Document:

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                                                                    EXHIBIT 10.4

                                                                    CONFIDENTIAL

                                  MYOGEN, INC.

                   SERIES B PREFERRED STOCK PURCHASE AGREEMENT

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                                                                    CONFIDENTIAL

                                  MYOGEN, INC.

                   SERIES B PREFERRED STOCK PURCHASE AGREEMENT

         THIS SERIES B PREFERRED STOCK PURCHASE AGREEMENT (the "Agreement") is
entered into as of December ___, 1999, by and among MYOGEN, INC., a Delaware
corporation (the "Company") and each of those persons and entities, severally
and not jointly, whose names are set forth on the Schedule of Purchasers
attached hereto as Exhibit A (which person and entities are hereinafter
collectively referred to as "Purchasers" and each individually as a
"Purchaser"), including future Purchasers as described in Section 7.16 below.

                                    RECITALS

         WHEREAS, the Company has authorized the sale and issuance of an
aggregate of eight hundred three thousand six hundred five (803,605) shares of
its Series B Preferred Stock (the "Shares") to Purchasers as set forth in the
agreement(s) by and between the Company and the Purchasers set forth opposite
each Purchaser's name on Exhibit A hereto (each, a "Patent Agreement") and
collectively, the "Patent Agreements"). All terms not otherwise defined herein
shall have the meanings assigned to them in the Patent Agreements;

         WHEREAS, Purchasers desire to grant rights to the Company pursuant to
the Patent Agreements, partly in consideration for the Shares on the terms and
conditions set forth herein; and

         WHEREAS, the Company desires to issue the Shares to Purchasers on the
terms and conditions set forth herein, as further inducement to the Purchasers
to enter into the Patent Agreements;

         NOW, THEREFORE, in consideration of the foregoing recitals and the
mutual promises hereinafter set forth, the parties hereto agree as follows:

         1. AGREEMENT TO SELL AND PURCHASE.

            1.1 AUTHORIZATION OF SHARES. On or prior to the Closing (as defined
in Section 2 below), the Company shall have authorized (a) the issuance to
Purchasers of the Shares and (b) the issuance of such shares of Common Stock to
be issued upon conversion of the Shares (the "Conversion Shares"). The Shares
shall have the rights, preferences, privileges and restrictions set forth in the
Restated Certificate of Incorporation, in the form attached hereto as Exhibit B
(the "Restated Charter").

            1.2 SALE AND PURCHASE. Subject to the terms and conditions hereof,
including those set forth in Section 7.16, at the Closing (as hereinafter
defined) the Company hereby agrees to issue to each Purchaser, and each
Purchaser agrees to enter into the respective Patent Agreement in exchange for
the Shares.

                                       1.
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                                                                    CONFIDENTIAL

         2. CLOSING, DELIVERY AND PAYMENT.

            2.1 CLOSING. The closing of the sale and purchase of the Shares
under this Agreement (the "Closing") shall take place at 5:00 p.m. on the date
hereof, at the offices of Cooley Godward LLP, 2595 Canyon Boulevard, Suite 250,
Boulder, CO 80302 or at such other time or place as the Company and Purchasers
may mutually agree (such date is hereinafter referred to as the "Closing Date"),
including as set forth in Section 7.16.

            2.2 DELIVERY. At the Closing, subject to the terms and conditions
hereof, the Company will deliver to each Purchaser a certificate representing
the number of Shares to be issued at the Closing to such Purchaser, contingent
upon the execution by the Company and the Purchaser of the respective Patent
Agreement.

         3. REPRESENTATIONS AND WARRANTIES OF THE COMPANY.

            Except as set forth on a Schedule of Exceptions delivered by the
Company to the Purchasers at the Closing, the Company hereby represents and
warrants to Purchasers as of the date of this Agreement as follows:

            3.1 ORGANIZATION, GOOD STANDING AND QUALIFICATION. The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Delaware. The Company has all requisite corporate power and
authority to own and operate its properties and assets, to execute and deliver
this Agreement, to issue and sell the Shares and the Conversion Shares, and to
carry out the provisions of this Agreement and the Restated Charter and to carry
on its business as presently conducted and as presently proposed to be
conducted. The Company is duly qualified and is authorized to do business and is
in good standing as a foreign corporation in all jurisdictions in which the
nature of its activities and of its properties (both owned and leased) makes
such qualification necessary, except for those jurisdictions in which failure to
do so would not have a material adverse effect on the Company or its business.

            3.2 CAPITALIZATION; VOTING RIGHTS. The authorized capital stock of
the Company, immediately prior to the Closing, will consist of Twenty-Seven
Million (27,000,000) shares of Common Stock, Four Million Three Hundred Seventy
Two Thousand Two Hundred Forty-Five (4,372,245) shares of which are issued and
outstanding and Two Million Two Hundred Eighty Thousand (2,280,000) shares of
which are reserved for future issuance to employees, of which One Million One
Hundred Thousand Five Hundred Thirty-Five (1,100,535) shares are subject to
options or purchase rights which have been issued to the Company's employees,
consultants and other service providers and One Million One Hundred Seventy-Nine
Thousand Four Hundred Sixty-Five Thousand (1,179,465) are reserved for options
which have not been granted as of the date hereof and Twenty Million
(20,000,000) shares of Preferred Stock, Six Million Seventy-Three Thousand
(6,073,000) of which are designated Series A Preferred Stock, Six Million
Thirty-Five Thousand (6,035,000) shares of which are issued and outstanding,
Eight Hundred Ten Thousand (810,000) of which are designated Series B Preferred
Stock, none of which are issued and outstanding, Thirteen Million One Hundred
Thousand (13,100,000) of which are designated Series C Preferred Stock, Six
Million Five Hundred Forty Five Thousand Four Hundred Fifty Five (6,545,455)
shares of which are issued and outstanding. There are no outstanding options,
warrants, rights (including conversion or preemptive rights and

                                       2.
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                                                                    CONFIDENTIAL

rights of first refusal), proxy or shareholder agreements, or agreements of any
kind for the purchase or acquisition from the Company of any of its securities.
When issued in compliance with the provisions of this Agreement and the Restated
Charter, the Shares and the Conversion Shares will be validly issued, fully paid
and nonassessable, and will be free of any liens or encumbrances; provided,
however, that the Shares and the Conversion Shares may be subject to
restrictions on transfer under state and/or federal securities laws as set forth
herein or as otherwise required by such laws at the time a transfer is proposed.

            3.3 AUTHORIZATION; BINDING OBLIGATIONS. All corporate action on the
part of the Company, its officers, directors and shareholders necessary for the
authorization of this Agreement, the performance of all obligations of the
Company hereunder at the Closing and the authorization, sale, issuance and
delivery of the Shares pursuant hereto and the Conversion Shares pursuant to the
Restated Charter has been taken or will be taken prior to the Closing. The
Agreement, when executed and delivered, will be a valid and binding obligation
of the Company enforceable in accordance with its terms, except (a) as limited
by applicable bankruptcy, insolvency, reorganization, moratorium or other laws
of general application affecting enforcement of creditors' rights, (b) general
principles of equity that restrict the availability of equitable remedies, and
(c) to the extent that the enforceability of the indemnification provisions in
paragraph 5 of Exhibit C hereto may be limited by applicable laws. The sale of
the Shares and the subsequent conversion of the Shares into Conversion Shares
are not and will not be subject to any preemptive rights or rights of first
refusal that have not been properly waived or complied with.

         4. REPRESENTATIONS AND WARRANTIES OF THE PURCHASER.

            Each Purchaser hereby represents and warrants to the Company as
follows (such representations and warranties do not lessen or obviate the
representations and warranties of the Company set forth in this Agreement):

            4.1 REQUISITE POWER AND AUTHORITY. Purchaser has all necessary power
and authority under all applicable provisions of law to execute and deliver this
Agreement and to carry out its provisions. All action on Purchaser's part
required for the lawful execution and delivery of this Agreement has been or
will be effectively taken prior to the Closing. Upon its execution and delivery,
this Agreement will be a valid and binding obligation of Purchaser, enforceable
in accordance with its terms, except (a) as limited by applicable bankruptcy,
insolvency, reorganization, moratorium or other laws of general application
affecting enforcement of creditors' rights, (b) general principles of equity
that restrict the availability of equitable remedies, and (c) to the extent that
the enforceability of the indemnification provisions of paragraph 5 of Exhibit A
hereto may be limited by applicable laws.

            4.2 INVESTMENT REPRESENTATIONS. Purchaser understands that neither
the Shares nor the Conversion Shares have been registered under the Securities
Act. Purchaser also understands that the Shares are being offered and sold
pursuant to an exemption from registration contained in the Securities Act based
in part upon Purchaser's representations contained in the Agreement. Purchaser
hereby represents and warrants as follows:

                                       3.
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                                                                    CONFIDENTIAL

                (A) PURCHASER BEARS ECONOMIC RISK. Purchaser has substantial
experience in evaluating and investing in private placement transactions of
securities in companies similar to the Company so that it is capable of
evaluating the merits and risks of its investment in the Company and has the
capacity to protect its own interests. Purchaser must bear the economic risk of
this investment indefinitely unless the Shares (or the Conversion Shares) are
registered pursuant to the Securities Act, or an exemption from registration is
available. Purchaser understands that the Company has no present intention of
registering the Shares, the Conversion Shares or any shares of its Common Stock.
Purchaser also understands that there is no assurance that any exemption from
registration under the Securities Act will be available and that, even if
available, such exemption may not allow Purchaser to transfer all or any portion
of the Shares or the Conversion Shares under the circumstances, in the amounts
or at the times Purchaser might propose.

                (B) ACQUISITION FOR OWN ACCOUNT. Purchaser is acquiring the
Shares and the Conversion Shares for Purchaser's own account for investment
only, and not with a view towards their distribution.

                (C) PURCHASER CAN PROTECT ITS INTEREST. Purchaser represents
that by reason of its, or of its management's, business or financial experience,
Purchaser has the capacity to protect its own interests in connection with the
transactions contemplated in this Agreement. Further, Purchaser is aware of no
publication of any advertisement in connection with the transactions
contemplated in the Agreement.

                (D) ACCREDITED INVESTOR. Purchaser represents that it is an
accredited investor within the meaning of Regulation D under the Securities Act.

                (E) COMPANY INFORMATION. Purchaser has received and read
sufficient information about the Company and has had an opportunity to discuss
the Company's business, management and financial affairs with directors,
officers and management of the Company and has had the opportunity to review the
Company's operations and facilities. Purchaser has also had the opportunity to
ask questions of and receive answers from, the Company and its management
regarding the terms and conditions of this investment.

                (F) RULE 144. Purchaser acknowledges and agrees that the Shares,
and, if issued, the Conversion Shares must be held indefinitely unless they are
subsequently registered under the Securities Act or an exemption from such
registration is available. Purchaser has been advised or is aware of the
provisions of Rule 144 promulgated under the Securities Act as in effect from
time to time, which permits limited resale of shares purchased in a private
placement subject to the satisfaction of certain conditions, including, among
other things: the availability of certain current public information about the
Company, the resale occurring following the required holding period under Rule
144 and the number of shares being sold during any three-month period not
exceeding specified limitations.

                (G) RESIDENCE. The office or offices of the Purchaser in which
its investment decision was made is located at the address or addresses of the
Purchaser set forth on the signature page hereto.

                                       4.
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                                                                    CONFIDENTIAL

         5. OTHER MATTERS

            5.1 REGISTRATION RIGHTS. Purchaser shall have the Registration
Rights set forth in Exhibit C hereto with respect to the Shares and the
Conversion Shares.

            5.2 BASIC FINANCIAL INFORMATION AND REPORTING.

                (A) The Company will maintain true books and records of account
in which full and correct entries will be made of all its business transactions
pursuant to a system of accounting established and administered in accordance
with generally accepted accounting principles consistently applied, and will set
aside on its books all such proper accruals and reserves as shall be required
under generally accepted accounting principles consistently applied.

