<PAGE>

                                                                Exhibit 10.10


                              EMPLOYMENT AGREEMENT

     THIS EMPLOYMENT AGREEMENT ("AGREEMENT") is made effective as of the 1st day
of November, 1999, by and between Photogen, Inc., a Tennessee corporation
("COMPANY"), and David D. Shaw, Ph.D. ("EMPLOYEE").

                                    RECITALS

     A.   Employee has substantial experience in development of medical
therapeutics and imaging agents; and

     B.   The Company desires to retain Employee and Employee hereby agrees to
serve in such capacity to assist it in various research projects, developing
product inventions and related matters, as hereinafter provided; subject to the
terms of this Agreement.

                                    AGREEMENT

     NOW, THEREFORE, in consideration of the mutual promises herein and for
other good and valuable consideration, the receipt and adequacy of which are
hereby acknowledged, the parties agree as follows:

     1.   DEFINITIONS. The following terms have the meanings set forth below:

     (a)  "COMPANY" means all affiliates of Photogen, Inc., including but not
          limited to Photogen Technologies, Inc. and Photogen Newco Ltd., except
          in the context of the entity directly employing Employee which shall
          mean Photogen, Inc.

     (b)  "COMPETITIVE BUSINESS" means engaging in the research, development,
          sale, lease, marketing, financing or distribution of technology,
          products or services similar to or competitive with the Company's
          products or services in the Field anywhere in the world.

     (c)  "CONFIDENTIALITY AGREEMENT" means that certain Employee
          Confidentiality and Inventions Agreement of even date herewith between
          the Employee and the Company attached hereto as Exhibit 4 and
          incorporated herein in its entirety by reference.

     (d)  "CONFIDENTIAL INFORMATION" has that meaning set forth in the
          Confidentiality Agreement.

     (e)  "FIELD" means the use of electromagnetic energy (including light
          generated by ultra fast lasers or other sources, x-rays and other
          sources of radiation), alone or in combination with photoactive
          compounds or targeting agents, to treat, diagnose or image human or
          animal tissue and/or disease.

<PAGE>

     (f)  "PATENTS" means patents, patent applications, and patent disclosures,
          together with all reissuances, continuations, continuations-in-part,
          revisions, extensions, and reexaminations thereof, and all
          improvements and inventions related thereto.

     2.   EMPLOYMENT SERVICES.

     (a)  During the Term of this Agreement, Employee shall use his best efforts
          to and shall devote 100% of his working time, except as provided in
          Section 4(b), to competently and faithfully promote the Company's
          interests, develop the Company's products and to perform the following
          services (collectively, the "EMPLOYMENT SERVICES"):

          (i)  assist the Company's Medical Director to direct, design,
     implement and complete, the preclinical and clinical activities for
     specific projects mutually agreed to between the Company and Employee,
     including but not limited to the following: (A) sentinel lymph node
     mapping, (B) enhanced radiation therapy through the use of
     radiosensitizers, (C) photodynamic therapy of deep tissue cancer, (D)
     evaluation of hair removal, psoriasis, and leg ulcer products; (E) new
     treatments for Barretts esophagus, and (F) other research projects assigned
     by the Company (individually and collectively, "RESEARCH PROJECTS");

          (ii) design, implement and complete clinical trial programs sufficient
     to obtain regulatory approval for sentinel node mapping and diagnostic
     products;

          (iii) design, implement and complete pre-clinical trials sufficient to
     evaluate PH-10 as a radiosensitizer for the treatment of various cancers;

          (iv) negotiate, prepare and monitor clinical contracts and Research
     Projects;

          (v) assist the Medical Director in developing new product inventions
     for the Company;

          (vi) design and assist in human clinical trials undertaken by the
     Company;

          (vii) assist the Company in obtaining patent or other protection for
     any technologies developed by Employee and arising out of his Employment
     Services ;

          (viii) assist the Company in obtaining licenses to technologies
     developed by third parties;


                                      -2-
<PAGE>

          (ix) assist Company in working with outside preclinical and clinical
     researchers and research organizations; and

          (x) perform such other duties assigned to Employee from time to time
     by the Company's Medical Director, the Company's Chief Executive Officer
     and/or Board of Directors.

