Document:

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                                                            Exhibit No. 10.68

AGREEMENT AND MUTUAL RELEASE AND DISCHARGE dated as of the day of
______________, 2001.

BY AND BETWEEN:             METHYLGENE INC.

                           (hereinafter referred to as the "METHYLGENE")

AND:                       HYBRIDON, INC.

                           (hereinafter referred to as "HYBRIDON")

WHEREAS Hybridon is currently a shareholder of Methylgene;

WHEREAS Hybridon wishes to sell shares it owns in the share capital of
Methylgene pursuant to an offer (the "OFFER") dated February 12, 2001 from Paul
Capital Partners (Paul Capital Partners and any affiliates thereof hereinafter
referred to as "PAUL CAPITAL") and accepted by Hybridon on February 14, 2001;

WHEREAS Hybridon may sell other shares it owns in the share capital of
Methylgene to certain existing shareholders or to others; and

WHEREAS Methylgene has agreed to cooperate and assist Hybridon in effecting
these transactions;

THEREFORE, FOR GOOD AND VALUABLE CONSIDERATION RECEIVED, THE PARTIES
HERETO AGREE AS FOLLOWS:

1.       COMPLIANCE - METHYLGENE

         1.1      Methylgene hereby certifies that it is and, to its knowledge,
                  Hybridon is in compliance with and not in default of any and
                  all agreements between Hybridon and Methylgene existing as at
                  the date hereof including, for greater certainty but not
                  limited to, the agreements listed in Schedule A attached
                  hereto;
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                                                                          Page 2

2.       COMPLIANCE - HYBRIDON

         2.1      Hybridon hereby certifies that it is and, to its knowledge,
                  Methylgene is in compliance with and not in default of any and
                  all agreements between Hybridon and Methylgene existing as at
                  the date hereof including, for greater certainty but not
                  limited to, the agreements listed in Schedule A attached
                  hereto;

3.       NON-COMPETITION

         3.1      The introductory paragraph of Section 13.1 of the Shareholders
                  Agreement (as defined in Schedule A) is amended by deleting
                  same and replacing it with the following:

                  "13.1    Hybridon undertakes in favour of Methylgene, whether
                           directly or indirectly, alone or in partnership,
                           association, joint venture or other collaboration
                           with any other person, company, partnership, business
                           or entity, as a principal, agent, shareholder,
                           employee, partner, consultant, subcontractor, unless
                           for the benefit of Methylgene, at the request of
                           Methylgene or with the consent of Methylgene, not
                           to:"

         3.2      Section 13.1.1 of the Shareholders Agreement is amended by
                  deleting same and replacing it with the following:

                  "13.1(A) from the date of the Agreement and Mutual Release and
                           Discharge entered into between Methylgene and
                           Hybridon until the expiry of a thirty (30) month
                           period starting the later of (a) May 31, 2001, (b)
                           the closing of the transaction contemplated by the
                           Offer and (c) Hybridon no longer having a nominee on
                           Methylgene's Board of Directors (the "COMMENCEMENT
                           DATE"), research, develop or market anywhere in the
                           world (i) any antisense compounds to inhibit DNA
                           Methyltransferases ("DNA METASES"), or (ii) any
                           method of inhibiting DNA MeTases for the treatment of
                           any indication;

                  13.1(B)  from the date of the Agreement and Mutual Release and
                           Discharge entered into between Methylgene and
                           Hybridon until the expiry of an eighteen (18) month
                           period starting on the Commencement Date (the
                           "RESTRICTIVE TERM"), not to engage in research or
                           development or marketing anywhere in the world
                           relating to the fields of histone deacetylases and
                           beta-lactamases. If prior to the expiration of the
                           Restrictive Term, Methylgene gives Hybridon written
                           notice designating a target gene or genes within
                           either histone deacetylases or beta-lactamases or
                           both, as the case may be, as the Second Molecular
                           Target and Third Molecular Target, respectively,
                           under the License Agreement dated January 4, 1996 as
                           amended and restated
<PAGE>   3
                                                                          Page 3

                           between the parties as of September 21, 2000 (the
                           "LICENSE AGREEMENT"), then Hybridon, for an
                           additional period of 18 months from the expiration of
                           the Restrictive Term, shall refrain from research or
                           development or marketing anywhere in the world
                           relating to either histone deacetylases or beta
                           lactamases, or both, depending upon which is the
                           location of the target gene or genes so designated by
                           Methylgene, and the designation of such target gene
                           or genes shall be deemed to have exhausted
                           Methylgene's rights under the License Agreement, to
                           specify a Second Molecular Target, if one designation
                           is made and a Third Molecular Target if two
                           designations are made;"

3.3      Sections 13.1.3 and 13.1.4 of the Shareholders Agreement are amended by
         deleting same and replacing them with the following:

         "13.1.3  from the date of the Agreement and Mutual Release and
                  Discharge entered into between Methylgene and Hybridon until
                  the expiry of a two (2) year period starting on the
                  Commencement Date, solicit entities which are customers or
                  suppliers of Methylgene on the relevant date with respect to
                  compounds or methods of the type described in 13.1(A) and
                  13.1(B); or

         13.1.4   from the date of the Agreement and Mutual Release and
                  Discharge entered into between Methylgene and Hybridon until
                  the expiry of a two (2) year period starting on the
                  Commencement Date, encourage any person employed by Methylgene
                  to leave Methylgene or solicit for employment any person who
                  is at the time of solicitation employed by Methylgene."

3.4      Section 13.1 of the Shareholders Agreement is amended by adding the
         following:

         "13.1.5  Nothing in the foregoing shall restrict the right of Hybridon
                  to own up to 5% of the outstanding capital stock of companies
                  whose shares are traded on a recognized stock exchange."

