Document:

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                                                                   Exhibit 10.49

                           THE ALTHEXIS COMPANY, INC.

                           FIRST AMENDED AND RESTATED
                       EXECUTIVE STOCK PURCHASE AGREEMENT

     This FIRST AMENDED AND RESTATED EXECUTIVE STOCK PURCHASE AGREEMENT (this
"Agreement") is made as of this 27th day of July, 2001, by and among The
 ---------
Althexis Company, Inc., a Delaware corporation (the "Company"), Mark Skaletsky
                                                     -------
(the "Purchaser"), and Paul Mellett, in his capacity as Assistant Secretary of
      ---------
the Company and escrow holder hereunder (the "Escrow Holder").
                                              -------------

     WHEREAS, the Purchaser is the Chairman, President and Chief Executive
Officer of the Company;

     WHEREAS, the Company sold to the Purchaser, and the Purchaser purchased
from the Company, 439,987 shares of the Company's common stock, par value $0.01
per share ("Common Stock"), according to the terms and conditions of that
            ------------
certain Executive Stock Purchase Agreement, dated as of April 18, 2001 (the
"Original Purchase Agreement");
 ---------------------------

     WHEREAS, the Company is entering into a certain Agreement and Plan of
Merger, of even date herewith (the "Merger Agreement"), by and among the
                                    ----------------
Company, Microcide Pharmaceuticals, Inc., a Delaware corporation ("Microcide"),
                                                                   ---------
and a wholly owned subsidiary of Microcide (the "Sub"), whereby the Sub will
                                                 ---
merge with and into the Company (the "Merger") with the Company as the surviving
                                      ------
entity and, as a condition to such Merger the Company has agreed to amend and
restate the terms of the Original Purchase Agreement; and

     WHEREAS, the parties to the Original Purchase Agreement (including a
secured promissory note and stock pledge agreement that were executed in
connection with the Original Purchase Agreement), desire to enter into this
First Amended and Restated Executive Stock Purchase Agreement to amend and
restate the terms of the Original Purchase Agreement (including the related note
and stock pledge agreement) in their entirety as set forth herein and therein.

     NOW, THEREFORE, in consideration of the promises and mutual covenants
herein set forth, and other good and valuable consideration, receipt of which is
hereby acknowledged, the parties hereto hereby mutually covenant and agree as
follows:

     1.   Definitions. For purposes of this Agreement, the following terms shall
          -----------
have the meanings provided therefor below in this Section 1 or elsewhere in this
Agreement as referred to below in this Section 1:

          "Additional 2002 Shares" shall have the meaning ascribed to such term
           ----------------------
          in Section 5 below.

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                                      -2-

          "Change in Control" shall mean any Sale Transaction in which the
           -----------------
          stockholders of the Company immediately prior to such Sale Transaction
          do not own or hold, directly or indirectly, immediately after
          consummation of such Sale Transaction, shares of capital stock of the
          surviving person or entity or acquiring person or entity, as
          applicable, in connection with such Sale Transaction representing at
          least a majority of the total voting power of the outstanding capital
          stock of such surviving person or entity or such acquiring person or
          entity, as the case may be, provided, however, that if any such Sale
                                      -----------------
          Transaction involves the sale by the Company of newly issued shares of
          capital stock of the Company, such Sale Transaction shall not
          constitute a Change in Control within the meaning of this definition
          if (i) the purchaser of such newly issued shares of capital stock in
          such Sale Transaction is a venture capital fund, any other public or
          private equity fund, a financial institution or other type or kind of
          financial investor, or (ii) the purchaser of such newly issued shares
          of capital stock in such Sale Transaction has entered into an
          agreement expressly prohibiting such purchaser from electing or
          appointing fifty percent (50%) or more of the members of the Board of
          Directors of the Company. For purposes of this calculation, only those
          shares of capital stock of the surviving person or entity or acquiring
          person or entity, as applicable, that were issued in exchange for
          outstanding securities of the Company shall be counted as being owned
          by the stockholders of the Company immediately after the consummation
          of the Sale Transaction.

          "Closing" shall have the meaning ascribed to such term in Section 6
           -------
          below.

          "Note" shall have the meaning ascribed to such term in Section 3
           ----
          below.

          "Notice" shall have the meaning ascribed to such term in Section 6
           ------
          below.

          "Option" shall have the meaning ascribed to such term in Section 6
           ------
          below.

          "Sale Transaction" shall mean shall mean any merger or consolidation
           ----------------
          of the Company with and into another person or entity, the sale or
          transfer of all or substantially all of the assets of the Company, or
          the sale or transfer of shares of capital stock of the Company, in
          each case in a single transaction or in a series of related
          transactions.

