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Exhibit 10.1

AMENDMENT AND PATENT ASSIGNMENT AGREEMENT

        This Amendment and Patent Assignment Agreement (this "Amendment"), dated
as of this 2nd day of July, 2004 (the "Amendment Date"), is made by and between
Sepracor Inc., a Delaware corporation ("Sepracor"), and Aventis Pharma SA
(successor in interest to Rhone-Poulenc Rorer SA), a corporation organized under
the laws of France ("Aventis").

RECITALS

A.    Sepracor and Aventis entered into that certain License and Assignment
Agreement dated as of September 30, 1999 (the "1999 Agreement").

B.    Aventis owns certain patents and patent applications, if any, as more
particularly described on Schedule 1.21, which are the foreign counterparts of
the RPR Application (as defined in the 1999 Agreement) assigned to Sepracor
pursuant to the 1999 Agreement and certain know-how relating to (+) zopiclone,
the racemate or the other enantiomer thereof, or metabolites of any of the
foregoing.

C.    Aventis is willing to assign the above-referenced patents and patent
applications, if any, to Sepracor and license to Sepracor exclusive rights under
the above-referenced know-how to make, have made, use, market, sell, offer for
sale, have sold and distribute pharmaceutical products containing (+) zopiclone
worldwide, on the terms and subject to the conditions set forth in the 1999
Agreement and this Amendment, and Sepracor is willing to accept such assignment
and license.

D.    Sepracor is willing to allow Aventis to assign certain rights under the
1999 Agreement to a third party.

E.    Sepracor and Aventis wish to amend certain provisions of the 1999
Agreement as set forth specifically in this Amendment.

        NOW THEREFORE, in consideration of the mutual promises, covenants and
agreements hereinafter set forth, the sufficiency of which is hereby
acknowledged, the parties to this Amendment mutually agree as follows:

ARTICLE 1

DEFINITIONS

        Capitalized terms used herein and not otherwise defined shall have the
meanings given to them in the 1999 Agreement, as amended.

ARTICLE 2

AMENDMENT OF 1999 AGREEMENT

        Effective from and after the date hereof, subject to Article 5 of this
Amendment, the 1999 Agreement shall be amended as follows:

         2.1  All references to the defined term "RPR" shall be replaced
throughout the 1999 Agreement by the term "Aventis" as defined in this
Amendment.

         2.2  Article 1 (Definitions) is hereby amended as follows:

      2.2.1  The definition of "Agreement" is hereby amended and restated in its
entirety as follows:

        "Agreement" shall mean this License and Assignment Agreement, as
amended.

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      2.2.2  The definition of the term "Product" is hereby amended by adding
the phrase ", provided, however, that in the Additional Territory, the term
"Product' shall not include (±) zopiclone" to the end of such definition.

      2.2.3  A new definition of "Additional Patents" is hereby added as
Section 1.21 as follows:

        "Additional Patents" shall mean French Patent Application No. 91-00490
and PCT International Patent Application No. PCT/FR92/00031 (collectively
"Priority Applications"), and any patents and patent applications (whether
pending, lapsed, or abandoned) in the Additional Territory claiming benefit of
at least one Priority Application, including without limitation, those described
on Schedule 1.21 hereto, and any and all additions, divisions, continuations,
continuations-in-part, reissues, reexaminations, substitutions, supplemental
protection certificates, extensions, patent term extensions, and renewals
thereof, and patents issued therefrom, which are the foreign counterparts of the
RPR Application.

      2.2.4  A new definition of "Additional Territory" is hereby added as
Section 1.22 as follows:

        "Additional Territory" shall mean all of the countries in the world and
their territories and possessions, other than the United States of America and
its territories and possessions.

      2.2.5  A new definition of "Amendment Date" is hereby added as
Section 1.23 as follows:

        "Amendment Date" shall mean July 2, 2004.

      2.2.6  A new definition of "Amendment Effective Date" is hereby added as
Section 1.24 as follows:

        "Amendment Effective Date" shall mean the date upon which the
shareholders of Aventis SA shall have sold to Sanofi-Synthelabo, and
Sanofi-Synthelabo shall have purchased from the shareholders of Aventis SA, more
than fifty percent (50%) of the shares and voting rights of Aventis SA in
furtherance of the tender offer launched by Sanofi-Synthelabo with respect to
Aventis SA on the terms set forth in the "Note d'Information" that received
approval of the French "Autorites des Marches Financiers", as the same may be
amended.