3.5      In the event of a merger or acquisition involving Hybridon and a
         non-affiliate third party (as the term "affiliate" is defined in the
         License Agreement), the provisions of Section 13 of the Shareholders'
         Agreement (until their expiration pursuant to the provisions of such
         Section 13), as hereinabove amended, and the provisions of Section
         4 below shall continue to apply to the surviving entity
         but shall not be construed in any way which would prejudice and shall
         not prejudice the right and ability of the surviving entity to continue
         to do any research and development activities which the non-affiliate
         third party was undertaking prior to the time of the merger.
<PAGE>   4
                                                                          Page 4

3.6      All other provisions of Section 13 of the Shareholders Agreement shall
         continue to apply mutatis mutandis. To the extent Hybridon is still a
         party to the Shareholders Agreement at the time of the initial public
         offering of the shares of Methylgene (the "IPO"), Hybridon undertakes
         that, pursuant to such IPO, it will agree to the termination of the
         Shareholders Agreement, save and except for the provisions of its
         Section 13, as amended hereby, which shall continue to apply for the
         relevant period.

4.       CONFIDENTIALITY

         4.1      Hybridon agrees, as long as such information or knowledge is
                  not part of the public domain or required to be disclosed in
                  accordance with applicable law, not to disclose, publish or
                  reveal in any manner whatsoever and to whomever, any
                  information or knowledge of a confidential nature concerning
                  the business operated by Methylgene, including, without
                  limiting the generality of the foregoing, trade secrets,
                  biological targets, chemical structures, inventions, software,
                  computer programs, patents, licences, manufacturing processes,
                  know-how, customer lists or contracts of Methylgene, whether
                  acquired by it in its capacity as a shareholder or through any
                  nominee on the Board of Directors, its representatives on the
                  scientific advisory board of Methylgene or from any other
                  means, Hybridon hereby expressly acknowledging that such trade
                  secrets, biological targets, chemical structures, inventions,
                  software, computer programs, patents, licences, manufacturing
                  processes, know-how, customer lists or contract and all other
                  information of a confidential nature concerning the business
                  operated by Methylgene has been disclosed to it at any time on
                  a confidential basis.

5.       UNDERTAKING - LOCK-UP AGREEMENT

         5.1      Hybridon shall, if it is a shareholder of Methylgene, at the
                  request of the underwriters engaged by Methylgene in
                  connection with its IPO, sign a lock-up agreement in their
                  favour in customary form under which Hybridon shall agree not
                  to sell or otherwise dispose of in any way any of its shares
                  in the share capital of Methylgene for the period applicable
                  to other significant shareholders of Methylgene following the
                  IPO, the whole as required by the underwriters. Furthermore,
                  Hybridon recognizes that it may be subject to certain escrow
                  requirements imposed by regulators in connection with an IPO
                  and agrees to comply therewith.

6.       RELEASE AND DISCHARGE - HYBRIDON

         6.1      Except for obligations of the parties set forth in any
                  agreement between them which are intended to survive the
                  execution of this agreement, Hybridon hereby releases and
                  discharges Methylgene from any claims, liabilities, actions or
                  demands of any kind which Hybridon now has or hereafter can,
                  shall or may
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                                                                          Page 5

                  have against Methylgene for or by reason of or in any way
                  arising out of any cause, matter or thing whatsoever existing
                  up to the present time.

7.       RELEASE AND DISCHARGE - METHYLGENE

         7.1      Except for obligations of the parties set forth in any
                  agreement between them which are intended to survive the
                  execution of this agreement, Methylgene hereby releases and
                  discharges Hybridon from any claims, liabilities, actions or
                  demands of any kind which Methylgene now has or hereafter can,
                  shall or may have against Hybridon for or by reason of or in
                  any way arising out of any cause, matter or thing whatsoever
                  existing up to the present time.

8.       HYBRIDON REPRESENTATIVE - BOARD OF DIRECTORS

         8.1      Notwithstanding any provision of the Shareholders' Agreement,
                  Hybridon agrees that, in the event it should own less than
                  350,000 shares in the share capital of Methylgene, Hybridon
                  shall no longer be entitled to appoint or elect a nominee on
                  Methylgene's Board of Directors and shall cause its nominee at
                  that time to resign and execute a full release and discharge
                  in customary form upon written notice by Methylgene to
                  Hybridon.

9.       EXPENSES

         9.1      Hybridon hereby agrees to pay any and all out-of-pocket
                  expenses (including, without limitation, the fees and
                  disbursements of Methylgene's legal and patents counsel)
                  incurred by Methylgene in connection with the proposed sale by
                  Hybridon of its shares in the share capital of Methylgene to
                  Paul Capital, to any other shareholder of Methylgene or to any
                  other person.

10.      OBLIGATIONS OF METHYLGENE

         10.1     Methylgene agrees to use its commercially reasonable efforts
                  to approve such measures as are necessary to combine its
                  presently outstanding class A and class B common shares into a
                  single class so as to eliminate any preference between them in
                  the distribution of assets as dividends or distributions upon
                  liquidation and in furtherance of this Methylgene shall
                  recommend that the holders of its class A and class B common
                  shares approve and agree to any action required of them to
                  implement the combination of the two classes of stock. Upon
                  such approval, Methylgene will see to the implementation of
                  these actions.

         10.2     Methylgene agrees to use its commercially reasonable efforts
                  to obtain the approval and consent of the other shareholders
                  of Methylgene to the amendment to the Shareholders Agreement
                  contemplated by Section 3 herein.
<PAGE>   6
                                                                          Page 6

                  Notwithstanding the foregoing however, Hybridon undertakes and
                  agrees to abide by the provisions of Section 3 of this
                  agreement, whether or not the other shareholders of Methylgene
                  give their approval and consent to the amendments to the
                  Shareholders Agreement contemplated thereby.

