<PAGE>   1
                                                                    EXHIBIT 10.3

                           LETTER OF CREDIT AGREEMENT


Date:  April 5, 1999                      ILEX Oncology, Inc.
                            Accepted by:  /s/ RICHARD L. LOVE
                                          ---------------------
                                          Richard L. Love, President
To:


DG BANK
Deutsche Genossenschaftsbank AG           Swift Code:  GENODEFF
EZD2: Mrs. Werner                         Telex-No.:   412 291 dg
Am Platz der Republik
60265 Frankfurt

Gentlemen,

Please advise Raiffeisenbank Buchloe-Kaufbeuren-Marktoberdorf eG,
Gutenbergstrasse 4,87600 Kaufbeuren the following Standby Letter of Credit in
their favour adding your confirmation.

Beneficiary:                              Applicant:
RAIFFEISENBANK                            ILEX Oncology, Inc.
Buchloe-Kaufbeuren-Marktoberdorf eG       11550 IH 10 West, Suite 300
Gutenbergstrasse 4                        San Antonio, Texas 78230
87600 Kaufbeuren/Germany

STANDBY LETTER OF CREDIT NO. __________ FOR 3.000.000,-DEM IN YOUR FAVOR

Gentlemen,

We hereby open our irrevocable Standby Letter of Credit No. ________ in your
favour for a maximum amount of
                           3.000.000,-DEM
                           (Three Million German Marks)

including any interest, commission and sundry expenses in support of the credit
facilities you are ready to grant to
                           Messrs. P.F.K. Pharma Forschung Kaufbeuren GmbH,
                           Schraderstrasse 4,87600 Kaufbeuren, account no. 6424.

This Standby Letter of Credit is available at sight and on first demand against
presentation of:

        your duly signed written statement certifying that Messrs. P.F.K. Pharma
        Forschung Kaufbeuren GmbH, Schraderstrasse 4,87600 Kaufbeuren, Germany,
        has not partially or totally fulfilled its obligations towards you in
        connection with the repayment of the credit facilities granted to it
        and that the amount you present to us for payment represents an amount
        past due.





                                  Page 1 of 2
<PAGE>   2
IRREVOCABLE STANDBY LETTERS OF CREDIT NO.


Any statement(s) are to be marked: "drawn under irrevocable Standby Letter of
Credit No.________".

Part applications against this Standby Letter of Credit are allowed, however
grand total not to exceed maximum of 3.000.000,-DEM.

This Standby Letter of Credit expires on May 3, 1999 in Frankfurt am
Main/Germany and is payable at the counters of DG Bank, Deutsche
Genossenschafts Bank AG, Frankfurt am Main/Germany.

All commission and/or fees including issuing banks and confirming bank fee's
are for beneficiary account.

Reimbursement: As per your instructions.

"This Standby Letter of Credit is subject to the Uniform Customs and Practice
for Documentary Credit (1993 Revision) International Chamber of Commerce
Publication No. 500".


-------------------------------
Authorized Signature, unquote


                                  Page 2 of 2
<PAGE>   3
                          COMMERCIAL PLEDGE AGREEMENT

<TABLE>
<CAPTION>
----------------------------------------------------------------------------------------------------------------------------------
PRINCIPAL       LOAN DATE      MATURITY       LOAN NO        CALL      COLLATERAL          ACCOUNT        OFFICER        INITIALS
<S>             <C>           <C>            <C>            <C>       <C>                 <C>            <C>            <C>
$1,841,730.00   04-05-1999    05-03-1999       0299          500          6064            2205524           006     
----------------------------------------------------------------------------------------------------------------------------------
                           References in the shaded area are for Lender's use only and do not limit the
                                  applicability of this document to any particular loan or item.
----------------------------------------------------------------------------------------------------------------------------------
</TABLE>

<TABLE>
<S>       <C>                                               <C>     <C>                         
BORROWER: ILEX ONCOLOGY, INC. (TIN: 74-2699185)             LENDER: THE FROST NATIONAL BANK
          11550 IH 10 WEST, SUITE 300                               P.O. BOX 1600
          SAN ANTONIO, TX 78230                                     SAN ANTONIO, TX 78296
==================================================================================================================================
</TABLE>

THIS COMMERCIAL PLEDGE AGREEMENT IS ENTERED INTO BETWEEN ILEX ONCOLOGY, INC.
(REFERRED TO BELOW AS "GRANTOR"); AND THE FROST NATIONAL BANK (REFERRED TO BELOW
AS "LENDER").