         2.3  The existing Section 2.1 shall be made Section 2.1(a) and a new
Section 2.1(b) is hereby added as follows:

        Within ten (10) Business Days of the Amendment Effective Date, Aventis
shall assign all right, title and interest to the Additional Patents to Sepracor
pursuant to an assignment substantially in the form attached hereto as
Schedule 2.1(b) and shall deliver such executed assignments to Sepracor within
such ten (10) Business Day period. Within twenty (20) Business Days of execution
and delivery by Aventis of such assignment, up to date copies of all underlying
prosecution history files of the RPR Application and the Additional Patents not
previously provided to Sepracor shall be transmitted to Sepracor. Sepracor may,
at its sole discretion, attend to filing and recordation of such assignments
with the applicable patent office(s) in the applicable jurisdiction(s).

         2.4  Section 2.2 is hereby amended by adding the following phrase at
the end of the first sentence:

        and, as of the Amendment Effective Date, an exclusive license (exclusive
even as to Aventis and its Affiliates except as provided under Section 2.9)
under the RPR Know-How and Improvements to develop, have developed, make, have
made, use, market, sell, offer for sale, have sold and distribute Product in the
Additional Territory.

         2.5  Section 2.2 is hereby further amended by (i) adding the phrase
"or, as of the Amendment Effective Date, the Additional Territory" after the
term "Territory" in the last sentence of Section 2.2

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and (ii) replacing the phrase "Section 2.7" with the phrase "Section 2.9" in the
first parenthetical in the first sentence of Section 2.2.

         2.6  Section 2.3 is hereby amended by adding the phrase "or, as of the
Amendment Effective Date, the Additional Territory" after the term "Territory"
in the last line of Section 2.3 and by deleting the word "European" in
Section 2.3.

         2.7  Section 2.7 is hereby deleted in its entirety.

         2.8  A new Section 2.9 is hereby added to the Agreement as follows:

        Aventis hereby reserves, and nothing in this Agreement shall impair or
limit, Aventis' or its Affiliates' or sublicensees' rights under the RPR
Know-How and Improvements to make or have made pharmaceutical products other
than Product, or subject to Article 26, to otherwise utilize the RPR Know-How
and Improvements for any purposes which are not expressly and exclusively
granted to Sepracor pursuant to the terms of this Agreement, including without
limitation for the purposes of meeting and otherwise fulfilling all of Aventis'
(and its Affiliates' and sublicensees') legal, regulatory and contractual
obligations with respect to (±) zopiclone in the Additional Territory.

         2.9  Section 3.2.4 is hereby deleted in its entirety.

       2.10  Section 5.2 is hereby amended by adding the phrase, "in the
Territory" after the phrase "to avoid loss of any rights" in the first sentence
of Section 5.2.

       2.11  Section 5.3 is hereby deleted in its entirety, and a new
Section 5.3 is added in lieu thereof which reads in its entirety as follows:

        After the Effective Date, Aventis shall provide Sepracor full access to
the information referenced in Section 2.3 hereof, and shall use good faith
reasonable efforts in fully cooperating with Sepracor in order to enable
Sepracor to obtain Regulatory Approval of Products. At Sepracor's request,
Aventis shall disclose to Sepracor in writing, or via mutually acceptable
electronic media, copies or reproductions of all written RPR Know-How reasonably
available to Aventis or its Affiliates in order to enable Sepracor to obtain
Regulatory Approval of Products. In addition, during the term of this Agreement,
Aventis shall promptly disclose to Sepracor in writing, or via mutually
acceptable electronic media, on an ongoing basis copies or reproductions of all
Improvements that are reasonably necessary to research, develop, register,
manufacture, market, use or sell Product. Such Improvements shall be
automatically deemed to be within the scope of the licenses granted herein
without payment of any additional compensation. Sepracor shall have the right to
use for all purposes in connection with Regulatory Approval (or other regulatory
approval in the Additional Territory) or any regulatory application for Product
all RPR Know-How and other information disclosed pursuant to this Section and
under this Agreement. Upon Sepracor's request, Aventis shall provide reasonable
technical assistance to enable Sepracor to utilize RPR Know-How to obtain
Regulatory Approval of Products, and from time to time at Sepracor's reasonable
request, Aventis shall use good faith reasonable efforts to assist Sepracor in
providing responses to questions that may be raised by regulatory authorities in
connection with Sepracor's applications for Regulatory Approval of Products.
Sepracor shall be solely responsible for obtaining all Regulatory Approvals (and
other regulatory approvals in the Additional Territory) related to the Product.