          "Signing Shares" shall have the meaning ascribed to such term in
           --------------
          Section 5 below.

          "Shares" shall have the meaning ascribed to such term in Section 2
           ------
          below.

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                                      -3-

          "Shares Certificate" shall have the meaning ascribed to such term in
           ------------------
          Section 4 below.

          "Stock Pledge Agreement" shall have the meaning ascribed to such term
           ----------------------
          in Section 3 below.

          "Third Party" shall mean any entity or person other than (i) the
           -----------
          Company, (ii) any subsidiary of the Company, (iii) any entity
          controlled by, or under common control with, the Company or any of its
          subsidiaries, (iv) the Purchaser, or (v) and any entity controlled by
          the Purchaser.

          "Time Vesting Shares" shall have the meaning ascribed to such term in
           -------------------
          Section 5 below.

          "Unvested Shares" shall mean, at the relevant time of reference
           ---------------
          thereto, those Shares that have not vested on or prior to such time
          pursuant to Section 5 or Section 6 hereof.

          "Vested Shares" shall mean, at the relevant time of reference thereto,
           -------------
          those Shares that have vested on or prior to such time pursuant to
          Section 5 or Section 6 hereof.

     2.   Sale of Stock. Pursuant to the Original Purchase Agreement, the
          -------------
Company has sold to the Purchaser, and the Purchaser has purchased from the
Company, an aggregate of 439,987 shares of Common Stock (subject to adjustment
pursuant to Section 10 hereof, the "Shares"), at the price of $0.50 per share,
                                    ------
for an aggregate purchase price of $219,993.50.

     3.   Payment of Purchase Price. Pursuant to the Original Purchase
          -------------------------
Agreement, the Purchaser paid $43,998.50 of the aggregate purchase price for the
Shares in cash and agreed to pay the remaining purchase price by the execution
and delivery to the Company of a secured promissory note, substantially in the
form of Exhibit A hereto (the "Note"), in the principal amount of $175,995
        ---------              ----
payable to the Company. The Purchaser shall continue to secure payment and
performance of his obligations under the Note by the pledge of a portion of the
Shares to the Company pursuant to, and in accordance with, the terms and
conditions of a Stock Pledge Agreement substantially in the form of Exhibit B
                                                                    ---------
hereto (the "Stock Pledge Agreement").
             ----------------------

     4.   Issuance of Shares. Pursuant to the terms of the Original Stock
          ------------------
Purchase Agreement, the Company prepared and duly executed a stock certificate,
registered in the name of the Purchaser, representing 439,987 shares of Common
Stock (the "Initial Shares Certificate"). The Initial Shares Certificate and any
            --------------------------
stock certificate prepared and duly executed by the Company, registered in the
name of the Purchaser, representing any Shares issued to the Purchaser at any
time after the date of this Agreement (each, an "Additional
                                                 ----------

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                                      -4-

Shares Certificate"), shall each be endorsed with the legend set forth in
------------------
Section 8(b) below for so long as such legends are applicable. The Initial
Shares Certificate and each Additional Shares Certificate, if any, each
accompanied by stock powers or other appropriate instruments of assignment
thereof duly executed in blank by the Purchaser, have been or shall be delivered
to the Escrow Holder to be held in escrow pursuant to the provisions of Section
7 hereof. The Company shall remove each legend from the Initial Shares
Certificate and each Additional Shares Certificate, if any, at such time as the
legend is no longer applicable to the shares represented by the certificate.

     5.   Vesting of Shares.
          -----------------

          (a)  Vesting Schedule. Subject to all of the provisions of this
               ----------------
Section 5 and to the vesting provisions of Section 6 below, the Shares shall
vest as follows:

               (i) 87,997 shares (subject to adjustment pursuant to Section 10
hereof, the "Signing Shares") shall vest on March 31, 2002;
             --------------

               (ii) an additional 87,997 shares (subject to adjustment pursuant
to Section 10 hereof, the "Additional 2002 Shares") shall also vest on March 31,
                           ----------------------
2002; and

               (iii) 263,993 shares (subject to adjustment pursuant to Section
10 hereof, the "Time Vesting Shares") shall vest in monthly installments, in as
                -------------------
equal number of shares as possible, at the end of each calendar month beginning
April 30, 2002 and ending March 31, 2005.

          (b)  Acceleration of Vesting.
               -----------------------

               (i) Notwithstanding anything in Section 5(a) above or in Section
5(c) below to the contrary, in the event that the Purchaser dies or the Company
or Microcide terminates the Purchaser's employment with the Company or Microcide
for any reason or no reason at any time prior to March 31, 2002, then the
vesting of all of the Signing Shares that are then outstanding shall be
accelerated such that all of such Signing Shares shall be fully vested on the
date of such death or termination.