GRANT OF SECURITY INTEREST. FOR VALUABLE CONSIDERATION, GRANTOR GRANTS TO LENDER
A SECURITY INTEREST IN THE COLLATERAL TO SECURE THE INDEBTEDNESS AND AGREES THAT
LENDER SHALL HAVE THE RIGHTS STATED IN THIS AGREEMENT WITH RESPECT TO THE
COLLATERAL, IN ADDITION TO ALL OTHER RIGHTS WHICH LENDER MAY HAVE BY LAW.

DEFINITIONS. The following words shall have the following meanings when used in
this Agreement:

     AGREEMENT. The word "Agreement" means this Commercial Pledge Agreement, as
     this Commercial Pledge Agreement may be amended or modified from time to
     time, together with all exhibits and schedules attached to this Commercial
     Pledge Agreement from time to time.

     COLLATERAL. The word "Collateral" means the following specifically
     described property, which Grantor has delivered or agrees to deliver (or 
     cause to be delivered) immediately to Lender, together with all Income and
     Proceeds as described below:

         ALL OF MY RIGHTS AND INTERESTS IN AN AMOUNT NOT LESS THAN $2,302,163.00
         NOW OWNED OR HEREAFTER ACQUIRED, IN AND TO ALL STOCKS, BONDS AND OTHER
         SECURITIES (INCLUDING BOTH CERTIFICATED AND UNCERTIFICATED SECURITIES),
         OPTIONS, CERTIFICATES OF DEPOSITS, ANNUITIES, NOTES, INSTRUMENTS,
         DOCUMENTS, COMMODITIES OR COMMODITIES FUTURES, CERTIFICATES OF TITLE OR
         OTHER DOCUMENTS EVIDENCING OWNERSHIP OF PROPERTY, CHATTEL PAPER,
         ACCOUNTS, CONTRACT RIGHTS, MONIES, CLAIMS, DEMANDS, INTERESTS IN
         PARTNERSHIPS, JOINT VENTURES OR LIMITED PARTNERSHIPS, GENERAL
         INTANGIBLES AND OTHER ASSETS OF ANY TYPE WHATSOEVER NOW OR HEREAFTER
         DEPOSITED OR HELD IN THAT CERTAIN CUSTODIAL ACCOUNT WITH ADVISORY
         SERVICE, ACCOUNT NO. F07555500 (THE "CUSTODIAL ACCOUNT"), WITH LENDER,
         WHICH CUSTODIAL ACCOUNT WITH ADVISORY SERVICE IS HELD IN MY NAME;
         TOGETHER WITH ALL SUBSTITUTES AND REPLACEMENT OF ANY SUCH ASSETS HELD
         IN THE CUSTODIAL ACCOUNT, AND ALL SECURITY FOR THE PAYMENT OF ANY OF 
         THE FOREGOING, AND ALL POLICIES OF INSURANCE RELATING TO ANY OF THE
         FOREGOING; AND ALL BOOKS AND RECORDS RELATING TO ANY OF THE FOREGOING.

     In addition, the word "Collateral" includes all property of Grantor, in the
     possession of Lender (or in the possession of a third party subject to the
     control of Lender), whether now or hereafter existing and whether tangible
     or intangible in character, including without limitation each of the
     following:

         (a) ALL PROPERTY TO WHICH LENDER ACQUIRES TITLE OR DOCUMENTS OF TITLE.

         (b) ALL PROPERTY ASSIGNED TO LENDER.

         (c) ALL PROMISSORY NOTES, BILLS OF EXCHANGE, STOCK CERTIFICATES, BONDS,
         SAVINGS PASSBOOKS, TIME CERTIFICATES OF DEPOSIT, INSURANCE POLICIES,
         AND ALL OTHER INSTRUMENTS AND EVIDENCES OF AN OBLIGATION.

         (d) ALL RECORDS RELATING TO ANY OF THE PROPERTY DESCRIBED IN THIS
         COLLATERAL SECTION, WHETHER IN THE FORM OF A WRITTEN, MICROFILM,
         MICROFICHE, OR ELECTRONIC MEDIA.

     EVENT OF DEFAULT. The words "Event of Default" mean and include without
     limitation any of the Events of Default set forth below in the section
     titled "Events of Default."

     GRANTOR. The word "Grantor" means ILEX ONCOLOGY, INC., its successors and
     assigns.

     GUARANTOR. The word "Guarantor" means and includes without limitation each
     and all of the guarantors, sureties, and accommodation parties in
     connection with the Indebtedness.