       2.12  Article 6 (Infringement) is hereby amended by adding the phrase "or
the Additional Patents, as applicable" after the term "RPR Applications" each
time such term appears in Article 6.

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       2.13  Article 8 (Term) is hereby deleted in its entirety, and a new
Article 8 is added in lieu thereof which reads in its entirety as follows:

ARTICLE 8—TERM

        This Agreement will commence as of the Effective Date and, unless sooner
terminated as provided hereunder, shall terminate on the expiration of the
royalty obligations of Article 3, after which time Sepracor will have a fully
paid-up, royalty-free and irrevocable exclusive license under RPR Know-How and
Improvements to develop, have developed, make, have made, use, market, sell,
offer for sale, have sold and distribute Product in the Territory and the
Additional Territory. Notwithstanding the foregoing, if Sepracor breaches its
obligations under Section 2.4, then this Agreement shall terminate ten
(10) years from the Effective Date, or thirty (30) days after Aventis gives
notice to Sepracor of such breach, as applicable. Upon any such early
termination of this Agreement, all rights to RPR Know-How and Improvements
licensed hereunder to Sepracor shall revert to Aventis, and Sepracor and its
Affiliates and Licensees shall make no further use of the same.

       2.14  Section 10.1.5 is hereby amended by adding the following clause at
the end of such Section 10.1.5:

        As of the Amendment Date, Aventis' right, title and interest in the
Additional Patents, or RPR Know-How as it relates to (+) zopiclone, in the
Additional Territory are not assigned, transferred, or conveyed to a Third Party
or, to Aventis' knowledge, otherwise encumbered by a Third Party;

       2.15  Section 10.1.6 is hereby amended by adding the following clause at
the end of such Section 10.1.6:

As of the Amendment Date, Aventis is the sole and exclusive owner of the
Additional Patents and, to Aventis' knowledge, the sole and exclusive owner or
licensee of the RPR Know-How as it relates to (+)zopiclone, all of which to the
knowledge of Aventis, are free and clear of any liens, charges and encumbrances
and, to the knowledge of Aventis, except for Aventis' Affiliates, no other
person, corporate or other private entity or governmental entity or subdivision
thereof has, or shall have, any claim of control with respect to the Additional
Patents, and RPR Know-How as it relates to (+) zopiclone, in the Additional
Territory;

       2.16  Section 10.1.7 is hereby amended by adding the following clause at
the end of such Section 10.1.7:

        As of the Amendment Date, in the Additional Territory, there are no
claims, judgments or settlements against or owed by Aventis pending or, to the
knowledge of Aventis, threatened with respect to the Additional Patents and the
RPR Know-How, as they relate to Zopiclone Technology or Product;

       2.17  Section 10.1.10 is hereby amended by adding the following clause at
the end of such Section 10.1.10:

        As of the Amendment Date, Aventis has not performed any searches to
determine if, and has no actual knowledge that, the development, manufacture,
use, distribution, marketing, promotion and sale of Compound in the Additional
Territory interferes or infringes on any intellectual property rights owned or
possessed by any Third Party; and

       2.18  A new Section 10.1.12 is hereby added as follows:

Aventis and its Affiliates (and any employee, consultant, or agent thereof)
shall not directly or indirectly take any action, or cause any action to be
taken, between the Amendment Date and

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the Amendment Effective Date that is inconsistent with or would alter the
correctness of Aventis' representations and warrants in this Agreement.

       2.19  Section 10.3 is hereby deleted in its entirety and hereby replaced
with a new Section 10.3 as follows:

        During the term of this Agreement, Aventis covenants that neither
Aventis nor any of its Affiliates will develop, use, market, promote, sell or
distribute (or agree with any third party to do any of the foregoing) in the
Territory any product containing Compound, the racemate or the other enantiomer
thereof, as an active ingredient.

       2.20  Article 11 (Adverse Events) is hereby amended and restated in its
entirety as follows:

        Following the Effective Date, Sepracor shall be solely responsible for
complying with all legal and/or regulatory obligations in the Territory
regarding the reporting of adverse events related to Product and following the
Amendment Effective Date, Sepracor shall be solely responsible for complying
with all legal and/or regulatory obligations in the Additional Territory
regarding the reporting of adverse events related to Product.

