Document:

<PAGE>   1
                                                                   EXHIBIT 10.1

CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY SQUARE
BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT TO RULE
406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                           MEMORANDUM OF UNDERSTANDING

THIS MEMORANDUM OF UNDERSTANDING ("MOU") is entered into as of the 26th day of
January, 2000 (the "EFFECTIVE DATE") by and between QUESTCOR PHARMACEUTICALS,
INC., a California corporation ("QUESTCOR"), and DAINIPPON PHARMACEUTICAL CO.,
LTD. ("DAINIPPON"), a corporation organized under the laws of Japan.

                                    RECITALS

A.      Dainippon and Questcor (formerly, RiboGene, Inc.) are parties to a
        research agreement dated January 27, 1998 (the "RESEARCH AGREEMENT") for
        the discovery of compounds which are active in certain Questcor Assays
        (as defined in Exhibit 6.2), and the parties wish to terminate the
        Research Agreement on mutually agreed upon terms.

B.      In connection with the termination of the Research Agreement, Dainippon
        desires to acquire certain rights in addition to the rights and licenses
        granted under the license agreement dated January 27, 1998 (the "LICENSE
        AGREEMENT"); and

C.      The parties wish to terminate the License Agreement and enter into a new
        license agreement as provided in this MoU.

                                    AGREEMENT

1.      LICENSE GRANT

Subject to the terms of this MoU, Questcor hereby grants Dainippon an exclusive
right and license, with the right to sublicense, under Licensed Technology, to
research, develop, make, have made, use and sell Products in the Territory.

Exhibit 1 hereto shall list all Licensed Patents existing as of the Effective
Date, updated from time to time to include new Licensed Patents. Dainippon
covenants and agrees that it will not use, directly or indirectly, the Licensed
Compounds or the Licensed Technology for any purpose other than researching,
developing, making, having made, using or selling Products in the Territory
under this MoU and the License Agreement.

For the avoidance of doubt, the rights and licenses granted to Dainippon under
this MoU shall not include any compounds screened or results generated in the
course of the screening of compounds provided to Questcor by ArQule,
Pharmacopeia or EnzyMed under separate agreements.

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

<PAGE>   2

2.      COMMERCIAL TERMS

2.1     LICENSE FEE.

In consideration of the license granted in Section 1, Dainippon shall pay to
Questcor two million U.S. dollars (US$ 2,000,000.00) (the "LICENSE FEE"), less
any applicable withholding tax not exceeding 10% of the License Fee. The License
Fee shall be due on the Effective Date and payable within thirty (30) days from
the Effective Date.

2.2     DEVELOPMENT MILESTONES.

Section 5.1 of the License Agreement is hereby deleted in its entirety and
replaced with the following:

(a)     Dainippon shall pay to Questcor the following amounts in the course of
        the development of each Questcor Product:

                                       [*]

(b)     Dainippon shall pay to Questcor [*] of the amounts set forth in
        subsection (a) above in the course of the development of each Derivative
        Questcor Product.

(c)     Dainippon shall pay to Questcor [*] of the amounts set forth in
        subsection (a) above in the course of the development of each
        Non-Antibacterial Questcor Product and each Dainippon Product.

(d)     Each milestone payment shall be made for each Product only once
        regardless of any substitution of a Licensed Back-up Compound for a
        Licensed Compound and any consequent repetition of a milestone event.
        Any withholding tax levied at source relating to the milestone payments
        payable to Questcor shall be borne by Dainippon. Questcor shall
        reasonably assist Dainippon with respect to the payment of the tax.

2.3     ROYALTIES.

Section 5.2 (a) of the License Agreement is hereby deleted in its entirety and
replaced with the following:

(a)     Dainippon shall pay to Questcor a royalty on Net Sales of each Questcor
        Product as follows:

          Royalty Rate in Percent (%)             Payable on portion of annual
          ---------------------------             ------------------------------
                                                  Net Sales (in U.S. $millions)
                                                  ------------------------------

                                       [*]

(b)     Dainippon shall pay to Questcor [*] of the royalties set forth in
        subsection (a) above in the course of the sale of each Derivative
        Questcor Product.