     INCOME AND PROCEEDS. The words "Income and Proceeds" mean all present and
     future income, proceeds, earnings, increases, and substitutions from or for
     the Collateral of every kind and nature, including without limitation all
     payments, interest, profits, distributions, benefits, rights, options,
     warrants, dividends, stock dividends, stock splits, stock rights,
     regulatory dividends, distributions, subscriptions, monies, claims for
     money due and to become due, proceeds of any insurance on the Collateral,
     shares of stock of different par value or no par value issued in
     substitution or exchange for shares included in the Collateral, and all
     other property Grantor is entitled to receive on account of such
     Collateral, including accounts, documents, instruments, chattel paper, and
     general intangibles.

     INDEBTEDNESS. The word "Indebtedness" means the indebtedness evidenced by
     the Note, including all principal and earned interest, together with all
     other indebtedness and costs and expenses for which Grantor is responsible
     under this Agreement or under any of the Related Documents. In addition,
     the word "Indebtedness" includes all other obligations, debts and
     liabilities, plus interest thereon, of Grantor, or any one or more of them,
     to Lender, as well as all claims by lender against Grantor, or any one or
     more of them, whether existing now or later; whether they are voluntary or
     involuntary, due or not due, direct or indirect, absolute or contingent,
     liquidated or unliquidated; whether Grantor may be liable individually or
     jointly with others; whether Grantor may be obligated as guarantor, surety,
     accommodation party or otherwise.

     LENDER. The word "Lender" means THE FROST NATIONAL BANK, its successors and
     assigns.

     NOTE. The word "Note" means the note or credit agreement dated April 5,
     1999, in the principal amount of $1,841,730.00 from ILEX ONCOLOGY, INC. to
     Lender, together with all renewals of, extensions of, modifications of,
     refinancings of, consolidations of and substitutions for the note or credit
     agreement.

     OBLIGOR. The word "Obligor" means and includes without limitation any and
     all persons or entities obligated to pay money or to perform some other act
     under the Collateral.

     RELATED DOCUMENTS. The words "Related Documents" mean and include without
     limitation all promissory notes, credit agreements, loan agreements,
     environmental agreements, guaranties, security agreements, mortgages, deeds
     of trust, and all other instruments, agreements and documents, whether now
     or hereafter existing, executed in connection with the Indebtedness.






<PAGE>   4
04-05-1999                COMMERCIAL PLEDGE AGREEMENT                     PAGE 2
LOAN NO 0299                      (CONTINUED)
================================================================================

RIGHT OF SETOFF. Grantor hereby grants Lender a contractual security interest in
and hereby assigns, conveys, delivers, pledges, and transfers all of Grantor's
right, title and interest in and to Grantor's accounts with Lender (whether
checking, savings, or some other account), including all accounts held jointly
with someone else and all accounts Grantor may open in the future, excluding,
however, all IRA and Keogh accounts, and all trust accounts for which the grant
of a security interest would be prohibited by law. Grantor authorizes Lender, to
the extent permitted by applicable law, to charge or setoff all Indebtedness
against any and all such accounts.

GRANTOR'S REPRESENTATIONS AND WARRANTIES WITH RESPECT TO THE COLLATERAL.
Grantor represents and warrants to Lender that:

     OWNERSHIP. Grantor is the lawful owner of the Collateral free and clear of
     all security interests, liens, encumbrances and claims of others except as
     disclosed to and accepted by Lender in writing prior to execution of this
     Agreement.

     RIGHT TO PLEDGE. Grantor has the full right, power and authority to enter
     into this Agreement and to pledge the Collateral.

     BINDING EFFECT. This Agreement is binding upon Grantor, as well as
     Grantor's heirs, successors, representatives and assigns, and is legally
     enforceable in accordance with its terms.

     NO FURTHER ASSIGNMENT. Grantor has not, and will not, sell, assign,
     transfer, encumber or otherwise dispose of any of Grantor's rights in the
     Collateral except as provided in this Agreement.

     NO DEFAULTS. There are no defaults existing under the Collateral, and there
     are no offsets or counterclaims to the same. Grantor will strictly and
     promptly perform each of the terms, conditions, covenants and agreements
     contained in the Collateral which are to be performed by Grantor, if any.

     NO VIOLATION. The execution and delivery of this Agreement will not violate
     any law or agreement governing Grantor or to which Grantor is a party, and
     its certificate or articles of incorporation and bylaws do not prohibit any
     term or condition of this Agreement.

LENDER'S RIGHTS AND OBLIGATIONS WITH RESPECT TO COLLATERAL. Lender may hold the
Collateral until all the Indebtedness has been paid and satisfied and thereafter
may deliver the Collateral to any Grantor. Lender shall have the following
rights in addition to all other rights it may have by law:

     MAINTENANCE AND PROTECTION OF COLLATERAL. Lender may, but shall not be
     obligated to, take such steps as it deems necessary or desirable to
     protect, maintain, insure, store, or care for the Collateral, including
     payment of any liens or claims against the Collateral. Lender may charge
     any cost incurred in so doing to Grantor.

