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                            AMERICAL SECURITIES, INC.
                               290 Seventh Avenue
                         San Francisco, California 94118

March 25, 2002

Taffy Williams, Ph.D.
President & CEO
Photogen Technologies, Inc.
140 Union Square Dr.
New Hope, PA 18938

Re: Mergers and Acquisitions and Certain Other Transactions

Dear Taffy:

         Reference is made to the letter agreement dated January 16, 2002 and
accepted January 17, 2002 between AmeriCal Securities, Inc. ("AmeriCal") and
Photogen Technologies, Inc. (the Company") (the "1/02 Agreement"). Thank you for
discussing a potential collaboration between the Company and AmeriCal. As
discussed we share you believe that the imaging of vulnerable plaque is the next
frontier in diagnostic cardiology. Based on the preliminary data that you have
provided us, PH-50 in conjunction with other diagnostic imaging modalities such
as electron beam tomography, appears a promising approach to this solve this
problem and we are confident that we can assist you in realizing your
objectives.

         As an incentive for AmeriCal to assist the Company in quickly raising
capital, the parties have agreed that the 1/02 Agreement is amended and restated
to read in its entirety as follows (as so amended and restated, and as the
agreement may be further amended from time to time, the "Agreement"):

         "This letter sets forth our agreement whereby AmeriCal will act as
exclusive agent to assist the Company in the acceleration of the development of
PH-50 diagnostics and therapeutics which may include one or more of the
following transactions: the sale of assets and/or equity securities of the
Company, the issuance or exchange of equity securities of the Company or an
affiliate in a transaction involving a partnership, limited liability company,
joint venture, merger, the sale or licensing of all or a portion of the
Company's rights to PH-50, written or oral contractual relationships relating to
the exploitation of PH-50, the acquisition or recapitalization of the Company or
any other form of corporate transaction with one or more third parties that may
achieve the Company's goal as stated below, excluding however, any transaction
covered by the separate agreement, dated the date hereof, between AmeriCal and
the Company (the "Financings Engagement Agreement") regarding financings (any
one or more of the foregoing included matters is individually or collectively
referred to as a "Transaction").

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[****]       Represents material which has been redacted pursuant to a request
             for confidential treatment pursuant to Rule 24B-2 under the
             Securities Exchange Act of 1934, as amended.

         While it is recognized that PH-50 has the potential to be developed as
a diagnostic and therapeutic agent, it is also recognized that to fully realize
this potential will take years and likely in excess of $100 million. Thus it is
thus likely that the optimal development of PH-50 therapeutics may require
greater financial and technical resources that are currently available to the
Company. The goal of this engagement is to identify target enterprises with
convergent interests and to structure a business relationship meeting the
objectives outlined above.

         Any Transaction is subject to the execution of definitive documents
satisfactory to the Company, the purchasers or other third parties to the
Transaction and AmeriCal. The definitive agreements will contain
representations, warranties and covenants of the parties, conditions and
indemnification provisions customary in transactions of similar type and size.

         The engagement of AmeriCal hereunder may be terminated upon 30 days
prior written notice given by the Company at any time after March 20, 2003 or at
any time immediately upon written notice from AmeriCal. Unless the engagement
had been previously terminated by AmeriCal in which case AmeriCal shall be
entitled to no compensation, if, prior to the termination of AmeriCal's
engagement hereunder or for a period of one year after such termination, the
Company enters into a definitive agreement with respect to a Transaction with
any person, firm or entity (whether or not such person, firm or entity is found
by, or such Transaction is facilitated by, AmeriCal) and such Transaction is
then or thereafter consummated, the Company shall pay to AmeriCal a fee equal to
[****]% of the Transaction Value (as hereinafter defined) deemed hereunder to
have been received by the Company under such Transaction; provided, however,
that AmeriCal shall not be entitled to any compensation under this Agreement in
respect of any transaction for which compensation is paid to AmeriCal under the
Financings Engagement Agreement.

         Fees shall be payable to AmeriCal not later than 10 days after the
related payments under a Transaction are received by the Company. However, in
the case of a sale of assets or securities of the Company, the fee shall be paid
simultaneously with such sale. For purposes of this Agreement, the term
"Transaction Value" of a Transaction means the aggregate payments deemed to be
received by the Company under and during the entire term of the Transaction,
including, but not limited to, payments made to the Company or any of the
Company's affiliates or designees; PROVIDED, HOWEVER, that in the case of
product sales or royalties received under license agreements only, after the
fifth anniversary of the date on which the first payment is received by the
Company, no further payments shall be considered to be part of the Transaction
Value.