         10.3     Methylgene shall and shall cause its counsel promptly to
                  review and comment upon the forms of waiver and waiver request
                  letter proposed by Hybridon for the purpose of Hybridon
                  soliciting the waiver by the holders of shares of Methylgene
                  capital stock of rights in the nature of preemption or first
                  refusal which are a precondition of Hybridon's selling its
                  shares to Paul Capital and to recommend such waivers to its
                  shareholders. Furthermore, and subject to compliance with the
                  terms of the Shareholders Agreement, Hybridon agrees, if one
                  or more offers are made to it prior to the later of May 31,
                  2001 and the closing of the transaction contemplated by the
                  Offer, to sell any other shares it owns in the capital of
                  Methylgene to the other shareholders, to Methylgene or to a
                  third party arranged by Methylgene at the same price and on
                  substantially the same commercial terms as set forth in the
                  Offer.

11.      This agreement shall terminate on April 30, 2001 if the transaction
         contemplated in the Offer does not close on or prior to April 30, 2001
         and, save and except for Section 9, no provision of this agreement
         shall survive such termination,.

12.      This mutual release and discharge may be executed in one or more
         counterparts, each of which shall be deemed an original, but all of
         which together shall constitute one and the same instrument. The
         agreements herein are in addition to any other agreements or
         understandings between the parties, which other agreements and
         understandings are not terminated hereby and continue to apply.

13.      The mutual release and discharge is governed by the laws of the
         Province of Quebec.

14.      The present release and discharge is being drafted in English language
         at the request of the parties hereto. La presente quittance a ete
         redigee en langue anglaise a la demande des parties aux presentes.

                                        METHYLGENE INC.

                                        Per:
                                              ----------------------------------

                                        HYBRIDON, INC.
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                                                                          Page 7

                                        Per:
                                              ----------------------------------
<PAGE>   8
                                   SCHEDULE A

   -  Unanimous Shareholders' Agreement dated January 4, 1996, as amended from
      time to time;

   -  Shareholders Agreement dated January 4, 1996, as amended from time to time
      as restated on September 21, 2000 (the "SHAREHOLDERS' AGREEMENT");

   -  License Agreement dated January 4, 1996, as amended from time to time;

   -  License Agreement dated March 12, 1999 between Hybridon and Integrated DNA
      Technologies Inc. ("IDT"), and the letter agreement dated July 26, 1999
      between Methylgene and IDT;

   -  License Agreement dated October 13, 1994 between McGill University and
      Hybridon, as amended from time to time;

   -  License Agreement dated December 15, 1995 between McGill University and
      Hybridon;

   -  License Agreement dated February 1, 1990 between the Worcester Foundation
      for Experimental Biology Inc. and Hybridon and restated September 8, 1993;

   -  Letter Agreement dated February 27, 1998 between Methylgene and McGill
      University;

   -  Letter Agreement dated July 1, 1999 between Methylgene and the University
      of Massachussetts (UMASS); and

   -  All licensed and sublicensed patents.

   -  Letter Agreement dated December 18, 1995 among Hybridon and Worcester
      Foundation for Biomedical Research, Inc. and into which Methylgene
      intervened.

   -  Various confidentiality agreements executed from time to time between
      Methylgene and Hybridon.<PAGE>   1
                                                              EXHIBIT NO. 10.69

                   OFFER TO EXCHANGE SERIES B PREFERRED STOCK
                         OF HYBRIDON, INC. ("HYBRIDON")
                 FOR ANY AND ALL OUTSTANDING PRINCIPAL AMOUNT OF
                          8% NOTES DUE 2002 OF HYBRIDON

                               DATED MARCH 5, 2001

THIS OFFER TO EXCHANGE WILL EXPIRE AT 12:00 MIDNIGHT, BOSTON TIME, ON MARCH 16,
2001, UNLESS THE OFFER IS EXTENDED.

Hybridon, Inc., a Delaware corporation ("Hybridon"), invites the holders of its
8% Notes due 2002 (the "Notes") to tender their Notes in exchange (the
"Exchange") for 1 share of Series B Preferred Stock, par value $.01 per share,
of Hybridon ("Series B Preferred Stock"), for each $100 in principal amount of
the Notes tendered, upon the terms of this Offer to Exchange. This Offer to
Exchange constitutes the "Offer."

Hybridon will pay all accrued but unpaid interest on the Notes due through the
date of the Exchange by issuing additional Notes in an aggregate principal
amount equal to the amount of accrued but unpaid interest. Notes which will be
paid in respect of accrued interest will be deemed tendered for exchange by a
Noteholder participating in the Offer to Exchange, unless said Noteholder
indicates otherwise in Box One below. The Offer is open to all holders of Notes.
The shares of Hybridon's Common Stock (the "Common Stock") into which the Notes
are convertible have been registered by Hybridon in its Registration Statement
on Form S-1 (No. 333-69649) (the "Registration Statement"), which became
effective under the Securities Act of 1933, as amended, on February 8, 2001.
Upon an amendment to the Registration Statement, the shares of Common Stock into
which shares of the Series B Preferred Stock are convertible will be registered.
As part of the Exchange, the registration rights granted to the holders of the
Notes in Section 12 of the Subscription Agreement between Hybridon and the
holders of the Notes, dated as of December 13, 1999 (the "Subscription
Agreement"), shall survive, and shall be applied in full force and effect to the
shares of Common Stock issuable upon conversion of the Series B Preferred Stock.
The definition of "Registrable Securities" in Section 12.1(f) of the
Subscription Agreement, as it will apply to the shares of Common Stock issuable
upon conversion of the Series B Preferred Stock, will substitute "shares of
Series B Preferred Stock" for "Notes."

DELIVERY OF THE OFFER TO EXCHANGE TO AN ADDRESS OTHER THAN THAT OF THE
DEPOSITARY AS SET FORTH HEREIN WILL NOT CONSTITUTE A VALID DELIVERY.

PLEASE READ THIS ENTIRE OFFER TO EXCHANGE CAREFULLY. QUESTIONS AND REQUESTS FOR
ASSISTANCE SHOULD BE DIRECTED TO
<PAGE>   2
THE DEPOSITARY AT THE ADDRESS AND TELEPHONE NUMBER SET FORTH HEREIN. REQUESTS
FOR ADDITIONAL COPIES OF THE OFFER TO EXCHANGE SHOULD BE DIRECTED TO THE
DEPOSITARY AT THE ADDRESS AND PHONE NUMBER LISTED HEREIN.

