<Page>
                                                                 Exhibit 10.8


THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933, AS AMENDED. THESE SECURITIES HAVE BEEN ACQUIRED FOR INVESTMENT AND
NOT WITH A VIEW TO DISTRIBUTION OR RESALE, AND MAY NOT BE SOLD, MORTGAGED,
PLEDGED, HYPOTHECATED OR OTHERWISE TRANSFERRED WITHOUT AN EFFECTIVE REGISTRATION
STATEMENT FOR SUCH SECURITIES UNDER THE SECURITIES ACT OF 1933, AS AMENDED, OR
AN OPINION OF COUNSEL REASONABLY SATISFACTORY TO THE CORPORATION THAT
REGISTRATION IS NOT REQUIRED UNDER SUCH ACT.

Warrant No. 3                                       Right to Purchase **76**
                                      COPY          Shares of Common Stock
                                                    of CombinatoRx, Incorporated

                            COMBINATORX, INCORPORATED

                          Common Stock Purchase Warrant

     COMBINATORX, INCORPORATED, a Massachusetts corporation (the "Corporation"),
hereby certifies that, for value received, Kristina Bieker-Brady of 142 Hinckley
Road, Milton, MA 02186, or her registered assigns (the "Holder"), is entitled,
subject to the terms set forth below, to purchase from the Corporation at any
time or from time to time before 5:00 P.M., Boston time, on or before May 1,
2005, the number of fully paid and nonassessable shares of Common Stock, each
with a par value of $0.001, of the Corporation set forth above at a purchase
price per share of $0.001 (such purchase price per share, as adjusted from time
to time as herein provided is referred to herein as the "Purchase Price"). The
number and character of such shares of Common Stock and the Purchase Price are
subject to adjustment as provided herein.

     This Warrant evidencing the right to purchase shares of Common Stock of the
Corporation is issued in connection with the terms of a letter agreement dated
April 25, 2000 between the Corporation and Clark & Elbing, LLP and a letter of
direction from Clark & Elbing, LLP dated December 8, 2000.

     As used herein the following terms, unless the context otherwise requires,
have the following respective meanings:

     (a) The term "Corporation" shall include CombinatoRx, Incorporated and any
corporation which shall succeed or assume the obligations of the Corporation
hereunder.

                                        1
<Page>

     (b) The term "Common Stock" means the Corporation's $0.001 par value Common
Stock and any other securities into which or for which any of such Common Stock
may be converted or exchanged pursuant to a plan of recapitalization,
reorganization, merger, sale of assets, or otherwise.

     1.      EXERCISE OF WARRANT.

             1. 1. FULL EXERCISE. This Warrant may be exercised in full by the
Holder hereof by surrender of this Warrant, with the form of subscription at the
end hereof duly executed by such Holder, to the Corporation at its principal
office, accompanied by payment, in cash or by certified or official bank check
payable to the order of the Corporation, in the amount obtained by multiplying
the number of shares of Common Stock for which this Warrant is then exercisable
by the Purchase Price then in effect.

             1.2. PARTIAL EXERCISE. This Warrant may be exercised in part by
surrender of this Warrant in the manner and at the place provided in Subsection
1. 1, except that the amount payable by the Holder on such partial exercise
shall be the amount obtained by multiplying (a) the number of shares of Common
Stock designated by the Holder in the subscription at the end hereof by (b) the
Purchase Price then in effect. On any such partial exercise the Corporation at
its expense will forthwith issue and deliver to or upon the order of the Holder
hereof a new Warrant or Warrants of like tenor, in the name of the Holder hereof
or as such Holder (upon payment by such Holder of any applicable transfer taxes)
may request, calling in the aggregate on the face or faces thereof for the
number of shares of Common Stock for which such Warrant or Warrants may still be
exercised.

