Document:

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                                                                     Exhibit 4.3

                           THE ALTHEXIS COMPANY, INC.
                             1999 STOCK OPTION PLAN

     1. Definitions. As used in this 1999 Stock Option Plan of The Althexis
        -----------
Company, Inc., the following terms shall have the following meanings:

          1.1. Board means the Company's Board of Directors.
               -----

          1.2. Code means the federal Internal Revenue Code of 1986, as amended.
               ----

          1.3. Committee means a committee appointed by the Board or, if no such
               ---------
     committee is appointed, the Board itself. Such committee or the Board, as
     the case may be, shall be responsible for the administration of the Plan,
     as provided in section 5 of the Plan.

          1.4. Company means The Althexis Company, Inc., a Delaware corporation.
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          1.5. Fair Market Value means (i) on any date prior to the Initial
               -----------------
     Public Offering Date, the value of a share of Stock on any such date as
     determined by the Committee, and (ii) on the Initial Public Offering Date
     or on any date thereafter, (A) the average of the closing bid and asked
     prices of a share of Stock on the over-the-counter market, as reported by
     the National Association of Securities Dealers Automated Quotation System
     ("NASDQ") on such date, or (B) the closing price of a share of Stock on an
     exchange or the Nasdaq Stock Market's National Market System, as the case
     may be, on such date.

          1.6. Grant Date means the date as of which an Option is granted, as
               ----------
     determined under Section 7.

          1.7. Incentive Option means an Option which by its terms is to be
               ----------------
     treated as an "incentive stock option" within the meaning of Section 422 of
     the Code.

          1.8. Initial Public Offering Date means the date of the closing of the
               -----------------------
     Company's initial public offering of Stock.

          1.9. Non-statutory Option means any Option that is not an Incentive
               --------------------
     Option.

          1.10. Option means an option to purchase shares of Stock granted under
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     the Plan.

          1.11. Option Agreement means an agreement between the Company and an
                ----------------
     Optionee, setting forth the terms and conditions of an Option.

          1.12. Option Price means the price paid by an Optionee for a share of
                ------------
     Stock upon exercise of an Option.

          1.13. Optionee means a person eligible to receive an Option, as
                --------
     provided in Section 6, to whom an Option shall have been granted under the
     Plan.

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          1.14. Plan means this 1999 Stock Option Plan of the Company, as
                ----
     amended from time to time.

          1.15. Stock means common stock, par value $0.01 per share, of the
                -----
     Company.

          1.16. Stockholders Agreement means the Stockholders Agreement, dated
                ----------------------
     as of January 1, 1999, among the Company and each of the stockholders party
     thereto, as amended, modified and supplemented from time to time.

          1.17. Ten Percent Owner means a person who owns, or is deemed within
                -----------------
     the meaning of Section 422(b)(6) of the Code to own, stock possessing more
     than 10% of the total combined voting power of all classes of stock of the
     Company (or its parent or subsidiary corporations). Whether a person is a
     Ten Percent Owner shall be determined with respect to each Option based on
     the facts existing immediately prior to the Grant Date of such Option.

          1.18. Vesting Year for any portion of any Incentive Option means the
                ------------
     calendar year in which that portion of the Option first becomes
     exercisable.

     2. Purpose. This Plan is intended to encourage ownership of Stock by
        -------
employees and consultants of the Company and its subsidiaries and to provide
additional incentives for them to promote the success of the Company's business.
The Plan is intended to be an incentive stock option plan within the meaning of
Section 422 of the Code but not all Options granted hereunder are required to be
Incentive Options.

     3. Term of the Plan. Options may be granted hereunder at any time in the
        ----------------
period commencing on the approval of the Plan by the Board and ending May 18,
2009.

     4. Stock Subject to the Plan. At no time shall the number of shares of
        -------------------------
Stock then outstanding which are attributable to the exercise of Options granted
under the Plan, plus the number of shares then issuable upon exercise of
outstanding Options granted under the Plan, exceed 400,000 shares, subject,
                                                                   -------
however, to the provisions of Section 17 of the Plan. Shares to be issued upon
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the exercise of Options granted under the Plan may be either authorized but
unissued shares or shares held by the Company in its treasury. If any Option
expires or terminates for any reason without having been exercised in full, the
shares not purchased thereunder shall again be available for Options thereafter
to be granted.

