Exhibit 10.21

AMENDMENT NO. 1 TO BRIDGE LOAN AGREEMENT AND NOTES

AMENDMENT NO. 1 TO BRIDGE LOAN AGREEMENT AND NOTES (this “Amendment”), dated as
of September 30, 2009, by and among Amarin Corporation plc (the “Company”) and
the Lenders party hereto.

WHEREAS, pursuant to that certain Bridge Loan Agreement, dated as of July 31,
2009, among the Company and the Lenders party thereto (the “Bridge Agreement”)
the Company issued Notes as of July 31, 2009 to the Lenders (the “Notes”;
undefined capitalized terms used herein, shall have the meanings ascribed
thereto in the Notes or the Bridge Agreement, as applicable);

WHEREAS, the Company and the Lenders party hereto, who constitute the Required
Lenders, have agreed, subject to the terms and conditions hereinafter set forth,
to amend certain provisions of the Bridge Agreement and the Notes as set forth
below;

NOW, THEREFORE, in consideration of the premises and covenants contained herein
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto, intending to be legally bound
hereby, agree as follows:

Section 1. Amendment to Bridge Agreement and Notes. The definition of “Term
Date” in the Notes is amended by replacing it in its entirety with the
following:

“Term Date” means October 16, 2009.

Section 2. Representations and Warranties. The Company represents and warrants
to the Lenders as of the date hereof:

(a) It is duly organized, validly existing and in good standing under the laws
of England and Wales and is duly qualified and in good standing to do business
in each jurisdiction where, because of the nature of its activities or
properties, such qualification is required.

(b) The execution and delivery of this Amendment is all within its corporate
powers, has been duly authorized by all necessary action, has, or by the time of
its execution and delivery shall have, received any necessary governmental or
regulatory approval, and does not and will not contravene or conflict with any
provision of (i) law, rule, regulation or ordinance, (ii) its certificate of
incorporation or by-laws; or (iii) any agreement binding upon it or any of its
properties, as the case may be.

(c) This Amendment executed by the Company is the legal, valid and binding
obligation of the Company, enforceable against it, in accordance with its
respective terms, except as limited by applicable bankruptcy, insolvency,
reorganization, moratorium, fraudulent conveyance, and any other laws of general
application affecting enforcement of creditors’ rights generally, and as limited
by laws relating to the availability of a specific performance, injunctive
relief, or other equitable remedies.

(d) The Company has a sufficient number of Ordinary Shares duly authorized and
reserved for issuance upon conversion of the Notes and exercise of the Warrants.
The Ordinary

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Shares issued upon conversion of the Notes and exercise of the Warrants in
accordance with the terms thereof will be validly issued, fully paid and
nonassessable and free from all taxes or liens created by the Company, with the
holders being entitled to all rights accorded to the holders of the Company’s
Ordinary Shares.

Section 3. Counterparts. This Amendment may be executed in any number of
counterparts and by different parties hereto on separate counterparts, each of
which when so executed and delivered shall be deemed to be an original, but all
of which when taken together shall constitute a single instrument. Delivery of
an executed counterpart of a signature page of this Amendment by facsimile
transmission or by email in Adobe “.pdf’ format shall be effective as delivery
of a manually executed counterpart hereof.

Section 4. Governing Law; Jurisdition; Waiver of Jury Trial; Service of Process.
THIS AMENDMENT SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF
THE STATE OF NEW YORK, WITHOUT REGARD TO CONFLICT-OF-LAW PROVISIONS. IN
CONNECTION WITH THE ADJUDICATION OF ANY DISPUTES RELATING TO THIS AMENDMENT,
EACH PARTY HEREBY IRREVOCABLY (A) SUBMITS TO THE EXCLUSIVE JURISDICTION OF THE
STATE AND FEDERAL COURTS SITTING IN NEW YORK, NEW YORK, BOROUGH OF MANHATTAN;
PROVIDED, THAT EACH PARTY SUBMITS TO THE JURISDICTION OF ANY OTHER COURT IN
WHICH A CLAIM RELATING TO THIS AMENDMENT IS VALIDLY BROUGHT BY ANY THIRD PARTY
AGAINST THE OTHER PARTY; (B) WAIVES, AND AGREES NOT TO ASSERT, (1) ANY CLAIM
THAT IT IS NOT SUBJECT TO THE JURISDICTION OF, OR ANY OBJECTION TO THE LAYING OF
VENUE IN, ANY SUCH COURT OR THAT SUCH ACTION HAS BEEN COMMENCED IN AN IMPROPER
OR INCONVENIENT FORUM AND (2) ANY RIGHT IT MAY HAVE TO TRIAL BY JURY; AND
(C) AGREES THAT SERVICE OF ANY PROCESS, SUMMONS, NOTICE OR DOCUMENT BY U.S.
REGISTERED MAIL TO SUCH PARTY’S ADDRESS AS PROVIDED HEREIN SHALL BE EFFECTIVE
WITH RESPECT TO ANY MATTER FOR WHICH IT HAS SUBMITTED TO JURISDICTION HEREBY. A
JUDGMENT IN ANY SUCH ACTION MAY BE ENFORCED IN ANY OTHER COURTS TO WHOSE
JURISDICTION THE APPLICABLE PARTY MAY BE SUBJECT. EACH PARTY (X) CERTIFIES THAT
NO AGENT OF ANY PARTY HAS REPRESENTED, EXPRESSLY OR OTHERWISE, THAT SUCH OTHER
PARTY WOULD NOT, IN THE EVENT OF LITIGATION, SEEK TO ENFORCE THE FOREGOING
WAIVER AND (Y) ACKNOWLEDGES THAT IT AND EACH OTHER PARTY HAS BEEN INDUCED TO
ENTER INTO THE AMENDMENT CONTEMPLATED HEREBY BY, AMONG OTHER THINGS, THE MUTUAL
WAIVERS, AGREEMENTS AND CERTIFICATIONS IN THIS SECTION.

Section 5. Headings. Captions contained in this Agreement are inserted only as a
matter of convenience and in no way define, limit or extend the scope or intent
of this Agreement or any provision of this Agreement and shall not affect the
construction of this Agreement.

[signature pages follow]

 

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IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be duly
executed as of the date first above written.

 

AMARIN CORPORATION PLC, By:  

/s/ Alan Cooke

Name:   Alan Cooke Title:   COO

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LENDERS:     SUNNINGHILL LIMITED   By:     /s/ Alan Cooke           /s/ SJ Kleis
Name:     C. Le Masurier  30.9.09           SJ Kleis 30.9.09 Title:     Director
          Director MIDSUMMER VENTURES, LP By:         Name:     Title:    
MIDSUMMER INVESTMENT, LIMITED By:         Name:     Title:    

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LENDERS: SUNNINGHILL LIMITED By:  

 

Name:   Title:   MIDSUMMER VENTURES, LP By:  

/s/ Joshua Thomas

Name:   Joshua Thomas Title:   Authorized Signatory MIDSUMMER INVESTMENT,
LIMITED By:  

/s/ Joshua Thomas

Name:   Joshua Thomas Title:   Authorized Signatory