ANY TENDER OF NOTES WHICH INVOLVES DENOMINATIONS OF LESS THAN $100 IN PRINCIPAL
AMOUNT THEREOF WILL BE EXCHANGED BY PAYMENT IN CASH OF AN AMOUNT EQUAL TO SAID
DENOMINATION.

Any holder of Notes (a "Noteholder") desiring to tender all or any portion of
the principal amount of such Notes should (i) sign this Offer to Exchange or a
facsimile thereof, and (ii) mail or deliver the signed Offer to Exchange to the
Depositary specified herein, along with either (A) the original Notes to the
Depositary, or, (B) for any Noteholder whose original Notes have been mutilated,
lost, stolen or destroyed, an affidavit of lost note, in substantially the same
form as Exhibit A attached hereto (the "Affidavit of Lost Note"), in lieu of
such Notes so mutilated, lost, stolen or destroyed, by the Expiration Date.

THIS TRANSACTION HAS NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS THE COMMISSION OR
ANY STATE SECURITIES COMMISSION PASSED UPON THE FAIRNESS OR MERITS OF THIS
TRANSACTION NOR UPON THE ACCURACY OR ADEQUACY OF THE INFORMATION CONTAINED IN
THIS DOCUMENT. ANY REPRESENTATION TO THE CONTRARY IS UNLAWFUL.

                         INFORMATION REGARDING HYBRIDON

Current information regarding Hybridon can be obtained by reference to its
prospectus (the "Prospectus") contained within the Registration Statement.
Please note that the Prospectus contains financial information about Hybridon
not later than September 30, 2000. Audited financial statements for the year
ended December 31, 2000 are not yet available. A copy of the prospectus can be
obtained from Robert Andersen, Chief Financial Officer of Hybridon, or by
accessing EDGAR.

                                     SUMMARY

This general summary is provided solely for the convenience of the Noteholders
and is qualified in its entirety by reference to the full text and more specific
details contained in this Offer to Exchange, and any amendments hereto.
<PAGE>   3
<TABLE>
<S>                              <C>
Hybridon....................     Hybridon, Inc., a Delaware corporation.

The Notes...................     8% Notes due 2002 of Hybridon.

Amount of Notes Sought......     All of the outstanding Notes.

Consideration...............     The consideration being offered per $100 in
                                 principal amount of the Notes tendered consists
                                 of 1 share of Series B Preferred Stock of
                                 Hybridon.

Conditions of Offer.........     The Offer is not conditioned upon any minimum
                                 of the principal amount of the Notes being
                                 tendered.

Expiration Date.............     March 16, 2001, at 12:00 Midnight, Boston time,
                                 unless extended.

How to Tender Notes.........     See "Procedure for Tendering Notes." For
                                 further information, call the Depositary.

Purpose of Offer............     The Offer is part of the restructuring of
                                 Hybridon's capital structure to reduce
                                 Hybridon's debt service obligations.

Exchange Date...............     As soon as practicable after the Expiration
                                 Date, which is expected to be no later than
                                 four business days after the Expiration Date.

Further Information.........     Additional copies of this Offer to Exchange may
                                 be obtained by contacting the Depositary at the
                                 address and telephone number shown herein.

Effect of Exchange on Rights     Although  shares  of  the  Series  B  Preferred
                                 of Shareholders are prior in liquidation to
                                 shares of Common Stock and Hybridon's Series A
                                 Preferred Stock, those Noteholders who exchange
                                 their Notes will be surrendering the security
                                 interest in Hybridon's assets securing the
                                 payment of their Notes. Under Hybridon's First
                                 Amended and Restated Subordination and
                                 Intercreditor Agreement dated June 29, 2000
                                 (the "Intercreditor Agreement"), to the extent
                                 that Noteholders accept this Offer to Exchange,
                                 the benefit of said security interest will
                                 inure largely to the lenders of a $6,000,000
                                 loan which was otherwise junior.
</TABLE>
<PAGE>   4
                               TERMS OF THE OFFER

Upon the terms described herein, Hybridon will exchange all Notes that are
validly tendered on or prior to the Expiration Date (as defined below) at the
rate of 1 share of Series B Preferred Stock for each $100 in principal amount of
the Notes tendered. Any tender of Notes which involves denominations of less
than $100 in principal amount thereof will be exchanged by payment in cash of an
amount equal to said denomination.

Hybridon will pay all accrued but unpaid interest on the Notes due through the
date of the Exchange by issuing additional Notes in an aggregate principal
amount equal to the amount of accrued but unpaid interest. Notes which will be
paid in respect of accrued interest will be deemed tendered for exchange by a
Noteholder participating in the Offer to Exchange, unless said Noteholder
indicates otherwise in Box One below.

                           CONSIDERATION BEING OFFERED

                            SERIES B PREFERRED STOCK

For a summary of the terms of the Series B Preferred Stock, and a comparison of
those terms with the terms of Hybridon's presently outstanding shares of Series
A Preferred Stock, see the summary attached hereto as Exhibit B. For a full
statement of the rights, preferences and privileges of the Series B Preferred
Stock, see Hybridon's Certificate of Designation for the Series B Preferred
Stock (the "Certificate") attached hereto as Exhibit C, which Hybridon will file
with the Delaware Secretary of State for purposes of amending its Certificate of
Incorporation and authorizing the issuance of the Series B Preferred Stock.