       2.21  Section 12.1 is hereby amended by adding the phrase "or the
Additional Territory" after the term "Territory" in Section 12.1.

       2.22  Article 15 (Notices) is hereby amended by replacing the existing
notice address for Aventis with the following address:

Aventis Pharma SA

c/o Aventis Pharmaceuticals Inc.
Mail Stop: BX2-716A
200 Crossing Boulevard, P.O. Box 6890
Bridgewater, NJ 08807-0890
Attn: Vice President Business Development/TL&A
Facsimile: (908) 231-4480

       2.23  Section 23.2 is hereby amended by deleting the phrase "in the
Territory," in the third line of Section 23.2.

       2.24  A new Article 26 is hereby added as follows:

ARTICLE 26—INFORMATION

        From and after the Amendment Effective Date, Aventis shall ensure that
Aventis and its Affiliates (and employees, consultants, and agents thereof) do
not, directly or indirectly, use or otherwise place in the public domain, any
information owned by, or in the possession or control of (including without
limitation information relating to Compound or any metabolites thereof provided
by Sepracor to Aventis or its Affiliates), Aventis or its Affiliates that is not
in the public domain, specifically relating to Compound (to the extent such
exists as of the Amendment Date) or (±) zopiclone, or any metabolite of either,
including but not limited to information relating to research, development,
pharmacological characteristics, clinical effect, clinical trials, manufacture,
marketing, commercialization, distribution, importation, exportation, cost,
pricing, supply, sales, sales support, or use thereof; provided, however, that
in the Additional Territory, any current or future employees and agents of
Aventis or its Affiliates involved in research, development, manufacture, sales,
marketing, promotion, medical communications, or regulatory maintenance of
pharmaceutical products containing (±) zopiclone in the Additional Territory,
shall be free to use any Aventis information specifically relating to (±)
zopiclone solely in connection with those activities in the Additional
Territory. For purposes of clarity, in no event shall the foregoing prohibit or
in any way limit (i) the rights of Aventis or its Affiliates to use, directly or

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indirectly, such information in connection with any actual or proposed sale,
assignment or other transfer of rights in (±) zopiclone from Aventis or its
Affiliates to a Third Party (a "Transferee") or (ii) the rights of any
Transferee to use, directly or indirectly, such information. Notwithstanding
anything to the contrary contained herein, (i) Aventis, its Affiliates and any
employee, agent, or consultant of Aventis, shall be allowed to use, directly or
indirectly, such information as required by any applicable law or any applicable
regulatory authority or other governmental entity in the Additional Territory
and (ii) the provisions of this Article 26 shall not exclude use of any
information independently developed by Sanofi-Synthelabo or its Affiliates prior
to the Amendment Effective Date.

       2.25  The following schedules are hereby added as schedules to the 1999
Agreement:

    2.25.1  Schedule 1.21 attached hereto is hereby added to the 1999 Agreement
as Schedule 1.21.

    2.25.2  Schedule 2.1(b) attached hereto is hereby added to the 1999
Agreement as Schedule 2.1(b).

       2.26  Except as otherwise set forth herein, the 1999 Agreement shall
remain in full force and effect.

ARTICLE 3

ADDITIONAL OBLIGATIONS OF AVENTIS

         3.1  Aventis shall, within twenty (20) Business Days of the Amendment
Effective Date, deliver copies of the documents described on Schedule 3.1 hereto
to Sepracor, to the extent Aventis has not previously delivered copies of such
documents, solely to the extent such documents are within the possession or
control of Aventis (or its Affiliates, agents, employees or consultants) on the
Amendment Date and are legally permitted to be disclosed.

         3.2  Within ten (10) Business Days after the Amendment Date, Aventis
shall provide Sepracor with (i) a current docket listing and description of the
status of each of the Additional Patents and copies of all related outstanding
actions, if any and (ii) a copy of the prosecution history of any pending
applications among the Additional Patents, if any. From and after the Amendment
Date until the earlier of (x) the Amendment Effective Date and (y) the date, if
any, that it is determined by Aventis and communicated to Sepracor that the
Amendment Effective Date will not occur, Aventis shall conduct maintenance and
prosecution of the Additional Patents consistent with Aventis' past practice and
otherwise in the ordinary course of business, provided, however, that Aventis
shall timely pay any taxes, annuities, or maintenance fees due between the
Amendment Date and the Amendment Effective Date.