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       2.
<PAGE>   3

(c)     Dainippon shall pay to Questcor [*] of the royalties set forth in
        subsection (a) above in the course of the sale of each Non-Antibacterial
        Questcor Product and each Dainippon Product.

3.      TECHNOLOGY TRANSFER

3.1     Questcor shall use commercially reasonable best efforts to transfer to
        Dainippon within thirty (30) days from the Effective Date the biological
        and chemical materials and the information listed in Exhibit 3.1.

3.2     The parties shall share in equal parts the direct and indirect costs
        actually incurred by Questcor in connection with the transfer of the
        technology as provided in Section 3.1; provided, however, that Dainippon
        shall have no obligation to pay more than forty thousand U.S. dollars
        (US$ 40,000.00) of such costs. Questcor shall provide Dainippon with an
        invoice of Dainippon's shared portion of such costs and details of such
        costs, which invoiced amount shall be due and payable without deductions
        within thirty (30) days from the date of invoice.

4.      TERMINATION OF RESEARCH AGREEMENT

4.1     The Research Agreement is hereby terminated and of no further force and
        effect, except as provided in Section 6.2 of this MoU, and except that
        Section 3.1 (other than (c)(iv)), Article 5 (Patent Prosecution and
        Defense) (except as amended by Section 6.3 of this MoU), 7
        (Representation and Warranties), 8 (Indemnification), and 9
        (Confidentiality) of the Research Agreement shall survive.

4.2     Questcor shall have no further obligation to conduct research activities
        under the Research Agreement, and Dainippon shall have no obligation to
        pay the installment of [*] for the third year of the Research Term (as
        defined in the Research Agreement) and to pay any and all costs of the
        full-time visiting research scientists of Dainippon incurred under
        Section 2.4 of the Research Agreement.

5.      APPLICABILITY AND RENEGOTIATION OF THE LICENSE AGREEMENT

5.1     Except to the extent modified by the terms of this MoU, the terms of the
        License Agreement shall continue to apply, including, without
        limitation, Article 3 of the License Agreement, until a new license
        agreement is executed as referred to in Section 5.2 below.

5.2     Promptly after the Effective Date, the parties shall renegotiate certain
        terms of the License Agreement. The parties shall make reasonable
        efforts to execute on or prior to February 29, 2000 (or such later date
        as mutually agreed by the parties) a new license agreement incorporating
        the terms contained in this MoU and such terms as the parties have
        agreed to amend in the course of the renegotiation of the License
        Agreement.

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       3.
<PAGE>   4

6.      MISCELLANEOUS

6.1     BINDING MOU.

This MoU is a binding agreement by and between the parties and enforceable
against each of the parties in accordance with its terms.

6.2     DEFINITIONS.

When used in this MoU, each of the terms listed in Exhibit 6.2 shall have the
meaning as defined therein. The defined terms of this MoU are deemed to amend
the corresponding terms defined in the License Agreement and the Research
Agreement. Any capitalized terms not defined in Exhibit 6.2 or in this MoU shall
have the meaning as defined in the License Agreement. The defined terms of
Article 1 of the Research Agreement which are not defined in Exhibit 6.2 or in
this MoU shall survive to the extent they are used or referenced in this MoU or
the License Agreement.

6.3     PATENT COSTS

Dainippon shall bear the costs relating to the filing, prosecution and
maintenance of the Licensed Patents, provided that Dainippon may at its own
discretion decide to or not to file, prosecute, maintain and abandon the
Licensed Patents in any country or territory, and provided further, that prior
to abandoning the filing, prosecution or maintenance of part or all of a
Licensed Patent, Dainippon shall give Questcor sixty (60) days' notice before
any relevant deadline, and Questcor shall have the right to pursue, at its own
expense, the filing, prosecution and maintenance of such Licensed Patent.

6.4     AMENDMENT.

No amendment or modification hereof shall be valid or binding upon the parties
unless made in writing and signed by both parties.