                          PROCEDURE FOR TENDERING NOTES

To validly tender the Notes pursuant to the Offer, the tendering Noteholder must
(i) sign this Offer to Exchange or a facsimile thereof, and (ii) mail or deliver
the signed Offer to Exchange to the Depositary (at its address set forth
herein), along with either (A) the original Notes to the Depositary, or (B) for
any Noteholder whose certificates for the Notes have been mutilated, lost,
stolen or destroyed, an Affidavit of Lost Note, in lieu of such Notes so
mutilated, lost, stolen or destroyed, on or prior to the Expiration Date.
<PAGE>   5
All questions as to the form of documents and the validity, eligibility
(including time of receipt) and acceptance for payment of any tender of Notes
will be determined by Hybridon, in its sole discretion, and its determination
shall be final and binding. Hybridon reserves the absolute right to reject any
or all tenders of Notes that (i) it determines are not in proper form, or (ii)
the acceptance for payments of or payment for which may, in the opinion of
Hybridon's counsel, be unlawful. Hybridon also reserves the absolute right to
waive any defect or irregularity in any tender of Notes. None of Hybridon, the
Depositary or any other person will be under any duty to give notice of any
defect or irregularity in tenders, nor shall any of them incur any liability for
failure to give any such notice.

                        ACCEPTANCE FOR EXCHANGE OF NOTES

Upon the terms of the Offer and as promptly as practicable after the Expiration
Date (which is expected to be within four (4) business days after the Expiration
Date), Hybridon will accept for exchange and exchange the Notes validly
tendered. Thereafter, exchange for all Notes validly tendered on or prior to the
Expiration Date and accepted for exchange pursuant to the Offer will be made by
the Depositary as promptly as practicable. In all cases, exchange pursuant to
the Offer will be made only after timely receipt by the Depositary of the
original Notes or an Affidavit of Lost Note, and a properly completed and duly
executed Offer to Exchange, or facsimile thereof. Notes not accepted for
exchange by Hybridon, or a replacement Note for the portion of the Notes not
tendered by the Noteholder or not accepted for exchange by Hybridon, will be
returned as promptly as practicable, without expense to the tendering
Noteholder.

                                    EXTENSION

Prior to the Expiration Date, Hybridon may extend the period of time during
which the Offer is open or otherwise amend or modify the Offer and may terminate
the Offer for any reason. There can be no assurance, however, that Hybridon will
extend the Offer. During any such extension, all Notes previously tendered will
remain subject to the Offer.

                   REPRESENTATIONS AND WARRANTIES OF HYBRIDON

In connection with this Offer and the Exchange, Hybridon represents and warrants
as follows:

                                  ORGANIZATION

Hybridon is a corporation duly organized, validly existing and in good standing
under the laws of the State of Delaware and has all requisite power and
authority
<PAGE>   6
to own and lease its property and to carry on its business as presently
conducted. Hybridon is duly qualified to do business as a corporation in all
jurisdictions in which the failure to be so qualified would have an adverse
affect on the financial or any other business condition of Hybridon.

                    AUTHORIZATION OF SERIES B PREFERRED STOCK

The issuance of shares of Series B Preferred Stock in accordance with the Offer
has been, or will be on or prior to the Exchange Date, duly authorized by all
necessary corporate action on the part of Hybridon, and all such shares of
Series B Preferred Stock have been duly reserved for issuance.

                             AUTHORIZATION OF OFFER

The execution, delivery and performance by Hybridon of this Offer and of the
transactions contemplated hereby have been duly authorized by all requisite
action on the part of Hybridon. This Offer has been duly executed and delivered
by the Company and constitutes a valid and binding obligation of Hybridon,
enforceable in accordance with its respective terms.

                             CONSENTS AND APPROVALS

No authorization, consent, approval or other order of, or declaration to or
filing with, any governmental agency or body (other than filings required to be
made under applicable federal and state securities laws) is required for the
valid authorization, execution, delivery and performance by Hybridon of this
Offer or the issuance of the shares of Series B Preferred Stock.

                                 SECURITIES LAWS

Based on the representations of the holders of the Notes set forth below, the
offer and issuance of the shares of Series B Preferred Stock will not be in
violation of the Securities Act of 1933, as amended (The "Securities Act").

                        REPRESENTATIONS AND WARRANTIES OF
                            THE HOLDERS OF THE NOTES

In connection with this Offer and the Exchange, as to himself, herself or
itself, the holders of the Notes represent and warrant as follows:
<PAGE>   7
                                INVESTMENT INTENT

Each Note holder recognizes that the Exchange involves a high degree of risk
including, but not limited to, the following: (i) Hybridon remains a development
stage business with limited operating history and requires substantial funds in
addition to any proceeds derived from the Exchange; (ii) an investment in
Hybridon is highly speculative, and only investors who can afford the loss of
their entire investment should consider investing in Hybridon, or the shares of
Series B Preferred Stock, or shares of Series B Preferred Stock-Underlying
Common Stock, (iii) such Note holder may not be able to liquidate his
investment; (iv) transferability of the Series B Preferred Stock is extremely
limited; and (v) in the event of a disposition of the Series B Preferred Stock
and the Series B Preferred Stock-Underlying Common Stock, such Note holder could
sustain the loss of his entire investment.

                                LACK OF LIQUIDITY

Each Note holder confirms that he or it is able (i) to bear the economic risk of
this investment, (ii) to hold the Series B Preferred Stock and any shares of
Series B Preferred Stock-Underlying Common Stock for an indefinite period of
time, and (iii) presently to afford a complete loss of his or its investment;
and represents that he or it has sufficient liquid assets so that the
illiquidity associated with this investment will not cause any undue financial
difficulties or affect such Note holder's ability to provide for his or its
current needs and possible financial contingencies, and that his or its
commitment to all speculative investments is reasonable in relation to his or
its net worth and annual income. Furthermore, each Note holder acknowledges that
the Series B Preferred Stock contains certain restrictions, as more particularly
set forth in the Certificate of Designation for the Series B Preferred Stock
attached hereto as Exhibit C.