                                       2

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[****]       Represents material which has been redacted pursuant to a request
             for confidential treatment pursuant to Rule 24B-2 under the
             Securities Exchange Act of 1934, as amended.

         For purposes of this Agreement, the Company shall be deemed to have
received Transaction Value in the amount of all payments actually received by
the Company, its affiliates or designees for the sales price of its securities
or assets or PH-50 or any royalty payment or fee actually received by the
Company under any license, joint venture or contractual arrangement relating to
PH-50, including any payment of cash or issuance of securities as a result of
the Company achieving any milestone; PROVIDED, HOWEVER, that there shall not be
included in Transaction Value any payment made to the Company in reimbursement
of the Company's expenses, including delivery charges or taxes.

         The Company also agrees to reimburse AmeriCal for all out-of-pocket
expenses of AmeriCal relating to the performance of AmeriCal's services
hereunder, including travel expenses and legal fees and disbursements of counsel
retained by AmeriCal in connection with the matters contemplated by this
Agreement, promptly after submission to the Company of reasonable documentation
of such expenses, provided that such fees and expenses shall not exceed $[****]
in the aggregate subject to the Company's authorization, not to be unreasonably
withheld, of any expense item in excess of $[****].

         The services to be rendered by AmeriCal are on a best efforts basis and
the Company acknowledges that AmeriCal is not making any representation or
guarantee that any person will purchase any assets or securities of the Company
at any time through the efforts of AmeriCal or otherwise.

         The Company agrees that it will not disclose, and will not include in
any public announcement the name of any purchaser or offeree, unless and until
such disclosure is required by applicable law or applicable regulation, and then
only to the extent of such requirements.

         Each party agrees to maintain all non-public and proprietary
information obtained hereunder confidential and not to use such information for
any purpose other than to evaluate the proposed transaction. Notwithstanding the
foregoing, AmeriCal may disclose such information pursuant to applicable law,
regulation, subpoena or administrative investigation. The foregoing restriction
shall not apply to any such information which becomes generally known to the
public for any reason whatsoever other than a breach hereof by AmeriCal.

         The Company agrees to indemnify AmeriCal in accordance with the
indemnification provisions (the "Indemnification Provisions") attached to this
Agreement which Indemnification Provisions are incorporated herein and made a
part hereof and which shall survive any amendment, restatement, termination or
expiration of this Agreement.

         This Agreement may not be amended or modified, except by an instrument
in writing signed by each of AmeriCal and the Company.

                                       3

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         This Agreement shall be governed by and construed in accordance with
the laws of the State of California, without regard to its conflicts of law
principles. Each party hereby irrevocably submits to the jurisdiction of any
court of the State of California, County of San Francisco or the United States
District Court for the Northern District of California in connection with any
suit, action or proceeding arising out of this Agreement.

         This Agreement shall be binding upon AmeriCal and the Company and shall
inure to the benefit of their respective successors and assigns. This Agreement
constitutes the entire understanding and agreement of the parties with respect
to the subject matter hereof and it supersedes all prior and/or contemporaneous
understandings and agreements with respect to such subject matter, all of which
are merged here.

         Please indicate your agreement with the terms of this letter by signing
a copy of this letter in the space set forth below and returning it to the
undersigned.

                                           Sincerely,

                                           AMERICAL SECURITIES, INC.

                                           By: /s/ Charles Engelberg
                                              ---------------------------------
                                                   Charles Engelberg, M.D.
                                                   Director of Corporate Finance

         The foregoing accurately sets forth our understanding and agreement
with respect to matters set forth herein.

Accepted this 25th day of March , 2002.

PHOTOGEN TECHNOLOGIES, INC.