     5. Administration. The Plan shall be administered by the Committee. Subject
        --------------
to the provisions of the Plan, the Committee shall have complete authority, in
its discretion, to make or to select the manner of making the following
determinations with respect to each Option to be granted by the Company: (a) the
employee or consultant to receive the Option; (b) whether the Option (if granted
to an employee) will be an Incentive Option or Non-statutory Option; (c) the
time of granting the Option; (d) the number of shares subject to the Option; (e)
the Option Price; (f) the Option period; (g) the Option exercise date or dates;
and (h) the effect of termination of employment, consulting or association with
the Company on the subsequent exercisability of the Option. In making such
determinations, the Committee may take into account the nature of the services
rendered by the respective employees and consultants, their present and
potential contributions to the success of the Company and its subsidiaries, and
such other factors as the Committee in its discretion shall deem relevant.
Subject to the provisions of the Plan, the Committee shall also have complete
authority to

<PAGE>

interpret the Plan, to prescribe, amend and rescind rules and regulations
relating to it, to determine the terms and provisions of the respective Option
Agreements (which need not be identical), and to make all other determinations
necessary or advisable for the administration of the Plan. The Committee's
determinations on the matters referred to in this Section 5 shall be conclusive.

     6. Eligibility. An Option shall be granted only to an employee, director or
        -----------
consultant of one or more of the Company or any subsidiary thereof.

     7. Time of Granting Options. The granting of an Option shall take place at
        ------------------------
the time specified in the Option Agreement. Only if expressly so provided in the
Option Agreement, shall the Grant Date be the date on which an Option Agreement
shall have been duly executed and delivered by the Company and the Optionee.

     8. Option Price. The Option Price under each Incentive Option shall be not
        ------------
less than 100% of the Fair Market Value of Stock on the Grant Date, or not less
than 110% of the Fair Market Value of Stock on the Grant Date if the Optionee is
then a Ten Percent Owner. The Option Price under each Non-statutory Option shall
not be so limited solely by reason of this Section 8.

     9. Option Period. No Incentive Option may be exercised later than the tenth
        -------------
anniversary of the Grant Date, but in any case not later than the fifth
anniversary of the Grant Date, if the Optionee is a Ten Percent Owner on the
Grant Date. The Option period under each Non-statutory Option shall not be so
limited solely by reason of this Section 9. An Option may become exercisable in
such installments, cumulative or non-cumulative, as the Committee may determine.
In the case of an Option not otherwise immediately exercisable in full, the
Committee may accelerate the exercisability of such Option in whole or in part
at any time, provided the acceleration of the exercisability of any Incentive
Option would not cause the Option to fail to comply with the provisions of
Section 422 of the Code.

     10. Limit on Incentive Option Characterization. No Incentive Option shall
         ------------------------------------------
be considered an Incentive Option to the extent that pursuant to its terms it
would permit the Optionee to purchase for the first time in any Vesting Year
under that Incentive Option more than the number of shares of Stock calculated
by dividing the current limit by the Option Price. The current limit for any
Optionee for any Vesting Year shall be $100,000 minus the aggregate Fair Market
Value at the applicable Grant Date of the number of shares of Stock available
for purchase for the first time in the Vesting Year under each other Incentive
Option granted to the Optionee under the Plan after December 31, 1986 and each
other incentive stock option granted to the Optionee after December 31, 1986
under any other incentive stock option plan of the Company (and any parent and
subsidiary corporations).

     11. Exercise of Option. An Option may be exercised by the Optionee giving
         ------------------
written notice, in the manner provided in Section 21, specifying the number of
shares with respect to which the Option is then being exercised. In the event
that such notice of exercise is given prior to the Initial Public Offering Date,
such notice shall be accompanied by (x) payment in the form of cash, or
certified or bank check payable to the order of the Company, in an amount equal
to the aggregate Option Price of the shares of Stock to be purchased, and (y) if
the exercising Optionee is not already a party to the Stockholders Agreement, an
executed counterpart of the Instrument of Adherence attached to the Stockholders
Agreement as Exhibit A thereto. In the event that such notice of exercise is
             -------
given on or after the Initial Public Offering Date, such notice of exercise
shall be accompanied by (x) payment in the form of (A) cash, or certified or
bank check payable to the order of the Company, in an amount equal to the
aggregate Option Price of the shares of Stock to be purchased, or (B) shares of
Stock having a Fair Market Value on the date the Option is exercised equal to
the aggregate Option Price of the shares of Stock to be

<PAGE>

purchased (provided that the use of shares of Stock to exercise an option shall
be subject to such restrictions as the Committee may reasonably require to avoid
adverse accounting effects), and (y) if the Stockholders Agreement is then still
in effect and if the exercising Optionee is not already a party to the
Stockholders Agreement, an executed counterpart of the Instrument of Adherence
attached to the Stockholders Agreement as Exhibit A thereto. Receipt by the
                                          -------
Company of such notice, payment and, if applicable, executed counterparts shall
constitute the exercise of the Option. Within 30 days thereafter but subject to
the remaining provisions of the Plan, the Company shall deliver or cause to be
delivered to the Optionee or his agent a certificate or certificates for the
number of shares then being purchased. Such shares shall be fully paid and
nonassessable.