6.5     CONSTRUCTION OF MOU AND CHOICE OF LAW; RESOLUTION OF DISPUTES.

(a)     This MoU and its terms and conditions shall be governed exclusively by
        and construed according to the laws of California, U.S.A., excluding its
        choice of law provisions and also excluding the United Nations
        Convention on Contracts for the International Sale of Goods. The
        official text of the MoU and any notices given or accounts or statements
        required hereby shall be in English.

(b)     All disputes which may arise between the parties hereto in relation to
        the interpretation or administration of this MoU shall be resolved by
        the agreement of the Chief Executive Officers or the Presidents of the
        respective parties.

(c)     Any disputes which cannot be resolved in this manner shall be finally
        settled by arbitration in accordance with the Conciliation and
        Arbitration Rules of the International

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       4.
<PAGE>   5
        Chamber of Commerce. The arbitration shall be held in San Francisco. The
        award rendered by the arbitration shall in any case be final and binding
        upon the parties hereto.

6.6     FORCE MAJEURE

Any delays in performance by any party under this MoU shall not be considered a
breach of this MoU if and to the extent caused by occurrences beyond the
reasonable control of the party affected, including but not limited to acts of
God, embargoes, governmental restrictions, strikes or other concerted acts of
workers, fire, flood, explosion, riots, wars, civil disorder, rebellion or
sabotage. The party suffering such occurrence shall immediately notify the other
party as soon as practicable and any time for performance hereunder shall be
extended by the actual time of delay caused by the occurrence.

6.7     SEVERABILITY.

If any term, condition or provision of this Agreement is held to be
unenforceable for any reason, it shall, if possible, be interpreted rather than
voided, in order to achieve the intent of the parties to this Agreement to the
extent possible. In any event, all other terms, conditions and provisions of
this Agreement shall be deemed valid and enforceable to the full extent.

6.8     PUBLICITY.

Within five (5) business days from the Effective Date, the parties shall agree
on a press release concerning this MoU. Except for that press release, or as
required by law, no party shall originate any publication, news release or other
public announcement, written or oral, whether in the public press, or
stockholders' reports, or otherwise, relating to the existence of or the
performance under this MoU, without the prior written approval of the other
party, which approval shall not be unreasonably withheld, but in no case shall
be withheld for longer than fifteen (15) days.

6.9     COUNTERPARTS.

This Agreement may be executed in two or more counterparts, each of which shall
be an original and all of which shall constitute together the same document.

6.10    ENTIRE AGREEMENT.

This MoU, the License Agreement, and any and all Exhibits referred to herein or
therein embody the entire understanding of the parties with respect to the
subject matter hereof and shall supersede all previous communications,
representations or understandings, either oral or written, between the parties
relating to the subject matter hereof.

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       5.
<PAGE>   6

IN WITNESS WHEREOF, both Dainippon and Questcor have executed this MoU, in
duplicate originals, by their respective officers hereunto duly authorized, as
of the day and year hereinabove written.

QUESTCOR PHARMACEUTICALS, INC.              DAINIPPON PHARMACEUTICAL CO., LTD.

By:                                         By:
Name:   Charles J. Casamento                Name:  Kenjiro Miyatake
Title:  Chairman, President and             Title: President
        Chief Executive Officer

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       6.
<PAGE>   7

                                    EXHIBIT 1

                                LICENSED PATENTS

[*]

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       7.
<PAGE>   8

                                   EXHIBIT 3.1

                           TECHNOLOGY TRANSFER PROGRAM

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       8.
<PAGE>   9

                                   EXHIBIT 6.2

                                   DEFINITIONS

"DAINIPPON PRODUCT" shall mean any Product containing a Dainippon Compound.

"DERIVATIVE QUESTCOR PRODUCT" shall mean any Product containing a Derivative
Questcor Program Compound.

"LICENSED BACK-UP COMPOUND" shall mean a Licensed Compound which is in the same
chemical class as a Licensed Compound, and which is developed to replace a
Licensed Compound whose development has been discontinued.

