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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">UNITED STATES</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">SECURITIES AND EXCHANGE COMMISSION</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Washington, D.C. 20549</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">FORM&nbsp;8-K</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">Current Report Pursuant to</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">Section&nbsp;13 or 15(d)&nbsp;of the</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">Securities Exchange Act of 1934</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Date of Report (Date of earliest event reported):</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">May&nbsp;21, 2013</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="5" face="Times New Roman" style="font-size:18.0pt;font-weight:bold;">IRONWOOD PHARMACEUTICALS,&nbsp;INC.</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Exact Name of Registrant as Specified in Charter)</font></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Delaware</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">001-34620</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">04-3404176</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(State   or Other Jurisdiction</font></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(IRS   Employer</font></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">of   Incorporation)</font></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Identification   No.)</font></p>    </td>   </tr>  </table>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">301 Binney Street</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">02142</font></b></p>    </td>   </tr>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Cambridge, Massachusetts</font></b></p>    </td>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">(617) 621-7722</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Registrant&#146;s telephone number,</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">including area code)</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Check the appropriate box below if the Form&nbsp;8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (<i>see </i>General Instruction A.2. below):</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="2" style="font-size:10.0pt;">&#160;&#160;&#160; Written communications pursuant to Rule&nbsp;425 under the Securities Act (17 CFR 230.425)</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="2" style="font-size:10.0pt;">&#160;&#160;&#160; Soliciting material pursuant to Rule&nbsp;14a-12 under the Exchange Act (17 CFR 240.14a-12)</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="2" style="font-size:10.0pt;">&#160;&#160;&#160; Pre-commencement communications pursuant to Rule&nbsp;14d-2(b)&nbsp;under the Exchange Act (17 CFR 240.14d-2(b))</font></p>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="2" style="font-size:10.0pt;">&#160;&#160;&#160; Pre-commencement communications pursuant to Rule&nbsp;13e-4(c)&nbsp;under the Exchange Act (17 CFR 240.13e-4(c))</font></p>
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<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item 8.01 Other Events</font></b></p>
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<p style="margin:0in 0in .0001pt;text-indent:24.5pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">On May&nbsp;21, 2013,&nbsp;Ironwood Pharmaceuticals,&nbsp;Inc. (the &#147;<u>Company</u>&#148;) entered into an underwriting agreement (the &#147;<u>Underwriting Agreement</u>&#148;) with the several underwriters set forth in Schedule 1 thereto (collectively, the &#147;<u>Underwriters</u>&#148;), for whom J.P. Morgan and BofA Merrill Lynch are acting as representatives, related to a public offering (the &#147;<u>Offering</u>&#148;) of 10,500,000 shares of the Company&#146;s Class&nbsp;A common stock, par value $0.001 par value per share, at a price to the public of $13.00 per share pursuant to a registration statement on Form&nbsp;S-3 (File No.&nbsp;333-179430) and a related prospectus supplement.&#160; In addition, the Company granted the Underwriters an option exercisable for 30 days from the date of the Underwriting Agreement to purchase, at the public offering price less any underwriting discounts and commissions, up to an additional 1,575,000 shares of Class&nbsp;A common stock.&#160; The foregoing description of the Underwriting Agreement is qualified in its entirety by reference to the Underwriting Agreement, which is filed as Exhibit&nbsp;1.1 hereto and incorporated herein by reference.&#160; J.P. Morgan and BofA Merrill Lynch served as joint book-running managers and as representatives of the Underwriters for the Offering, and Morgan Stanley served as book-running manager for the Offering. Cowen and Company, Ladenburg Thalmann &amp; Co. Inc. and Mizuho Securities served as co-managers for the Offering. The Offering closed on May&nbsp;24, 2013. On May&nbsp;21, 2013, the Company issued a press release announcing the pricing of the Offering.&#160; A copy of the Company&#146;s press release is attached hereto as Exhibit&nbsp;99.1 and is incorporated herein by reference.</font></p>
<p style="margin:0in 0in .0001pt;text-indent:24.5pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;text-indent:24.5pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">An opinion dated May&nbsp;24, 2013 regarding the validity of the shares of Class&nbsp;A common stock offered in the Offering is filed as Exhibit&nbsp;5.1 to this Current Report on Form&nbsp;8-K.</font></p>