     INCOME AND PROCEEDS FROM THE COLLATERAL. Lender may receive all Income and
     Proceeds and add it to the Collateral. Grantor agrees to deliver to Lender
     immediately upon receipt, in the exact form received and without 
     commingling with other property, all Income and Proceeds from the
     Collateral which may be received by, paid, or delivered to Grantor or for
     Grantor's account, whether as an addition to, in discharge of, in
     substitution of, or in exchange for any of the Collateral.

     APPLICATION OF CASH. At Lender's option, Lender may apply any cash, whether
     included in the Collateral or received as Income and Proceeds or through
     liquidation, sale, or retirement, of the Collateral, to the satisfaction of
     the Indebtedness or such portion thereof as Lender shall choose, whether or
     not matured.

     TRANSACTIONS WITH OTHERS. Lender may (a) extend time for payment or other
     performance, (b) grant a renewal or change in terms or conditions, or (c)
     compromise, compound or release any obligation, with any one or more
     Obligors, endorsers, or Guarantors of the Indebtedness as Lender deems
     advisable, without obtaining the prior written consent of Grantor, and no
     such act or failure to act shall affect Lender's rights against Grantor or
     the Collateral.

     ALL COLLATERAL SECURES INDEBTEDNESS. All Collateral shall be security for
     the Indebtedness, whether the Collateral is located at one or more offices
     or branches of Lender and whether or not the office or branch where the
     Indebtedness is created is aware of or relies upon the Collateral.

     COLLECTION OF COLLATERAL. Lender, at Lender's option, may, but need not,
     collect directly from the Obligors on any of the Collateral all Income and
     Proceeds or other sums of money and other property due and to become due
     under the Collateral, and Grantor authorizes and directs the Obligors, if
     Lender exercises such option, to pay and deliver to Lender all Income and
     Proceeds and other sums of money and other property payable by the terms of
     the Collateral and to accept Lender's receipt for the payments.

     POWER OF ATTORNEY. Grantor irrevocably appoints Lender as Grantor's
     attorney-in-fact, with full power of substitution, (a) to demand, collect,
     receive, receipt for, sue and recover all Income and Proceeds and other
     sums of money and other property which may now or hereafter become due,
     owing or payable from the Obligors in accordance with the terms of the
     Collateral; (b) to execute, sign and endorse any and all instruments,
     receipts, checks, drafts and warrants issued in payment for the Collateral;
     (c) to settle or compromise any and all claims arising under the
     Collateral, and in the place and stead of Grantor, execute and deliver
     Grantor's release and acquittance for Grantor; (d) to file any claim or
     claims or to take any action or institute or take part in any proceedings,
     either in Lender's own name or in the name of Grantor, or otherwise, which
     in the discretion of Lender may seem to be necessary or advisable; and (e)
     to execute in Grantor's name and to deliver to the Obligors on Grantor's
     behalf, at the time and in the manner specified by the Collateral, any
     necessary instruments or documents.

     PERFECTION OF SECURITY INTEREST. Upon request of Lender, Grantor will
     deliver to Lender any and all of the documents evidencing or constituting
     the Collateral. When applicable law provides more than one method of
     perfection of Lender's security interest, Lender may choose the method(s)
     to be used. Upon request of Lender, Grantor will sign and deliver any
     writings necessary to perfect Lender's security interest. Grantor hereby
     appoints Lender as Grantor's irrevocable attorney-in-fact for the purpose
     of executing any documents necessary to perfect or to continue the security
     interest granted in this Agreement. THIS IS A CONTINUING SECURITY AGREEMENT
     AND WILL CONTINUE IN EFFECT EVEN THOUGH ALL OR ANY PART OF THE INDEBTEDNESS
     IS PAID IN FULL AND EVEN THOUGH FOR A PERIOD OF TIME GRANTOR MAY NOT BE
     INDEBTED TO LENDER.

EXPENDITURES BY LENDER. If not discharged or paid when due, Lender may (but
shall not be obligated to) discharge or pay any amounts required to be
discharged or paid by Grantor under this Agreement, including without limitation
all taxes, liens, security interests, encumbrances, and other claims, at any
time levied or placed on the Collateral. Lender also may (but shall not be
obligated to) pay all costs for insuring, maintaining and preserving the
Collateral. All such expenditures incurred or paid by Lender for such purposes
will then bear Interest at the Note rate from the date incurred or paid by
Lender to the date of repayment by Grantor. All such expenses shall become a
part of the Indebtedness and, at Lender's option, will (a) be payable on demand,
(b) be added to the balance of the Note and be apportioned among and be payable
with any installment payments to become due during either (i) the term of any
applicable insurance policy or (ii) the remaining term of the Note, or (c) be
treated as a balloon payment which will be due and payable at the Note's
maturity. This Agreement also will secure payment of these amounts. Such right
shall be in addition to all other rights and remedies to which Lender may be
entitled upon the occurrence of an Event of Default.