     12. Restrictions on Issue of Shares.
         -------------------------------

          (a) Notwithstanding any other provision of the Plan, if, at any time,
in the reasonable opinion of the Company the issuance of shares of Stock covered
by the exercise of any Option would constitute a violation of law (including,
without limitation, a violation of federal or state securities law), the Company
shall not be required to issue such shares of Stock unless and until the Company
and/or the exercising holder of such Option shall have taken any and all steps
reasonably necessary or required to ensure that such issuance would not cause or
result in any such violation of law. Such reasonably necessary or required steps
may include, without limitation, (i) seeking and obtaining any approvals from,
or making filings with, such governmental agencies as may be required under any
applicable law, rule, or regulation, (ii) registering under the Securities Act
of 1933, as amended (the "Securities Act"), the shares of Stock to be issued
upon exercise of such Option or (iii) causing any such proposed issuance to
qualify for or come within any exemption from the registration requirements of
the Securities Act or any applicable state securities laws.

          (b) Each certificate representing shares issued upon the exercise of
an Option will bear restrictive legends which may refer to this Plan and to
applicable restrictions under the Stockholders Agreement.

     13. Investment Representations; Subsequent Registration.
         ---------------------------------------------------

          (a) Unless the shares to be issued upon exercise of an Option granted
under the Plan have been effectively registered under the Securities Act, the
Company may, as a condition precedent to the exercise of such Option, require
that the exercising holder of such Option make such written representations and
warranties to the Company as the Company shall deem necessary or appropriate for
purposes of confirming that the issuance of such shares shall be exempt from the
registration requirements of the Securities Act or any applicable state
securities laws and that the issuance of such shares is otherwise in compliance
with all applicable laws, rules and regulations, including, without limitation,
federal and state securities laws. The Company shall be under no obligation to
issue any of such shares unless and until the exercising holder of such Option
makes such written representations and warranties to the Company, and the
Company shall also be under no obligation to issue any of such shares if any of
such written representations and warranties are not true, correct or complete
when made.

          (b) Each share of Stock issued pursuant to the exercise of an Option
granted pursuant to this Plan may bear a reference to the fact that no
registration statement has been filed with the Securities and Exchange
Commission in respect such share and that the ability to resell such share

<PAGE>

will be subject to certain restrictions on transfer imposed under applicable
federal and state securities laws.

          (c) If the Company shall deem it necessary or desirable to register
under the Securities Act or other applicable statutes any shares with respect to
which an Option shall have been granted, or to qualify any such shares for
exemption from the Securities Act or other applicable statutes, then the Company
shall take such action at its own expense. The Company may require from each
Option holder, or each holder of shares of Stock acquired pursuant to the Plan,
such information in writing for use in any registration statement, prospectus,
preliminary prospectus or offering circular as is reasonably necessary for such
purpose and may require reasonable indemnity to the Company and its officers and
directors from such holder against all losses, claims, damage and liabilities
arising from such use of the information so furnished and caused by any untrue
statement of any material fact therein or caused by the omission to state a
material fact required to be stated therein or necessary to make the statements
therein not misleading in the light of the circumstances under which they were
made.

     14. Withholding; Notice of Disposition of Stock Prior to Expiration of
         ------------------------------------------------------------------
Specified Holding Period.
------------------------

          (a) Whenever shares are to be issued in satisfaction of an Option
granted hereunder, the Company shall have the right to require the Optionee to
remit to the Company an amount sufficient to satisfy federal, state, local or
other withholding tax requirements if and to the extent required by law (whether
so required to secure for the Company an otherwise available tax deduction or
otherwise) prior to the delivery of any certificate or certificates for such
shares.

          (b) The Company may require as a condition to the issuance of shares
covered by any Incentive Option that the party exercising such Option give a
written representation to the Company which is satisfactory in form and
substance to its counsel and upon which the Company may reasonably rely, that he
or she will report to the Company any disposition of such shares prior to the
expiration of the holding periods specified by Section 422(a)(1) of the Code. If
and to the extent that the realization of income in such a disposition imposes
upon the Company federal, state, local or other withholding tax requirements, or
any such withholding is required to secure for the Company an otherwise
available tax deduction, the Company shall have the right to require that the
recipient remit to the Company an amount sufficient to satisfy those
requirements; and the Company may require as a condition to the issuance of
shares covered by an Incentive Option that the party exercising such option give
a satisfactory written representation promising to make such a remittance.