                            KNOWLEDGE AND EXPERIENCE

Each Note holder hereby acknowledges and represents that such Note holder has
prior investment experience, including investment in securities that are
non-listed, unregistered and are not traded on the Nasdaq National or SmallCap
Market, nor on the National Association of Securities Dealers, Inc.'s (the
"NASD") automated quotation system, or such Note holder has employed at its own
expense the services of an investment advisor, attorney and/or accountant to
request documents from Hybridon and to read all of the documents furnished or
made available by Hybridon to such Note holder and to evaluate the investment,
tax and legal merits and the consequences and risks of such a transaction on
such Note holder's behalf, that such Note holder or such professional advisor
has such knowledge and experience in financial and business matters and that
such Note holder or such professional
<PAGE>   8
advisor is capable of evaluating the merits and risks of the prospective
investment and, if applicable, satisfies the conditions set out in Rule 501(h)
under the Securities Act.

                              NOTE HOLDER CAPACITY

Each Note holder hereby represents that such Note holder either by reason of
such Note holder's business or financial experience, or the business or
financial experience of such Note holder's professional advisors (who are
unaffiliated with, and who are not compensated by, Hybridon or any affiliate or
selling agent of Hybridon, directly or indirectly), has the capacity to protect
such Note holder's own interests in connection with the transaction contemplated
hereby.

                                  REGISTRATION

Each Note holder hereby acknowledges that the Offer has not been reviewed by the
Securities and Exchange Commission or any state regulatory authority. No Note
holder shall sell or otherwise transfer the Series B Preferred Stock or any
Series B Preferred Stock-Underlying Common Stock unless such securities are
registered under the Securities Act or unless an exemption from such
registration is available.

                                     LEGEND

Each Note holder consents to the placement of the legend set forth below on any
certificate or other document evidencing the shares of the Series B Preferred
Stock: THESE SECURITIES HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
1933, AS AMENDED (THE "SECURITIES ACT"), OR APPLICABLE STATE SECURITIES LAWS.
THESE SECURITIES HAVE BEEN ACQUIRED FOR INVESTMENT AND NOT WITH A VIEW TO THEIR
DISTRIBUTION OR RESALE, AND MAY NOT BE SOLD, PLEDGED, OR OTHERWISE TRANSFERRED
WITHOUT AN EFFECTIVE REGISTRATION STATEMENT FOR SUCH SECURITIES UNDER THE
SECURITIES ACT AND APPLICABLE STATE SECURITIES LAWS, OR AN OPINION OF COUNSEL
SATISFACTORY TO THE COMPANY TO THE EFFECT THAT SUCH REGISTRATION IS NOT
REQUIRED.

                               POWER AND AUTHORITY

Each Note holder represents that such Note holder has full power and authority
(corporate, statutory and otherwise) to execute and deliver this Offer and to
exchange their Notes for shares of Series B Preferred Stock and any shares of
Series B Preferred Stock-Underlying Common Stock. This Agreement constitutes the
legal,
<PAGE>   9
valid and binding obligation of each Note holder, enforceable against such Note
holder in accordance with its terms.

                              ACCREDITED INVESTOR

Each Note holder represents that it is an "accredited investor" as such term is
defined in Rule 501 of Regulation D.

                                   DEPOSITARY

                 The Depositary for this Offer to Exchange is:

                              Holland & Knight LLP
                           Attn: Sean B. Leonard, Esq.
                               10 St. James Avenue
                                Boston, MA 02116
                            Telephone: (617) 523-2700
                            Facsimile: (617) 523-6850
<PAGE>   10
                 SIGNATURE(S) MUST BE PROVIDED IN BOX TWO BELOW

[_]      CHECK HERE IF TENDERED NOTES ARE ENCLOSED HEREWITH.
[_]      CHECK HERE IF A CERTIFICATE OF LOST NOTE IS ENCLOSED HEREWITH.
--------------------------------------------------------------------------------
                                     BOX ONE
--------------------------------------------------------------------------------

LIST BELOW THE NOTES TO WHICH THIS OFFER TO EXCHANGE RELATE. IF THE SPACE
PROVIDED BELOW IS INADEQUATE, THE NOTE NUMBERS AND PRINCIPAL AMOUNTS SHOULD BE
LISTED ON A SEPARATE SIGNED SCHEDULE AFFIXED HERETO. NOTEHOLDERS WHO WISH TO
TENDER THEIR NOTES MUST COMPLETE THE TABLE BELOW AND COMPLETE AND SIGN IN BOX
TWO.

HYBRIDON WILL PAY ALL ACCRUED BUT UNPAID INTEREST ON THE NOTES DUE BY ISSUING
ADDITIONAL NOTES IN AN AGGREGATE PRINCIPAL AMOUNT EQUAL TO THE AMOUNT OF ACCRUED
BUT UNPAID INTEREST. NOTES WHICH WILL BE PAID IN RESPECT OF ACCRUED INTEREST
SHALL BE DEEMED TENDERED FOR EXCHANGE, UNLESS SPECIFIED TO THE CONTRARY BY
CHECKING THE BOX BELOW. CHECK THE BOX BELOW IF YOU DO NOT WANT NOTES WHICH WILL
BE PAID IN RESPECT OF ACCRUED INTEREST DEEMED TENDERED FOR EXCHANGE.

[_]      CHECK HERE IF YOU DO NOT WANT NOTES WHICH WILL BE PAID IN RESPECT OF
         ACCRUED INTEREST DEEMED TENDERED FOR EXCHANGE.
--------------------------------------------------------------------------------
                              DESCRIPTION OF NOTES
--------------------------------------------------------------------------------
<TABLE>
<CAPTION>
       1                         2                    3                4

NAME(S) AND                 NOTE NUMBER(S)         AGGREGATE         PRINCIPAL
ADDRESS(ES) OF              (ATTACH LIST IF        PRINCIPAL         AMOUNT
REGISTERED HOLDER(S)        NECESSARY)*            AMOUNT(S)         TENDERED**
--------------------------------------------------------------------------------
<S>                         <C>                    <C>              <C>