     (b)  Employee will perform the Employment Services substantially in the
          Boston, Massachusetts area. Employee may also be required to travel
          from time to time to Knoxville, Tennessee and elsewhere as reasonably
          necessary to perform the Employment Services.

     3.   COMPENSATION.

     (a)  Company shall pay Employee an annual base gross salary as set forth in
          Exhibit 1, payable in accordance with normal Company payroll
          procedures and subject to deductions and withholdings required by law
          or by agreement with Employee. Company will reimburse Employee for
          reasonable and necessary business expenses in accordance with
          Company's policies.

     (b)  Concurrently with the execution of this Agreement, Company and
          Employee will execute an Incentive Stock Option Award Agreement
          ("Award Agreement") providing for the Company to grant Employee a
          total of 150,000 options to acquire Company common stock, pursuant to
          Company's 1998 Long Term Incentive Compensation Plan. The options will
          vest in accordance with and in all other respects be subject to the
          provisions of the Award Agreement.

     (c)  Employee will be eligible to participate in and receive benefits
          provided under Company's insurance and benefit plans as set forth on
          Exhibit 1, in accordance with their respective provisions. Company
          reserves the right to amend all insurance and benefit plans at any
          time as necessary to accomplish Company's business objectives.

     4.   COVENANTS AND REPRESENTATIONS. Employee represents and warrants to,
and covenants with, the Company as follows:

     (a)  During the Term, Employee will render services (similar to the
          Employment Services or otherwise) in the Field only to the Company and
          will not enter into any other agreement, arrangement, understanding,
          or other relationship pursuant to which Employee is obligated to
          render advice and services in the Field to any third party.


                                      -3-
<PAGE>

     (b)  Employee is under no contractual or other obligation or restriction
          which is inconsistent with Employee's obligations under this Agreement
          or the performance of the Employment Services. Employee's pre-existing
          relationships with commercial, educational or other research
          institutions are listed in Exhibit 2 attached hereto. Employee will
          terminate all employment, consulting and similar affiliations with
          commercial entities in the Field that are in competition with Company.
          Notwithstanding the terms and conditions of the Confidentiality
          Agreement, Employee may act as a consultant for a maximum of three (3)
          days per month inside or outside the Field with any entity that is not
          in competition with the Company. Employee will arrange to perform any
          responsibilities to third parties in a manner that will not conflict
          with Employee's responsibilities under this Agreement. Employee agrees
          to indemnify, defend and hold Company and its affiliates, directors,
          officers, employees and independent contractors harmless from all
          claim, allegation, loss, liability (including settlement payments),
          expenses (including reasonable attorneys' fees and expenses) any of
          them incurs that arise out of or relate to (i) any wrongful act or
          omission of Employee while he is engaged in such activities with third
          parties, or (ii) the acts or omissions of those third parties.

     (c)  Employee has set forth on Exhibit 3 attached hereto, a complete list
          of any patents or patent applications in the Field of which Employee
          is the joint or sole inventor.

     5.   CONFIDENTIALITY.

     (a)  The Company has developed and will develop Confidential Information
          over a substantial period of time and at substantial expense. The
          Confidential Information is of great importance to the Company's
          business. During the Term, Employee may develop, become aware of, or
          have access to the Confidential Information. In consideration of the
          foregoing and as an inducement for Company to extend employment to
          Employee hereunder, Employee has executed the Confidentiality
          Agreement and Employee acknowledges and agrees the Company is and
          shall at all times remain the sole owner of the Confidential
          Information.