By: /s/ Taffy Williams
   --------------------------------
       Taffy Williams, Ph.D
       President and CEO

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                           INDEMNIFICATION PROVISIONS

         Photogen Technologies ("PHGN") agrees to indemnify and hold harmless
AmeriCal Securities, Inc. ("AmeriCal") to the fullest extent permitted by law,
from and against, and AmeriCal shall not have any liability to PHGN, or its
owners, parents, creditors or security holders for, any and all losses, claims,
damages, obligations, penalties, judgments, awards, liabilities, costs, expenses
and disbursements (and any and all actions, suits, proceedings and
investigations in respect thereof and any and all reasonable legal and other
costs, expenses or disbursements in giving testimony or furnishing documents in
response to a subpoena or otherwise), including, without limitation, the costs,
expenses and disbursements, as and when incurred, of investigating, preparing or
defending any such action, suit proceeding or investigation (whether or not in
connection with litigation in which AmeriCal is a party), directly or
indirectly, caused by, relating to, based upon, arising out of or in connection
with (a) AmeriCal's acting for PHGN, including, without limitation, any act or
omission by AmeriCal in connection with its acceptance of or the performance or
non-performance of its obligations under the agreement dated March 25, 2002,
between PHGN and AmeriCal, as it may be amended from time to time (the
"AGREEMENT"), (b) any transaction, including the sale of the assets or
securities of PHGN, (c) any untrue statement or alleged untrue statement of a
material fact contained in, or omissions or alleged omissions from any
disclosure document used in connection with the sale of securities of PHGN or
similar statements or omissions in or from any other information furnished by
PHGN to AmeriCal or any prospective purchaser or (d) any use of proceeds;
PROVIDED, HOWEVER, such indemnity agreement shall not apply to (i) AmeriCal for
any portion of any such loss, claim, damage, obligation, penalty, judgment,
award, liability, cost, expense or disbursement to the extent it is found in a
final judgment by a court of competent jurisdiction (not subject to further
appeal) to have resulted primarily and directly from the gross negligence or
willful misconduct of AmeriCal or to have resulted primarily and directly from
written information furnished to PHGN by or on behalf of AmeriCal with respect
to AmeriCal expressly for use in any securities offering document of PHGN.

         These Indemnification Provisions shall be in addition to any liability
which PHGN may otherwise have to AmeriCal and shall extend to all of AmeriCal's
affiliated entities, directors, officers, employees, legal counsel, agents and
controlling persons (within the meaning of the federal securities laws). All
references to AmeriCal in these Indemnification Provisions shall be understood
to include any and all of the foregoing.

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         If any action, suit, proceeding or investigation is commenced, as to
which AmeriCal proposes to demand indemnification, it shall notify PHGN with
reasonable promptness; PROVIDED, HOWEVER, that any failure by AmeriCal to notify
PHGN shall not relieve PHGN from its obligations hereunder unless such failure
prevents the presentation of material defenses thereto. AmeriCal shall have the
right to retain counsel of its own choice to represent it, and PHGN shall pay
the fees, expenses and disbursements of such counsel; and such counsel shall, to
the extent consistent with their professional responsibilities, cooperate with
PHGN and any counsel designated by PHGN. PHGN shall be liable for any settlement
of any claim against AmeriCal made with PHGN's written consent, which consent
shall not be unreasonably withheld. PHGN shall not, without the prior written
consent of AmeriCal, settle or compromise any claim, or permit a default or
consent to the entry of any judgment in respect thereof, unless such settlement,
compromise or consent includes, as an unconditional term thereof, the giving by
the claimant to AmeriCal of an unconditional and irrevocable release from all
liability in respect of such claim.

         In order to provide for just and equitable contribution, if a claim for
indemnification pursuant to these Indemnification Provisions is made but it is
found in a final judgment by a court of competent jurisdiction (not subject to
further appeal) that such indemnification may not be enforced in such case, even
though the express provisions hereof provide for indemnification in such case,
then PHGN and AmeriCal shall contribute to the losses, claims, damages,
obligations, penalties, judgments, awards, liabilities, costs, expenses and
disbursements to which the indemnified persons may be subject in accordance with
the relative benefits received by them, and also their relative fault in
connection with the statements, acts or omissions which resulted in such losses,
claims, damages, obligations, penalties, judgments, awards, liabilities, costs,
expenses and disbursements and the relevant equitable considerations shall also
be considered. No person found liable for a fraudulent misrepresentation shall
be entitled to contribution from any person who is not also found liable for
such fraudulent misrepresentation. Notwithstanding the foregoing, AmeriCal shall
not be obligated to contribute any amount hereunder that exceeds the amount of
fees previously received by AmeriCal pursuant to the Agreement.

         Neither termination nor completion of the engagement of AmeriCal
referred to above shall affect these Indemnification Provisions which shall
remain operative and in full force and effect."