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------
</TABLE>
* NEED NOT BE COMPLETED BY NOTEHOLDERS TENDERING BY AFFIDAVIT OF LOST NOTE IF
NOTE NUMBER NOT AVAILABLE.
** UNLESS OTHERWISE INDICATED, THE NOTEHOLDER WILL BE DEEMED TO HAVE TENDERED
THE ENTIRE PRINCIPAL AMOUNT OF NOTES REPRESENTED BY TENDERED NOTES
<PAGE>   11
--------------------------------------------------------------------------------
                                     BOX TWO
--------------------------------------------------------------------------------

                         ACCEPTANCE OF OFFER TO EXCHANGE

                                PLEASE SIGN HERE
                 TO BE COMPLETED BY ALL HOLDERS TENDERING NOTES
          (WHETHER OR NOT NOTES ARE BEING PHYSICALLY TENDERED OR BEING
                      TENDERED BY AFFIDAVIT OF LOST NOTE)

I have read this Offer to Exchange, and hereby accept such Offer to Exchange,
and further agree to be bound by the foregoing.

----------------------------------------------------------------
Signature(s) of Registered Noteholder(s) or Authorized Signatory

----------------------------------------------------------------
Type or Print Name

Dated:                   , 2001
      ---------- --------

Area Code and Telephone No(s):
                              ------------------------------------------------

Must be signed by the registered Noteholder(s) exactly as the name(s) appear(s)
on the certificate and by person(s) authorized to become registered
Noteholder(s) as evidenced by endorsements and documents.
<PAGE>   12
                                    EXHIBIT A

                             AFFIDAVIT OF LOST NOTE
<PAGE>   13
                             AFFIDAVIT OF LOST NOTE

The undersigned (the "Noteholder"), a Noteholder of Hybridon, Inc. (the
"Company"), does hereby certify:

1. The Noteholder is the beneficial owner of that certain 8% Note due 2002
Number ___ of the Company (the "Note") dated December 13, 1999, and registered
in the name of the Noteholder, in the principal amount of $___________, and is
entitled to the full and exclusive possession of the Note;

2. The Noteholder has made an extensive search to locate the Note but it appears
to have been mutilated, lost, stolen or destroyed so that it cannot be found or
produced for exchange for shares of the Company's Series B Preferred Stock;

3. Neither the Note nor the rights of the Noteholder, therein, have in whole or
in part been assigned, transferred, pledged or otherwise hypothecated by the
Noteholder;

4. The Note was not endorsed;

5. The Noteholder hereby requests and this Affidavit of Lost Note is made for
the purpose of inducing the Company and its transfer agent (i) to refuse to
recognize any person other than the Noteholder as the owner of the Note, (ii) to
refuse to make any payment, transfer, registration, delivery or exchange in
connection with the Note to any other person other than the Noteholder, and
(iii) to refuse to take any other action pursuant to the request or demand of
any person other than the Noteholder with respect to the Note;

6. In consideration for the Company's acceptance of this Affidavit of Lost Note,
the Noteholder agrees to indemnify and hold the Company harmless against any
person, firm or entity now or hereafter acting as the Company's transfer agent,
or any successors and assigns of such person, firm, entity in such capacity,
from and against any and all claims, demands, liabilities, losses, damages and
expenses, including attorney's fees and costs of suits, incurred in connection
with or arising out of their compliance with this Affidavit of Lost Note, as set
forth therein;

7. If the original Note comes into the possession of the Noteholder, it will
promptly be delivered to the Company for cancellation.

                  [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]
<PAGE>   14
Dated this        day of            , 2001.
           ------        -----------

X
 -------------------------------------------
      [Signature of Noteholder]

Name:
     ---------------------------------------

Address:
        ------------------------------------

        ------------------------------------

        ------------------------------------

Note Number:
            ------------------------------------------
Principal Amount of Note:
                         -----------------------------
<PAGE>   15
                                    EXHIBIT B

                       SUMMARY OF SERIES B PREFERRED STOCK
<PAGE>   16
                      SERIES B CONVERTIBLE PREFERRED STOCK

                    ISSUED IN CONNECTION WITH THE EXCHANGE OF
                                8% NOTES DUE 2002

Set forth below is a summary of the principal terms of a Series B Convertible
Preferred Stock (the "Preferred B") of Hybridon, Inc. (the "Company"), to be
offered in exchange for the 8% Notes of the Company due 2002. The terms of the
Company's Series A Convertible Preferred Stock (the "Preferred A") are set forth
for comparison. Differences between the Preferred A and Preferred B are
highlighted in the Preferred B column. This general summary is provided solely
for the convenience of the Noteholders and is qualified in its entirety by
reference to the full text and more specific details contained in the
Certificate of Designation for the Preferred B.

<TABLE>
<CAPTION>
Rights, Preferences, Privileges              Preferred A                        Preferred B
-------------------------------              -----------                        -----------
<S>                                <C>                                 <C>
(1) Dividend Provisions            -    dividends on semi-annual       -    dividends on semi-annual
                                        basis at rate of 6.5% of            basis at rate of 8% of
                                        Dividend Base Amount                Dividend Base Amount
                                        ($100 plus accrued but              ($100 plus accrued but
                                        unpaid dividends)                   unpaid dividends)

                                                                       -    payable in cash, or at
                                   -    payable in cash, or at              Company's option, PIK
                                        Company's option, PIK
                                                                       -    in preference to
                                   -    in preference to                    dividends paid on Common
                                        dividends paid on Common            AND PREFERRED A

                                   -    Preferred A valued at          -    Preferred B valued at
                                        $100.00/share for                   $100.00/share for
                                        calculating PIK                     calculating PIK

                                   -    payable if declared by         -    payable if declared by
                                        Board                               Board