     (b)  During the Term and thereafter, Employee shall exercise all
          commercially reasonable precautions to physically protect the
          integrity and confidentiality of the Confidential Information.
          Employee will not remove any Confidential Information or copies
          thereof from the Company's premises or the other places Employee is
          authorized to perform the Employment Services to the extent necessary
          to perform such Employment Services, and then only with the Company's
          prior written consent.


                                      -4-
<PAGE>

     (c)  Employee will comply with Company's policies concerning publication
          with respect to his work for the Company.

     6.   NONCOMPETITION; NONSOLICITATION.

     (a)  During the Term, and for 12 months after termination of the Term, the
          Employee agrees not to, directly or indirectly through any other
          person or entity, (i) own, manage, control, participate in, consult
          with, be employed by, render services for, any person or entity
          engaged in a Competitive Business or in any manner or in any capacity
          (except as owner of 2% or less of stock of a publicly registered and
          traded entity) engage in any Competitive Business, (ii) solicit,
          induce or attempt to influence any other person or entity to engage in
          any Competitive Business or to curtail or cease any business or
          business relationship with the Company, its affiliates, employees or
          independent contractors, (iii) solicit any other employee or
          independent contractor to terminate any employment or engagement with
          the Company and engage in a Competitive Business, or (iv) disparage
          the Company, its affiliates, employees, independent contractors or
          their services or products.

     (b)  The Company consents to Employee performing services for the entities
          listed in paragraph 4(b), provided that (i) Employee keeps the Company
          advised on a current basis of his activities for such other entities
          (or, to the extent Employee is unable to keep the Company advised
          because of a confidentiality agreement with such other entity,
          Employee will disclose as much information as possible without
          breaching the confidentiality agreement and seek the other entity's
          permission to make further limited disclosure to the Company), (ii)
          Employee strictly observes the confidentiality obligations in
          paragraph 7, above, and (iii) Employee uses his best efforts to assure
          that those other entities do not obtain any interest in Developments
          or the Company's Patents. Upon written notice by one party hereto to
          the other of a conflict situation, Employee and the Company will
          negotiate in good faith for a period of 30 days (or longer by mutual
          agreement) to modify Employee's responsibilities under this Agreement
          or to develop other protections and procedures to minimize the
          possibility of competitive conflict, the disclosure of Confidential
          Information and uncertainty as to the Company's ownership of
          Developments or the Company's Patents. If Employee and the Company are
          unable to agree on such modifications, either of them may terminate
          this Agreement upon written notice to the other.


                                      -5-
<PAGE>

     7.   TERM AND TERMINATION.

     (a)  Unless terminated earlier pursuant to paragraph 7(b) below, this
          Agreement shall be in effect for an initial term of four years
          beginning on the effective date hereof (the "INITIAL TERM"). This
          Agreement may be renewed for successive renewal terms lasting for one
          year (the "RENEWAL TERMS") upon mutual written agreement of the
          parties. The Initial Term and any subsequent Renewal Terms are
          collectively referred to as the "TERM."

     (b)  This Agreement and Employee's employment by the Company may be
          terminated before the expiration of any Term as follows:

          (i)  By the Company in the event:

               (1)  Employee commits a material breach of this Agreement where
                    such breach, if curable, is not remedied to the Company's
                    reasonable satisfaction within thirty (30) days after
                    written notice to Employee (and termination shall be
                    effective as of the end of such 30-day period); or

               (2)  Employee is convicted for committing an act of fraud,
                    embezzlement, theft or another act constituting a felony
                    (and termination shall be effective upon written notice to
                    Employee); or

               (3)  Employee dies or becomes mentally or physically disabled
                    such that the Employee cannot, in the opinion of an
                    independent physician selected by the Company, perform the
                    Employment Services (with reasonable accommodation to the
                    extent required by law) for a period of 12 months (and
                    termination shall be effective on the date Employee dies or
                    upon written notice the Company has determined he is
                    disabled under the foregoing criteria);

               in which case: (A) the Company shall pay Employee his base salary
               and any other amounts required by applicable law to be paid
               through the effective date of termination but the Company shall
               have no other obligations under this Agreement as of the
               effective date of the termination, and (B) the Company shall
               permit the Employee or his beneficiary to exercise vested options
               to acquire Company stock in accordance with and subject to the
               Award Agreement.