               (ii) Notwithstanding anything in Section 5(a) above to the
contrary but subject to the provisions of Section 5(c) below, in the event a
Change in Control of Microcide occurs after March 31, 2002, then the vesting of
all of those Time Vesting Shares that are outstanding immediately prior to such
Change in Control and that have not already previously vested shall be
accelerated such that all of such Time Vesting Shares shall be fully vested
immediately prior to the occurrence of such Change in Control.

          (c)  No Further Vesting Following Termination. Except if and to the
extent otherwise expressly provided in Section 5(b)(i) or Section 6, upon
termination of the

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                                      -5-

Purchaser's employment with the Company or Microcide for any reason or for no
reason, regardless of whether such termination is effected by the Company or
Microcide by the Purchaser (whether voluntarily or involuntarily) or upon the
Purchaser's death, none of the Unvested Shares owned of record or beneficially
by the Purchaser on the date of termination shall thereafter vest.

          (d)  Change in Control. In the event that, in connection with a Change
               -----------------
in Control transaction, the Purchaser receives or acquires any shares of stock
in any person or entity or other property or assets of any kind (collectively,
the "New Property") in exchange for shares of stock in the Company (including,
     ------------
without limitation, the Shares) or other property or assets, in either case that
is owned or held by the Purchaser subject to this Agreement (collectively, the
"Subject Property"), then, following the Change in Control, the provisions of
 ----------------
this Agreement shall apply to such New Property to the same extent as they would
have applied to the Subject Property and the Purchaser shall hold such New
Property subject to all of the provisions of this Agreement and shall comply and
perform with all of his obligations under this Agreement with respect to such
New Property and such New Property shall thereafter be deemed to be and treated
as Subject Property.

          (e)  Delivery of Vested Shares. Vested Shares shall, at the request of
               -------------------------
the Purchaser, be released from the escrow provided for in Section 7 hereof and
shall be delivered to the Purchaser.

          (f)  Escrow of Unvested Shares. All Unvested Shares shall be held in
               -------------------------
escrow pursuant to Section 7 below.

     6.   Repurchase Right.
          ----------------

          (a)  In the event of the termination of the Purchaser's employment
with the Company or Microcide for any reason or for no reason at any time,
regardless of whether such termination is effected by the Company or Microcide
by the Purchaser (whether voluntarily or involuntarily) or upon the Purchaser's
death, the Company shall have the right, but not the obligation, to repurchase
all or any number of the Shares that are then Unvested Shares (after giving
effect to the acceleration of vesting provisions of Section 5(b), if applicable)
subject to and in accordance with the terms of this Section 6. The Company may
exercise its repurchase rights under this Section 6(a), by giving written notice
of exercise in accordance with the provisions of Section 6(c) below, within
thirty (30) days following the date of the termination or death of the
Purchaser.

          (b)  The Company may exercise its repurchase rights in this Section 6
by delivering to the Purchaser, within the time periods specified in Section
6(a), a written notice (the "Notice") of its intention to exercise its
                             ------
repurchase right under this Section 6, specifying the number of Unvested Shares
that the Company desires to repurchase, whereupon, subject to the provisions of
this Section 6, the Company shall become legally obligated to repurchase

<PAGE>
                                      -6-

from the Purchaser, and the Purchaser shall become legally obligated to sell to
the Company, at the Closing (as such term is defined below), the number of
Unvested Shares referred to in the Notice. The purchase price per share for all
of the Unvested Shares repurchased by the Company pursuant to this Section 6
shall be $0.50 (subject to adjustment pursuant to Section 10 hereof and subject
to adjustment after the Merger taking into account the Exchange Ratio (as such
term is defined in the Merger Agreement), payable, at the election of the
Company, in cash or through the cancellation of indebtedness. The closing (the
"Closing") of the repurchase by the Company of all or any number of Unvested
 -------
Shares pursuant to this Section 6 shall take place at the offices of Bingham
Dana LLP, 150 Federal Street, Boston, MA 02110 at such time and on such date as
the Company shall specify in the Notice, provided that the date of the Closing
                                         --------
shall in no event be later than sixty (60) days after the date of the Notice. At
the Closing, the Purchaser shall deliver to the Company a certificate or
certificates evidencing the number of Unvested Shares to be repurchased, duly
endorsed for transfer or accompanied by duly executed stock powers, against
payment by the Company of the purchase price therefor in accordance with the
terms of this Section 6. In the event that the Company has a right to repurchase
any Unvested Shares pursuant to this Section 6 and elects not to, or fails to,
repurchase all or a portion of such Unvested Shares in accordance with the
provisions of this Section 6, then all of the Unvested Shares not so repurchased
shall automatically become fully vested and, thereafter, shall be treated as
Vested Shares for all purposes of this Agreement.