             1.3. TRUSTEE FOR WARRANT HOLDERS. In the event that a bank or
trust Corporation shall have been appointed as trustee for the Holder of the
Warrant pursuant to Subsection 3.2, such bank or trust Corporation shall have
all the powers and duties of a warrant agent appointed pursuant to Section 10
and shall accept, in its own name for the account of the Corporation or such
successor person as may be entitled thereto, all amounts otherwise payable to
the Corporation or such successor, as the case may be, on exercise of this
Warrant pursuant to this Section 1.

     2.      DELIVERY OF STOCK CERTIFICATES, ETC., ON EXERCISE. As soon as
practicable after the exercise of this Warrant, in full or in part, and in any
event within 10 days thereafter, the Corporation at its expense (including the
payment by it of any applicable issue taxes) will cause to be issued in the name
of and delivered to the Holder hereof, or as such Holder (upon payment by such
Holder of any applicable transfer taxes) may direct, a certificate or
certificates for the number of fully paid and nonassessable shares of Common
Stock to which such Holder shall be entitled on such exercise, plus, in lieu of
any fractional share to which such Holder would otherwise be entitled, cash
equal to such fraction multiplied by the then current market value of one full
share, together with any other stock or other securities and property (including
cash, where applicable) to which such Holder is entitled upon such exercise
pursuant to Section 1 or otherwise.

     3.      ADJUSTMENT FOR REORGANIZATION, CONSOLIDATION, MERGER, ETC.

                                        2
<Page>

             3.1. ADJUSTMENT. In case at any time or from time to time, the
Corporation shall: (a) effect a reorganization, (b) consolidate with or merge
into any other person, or (c) transfer all or substantially all of its
properties or assets to any other person under any plan or arrangement
contemplating the dissolution of the Corporation, then, in each such case, the
Holder of this Warrant, on the exercise hereof as provided in Section 1 at any
time after the consummation of such reorganization, consolidation or merger or
the effective date of such dissolution, as the case may be, shall receive, in
lieu of the Common Stock issuable on such exercise prior to such consummation or
such effective date, the stock and other securities and property (including
cash) to which such Holder would have been entitled upon such consummation or in
connection with such dissolution, as the case may be, if such Holder had so
exercised this Warrant, immediately prior thereto, all subject to further
adjustment thereafter as provided in Section 4.

             3.2. DISSOLUTION. In the event of any dissolution of the
Corporation following the transfer of all or substantially all of its properties
or assets, the Corporation, prior to such dissolution, shall at its expense
deliver or cause to be delivered the stock and other securities and property
(including cash, where applicable) receivable by the Holder of this Warrant
after the effective date of such dissolution pursuant to this Section 3 to a
bank or trust Corporation, having its principal office in Boston, Massachusetts,
as trustee for the Holder or Holders of this Warrant.

             3.3. CONTINUATION OF TERMS. Upon any reorganization, consolidation,
merger or transfer (and any dissolution following any transfer) referred to in
this Section 3, this Warrant shall continue in full force and effect and the
terms hereof shall be applicable to the shares of stock and other securities and
property receivable on the exercise of this Warrant after the consummation of
such reorganization, consolidation or merger or the effective date of
dissolution following any such transfer, as the case may be, and shall be
binding upon the issuer of any such stock or other securities, including, in the
case of any such transfer, the person acquiring all or substantially all of the
properties or assets of the Corporation, whether or not such person shall have
expressly assumed the terms of this Warrant as provided in Section 5.