ARTICLE 4

CONSENT TO ASSIGNMENT

         4.1  Pursuant to the terms of Section 18.1 of the 1999 Agreement,
Sepracor hereby consents to the assignment by Aventis of Aventis' rights, in
whole or in part, under Article 3 and Article 4 of the 1999 Agreement, as
amended by this Amendment, including, without limitation, its right to receive
Royalties pursuant to Section 3.1, its right to receive Royalty Statements
pursuant to Section 4.1 and its right to conduct audits pursuant to Section 4.5.

         4.2  In connection with the foregoing consent to assignment, Aventis
shall have the right to provide complete and unredacted copies of the 1999
Agreement and this Amendment to any potential assignee, provided, however, that
such potential assignee agrees to be bound by confidentiality provisions
substantially similar to those set forth in Article 7 of the 1999 Agreement.

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ARTICLE 5

EFFECTIVE DATE

         5.1  Except with respect to the provisions of Sections 2.18, 2.22, 3.2
and 4.2 of this Amendment which shall be effective as of the Amendment Date,
this Amendment shall become effective on the date upon which the shareholders of
Aventis SA shall have sold to Sanofi-Synthelabo, and Sanofi-Synthelabo shall
have purchased from the shareholders of Aventis SA, more than fifty percent
(50%) of the shares and voting rights of Aventis SA in furtherance of the tender
offer launched by Sanofi-Synthelabo with respect to Aventis SA on the terms set
forth in the "Note d'Information" that received approval of the French
"Autorites des Marches Financiers", as the same may be amended (the "Purchase
Event"). Notwithstanding the foregoing, to the extent an event occurs which
would require payment of $5,000,000 under Section 3.2.4 of the 1999 Agreement
following the Amendment Date, but prior to the Amendment Effective Date, no such
payment shall be made unless it is determined by Aventis and communicated to
Sepracor that the Amendment Effective Date will not occur (the date such notice
is communicated, the "Termination Date"), in which event such payment shall be
made within (10) Business Days following the Termination Date.

[NO FURTHER TEXT ON THIS PAGE]

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IN WITNESS WHEREOF, the Parties have executed this Amendment as of the date
first above written.

AVENTIS PHARMA SA
By:
 
/s/  DIRK OLDENBURG      

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Name:   Dirk Oldenburg Title:   General Counsel
SEPRACOR INC.
By:
 
/s/  WILLIAM E. YELLE      

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Name:   William E. Yelle Title:   VP, Business Development