                                                                       -    NO DIVIDEND OR
                                                                            DISTRIBUTION PAID TO
                                                                            COMMON OR PREFERRED A
                                                                            UNLESS PAID TO PREFERRED
                                                                            B FIRST
</TABLE>
<PAGE>   17
<TABLE>
<S>                                <C>                                 <C>
(2) Liquidation Preference         -    upon (i) liquidation,          -    upon (i) liquidation,
                                        dissolution, or winding             dissolution, or winding
                                        up, (ii) sale of all or             up, (ii) sale of all or
                                        substantially all assets,           substantially all assets,
                                        or (iii) any merger                 or (iii) any merger
                                        transaction, entitled to            transaction, entitled to
                                        receive Dividend Base               receive Dividend Base
                                        Amount prior to any                 Amount prior to any
                                        distribution to Common              distribution to Common OR
                                                                            PREFERRED A

(3) Conversion                     -    "Conversion Price" is          -    "Conversion Price" is
                                        $4.25, subject to                   $.50, subject to
                                        adjustment in Section 4             adjustment in Section 4

                                   -    "Conversion Rate"              -    "Conversion Rate"
                                        determined by dividing              determined by dividing
                                        then-existing Conversion            then-existing Conversion
                                        Price into Dividend Base            Price into Dividend Base
                                        Amount                              Amount

(4) Antidilution                   Conversion Price subject to         Conversion Price subject to
                                   proportional adjustment if          proportional adjustment if
                                   Company:                            Company:

                                   -    pays dividend or makes         -    pays dividend or makes
                                        distribution on any class           distribution on any class
                                        of capital stock in                 of capital stock in
                                        shares of Common;                   shares of Common;

                                   -    subdivides outstanding         -    subdivides outstanding
                                        Common into greater                 Common into greater
                                        number of shares;                   number of shares;
</TABLE>
<PAGE>   18
<TABLE>
<S>                                <C>                                 <C>
                                   -    combines outstanding           -    combines outstanding
                                        Common into smaller                 Common into smaller
                                        number of shares;                   number of shares;

                                   -    issues shares of a series      -    issues shares of a series
                                        or class of capital stock           or class of capital stock
                                        to any holder of Common,            to any holder of Common,
                                        Preferred A or Preferred            Preferred A or Preferred
                                        B or rights to acquire              B or rights to acquire
                                        shares of a series or               shares of a series or
                                        class of capital stock at           class of capital stock at
                                        price per share less than           price per share less than
                                        market price;                       market price;

                                   -    pays or distributes to         -    pays or distributes to
                                        the holders of a series             the holders of a series
                                        or class of capital stock           or class of capital stock
                                        assets, properties, or              assets, properties, or
                                        rights to acquire                   rights to acquire
                                        Company's capital stock             Company's capital stock
                                        at price per share less             at price per share less
                                        than market price;                  than market price;

                                   -    or makes distribution          -    or makes distribution
                                        consisting solely of cash           consisting solely of cash
                                        to holders of any class             to holders of any class
                                        of capital stock where,             of capital stock where,
                                        during specified 12 month           during specified 12 month
                                        period, cash distribution           period, cash distribution
                                        exceeds 10% of product of           exceeds 10% of product of
                                        market price of Common              market price of Common
                                        multiplied by total                 multiplied by total
                                        outstanding shares of               outstanding shares of
                                        Common                              Common
</TABLE>

<PAGE>   19

<TABLE>
<S>                                <C>                                 <C>
(5) Mandatory Conversion           -    converted into shares of       -    CONVERTED INTO SHARES OF
                                        Common using Conversion             COMMON IF ALL OF
                                        Price of $4.00 if closing           PREFERRED A IS CONVERTED,
                                        bid price of Common                 USING A CONVERSION PRICE
                                        equals or exceeds 250% of           OF $.50
                                        Conversion Price for at
                                        least 20 trading days in
                                        any period of 30
                                        consecutive trading days

(6) Redemption                     -    at Company's option,           -    at Company's option,
                                        redeemable for cash equal           redeemable for cash equal
                                        to the Dividend Base                to the Dividend Base
                                        Amount if closing bid               Amount if closing bid
                                        price of Common equals or           price of Common equals or
                                        exceeds 250% of                     exceeds 250% of
                                        Conversion Price of                 Conversion Price of
                                        Preferred A for at least            Preferred A for at least
                                        20 trading days in any              20 trading days in any
                                        period of 30 consecutive            period of 30 consecutive
                                        trading days                        trading days

(7) Voting Rights                  Company needs 50% vote of all       Company needs 50% vote of all
                                   outstanding Preferred A to:         outstanding PREFERRED B to:

                                   -    amend, alter or repeal         -    amend, alter or repeal
                                        any provision of                    any provision of
                                        Certificate of                      Certificate of
                                        Incorporation or Bylaws             Incorporation or Bylaws
                                        adversely affecting                 adversely affecting
                                        relative rights,                    relative rights,
                                        preferences,                        preferences,
                                        qualifications,                     qualifications,
                                        limitations or                      limitations or
</TABLE>
<PAGE>   20
<TABLE>
<S>                                <C>                                 <C>
                                        restrictions of Preferred           restrictions of PREFERRED
                                        A (issuance of securities           B;
                                        ranking prior to, or pari
                                        passu with Preferred A         -    ISSUE SECURITIES RANKING
                                        upon Liquidation Event or           PRIOR TO, OR PARI PASSU
                                        with respect to payment             WITH PREFERRED B UPON
                                        of dividends does not               LIQUIDATION EVENT OR WITH
                                        adversely affect such);             RESPECT TO PAYMENT OF
                                        or                                  DIVIDENDS; OR

                                   -    authorize or issue, or         -    authorize or issue, or
                                        increase authorized amount          increase authorized
                                        of, Preferred A, other than         amount of, PREFERRED B,
                                        Preferred A issuable as             other than PREFERRED B
                                        dividends on Preferred A or         ISSUABLE AS DIVIDENDS ON
                                        in exchange for 9% Notes            PREFERRED B OR IN
                                                                            EXCHANGE FOR 8% NOTES
</TABLE>
<PAGE>   21
                                    EXHIBIT C

                         CERTIFICATE OF DESIGNATION FOR
                      SERIES B PREFERRED STOCK OF HYBRIDON

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