LIMITATIONS ON OBLIGATIONS OF LENDER. Lender shall use ordinary reasonable care
in the physical preservation and custody of the Collateral in Lender's
possession, but shall have no other obligation to protect the Collateral or its
value. In particular, but without limitation, Lender shall have no
responsibility for (a) any depreciation in value of the Collateral or for the
collection or protection of any Income and Proceeds from the Collateral, (b)
preservation of rights against parties to the Collateral or against third
persons, (c) ascertaining any maturities, calls, conversions, exchanges, offers,
lenders, or similar matters relating to any of the Collateral, or (d) informing
Grantor about any of the above, whether or not Lender has or is deemed to have
knowledge of such matters. Except as provided above, Lender shall have no
liability for depreciation or deterioration of the Collateral. 
<PAGE>   5
04-05-1999                COMMERCIAL PLEDGE AGREEMENT                     PAGE 3
LOAN NO 0299                       (CONTINUED)              
================================================================================

EVENTS OF DEFAULT. Each of the following shall constitute an Event of Default
under this Agreement:

     DEFAULT ON INDEBTEDNESS. Failure of Grantor to make any payment when due
     on the indebtedness.

     OTHER DEFAULTS. Failure of Grantor to comply with or to perform any other
     term, obligation, covenant or condition contained in this Agreement or in
     any of the Related Documents or in any other agreement between Lender and
     Grantor.

     DEFAULT IN FAVOR OF THIRD PARTIES. Should Borrower or any Grantor default
     under any loan, extension of credit, security agreement, purchase or sales
     agreement, or any other agreement, in favor of any other creditor or person
     that may materially affect any of Borrower's property or Borrower's or any
     Grantor's ability to repay the Loans or perform their respective
     obligations under this Agreement or any of the Related Documents.

     FALSE STATEMENTS. Any warranty, representation or statement made or
     furnished to Lender by or on behalf of Grantor under this Agreement, the
     Note or the Related Documents is false or misleading in any material
     respect, either now or at the time made or furnished.

     DEFECTIVE COLLATERALIZATION. This Agreement or any of the Related Documents
     ceases to be in full force and effect (including failure of any collateral
     documents to create a valid and perfected security interest or lien) at any
     time and for any reason.

     INSOLVENCY. The dissolution or termination of Grantor's existence as a
     going business, the insolvency of Grantor, the appointment of a receiver
     for any part of Grantor's property, any assignment for the benefit of
     creditors, any type of creditor workout, or the commencement of any
     proceeding under any bankruptcy or insolvency laws by or against Grantor. 

     CREDITOR OR FORFEITURE PROCEEDINGS. Commencement of foreclosure or
     forfeiture proceedings, whether by judicial proceeding, self-help,
     repossession or any other method, by any creditor of Grantor or by any
     governmental agency against the Collateral or any other collateral securing
     the Indebtedness. This includes a garnishment of any of Grantor's deposit
     accounts with Lender.

     EVENTS AFFECTING GUARANTOR. Any of the preceding events occurs with
     respect to any Guarantor of any of the Indebtedness or such Guarantor dies
     or becomes incompetent.

     ADVERSE CHANGE. A material adverse change occurs in Grantor's financial
     condition, or Lender believes the prospect of payment or performance of
     the Indebtedness is impaired.

     INSECURITY. Lender, in good faith, deems itself insecure.

RIGHTS AND REMEDIES ON DEFAULT. If an Event of Default occurs under this
Agreement, at any time thereafter, Lender may exercise any one or more of the 
following rights and remedies:

     ACCELERATE INDEBTEDNESS. Declare all Indebtedness immediately due and
     payable, without notice of any kind to Grantor.

     COLLECT THE COLLATERAL. Collect any of the Collateral and, at Lender's
     option and to the extent permitted by applicable law, retain possession of
     the Collateral while suing on the Indebtedness.