     7.   Escrow of Unvested Shares.
          -------------------------

          (a)  Escrow Holder. Each stock certificate representing Unvested
               -------------
Shares shall be held in escrow by the Escrow Holder, together with a stock
assignment, in the form attached hereto as Exhibit C, executed in blank by the
                                           ---------
Purchaser with respect to the Unvested Shares represented by such stock
certificate. Subject to the provisions of Section 5(e) above, each stock
certificate representing Unvested Shares shall be held in escrow pursuant to
this Section 7 until all of such Unvested Shares become fully vested pursuant
to, and in accordance with, the provisions of Section 5 or Section 6 hereof or
until all of such Unvested Shares are repurchased by the Company pursuant to,
and in accordance with, the provisions of Section 6 hereof, whichever occurs
earlier.

          (b)  Rights of Purchaser with respect to Unvested Shares held in
               -----------------------------------------------------------
Escrow. Subject to the terms hereof, the Purchaser shall have all the rights of
------
a stockholder with respect to the Unvested Shares while they are held in escrow,
including without limitation, the right to vote such Unvested Shares and receive
any cash dividends declared thereon. If there is (i) any stock dividend, stock
split or other change in the Unvested Shares, or (ii) any merger or sale of all
or substantially all of the assets or other acquisition of the Company, then,
subject to the acceleration provisions set forth in Section 5(b) hereof, any and
all new, substituted or additional securities to which the Purchaser is entitled
by reason of his ownership of the Unvested Shares shall be immediately subject
to this escrow, deposited with

<PAGE>
                                      -7-

the Escrow Holder and included thereafter as "Unvested Shares" for purposes of
this Agreement.

          (c)  Obligations and Liabilities of the Escrow Holder. The Escrow
               ------------------------------------------------
Holder shall be obligated only for the performance of such duties as are
specifically set forth herein and may rely and shall be protected in relying or
refraining from acting on any instrument reasonably believed by him to be
genuine and to have been signed or presented by the proper party or parties. The
Escrow Holder shall not be personally liable for any act he may do or refrain
from doing hereunder as Escrow Holder or as attorney-in-fact for the Purchaser,
provided that the Escrow Holder acts or refrains from acting in good faith and
--------
in the exercise of his own good judgment, and any act that he does or refrains
from doing pursuant to the advice of his own attorneys, who may be counsel to
the Company, shall be conclusive evidence of such good faith.

          (d)  Duties of the Escrow Holder.
               ---------------------------

               (i) In the event of any repurchase of Unvested Shares pursuant
to, and in accordance with, the provisions of Section 6 hereof, the Escrow
Holder shall take all steps necessary to consummate such repurchase, including,
but not limited to, presentment of stock certificates representing the Unvested
Shares subject to such repurchase, together with stock powers executed by or in
the name of the Purchaser appropriately completed by the Escrow Holder, to the
Company or its transfer agent with irrevocable instructions to register the
transfer of such Unvested Shares into the name of the Company or its designee.
The Purchaser hereby appoints the Escrow Holder his irrevocable attorney-in-fact
to execute in his name, acknowledge and deliver all stock powers and other
instruments as may be necessary or desirable with respect to the repurchase of
any Unvested Shares pursuant to, and in accordance with, the provisions of
Section 6 hereof.

               (ii) Upon the vesting of any Unvested Shares, the Escrow Holder
shall, at the request of the Purchaser, either (i) promptly deliver to the
Purchaser the certificate or certificates representing such Unvested Shares that
have become vested or (ii) promptly cause a new certificate endorsed with the
appropriate legends to be issued for such Unvested Shares that have become
vested and shall deliver such certificate to the Purchaser.

               (iii) The Escrow Holder may, but need not, submit a memorandum to
the Purchaser and to the Company setting forth action the Escrow Holder intends
to take with respect to the escrow of the Unvested Shares and requesting the
parties to acknowledge the propriety of the intended action. If, in any such
case, either party fails or refuses to acknowledge the propriety of the intended
action, the Escrow Holder may seek the advice of counsel, who

<PAGE>
                                      -8-

may be counsel to the Company, and any action taken in accordance with the
written advice of such counsel shall be full protection to the Escrow Holder in
respect thereto against any person. It is agreed that in any event the Escrow
Holder shall not be liable for any action or failure to act taken in good faith,
and that his liability shall be limited to actions or inaction constituting
gross negligence or willful misconduct.