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SCHEDULE 1.21

ADDITIONAL PATENTS

Country

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  Application Date

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  Application Number

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  Grant date

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  Patent Number

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  Expiration date

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AR   16/01/1992   321649   31/05/1995   248024   31/05/2010 AT   16/01/1992  
92903994.9-2101   12/04/1995   E 121089   16/01/2012 AU   16/01/1992   92 12264
  07/01/1997   671797   16/01/2012 BD   15/01/1992   92 5   14/01/1993   1002434
  17/01/2007 BE   16/01/1992   92903994.9-2101   12/04/1995   609210  
16/01/2012 BY   17/05/1994   1681   14/05/1997   1854   16/01/2012 CA  
16/01/1992   2099782   03/12/2003   2099782   16/01/2012 CH   16/01/1992  
92903994.9-2101   12/04/1995   609210   16/01/2012 CZ   16/01/1992   93 1380  
02/04/1996   281011   16/01/2012 DE   16/01/1992   92903994.9-2101   12/04/1995
  69202060.8   16/01/2012 DK   16/01/1992   92903994.9-2101   12/04/1995  
609210   16/01/2012 ES   16/01/1992   92903994.9-2101   12/04/1995   2071486  
16/01/2012 FI   16/01/1992   93 3248   14/11/1997   100331   16/01/2012 FR  
16/01/1992   92903994.9-2101   12/04/1995   609210   16/01/2012 GB   16/01/1992
  92903994.9-2101   12/04/1995   609210   16/01/2012 GR   16/01/1992  
92903994.9-2101   12/04/1995   609210   16/01/2012 HU   16/01/1992   P93 02063  
02/10/2000   218928   16/01/2012 IE   16/01/1992   92 126   21/11/1995   66110  
16/01/2012 IL   16/01/1992   100677   28/02/1996   100677   16/01/2012 IT  
16/01/1992   92903994.9-2101   12/04/1995   609210   16/01/2012 LU   16/01/1992
  92903994.9-2101   12/04/1995   609210   16/01/2012 MA   15/01/1992   22677  
01/10/1992   22392   15/01/2012 MX   16/01/1992   92 180   12/08/1994   175679  
16/01/2012 NG   16/01/1992   92 15   30/07/1992   RP 11252   16/01/2012 NL  
16/01/1992   92903994.9-2101   12/04/1995   609210   16/01/2012 NO   16/01/1992
  93 1919   08/01/1997   179911   16/01/2012 NZ   15/01/1992   241313  
20/07/1993   241313   15/01/2012 OA   16/01/1992   60387   15/04/1994   9807  
16/01/2012 PH   16/01/1992   43784   23/12/1997   30982   23/12/2014 PK  
16/01/1992   92 22   19/12/1993   132928   16/01/2008 PL   16/01/1992   P 299834
  01/02/1995   166976   16/01/2012 PT   16/01/1992   92903994.9-2101  
12/04/1995   609210   16/01/2012 RU   16/01/1992   93 051787   10/05/1998  
2110519   16/01/2012 SE   16/01/1992   92903994.9-2101   12/04/1995   609210  
16/01/2012 SK   16/01/1992   93 719   18/02/1998   279060   16/01/2012 TN  
17/01/1992   92 004   06/10/1992   16531   17/01/2012 YU   17/01/1992   92P 55  
11/06/2002   48 878   17/01/2012 ZA   15/01/1992   92 302   28/10/1992   92 302
  15/01/2012

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SCHEDULE 2.1(b)

ASSIGNMENT OF ADDITIONAL PATENTS

ASSIGNMENT

WHEREAS, Aventis Pharma SA ("Aventis"), a French corporation having a place of
business at 20, avenue Raymond-Aron, F-92160 Antony, France, owns, by
assignment, all right, title, and interest in and to French Patent Application
Serial No. 91-00490, entitled "Optically Active 5H-Pyrrolo[3,4-b] pyrazine
Derivative, Its Preparation and Pharmaceutical Compositions Containing Same",
and PCT International Patent Application No. PCT/FR92/00031, and any and all
non-United States patents or patent applications related to either of the
aforesaid applications, including without limitation the patents and patent
applications listed in Attachment A, and any invention described therein
(collectively hereinafter "Eszopiclone Foreign Patents"); and

WHEREAS, Sepracor Inc. ("Sepracor"), a Delaware corporation having a place of
business at 84 Waterford Drive, Marlborough, Massachusetts USA 01752, is
desirous of obtaining Aventis' entire right, title, and interest in, to, and
under the Eszopiclone Foreign Patents, and Aventis desires to assign to Sepracor
its entire right, title and interest in the same.

NOW THEREFORE, be it known that, for good and valuable consideration to Aventis,
the receipt and sufficiency of which is hereby acknowledged, Aventis hereby
sells, assigns, transfers, and sets over to Sepracor, its lawful successors and
assigns, Aventis' entire right, title, and interest in, to, and under
Eszopiclone Foreign Patents (all of which are hereby incorporated by reference
as if fully set forth herein), any inventions described or claimed therein, all
rights to claim priority on the basis of any Eszopiclone Foreign Patents, and
all applications for Letters Patent that may be filed for such invention in any
country other than the United States and all Letters Patent that may be granted
on any such application, and any and all extensions, supplemental protection
certificates, additions, divisions, substitutions, and renewals of any of the
foregoing, in each case, as fully and entirely as the same would have been held
and enjoyed by Aventis if this assignment had not been made.

AND, AVENTIS HEREBY further convenants that Aventis shall, from time to time
after the date hereof upon the request of Sepracor, execute and deliver to
Sepracor such further conveyance instruments or confirmatory assignments as may
be necessary or desirable to evidence more fully the transfer of ownership of
all Eszopiclone Foreign Patents to Sepracor, or the original ownership of all
the Eszopiclone Foreign Patents on the part of Aventis, to the fullest extent
possible, and to perform any other acts deemed necessary to carry out the intent
of this instrument.