                (B) So long as each Purchaser (with its affiliates) shall own
shares of Registrable Securities (as defined in Exhibit C hereto), as soon as
practicable after the end of each fiscal year of the company, and in any event
within ninety (90) days thereafter, the Company will furnish each Purchaser a
consolidated balance sheet of the Company, as at the end of such fiscal year,
and a consolidated statement of income and a consolidated statement of cash
flows of the Company, for such year, all prepared in accordance with generally
accepted accounting principles consistently applied and setting forth in each
case in comparative form the figures for the previous fiscal year, all in
reasonable detail. Such financial statements shall be accompanied by a report
and opinion thereon by independent public accountants of national standing
selected by the Company's Board of Directors, and a Company-prepared comparison
to the Company's operating plan for such year.

                (C) So long as each Purchaser (with its affiliates) shall own
shares of Registrable Securities, the Company will furnish such Purchaser, as
soon as practicable after the end of the first, second and third quarterly
accounting periods in each fiscal year of the Company and in any event within
forty-five (45) days thereafter, a consolidated balance sheet of the Company as
of the end of each such quarterly period, and a consolidated statement of income
and a consolidated statement of cash flows of the Company for such period and
for the current fiscal year to date, prepared in accordance with generally
accepted accounting principles, with the exception that no notes need be
attached to such statements and year-end audit adjustments may no have been
made.

                (D) So long as each Purchaser (with its affiliates) shall own
shares of Registrable Securities, the Company will furnish such Purchaser (i) at
least thirty (30) days prior to the beginning of each fiscal year an annual
budget and operating plans for such fiscal year (and as soon as available, any
subsequent revisions thereto); and (ii) as soon as practicable after the end of
each month, and in any event within twenty (20) days thereafter, a consolidated
balance sheet of the Company as of the end of each such month, and a
consolidated statement of income and a consolidated statement of cash flows of
the Company for such month and for the current fiscal year to date, including a
comparison to plan figures for such period, prepared in accordance with
generally accepted accounting principles consistently applied, with the
exception that no notes need be attached to such statements and year-end audit
adjustments may not have been made.

                                       5.
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                                                                    CONFIDENTIAL

            5.3 INSPECTION RIGHTS. Each Purchaser shall have the right to visit
and inspect any of the properties of the Company or any of its subsidiaries, and
to discuss the affairs, finances and accounts of the Company or any of its
subsidiaries with its officers, and to review such information as is reasonably
requested all at such reasonable times and as often as may be reasonably
requested; provided, however, that the Company shall not be obligated under this
Section 5.3 with respect to a competitor of the Company or with respect to
information which the Board of Directors determines in good faith is
confidential and should not, therefore, be disclosed.

            5.4 CONFIDENTIALITY OF RECORDS. Each Purchaser agrees to use, and to
use its best efforts to insure that its authorized representatives use, the same
degree of care as Purchaser uses to protect its own confidential information to
keep confidential any information furnished to it which the Company identifies
as being confidential or proprietary (so long as such information is not in the
public domain), except that such Purchaser may disclose such proprietary or
confidential information to any partner, subsidiary or parent of such Purchaser
for the purpose of evaluating its investment in the Company as long as such
partner, subsidiary or parent is advised of the confidentiality provisions of
this Section 5.4.

            5.5 RIGHT OF FIRST REFUSAL. No Purchaser shall not sell, assign,
pledge, or in any manner transfer any of the Shares or Conversion Shares or any
right or interest therein, whether voluntarily or by operation of law, or by
gift or otherwise, except by a transfer which meets the following requirements:

                (A) If a Purchaser desires to sell or otherwise transfer any of
the Shares or Conversion Shares, then such Purchaser shall first give written
notice thereof to the Company. The notice shall name the proposed transferee and
state the number of the Shares or Conversion Shares to be transferred, the
proposed consideration, and all other terms and conditions of the proposed
transfer.

                (B) For thirty (30) days following receipt of such notice, the
Company shall have the option to purchase all of the Shares or Conversion
Shares, or any portion thereof, specified in the notice at the price and upon
the terms set forth in such notice. In the event of a gift, property settlement
or other transfer in which the proposed transferee is not paying the full price
for the Shares or Conversion Shares, and that is not otherwise exempted from the
provisions of this Section 5.5, the price shall be deemed to be the fair market
value of the Shares or Conversion Shares at such time as determined in good
faith by the Board of Directors. In the event the Company elects to purchase all
of the Shares or Conversion Shares, or any portion thereof, it shall give
written notice to the Purchaser of its election and settlement for said shares
shall be made as provided below in Section 5.5(d).

                (C) The Company may assign its rights hereunder.

                (D) In the event the Company and/or its assignee(s) elect to
acquire any of the Shares or Conversion Shares as specified in Purchaser's
notice, the Secretary of the Company shall so notify Purchaser and settlement
thereof shall be made in cash within thirty (30) days after the Secretary of the
Company receives Purchaser's notice; provided that if the terms of payment set
forth in Purchaser's notice were other than cash against delivery, the

                                       6.
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                                                                    CONFIDENTIAL

Company and/or its assignee(s) shall pay for said Shares or Conversion Shares on
the same terms and conditions set forth in Purchaser's notice.

                (E) In the event the Company and/or its assignees(s) do not
elect to acquire all of the Shares or Conversion Shares, or any portion thereof,
specified in Purchaser's notice, Purchaser may, within the sixty-day period
following the expiration of the option rights granted to the Company and/or its
assignees(s) herein, transfer the Shares or Conversion Shares specified in
Purchaser's notice which were not acquired by the Company and/or its
assignees(s) as specified in Purchaser's notice. All Shares or Conversion Shares
so sold by Purchaser shall continue to be subject to the provisions of this
section in the same manner as before said transfer.

                (F) Anything to the contrary contained herein notwithstanding,
the following transactions shall be exempt from the provisions of this Section
5.5:

                    (I) Purchaser's transfer of any or all of the Shares or
Conversion Shares to the Company or to any other stockholder of the Company.

                    (II) Purchaser's transfer of any or all of the Shares or
Conversion Shares to a person who, at the time of such transfer, is an officer
or director of the Company.

            In any such case, the transferee, assignee, or other recipient shall
receive and hold such stock subject to the provisions of this section, and there
shall be no further transfer of such stock except in accord with this section.

                (G) The provisions of this Section 5.5 may be waived with
respect to any transfer by the Company, upon duly authorized action of its Board
of Directors.

                (H) Any sale or transfer, or purported sale or transfer, of the
Shares or Conversion Shares shall be null and void unless the terms, conditions,
and provisions of this Section 5.5 are strictly observed and followed.

                (I) The foregoing right of first refusal shall terminate upon
the date securities of the Company are first offered to the public pursuant to a
registration statement filed with, and declared effective by, the United States
Securities and Exchange Commission under the Securities Act of 1933, as amended.

            5.6 TERMINATION OF PATENT AGREEMENT. Each Purchaser and each
Inventor agree that they will return to the Company any Shares or Conversion
Shares held by them and they will hold no other equity position in the Company,
without payment of additional consideration by the Company, if the respective
Patent Agreement for which it transferred rights to the Company in exchange for
the Shares is terminated for any reason, other than as set forth in Section 8.1
of the Patent Agreement with the University of Texas System.

            5.7 TRANSFER OF SHARES TO INVENTOR. Notwithstanding anything herein
to the contrary, Purchasers shall be permitted to transfer a portion of the
Shares to Inventor pursuant to the Patent Agreement with the University of Texas
System, provided, however, that for such

                                       7.
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                                                                    CONFIDENTIAL

transfer to be valid Inventor must execute a signature page to this Agreement,
and agree to be bound by all provisions of this Agreement.

            5.8 "MARKET STAND-OFF" AGREEMENT. If requested by the Company or the
representative of the underwriters of Common Stock (or other securities) of the
Company, each Purchaser shall not sell or otherwise transfer or dispose of any
Common Stock (or other securities) of the Company held by such Purchaser (other
than those included in a registration pursuant to paragraph 1 of Exhibit C
hereto) for a period specified by the representative of the underwriters not to
exceed one hundred eighty (180) days following the effective date of a
registration statement of the Company filed under the Securities Act. The
Company may impose stop-transfer instructions with respect to the shares of
Common Stock (or other securities) subject to the foregoing restriction until
the end of said one hundred eighty (180) day period.

         6. CONDITIONS TO CLOSING.

            6.1 CONDITIONS TO PURCHASER'S OBLIGATIONS AT THE CLOSING.
Purchasers' obligations to purchase the Shares at the Closing are subject to the
satisfaction, at or prior to the Closing Date, of the following conditions:

                (A) REPRESENTATIONS AND WARRANTIES TRUE; PERFORMANCE OF
OBLIGATIONS. The representations and warranties made by the Company in Section 3
hereof shall be true and correct in all material respects as of the Closing Date
with the same force and effect as if they had been made as of the Closing Date,
and the Company shall have performed all obligations and conditions herein
required to be performed or observed by it on or prior to the Closing.

                (B) CONSENTS, PERMITS, AND WAIVERS. The Company shall have
obtained any and all consents, permits and waivers necessary or appropriate for
consummation of the transactions contemplated by the Agreement (except for such
as may be properly obtained subsequent to the Closing).

                (C) FILING OF RESTATED CHARTER. The Restated Charter shall have
been filed with the Secretary of State of the State of Delaware and shall
continue to be in full force and effect as of the Closing Date.

                (D) CORPORATE DOCUMENTS. The Company shall have delivered to
each Purchaser or its counsel, copies of all corporate documents of the Company
as such Purchaser shall reasonably request.

                (E) RESERVATION OF CONVERSION SHARES. The Conversion Shares
issuable upon conversion of the Shares shall have been duly authorized and
reserved for issuance upon such conversion.

                (F) PATENT AGREEMENT. The Company and Purchaser shall have
entered into the respective Patent Agreement indicated on Exhibit A.

                (G) PROCEEDINGS AND DOCUMENTS. All corporate and other
proceedings in connection with the transactions contemplated at the Closing
hereby and all

                                       8.
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                                                                    CONFIDENTIAL

documents and instruments incident to such transactions shall be reasonably
satisfactory in substance and form to the Purchasers and their special counsel,
and the Purchasers and their special counsel shall have received all such
counterpart originals or certified or other copies of such documents as they may
reasonably request.

            6.2 CONDITIONS TO OBLIGATIONS OF THE COMPANY. The Company's
obligation to issue and sell the Shares at each Closing is subject to the
satisfaction, on or prior to such Closing, of the following conditions:

                (A) REPRESENTATIONS AND WARRANTIES TRUE. The representations and
warranties in Section 4 made by each Purchaser shall be true and correct in all
material respects at the date of the Closing, with the same force and effect as
if they had been made on and as of said date.

                (B) PERFORMANCE OF OBLIGATIONS. Each Purchaser shall have
performed and complied with all agreements and conditions herein required to be
performed or complied with by such Purchaser on or before the Closing.

                (C) FILING OF RESTATED CHARTER. The Restated Charter shall have
been filed with the Secretary of State of the State of Delaware and shall
continue to be in full force and effect as of the Closing Date.

                (D) PATENT AGREEMENT. The Company and each Purchaser shall have
entered into the respective Patent Agreement indicated on Exhibit A.

                (E) CONSENTS, PERMITS, AND WAIVERS. The Company shall have
obtained any and all consents, permits and waivers necessary or appropriate for
consummation of the transactions contemplated by the Agreement and the Patent
Agreement (except for such as may be properly obtained subsequent to the
Closing).

         7. MISCELLANEOUS.

            7.1 GOVERNING LAW. Except for sections 7.10 and 7.14, this Agreement
shall be governed in all respects by the laws of the State of Colorado as such
laws are applied to agreements between Colorado residents entered into and
performed entirely in Colorado.

            7.2 SURVIVAL. The representations, warranties, covenants and
agreements made herein shall survive any investigation made by Purchasers and
the closing of the transactions contemplated hereby for a period of one (1) year
following the Closing. All statements as to factual matters contained in any
certificate or other instrument delivered by or on behalf of the Company
pursuant hereto in connection with the transactions contemplated hereby shall be
deemed to be representations and warranties by the Company hereunder solely as
of the date of such certificate or instrument.

