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Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 Form 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31 , 2020 TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from Commission file number 001-36576 to Delaware ( State or other jurisdiction of incorporation or organization ) Title of Each Class Common Stock , par value $ 0.001 per share MARINUS PHARMACEUTICALS Marinus Pharmaceuticals , Inc. ( Exact name of registrant as specified in its charter ) 5 Radnor Corporate Center , Suite 500 100 Matsonford Road Radnor , PA 19087 ( Address of principal executive offices including zip code ) ( 484 ) 801-4670 ( Registrant's telephone number , including area code ) Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) MRNS 20-0198082 ( I.R.S. Employer Identification No. ) Name of Each Exchange on Which Registered Nasdaq Global Market Securities registered pursuant to Section 12 ( g ) of the Act : None . Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes No Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Exchange Act . Yes No Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes No Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( § 232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes No □ Non - accelerated filer Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of “ large accelerated filer , ” “ accelerated filer , ” “ smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Accelerated filer Smaller reporting company Large accelerated filer Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . O Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act ) . Yes No The aggregate market value of the registrant's common stock ( the only common equity of the registrant ) held by non - affiliates of the registrant on the last business day of the registrant's most recent completed second fiscal quarter ( June 30 , 2020 ) was $ 303,163,851 , based on the closing price reported on the Nasdaq Global Market on June 30 , 2020 . The total number of shares of the registrant's common stock , par value $ 0.001 per share , outstanding as of March 8 , 2021 was 36,578,460 . Documents Incorporated by Reference Certain portions of the registrant's Definitive Proxy Statement for its 2021 Annual Meeting of the Stockholders , which is expected to be filed with the U.S. Securities and Exchange Commission within 120 days after the end of the registrant's fiscal year ended December 31 , 2020 , are incorporated by reference into Part III , Items 10-14 of this Annual Report on Form 10 - K .