AND, AVENTIS HEREBY authorizes and requests that any official of any country
whose duty it is to issue patents or other evidence or forms of industrial
property protection on applications as aforesaid, to issue all patents or
Letters Patents for this invention to Sepracor, its successors and assigns, in
accordance with the terms of this instrument.

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        IN WITNESS WHEREOF, Aventis has caused its authorized representative to
execute this Assignment

AVENTIS PHARMA SA        
By:
 
 
 
Date:
 
     

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Name:             Title:            

 

State of   )         )   SS.: County of   )    

        On this            day of                        , 2004, before me, a
Notary Public in and for the State and County aforesaid, personally
appeared                        , to me known and known to me to be the person
of that name, who signed and sealed the foregoing instrument, and he
acknowledged the same to be his free act and deed.

   

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  ,     Notary Public.

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ATTACHMENT A

1.   French Patent Application No. 91-00490, filed 17 January 1991; 2.   PCT
International Patent Application No. PCT/FR92/00031, filed 16 January 1992; 3.  
European Patent Application No. 92903994.9-2101, filed 16 January 1992; 4.  
Argentinian Patent No. 248024, granted 31 May 1995; 5.   Austrian Patent No. E
121089, granted 12 April 1995; 6.   Australian Patent No. 671797, granted 7
January 1997; 7.   Bangladesh Patent No. 1002434, granted 14 January 1993; 8.  
Belgian Patent No. 609210, granted 12 April 1995; 9.   Belarus Patent No. 1854,
granted 14 May 1997; 10.   Canadian Patent No. 2,099,782, granted 3
December 2003; 11.   Swiss Patent No. 609210, granted 12 April 1995; 12.   Czech
Republic Patent No. 281011, granted 2 April 1996; 13.   German Patent
No. 69202060.8, granted 12 April 1995; 14.   Danish Patent No. 609210, granted
12 April 1995; 15.   Spanish Patent No. 2071486, granted 12 April 1995; 16.  
Finnish Patent No. 100331, granted 14 November 1997; 17.   French Patent
No. 609210, granted 12 April 1995; 18.   British Patent No. 609210, granted 12
April 1995; 19.   Greek Patent No. 609210, granted 12 April 1995; 20.  
Hungarian Patent No. 218928, granted 2 October 2000; 21.   Ireland Patent
No. 66110, granted 21 November 1995; 22.   Israeli Patent No. 100677, granted 28
February 1996; 23.   Italian Patent No. 609210, granted 12 April 1995; 24.  
Japanese Patent Application No. 1992-504006, filed 16 January 1992; 25.  
Luxembourg Patent No. 609210, granted 12 April 1995; 26.   Moroccan Patent
No. 22392, granted 1 October 1992; 27.   Mexican Patent No. 175679, granted 12
August 1994; 28.   Nigerian Patent No. RP 11252, granted 30 July 1992; 29.  
Netherlands Patent No. 609210, granted 12 April 1995; 30.   Norwegian Patent
No. 179911, granted 8 January 1997; 31.   New Zealand Patent No. 241313, granted
20 July 1993; 32.   OAPI Patent No. 9807, granted 15 April 1994; 33.  
Philippine Patent No. 30982, granted 23 December 1997; 34.   Pakistan Patent
No. 132928, granted 19 December 1993; 35.   Polish Patent No. 166976, granted 1
February 1995; 36.   Portuguese Patent No. 609210, granted 12 April 1995; 37.  
Russian Federation Patent No. 2110519, granted 10 May 1998; 38.   Swedish Patent
No. 609210, granted 12 April 1995; 39.   Slovakian Patent No. 279060, granted 18
February 1998;      

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40.   Tunisian Patent No. 16531, granted 6 October 1992; 41.   Yugoslavian
Patent No. 48 878, granted 11 June 2002; and 42.   South African Patent No. 92
302, granted 28 October 1992.

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SCHEDULE 3.1

ADDITIONAL DOCUMENTATION

        Draft or completed reports of any clinical or nonclinical studies or
experiments not published or previously provided to Sepracor as of the Amendment
Effective Date, including postmarketing reviews and safety surveillance reports
relating to Compound, the racemate, the other enantiomer thereof, or metabolites
of any of the foregoing.

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QuickLinks

Exhibit 10.1

AMENDMENT AND PATENT ASSIGNMENT AGREEMENT