"LICENSED COMPOUND" shall mean:

        a)      any compounds from the Questcor library which have been shown to
                be active in Questcor Assays;

        b)      any compounds designed or synthesized by Questcor wholly or in
                part in the course of the Research Program;

                        (each compound falling within the categories a) through
                        b) being a "QUESTCOR PROGRAM COMPOUND")

        c)      any compounds claimed in a Licensed Patent;

                        (each compound falling within the categories a) through
                        c) above being a "QUESTCOR COMPOUND")

        d)      any analogs or derivatives of compounds falling within (a) or
                (b) (each a "DERIVATIVE QUESTCOR PROGRAM COMPOUND");

        e)      any compounds tested in Questcor Assays under this MoU and with
                activity in Questcor Assays, other than those falling within (a)
                through (d) above (each a "DAINIPPON COMPOUND"). For avoidance
                of doubt, the parties agree that any compounds which Dainippon
                has discovered using assays other than Questcor Assays and
                develops on the basis of activity other than Targets shall not
                be included in "Dainippon Compound".

"LICENSED PATENT" shall mean any patent claiming inventions or discoveries
within the Licensed Technology made prior to termination of the Research
Agreement, including, without limitation, any substitutions, extensions,
reissues, renewals, supplementary protection certificates and inventors'
certificates, which have not been held invalid or unenforceable by a
non-appealable or non-appealed decision of a court of competent jurisdiction,
and any patent applications claiming inventions or discoveries within the
Licensed Technology made prior to termination of the Research Agreement,
including, without limitation, any provisionals, divisionals, continuations,
continuations-in-part.

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                       9.
<PAGE>   10

"LICENSED TECHNOLOGY" shall mean RiboGene Technology and Collaboration
Technology (both as defined in the Research Agreement).

"PRODUCT" shall mean any pharmaceutical product, including all formulations,
line extensions or modes of administration thereof, which contains a Licensed
Compound as an active ingredient.

"QUESTCOR ASSAYS" shall mean any Target specific assays.

"NON-ANTIBACTERIAL QUESTCOR PRODUCT" shall mean any Product containing a
Questcor Program Compound developed or sold for uses other than therapeutic
treatment of bacterial infections in humans.

"QUESTCOR PRODUCT" shall mean any Product containing a Questcor Compound.

"TARGET(s)" shall mean ppGpp and Deformylase.

[ ] = CERTAIN CONFIDENTIAL INFORMATION CONTAINED IN THIS DOCUMENT, MARKED BY
SQUARE BRACKETS, IS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION PURSUANT
TO RULE 406 OF THE SECURITIES ACT OF 1933, AS AMENDED.

                                      10.<PAGE>   1
                                                                  EXHIBIT 10.29

                                OPTION AGREEMENT

        This OPTION AGREEMENT (together with the exhibits hereto the
"Agreement") is entered into the 1st day of March, 1997 (the "Effective Date")
by and between CORIXA CORPORATION, a Delaware corporation with its principal
place of business located at 1124 Columbia Street, Suite 200, Seattle,
Washington 98104 ("Corixa") and SmithKline Beecham Biologicals Manufacturing
S.A., a Belgian corporation with its principal place of business at Rue de
l'Institut 89, B-1330 Rixensart, Belgium ("SB").

                              W I T N E S S E T H:

        WHEREAS, Corixa and SB are parties to a Prostate Cancer Collaboration
and License Agreement (the "Prostate Cancer Agreement") and a Breast Cancer
Collaboration and License Agreement (the "Breast Cancer Agreement") both of even
date herewith (each, the "Collaboration Agreement") whereby the parties have
agreed to collaborate in the research, development and license of antigens for
vaccine products for the prevention and/or treatment of prostate and breast
cancers, respectively;

        WHEREAS, SB desires to acquire from Corixa an option to enter into
additional agreements for antigens for vaccine products for the prevention
and/or treatment of colon and/or ovarian cancers that take the form of the
Collaboration Agreements with appropriate revisions to reflect a modification of
the subject matter to cover colon and/or ovarian cancers.