     4.      ADJUSTMENT IN CASE OF EXTRAORDINARY EVENTS. In the event that the
Corporation shall (i) issue additional shares of the Common Stock as a dividend
or other distribution on outstanding Common Stock, (ii) subdivide its
outstanding shares of Common Stock or (iii) combine its outstanding shares of
the Common Stock into a smaller number of shares of the Common Stock, then, in
each such event, the Purchase Price shall, simultaneously with the happening of
such event, be adjusted by multiplying the then Purchase Price by a fraction,
the numerator of which shall be the number of shares of Common Stock outstanding
immediately prior to such event and the denominator of which shall be the number
of shares of Common Stock outstanding immediately after such event, and the
product so obtained shall thereafter be the Purchase Price then in effect. The
Purchase Price, as so adjusted, shall be readjusted in the same manner upon the
happening of any successive event or events described

                                        3
<Page>

herein in this Section 4. The Holder of this Warrant shall thereafter, on the
exercise hereof as provided in Section 1, be entitled to receive that number of
shares of Common Stock determined by multiplying the number of shares of Common
Stock which would otherwise (but for the provisions of this Section 4) be
issuable on such exercise by a fraction of which (i) the numerator is the
Purchase Price which would otherwise (but for the provisions of this Section 4}
be in effect, and (ii) the denominator is the Purchase Price in effect on the
date of such exercise.

     5.      NO DILUTION OR IMPAIRING . The Corporation will not, by amendment
of its Articles of Organization or through any reorganization, transfer of
assets, consolidation, merger, dissolution, issue or sale of securities or any
other voluntary action, avoid or seek to avoid the observance or performance of
any of the terms of this Warrant, but will at all times in good faith assist in
the carrying out of all such terms and in the taking of all such action as may
be necessary or appropriate in order to protect the rights of the Holders of
this Warrant against dilution or other impairment. Without limiting the
generality of the foregoing, the Corporation (a) will not increase the par value
of any shares of stock receivable on the exercise of this Warrant above the
amount payable therefor on such exercise, (b) will take all such action as may
be necessary or appropriate in order that the Corporation may validly and
legally issue fully paid and nonassessable shares of stock on the exercise of
this Warrant from time to time outstanding, and (c) will not transfer all or
substantially all of its properties and assets to any other person (corporate or
otherwise), or consolidate with or merge into any other person or permit any
such person to consolidate with or merge into the Corporation (if the
Corporation is not the surviving person), unless such other person shall
expressly assume in writing and will be bound by all the terms of the Warrant.

     6.      NOTICES OF RECORD DATE, ETC. In the event of;

     (a) any capital reorganization of the Corporation, any reclassification or
recapitalization of the capital stock of the Corporation or any transfer of all
or substantially all the assets of the Corporation to or consolidation or merger
of the Corporation with or into any other person, or

     (b) any voluntary or involuntary dissolution, liquidation or winding-up of
the Corporation,

then, and in each such event, the Corporation will mail or deliver, or cause to
be mailed or delivered, to each Holder of a Warrant a notice specifying the date
on which any such reorganization, reclassification, recapitalization, transfer,
consolidation, merger, dissolution, liquidation or winding-up is to take place,
and the time, if any is to be fixed, as of which the Holders of record of Common
Stock shall be entitled to exchange their shares of Common Stock for securities
or other property deliverable on such reorganization, reclassification,
recapitalization, transfer, consolidation, merger, dissolution, liquidation or
winding-up.

                                        4
<Page>

     7.      RESERVATION OF STOCK, ETC., ISSUABLE ON EXERCISE OF WARRANT. The
Corporation will at all times reserve and keep available, solely for issuance
and delivery on the exercise of this Warrant, all shares of Common Stock from
time to time issuable on the exercise of this Warrant.

     8.      EXCHANGE OF WARRANT. On surrender for exchange of this Warrant
properly endorsed, to the Corporation, the Corporation at its expense will issue
and deliver to or on the order of the Holder thereof a new Warrant or Warrants
of like tenor, in the name of such Holder or as such Holder (on payment by such
Holder of any applicable transfer taxes) may direct, calling in the aggregate on
the face or faces thereof for the number of shares of Common Stock called for on
the face or faces of the Warrant so surrendered.