            7.3 SUCCESSORS AND ASSIGNS. Except as otherwise expressly provided
herein, the provisions hereof shall inure to the benefit of, and be binding
upon, the successors, assigns, heirs, executors and administrators of the
parties hereto and shall inure to the benefit of and be enforceable by each
person who shall be a holder of the Shares from time to time.

                                       9.
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                                                                    CONFIDENTIAL

            7.4 ENTIRE AGREEMENT. This Agreement, the Exhibits and Schedules
hereto, the Patent Agreements and the other documents delivered pursuant hereto
constitute the full and entire understanding and agreement between the parties
with regard to the subjects hereof and no party shall be liable or bound to any
other in any manner by any representations, warranties, covenants and agreements
except as specifically set forth herein and therein.

            7.5 SEVERABILITY. In case any provision of the Agreement shall be
invalid, illegal or unenforceable, the validity, legality and enforceability of
the remaining provisions shall not in any way be affected or impaired thereby.

            7.6 AMENDMENT AND WAIVER.

                (A) This Agreement may be amended or modified only upon the
written consent of the Company and holders of at least a majority of the Shares
(treated as if converted and including any Conversion Shares into which the
Shares have been converted that have not been sold to the public).

                (B) The obligations of the Company and the rights of the holders
of the Shares and the Conversion Shares under the Agreement may be waived only
with the written consent of the holders of at least a majority of the Shares
(treated as if converted and including any Conversion Shares into which the
Shares have been converted that have not been sold to the public).

            7.7 DELAYS OR OMISSIONS. It is agreed that no delay or omission to
exercise any right, power or remedy accruing to any party, upon any breach,
default or noncompliance by another party under this Agreement or the Restated
Charter, shall impair any such right, power or remedy, nor shall it be construed
to be a waiver of any such breach, default or noncompliance, or any acquiescence
therein, or of or in any similar breach, default or noncompliance thereafter
occurring. It is further agreed that any waiver, permit, consent or approval of
any kind or character on any Purchaser's part of any breach, default or
noncompliance under this Agreement or under the Restated Charter or any waiver
on such party's part of any provisions or conditions of the Agreement or the
Restated Charter must be in writing and shall be effective only to the extent
specifically set forth in such writing. All remedies, either under this
Agreement the Restated Charter, by law, or otherwise afforded to any party,
shall be cumulative and not alternative.

            7.8 NOTICES. All notices required or permitted hereunder shall be in
writing and shall be deemed effectively given: (a) upon personal delivery to the
party to be notified, (b) when sent by confirmed telex or facsimile if sent
during normal business hours of the recipient, if not, then on the next business
day, (c) five (5) days after having been sent by registered or certified mail,
return receipt requested, postage prepaid, or (d) one (1) day after deposit with
a nationally recognized overnight courier, specifying next day delivery, with
written verification of receipt. All communications shall be sent to the Company
at the address as set forth on the signature page hereof and to Purchaser at the
address set forth on the signature hereto or at such other address as the
Company or Purchaser may designate by ten (10) days advance written notice to
the other parties hereto.

                                      10.
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                                                                    CONFIDENTIAL

            7.9 EXPENSES. Each party shall pay all costs and expenses that it
incurs with respect to the negotiation, execution, delivery and performance of
the Agreement.

            7.10 ATTORNEYS' FEES. Subject to the statutory authority of the
Texas Attorney General, in the event that any suit or action is instituted to
enforce any provision in this Agreement, the prevailing party in such dispute
shall be entitled to recover from the losing party all fees, costs and expenses
of enforcing any right of such prevailing party under or with respect to this
Agreement, including without limitation, such reasonable fees and expenses of
attorneys and accountants, which shall include, without limitation, all fees,
costs and expenses of appeals.

            7.11 TITLES AND SUBTITLES. The titles of the sections and
subsections of the Agreement are for convenience of reference only and are not
to be considered in construing this Agreement.

            7.12 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

            7.13 BROKER'S FEES. Each party hereto represents and warrants that
no agent, broker, investment banker, person or firm acting on behalf of or under
the authority of such party hereto is or will be entitled to any broker's or
finder's fee or any other commission directly or indirectly in connection with
the transactions contemplated herein. Each party hereto further agrees to
indemnify each other party for any claims, losses or expenses incurred by such
other party as a result of the representation in this Section 7.14 being untrue.

            7.14 CONFIDENTIALITY. Subject to the Texas Public Information Act,
each party hereto agrees that, except with the prior written consent of the
other party, it shall at all times keep confidential and not divulge, furnish or
make accessible to anyone any confidential information, knowledge or data
concerning or relating to the business or financial affairs of the other parties
to which such party has been or shall become privy by reason of this Agreement
or the Patent Agreement, discussions or negotiations relating to this Agreement
or the Patent Agreement, the performance of its obligations hereunder or the
ownership of the Shares purchased hereunder. The provisions of this Section 7.14
shall be in addition to, and not in substitution for, the provisions of any
separate nondisclosure agreement executed by the parties hereto.

            7.15 PRONOUNS. All pronouns contained herein, and any variations
thereof, shall be deemed to refer to the masculine, feminine or neutral,
singular or plural, as to the identity of the parties hereto may require.

            7.16 SUBSEQUENT SALES OF SHARES. Notwithstanding any other provision
set forth herein, the parties hereto acknowledge and agree that each Patent
Agreement indicated on Exhibit A hereto may not be executed on the date hereof
and that the Company may sell the respective Shares to the Purchasers indicated
on Exhibit A in the amounts indicated upon execution of the relevant Patent
Agreement after the date hereof, without any further action other than the
execution of additional signature pages hereto. For purposes of this Agreement,
each date of the sale of Shares hereunder shall be a "Closing." The sales of any
Shares pursuant to the the

                                      11.
<PAGE>

                                                                    CONFIDENTIAL

same terms and conditions set forth in this Agreement, including, without
limitation, the representations and warranties by such Purchasers as set forth
in Section 5 hereof. Any Shares sold pursuant to this Section 7.16 shall be
deemed to be "Shares" for all purposes under this Agreement, any party which
acquires such shares shall be deemed a "Purchaser" for all purposes under this
Agreement, and the Patent Agreements listed on Exhibit A hereto shall be a
condition of closing such sale.

              [THE REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                      12.
<PAGE>

                                                                    CONFIDENTIAL

         IN WITNESS WHEREOF, the parties hereto have executed the SERIES B
PREFERRED STOCK PURCHASE AGREEMENT as of the date set forth in the first
paragraph hereof.

COMPANY:                                      PURCHASER:

MYOGEN, INC.                                  UNIVERSITY OF TEXAS SYSTEM

By: /s/ J. William Freytag                    By: /s/ Ray Farabee
   -------------------------------               -------------------------------
    President                                     Ray Farabee
    J. William Freytag                        Title: Vice Chancellor and
                                                     General Counsel
                                                    ----------------------------

Address:                                      Address: 201 West 7th Street
                                                       Austin, Texas 78701

                                              THE UNIVERSITY OF NORTH TEXAS
                                              HEALTH SCIENCE CENTER AT FORTH
                                              WORTH

                                              By: /s/ Benjamin L. Cohen, DO
                                                 -------------------------------

                                              Name: Benjamin L Cohen, DO
                                                   -----------------------------

                                              Title:   Act. Pres & Exec Dean
                                                    ----------------------------

                                              MANTEX BIOTECH INC.

                                              By: /s/ Albert D. Friesen
                                                 -------------------------------

                                              Name: Albert D. Friesen
                                                   -----------------------------

                                              Title: President
                                                    ----------------------------

                                              The University of North Texas
                                              Health Science Center at Fort
                                              Worth

                                              By: /s/ Steven L. Russell
                                                 -------------------------------

                                              Name: Steven L. Russell
                                                   -----------------------------

                                              Title: Vice President for Fiscal
                                                     Affairs
                                                    ----------------------------

                   SERIES B PREFERRED STOCK PURCHASE AGREEMENT
                                 SIGNATURE PAGE
<PAGE>

                                                                    CONFIDENTIAL

                                    EXHIBIT A

                             SCHEDULE OF PURCHASERS

<TABLE>
<CAPTION>
----------------------------------------------------------------------------------------------------------------------
                 NAME                              NUMBER OF SHARES                      PATENT AGREEMENT(S)
----------------------------------------------------------------------------------------------------------------------
<S>                                                <C>                          <C>
University of Texas System                              482,163                 Patent and Technology Agreement
                                                                                between University of Texas System
                                                                                and the Company ("UT Agreement")
----------------------------------------------------------------------------------------------------------------------
University of Texas System                              28,929                  Assignment and Exchange Agreement
                                                                                between ManTex Biotech Inc., its
                                                                                shareholders and the Company
                                                                                ("ManTex Agreement")
----------------------------------------------------------------------------------------------------------------------
The University of North Texas Health                    19,287                  ManTex Agreement
Science Center at Forth Worth
----------------------------------------------------------------------------------------------------------------------
The University of North Texas Health                    120,541                 Sponsored Research Program with
Science Center at Forth Worth                                                   UNTHSC and the Company
----------------------------------------------------------------------------------------------------------------------
ManTex Biotech Inc.                                     152,685                 ManTex Agreement
----------------------------------------------------------------------------------------------------------------------
</TABLE>

<PAGE>
                                                                    CONFIDENTIAL

                                   EXHIBIT B

                     RESTATED CERTIFICATE OF INCORPORATION

                            [INTENTIONALLY OMITTED]
<PAGE>

                                                                    CONFIDENTIAL

                                    EXHIBIT C

                               REGISTRATION RIGHTS

1. PIGGYBACK REGISTRATIONS. The Company shall notify the Purchaser in writing at
least thirty (30) days prior to the filing of any registration statement under
the Securities Act for purposes of a public offering of securities of the
Company (including, but not limited to, registration statements relating to
secondary offerings of securities of the Company, but excluding registration
statements relating to the Initial Public Offering (as defined in paragraph 3 of
this Exhibit C), employee benefit plans or with respect to corporate
reorganizations or other transactions under Rule 145 of the Securities Act) and
will afford the Purchaser an opportunity to include in such registration
statement all or part of the Conversion Shares (the "Registrable Securities").
If the Purchaser desires to include in any such registration statement all or
any part of the Registrable Securities held by it, Purchaser shall, within
fifteen (15) days after the above-described notice from the Company, so notify
the Company in writing. Such notice shall state the number of shares of
Registrable Securities that Purchaser wishes to include in such registration
statement. If Purchaser decides not to include all of its Registrable Securities
in any registration statement thereafter filed by the Company, Purchaser shall
nevertheless continue to have the right to include any Registrable Securities in
any subsequent registration statement or registration statements as may be filed
by the Company with respect to offerings of its securities, all upon the terms
and conditions set forth herein.

   (a) UNDERWRITING. If the registration statement under which the Company gives
notice under this paragraph 1 is for an underwritten offering, the Company shall
so advise Purchaser. In such event, the right of Purchaser to be included in a
registration pursuant to paragraph 1 shall be conditioned upon Purchaser's
participation in such underwriting and the inclusion of Purchaser's Registrable
Securities in the underwriting to the extent provided herein. Purchaser shall
enter into an underwriting agreement in customary form with the underwriter or
underwriters selected for such underwriting by the Company. If the underwriter
determines in good faith that marketing factors require a limitation of the
number of shares to be underwritten, the number of shares that may be included
in the underwriting shall be allocated, first, to the Company; second, to the
Holders (as defined in the Company's Second Amended and Restated Investor
Rights Agreement, dated as of November 23, 1999), if any, seeking registration
on a pro rata basis based on the total number of Registrable Securities held by
the Holders; and third, to Purchaser. No such reduction shall reduce the
securities being offered by the Company for its own account to be included in
the registration and underwriting, and in no event shall the amount of
securities of the selling Holders included in the registration be reduced unless
such registration does not include shares of any other selling stockholders,
including the Purchaser. If in the sole judgement of the underwriter the
inclusion of any Registrable Securities held by the Purchaser in a Registration
would adversely affect the offering, the Company and the underwriter may exclude
all shares of Registrable Securities held by Purchaser requested to be included
in such registration.