        NOW, THEREFORE, for and in consideration of the mutual observance of the
covenants hereinafter set forth and other good and valuable consideration, the
receipt of which is hereby acknowledged, the parties hereto agree as follows:

        1. DEFINITIONS

           "Colon Collaboration Agreement" shall have the meaning assigned
thereto in Section 2(a).

           "Colon Field" shall mean any and all in vivo administered
prophylactic and/or therapeutic colon cancer Vaccines (as defined in the
Collaboration Agreements) for use in humans.

           "Colon Option" means the option granted by Corixa to SB pursuant to
Section 2 hereof.

           "Colon Option Period" means the period commencing on the Effective
Date and continuing until the last day of the eighteenth (18th) month following
the Effective Date, as such Colon Option Period may be extended pursuant to
Section 2(c) hereof.

<PAGE>   2

           "National Exchange" shall mean the Nasdaq National Market or any
other national exchange on which the Common Stock of Corixa is listed.

           "Ovarian Collaboration Agreement" shall have the meaning assigned
thereto in Section 3(a).

           "Ovarian Field" shall mean any and all in vivo administered
prophylactic and/or therapeutic ovarian cancer Vaccines as defined in the
Collaboration Agreement for use in humans.

           "Ovarian Option" means the option granted by Corixa to SB pursuant to
Section 3 hereof.

           "Ovarian Option Period" means the period commencing on the Effective
Date and continuing until the last day of the twelfth (12th) month following the
Effective Date, as such Ovarian Option Period may be extended pursuant to
Section 3(c) hereof.

        2. GRANT OF OPTION FOR COLON FIELD

           (a) Grant of Option

               Subject to the provisions of this Agreement, Corixa hereby grants
to SB an option, exercisable during the Colon Option Period, to enter into a
collaboration and license agreement to cover the Colon Field (the "Colon
Option"), on substantially the same terms and conditions set forth in the
Collaboration Agreement; provided, that the parties acknowledge and agree that
the amounts allocated to research funding and other modifications that reflect
the nature of the disease target may be required (the "Colon Collaboration
Agreement"). SB may exercise the Colon Option at any time on or before the
expiration of the Colon Option Period (i) by providing written notice of
exercise to Corixa prior to the expiration of the Colon Option Period, and (ii)
paying to Corixa a lump sum payment of Five Hundred Thousand United States
Dollars (U.S.$500,000) by wire transfer of immediately available funds, which
payment shall be subject to credit or conversion as set forth in Section 4
below; provided, however, that Corixa may begin discussing possible
collaborations with parties other than SB at any time after ninety (90) days
prior to the expiration of the Colon Option Period; such discussions shall be
terminated forthwith by Corixa if and when SB exercises the Colon Option or
extends the Colon Option Period pursuant to Section 2(c) hereof.

           (b) Initial Option Fee

               Upon execution and delivery of this Agreement, SB shall pay to
Corixa a lump sum payment of One Million Five Hundred Thousand United States
Dollars (US$1,500,000) by wire transfer of immediately available funds, which
payment shall be subject to credit, conversion or repayment as set forth in
Section 4 below.

                                      -2-
<PAGE>   3

           (c) Extension of the Option Period

               SB will have the right to extend the Colon Option Period for one
(1) additional twelve (12) month period by (i) providing written notice to
Corixa of SB's intention to extend the Colon Option Period prior to the
expiration of the initial Colon Option Period, and (ii) making the payment set
forth in Section 2(d) below.

           (d) Option Extension Fee

               If SB elects to extend the Colon Option Period pursuant to
Section 2(c) above, on or before the expiration of the initial Colon Option
Period, SB shall pay to Corixa a lump sum payment of Two Million United States
Dollars (US$2,000,000) by wire transfer of immediately available funds, which
payment shall be subject to credit, conversion or repayment as set forth in
Section 4 below.

