warrant may be executed in counterparts, all of which together shall constitute one and the same agreement.

5.10
Governing Law. This Warrant shall be governed by and construed in accordance with the laws of the State of Delaware, without giving effect to its principles regarding conflicts of law.

[Signature page follows.]



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|                     |  |                                     |  |       |  |         |  |                                           
|-------------------- |  | ----------------------------------- |  | ----- |  | ------- |  | ------------------------------------------
|COMPANY            |  |                                     |  | Date: |  | 8/28/08
|                     |  |                                     | 
|ARCA BIOPHARMA, INC. |  |                                     |  |       |  |        
|                     |  |                                     |  |      
|By:                  |  | /s/ Richard B. Brewer               |  |       |  | By:     |  | /s/ Patrick Wheeler                       
|Name:                |  | Richard B. Brewer                   |  |       |  | Name:   |  | Patrick Wheeler                           
|                     |  | (Print)                             |  |       |  |         |  | (Print)                                   
|Title:               |  | Chairman of the Board, President or |  |       |  | Title:  |  | Chief Financial Officer, Secretary,       
|                     |  | Vice President                      |  |       |  |         |  | Assistant Treasurer or Assistant Secretary



|                    |  |                     
|------------------- |  | --------------------
|HOLDER           
|                   
|SILICON VALLEY BANK
|                    | 
|By:                 |  | /s/ Bret J. Turner  
|Name:               |  | Bret J. Turner      
|                    |  | (Print)             
|Title:              |  | Relationship Manager