   (b) RIGHT TO TERMINATE REGISTRATION. The Company shall have the right to
terminate or withdraw any registration initiated by it under paragraph 1 prior
to the effectiveness of such registration whether or not Purchaser has elected
to include securities in such registration. The Registration Expenses of such
withdrawn registration shall be borne by the Company in accordance with
paragraph 2 hereof.

2. EXPENSES OF REGISTRATION. All Registration Expenses incurred in connection
with any registration, qualification or compliance pursuant to paragraph 1 shall
be borne proportionately by Purchaser and the Company. For the purposes of this
paragraph 2, "Registration Expenses" shall mean all expenses incurred by the
Company in complying with paragraph 1 hereof, including, without limitation, all
registration and filing fees, printing expenses, fees and disbursements of
counsel for the Company,

<PAGE>

                                                                    CONFIDENTIAL

blue sky fees and expenses and the expense of any special audits incident to or
required by any such registration (but excluding the compensation of regular
employees of the Company which shall be paid in any event by the Company). All
underwriting discounts and selling commissions applicable to the sale incurred
in connection with any registrations of Registrable Securities hereunder, shall
be borne by Purchaser, including expenses of any special counsel retained by
Purchaser in connection with the registration.

3. TERMINATION OF REGISTRATION RIGHTS. All registration rights granted under
paragraph 1 shall terminate and be of no further force and effect three (3)
years after the effective date of the Company's first firm commitment
underwritten public offering of its Common Stock pursuant to an effective
registration under the Securities Act (the "Initial Public Offering"). In
addition, Purchaser's registration rights shall expire if (i) the Company has
completed its Initial Public Offering and is subject to the provisions of the
Exchange Act of 1934, as amended (the "Exchange Act"), and (ii) all Registrable
Securities held by and issuable to Purchaser may be sold under Rule 144 during
any ninety (90) day period.

4. PRIORITY OF REGISTRATION RIGHTS. The registration rights of Purchaser shall
be subordinate to those registration rights granted to the Holders under the
Company's Amended and Restated Investor Rights Agreement.

5. INDEMNIFICATION. In the event any Registrable Securities are included in a
registration statement:

   (a) To the extent permitted by law, the Company will indemnify and hold
harmless the Purchaser, the partners, officers, directors, and legal counsel of
Purchaser, any underwriter (as defined in the Securities Act of 1933, as amended
(the "Securities Act")) for Purchaser and each person, if any, who controls
Purchaser or such underwriter within the meaning of the Securities Act or the
Exchange Act, against any losses, claims, damages, or liabilities (joint or
several) to which they may become subject under the Securities Act, the Exchange
Act or other federal or state law, insofar as such losses, claims, damages or
liabilities (or actions in respect thereof) arise out of or are based upon any
of the following statements, omissions or violations (collectively, a
"Violation") by the Company: (i) any untrue statement or alleged untrue
statement of a material fact contained in such registration statement, including
any preliminary prospectus or final prospectus contained therein or any
amendments or supplements thereto, (ii) the omission or alleged omission to
state therein a material fact required to be stated therein, or necessary to
make the statements therein not misleading, or (iii) any violation or alleged
violation by the Company of the Securities Act, the Exchange Act, any state
securities law or any rule or regulation promulgated under the Securities Act,
the Exchange Act or any state securities law in connection with the offering
covered by such registration statement; and the Company will reimburse Purchaser
or each such partner, officer or director, underwriter or controlling person for
any legal or other expenses reasonably incurred by them in connection with
investigating or defending any such loss, claim, damage, liability or action;
provided, however, that the indemnity agreement contained in this paragraph 5
shall not apply to amounts paid in settlement of any such loss, claim, damage,
liability or action if such settlement is effected without the consent of the
Company, which consent shall not be unreasonably withheld, nor shall the Company
be liable in any such case for any such loss, claim, damage, liability or action
to the extent that it arises out of or is based upon a Violation which occurs in
reliance upon and in conformity with written information furnished expressly for
use in connection with such registration by Purchaser or such partner, officer,
director, underwriter or controlling person of Purchaser.

   (b) To the extent permitted by law, Purchaser will, if Registrable Securities
held by Purchaser are included in the securities as to which such registration
qualifications or compliance is being effected, indemnify and hold harmless the
Company, each of its directors, its officers, and legal counsel

<PAGE>

                                                                    CONFIDENTIAL

and each person, if any, who controls the Company within the meaning of the
Securities Act, any underwriter and any Holder selling securities under such
registration statement or any of such Holder's partners, directors or officers
or any person who controls such Holder, against any losses, claims, damages or
liabilities (joint or several) to which the Company or any such director,
officer, controlling person, underwriter or such Holder, or partner, director,
officer or controlling person of such Holder may become subject under the
Securities Act, the Exchange Act or other federal or state law, insofar as such
losses, claims, damages or liabilities (or actions in respect thereto) arise out
of or are based upon any Violation, in each case to the extent (and only to the
extent) that such Violation occurs in reliance upon and in conformity with
written information furnished by Purchaser under an instrument duly executed by
Purchaser and stated to be specifically for use in connection with such
registration; and Purchaser will reimburse any legal or other expenses
reasonably incurred by the Company or any such director, officer, controlling
person, underwriter or Holder, or partner, officer, director or controlling
person of such other Holder in connection with investigating or defending any
such loss, claim, damage, liability or action if it is judicially determined
that there was such a Violation; provided, however, that the indemnity agreement
contained in this paragraph 5 shall not apply to amounts paid in settlement of
any such loss, claim, damage, liability or action if such settlement is effected
without the consent of the Purchaser, which consent shall not be unreasonably
withheld; provided, further, that in no event shall any indemnity under this
paragraph exceed the net proceeds from the offering received by Purchaser.

   (c) Promptly after receipt by an indemnified party under this paragraph 5 of
notice of the commencement of any action (including any governmental action),
such indemnified party will, if a claim in respect thereof is to be made against
any indemnifying party under this paragraph 5, deliver to the indemnifying party
a written notice of the commencement thereof and the indemnifying party shall
have the right to participate in, and, to the extent the indemnifying party so
desires, jointly with any other indemnifying party similarly noticed, to assume
the defense thereof with counsel mutually satisfactory to the parties; provided,
however, that an indemnified party shall have the right to retain its own
counsel, with the fees and expenses to be paid by the indemnifying party, if
representation of such indemnified party by the counsel retained by the
indemnifying party would be inappropriate due to actual or potential differing
interests between such indemnified party and any other party represented by such
counsel in such proceeding. The failure to deliver written notice to the
indemnifying party within a reasonable time of the commencement of any such
action, if materially prejudicial to its ability to defend such action, shall
relieve such indemnifying party of any liability to the indemnified party under
this paragraph 5 only to the extent so prejudiced, but the omission so to
deliver written notice to the indemnifying party will not relieve it of any
liability that it may have to any indemnified party otherwise than under this
paragraph 5.

   (d) If the indemnification provided for in this paragraph 5 is held by a
court of competent jurisdiction to be unavailable to an indemnified party with
respect to any losses, claims, damages or liabilities referred to herein, the
indemnifying party, in lieu of indemnifying such indemnified party hereunder,
shall to the extent permitted by applicable law contribute to the amount paid or
payable by such indemnified party as a result of such loss, claim, damage or
liability in such proportion as is appropriate to reflect the relative fault of
the indemnifying party on the one hand and of the indemnified party on the other
in connection with the Violation(s) that resulted in such loss, claim, damage or
liability, as well as any other relevant equitable considerations. The relative
fault of the indemnifying party and of the indemnified party shall be determined
by a court of law by reference to, among other things, whether the untrue or
alleged untrue statement of a material fact or the omission to state a material
fact relates to information supplied by the indemnifying party or by the
indemnified party and the parties' relative intent, knowledge, access to
information and opportunity to correct or prevent such statement or omission;
provided, that in no event shall any contribution by Purchaser hereunder exceed
the net proceeds from the offering received by Purchaser.

<PAGE>

                                                                    CONFIDENTIAL

   (e) The obligations of the Company and Purchaser under this paragraph 5 shall
survive completion of any offering of Registrable Securities in a registration
statement. No indemnifying party, in the defense of any such claim or
litigation, shall, except with the consent of each indemnified party, consent to
entry of any judgment or enter into any settlement which does not include as an
unconditional term thereof the giving by the claimant or plaintiff to such
indemnified party of a full release from all liability in respect to such claim
or litigation.

   (f) Notwithstanding the foregoing, to the extent that the provisions on
indemnification and contribution contained in the underwriting agreement entered
into in connection with the underwritten public offering are in conflict with
any of the foregoing provisions, the provisions in the underwriting agreement
shall control.<PAGE>
                                                                    EXHIBIT 10.5
                                  MYOGEN, INC.

                   SERIES C PREFERRED STOCK PURCHASE AGREEMENT

     THIS SERIES C PREFERRED STOCK PURCHASE AGREEMENT (the "AGREEMENT") is
entered into as of this 23 day of November, 1999, by and among MYOGEN, INC., a
Delaware corporation (the "COMPANY") and EACH OF THOSE PERSONS AND ENTITIES,
SEVERALLY AND NOT JOINTLY, WHOSE NAMES ARE SET FORTH ON THE SCHEDULE OF
PURCHASERS ATTACHED HERETO AS EXHIBIT A (which persons and entities are
hereinafter collectively referred to as "PURCHASERS" and each individually as a
"PURCHASER").

                                    RECITALS

     WHEREAS, the Company has authorized the sale and issuance of up to an
aggregate of Thirteen Million Ninety Thousand Nine Hundred Ten (13,090,910)
shares of its Series C Preferred Stock (the "SERIES C STOCK");

     WHEREAS, Purchasers desire to purchase the Series C Stock on the terms and
conditions set forth herein; and

     WHEREAS, the Company desires to issue and sell the Shares to Purchasers on
the terms and conditions set forth herein;

     NOW, THEREFORE, in consideration of the foregoing recitals and the mutual
promises hereinafter set forth, the parties hereto agree as follows:

1. AGREEMENT TO SELL AND PURCHASE

     1.1 AUTHORIZATION OF SHARES. On or prior to the Closing (as defined in
Section 2 below), the Company shall have authorized (i) the sale and issuance to
Purchasers of the Shares having the rights, preferences, privileges and
restrictions set forth in the Restated Certificate of Incorporation of the
Company, as amended, in the form attached hereto as Exhibit B (the "RESTATED
CERTIFICATE") and (ii) the issuance of such shares of Common Stock to be issued
upon the conversion of the Shares (the "CONVERSION SHARES").

     1.2 SALE AND PURCHASE. Subject to the terms and conditions hereof, the
Company hereby agrees to issue and sell to each Purchaser and each Purchaser
agrees to purchase from the Company, the number of Shares set forth opposite
such Purchaser's name on Exhibit A at a purchase price of One Dollar and
Thirty-Seven and One-Half Cents ($1.375) per share (the "SHARES").

2. CLOSING, DELIVERY AND PAYMENT

     2.1 CLOSING. The closing of the sale and purchase of the Shares under this
Agreement shall take place on November 23, 1999 (the "FIRST CLOSING") and on the
date of the Second Closing (as defined below) at the offices of Cooley Godward
LLP, 2595 Canyon Boulevard, Suite 250, Boulder, Colorado 80302 or at such other
time or place as the Company and Purchasers

                                       1.
<PAGE>

may mutually agree (each such date is hereinafter referred to as a "CLOSING
DATE"). The Purchasers shall be obligated, within 15 days after the date on
which a majority of the Company's Board of Directors approves an amendment to
the License Agreement (Enoximone) By and Between Hoechst Marion Roussel, Inc.
and the Company, dated October 1, 1998 (the "HMR AGREEMENT") which cures the
Company's default under the HMR Agreement, without any further action on the
part of the Company and regardless of any other development with respect to the
Company (unless such development is a Material Adverse Development (as defined
below)), to purchase the Shares indicated on Exhibit A hereto at the Second
Closing and all Shares purchased at such Second Closing shall be considered
Shares for the purposes of this Agreement upon the Company and the Purchasers
executing additional signature pages to this document as of the date of such
Second Closing and the payment by the Purchasers of the purchase price of the
Shares. At each Closing, subject to the terms and conditions hereof, the Company
will deliver to the Purchasers a certificate representing the number of Shares
to be purchased at the Closing by each Purchaser, against payment of the
purchase price therefor by certified check, cancellation of indebtedness from
the Company or wire transfer of immediately available funds. The "Second
Closing" shall be the date which is within 15 days after date on which the
Company's Board of Directors approves the amendment to the HMR Agreement, as
mutually agreeable by the parties hereto. "Material Adverse Development" with
respect to this Section 2.1 and Section 5.2 shall mean any change or effect that
is materially adverse to the business, properties, assets, condition, prospects,
or results of operations of the Company taken as a whole.