        3. GRANT OF OPTION FOR OVARIAN FIELD

           (a) Grant of Option

               Subject to the provisions of this Agreement, Corixa hereby grants
to SB an option, exercisable during the Ovarian Option Period, to enter into a
collaboration and license agreement to cover the Ovarian Field (the "Ovarian
Option"), on substantially the same terms and conditions set forth in the
Collaboration Agreement, provided, that the parties acknowledge and agree that
the amounts allocated to research funding and other modifications that reflect
the nature of the disease target may be required (the "Ovarian Collaboration
Agreement"). SB may exercise the Ovarian Option at any time on or before the
expiration of the Ovarian Option Period (i) by providing written notice of
exercise to Corixa prior to the expiration of the Ovarian Option Period and (b)
paying to Corixa a lump sum payment of Five Hundred Thousand United States
Dollars (U.S.$500,000) by wire transfer of immediately available funds, which
payment shall be subject to credit or conversion as set forth in Section 4
below; provided, however, that Corixa may begin discussing possible
collaborations with parties other than SB at any time after ninety (90) days
prior to the expiration of the Ovarian Option Period. Said discussions shall be
terminated forthwith by Corixa if and when SB exercises the Ovarian Option or
extends the Ovarian Option Period pursuant to Section 3(c) hereof.

           (b) Initial Option Fee

               Upon execution and delivery of this Agreement, SB shall pay to
Corixa a lump sum payment of One Million Five Hundred Thousand United States
Dollars (US$1,500,000) by wire transfer of immediately available funds, which
payment shall be subject to credit, conversion or repayment as set forth in
Section 4 below.

           (c) Extension of the Option Period

                                      -3-
<PAGE>   4

               SB will have the right to extend the Ovarian Option Period for
one (1) additional eighteen (18) month period by (i) providing written notice to
Corixa of SB's intention to extend the initial Ovarian Option Period prior to
the expiration of the initial Ovarian Option Period, and (ii) making the payment
set forth in Section 3(d) below.

           (d) Option Extension Fee

               If SB elects to extend the Ovarian Option Period pursuant to
Section 3(c) above, on or before the expiration of the initial Ovarian Option
Period, SB shall pay to Corixa a lump sum payment of Two Million United States
Dollars (US$2,000,000) by wire transfer of immediately available funds, which
payment shall be subject to credit, conversion or repayment as set forth in
Section 4 below.

        4. OPTION PAYMENTS. All payments made by SB to Corixa pursuant to
Sections 2 and 3 above shall be subject to the following terms and conditions:

           (a) Colon Option Payments. If SB exercises the Colon Option, then all
payments made by SB to Corixa pursuant to Section 2 shall be, at the election of
SB, either (i) credited against future milestones in the Colon Collaboration
Agreement in accordance with Section 4(c) below or (ii) if Corixa's Common Stock
is listed on a National Exchange and Corixa has a market capitalization of not
less than US$128 million (assuming exercise or conversion of all outstanding
options, warrants and other securities exercisable for or convertible into
Corixa's Common Stock), converted into Corixa's Common Stock in accordance with
Section 4(d) below. If SB does not exercise the Colon Option, then all payments
made by SB to Corixa pursuant to Section 2 shall be repaid by Corixa in
accordance with Section 4(e) below.

           (b) Ovarian Option Payments. If SB exercises the Ovarian Option, then
all payments made by SB to Corixa pursuant to Section 3 shall be, at the
election of SB, either (i) credited against future milestones in the Ovarian
Collaboration Agreement in accordance with Section 4(c) below or (ii) if
Corixa's Common Stock is listed on a National Exchange and Corixa has a market
capitalization of not less than US$128 million (assuming exercise or conversion
of all outstanding options, warrants and other securities exercisable for or
convertible into Corixa's Common Stock), converted into shares of Corixa's
Common Stock in accordance with Section 4(d) below. If SB does not exercise the
Ovarian Option, then all payments made by SB to Corixa pursuant to Section 3
shall be repaid by Corixa in accordance with Section 4(e) below.