               (iv) It is understood and agreed that should any dispute arise
with respect to the delivery, ownership or right of possession of the Unvested
Shares or other securities held by the Escrow Holder hereunder, he is authorized
and directed to retain in his possession without liability to anyone all or any
part of said Unvested Shares or other securities until such dispute shall have
been settled either by mutual written agreement of the parties concerned or by a
final order, decree or judgment of a court of competent jurisdiction after the
time for appeal has expired and no appeal has been perfected, but he shall be
under no duty whatsoever to institute or defend any such proceedings.

               (v) The Escrow Holder is hereby expressly authorized to comply
with and obey orders, judgments or decrees of any court. In case the Escrow
Holder obeys or complies with any such order, judgment or decree, he shall not
be liable to any of the parties hereto or to any other person, firm or
corporation by reason of such compliance, notwithstanding any such order,
judgment or decree being subsequently reversed, modified, annulled, set aside,
vacated or found to have been entered without jurisdiction.

               (vi) By signing this Agreement, the Escrow Holder becomes a party
to this Agreement only for the purposes of this Section 7.

          (e)  Change of Duties. The Escrow Holder's duties hereunder may be
               ----------------
altered, amended, modified, or revoked only by a writing signed by all of the
parties hereto; provided, however, that the Company may at any time, at its
                -----------------
option, elect to terminate this escrow by notice to the Purchaser and the Escrow
Holder.

          (f)  Costs and Fees. All reasonable costs, fees and disbursements
               --------------
incurred by the Escrow Holder in connection with the performance of his duties
hereunder shall be borne by the Company.

          (g)  Resignation. The Escrow Holder reserves the right, upon notice to
               -----------
the Company and the Purchaser, to resign from his duties as Escrow Holder and to
appoint a substitute Escrow Holder.

<PAGE>
                                      -9-

     8.   Restrictions on Transfer.
          ------------------------

          (a)  No Transfers of Unvested Shares. Except for (i) the escrow
               -------------------------------
described in Section 7 above or (ii) the transfer of any Unvested Shares to the
Company or in connection with a Change in Control of the Company as contemplated
by this Agreement, none of the Unvested Shares or any beneficial interest
therein shall be transferred, encumbered or otherwise disposed of in any way
until such Unvested Shares have become vested pursuant to, and in accordance
with, the provisions of Section 5 or Section 6 hereof.

          (b)  Legend for Unvested Shares. The certificates evidencing any of
               --------------------------
the Unvested Shares shall be endorsed with a legend substantially as follows:

               "THIS CERTIFICATE AND THE SHARES REPRESENTED BY THIS CERTIFICATE
               ARE SUBJECT TO A FIRST AMENDED AND RESTATED EXECUTIVE STOCK
               PURCHASE AGREEMENT AND TO THE RESTRICTIONS UPON TRANSFER
               CONTAINED THEREIN. A COPY OF SUCH EXECUTIVE STOCK PURCHASE
               AGREEMENT WILL BE FURNISHED TO ANY INTERESTED PARTY UPON WRITTEN
               REQUEST FREE OF CHARGE."

     9.   Investment Intent. The Purchaser represents and warrants that any
          -----------------
Shares acquired by him or that may be acquired by him are being or will be, as
the case may be, acquired for his own account for the purpose of investment and
not with a view to the resale or distribution thereof.

     10.  Adjustment for Stock Splits, etc.
          --------------------------------

          (a) In the event that at any time after the date of this Agreement the
Company implements any stock dividend, reclassification, recapitalization or
other change in or with respect to the Common Stock resulting in shares of any
series of capital stock of the Company (other than Common Stock) or other
securities, property or assets of the Company being distributed in respect of,
or being exchanged for, shares of Common Stock subject to this Agreement, then
all references in this Agreement to such shares of Common Stock and the
repurchase price set forth in Section 6(b) shall be appropriately adjusted to
take into account any such shares of any series of capital stock of the Company
(other than Common Stock) or other securities, property or assets of the Company
distributed in respect of, or exchanged for, outstanding shares of Common Stock
and all of such shares of any series of capital stock of the Company (other than
Common Stock) or other securities, property or assets of the Company shall be
held subject to all of the terms and conditions of this Agreement to the same
extent as if they were shares of Common Stock subject to this Agreement.

          (b) In the event that at any time after the date of this Agreement the
Company implements any stock split, stock dividend or reverse stock split in or
with respect to the

<PAGE>
                                      -10-

Common Stock resulting in an increase or decrease in the outstanding shares of
Common Stock of the Company, then all references in this Agreement to any number
of shares of Common Stock shall be appropriately adjusted to reflect any such
stock split, stock dividend, or reverse stock split.