          (ii) By the Company in the event the Company is dissatisfied in its
     reasonable judgment with the Employee's performance of the Employment
     Services or the results thereof which, if curable, are not remedied to the
     Company's reasonable satisfaction


                                      -6-
<PAGE>

     within forty-five (45) days after specific written notice thereof has been
     delivered to the Employee (and termination shall be effective as of the end
     of such 45-day period); in which case: (A) the Company shall pay Employee
     his base salary and any other amounts required by applicable law to be paid
     through the effective date of termination but the Company shall have no
     other obligations under this Agreement as of the effective date of the
     termination, and (B) the Company shall permit the Employee or his
     beneficiary to exercise vested options to acquire Company stock in
     accordance with and subject to the Award Agreement.

          (iii) By the Employee, provided the Employee shall give the Company at
     least 60 days prior written notice thereof (and termination shall be
     effective as of the end of such 60-day or longer period); in which case (A)
     the Company shall pay Employee his base salary and any other amounts
     required by applicable law to be paid through the effective date of
     termination but the Company shall have no other obligations under this
     Agreement as of the effective date of the termination, and (B) the Company
     shall permit the Employee to exercise vested options to acquire Company
     stock in accordance with and subject to the Award Agreement.

     (c)  Notwithstanding anything to the contrary, the obligations under this
          Agreement which by their terms survive termination, including, without
          limitation, the applicable Confidentiality, Noncompetition, and
          Nonsolicitation provisions of this Agreement as set forth in
          paragraphs 5 and 6 hereof and the Confidentiality Agreement, shall
          survive termination; and the representations and warranties, including
          without limitation the provisions of paragraph hereof, shall survive
          termination. Upon termination, and in any case upon the Company's
          request, Employee shall return immediately to the Company all
          Confidential Information and copies thereof and not retain any copies
          thereof.

     8.   MISCELLANEOUS.

     (a)  This Agreement and the rights and obligations hereunder may not be
          assigned, delegated or transferred by Employee without the prior
          written consent of the Company. The Company may assign this Agreement
          to an affiliate of the Company or to an acquirer or successor in
          connection with the merger, consolidation, or sale of all or
          substantially all of its assets, and this Agreement shall inure to the
          benefit of the Company's assignee.

     (b)  This Agreement may not be amended or modified, or any provision hereof
          waived, except by a written instrument duly signed by both parties
          contemporaneously or after the date of this Agreement. This Agreement
          (together with the Award Agreement and the Agreement regarding certain
          conditions) constitutes the entire agreement between the parties
          concerning the subject matter


                                      -7-
<PAGE>

          hereof and thereof, and supersedes all prior written or oral
          negotiations, representations and agreements.

     (c)  The Employee acknowledges that a breach of paragraphs 5, 6 or the
          Confidentiality Agreement may cause the Company and its affiliates
          irreparable harm. Accordingly, in the event of a breach or threatened
          breach by the Employee of any of the provisions of paragraphs 5, 6 or
          the Confidentiality Agreement, the Employee agrees that the Company
          may be entitled to injunctive relief restraining the Employee and any
          individual or entity from participating in such breach or threatened
          breach. The Employee waives any provision of law requiring the Company
          to post a bond for any such injunctive relief. Nothing herein shall be
          construed as prohibiting the Company from pursuing any other remedies
          available at law or in equity for any breach or threatened breach of
          this Agreement.

     (d)  Any waiver of a breach of any provision of this Agreement shall not
          operate as a waiver of any other breach of such provision or any other
          provision, nor shall any failure to enforce any provision hereof
          operate as a waiver of such provision or of any other provision.