           (c) Credit Against Milestones. From and after receipt of written
notice from SB to Corixa (the "Credit Notice") of SB's intent to credit payments
made by it pursuant to Section 2 and/or Section 3 of this Agreement, as
applicable (the "Credit Amount"), which Credit Notice shall be made within
fifteen (15) days of the exercise of the Colon Option and/or the Ovarian Option,
as applicable, Corixa shall repay the Credit Amount by applying such Credit
Amount against future milestones that become payable by SB to Corixa in
accordance with the terms of the Colon Collaboration Agreement

                                      -4-
<PAGE>   5

and/or the Ovarian Collaboration Agreement, as applicable, during the period
beginning the date of the Credit Notice(s) and ending on the fifth anniversary
date of the Credit Notice(s). On the fifth anniversary date of the Credit
Notice, Corixa shall pay to SB the balance, if any, of the Credit Amount not
credited against such milestones pursuant to this Section 4(c). However, the
entire Credit Amount or any portion thereof may be prepaid by Corixa at any time
in its sole discretion.

           (d) Conversion to Common Stock. Within thirty (30) days (the
"Conversion Date") following the Corixa's receipt of a written notice from SB
(the "Conversion Notice Date") of SB's intent to convert payments made by it
pursuant to Section 2 and/or Section 3 of this Agreement as applicable into
shares of Corixa's Common Stock, (the "Conversion Amount"), Corixa shall issue
to SB the number of shares of Corixa's Common Stock equal to the Conversion
Amount divided by the "Common Stock Price." For purposes of this Section 4(d),
the "Common Stock Price" shall equal 111.25% of the average closing price for
Corixa's Common Stock during the period beginning six (6) trading days prior to
the Conversion Notice Date and ending on the trading day prior to the date of
the Conversion Notice Date, as reported on the National Exchange.

           (e) Repayment. Within thirty (30) days following expiration of the
Colon Option Period, or the Ovarian Option Period, as the case may be (the "Note
Issuance Date"), to the extent SB has not exercised the Colon Option, or the
Ovarian Option, as applicable, Corixa shall issue to SB an interest-free
promissory note in principal amount equal to the payments made by SB pursuant to
Section 2 in the case of the Colon Option and Section 3 in the case of the
Ovarian Option, which will not bear interest, payable in three (3) equal annual
payments on the third, fourth and fifth anniversaries of the date of this
Agreement

        5. MISCELLANEOUS

           (a) Development Efforts. Upon exercise of the Colon Option and/or
Ovarian Option, the parties will collaborate in the discovery and development of
Antigens for use in Vaccine(s) for the prevention and/or treatment of colon
cancer and/or ovarian cancer, as applicable, by using commercially reasonable
efforts to conduct the activities set forth in a research program plan to be
prepared in good faith jointly by the parties as soon as practicable after the
exercise of the Colon Option and/or Ovarian Option.

           (b) Entire Agreement. This Agreement, entered into as of the date
written above, constitutes the entire agreement between the parties relating to
the subject matter hereof and supersedes all previous writings and
understandings, except that the Non Disclosure Agreements dated 17 February 1995
and 10 August 1995, the Tuberculosis Collaboration and License Agreement dated
October 6, 1995 between Corixa Corporation and SmithKline Beecham Biologicals
S.A., the Prostate Cancer

                                      -5-
<PAGE>   6

Collaboration and License Agreement of even date herewith and the Breast Cancer
Collaboration and License Agreement of even date herewith remain in full force
and effect. No terms or provisions of this Agreement shall be varied or modified
by any prior or subsequent statement, conduct or act of either of the parties,
except that the parties may mutually amend this Agreement by written instruments
specifically referring to and executed in the same manner as this Agreement.