     9.      REPLACEMENT OF WARRANT. On receipt of evidence reasonably
satisfactory to the Corporation of the loss, theft, destruction or mutilation of
this Warrant and, in the case of any such loss, theft or destruction of this
Warrant, on delivery of an indemnity agreement or security reasonably
satisfactory in form and amount to the Corporation or, in the case of any such
mutilation, on surrender and cancellation of such Warrant, the Corporation at
its expense will execute and deliver, in lieu thereof, a new Warrant of like
tenor.

     10.     WARRANT AGENT. The Corporation may, by written notice to each
Holder of a Warrant, appoint an agent having an office in Boston, Massachusetts
for the purpose of issuing Common Stock on the exercise of this Warrant pursuant
to Section 1, exchanging this Warrant pursuant to Section 8 and replacing this
Warrant pursuant to Section 9, or any of the foregoing, and thereafter any such
issuance, exchange or replacement, as the case may be, shall be made at such
office by such agent.

     11.     NEGOTIABILITY, ETC. This Warrant is issued upon the following
terms, to all of which each Holder hereof by the taking hereof consents and
agrees:

     (a) subject to compliance with applicable federal and state securities
laws, title to this Warrant may be transferred by endorsement (by the Holder
hereof executing the form of assignment at the end hereof) and delivery in the
same manner as in the case of a negotiable instrument transferable by
endorsement and delivery;

     (b) any person in possession of this Warrant, properly endorsed, is
authorized to represent himself as absolute owner hereof and is empowered to
transfer absolute title hereto by endorsement and delivery hereof to a bona fide
purchaser hereof for value; each prior taker or owner waives and renounces all
of his equities or rights in this Warrant in favor of each such bona fide
purchaser, and each such bona fide purchaser shall acquire absolute title hereto
and to all rights represented hereby; and

     (c) until this Warrant is transferred on the books of the Corporation, the
Corporation may treat the registered Holder hereof as the absolute owner hereof
for all purposes, notwithstanding any notice to the contrary.

                                        5
<Page>

     12.     NOTICES, ETC. All notices and other communications from the
Corporation to the Holder of this Warrant shall be mailed by first class
registered or certified mail, postage prepaid, or sent by a reputable overnight
deliver service, to such address as may have been furnished to the Corporation
in writing by such Holder or, until any such Holder furnishes to the Corporation
an address, then to, and at the address of, the last Holder of this Warrant who
has so furnished an address to the Corporation.

     13.     MISCELLANEOUS. This Warrant and any term hereof may be changed,
waived, discharged or terminated only by an instrument in writing signed by the
party against which enforcement of such change, waiver, discharge or termination
is sought. This Warrant shall be construed and enforced in accordance with and
governed by the laws of the Commonwealth of Massachusetts. The headings in this
Warrant are for purposes of reference only, and shall not limit or otherwise
affect any of the terms hereof. This Warrant is being executed as an instrument
under seal. The invalidity or unenforceability of any provision hereof shall in
no way affect the validity or enforceability of any other provision.

     14.     EXPIRATION. The right to exercise this Warrant shall expire at 5:00
P.M., Boston time, on May 1, 2005.

             EXECUTED this 8th day of December, 2000.

                                        COMBINATORX, INCORPORATED

                                        COPY

                                        By: /s/ Alexis Borisy
                                           ------------------------------
                                        Alexis Borisy, President
                                        Hereunto Duly Authorized

<Page>

(Signature must conform to name
of Holder as specified on the                  ----------------------------
face of the Warrant)                           Kristina Bieker-Brady

                                      COPY

                               FORM OF ASSIGNMENT
                   (To be signed only on transfer of Warrant)

     For value received, the undersigned hereby sells, assigns, and transfers
unto _____________________________ the right represented by the within Warrant
to purchase _________ shares of Common Stock of COMBINATORX, INCORPORATED to
which the within Warrant relates, and appoints ______________ Attorney to
transfer such right on the books of COMBINATORX, INCORPORATED with full power of
substitution in the premises.

     Dated:_______________


                                               ----------------------------
                                               Kristina Bieker-Brady

Signed in the presence of:


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

                                        7