<html>
<head>


  </head>
<body lang="EN-US">
<div style="font-family:Times New Roman;">
<div style="border-bottom:solid windowtext 1.0pt;border-left:none;border-right:none;border-top:solid windowtext 3.0pt;padding:1.0pt 0in 1.0pt 0in;">
<p style="border:none;margin:0in 0in .0001pt;padding:0in;"><font size="1" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>
</div>
<p style="margin:0in 0in .0001pt;"><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="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.&nbsp;20549</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 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>
<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="3" face="Times New Roman" style="font-size:12.0pt;font-weight:bold;">CURRENT REPORT</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;">PURSUANT TO SECTION&nbsp;13 OR 15(d)</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;">OF THE SECURITIES EXCHANGE ACT OF 1934</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 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;">July&nbsp;30, 2013</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 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;">CUBIST 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>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<div align="center">
<table border="0" cellspacing="0" cellpadding="0" width="100%" style="border-collapse:collapse;width:100.0%;">
<tr>
<td width="32%" valign="top" style="padding:0in 0in 0in 0in;width:32.0%;">
<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>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.0%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:1.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="32%" valign="top" style="padding:0in 0in 0in 0in;width:32.0%;">
<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;">0-21379</font></b></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.0%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:1.0pt;font-weight:bold;">&nbsp;</font></b></p>    </td>
<td width="32%" valign="top" style="padding:0in 0in 0in 0in;width:32.0%;">
<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;">22-3192085</font></b></p>    </td>   </tr>
<tr>
<td width="32%" valign="top" style="padding:0in 0in 0in 0in;width:32.0%;">
<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<br>   of Incorporation)</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="32%" valign="top" style="padding:0in 0in 0in 0in;width:32.0%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(Commission File Number)</font></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="32%" valign="top" style="padding:0in 0in 0in 0in;width:32.0%;">
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">(I.R.S. Employer<br>   Identification No.)</font></p>    </td>   </tr>  </table>
</div>
<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;">65 Hayden Avenue</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;">Lexington, Massachusetts 02421</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;">(Address of Principal Executive Offices, including Zip Code)</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;">&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;">(781) 860-8660</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, including area code)</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;">&nbsp;</font></p>
<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:</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 .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">x</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Written communications pursuant to Rule&nbsp;425 under the Securities Act (17 CFR 230.425)</font></p>
<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Soliciting material pursuant to Rule&nbsp;14a-12 under the Exchange Act (17 CFR 240.14a-12)</font></p>
<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Pre-commencement communications pursuant to Rule&nbsp;14d-2(b)&nbsp;under the Exchange Act (17 CFR 240.14d-2(b))</font></p>
<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt .25in;text-indent:-.25in;"><font size="2" face="Wingdings" style="font-size:10.0pt;">o</font><font size="1" style="font-size:3.0pt;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font><font size="2" style="font-size:10.0pt;">Pre-commencement communications pursuant to Rule&nbsp;13e-4(c)&nbsp;under the Exchange Act (17 CFR 240.13e-4(c))</font></p>
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<div style="border-bottom:solid windowtext 3.0pt;border-left:none;border-right:none;border-top:solid windowtext 1.0pt;padding:1.0pt 0in 1.0pt 0in;">
<p style="border:none;margin:0in 0in .0001pt;padding:0in;"><font size="1" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>
</div>
<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>
<!-- SEQ.=1,FOLIO='',FILE='C:\JMS\kdefran\13-17558-3\task6164143\17558-3-ba.htm',USER='kdefran',CD='Jul 30 16:18 2013' -->

<br clear="all" style="page-break-before:always;">
<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 style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item&nbsp;8.01</font></b><b><font size="1" style="font-size:8.5pt;font-weight:bold;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font></b><b><font size="2" style="font-size:10.0pt;font-weight:bold;">Other Events</font></b></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;text-indent:.5in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">On July&nbsp;30, 2013, Cubist Pharmaceuticals,&nbsp;Inc. (&#147;Cubist&#148;) and Optimer Pharmaceuticals,&nbsp;Inc. (&#147;Optimer&#148;) issued a joint press release announcing the execution of an Agreement and Plan of Merger, dated as of July&nbsp;30, 2013, providing for the proposed merger of a wholly-owned subsidiary of Cubist with and into Optimer, with Optimer surviving the proposed merger as a wholly-owned subsidiary of Cubist. A copy of the 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:.5in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt 12.2pt;"><b><i><font size="2" face="Times New Roman" style="font-size:10.0pt;font-style:italic;font-weight:bold;">Important Additional Information Will Be Filed with the Securities and Exchange Commission (SEC)</font></i></b></p>
