UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

x

Rule 13d-1(b)

o

Rule 13d-1(c)

o

Rule 13d-1(d)

CUSIP No. 46333X108

1.

Names of Reporting Persons

Sectoral Asset Management Inc.

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization

Canada

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power

3,943,762

6.

Shared Voting Power

-0-

7.

Sole Dispositive Power

5,084,999

8.

Shared Dispositive Power

-0-

9.

Aggregate Amount Beneficially Owned by Each Reporting Person

5,084,999

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)

5.2%

12.

Type of Reporting Person (See Instructions)

IA

CUSIP No. 46333X108

1.

Names of Reporting Persons

Jérôme G. Pfund

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization

Swiss

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power

3,943,762

6.

Shared Voting Power

-0-

7.

Sole Dispositive Power

5,084,999

8.

Shared Dispositive Power

-0-

9.

Aggregate Amount Beneficially Owned by Each Reporting Person

5,084,999

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)

5.2%

12.

Type of Reporting Person (See Instructions)

IN

CUSIP No. 46333X108

1.

Names of Reporting Persons

Michael L. Sjöström

2.

Check the Appropriate Box if a Member of a Group (See Instructions)

(a)

o

(b)

o

3.

SEC Use Only

4.

Citizenship or Place of Organization

Swiss

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

5.

Sole Voting Power

3,943,762

6.

Shared Voting Power

-0-

7.

Sole Dispositive Power

5,084,999

8.

Shared Dispositive Power

-0-

9.

Aggregate Amount Beneficially Owned by Each Reporting Person

5,084,999

10.

Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions)  o

11.

Percent of Class Represented by Amount in Row (9)

5.2%

12.

Type of Reporting Person (See Instructions)

IN

Item 1.

(a)

Name of Issuer
Ironwood Pharmaceuticals Inc.

(b)

Address of Issuer’s Principal Executive Offices
301 Binney Street, Cambridge, MA 02142, United States

Item 2.

(a)

Name of Person(s) Filing
Sectoral Asset Management Inc.
Jérôme G. Pfund

Michael L. Sjöström

(b)

Address of Principal Business Office or, if none, Residence
2120-1000 Sherbrooke St. West Montreal PQ H3A 3G4 Canada

(c)

Citizenship
Sectoral Asset Management Inc. is a Canadian corporation

Jérôme G. Pfund is a Swiss citizen

Michael L. Sjöström is a Swiss citizen

(d)

Title of Class of Securities
Common Stock

(e)

CUSIP Number
46333X108

Item 3.

If this statement is filed pursuant to §§240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:

(a)

o

Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);

(b)

o

Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);

(c)

o

Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);

(d)

o

Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);

(e)

x

An investment adviser in accordance with §240.13d-1(b)(1)(ii)(E);

(f)

o

An employee benefit plan or endowment fund in accordance with §240.13d-1(b)(1)(ii)(F);

(g)

x

A parent holding company or control person in accordance with §240.13d-1(b)(1)(ii)(G);

(h)

o

A savings association as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);

(i)

o

A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);

(j)

o

A non-U.S. institution in accordance with § 240.13d–1(b)(1)(ii)(J);

(k)

o

Group, in accordance with § 240.13d–1(b)(1)(ii)(K). If filing as a non-U.S. institution in accordance with

§ 240.13d–1(b)(1)(ii)(J), please specify the type of institution:____________________________

Item 4.

Ownership

Sectoral Asset Management Inc, in its capacity as an investment adviser, has the sole right to dispose of or vote the number of shares of common stock of the Issuer set forth in this filing.  Jérôme G. Pfund and Michael L. Sjöström, together, hold majority of shares of Sectoral Asset Management Inc.  Sectoral Asset Management, Inc. and Messrs. Pfund and Sjöström disclaim beneficial ownership of the Issuer’s common stock held by Sectoral Asset Management Inc.

Provide the following information regarding the aggregate number and percentage of the class of securities of the issuer identified in Item 1.

(a)

Amount beneficially owned:   

Sectoral Asset Management Inc.:  5,084,999 shares
Jérôme G. Pfund:  5,084,999 shares
Michael L. Sjöström:  5,084,999 shares

(b)

Percent of class:   

Sectoral Asset Management Inc.:  5.2%
Jérôme G. Pfund:  5.2%
Michael L. Sjöström:  5.2%

(c)

Number of shares as to which the person has:

(i)

Sole power to vote or to direct the vote   

Sectoral Asset Management Inc.:  3,943,762 shares
Jérôme G. Pfund:  3,943,762 shares
Michael L. Sjöström:  3,943,762 shares

(ii)

Shared power to vote or to direct the vote    

Sectoral Asset Management Inc.: -0- shares
Jérôme G. Pfund:  -0- shares
Michael L. Sjöström:  -0- shares

(iii)

Sole power to dispose or to direct the disposition of   

Sectoral Asset Management Inc.:  5,084,999 shares
Jérôme G. Pfund:  5,084,999 shares
Michael L. Sjöström:  5,084,999 shares

(iv)

Shared power to dispose or to direct the disposition of   

Not applicable.

Item 5.

Ownership of Five Percent or Less of a Class

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following   o.

Item 6.

Ownership of More than Five Percent on Behalf of Another Person

Various persons, as investment advisory clients of Sectoral Asset Management, Inc.,  have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the common stock of the Issuer.  To the knowledge of Sectoral Asset Management, Inc., no one such person’s interest in the common stock of the Issuer is more than five percent of the total outstanding common stock of the Issuer.

Item 7.

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company or Control Person

Not applicable.

Item 8.

Identification and Classification of Members of the Group

Not applicable.

Item 9.

Notice of Dissolution of Group

Not applicable.

Item 10.

Certification

By signing below the undersigned each certifies that, to the best knowledge and belief of the undersigned, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect.

Date: February 7, 2014

SECTORAL ASSET MANAGEMENT INC.

/s/ Jérôme G. Pfund

By:

Jérôme G. Pfund

Its:

CEO

Date: February 7, 2014

/s/ Jérôme G. Pfund

Jérôme G. Pfund

Date: February 7, 2014

/s/ Michael L. Sjöström

Michael L. Sjöström

ATTENTION

Intentional misstatements or omissions of fact constitute Federal Criminal Violations (See 18 U.S.C. 1001).

SECTORAL ASSET MANAGEMENT INC.

/s/ Jérôme G. Pfund

By:

Jérôme G. Pfund

Its:

CEO

/s/ Jérôme G. Pfund

Jérôme G. Pfund

/s/ Michael L. Sjöström

Michael L. Sjöström