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<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item 9.01 Financial Statements and Exhibits</font></b></p>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">Description</font></b></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">1.1</font></p>    </td>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Underwriting   Agreement, dated as of May&nbsp;21, 2013, by and between Ironwood   Pharmaceuticals,&nbsp;Inc., J.P. Morgan Securities LLC and Merrill Lynch,   Pierce, Fenner&nbsp;&amp; Smith Incorporated, as representatives of the   several Underwriters named in Schedule 1 thereto.</font></p>    </td>   </tr>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
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<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">5.1</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Opinion   of Ropes&nbsp;&amp; Gray LLP, with respect to the validity of the   Class&nbsp;A common stock registered.</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">99.1</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Ironwood   Pharmaceuticals,&nbsp;Inc. Press Release dated May&nbsp;21, 2013, announcing   the pricing of an offering of Class&nbsp;A common stock.</font></p>    </td>   </tr>  </table>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
</div>
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<div style="font-family:Times New Roman;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">SIGNATURES</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;text-indent:24.5pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.</font></p>
<p style="margin:0in 0in .0001pt;text-indent:24.5pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt;text-indent:24.5pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<table border="0" cellspacing="0" cellpadding="0" width="100%" style="border-collapse:collapse;width:100.0%;">
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.26%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="49%" colspan="2" valign="top" style="padding:0in 0in 0in 0in;width:49.74%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">IRONWOOD   PHARMACEUTICALS,&nbsp;INC.</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.26%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="49%" colspan="2" valign="top" style="padding:0in 0in 0in 0in;width:49.74%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.26%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.72%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.02%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.26%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Dated:   May&nbsp;24, 2013</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.72%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">By:</font></p>    </td>
<td width="45%" valign="top" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:45.02%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">/s/   </font><font size="2" style="font-size:10.0pt;">Halley   E. Gilbert</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.26%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.72%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:45.02%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Name:   Halley E. Gilbert</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.26%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="4%" valign="top" style="padding:0in 0in 0in 0in;width:4.72%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="45%" valign="top" style="padding:0in 0in 0in 0in;width:45.02%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Title:   Vice President, Legal Affairs and General Counsel</font></p>    </td>   </tr>  </table>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
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<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">EXHIBIT&nbsp;INDEX</font></b></p>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">&nbsp;</font></b></p>
<table border="0" cellspacing="0" cellpadding="0" width="100%" style="border-collapse:collapse;width:100.0%;">
<tr>
<td width="9%" valign="bottom" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">Exhibit&nbsp;No.</font></b></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="1" face="Times New Roman" style="font-size:1.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="88%" valign="bottom" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:88.82%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">Description</font></b></p>    </td>   </tr>
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<td width="9%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="88%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">1.1</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Underwriting   Agreement, dated as of May&nbsp;21, 2013, by and between Ironwood   Pharmaceuticals,&nbsp;Inc., J.P. Morgan Securities LLC and Merrill Lynch,   Pierce, Fenner&nbsp;&amp; Smith Incorporated, as representatives of the   several Underwriters named in Schedule 1 thereto.</font></p>    </td>   </tr>
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<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">5.1</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Opinion   of Ropes&nbsp;&amp; Gray LLP, with respect to the validity of the   Class&nbsp;A common stock registered.</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>   </tr>
<tr>
<td width="9%" valign="top" style="padding:0in 0in 0in 0in;width:9.04%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">99.1</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.14%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="88%" valign="top" style="padding:0in 0in 0in 0in;width:88.82%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Ironwood   Pharmaceuticals,&nbsp;Inc. Press Release dated May&nbsp;21, 2013, announcing   the pricing of an offering of Class&nbsp;A common stock.</font></p>    </td>   </tr>  </table>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
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UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
FORM 8-K
Current Report Pursuant to
Section 13 or 15(d) of the
Securities Exchange Act of 1934
Date of Report (Date of earliest event reported):
May 21, 2013
IRONWOOD PHARMACEUTICALS, INC.