<p style="margin:0in 0in .0001pt 12.2pt;"><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;">This communication is being made in respect of the proposed merger involving Cubist and Optimer. In connection with the proposed transaction, Cubist intends to file with the SEC a registration statement on Form&nbsp;S-4, which will include a proxy statement/prospectus and other relevant materials in connection with the proposed merger, and each of Cubist and Optimer intend to file with the SEC other documents regarding the proposed transaction. The proxy statement/prospectus and this Current Report are not offers to sell Cubist securities and are not soliciting an offer to buy Cubist securities in any state where the offer and sale is not permitted. The final proxy statement/prospectus will be mailed to the stockholders of Optimer. INVESTORS AND SECURITY HOLDERS OF OPTIMER ARE URGED TO READ THE PROXY STATEMENT/PROSPECTUS (INCLUDING ANY AMENDMENTS OR SUPPLEMENTS THERETO) AND THE OTHER RELEVANT MATERIAL CAREFULLY IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT CUBIST AND OPTIMER AND THE PROPOSED MERGER.</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;">The proxy statement/prospectus and other relevant materials (when they become available), and any and all documents filed with the SEC, may be obtained free of charge at the SEC&#146;s web site at <i>www.sec.gov</i>. In addition, investors and security holders may obtain free copies of the documents filed with the SEC by Cubist by directing a written request to Cubist Pharmaceuticals,&nbsp;Inc., 65 Hayden Avenue, Lexington, Massachusetts 02421, Attention: Investor Relations, and by Optimer by directing a written request to Optimer Pharmaceuticals,&nbsp;Inc., 101 Hudson Street, Suite&nbsp;3501, Jersey City, NJ 07302, Attention: Investor Relations. In addition, investors and security holders may obtain free copies of the documents filed with the SEC by Cubist by going to Cubist&#146;s investor information web site at http://investors.cubist.com/corporateprofile.aspx?iid=4093793 and by Optimer by going to Optimer&#146;s investor information web site at: http://investor.optimerpharma.com/index.cfm.</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;">Optimer and its executive officers and directors and other persons, may be deemed to be participants in the solicitation of proxies from its stockholders in connection with the proposed merger. Information about the executive officers and directors of Optimer and their ownership of Optimer common stock is set forth in its annual report on Form&nbsp;10-K for the year ended December&nbsp;31, 2012, filed with the SEC on March&nbsp;18, 2013 and the proxy statement for Optimer&#146;s 2013 annual meeting of stockholders, filed with the SEC on April&nbsp;12, 2013 and as amended on April&nbsp;19, 2013. Certain executive officers and directors of Optimer and other persons may have direct or indirect interests in the proposed merger due to securities holdings, pre-existing or future indemnification arrangements and rights to severance payments if their employment or directorship, as applicable, is terminated prior to or following the proposed merger. If and to the extent that any of the Optimer participants will receive any additional benefits in connection with the proposed merger, the details of those benefits will be described in the proxy statement/prospectus relating to the proposed merger. Investors and security holders may obtain additional information regarding the direct and indirect interests of Optimer and its executive officers and directors in the proposed merger by reading the proxy statement/prospectus regarding the proposed merger when it becomes available.</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;">&nbsp;</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;">2<a name="PB_2_005553_7056"></a></font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
</div>
<!-- SEQ.=1,FOLIO='2',FILE='C:\JMS\kdefran\13-17558-3\task6164143\17558-3-ba.htm',USER='kdefran',CD='Jul 30 16:18 2013' -->

<br clear="all" style="page-break-before:always;">
<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 style="margin:0in 0in .0001pt .8in;text-indent:-.8in;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">Item&nbsp;9.01.</font></b><b><font size="1" style="font-size:3.0pt;font-weight:bold;">&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; </font></b><b><font size="2" style="font-size:10.0pt;font-weight:bold;">Financial Statements and Exhibits.</font></b></p>
<p style="margin:0in 0in .0001pt .8in;text-indent:-.8in;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>
<p style="margin:0in 0in .0001pt 24.5pt;"><i><font size="2" face="Times New Roman" style="font-size:10.0pt;font-style:italic;">(d)</font></i><i><font size="1" style="font-size:8.5pt;font-style:italic;">&#160;&#160;&#160; </font></i><i><font size="2" style="font-size:10.0pt;font-style:italic;"> Exhibits</font></i></p>
<p style="margin:0in 0in .0001pt;"><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="6%" valign="top" style="padding:0in 0in 0in 0in;width:6.0%;">
<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:2.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="92%" valign="top" style="padding:0in 0in 0in 0in;width:92.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Joint   Press Release dated July&nbsp;30, 2013.</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>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">3<a name="PB_3_005709_5335"></a></font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
</div>