     SELL THE COLLATERAL. Sell the Collateral, at Lender's discretion, as a unit
     or in parcels, at one or more public or private sales. Unless the
     Collateral is perishable or threatens to decline speedily in value or is of
     a type customarily sold on a recognized market, Lender shall give or mail
     to Grantor, or any of them, notice at least ten (10) days in advance of the
     time and place of any public sale, or of the date after which any private
     sale may be made. Grantor agrees that any requirement of reasonable notice
     is satisfied if Lender mails notice by ordinary mail addressed to Grantor,
     or any of them, at the last address Grantor has given Lender in writing. If
     a public sale is held, there shall be sufficient compliance with all
     requirements of notice to the public by a single publication in any
     newspaper of general circulation in the county where the Collateral is
     located, setting forth the time and place of sale and a brief description
     of the property to be sold. Lender may be a purchaser at any public sale.

     REGISTER SECURITIES. Register any securities included in the Collateral in
     Lender's name and exercise any rights normally incident to the ownership
     of securities.

     SELL SECURITIES. Sell any securities included in the Collateral in a
     manner consistent with applicable federal and state securities laws,
     notwithstanding any other provision of this or any other agreement. If,
     because of restrictions under such laws, Lender is or believes it is
     unable to sell the securities in an open market transaction, Grantor
     agrees that Lender shall have no obligation to delay sale until the
     securities can be registered, and may make a private sale to one or more
     persons or to a restricted group of persons, even though such sale may
     result in a price that is less favorable than might be obtained in an open
     market transaction, and such a sale shall be considered commercially
     reasonable. If any securities held as Collateral are "restricted
     securities" as defined in the Rules of the Securities and Exchange
     Commission (such as Regulation D or Rule 144) or state securities
     departments under state "Blue Sky" laws, or if Grantor is an affiliate of
     the issuer of the securities, Grantor agrees that neither Grantor nor any
     member of Grantor's family will sell or dispose of any securities of such
     issuer without obtaining Lender's prior written consent.

     FORECLOSURE. Maintain a judicial suit for foreclosure and sale of the
     Collateral.

     TRANSFER TITLE. Effect transfer of title upon sale of all or part of the
     Collateral. For this purpose, Grantor irrevocably appoints Lender as its
     attorney-in-fact to execute endorsements, assignments and instruments in
     the name of Grantor and each of them (if more than one) as shall be
     necessary or reasonable.

     OTHER RIGHTS AND REMEDIES. Have and exercise any or all of the rights and
     remedies of a secured creditor under the provisions of the Uniform
     Commercial Code, at law, in equity, or otherwise.

     APPLICATION OF PROCEEDS. Apply any cash which is part of the Collateral,
     or which is received from the collection or sale of the Collateral, to
     reimbursement of any expenses, including any costs for registration of
     securities, commissions incurred in connection with a sale, attorney fees
     as provided below, and court costs, whether or not there is a lawsuit and
     including any fees on appeal, incurred by Lender in connection with the
     collection and sale of such Collateral and to the payment of the
     indebtedness of Grantor to Lender, with any excess funds to be paid to
     Grantor as the interests of Grantor may appear. Grantor agrees, to the
     extent permitted by law, to pay any deficiency after application of the
     proceeds of the Collateral to the Indebtedness.

     CUMULATIVE REMEDIES. All of Lender's rights and remedies, whether
     evidenced by this Agreement or by any other writing, shall be cumulative
     and may be exercised singularly or concurrently. Election by Lender to
     pursue any remedy shall not exclude pursuit of any other remedy, and an
     election to make expenditures or to take action to perform an obligation
     of Grantor under this Agreement, after Grantor's failure to perform, shall
     not affect Lender's right to declare a default and to exercise its
     remedies.

MISCELLANEOUS PROVISIONS. The following miscellaneous provisions are a part of
this Agreement:

     AMENDMENTS. This Agreement, together with any Related Documents,
     constitutes the entire understanding and agreement of the parties as to
     the matters set forth in this Agreement. No alteration of or amendment to
     this Agreement shall be effective unless given in writing and signed by
     the party or parties sought to be charged or bound by the alteration or
     amendment.

     APPLICABLE LAW. THIS AGREEMENT HAS BEEN DELIVERED TO LENDER AND ACCEPTED
     BY LENDER IN THE STATE OF TEXAS. IF THERE IS A LAWSUIT, AND IF



<PAGE>   6
04-05-1999                    COMMERCIAL PLEDGE AGREEMENT                 PAGE 4
LOAN NO 0299                        (CONTINUED)

================================================================================

     THE TRANSACTION EVIDENCED BY THIS AGREEMENT OCCURRED IN BEXAR COUNTY,
     GRANTOR AGREES UPON LENDER'S REQUEST TO SUBMIT TO THE JURISDICTION OF THE
     COURTS OF BEXAR COUNTY, THE STATE OF TEXAS. THIS AGREEMENT SHALL BE
     GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS
     AND APPLICABLE FEDERAL LAWS.