     15. Termination of Association with the Company. If the Optionee's
         -------------------------------------------
employment or consulting relationship with the Company is terminated, whether
voluntarily or otherwise, the Option, to the extent the Option is exercisable on
the date of termination, may be exercised by the Optionee, but only within 90
                                                           ------------------
days after he or she ceases to be an employee of or consultant to the Company,
----
unless terminated earlier by its terms. Notwithstanding the foregoing, in the
event that the applicable Option Agreement with respect to an Option shall
contain specific provisions governing the effect that any such termination shall
have on the exercisability of such Option, such provisions in the Option
Agreement shall, to the extent that they are inconsistent with the provisions of
this Section 15, control and deem to supersede the provisions of this Section
15. For purposes of this Section 15, military or sick leave shall not be deemed
a termination of employment, provided that it does not exceed the longer of 120
                             --------
days or the period during which the absent Optionee's reemployment rights, if
any, are guaranteed by statute or by contract.

<PAGE>

     16. Transferability of Options. Incentive Options shall not be
         --------------------------
transferable, otherwise than by will or the laws of descent and distribution,
and may be exercised during the life of the Optionee only by the Optionee.
Nonstatutory Options shall not be transferable, otherwise than by will or the
laws of descent and distribution, and may be exercised during the life of the
Optionee only by the Optionee; provided, however, that Nonstatutory Options may
                               -----------------
be transferred to a third party if and to the extent authorized and permitted by
the Committee, whereupon any such third party may exercise such Nonstatutory
Option, at any time and from time to time thereafter, subject to and upon the
terms and conditions set forth in this Plan and the Option Agreement relating to
such Nonstatutory Option. In granting its authorization and permission to any
proposed transfer of a Nonstatutory Option to a third party, the Committee may
impose conditions or requirements that must be satisfied by the transferor or
the third party transferee prior to or in connection with such transfer,
including, without limitation, any conditions or requirements that may be
necessary or desirable, in the sole and absolute discretion of the Committee, to
ensure that such proposed transfer complies with applicable securities laws or
to prevent the Company, such transferor or such third party transferee from
violating or otherwise not being in compliance with applicable securities laws
as a result of such transfer. For purposes of this Section 16, the term
Nonstatutory Option shall include an Option that was an Incentive Option at the
time of grant, but that has been subsequently been disqualified or otherwise
lost its status as an Incentive Option.

     17. Adjustment of Number and Kind of Option Shares. In the event of any
         ----------------------------------------------
stock dividend payable in Stock or any split-up or contraction in the number of
shares of Stock after the date of an Option Agreement and prior to the exercise
in full of the Option, the number of shares subject to such Option Agreement and
the price to be paid for each share subject to the Option shall be
proportionately adjusted. In the event of any reclassification, exchange,
conversion or change of outstanding shares of Stock, or in case of any
consolidation or merger of the Company with or into another company, or in case
of any sale or conveyance to another company or entity of the property of the
Company as a whole or substantially as a whole, shares of stock or other
securities equivalent in kind and value to those shares an Optionee would have
received if he or she had held the full number of shares of Stock subject to the
Option immediately prior to such reclassification, exchange, conversion, change,
consolidation, merger, sale or conveyance and had continued to hold those shares
(together with all other shares, stock and securities thereafter issued in
respect thereof) to the time of the exercise of the Option shall thereupon be
subject to the Option. Upon dissolution or liquidation of the Company, the
Option shall terminate, but the Optionee (if at the time in the employ of or
retained as a consultant to the Company or any of its subsidiaries) shall have
the right, immediately prior to such dissolution or liquidation, to exercise the
Option to the extent exercisable on the date of such dissolution or liquidation.
No fraction of a share shall be purchasable or deliverable upon exercise, but in
the event any adjustment hereunder of the number of shares covered by the Option
shall cause such number to include a fraction of a share, such number of shares
shall be adjusted to the nearest smaller whole number of shares. In the event of
changes in the outstanding Stock by reason of any stock dividend, split-up,
contraction, reclassification, or change of outstanding shares of Stock of the
nature contemplated by this Section 17, the number of shares of Stock available
for the purpose of the Plan as stated in Section 4 shall be correspondingly
adjusted.

     18. Reservation of Stock. The Company shall at all times during the term of
         --------------------
each Option reserve or otherwise keep available such number of shares of Stock
as will be sufficient to satisfy the requirements of the Plan and shall pay all
fees and expenses necessarily incurred by the Company in connection therewith.