     2.2 DELIVERY. At each Closing, subject to the terms and conditions hereof,
the Company will deliver to the Purchasers certificates representing the number
of Shares to be purchased at the Closing by each Purchaser, against payment of
the purchase price therefor by check, wire transfer made payable to the order of
the Company, or cancellation of indebtedness.

3. REPRESENTATIONS AND WARRANTIES OF THE COMPANY

     Except as set forth on the Schedule of Exceptions, the Company hereby
represents and warrants to each Purchaser as of the First Closing as follows:

     3.1 ORGANIZATION, GOOD STANDING AND QUALIFICATION. The Company is a
corporation duly organized, validly existing and in good standing under the laws
of the State of Delaware. The Company has all requisite corporate power and
authority to own and operate its properties and assets, to execute and deliver
this Agreement, the Second Amended and Restated Investor Rights Agreement in the
form attached hereto as Exhibit C (the "INVESTOR RIGHTS AGREEMENT") and the
Second Amended Shareholders Agreement attached hereto as Exhibit D (the
"SHAREHOLDERS AGREEMENT"), to issue and sell the Shares, the Conversion Shares,
upon conversion thereof, and to carry out the provisions of this Agreement, the
Investor Rights Agreement, the Shareholders Agreement and the Restated
Certificate and to carry on its business as presently conducted and as presently
proposed to be conducted. The Company is duly qualified and is authorized to do
business and is in good standing as a foreign corporation in Colorado and in all
jurisdictions in which the nature of its activities and of its properties (both
owned and leased) makes such qualification necessary, except for those
jurisdictions in which failure to do so would not have a material adverse effect
on the Company or its business. The

                                       2.
<PAGE>

Company does not have any subsidiaries and is not a participant in any joint
venture, partnership or similar arrangement.

     3.2 CAPITALIZATION; VOTING RIGHTS. The authorized capital stock of the
Company, immediately prior to the First Closing, will consist of Twenty-Seven
Million (27,000,000) shares of Common Stock, Four Million Three Hundred
Seventy-Two Thousand Two Hundred Forty-Five (4,372,245) shares of which are
issued and outstanding and Two Million Two Hundred Eighty Thousand (2,280,000)
shares of which are reserved for future issuance to employees, of which One
Million One Hundred Thousand Five Hundred Thirty-Five (1,100,535) shares are
subject to options or purchase rights which have been issued to the Company's
employees, consultants and other service providers and One Million One Hundred
Seventy-Nine Thousand Four Hundred Sixty-Five Thousand (1,179,465) are reserved
for options which have not been granted as of the date hereof and Twenty Million
(20,000,000) shares of Preferred Stock, Six Million Seventy-Three Thousand
(6,073,000) of which are designated Series A Preferred Stock, Six Million
Thirty-Five Thousand (6,035,000) shares of which are issued and outstanding,
Eight Hundred Ten Thousand (810,000) of which are designated Series B Preferred
Stock, none of which are issued and outstanding, Thirteen Million One Hundred
Thousand (13,100,000) of which are designated Series C Preferred Stock, none of
which are issued and outstanding. All issued and outstanding shares of the
Company's Common Stock (i) have been duly authorized and validly issued to the
persons listed on Exhibit E hereto, (ii) are fully paid and nonassessable, and
(iii) were issued in compliance with all applicable state and federal laws
concerning the issuance of securities. The rights, preferences, privileges and
restrictions of the Shares are as stated in the Restated Certificate. The
Conversion Shares have been duly and validly reserved for issuance. Other than
as set forth on Exhibit E, and except as may be granted pursuant to the Investor
Rights Agreement and the Shareholders Agreement, there are no outstanding
options, warrants, rights (including conversion or preemptive rights and rights
of first refusal), proxy or shareholder agreements, or agreements of any kind
for the purchase or acquisition from the Company of any of its securities. When
issued in compliance with the provisions of this Agreement and the Certificate,
the Shares and the Conversion Shares will be validly issued, fully paid and
nonassessable, and will be free of any liens or encumbrances; provided, however,
that the Shares and the Conversion Shares may be subject to restrictions on
transfer under state and/or federal securities laws as set forth herein or as
otherwise required by such laws at the time a transfer is proposed.

     3.3 AUTHORIZATION; BINDING OBLIGATIONS. All corporate action on the part of
the Company, its officers, directors and shareholders necessary for the
authorization of this Agreement, the Investor Rights Agreement and the
Shareholders Agreement, the performance of all obligations of the Company
hereunder and thereunder at the Closing and the authorization, sale, issuance
and delivery of the Shares pursuant hereto and the Conversion Shares pursuant to
the Restated Certificate has been taken or will be taken prior to the Closing.
The Agreement, the Investor Rights Agreement and the Shareholders Agreement,
when executed and delivered, will be valid and binding obligations of the
Company enforceable in accordance with their terms, except (i) as limited by
applicable bankruptcy, insolvency, reorganization moratorium or other laws of
general application affecting enforcement of creditors' rights; (ii) general
principles of equity that restrict the availability of equitable remedies; and
(iii) to the extent that the enforceability of the indemnification provisions in
Section 2.9 of the Investor Rights Agreement may be limited by applicable laws.
The sale of the Shares and the subsequent conversion of

                                       3.
<PAGE>

Shares into Conversion Shares are not and will not be subject to any preemptive
rights or rights of first refusal that have not been properly waived or complied
with.

     3.4 FINANCIAL STATEMENTS. The Company has delivered to each Purchaser its
(i) unaudited balance sheet as at September 30, 1999 (the "FINANCIAL
STATEMENTS"), copies of which are attached hereto as Exhibit F. The Financial
Statements are complete and correct in all material respects, and present fairly
the financial condition and position of the Company as of September 30, 1999.

     3.5 LIABILITIES. The Company has no material liabilities and, to the best
of its knowledge, knows of no material contingent liabilities not disclosed in
the Financial Statements, except current liabilities incurred in the ordinary
course of business subsequent to September 30, 1999, which have not been, either
in any individual case or in the aggregate, materially adverse to the financial
condition or operating results of the Company,

     3.6 AGREEMENTS; ACTION.

         (a) Except for agreements explicitly contemplated hereby and agreements
between the Company and its employees with respect to the sale of the Company's
Common Stock and except as set forth on the Schedule of Exceptions, there are no
agreements, understandings or proposed transactions between the Company and any
of its officers, directors, affiliates or any affiliate thereof.

         (b) Except as set forth on the Schedule of Exceptions, there are no
agreements, understandings, instruments, contracts, proposed transactions,
judgments, orders, writs or decrees to which the Company is a party or to its
knowledge by which it is bound which may involve (i) obligations (contingent or
otherwise) of, or payments to, the Company in excess of $10,000 or (ii) the
license of any patent, copyright, trade secret or other proprietary right to or
from the Company (other than licenses arising from the purchase of "off the
shelf" or other standard products), or (iii) provisions restricting or affecting
the development, manufacture or distribution of the Company's products or
services, or (iv) indemnification by the Company with respect to infringements
of proprietary rights (other than indemnification obligations arising from
purchase or sale agreements entered into in the ordinary course of business).

         (c) Since the date of the Financial Statements, the Company has not (i)
declared or paid any dividends, or authorized or made any distribution upon or
with respect to any class or series of its capital stock, (ii) incurred any
indebtedness for money borrowed or any other liabilities (other than with
respect to dividend obligations, distributions, indebtedness and other
obligations incurred in the ordinary course of business or as disclosed in the
Balance Sheet) individually in excess of $10,000 or, in the case of indebtedness
and/or liabilities individually less than $10,000, in excess of $25,000 in the
aggregate, (iii) made any loans or advances to any person, other than ordinary
advances for travel and business expenses, or (iv) sold, exchanged or otherwise
disposed of any of its assets or rights, other than the sale of its inventory in
the ordinary course of business,

         (d) For the purposes of subsections (b) and (c) above, all
indebtedness, liabilities, agreements, understandings, instruments, contracts
and proposed transactions

                                       4.
<PAGE>

involving the same person or entity (including persons or entities the Company
has reason to believe are affiliated therewith) shall be aggregated for the
purpose of meeting the individual minimum dollar amounts of such subsections.

         (e) Except as set forth on the Schedule of Exceptions, the Company has
not engaged in the past three months in any discussion (i) with any
representative of any corporation or corporations regarding the consolidation or
merger of the Company with or into any such corporation or corporations, (ii)
with any corporation, partnership, association or other business entity or any
individual regarding the sale, conveyance or disposition of all or substantially
all of the assets of the Company, or a transaction or series of related
transactions in which more than fifty percent (50%) of the voting power of the
Company is disposed of, or (iii) regarding any other form of acquisition,
liquidation, dissolution or winding up of the Company.

     3.7 OBLIGATIONS TO RELATED PARTIES. Except as set forth on the Schedule of
Exceptions, there are no obligations of the Company to owners, directors,
shareholders, or employees of the Company other than (a) for payment of salary
for services rendered, (b) reimbursement for reasonable expenses incurred on
behalf of the Company and (c) for other standard employee benefits made
generally available to all employees (including stock option agreements
outstanding under any stock option plan approved by the Board of Directors of
the Company). None of the officers, directors or shareholders of the Company, or
any members of their immediate families, are indebted to the Company or have any
direct or indirect ownership interest in any firm or corporation with which the
Company is affiliated or with which the Company has a business relationship, or
any firm or corporation which competes with the Company (except as a holder of
securities of a publicly traded company to the extent of not more than two
percent (2%) of the issued and outstanding securities of such corporation). No
officer, director or stockholder or any member of their immediate families is,
directly or indirectly, interested in any material contract with the Company.
Except as may be disclosed in the Financial Statements, the Company is not a
guarantor or indemnitor of any indebtedness of any other person, firm or
corporation.

     3.8 CHANGES. Since September 30, 1999 there has not been to the Company's
knowledge:

         (a) Any change in the assets, liabilities, financial condition or
operations of the Company from that reflected in the Financial Statements, other
than changes in the ordinary course of business, none of which individually or
in the aggregate has had or is expected to have a material adverse effect on
such assets, liabilities, financial condition or operations of the Company;

         (b) Any resignation or termination of any key officers of the Company;
and the Company, to the best of its knowledge, does not know of the impending
resignation or termination of employment of any such officer;

         (c) Any material change, except in the ordinary course of business, in
the contingent obligations of the Company by way of guaranty, endorsement,
indemnity, warranty or otherwise;

                                       5.
<PAGE>

         (d) Any damage, destruction or loss, whether or not covered by
insurance, materially and adversely affecting the properties, business or
prospects or financial condition of the Company;

         (e) Any waiver by the Company of a valuable right or of a material debt
owed to it;

         (f) Any direct or indirect loans made by the Company to any
stockholder, employee, officer or director of the Company, other than advances
made in the ordinary course of business;

         (g) Any material change in any compensation arrangement or agreement
with any employee, officer, director or shareholder;

         (h) Any declaration or payment of any dividend or other distribution of
the assets of the Company;

         (i) Any labor organization activity;

         (j) Any debt, obligation or liability incurred, assumed or guaranteed
by the Company, except for those immaterial amounts and for current liabilities
incurred in the ordinary course of business;

         (k) Any sale, assignment or transfer of any patents, trademarks,
copyrights, trade secrets or other intangible assets;

         (l) Any change in any material agreement to which the Company is a
party or by which it is bound which materially and adversely affects the
business, assets, liabilities, financial condition, operations or prospects of
the Company, including compensation agreements with the Company's employees; or

         (m) Any other event or condition of any character that, either
individually or cumulatively, has materially and adversely affected the
business, assets, liabilities, financial condition, operations or prospects of
the Company.