     ATTORNEY'S FEES AND OTHER COSTS. Lender may hire an attorney to help
     collect the Note if Grantor does not pay, and Grantor will pay Lender's
     reasonable attorneys' fees. Grantor also will pay Lender all other amounts
     actually incurred by Lender as court costs, lawful fees for filing,
     recording, or releasing to any public office any instrument securing the
     Note; the reasonable cost actually expended for repossessing, storing,
     preparing for sale, and selling any security; and fees for noting a lien on
     or transferring a certificate of title to any motor vehicle offered as 
     security for the Note, or premiums or identifiable charges received in
     connection with the sale of authorized insurance.

     CAPTION HEADINGS. Caption headings in this Agreement are for convenience
     purposes only and are not to be used to interpret or define the provisions
     of this Agreement.

     NOTICES. All notices required to be given under this Agreement shall be
     given in writing, may be sent by telefacsimile (unless otherwise required
     by law), and shall be effective when actually delivered or when deposited
     with a nationally recognized overnight courier or deposited in the United
     States mail, first class, postage prepaid, addressed to the party to whom
     the notice is to be given at the address shown above. Any party may change
     its address for notices under this Agreement by giving formal written
     notice to the other parties, specifying that the purpose of the notice is
     to change the party's address. To the extent permitted by applicable law,
     if there is more than one Grantor, notice to any Grantor will constitute
     notice to all Grantors. For notice purposes, Grantor will keep Lender
     informed at all times of Grantor's current address(es).

     SEVERABILITY. If a court of competent jurisdiction finds any provision of
     this Agreement to be invalid or unenforceable as to any person or
     circumstance, such finding shall not render that provision invalid or
     unenforceable as to any other persons or circumstances. If feasible, any
     such offending provision shall be deemed to be modified to be within the
     limits of enforceability or validity; however, if the offending provision
     cannot be so modified, it shall be stricken and all other provisions of
     this Agreement in all other respects shall remain valid and enforceable.

     SUCCESSOR INTERESTS. Subject to the limitations set forth above on transfer
     of the Collateral, this Agreement shall be binding upon and inure to the
     benefit of the parties, their successors and assigns.

     WAIVER. Lender shall not be deemed to have waived any rights under this
     Agreement unless such waiver is given in writing and signed by Lender. No
     delay or omission on the part of Lender in exercising any right shall
     operate as a waiver of such right or any other right. A waiver by Lender of
     a provision of this Agreement shall not prejudice or constitute a waiver of
     Lender's right otherwise to demand strict compliance with that provision or
     any other provision of this Agreement. No prior waiver by Lender, nor any
     course of dealing between Lender and Grantor, shall constitute a waiver of
     any of Lender's rights or of any of Grantor's obligations as to any future
     transactions. Whenever the consent of Lender is required under this
     Agreement, the granting of such consent by Lender in any instance shall not
     constitute continuing consent to subsequent instances where such consent is
     required and in all cases such consent may be granted or withheld in the
     sole discretion of Lender.

OTHER PARTIES AND OTHER COLLATERAL. No renewal or extension of or any other
indulgence with respect to the indebtedness or any part thereof, no release of
any security, no release of any person (including any maker, endorser, guarantor
or surety) liable on the Indebtedness, no delay in enforcement of payment, and
no delay or omission or lack of diligence or care in exercising any right or
power with respect to the Indebtedness or any security thereof or under this
Agreement shall in any manner impair or affect the rights of Lender under the
law, hereunder, or under any agreement pertaining to the Collateral. Lender need
not file suit or assert a claim for personal judgment against any person for any
part of the Indebtedness or seek to realize upon any other security for the
Indebtedness, before foreclosing upon the Collateral for the purpose of paying
the Indebtedness. I waive any right to the benefit of or to require or control
application of any other security or proceeds thereof, and agree that Lender
shall have no duty or obligation to me to apply to the Indebtedness any such
other security or proceeds thereof.

PARTIES BOUND. Lender's rights hereunder shall inure to the benefit of its
successors and assigns, and in the event of any assignment or transfer of the
Note or the Property, Lender thereafter shall be fully discharged from any
responsibility with respect to the Collateral so assigned or transferred, but
Lender shall retain all rights and powers hereby given with respect to the note
or Collateral not so assigned or transferred. All my representations, warranties
and agreements, if more than one, are joint and several, and all shall be
binding upon my personal representatives, heirs, successors and assigns. I may
not assign any rights, duties or obligations hereunder without the prior written
consent of Lender.

