<SEC-DOCUMENT>0001123292-16-001876.txt : 20160302
<SEC-HEADER>0001123292-16-001876.hdr.sgml : 20160302
<ACCEPTANCE-DATETIME>20160302175727
ACCESSION NUMBER:		0001123292-16-001876
CONFORMED SUBMISSION TYPE:	3
PUBLIC DOCUMENT COUNT:		2
CONFORMED PERIOD OF REPORT:	20160302
FILED AS OF DATE:		20160302
DATE AS OF CHANGE:		20160302

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			Syndax Pharmaceuticals Inc
		CENTRAL INDEX KEY:			0001395937
		STANDARD INDUSTRIAL CLASSIFICATION:	PHARMACEUTICAL PREPARATIONS [2834]
		IRS NUMBER:				320162505
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		460 TOTTEN POND ROAD
		STREET 2:		SUITE 650
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
		BUSINESS PHONE:		781-419-1400

	MAIL ADDRESS:	
		STREET 1:		460 TOTTEN POND ROAD
		STREET 2:		SUITE 650
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Metzger Michael A
		CENTRAL INDEX KEY:			0001507220

	FILING VALUES:
		FORM TYPE:		3
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-37708
		FILM NUMBER:		161478729

	MAIL ADDRESS:	
		STREET 1:		C/O SYNDAX PHARMACEUTICALS, INC.
		STREET 2:		400 TOTTEN POND ROAD,
		CITY:			WALTHAM
		STATE:			MA
		ZIP:			02451
</SEC-HEADER>
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<TYPE>3
<SEQUENCE>1
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<DESCRIPTION>PRIMARY DOCUMENT
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<ownershipDocument>

    <schemaVersion>X0206</schemaVersion>

    <documentType>3</documentType>

    <periodOfReport>2016-03-02</periodOfReport>

    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0001395937</issuerCik>
        <issuerName>Syndax Pharmaceuticals Inc</issuerName>
        <issuerTradingSymbol>SNDX</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001507220</rptOwnerCik>
            <rptOwnerName>Metzger Michael A</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O SYNDAX PHARMACEUTICALS, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>400 TOTTEN POND ROAD, SUITE 110</rptOwnerStreet2>
            <rptOwnerCity>WALTHAM</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02451</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President and COO</officerTitle>
        </reportingOwnerRelationship>
    </reportingOwner>

    <derivativeTable>
        <derivativeHolding>
            <securityTitle>
                <value>Employee Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>7.20</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F1"/>
            </exerciseDate>
            <expirationDate>
                <value>2025-06-01</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>328933</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
        <derivativeHolding>
            <securityTitle>
                <value>Employee Stock Option (right to buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>10.90</value>
            </conversionOrExercisePrice>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2025-09-09</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>157307</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value></value>
                </natureOfOwnership>
            </ownershipNature>
        </derivativeHolding>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Thirty seven and one half percent (37.5%) of the total number of shares underlying the option shall vest on the earlier of (i) May 5, 2016 and (ii) the completion of an initial public offering of the Issuer; and one-thirty-sixth (1/36th) of the remaining shares underlying the option shall vest monthly thereafter on the last day of each month, subject to the reporting person's continuous service on the relevant vesting dates.</footnote>
        <footnote id="F2">Thirty-seven and one half percent (37.5%) of the total number of shares underlying the option shall vest on the earlier of (i) May 5, 2016 and (ii) the completion of an initial public offering of the Issuer; and one-thirty-sixth (1/36th) of the remaining shares underlying the option shall vest monthly thereafter on the last day of each month, subject to the reporting person's continuous service on the relevant vesting dates.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Michael A. Metzger</signatureName>
        <signatureDate>2016-03-02</signatureDate>
    </ownerSignature>
</ownershipDocument>
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<div style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif; TEXT-ALIGN: left; MARGIN-TOP: 12pt; TEXT-INDENT: 36pt">The undersigned, an officer of Syndax Pharmaceuticals, Inc., a Delaware corporation (the "<font style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif"><u>Company</u></font>"), hereby constitutes and appoints Briggs Morrison and John Pallies, for so long as each such person is an employee of the Company, and Laura Berezin, for so long as she is a partner of a law firm representing the Company, the undersigned's true and lawful attorney-in-fact, each with the power to act alone for the undersigned and in the undersigned's name, place and stead, to:</div>

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<div style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif; TEXT-ALIGN: left; MARGIN-TOP: 12pt">Prepare, complete and execute Forms 3, 4 and 5 under the Securities Exchange Act of 1934, as amended (the "<font style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif"><u>Exchange Act</u></font>"), and other forms and all amendments thereto on the undersigned's behalf as such attorney-in-fact shall in his or her discretion determine to be required or advisable pursuant to Section 16 of the Exchange Act and the rules and regulations promulgated thereunder, or any successor laws and regulations, as a consequence of the undersigned's beneficial ownership of securities of the Company or changes in such beneficial ownership; and</div>
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<div style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif; TEXT-ALIGN: left; MARGIN-TOP: 12pt">Do all acts necessary in order to file such forms and amendments with the Securities and Exchange Commission, any securities exchange or national association, the Company and such other persons or agencies as the attorney-in-fact shall deem appropriate.</div>
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<div style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif; TEXT-ALIGN: left; MARGIN-TOP: 12pt; TEXT-INDENT: 36pt">The undersigned hereby ratifies and confirms all that said attorneys-in-fact and agents shall do or cause to be done by virtue hereof. The undersigned acknowledges that the foregoing attorneys-in-fact, in serving in such capacity at the request of the undersigned, are not assuming, nor is the Company assuming, any of the undersigned's responsibilities to comply with or liability under Section 16 of the Exchange Act or any rules and regulations promulgated thereunder or any successor laws and regulations.</div>

<div style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif; TEXT-ALIGN: left; MARGIN-TOP: 12pt; TEXT-INDENT: 36pt">This Power of Attorney shall remain in full force and effect until the undersigned is no longer required to file Forms 3, 4, or 5 with respect to the undersigned's beneficial ownership of securities of the Company or changes in such beneficial ownership, unless earlier revoked by the undersigned in a signed writing delivered to the Company and the foregoing attorneys-in-fact.</div>

<div style="FONT-SIZE: 12pt; FONT-FAMILY: 'Times New Roman', Times, serif; TEXT-ALIGN: left; MARGIN-TOP: 12pt; TEXT-INDENT: 36pt">The undersigned has executed this Power of Attorney as of September 24, 2015.</div>
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