     3.9 TITLE TO PROPERTIES AND ASSETS; LIENS, ETC. The Company has good and
marketable title to its properties and assets, including the properties and
assets reflected in the Financial Statements and good title to its leasehold
estates, to the Company's knowledge, in each case subject to no mortgage,
pledge, lien, lease, encumbrance or charge, other than (i) those resulting from
taxes which have not yet become delinquent, (ii) minor liens and encumbrances
which do not materially detract from the value of the property subject thereto
or materially impair the operations of the Company, and (iii) those that have
otherwise arisen in the ordinary course of business. All Facilities, machinery,
equipment, fixtures, vehicles and other properties owned, leased or used by the
Company are in good operating condition and repair and are reasonably fit and
usable for the purposes for which they are being used.

     3.10 PATENTS AND TRADEMARKS. The Company owns or possesses sufficient legal
rights to all patents, trademarks, service marks, trade names, copyrights, trade
secrets,

                                       6.
<PAGE>

information and other proprietary rights and processes ("INTELLECTUAL PROPERTY")
necessary for its business as now conducted and as proposed to be conducted,
without any known infringement of the rights of others. Except as set forth on
the Schedule of Exceptions, there are no outstanding options, licenses or
agreements of any kind relating to the foregoing, nor is the Company bound by or
a party to any options, licenses or agreements of any kind with respect to the
patents, trademarks, service marks, trade names, copyrights, trade secrets,
licenses, information and other proprietary rights and processes of any other
person or entity other than such licenses or agreements arising from the
purchase of "off the shelf" or standard products. Except as set forth on the
Schedule of Exceptions, at no time during the conception or reduction to
practice of any of the Company's Intellectual Property was any developer,
inventor or other contributor to such Intellectual Property operating under any
grants from any governmental entity or agency or private source, performing
research sponsored by any governmental entity or agency or private source or
subject to any employment agreement or invention assignment or nondisclosure
agreement or other obligation with any third party that could adversely affect
the Company's rights in such Intellectual Property. Except as set forth on the
Schedule of Exceptions, the Company is not obligated to make any payments by the
way of royalties, fees or otherwise to any owner or licensor of any patent,
trademark, trade name, copyright or other intangible asset, with respect to the
use thereof or in connection with the conduct of its business, or otherwise.
Except as set forth on the Schedule of Exceptions, the Company has not granted
to any third party any option, license or other right of any kind to its
Intellectual Property. Except as set forth on the Schedule of Exceptions, the
Company does not license any technology from any third party other than for
internal use. The Company has not received any communications alleging that the
Company has violated or, by conducting its business as proposed, would violate
any of the patents, trademarks, service marks, trade names, copyrights or trade
secrets or other proprietary rights of any other person or entity. The Company
is not aware of any violation by a third party of any of the Company's patents,
trademarks, service marks, trade names, copyrights, trade secrets, information
or other proprietary rights and processes. Except as set forth on the Schedule
of Exceptions, the Company is not aware that any of its officers, employees,
consultants, contractors or shareholders is obligated under any contract
(including licenses, covenants or commitments of any nature) or other agreement,
or subject to any judgment, decree or order of any court or administrative
agency, that would interfere with their duties to the Company or that would
conflict with the Company's business as proposed to be conducted or that would
prevent such officers, employees, consultants or contractors from assigning
inventions to the Company. Neither the execution nor delivery of this Agreement,
nor the carrying on of the Company's business by the employees or consultants of
the Company, nor the conduct of the Company's business as proposed, will, to the
Company's knowledge, conflict with or result in a breach of the terms,
conditions or provisions of, or constitute a default under, any contract,
covenant or instrument under which any employee or consultant is now obligated.
The Company does not believe it is or will be necessary to utilize any
inventions, trade secrets or proprietary information of any of its employees
made prior to their employment by the Company, except for inventions, trade
secrets or proprietary information that have been assigned or licensed to the
Company.

     3.11 COMPLIANCE WITH OTHER INSTRUMENTS. The Company is not in violation or
default of any term of its Restated Certificate or Bylaws, or of any provision
of any mortgage, indenture, contract, agreement, instrument or contract to which
it is party or by which it is bound or of any judgment, decree, order, writ or,
to its knowledge, any statute, rule or regulation

                                       7.
<PAGE>

applicable to the Company which would materially and adversely affect the
business, assets, liabilities, financial condition, operations or prospects of
the Company. The execution, delivery, and performance of and compliance with
this Agreement, the Investor Rights Agreement and the Shareholders Agreement,
and the issuance and sale of the Shares pursuant hereto and of the Conversion
Shares pursuant to the Restated Certificate, will not result in any such
material violation, or be in conflict with or constitute a default under any
such term, or result in the creation of any mortgage, pledge, lien, encumbrance
or charge upon any of the properties or assets of the Company or the suspension,
revocation impairment, forfeiture or nonrenewal of any permit license,
authorization or approval applicable to the Company, its business or operations
or any of its assets or properties.

     3.12 LITIGATION. There is no action, suit, proceeding or investigation
pending or to the Company's knowledge currently threatened against the Company
that questions the validity of this Agreement, the Investor Rights Agreement or
the Shareholders Agreement or the right of the Company to enter into any of such
agreements, or to consummate the transactions contemplated hereby or thereby, or
which might result, either individually or in the aggregate, in any material
adverse change in the assets, condition, affairs or prospects of the Company,
financially or otherwise, or any change in the current equity ownership of the
Company, nor is the Company aware that there is any basis for the foregoing. The
foregoing includes, without limitation, actions pending or threatened (or any
basis therefor known to the Company) involving the prior employment of any of
the Company's employees, their use in connection with the Company's business of
any information or techniques allegedly proprietary to any of their former
employers, or their obligations under any agreements with prior employers. The
Company is not a party or subject to the provisions of any order, writ,
injunction, judgment or decree of any court or government agency or
instrumentality. There is no action, suit, proceeding or investigation by the
Company currently pending or which the Company intends to initiate.

     3.13 TAX RETURNS AND PAYMENTS. The Company has filed all tax returns
(federal, state and local) required to be filed by it. All taxes shown to be due
and payable on such returns, any assessments imposed, and to the Company's
knowledge all other taxes due and payable by the Company on or before the
Closing have been paid or will be paid prior to the time they become delinquent.
The Company has not been advised (i) that any of its returns, federal, state or
other, have been or are being audited as of the date hereof, or (ii) of any
deficiency in assessment or proposed judgment to its federal, state or other
taxes. The Company has no knowledge of any liability of any tax to be imposed
upon its properties or assets of the date of this Agreement that is not
adequately provided for.

     3.14 EMPLOYEES. The Company has no collective bargaining agreements with
any of its employees. There is no labor union organizing activity pending or, to
the Company's knowledge, threatened with respect to the Company. No employee has
any agreement or contract, written or verbal, regarding his employment. To the
Company's knowledge, no employee of the Company, nor any consultant with whom
the Company has contracted, is in violation of any term of any employment
contract, patent disclosure agreement or any other agreement relating to the
fight of any such individual to be employed by, or to contract with, the Company
because of the nature of the business to be conducted by the Company; and to the
Company's knowledge the continued employment by the Company of its present
employees, and the performance of the Company's contracts with its independent
contractors, will not result in

                                       8.
<PAGE>

any such violation. The Company has not received any notice alleging that any
such violation has occurred. No employee of the Company has been granted the
right to continued employment by the Company or to any material compensation
following termination of employment with the Company. The Company is not aware
that any officer or key employee, or that any group of key employees, intends to
terminate their employment with the Company, nor does the Company have a present
intention to terminate the employment of any officer, key employee or group of
key employees.

     3.15 PROPRIETARY INFORMATION AND INVENTIONS AGREEMENTS. Each employee,
officer, consultant and contractor of the Company has executed either a
Proprietary Information and Inventions Agreement or a Consulting Agreement
substantially in the forms of Exhibit G-1 and Exhibit G-2 attached hereto.

     3.16 REGISTRATION RIGHTS. Except as required pursuant to the Investor
Rights Agreement and the Series B Preferred Stock Purchase Agreement proposed to
be entered into by and between the Company and those persons and entities
identified on Exhibit A thereto, the Company is presently not under any
obligation, and has not granted any rights, to register (as defined in Section
1.2 of the Investor Rights Agreement) any of the Company's presently outstanding
securities or any of its securities that may hereafter be issued.

     3.17 COMPLIANCE WITH LAWS; PERMITS. To its knowledge, the Company is not in
violation of any applicable statute, rule, regulation, order or restriction of
any domestic or foreign government or any instrumentality or agency thereof in
respect of the conduct of its business or the ownership of its properties which
violation would materially and adversely affect the business, assets,
liabilities, financial condition operations or prospects of the Company. No
governmental orders, permissions, consents, approvals or authorizations are
required to be obtained and no registrations or declarations are required to be
filed in connection with the execution and delivery of this Agreement and the
issuance of the Shares or the Conversion Shares, except such as has been duly
and validly obtained or filed, or with respect to any filings that must be made
after the Closing, as will be filed in a timely manner. The Company has all
franchises, permits, licenses and any similar authority necessary for the
conduct of its business as now being conducted by it, the lack of which could
materially and adversely affect the business, properties, prospects or financial
condition of the Company and believes it can obtain, without undue burden or
expense, any similar authority for the conduct of its business as planned to be
conducted.

     3.18 ENVIRONMENTAL AND SAFETY LAWS. To its knowledge, the Company is not in
violation of any applicable statute, law or regulation relating to the
environment or occupational health and safety, and to its knowledge, no material
expenditures are or will be required in order to comply with any such existing
statute, law or regulation.

     3.19 OFFERING VALID. Assuming the accuracy of the representations and
warranties of the Purchasers contained in Section 5.2 hereof, the offer, sale
and issuance of the Shares and the Conversion Shares will be exempt from the
registration requirements of the Securities Act of 1933, as amended (the
"SECURITIES ACT") and will have been registered or qualified (or are exempt from
registration and qualification) under the registration, permit or qualification
requirements of all applicable state securities laws. Neither the Company nor
any agent on its

                                       9.
<PAGE>

behalf has solicited or will solicit any offers to sell or has offered to sell
or will offer to sell all or any part of the Shares to any person or persons so
as to bring the sale of such Shares by the Company within the registration
provisions of the Securities Act.

     3.20 FULL DISCLOSURE. The Company has provided each Purchaser with all
information which such Purchaser has requested for deciding whether to purchase
the Shares and all information which the Company believes is reasonably
necessary to enable the Purchasers to make such decision. This Agreement, the
Exhibits hereto, the Investor Rights Agreement, the Shareholders Agreement and
all other documents delivered by the Company to Purchasers or their attorneys or
agents in connection herewith or therewith or with the transactions contemplated
hereby or thereby, do not contain any untrue statement of a material fact nor,
to the Company's knowledge, omit to state a material fact necessary in order to
make the statements contained herein or therein not misleading. To the Company's
knowledge, there are no facts which (individually or in the aggregate)
materially adversely affect the business, assets, liabilities, financial
condition, prospects or operations of the Company that have not been set forth
in the Agreement, the Exhibits hereto, or in other documents delivered to
Purchasers or their attorneys or agents in connection herewith.

     3.21 CORPORATE DOCUMENTS; MINUTE BOOKS. The Certificate and Bylaws of the
Company are in the form previously provided to special counsel to the
Purchasers. The minute books of the Company provided to the Purchaser's legal
counsel contain a complete minutes of all meetings of and records of all actions
taken by directors and shareholders since the time of incorporation.

     3.22 REAL PROPERTY HOLDING CORPORATION. The Company is not a real property
holding corporation within the meaning of Internal Revenue Code Section
897(c)(2) and any regulations promulgated thereunder.