          (c) In the event that any stock split, stock dividend, reverse stock
split, reclassification, recapitalization or other change in or with respect to
the Common Stock which may be adopted by the Company after the date of this
Agreement would cause an adjustment in any number of shares of Common Stock or
any number of shares of any other series or class of capital stock of the
Company that would otherwise result in fractional shares of Common Stock or such
other series or class of capital stock of the Company, or in the event that any
calculation or determination of the number of Shares that are or have vested or
that are subject to vesting under this Agreement would otherwise result in
fractional shares of Common Stock or any other series or class of capital stock
of the Company, then such fractional shares shall be disregarded by rounding
down to the nearest whole number of shares.

          (d) In the event that at any time after the date of this Agreement the
Company implements any stock split, stock dividend, reverse stock split,
reclassification, recapitalization or other change in or with respect to the
Common Stock, then all references in this Agreement to the purchase price for
any shares of Common Stock shall be appropriately adjusted to take into account
any such stock split, stock dividend, reverse stock split, reclassification,
recapitalization or other change in or with respect to the Common Stock.
Notwithstanding the foregoing, the purchase price with respect to any shares of
Common Stock shall never be reduced to a value below the par value of such
shares of Common Stock.

          (e) The foregoing provisions of this Section 10 shall apply
successively to one or more stock splits, stock dividends, reverse stock splits,
reclassifications, recapitalizations or other changes in or with respect to the
Common Stock.

     11.  Tax Consequences.
          ----------------

          (a)  Withholding Taxes. It is understood by the parties hereto that
               -----------------
the issuance and/or sale of any of the Shares to the Purchaser may be deemed
compensatory in purpose and in effect and that, as a result, the Purchaser may
be obligated to advance to the Company amounts sufficient to pay withholding
taxes in respect of such Shares at the time Purchaser becomes subject to Federal
income taxation with respect to the receipt of such Shares. In the event that
such withholding tax obligations arise, the parties hereby agree that the
Company shall have no obligation to pay such withholding taxes, that payment of
such withholding taxes shall be the exclusive obligation of the Purchaser and
that the Company shall be entitled to withhold amounts from other sources of
compensation otherwise due to the Purchaser by the Company for purposes of
satisfying such withholding taxes if and to the extent that such withholding
taxes have not been otherwise paid or satisfied by the Purchaser.

<PAGE>
                                      -11-

The Purchaser agrees on his behalf, and on behalf of his successors and assigns,
to indemnify the Company with respect to any withholding tax payment that the
Company is required to make that arises from the issuance and/or sale of any of
the Shares to the Purchaser.

          (b)  Section 83(b) Election. Purchaser hereby acknowledges delivery to
               ----------------------
the Company of a signed copy of any instrument, letter or other document
executed and filed with the Internal Revenue Service evidencing his election
under Section 83(b)(2) of the Internal Revenue Code of 1986, as amended, to
treat his receipt of any of the Shares under the Original Purchase Agreement as
included in the Purchaser's gross income in the year of receipt.

     12.  General Provisions.
          ------------------

          (a)  Governing Law. This Agreement shall be governed by the internal
               -------------
substantive laws of The Commonwealth of Massachusetts, without reference to any
conflict of laws provisions thereof that would implicate the substantive or
procedural laws of any other jurisdiction.

          (b)  Entire Agreement. This Agreement represents the entire agreement
               ----------------
between the parties with respect to the purchase of Common Stock by the
Purchaser from the Company and supersedes all prior written and oral agreements
and understandings between the parties (including without limitation the
Original Purchase Agreement) to the extent that such prior written and oral
agreements relate or pertain to the subject matter of this Agreement. This
Agreement may only be modified or amended pursuant to a written agreement or
instrument signed by the Company and the Purchaser or, with respect to Section
7, the Company, the Purchaser and the Escrow Holder.