<!-- SEQ.=1,FOLIO='3',FILE='C:\JMS\kdefran\13-17558-3\task6164143\17558-3-ba.htm',USER='kdefran',CD='Jul 30 16:18 2013' -->

<br clear="all" style="page-break-before:always;">
<div>
<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;">SIGNATURE</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;"><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;"><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="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:50.0%;">
<p style="margin:0in 0in .0001pt 58.75pt;text-indent:-10.0pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="50%" colspan="2" valign="top" style="padding:0in 0in 0in 0in;width:50.0%;">
<p style="margin:0in 0in .0001pt;"><b><font size="2" face="Times New Roman" style="font-size:10.0pt;font-weight:bold;">CUBIST PHARMACEUTICALS,&nbsp;INC.</font></b></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.0%;">
<p style="margin:0in 0in .0001pt 10.0pt;text-indent:-10.0pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="6%" valign="top" style="padding:0in 0in 0in 0in;width:6.66%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="43%" valign="top" style="padding:0in 0in 0in 0in;width:43.34%;">
<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:50.0%;">
<p style="margin:0in 0in .0001pt 58.75pt;text-indent:-10.0pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="6%" valign="top" style="padding:0in 0in 0in 0in;width:6.66%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">By:</font></p>    </td>
<td width="43%" valign="top" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:43.34%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">/s/   Thomas J. DesRosier</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.0%;">
<p style="margin:0in 0in .0001pt 58.75pt;text-indent:-10.0pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="6%" valign="top" style="padding:0in 0in 0in 0in;width:6.66%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Name:</font></p>    </td>
<td width="43%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:43.34%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Thomas   J. DesRosier</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.0%;">
<p style="margin:0in 0in .0001pt 58.75pt;text-indent:-10.0pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="6%" valign="top" style="padding:0in 0in 0in 0in;width:6.66%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Title:</font></p>    </td>
<td width="43%" valign="top" style="padding:0in 0in 0in 0in;width:43.34%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Senior   Vice President, Chief Legal Officer, General Counsel and Secretary</font></p>    </td>   </tr>
<tr>
<td width="50%" valign="top" style="padding:0in 0in 0in 0in;width:50.0%;">
<p style="margin:0in 0in .0001pt 58.75pt;text-indent:-10.0pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">&nbsp;</font></p>    </td>
<td width="50%" colspan="2" valign="top" style="padding:0in 0in 0in 0in;width:50.0%;">
<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:50.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Date:   July&nbsp;30, 2013</font></p>    </td>
<td width="50%" colspan="2" valign="top" style="padding:0in 0in 0in 0in;width:50.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</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>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">4<a name="PB_4_005832_5796"></a></font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
</div>
<!-- SEQ.=1,FOLIO='4',FILE='C:\JMS\kdefran\13-17558-3\task6164143\17558-3-ba.htm',USER='kdefran',CD='Jul 30 16:18 2013' -->

<br clear="all" style="page-break-before:always;">
<div>
<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;"><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="8%" valign="bottom" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:8.0%;">
<p style="margin:0in 0in .0001pt;"><b><font size="1" face="Times New Roman" style="font-size:8.0pt;font-weight:bold;">Exhibit<br>   No.</font></b></p>    </td>
<td width="2%" valign="bottom" style="padding:0in 0in 0in 0in;width:2.0%;">
<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="90%" valign="bottom" style="border:none;border-bottom:solid windowtext 1.0pt;padding:0in 0in 0in 0in;width:90.0%;">
<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>
<tr>
<td width="8%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:8.0%;">
<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:2.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:1.0pt;">&nbsp;</font></p>    </td>
<td width="90%" valign="top" style="border:none;padding:0in 0in 0in 0in;width:90.0%;">
<p style="margin:0in 0in .0001pt;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">Joint   Press Release dated July&nbsp;30, 2013.</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>
<p align="center" style="margin:0in 0in .0001pt;text-align:center;"><font size="2" face="Times New Roman" style="font-size:10.0pt;">5<a name="PB_5_005905_2897"></a></font></p>
<div style="margin:0in 0in .0001pt;"><hr size="3" width="100%" noshade color="#010101" align="left"></div>
</div>
<!-- SEQ.=1,FOLIO='5',FILE='C:\JMS\kdefran\13-17558-3\task6164143\17558-3-ba.htm',USER='kdefran',CD='Jul 30 16:18 2013' -->

</body>
</html>
[{"type": "8-K", "sequence": "1", "filename": "a13-17558_38k.htm", "description": "8-K", "secsgml_size_bytes": 28574}]
{"company-data": {"conformed-name": "CUBIST PHARMACEUTICALS INC", "cik": "0000912183", "assigned-sic": "2834", "irs-number": "223192085", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "8-K", "act": "34", "file-number": "000-21379", "film-number": "13996877"}, "business-address": {"street1": "65 HAYDEN AVENUE", "city": "LEXINGTON", "state": "MA", "zip": "02421", "phone": "781-860-8660"}, "mail-address": {"street1": "65 HAYDEN AVENUE", "city": "LEXINGTON", "state": "MA", "zip": "02421"}}
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)
July 30, 2013
CUBIST PHARMACEUTICALS, INC.