     3.23 MANUFACTURING AND MARKETING RIGHTS. The Company has not granted rights
to manufacture, produce, assemble, license, market or sell its products to any
third party, and is not bound by any agreement that affects the Company's
exclusive right to develop, manufacture, assemble, distribute, market or sell
its products.

     3.24 VOTING AGREEMENTS. Except as set forth in the Schedule of Exceptions,
the Shareholders Agreement, the Restated Certificate or the Investors Rights
Agreement, the Company has no agreement, obligation or commitment with respect
to the election of any individual or individuals to the Board of Directors, and
to the best of the Company's knowledge, there is no voting agreement or other
arrangement among its stockholders with respect to the election of any
individual or individuals to the Board of Directors.

     3.25 INSURANCE. The Company has in full force and effect fire and casualty
insurance policies, with extended coverage, and insurance against other hazards,
risks and liabilities to persons and property to the extent and in the manner
customary for companies in similar businesses similarly situated.

                                      10.
<PAGE>

     3.26 DISTRIBUTIONS. There has been no declaration or payment by the Company
of any dividend, nor any distribution by the Company of any assets of any kind,
to any class or series of its capital stock.

     3.27 BROKERS OR FINDERS. No broker or finder is entitled to any brokerage
or other fee from the Company based upon arrangements made by or on behalf of
the Company in connection with the transactions contemplated by this Agreement

4. REPRESENTATIONS AND WARRANTIES OF THE PURCHASERS

     Each Purchaser hereby represents and warrants to the Company as follows
(such representations and warranties do not lessen or obviate the
representations and warranties of the Company set forth in this Agreement):

     4.1 REQUISITE POWER AND AUTHORITY. Purchaser has all necessary power and
authority under all applicable provisions of law to execute and deliver this
Agreement, the Investor Rights Agreement and the Shareholders Agreement and to
carry out their provisions. All action on Purchaser's part required for the
lawful execution and delivery of this Agreement, the Investor Rights Agreement
and the Shareholders Agreement have been or will be effectively taken prior to
the Closing. Upon their execution and delivery, this Agreement, the Investor
Rights Agreement and the Shareholders Agreement will be valid and binding
obligations of Purchaser, enforceable in accordance with their terms, except (i)
as limited by applicable bankruptcy, insolvency, reorganization, moratorium or
other laws of general application affecting enforcement of creditors' rights,
(ii) general principles of equity that restrict the availability of equitable
remedies, and (iii) to the extent that the enforceability of the indemnification
provisions of Section 2.9 of the Investor Rights Agreement may be limited by
applicable laws.

     4.2 INVESTMENT REPRESENTATIONS. Purchaser understands that neither the
Shares nor the Conversion Shares have been registered under the Securities Act,
Purchaser also understands that the Shares are being offered and sold pursuant
to an exemption from registration contained in the Securities Act based in part
upon Purchaser's representations contained in the Agreement. Purchaser hereby
represents and warrants as follows:

         (a) PURCHASER BEARS ECONOMIC RISK. Purchaser has substantial experience
in evaluating and investing in private placement transactions of securities in
companies similar to the Company so that it is capable of evaluating the merits
and risks of its investment in the Company and has the capacity to protect its
own interests. Purchaser must bear the economic risk of this investment
indefinitely unless the Shares (or the Conversion Shares) are registered
pursuant to the Securities Act, or an exemption from registration is available.
Purchaser understands that the Company has no present intention of registering
the Shares, the Conversion Shares or any shares of its Common Stock. Purchaser
also understands that there is no assurance that any exemption from registration
under the Securities Act will be available and that, even if available, such
exemption may not allow Purchaser to transfer all or any portion of the Shares
or the Conversion Shares under the circumstances, in the amounts or at the times
Purchaser might propose.

                                      11.
<PAGE>

         (b) ACQUISITION FOR OWN ACCOUNT. Purchaser is acquiring the Shares and
the Conversion Shares for Purchaser's own distribution.

         (c) PURCHASER CAN PROTECT ITS INTEREST. Purchaser represents that by
reason of its, or of its management's, business or financial experience,
Purchaser has the capacity to protect its own interests in connection with the
transactions contemplated in this Agreement, the Investor Rights Agreement and
the Shareholders Agreement. Further, Purchaser is aware of no publication of any
advertisement in connection with the transactions contemplated in the Agreement.

         (d) ACCREDITED INVESTOR. Purchaser represents that it is an accredited
investor within the meaning of Regulation D under the Securities Act.

         (e) COMPANY INFORMATION. Purchaser has received and read the Financial
Statements and has had an opportunity to discuss the Company's business,
management and financial affairs with directors, officers and management of the
Company and has had the opportunity to review the Company's operations and
facilities. Purchaser has also had the opportunity to ask questions of and
receive answers from, the Company and its management regarding the terms and
conditions of this investment.

         (f) RULE 144. Purchaser acknowledges and agrees that the Shares, and,
if issued, the Conversion Shares must be held indefinitely unless they are
subsequently registered under the Securities Act or an exemption from such
registration is available. Purchaser has been advised or is aware of the
provisions of Rule 144 promulgated under the Securities Act, which permits
limited resale of shares purchased in a private placement subject to the
satisfaction of certain conditions, including, among other things, the
availability of certain current public information about the Company, the resale
occurring not less than one year after a party has purchased and paid for the
security to be sold, the sale being through an unsolicited brokers transaction
or in transactions directly with a market maker (as defined under the Securities
Exchange Act of 1934, as amended) and the number of shares being sold during any
three-month period not exceeding specified limitations.

     4.3 TRANSFER RESTRICTIONS. Each Purchaser acknowledges and agrees that the
Shares and, if issued, the Conversion Shares, are subject to restrictions on
transfer as set forth in the Investor Rights Agreement.

5. CONDITIONS TO CLOSING

     5.1 CONDITIONS TO PURCHASERS' OBLIGATIONS AT THE FIRST CLOSING. Purchasers'
obligations to purchase the Shares at the First Closing are subject to the
satisfaction, at or prior to the First Closing, of the following conditions:

         (a) REPRESENTATIONS AND WARRANTIES TRUE; PERFORMANCE OF OBLIGATIONS.
The representations and warranties made by the Company in Section 3 hereof shall
be true and correct in all material respects as of the First Closing Date with
the same force and effect as if they had been made as of such First Closing
Date, and the Company shall have performed all obligations and conditions herein
required to be performed or observed by it on or prior to the First Closing.

                                      12.
<PAGE>

         (b) LEGAL INVESTMENT. On the First Closing Date, the sale and issuance
of the Shares and the proposed issuance of the Conversion Shares shall be
legally permitted by all laws and regulations to which Purchasers and the
Company are subject.

         (c) CONSENTS, PERMITS, AND WAIVERS. The Company shall have obtained any
and all consents, permits and waivers necessary or appropriate for consummation
of the transactions contemplated by the Agreement, the Investor Rights Agreement
and the Shareholders Agreement (except for such as may be properly obtained
subsequent to each Closing).

         (d) CORPORATE DOCUMENTS. The Company shall have delivered to Purchasers
or their counsel, copies of all corporate documents of the Company as Purchasers
shall reasonably request.

         (e) COMPLIANCE CERTIFICATE. The Company shall have delivered to
Purchasers a Compliance Certificate, executed by the President of the Company,
dated the date of the First Closing, to the effect that the conditions specified
in subsections (a) through (n) of this Section 5.1 have been satisfied. The
Company shall have delivered to Purchasers a Secretary's Certificate, executed
by the Secretary of the Company, dated as of the First Closing Date, in form and
substance reasonably satisfactory to Purchasers.

         (f) RESERVATION OF CONVERSION SHARES. The Conversion Shares issuable
upon conversion of the Shares shall have been duly authorized and reserved for
issuance upon such conversion.

         (g) INVESTOR RIGHTS AGREEMENT. The Investor Rights Agreement shall have
been executed and delivered by the parties thereto.

         (h) SHAREHOLDERS AGREEMENT. The Shareholders Agreement shall have been
executed and delivered by the parties thereto.

         (i) BOARD OF DIRECTORS. Upon the Closing, the authorized size of the
Board of Directors of the Company shall be seven (7) members as set forth in
Shareholders Agreement.

         (j) RESTATED CERTIFICATE. The Company shall have filed with the
Secretary of State of the State of Delaware the Restated Certificate.

         (k) LEGAL OPINION. The Purchasers shall have received from legal
counsel to the Company an opinion addressed to them, dated as of the First
Closing, in substantially the form attached hereto as Exhibit H.

         (l) PROCEEDINGS AND DOCUMENTS. All corporate and other proceedings in
connection with the transactions contemplated at the First Closing hereby and
all documents and instruments incident to such transactions shall be reasonably
satisfactory in substance and form to the Purchasers and their special counsel
and the Purchasers and their special counsel shall have received all such
counterpart originals or certified or other copies of such documents as they may
reasonably request.

                                      13.
<PAGE>

         (m) KEY PERSON LIFE INSURANCE. The Company agrees to use its best
efforts to obtain from financially sound and reputable insurers term life
insurance on the life of J. William Freytag in the amount of $2,000,000. Once
obtained, the Company will use its best efforts to maintain the term life
insurance required by this Section 5.1(m). Such insurance policies shall name
the Company as loss payee and shall not be cancelable by the Company without the
prior approval of the Board of Directors.

         (n) DUE DILIGENCE. The Purchasers shall (i) have completed their due
diligence of the Company, and (ii) be satisfied, in their sole discretion, with
the results of such due diligence review.

     5.2 CONDITIONS TO PURCHASERS' OBLIGATIONS AT THE SECOND CLOSING.
Purchasers' obligations to purchase the Shares at the Second Closing are subject
to the satisfaction, at or prior to the Second Closing, of the following
conditions:

         (a) EXECUTION OF THE AMENDMENT TO THE HMR AGREEMENT. A majority of the
Company's Board of Directors must have approved an amendment to the HMR
Agreement which cures the Company's default under the HMR Agreement and such
amendment shall have been executed by all necessary parties thereto and there
shall have been no Material Adverse Development with respect to the Company.

         (b) LEGAL INVESTMENT. On the Second Closing Date, the sale and issuance
of the Shares and the proposed issuance of the Conversion Shares shall be
legally permitted by all laws and regulations to which Purchasers and the
Company are subject.

         (c) COMPLIANCE CERTIFICATE. The Company shall have delivered to
Purchasers a Compliance Certificate, executed by the President of the Company,
dated the date of the Second Closing, to the effect that the conditions
specified in subsections (a) through (e) of this Section 5.2 have been
satisfied.

         (d) LEGAL OPINION. The Purchasers shall have received from legal
counsel to the Company an opinion addressed to them, dated as of the Second
Closing, in substantially the form attached hereto as Exhibit H except for such
changes as may be required to update such opinion to the facts concerning the
Company or changes to applicable law which may occur prior to the Second
Closing.

         (e) PROCEEDINGS AND DOCUMENTS. All corporate and other proceedings in
connection with the transactions contemplated at the Second Closing hereby and
all documents and instruments incident to such transactions shall be reasonably
satisfactory in substance and form to the Purchasers and their special counsel
and the Purchasers and their special counsel shall have received all such
counterpart originals or certified or other copies of such documents as they may
reasonably request.

                                      14.
<PAGE>

     5.3 CONDITIONS TO OBLIGATIONS OF THE COMPANY. The Company's obligation to
issue and sell the Shares at each Closing is subject to the satisfaction, on or
prior to the Closing, of the following conditions:

         (a) REPRESENTATIONS AND WARRANTIES TRUE. The representations and
warranties made by Purchasers in Section 4 hereof shall be true and correct in
all material respects at the date of each Closing, with the same force and
effect as if they had been made on and as of said date.

         (b) PERFORMANCE OF OBLIGATIONS. Purchasers shall have performed and
complied with all agreements and conditions herein required to be performed or
complied with by Purchasers on or before the Closing.

         (c) INVESTOR RIGHTS AGREEMENT. An Investor Rights Agreement
substantially in the form attached hereto as Exhibit D shall have been executed
and delivered by the Purchasers.