     (e)  If any provision of this Agreement is deemed in a final order by a
          court of competent jurisdiction to be unenforceable or invalid, the
          enforceability and validity of all other provisions shall not be
          affected and that court shall modify the unenforceable or invalid
          provision to the extent necessary to render it enforceable and valid
          and that provision shall be enforced as modified. Employee agrees that
          the time period and scope of the covenants in paragraphs 6, 7 and 8 of
          this Agreement are reasonable and appropriate under the circumstances
          of the Company's business.


                                      -8-
<PAGE>

         IN WITNESS WHEREOF, Employee and the President of the Company have
signed this Agreement as of the Effective Date.

                                      -9-
<PAGE>

COMPANY                                      EMPLOYEE

Photogen, Inc.

By:  /s/ John T. Smolik                      By:  /s/ David D. Shaw
   -------------------------------              -------------------------------
     John T. Smolik, President                    David D. Shaw, Ph.D.

Address:  7327 Oak Ridge Highway             Address:  600 Fairville Road
          Suite B                                      Chadds Ford, PA 19317
          Knoxville, TN  37931
          Telephone:  423/769-4011                     Telephone:______________
          Telecopier: 423/769-4013                     Telecopier:_____________
          E-mail:     smolik@photogen.com              E-mail: ________________


                                      -10-
<PAGE>

                                    EXHIBIT 1

                             TITLE AND COMPENSATION

Title:         Vice President of Clinical Research and Technology Development

Gross Salary:  Starting salary of $125,000 per year. Salary subject to  periodic
               review.

Bonus/Options: Bonus will be available and paid at the discretion of the Board
               or its Compensation Committee.

Benefits:      Medical/Dental/Term Life Insurance
               Long term disability insurance
               Paid vacation (4 weeks, 9 Company holidays and 2 floating days)
               Benefit programs will be developed and implemented as approved by
               the Board 401(k) salary deferral plan (non-contributory by
               Company)
               Relocation Assistance as follows:

                    -    all customary and reasonable moving expenses (including
                         packing charges);

                    -    two (2) three day house hunting trips from Pennsylvania
                         to Massachusetts;

                    -    final transportation expenses to Massachusetts; and o
                         reimbursement up to a maximum of $15,000 for expenses
                         in connection with re-selling Employee's current home
                         located in Chadds Ford, Pennsylvania upon reasonable
                         documentation submitted to the Company

                    -    one (1) month's salary to offset miscellaneous
                         non-reimbursed moving expenses

<PAGE>

                                    EXHIBIT 2

                      EMPLOYEE'S PREEXISTING RELATIONSHIPS

<TABLE>
<CAPTION>
--------------------------------------------------------------------------------
                                                              SUBJECT TO WRITTEN
                                                                  AGREEMENT
                                                                   (Y OR N)
                                                               IF "YES" ATTACH A
     ENTITY              SUMMARY OF RELATIONSHIP               COPY OF AGREEMENT
                                                                 (REDACTED AS
                                                                   NECESSARY)
--------------------------------------------------------------------------------
<S>                 <C>                                     <C>
None
--------------------------------------------------------------------------------

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

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

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

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

--------------------------------------------------------------------------------
</TABLE>

                                       -1-

<PAGE>

                                    EXHIBIT 3

                        EMPLOYEE'S INTELLECTUAL PROPERTY

<TABLE>
<CAPTION>
--------------------------------------------------------------------------------
          DESCRIPTION                                            STATUS
--------------------------------------------------------------------------------
<S>                                               <C>
[List any patents or patent applications]
--------------------------------------------------------------------------------
None
--------------------------------------------------------------------------------

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

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

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

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

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

--------------------------------------------------------------------------------
</TABLE>

<PAGE>

                                    EXHIBIT 4

         See attached Employee Confidentiality and Inventions Agreement.