           (c) Notices. Any notice required or permitted under this Agreement
shall be deemed given if delivered (i) personally, (ii) by facsimile
transmission (receipt verified), (iii) by registered or certified mail (return
receipt requested), postage prepaid, or (iv) sent by express courier service
(receipt verified), to the following addresses of the parties:

           If to Corixa:     Corixa Corporation
                             1124 Columbia Street, Suite 200
                             Seattle, WA  98104
                             Attention: Chief Operating Officer
                             Telephone: (206) 667-5711
                             Telecopy:  (206) 667-5715

           with a copy to:   Venture Law Group
                             4750 Carillon Point
                             Kirkland, Washington 98033
                             Attention: William W. Ericson
                             Telephone: (206) 739-8700
                             Telecopy:  (206) 739-8750

           If to SB:         SmithKline Beecham Biologicals
                             Manufacturing S.A.
                             Rue de l'Institut 89
                             1330 Rixensart, Belgium
                             Attention: Senior Vice President,
                             General Manager
                             Telephone: 32-2-6568250
                             Telecopy:  32-2-6568025

Any notice required or permitted to be given concerning this Agreement shall be
effective upon receipt by the party to whom it is addressed.

           (d) Assignment. Subject to the other provisions of this Section 5(d),
this Agreement and the Options herein granted shall be binding upon and inure to
the benefit of the successors in interest of the respective parties. Neither
this Agreement nor any interest hereunder shall be assignable by either party
without the written consent of the other provided, however, that either party
may assign this Agreement to any corporation or other entity with which it may
merge or consolidate, and/or to any corporation or other entity to which it may
transfer all or substantially all of its assets,

                                      -6-
<PAGE>   7

without obtaining the consent of the other party; provided that the consent of
the non-transferring party (including, without limitation, by transfer of
assets, merger or consolidation or otherwise) shall be required for any transfer
of all or substantially all assets that materially alters the rights of such
non-transferring party under this Section 5(d).

           (e) Governing Law; Arbitration. This Agreement will be governed by
the laws of the State of Washington, USA. Any dispute, controversy or claim
arising out of or in relation to this Agreement or the breach, termination or
invalidity thereof, that cannot be settled amicably by agreement of the parties
hereto, shall be finally settled by arbitration in accordance with the
arbitration rules of the American Arbitration Association ("AAA"), then in
force, by one or more arbitrators appointed in accordance with said rules;
provided that the appointed arbitrators shall have appropriate experience in the
bio-pharmaceutical industry; provided further, however, that arbitration
proceedings may not be instituted until the party alleging breach of this
Agreement by the other party has given the other party not less than sixty (60)
days notice to remedy any alleged breach and the other party has failed to do
so. The place of arbitration shall be Seattle, Washington, USA if arbitration is
initiated by SB and New York, New York if initiated by Corixa. The award
rendered shall be final and binding upon both parties. The judgment rendered by
the arbitrator shall include costs of arbitration, reasonable attorneys' fees
and reasonable costs for any expert and other witnesses. The arbitration in such
proceeding may expressly consider the amounts paid pursuant to Sections 2 and 3
hereof in considering any claim of damages. Nothing in this Agreement shall be
deemed as preventing either party from seeking injunctive relief (or any other
provisional remedy) from any court having jurisdiction over the parties and the
subject matter of the dispute as necessary to protect either party's name,
proprietary information, trade secrets, know-how or any other proprietary
rights. Judgment upon the award may be entered in any court having jurisdiction,
or application may be made to such court for judicial acceptance of the award
and/or an order of enforcement as the case may be.

           (f) Severability; Waiver If any provision of this Agreement is
finally held to be invalid, illegal or unenforceable by a court of competent
jurisdiction, the validity, legality and enforceability of the remaining
provisions shall not be affected or impaired in any way. Any delay in enforcing
a party's rights under this Agreement or any waiver as to a particular default
or other matter shall not constitute a waiver of a party's right to the future
enforcement of its rights under this Agreement.

           (g) [Intentionally omitted.]

                                      -7-
<PAGE>   8

        IN WITNESS WHEREOF, each of the parties hereto has caused this Agreement
to be executed by its duly authorized officer as of the date first written
above.

Agreed to and accepted by:                     Agreed to and accepted by:
CORIXA CORPORATION                             SMITHKLINE BEECHAM BIOLOGICALS
MANUFACTURING S.A.

/s/  STEVEN GILLIS                             /s/  JEAN STEPHENNE
------------------------------                 ------------------------------
Steven Gillis                                  Jean Stephenne
President and CEO                              Senior Vice President and
                                               General Manager

                                      -8-

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