         (d) CONSENTS, PERMITS, AND WAIVERS. The Company shall have obtained any
and all consents, permits and waivers necessary or appropriate for consummation
of the transactions contemplated by the Agreement, the Investor Rights Agreement
and the Shareholders Agreement (except for such as may be properly obtained
subsequent to the Closing).

6. MISCELLANEOUS

     6.1 GOVERNING LAW. This Agreement shall be governed in all respects by the
laws of the State of Colorado as such laws are applied to agreements between
Colorado residents entered into and performed entirely in Colorado.

     6.2 SURVIVAL. The representations, warranties, covenants and agreements
made herein shall survive any investigation made by any Purchaser and the
closing of the transactions contemplated hereby. All statements as to factual
matters contained in any certificate or other instrument delivered by or on
behalf of the Company pursuant hereto in connection with the transactions
contemplated hereby shall be deemed to be representations and warranties by the
Company hereunder solely as of the date of such certificate or instrument.

     6.3 SUCCESSORS AND ASSIGNS. Except as otherwise expressly provided herein,
the provisions hereof shall inure to the benefit of, and be binding upon, the
successors, assigns, heirs, executors and administrators of the parties hereto
and shall inure to the benefit of and be enforceable by each person who shall be
a holder of the Shares from time to time.

     6.4 ENTIRE AGREEMENT. This Agreement, the Exhibits and Schedules hereto,
the Investor Rights Agreement and the Shareholders Agreement and the other
documents delivered pursuant hereto constitute the full and entire understanding
and agreement between the parties with regard to the subjects hereof and no
party shall be liable or bound to any other in any manner by any
representations, warranties, covenants and agreements except as specifically set
forth herein and therein.

                                      15.
<PAGE>

     6.5 SEVERABILITY. In case any provision of the Agreement shall be invalid,
illegal or unenforceable, the validity, legality and enforceability of the
remaining provisions shall not in any way be affected or impaired thereby.

     6.6 AMENDMENT AND WAIVER.

         (a) This Agreement may be amended or modified only upon the written
consent of the Company and holders of more than sixty-five percent (65%) of the
holders of Series C Stock sold pursuant to this Agreement (treated as if
converted and including any Shares issued upon the conversion of Series C Stock
into Common Stock that have been converted that have not been sold to the
public).

         (b) The obligations of the Company and the rights of the holders of the
Shares and the Conversion Shares under the Agreement may be waived only with the
written consent of the holders of more than sixty-five percent (65%) of the
holders of Series C Stock sold pursuant to this Agreement (treated as if
converted and including any Shares issued upon the conversion of Series C Stock
into Common Stock that have not been sold to the public).

     6.7 DELAYS OR OMISSIONS. It is agreed that no delay or omission to exercise
any right, power or remedy accruing to any party, upon any breach, default or
noncompliance by another party under this Agreement, the Investor Rights
Agreement and the Shareholders Agreement or the Restated Certificate, shall
impair any such right, power or remedy, nor shall it be construed to be a waiver
of any such breach, default or noncompliance, or any acquiescence therein, or of
or in any similar breach, default or noncompliance thereafter occurring. It is
further agreed that any waiver, permit, consent or approval of any kind or
character on any Purchaser's part of any breach, default or noncompliance under
this Agreement or under the Restated Certificate or any waiver on such party's
part of any provisions or conditions of the Agreement must be in writing and
shall be effective only to the extent specifically set forth in such writing.
All remedies, either under this Agreement, the Restated Certificate, by-law or
otherwise afforded to any party, shall be cumulative and not alternative.

     6.8 NOTICES. All notices required or permitted hereunder shall be in
writing and shall be deemed effectively given: (i) upon personal delivery to the
party to be notified; (ii) when sent by confirmed facsimile if sent during
normal business hours of the recipient, or if not, then on the next business
day; (iii) five (5) days after having been sent by registered or certified mail,
return receipt requested, postage prepaid; or (iv) one (1) day after deposit
with a nationally recognized overnight courier, specifying next day delivery,
with written verification of receipt. All communications shall be sent to the
Company at the address as set forth on the signature page hereof and to
Purchaser at the address set forth on Exhibit A attached hereto or at such other
address as the Company or Purchaser may designate by ten (10) days advance
written notice to the other parties hereto.

     6.9 EXPENSES. The Company shall pay all costs and expenses that it incurs
with respect to the negotiation, execution, delivery and performance of the
Agreement. The Company shall, at the Closing, reimburse the reasonable fees and
expenses, not to exceed $15,000, of Georgopoulos & Pahlavan, LLP, special
counsel for the Purchasers, incurred in connection with the negotiation,
execution, delivery and performance of this Agreement.

                                      16.
<PAGE>

     6.10 ATTORNEYS' FEES. In the event that any dispute among the parties to
this Agreement should result in litigation, the prevailing party in such dispute
shall be entitled to recover from the losing party all fees, costs and expenses
of enforcing any right of such prevailing party under or with respect to this
Agreement, including without limitation, such reasonable fees and expenses of
attorneys and accountants, which shall include, without limitation, all fees,
costs and expenses of appeals.

     6.11 TITLES AND SUBTITLES. The titles of the sections and subsections of
the Agreement are for convenience of reference only and are not to be considered
in construing this Agreement.

     6.12 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

     6.13 BROKER'S FEES. Each party hereto represents and warrants that no
agent, broker, investment banker, person or firm acting on behalf of or under
the authority of such party hereto is or will be entitled to any broker's or
finder's fee or any other commission directly or indirectly in connection with
the transactions contemplated herein. Each party hereto further agrees to
indemnify each other party for any claims, losses or expenses incurred by such
other party as a result of the representation in this Section 6.13 being untrue

     6.14 COUNTERPARTS. This Agreement may be executed in any number of
counterparts, each of which shall be an original, but all of which together
shall constitute one instrument.

              [THE REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK)

                                      17.
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this SERIES C
PREFERRED STOCK PURCHASE AGREEMENT as of the date set forth in the first
paragraph hereof.

COMPANY                                INVESTORS:

MYOGEN, INC.                           SEQUEL LIMITED PARTNERSHIP

                                       By: Sequel Venture Partners, LLC
                                           its General Partner

By: /s/ J. William Freytag             By: /s/ Dan Mitchell
    -------------------------------        --------------------------------
    J. William Freytag                     Manager
    President

                                       SEQUEL EURO LIMITED PARTNERSHIP

                                       By: Sequel Venture Partners, LLC
                                           its General Partner

                                       By: /s/ Dan Mitchell
                                           --------------------------------
                                           Manager

                                       CROSSPOINT VENTURE PARTNERS 1997

                                       By: /s/ Donald B. Milder
                                           --------------------------------
                                           its General Partner

                                       By:
                                           --------------------------------

                                       INTERWEST PARTNERS VI, LP

                                       By: /s/ Arnold Oronsky
                                           --------------------------------
                                           General Partner

                   SERIES C PREFERRED STOCK PURCHASE AGREEMENT
<PAGE>

                                       INTERWEST INVESTORS VI, LP

                                       By: /s/ Arnold Oronsky
                                           --------------------------------
                                           General Partner

                                       NEW ENTERPRISE ASSOCIATES IX,
                                       LIMITED PARTNERSHIP

                                       By: NEA Partners IX, Limited Partnership

                                       By: /s/ Signature Illegible
                                           --------------------------------
                                           General Partner

                                       NEA PRESIDENTS FUND, L.P.

                                       By: NEA General Partners, L.P.

                                       By: /s/ Signature Illegible
                                           --------------------------------
                                           General Partner

                                       NEA VENTURES 1999, LIMITED PARTNERSHIP

                                       By: /s/ Susie Greathouse
                                           --------------------------------
                                           Vice President

                                       NEW VENTURE PARTNERS IV L.P.

                                       By: /s/ Signature Illegible
                                           --------------------------------
                                           General Partner

                                       CMEA LIFE SCIENCES FUND, L.P.

                                       By: /s/ Thomas R. Baruch
                                           --------------------------------
                                           General Partner

                   SERIES C PREFERRED STOCK PURCHASE AGREEMENT
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this SERIES C
PREFERRED STOCK PURCHASE AGREEMENT as of this 8th day of December.

                                       INVESTORS:

                                       SEQUEL LIMITED PARTNERSHIP

                                       By: Sequel Venture Partners, LLC
                                           its General Partner

                                       By: /s/ Dan Mitchell
                                           --------------------------------
                                           Manager

                                       SEQUEL EURO LIMITED PARTNERSHIP

                                       By: Sequel Venture Partners, LLC
                                           its General Partner

                                       By: /s/ Dan Mitchell
                                           --------------------------------
                                           Manager

                                       CROSSPOINT VENTURE PARTNERS 1997

                                       By: /s/ Donald B. Milder
                                           --------------------------------
                                           its General Partner

                                       By:
                                           --------------------------------

                                       INTERWEST PARTNERS VI, LP

                                       By: /s/ Arnold Oronsky
                                           --------------------------------
                                           General Partner

                     SERIES C SECOND CLOSING SIGNATURE PAGES
                            STOCK PURCHASE AGREEMENT
<PAGE>

                                       INTERWEST INVESTORS VI, LP

                                       By: /s/ Arnold Oronsky
                                           -----------------------------------
                                           General Partner

                                       NEW ENTERPRISE ASSOCIATES 9,
                                       LIMITED PARTNERSHIP

                                       By: NEA Partners 9, Limited Partnership

                                       By: /s/ Signature Illegible
                                           -----------------------------------
                                           General Partner

                                       NEW VENTURE PARTNERS IV L.P.

                                       By: /s/ Signature Illegible
                                           -----------------------------------
                                           General Partner

                                       CMEA LIFE SCIENCES FUND, L.P.

                                       By: /s/ Signature Illegible
                                           -----------------------------------
                                           General Partner

                     SERIES C SECOND CLOSING SIGNATURE PAGES
                            STOCK PURCHASE AGREEMENT
<PAGE>

                                    EXHIBIT A

                             SCHEDULE OF PURCHASERS

<Table>
<Caption>
                                                           FIRST CLOSING                       SECOND CLOSING
                                                      ---------------------------         ---------------------------
         NAME AND ADDRESS                              SHARES          PRICE               SHARES          PRICE
         ----------------                             ---------     -------------         ---------     -------------

<S>                                                   <C>           <C>                   <C>           <C>
New Enterprise Associates 9, Limited                  2,887,274     $3,970,001.62         2,887,274     $3,970,001.62
Partnership
2490 Sand Hill Road
Menlo Park, CA 94025

NEA Ventures 1999, Limited Partnership                    3,636          4,999.50             3,636          4,999.50
2490 Sand Hill Road
Menlo Park, CA 94025

NEA Presidents Fund, L.P.                                18,181         24,998.88            18,181         24,998.88
2490 Sand Hill Road
Menlo Park, CA 94025

Sequel Limited Partnership                              442,719        608,738.63           442,719        608,738.63
4430 Arapahoe Avenue, Suite 220
Boulder, CO 80303

Sequel Euro Limited Partnership                         193,644        266,260.50           193,644        266,260.50
4430 Arapahoe Avenue, Suite 220
Boulder, CO 80303

Crosspoint Venture Partners 1997                      1,090,910      1,500,001.25         1,090,910      1,500,001.25
18552 MacArthur Boulevard, Suite 400
Irvine, CA 92612

InterWest Partners VI, L.P.                             881,455      1,212,000.63           881,455      1,212,000.63
3000 Sand Hill Road
Building 3, Suite 255
Menlo Park, CA 94025

InterWest Investors VI, L.P.                             27,636         37,999.50            27,636         37,999.50
3000 Sand Hill Road
Building 3, Suite 255
Menlo Park, CA 94025

New Venture Partners IV L.P.                            272,728        375,001.00           272,728        375,001.00
1119 St. Paul Street
Baltimore, MD 21202

CMEA Life Sciences Fund, L.P.                           727,272        999,999.00           727,272        999,999.00
235 Montgomery Street, Suite 920
San Francisco, CA 94104
Attn: Meryl L. Schreibstein
                                                      ---------     -------------         ---------     -------------
                                       TOTAL:         6,545,455     $9,000,000.51         6,545,455     $9,000,000.51
</Table>

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