CONTINUATION OF SECURITY INTEREST. It is expressly agreed that upon any
termination of the Custodial Account and/or the Credit Line Agreement, the
security interest herein granted in the Property shall remain in full force and
effect until the Indebtedness has been fully paid and discharged,
notwithstanding the termination of the Custodial Account and/or the Credit Line
Agreement or the fact that the assets comprising the Property may no longer be
held in the Custodial Account. I irrevocably appoint Lender, its agents or
employees, as my agent and attorney-in-fact to execute and file any documents
and give any notices which may be necessary to evidence the continuation of
said security interest after termination of the Custodial Account and/or the
Credit Line Agreement.

ENFORCEABILITY. This Pledge Agreement shall remain in full force and effect
regardless of whether the liability of any other Obligor or any other party
liable upon the Indebtedness may have ceased, or irrespective of the validity
or enforceability of any other instrument executed in connection with the
Indebtedness, and notwithstanding the reorganization, death, incapacity or
bankruptcy of any other party liable on the Indebtedness or the reorganization,
my death, incapacity or bankruptcy or any other event or proceeding affecting
me or any other party liable on the Indebtedness. The grant of a security
interest hereunder and all of Lender's rights, powers and remedies in connection
therewith shall remain in full force and effect until Lender has executed a
written termination statement and reassigned or released to me, without
recourse, the Property and all rights conveyed hereby.

FACSIMILE DOCUMENTS AND SIGNATURES. For purposes of negotiating and finalizing
this document, if this document is transmitted by facsimile machine ("fax"), it
shall be treated for all purposes as an original document. Additionally, the
signature of any party on this document transmitted by way of a fax machine
shall be considered for all purposes as an original signature. Any such faxed
document shall be considered to have the same binding legal effect as an
original document. At the request of any party, any faxed document shall be
re-executed by each signatory party in an original form. 



 
<PAGE>   7
04-05-1999               COMMERCIAL PLEDGE AGREEMENT                     PAGE 5
LOAN NO 0299                  (CONTINUED)
===============================================================================

GRANTOR ACKNOWLEDGES HAVING READ ALL THE PROVISIONS OF THIS PLEDGE AGREEMENT,
AND GRANTOR AGREES TO ITS TERMS. THIS AGREEMENT IS DATED APRIL 5, 1999.

GRANTOR:

ILEX ONCOLOGY, INC.

By: /s/ RICHARD L. LOVE
   ----------------------------------------
   RICHARD L. LOVE, President

===============================================================================
<PAGE>   8
M023

TX-DDD 1436 CST 04/29/99
18
TRT
*166995 FROSTBK
841412291?
088857 1536 04/29
GA
41229-1 DG D
*166995 FROSTBK
TO:  DG Bank
     DEUTSCHE GENOSSENSCHAFTSBANK AG
     EZD2: MRS. WERNER
     AM PLATZ DER REPUBLIK
     60265 FRANKFURT

MESSAGE NO.   18

THIS TEST IS TESTED WITH CITIBANK N.A., NEW YORK TESTING KEYS FOR
AUTHENTICATION PURPOSES ONLY. PLEASE VERIFY THIS TEST NUMBER WITH THEM. THEIR
TELEX NUMBER NO. 669720

TEST NO.            For No Amount
                    Dated April 29, 1999

Attn:     Documentary Credits

We hereby issue Amendment No. 1 to our Irrevocable Standby Letter of Credit No.
22874-S, in the amount of 3,000,000.-DEM in favor of RAIFFEISENBANK,
Buchloe-Kaufbeuren-Marktoberdorf eG Gutenbergstrasse 4, 87600
Kaufbeuren/Germany, for the account of Ilex Oncology, Inc., 11550 IH 10
West-Suite 300, San Antonio, Texas 78230, as follows:

The expiration date is extended to June 30, 1999.

All other terms and conditions remain unchanged.

Note: "If you are unable to comply with this instrument, please advise The
Frost National Bank, San Antonio, Texas in writing within ten (10) business
days. This Amendment is considered accepted if The Frost National Bank is not
notified within ten (10) business days."

Please obtain acceptance of Amendment No. 1 and advise us by tested message of
the acceptance. Thank you for your assistance.

This Telex is the operative instrument as per Article 11-A. No airmail
confirmation will follow.

"This credit is subject to the current Uniform Customs and Practice for
Documentary Credits (1993 Revision) issued by the International Chamber of
Commerce Publication No. 500."

Regards,

Gloria A. Munoz, Vice President 
Letters of Credit Dept.
The Frost National Bank, San Antonio, Texas USA
April 29, 1999    GAM/vmb