          (c)  Notices. Any notice, demand, request or other communication
               -------
hereunder to any party shall be deemed to be sufficient if contained in a
written instrument delivered in person or duly sent by first class registered,
certified or overnight mail, postage prepaid, or telecopied with a confirmation
copy by regular, certified or overnight mail, postage prepaid, or sent by
electronic mail with a confirmation copy by regular, certified or overnight
mail, postage prepaid, to such party at the address, telecopier number or email
address, as the case may be, set forth below or such other address, telecopier
number, or email address, as the case may be, as may hereafter be designated in
writing by the addressee to the addressor listing all parties:

<PAGE>
                                      -12-

     if to the Company, to:

          The Althexis Company, Inc.
          1365 Main Street
          Waltham, Massachusetts  02154
          Attention:  Paul Mellett, Chief Financial Officer
          Facsimile:  (781) 647-5552

          with a copy to:

          Bingham Dana LLP
          150 Federal Street
          Boston, MA  02110
          Attention:  Julio E. Vega, Esq.
          Facsimile:  (617) 951-8736

     if to the Purchaser, to:

          Mark Skaletsky
          363 Willis Road
          Sudbury,  MA  01776
          Facsimile: (978) 440-8196

          with a copy to:

          Elizabeth Grammer, Esq.
          26 High Ridge Rd.
          Topsfield, MA  01983
          Facsimile:  (978) 887-1271

     if to the Escrow Holder, to:

          The Althexis Company, Inc.
          1365 Main Street
          Waltham, Massachusetts  02154
          Attention:  Paul Mellett
          Facsimile:  (781) 647-5552

     All such notices, requests and other communications shall be deemed to have
been received: (i) in the case of personal delivery, on the date of such
delivery; (ii) in the case of mail, on the third day following deposit into the
mail; and (iii) in the case of facsimile transmission, when confirmed by
facsimile machine report.

<PAGE>
                                      -13-

          (d)  Binding Effect. This Agreement shall inure to the benefit of, be
               --------------
binding upon, the heirs, personal representatives, executors, administrators,
successors and/or permitted assigns of the parties. This Agreement shall also
inure to the benefit of, and be binding upon, any transferee of the Shares.

          (e)  Assignment. The rights, benefits and obligations of the Company
               ----------
under this Agreement shall be transferable to any one or more persons or
entities, and all covenants and agreements hereunder shall inure to the benefit
of, and be enforceable by, the Company's successors and assigns, provided that
                                                                 -------------
any assignee agrees in writing to be bound by the terms and obligations of the
Company hereunder. The Purchaser may not delegate any of its obligations under
this Agreement without the Company's prior written consent.

          (f)  No Waiver. Either party's failure to enforce any provision or
               ---------
provisions of this Agreement shall not in any way be construed as a waiver of
any such provision or provisions, nor prevent the party thereafter from
enforcing each and every other provision of this Agreement. The rights granted
both parties herein are cumulative and shall not constitute a waiver of either
party's right to assert all other legal remedies available to it under the
circumstances.

          (g)  Severability. If any provision of this Agreement shall be held
               ------------
illegal, invalid or unenforceable, such illegality, invalidity or
unenforceability shall attach only to such provision and shall not in any manner
affect or render illegal, invalid or unenforceable any other severable
provisions of this Agreement.

          (h)  Headings. Headings are for convenience only and are not deemed to
               --------
be part of this Agreement.

          (i)  Further Assurances. The Purchaser agrees upon request to execute
               ------------------
any further documents or instruments necessary or desirable to carry out the
purposes or intent of this Agreement.

          (j)  Counterparts. This Agreement may be executed in counterparts, all
               ------------
of which together shall for all purposes constitute one Agreement, binding on
each of the parties hereto notwithstanding that each such party shall not have
signed the same counterpart.

          (k)  Relationship with Purchaser. The Company is not by reason of this
               ---------------------------
Agreement or the issuance of any Shares obligated to continue the Purchaser's
association with the Company as an officer, director, employee, or in any other
capacity (other than as a stockholder).

          (l)  Consent to Jurisdiction. In case of any dispute hereunder, the
               -----------------------
parties will submit to the exclusive jurisdiction and venue of any court of
competent jurisdiction

<PAGE>
                                      -14-

sitting in Suffolk County, Massachusetts, and will comply with all requirements
necessary to give such court jurisdiction over the parties and the controversy.

          (m)  Effectiveness of Agreement. This Agreement shall only take effect
               --------------------------
at, and simultaneous with, the Effective Time (as such term is defined in the
Merger Agreement). In the event that the Merger is not consummated, this
Agreement shall be null and void and of no further effect, in which case the
terms of the Original Purchase Agreement shall continue in full force and effect
to govern the agreement of the parties with respect to the matters contemplated
thereby.

               [Remainder of this page intentionally left blank.]

<PAGE>

     IN WITNESS WHEREOF, the parties have duly executed this First Amended and
Restated Executive Stock Purchase Agreement under seal as of the day and year
first set forth above.

PURCHASER

    /s/ Mark Skaletsky
------------------------------------
Mark Skaletsky

THE ALTHEXIS COMPANY, INC.