(Exact Name of Registrant as Specified in Charter)
Delaware
001-34620
04-3404176
(State   or Other Jurisdiction
(Commission   File Number)
(IRS   Employer
of   Incorporation)
Identification   No.)
301 Binney Street
02142
Cambridge, Massachusetts
(Zip   code)
(Address   of principal
executive offices)
(617) 621-7722
(Registrant's telephone number,
including area code)
Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions (
see
General Instruction A.2. below):
o
Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)
o
Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)
o
Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))
o
Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))
Item 8.01 Other Events
On May 21, 2013, Ironwood Pharmaceuticals, Inc. (the "
Company
") entered into an underwriting agreement (the "
Underwriting Agreement
") with the several underwriters set forth in Schedule 1 thereto (collectively, the "
Underwriters
"), for whom J.P. Morgan and BofA Merrill Lynch are acting as representatives, related to a public offering (the "
Offering
") of 10,500,000 shares of the Company's Class A common stock, par value $0.001 par value per share, at a price to the public of $13.00 per share pursuant to a registration statement on Form S-3 (File No. 333-179430) and a related prospectus supplement.  In addition, the Company granted the Underwriters an option exercisable for 30 days from the date of the Underwriting Agreement to purchase, at the public offering price less any underwriting discounts and commissions, up to an additional 1,575,000 shares of Class A common stock.  The foregoing description of the Underwriting Agreement is qualified in its entirety by reference to the Underwriting Agreement, which is filed as Exhibit 1.1 hereto and incorporated herein by reference.  J.P. Morgan and BofA Merrill Lynch served as joint book-running managers and as representatives of the Underwriters for the Offering, and Morgan Stanley served as book-running manager for the Offering. Cowen and Company, Ladenburg Thalmann & Co. Inc. and Mizuho Securities served as co-managers for the Offering. The Offering closed on May 24, 2013. On May 21, 2013, the Company issued a press release announcing the pricing of the Offering.  A copy of the Company's press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference.
An opinion dated May 24, 2013 regarding the validity of the shares of Class A common stock offered in the Offering is filed as Exhibit 5.1 to this Current Report on Form 8-K.
Item 9.01 Financial Statements and Exhibits
Exhibit No.
Description
1.1
Underwriting   Agreement, dated as of May 21, 2013, by and between Ironwood   Pharmaceuticals, Inc., J.P. Morgan Securities LLC and Merrill Lynch,   Pierce, Fenner & Smith Incorporated, as representatives of the   several Underwriters named in Schedule 1 thereto.
5.1
Opinion   of Ropes & Gray LLP, with respect to the validity of the   Class A common stock registered.
99.1
Ironwood   Pharmaceuticals, Inc. Press Release dated May 21, 2013, announcing   the pricing of an offering of Class A common stock.
SIGNATURES
Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.
IRONWOOD   PHARMACEUTICALS, INC.
Dated:   May 24, 2013
By:
/s/
Halley   E. Gilbert
Name:   Halley E. Gilbert
Title:   Vice President, Legal Affairs and General Counsel
EXHIBIT INDEX
Exhibit No.
Description
1.1
Underwriting   Agreement, dated as of May 21, 2013, by and between Ironwood   Pharmaceuticals, Inc., J.P. Morgan Securities LLC and Merrill Lynch,   Pierce, Fenner & Smith Incorporated, as representatives of the   several Underwriters named in Schedule 1 thereto.
5.1
Opinion   of Ropes & Gray LLP, with respect to the validity of the   Class A common stock registered.
99.1
Ironwood   Pharmaceuticals, Inc. Press Release dated May 21, 2013, announcing   the pricing of an offering of Class A common stock.