<PAGE>

                    [PHOTOGEN TECHNOLOGIES, INC. LETTERHEAD]


                                November 1, 1999

David P. Shaw, Ph.D.
600 Fairville Road
Chadds Ford, PA 19317

          RE:  EMPLOYEE CONFIDENTIALITY AND INVENTIONS AGREEMENT ("AGREEMENT")

Dear Dr. Shaw:

          In the course of your work for Photogen, Inc., its parent company
Photogen Technologies, Inc. or any of their affiliates (collectively,
"Photogen"), you may have access to Photogen's confidential and proprietary
information and/or you may create Developments (defined below). As a condition
to Photogen hiring and employing you (and for other legally sufficient
consideration, the receipt and adequacy of which you acknowledge), you and
Photogen agree as follows:

          1.   CONFIDENTIAL INFORMATION.

          (a)  During your employment by Photogen and for a period of three (3)
               years following termination of your employment, for any reason,
               you will hold in trust, keep confidential and not disclose,
               directly or indirectly, to any third parties or make any use of
               Confidential Information (defined below) for any purpose except
               for the benefit of Photogen in the performance of your duties.
               Confidential Information will not be subject to these
               restrictions if it becomes generally known to the public or in
               the industry without any fault by you or any other person or
               entity, or if Photogen ceases to have a legally protectable
               interest in it. Upon termination of your employment (regardless
               of the reason for termination), you will immediately return to
               Photogen all tangible Confidential Information and any other
               material made or derived from Confidential Information, including
               information stored in electronic format and handwritten notes,
               which is in your possession or which you delivered to others.

          (b)  "Confidential Information" means any and all information that has
               or could have value or utility to Photogen, whether or not
               reduced to written or other tangible form and all copies thereof,
               relating to Photogen's private or proprietary matters,

<PAGE>

[LOGO]

David Shaw, Ph.D.
November 1, 1999
Page 2

               confidential matters or trade secrets. Confidential Information
               includes, but is not limited to, the following:

               (i)  technical information (whether or not subject to patent
          registration or protection), such as research and development,
          methods, trade secrets, know-how, formulas, compositions, protocols,
          processes and techniques, discoveries, machines, inventions, ideas,
          computer programs (including software and data used in all such
          programs), drawings, specifications;

               (ii) except to the extent publicly disclosed by Photogen without
          any fault by you or any other person or entity, information relating
          to Photogen's patents, patent applications, and patent disclosures,
          together with all reissuances, continuations, continuations-in-part,
          revisions, extensions, and reexaminations thereof, and all
          improvements and inventions related thereto;

               (iii) business information, such as information concerning any
          products, customers, suppliers, production, developments, costs,
          purchasing, pricing, profits, markets, sales, accounts, customers,
          financing, acquisitions, strategic alliances or collaborations,
          expansions; and

               (iv) other information relating to Photogen's business practices,
          strategies or policies which are deemed a "trade secret" under
          applicable laws.

          2.   DEVELOPMENTS.

          (a)  You agree to promptly and fully disclose in writing to Photogen,
               Inc.'s President all Developments that are related to or useful
               in the business or demonstrably anticipated business of Photogen,
               or result from duties assigned to you by Photogen or from the use
               of any of Photogen's assets or facilities. "Developments" means
               any and all inventions, discoveries, improvements, know-how,
               works or other intellectual property (whether or not subject to
               registration with any governmental office) you conceive, reduce
               to practice, discover or make, alone or with others, during your
               employment and for twelve (12) months after termination
               (regardless of the reason for termination).

          (b)  You hereby assign and transfer to Photogen, Inc. all of your
               right, title and interest in and to all Developments. You agree
               to sign and deliver to Photogen, Inc. (during and after
               employment) other documents Photogen, Inc. considers

<PAGE>

[LOGO]