(Exact name of registrant as specified in charter)
Delaware
0-21379
22-3192085
(State or Other Jurisdiction
of Incorporation)
(Commission File Number)
(I.R.S. Employer
Identification No.)
65 Hayden Avenue
Lexington, Massachusetts 02421
(Address of Principal Executive Offices, including Zip Code)
(781) 860-8660
(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:
x
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 July 30, 2013, Cubist Pharmaceuticals, Inc. ("Cubist") and Optimer Pharmaceuticals, Inc. ("Optimer") issued a joint press release announcing the execution of an Agreement and Plan of Merger, dated as of July 30, 2013, providing for the proposed merger of a wholly-owned subsidiary of Cubist with and into Optimer, with Optimer surviving the proposed merger as a wholly-owned subsidiary of Cubist. A copy of the press release is attached hereto as Exhibit 99.1 and is incorporated herein by reference.
Important Additional Information Will Be Filed with the Securities and Exchange Commission (SEC)
This communication is being made in respect of the proposed merger involving Cubist and Optimer. In connection with the proposed transaction, Cubist intends to file with the SEC a registration statement on Form S-4, which will include a proxy statement/prospectus and other relevant materials in connection with the proposed merger, and each of Cubist and Optimer intend to file with the SEC other documents regarding the proposed transaction. The proxy statement/prospectus and this Current Report are not offers to sell Cubist securities and are not soliciting an offer to buy Cubist securities in any state where the offer and sale is not permitted. The final proxy statement/prospectus will be mailed to the stockholders of Optimer. INVESTORS AND SECURITY HOLDERS OF OPTIMER ARE URGED TO READ THE PROXY STATEMENT/PROSPECTUS (INCLUDING ANY AMENDMENTS OR SUPPLEMENTS THERETO) AND THE OTHER RELEVANT MATERIAL CAREFULLY IN THEIR ENTIRETY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT CUBIST AND OPTIMER AND THE PROPOSED MERGER.
The proxy statement/prospectus and other relevant materials (when they become available), and any and all documents filed with the SEC, may be obtained free of charge at the SEC's web site at
www.sec.gov
. In addition, investors and security holders may obtain free copies of the documents filed with the SEC by Cubist by directing a written request to Cubist Pharmaceuticals, Inc., 65 Hayden Avenue, Lexington, Massachusetts 02421, Attention: Investor Relations, and by Optimer by directing a written request to Optimer Pharmaceuticals, Inc., 101 Hudson Street, Suite 3501, Jersey City, NJ 07302, Attention: Investor Relations. In addition, investors and security holders may obtain free copies of the documents filed with the SEC by Cubist by going to Cubist's investor information web site at http://investors.cubist.com/corporateprofile.aspx?iid=4093793 and by Optimer by going to Optimer's investor information web site at: http://investor.optimerpharma.com/index.cfm.
Optimer and its executive officers and directors and other persons, may be deemed to be participants in the solicitation of proxies from its stockholders in connection with the proposed merger. Information about the executive officers and directors of Optimer and their ownership of Optimer common stock is set forth in its annual report on Form 10-K for the year ended December 31, 2012, filed with the SEC on March 18, 2013 and the proxy statement for Optimer's 2013 annual meeting of stockholders, filed with the SEC on April 12, 2013 and as amended on April 19, 2013. Certain executive officers and directors of Optimer and other persons may have direct or indirect interests in the proposed merger due to securities holdings, pre-existing or future indemnification arrangements and rights to severance payments if their employment or directorship, as applicable, is terminated prior to or following the proposed merger. If and to the extent that any of the Optimer participants will receive any additional benefits in connection with the proposed merger, the details of those benefits will be described in the proxy statement/prospectus relating to the proposed merger. Investors and security holders may obtain additional information regarding the direct and indirect interests of Optimer and its executive officers and directors in the proposed merger by reading the proxy statement/prospectus regarding the proposed merger when it becomes available.
2
Item 9.01.
Financial Statements and Exhibits.
(d)
Exhibits
99.1
Joint   Press Release dated July 30, 2013.
3
SIGNATURE
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.
CUBIST PHARMACEUTICALS, INC.
By:
/s/   Thomas J. DesRosier
Name:
Thomas   J. DesRosier
Title:
Senior   Vice President, Chief Legal Officer, General Counsel and Secretary
Date:   July 30, 2013
4
EXHIBIT INDEX
Exhibit
No.
Description
99.1
Joint   Press Release dated July 30, 2013.
5