UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Item 5.

Interest in Securities of the Issuer

“Item 5.  Interest in Securities of the Issuer” is hereby amended and restated as follows:

(a) — (b) As of this date of this filing, the Reporting Persons may be deemed, directly or indirectly, including by reason of their mutual affiliation, to be the beneficial owners of the shares of Common Stock held by the OrbiMed Stockholders, OPI III and Associates III. As of the date of this filing, OPI III directly holds 2,705,700 shares of Common Stock and warrants to purchase an additional 78,287 shares of Common Stock and Associates III directly holds 25,800 shares of Common Stock and warrants to purchase an additional 744 shares of Common Stock.

OrbiMed Advisors LLC, pursuant to its authority as the sole managing member of OrbiMed Capital GP III LLC, the sole general partner of OPI III, and as the general partner of Associates III, may be deemed to indirectly beneficially own the securities of the Issuer held by OPI II and Associates III, which represent an aggregate of 2,731,500 shares of Common Stock and warrants to purchase 79,031 shares of Common Stock and constitute approximately 11.08% of the Common Stock(1). Isaly, pursuant to his authority as the managing member of, and owner of a controlling interest in, OrbiMed Advisors LLC, may be deemed to also indirectly beneficially own the foregoing securities of the Issuer attributable to OrbiMed Advisors LLC.

OrbiMed Capital GP III LLC, pursuant to its authority as the general partner of OPI III, may be deemed to indirectly beneficially own the securities of the Issuer held by OPI III, which represent an aggregate of 2,705,700 shares of Common Stock and warrants to purchase 78,287 shares of Common Stock and constitute approximately 10.98% of the Common Stock(1).

As a result of the agreements and relationships described in Items 2 and 3 above and Item 6 below, the Reporting Persons have shared discretionary power to direct the vote and the disposition of the securities of the Issuer held by OPI III, and OrbiMed Advisors LLC and Isaly have shared discretionary power to direct the vote and the disposition of the securities of the Issuer held by Associates III.

(c) The information provided in Item 3 is incorporated by reference herein.  In addition, on February 8, 2011 the OrbiMed Stockholders acquired 1,487,680 shares of Common Stock upon automatic conversion of the Series A convertible preferred stock of the Issuer and 980,424 shares of Common Stock upon the conversion of convertible promissory notes of the Issuer, in each case in connection with the closing of the Issuer’s initial public offering of Common Stock on February 8, 2011. The convertible promissory notes of the Issuer that were automatically converted into Common Stock at the closing of the initial public offering were acquired by the OrbiMed Stockholders in multiple private placement transactions with the Issuer that occurred beginning in January 2009 through December 2010.

(d) Not applicable.

(e) Not applicable.

Item 6.

Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer

“Item 6 Contracts, Arrangements, Understandings or Relationships with Respect to Securities of the Issuer” is hereby amended and restated as follows:

In addition to the relationships between the Reporting Persons described in Items 2, 3 and 5, OrbiMed Capital GP III LLC is the sole general partner of OPI III, pursuant to the terms of the limited partnership agreement of OPI III, and OrbiMed Advisors LLC is the sole managing member of OrbiMed Capital GP III LLC, pursuant to the terms of the limited liability company agreement of OrbiMed Capital GP III LLC.  OrbiMed Advisors LLC is also the sole general partner of Associates III, pursuant to the terms of the limited partnership agreement of Associates III.  Pursuant to these agreements and relationships, OrbiMed Advisors LLC and OrbiMed Capital GP III LLC have shared discretionary investment management authority with respect to the assets of OPI III.  Such authority includes the power to direct the vote and the disposition of the securities of the Issuer held by OPI III and Associates III.  OrbiMed Advisors LLC also has discretionary investment management authority with respect to the assets of Associates III and, as a result, has the power to direct the vote and disposition of the securities of the Issuer held by Associates III. Isaly, pursuant to his authority as the managing member of, and owner of a controlling interest in, OrbiMed Advisors LLC, also has shared discretionary power to direct the vote and the disposition of the securities of the Issuer held by OPI III and Associates III.

On June 2, 2011, Carl L. Gordon, who is a member of OrbiMed Advisors LLC, resigned as a member of the board of directors of the Issuer and since June 2, 2011, the Reporting Persons have had no representation on the Issuer’s board of directors.

Investors’ Rights Agreement

The Reporting Persons and other stockholders of the Issuer have entered into an Investors’ Rights Agreement with the Issuer, dated March 23, 2007 (the “Investors’ Rights Agreement”).  Subject to the terms of the Investors’ Rights Agreement, holders of shares of Common Stock having registration rights (“Registrable Securities”) can demand that the Issuer file a registration statement or request that their shares be covered by a registration statement that the Issuer is otherwise filing.