David Shaw, Ph.D.
November 1, 1999
Page 3

               necessary or desirable to evidence its ownership of
               Developments. All copyrightable works that are Developments,
               whether or not works made for hire (as defined in 17 U.S.C.
               Section 101), shall be owned by Photogen, Inc. and it may file
               and own the same as the author throughout the world. If
               Photogen, Inc. is unable for any reason to secure your
               signature on any document necessary or desirable to apply for,
               prosecute, obtain, or enforce any patent, trademark, service
               mark, copyright, or other right or protection relating to any
               Development, you hereby irrevocably designate and appoint
               Photogen, Inc. and each of its duly authorized officers and
               agents, as your agent and attorney-in-fact to act for and in
               your behalf and stead to execute and file any such document
               and to do all other lawfully permitted acts to further the
               prosecution, issuance, and enforcement of patents, trademarks,
               service marks, copyrights, or other rights or protections with
               the same force and effect as if personally executed and
               delivered by you. You agree that this power of attorney is
               irrevocable and is coupled with an interest and thereby
               survives your death or disability.

          3.   NO CONFLICTS. You represent and warrant to, and agree with
               Photogen that:

          (a)  You are under no contractual or other obligation or restriction
               which is inconsistent with your obligations under this Agreement
               or the performance of you duties for Photogen. Without limiting
               the generality of the foregoing, performing your duties for
               Photogen will not cause you to breach any agreement or other duty
               which obligates you to keep in confidence any confidential or
               proprietary information or trade secret of any third party or to
               refrain from competing, directly or indirectly, with the business
               of any third party.

          (b)  Neither you nor any third party has any ownership or other
               interest in any idea, invention or other item of intellectual
               property that will be used in performing your duties for
               Photogen, and all Developments will be free and clear of any
               encumbrances or claims of third parties. In performing your
               duties for Photogen, you will not disclose to Photogen or use any
               confidential or proprietary information or trade secret of any
               third party, and you will not interfere with the business of any
               third party in any way contrary to applicable law.

          (c)  In performing your duties for Photogen, you will comply with
               applicable laws and regulations.

<PAGE>

[LOGO]

David Shaw, Ph.D.
November 1, 1999
Page 4

          4.   NO EMPLOYMENT RIGHTS. Nothing in this Agreement shall affect your
or Photogen's right to terminate your employment or Photogen's right modify your
terms of employment, or to confer on you any other rights or benefits in
connection with your employment.

          5.   SEVERABILITY; CHOICE OF LAW; INJUNCTION. If any provision of this
Agreement is deemed by a court of competent jurisdiction to be unenforceable or
invalid, the enforceability and validity of all other provisions shall not be
affected and that court shall modify the unenforceable or invalid provision to
the extent necessary to render it enforceable and valid and that provision shall
be enforced as modified. You agree that the time period and scope of the
covenants in this Agreement are reasonable and appropriate under the
circumstances of Photogen's business. This Agreement shall be governed and
interpreted in accordance with the laws of the State of Tennessee without regard
to its provisions on conflicts of law. Without limiting any other available
remedies at law or in equity, Photogen will be entitled to injunctive relief
restraining any individual or entity from participating in any breach or
threatened breach of this Agreement without having to post a bond or security.

          6.   MISCELLANEOUS. This Agreement may not be amended or modified
except by a written instrument signed by both parties after the date of this
Agreement. This Agreement may be assigned by Photogen and shall inure to the
benefit of Photogen, its successors and assigns, but may not be assigned or
delegated by you. This Agreement supersedes all prior agreements, negotiations
and representations, written or oral, between the parties with respect to the
subject matter contained herein. Any waiver of any breach of, or failure to
enforce, any of the provisions of this Agreement shall not operate as a waiver
of any other breach or waiver of performance of such provisions or any other
provisions. Your obligations under this Agreement survive termination of your
employment, regardless of the manner or reason for termination.

<PAGE>

[LOGO]

David Shaw, Ph.D.
November 1, 1999
Page 5

          Please indicate your agreement to the foregoing by signing a copy of
this letter below and returning it to me. I look forward to working with you.

                                   Very truly yours,

                                        /s/ John Smolik

                                   John T. Smolik, President


Accepted and Agreed to as of
November 1, 1999.

By:  /s/ David D. Shaw
   --------------------------
     David D. Shaw, Ph.D.