<PAGE>

     19. Limitation of Rights in Stock; No Special Employment or Other Rights.
         --------------------------------------------------------------------
The Optionee shall not be deemed for any purpose to be a stockholder of the
Company with respect to any of the shares of Stock covered by an Option, except
to the extent that such Option shall have been exercised with respect thereto
and, in addition, a certificate shall have been issued therefor and delivered to
the Optionee or his agent. Any Stock issued pursuant to the exercise of any
Option shall be subject to all restrictions upon the transfer thereof which may
be now or hereafter imposed by the Certificate of Incorporation, the By-laws of
the Company, or the Stockholders Agreement. Nothing contained in the Plan or in
any Option shall confer upon any Optionee any right with respect to the
continuation of his or her employment with, or retention as a consultant by, the
Company (or any subsidiary), or interfere in any way with the right of the
Company (or any subsidiary), subject to the terms of any separate employment or
consulting agreement or provision of law or corporate articles or by-laws to the
contrary, at any time to terminate such employment or consulting agreement or to
increase or decrease the compensation of the Optionee from the rate in existence
at the time of the grant of an Option.

     20. Termination and Amendment of the Plan. The Board may at any time
         -------------------------------------
terminate the Plan or make such modifications of the Plan as it shall deem
advisable. No termination or amendment of the Plan may, without the consent of
the Optionee to whom any Option shall theretofore have been granted, adversely
affect the rights of such Optionee under such Option.

     21. Notices and Other Communications. All notices and other communications
         --------------------------------
required or permitted under the Plan shall be effective if in writing and if
delivered or sent by certified or registered mail, return receipt requested (a)
if to the Optionee, at his or her residence address last filed with the Company,
and (b) if to the Company, at 1365 Main Street, Waltham, Massachusetts, 02451,
Attention: Manuel Navia, President, with a copy to the Assistant Secretary of
the Company, presently, Julio E. Vega, at Bingham Dana LLP, 150 Federal Street,
Boston, MA 02110, or to such other persons or addresses as the Optionee or the
Company may specify by a written notice to the other from time to time.<PAGE>

                                                                     Exhibit 4.4

                           THE ALTHEXIS COMPANY, INC.

                                 FIRST AMENDMENT
                                     TO THE
                             1999 STOCK OPTION PLAN
                             ----------------------

         This FIRST AMENDMENT (this "Amendment") to the 1999 Stock Option Plan
(the "Plan") of The Althexis Company, Inc., a Delaware corporation (the
"Company"), is being adopted by resolution of the Board of Directors by written
consent in lieu of a meeting as of May 14, 2001 (the "Effective Date").
Effective from and after the Effective Date, the Plan is hereby amended as
follows:

         1.     Section 5 of the Plan hereby is amended by deleting such Section
 in its entirety and replacing it with the following:

         "5. Administration. The Plan shall be administered by the Committee,
          -----------------
         provided, however, that the Committee may delegate to an executive
         officer or officers the authority to grant Options hereunder to
         employees who are not officers, and to consultants, in accordance with
         such guidelines as the Committee shall set forth at any time or from
         time to time. Subject to the provisions of the Plan, the Committee or
         its designee shall have complete authority, in its discretion, to make
         or to select the manner of making the following determinations with
         respect to each Option to be granted by the Company: (a) the employee
         or consultant to receive the Option; (b) whether the Option (if granted
         to an employee) will be an Incentive Option or Non-statutory Option;
         (c) the time of granting the Option; (d) the number of shares subject
         to the Option; (e) the Option Price; (f) the Option period; (g) the
         Option exercise date or dates; and (h) the effect of termination of
         employment, consulting or association with the Company on the
         subsequent exercisability of the Option. In making such determinations,
         the Committee may take into account the nature of the services rendered
         by the respective employees and consultants, their present and
         potential contributions to the success of the Company and its
         subsidiaries, and such other factors as the Committee in its discretion
         shall deem relevant. Subject to the provisions of the Plan, the
         Committee shall also have complete authority to interpret the Plan, to
         prescribe, amend and rescind rules and regulations relating to it, to
         determine the terms and provisions of the respective Option Agreements
         (which need not be identical), and to make all other determinations
         necessary or advisable for the administration of the Plan. The
         Committee's determinations on the matters referred to in this Section 5
         shall be conclusive."

         Except to the extent amended hereby, all of the terms, provisions and
conditions set forth in the Plan are hereby ratified and confirmed and shall
remain in full force and effect. The Plan and this Amendment shall be read and
construed together as a single instrument.

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