Demand Registration Rights

If the Issuer shall receive a written request from the Holders (as defined in the Investors’ Rights Agreement) of at least 30% of the Registrable Securities then outstanding (the “Initiating Holders”) that the Issuer file a Form S-1 registration statement under the Securities Act of 1933, as amended (the “Securities Act”) with respect to at least 20% of the Registrable Securities then outstanding (or a lesser percent if the anticipated aggregate offering price, net of selling expenses, would exceed $10,000,000), then the Issuer shall (i) within ten (10) days after the date such request is given, give notice thereof to all Holders other than the Initiating Holders; and (ii) as soon as practicable, and in any event within sixty (60) days after the date such request is given by the Initiating Holders, file a Form S-1 registration statement under the Securities Act covering all Registrable Securities that the Initiating Holders requested to be registered and any additional Registrable Securities requested to be included in such registration by any other Holders.

If at any time when it is eligible to use a Form S-3 registration statement, the Issuer receives a request from Holders of at least twenty percent (20%) of the Registrable Securities then outstanding that the Issuer file a Form S-3 registration statement with respect to outstanding Registrable Securities of such Holders having an anticipated aggregate offering price, net of selling expenses, of at least $1,000,000, then the Issuer shall

(i) within ten (10) days after the date such request is given, give notice to all Holders other than the Initiating Holders; and (ii) as soon as practicable, and in any event within forty-five (45) days after the date such request is given by the Initiating Holders, file a Form S-3 registration statement under the Securities Act covering all Registrable Securities requested to be included in such registration by any other Holders.

The Issuer shall not be obligated to effect, or to take any action to effect, any demand registration after the Issuer has effected three Form S-1 demand registrations.

The Issuer shall not be obligated to effect, or to take any action to effect, any Form S-3 demand registration if the Issuer has effected two Form S-3 demand registrations within the twelve month period immediately preceding the date of such request.

Piggyback Registration Rights

If the Issuer proposes to register any of its stock in connection with the public offering of such securities by the Issuer or on behalf of selling stockholders, the Issuer shall, at such time, promptly give each Holder written notice of such registration. Upon the written request of each Holder, the Issuer shall, subject to the certain limitations, use its reasonable best efforts to cause to be registered all of the Registrable Securities that each such Holder has requested to be registered.

Expenses of Registration

Subject to certain limitations, the Issuer will pay all registration expenses, other than underwriting discounts and commissions, related to any registration effected pursuant to the Investor Rights Agreement.

Indemnification

The Investor Rights Agreement contains customary cross-indemnification provisions, pursuant to which the Issuer is obligated to indemnify the selling stockholders in the event of material misstatements or omissions in the registration statement attributable to the Issuer, and the selling stockholders are obligated to indemnify the Issuer for material misstatements or omissions attributable to them.

Termination

No Holder shall be entitled to exercise any registration rights after the earlier of:

·      when all of such Holder’s Registrable Securities could be sold without restriction during any 90-day period and such Holder owns less than 1% of the then outstanding capital stock of the Issuer; and

·      February 8, 2016.

The foregoing description of the terms of the Investor Rights Agreement is intended as a summary only and is qualified in its entirety by reference to the Investor Rights Agreement, which is filed as an exhibit to the Schedule 13D originally filed by the Reporting Persons and incorporated by reference herein.

Other than as described in this Schedule 13D, to the best of the Reporting Persons’ knowledge, there are no other contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 and between such persons and any person with respect to any securities of the Issuer.

Item 7.

Material to be Filed as Exhibits

A.            Agreement regarding filing of joint Schedule 13D.

OrbiMed Advisors LLC

By:

/s/ Samuel D. Isaly

Name: Samuel D. Isaly

Title: Managing Member

OrbiMed Capital GP III LLC

By:

OrbiMed Advisors LLC,

its Managing Member

By:

/s/ Samuel D. Isaly

Name: Samuel D. Isaly

Title: Managing Member

By:

/s/ Samuel D. Isaly

Name: Samuel D. Isaly

Name

Position with Reporting
Person

Principal Occupation

Samuel D. Isaly

Managing Member

Managing Member

OrbiMed Advisors LLC

Michael B. Sheffery

Member

Member

OrbiMed Advisors LLC

Carl L. Gordon

Member

Member

OrbiMed Advisors LLC

Sven H. Borho

German and Swedish Citizen

Member

Member

OrbiMed Advisors LLC

Jonathan T. Silverstein

Member

Member

OrbiMed Advisors LLC

W. Carter Neild

Member

Member

OrbiMed Advisors LLC

Geoffrey C. Hsu

Member

Member

OrbiMed Advisors LLC

Evan D. Sotiriou

Chief Financial Officer

Chief Financial Officer

OrbiMed Advisors LLC

OrbiMed Advisors LLC

By:

/s/ Samuel D. Isaly

Name: Samuel D. Isaly

Title: Managing Member

OrbiMed Capital GP III LLC

By:

OrbiMed Advisors LLC,

its Managing Member

By:

/s/ Samuel D. Isaly

Name: Samuel D. Isaly

Title: Managing Member

By:

/s/ Samuel D. Isaly

Name: Samuel D. Isaly