By:  /s/ Paul Mellett
   ---------------------------------
     Paul Mellett
     Chief Financial Officer

ESCROW HOLDER

    /s/ Paul Mellett
------------------------------------
Paul Mellett, Assistant Secretary of
The Althexis Company, Inc.<PAGE>

                                                                     Exhibit 4.3

                           ESSEX PROPERTY TRUST, INC.

                          AMENDMENT TO RIGHTS AGREEMENT
                          -----------------------------

     Amendment No. 1 (the "Amendment"), dated as of February 28, 2002 among
Essex Property Trust, Inc. (the "Company"), BankBoston, N.A. ("BankBoston") and
Computershare Investor Services, LLC ("Computershare"), to the Rights Agreement
(the "Rights Agreement") dated as of November 11, 1998 between the Company and
BankBoston.

                               W I T N E S S E T H

     WHEREAS, the Company and BankBoston previously entered into the Rights
Agreement, pursuant to which the BankBoston was appointed to serve as the Rights
Agent under the Rights Agreement; and

     WHEREAS, the Company desires to appoint Computershare as successor Rights
Agent under the Rights Agreement, effective as of February 28, 2002, and as of
such date, BankBoston will be relieved of its duties as Rights Agent under the
Rights Agreement; and

     WHEREAS, in connection with the resignation of BankBoston as Rights Agent
and the appointment of Computershare as successor Rights Agent, the Company,
BankBoston and Computershare desire to amend the Rights Agreement in certain
respects.

     NOW, THEREFORE, in consideration of the premises and the mutual agreements
herein set forth, the parties hereby agree as follows:

     Section 1. Resignation of Rights Agent. BankBoston hereby resigns as Rights
Agent under the Rights Agreement, effective as of February 28, 2002, and the
Company hereby accepts such resignation.

     Section 2. Appointment of the Successor Rights Agent. The Company hereby
appoints Computershare as successor Rights Agent under the Rights Agreement,
effective as of February 28, 2002, and Computershare hereby accepts such
appointment.

     Section 3. Amendment of Rights Agreement. Effective as of the date of
appointment of Computershare as successor Rights Agent, the Rights Agreement
shall be amended as follows:

     (a) Section 21 of the Rights Agreement is hereby amended by deleting the
sentence that begins on page 44 with "Any successor Rights Agent. . . ." and
ends on page 45 with ". . . an affiliate of a legal business entity described
above in clause (a) of this sentence." and submitting in lieu thereof the
following sentence:

          "Any successor Rights Agent, whether appointed by the Company or by
     such a court, shall be (a) a corporation, limited liability company or
     trust company (or similar form of entity under the laws of any state of the
     United States or a foreign

<PAGE>

     jurisdiction) authorized to conduct business under the laws of the United
     States or any state of the United States, which is authorized under such
     laws to exercise corporate trust, fiduciary or stockholder services powers
     and is subject to supervision or examination by a federal or state
     authority and which has at the time of its appointment as Rights Agent a
     combined capital and surplus of at least $10,000,000 or (b) an Affiliate
     controlled by an entity described in clause (a) of this sentence."

     (b) Section 26 of the Rights Agreement is hereby amended by deleting the
address for notice or demand to be given to the Rights Agent therein and
substituting in lieu thereof the following:

         "Computershare Investor Services, LLC
         Two North LaSalle Street
         Chicago, Illinois 60602
         Attention:  Keith Bradley

         with a copy to:

         Computershare Investor Services, LLC
         Two North LaSalle Street
         Chicago, Illinois 60602
         Attention:  Steven Rothbloom"

     (c) All references in the Rights Agreement to "BankBoston, N.A." as Rights
Agent shall for all purposes be deemed to refer to "Computershare Investor
Services, LLC."

     Section 4. Continued Effectiveness. Except as amended hereby, the Rights
Agreement shall remain in full force and effect.

     Section 5. Execution in Counterparts. This Amendment may be executed in any
number of counterparts and each of such counterparts shall for all purposes be
deemed to be an original, and all such counterparts shall together constitute
one and the same instrument.

     Section 6. Except as otherwise expressly provided herein, or unless the
context otherwise requires, all terms used herein have the meanings assigned to
them in the Rights Agreement.

                            [Signature Page Follows]

<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed as of the day and year above written.

                                Essex Property Trust, Inc.

                                By
                                   -----------------------------------
                                Its
                                    ----------------------------------

                                BankBoston, N.A.

                                By
                                   -----------------------------------
                                Its
                                    ----------------------------------

                                COMPUTERSHARE INVESTOR
                                SERVICES, LLC

                                By
                                   -----------------------------------
                                Its
                                    ----------------------------------

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