<PAGE>   1
                                                                   EXHIBIT 10.15


                   THE TRANSFER OF THIS WARRANT IS SUBJECT TO
                   RESTRICTIONS CONTAINED HEREIN. THIS WARRANT
                      HAS BEEN ISSUED IN RELIANCE UPON THE
                  REPRESENTATION OF THE HOLDER THAT IT HAS BEEN
                  ACQUIRED FOR INVESTMENT PURPOSES AND NOT WITH
                 A VIEW TOWARD THE RESALE OR OTHER DISTRIBUTION
                  THEREOF. NEITHER THIS WARRANT NOR THE SHARES
                   ISSUABLE UPON THE EXERCISE OF THIS WARRANT
                    HAVE BEEN REGISTERED UNDER THE SECURITIES
                    ACT OF 1933 OR ANY STATE SECURITIES LAWS.


                                      IRORI

                          Common Stock Purchase Warrant


To Subscribe for and Purchase                                      July 18, 1995
150,000 Shares of Common Stock
of Irori


        THIS CERTIFIES that, for value received, Enterprise Partners III, L.P.,
or its registered assigns (the "Holder"), is entitled to subscribe for and
purchase from Irori, a California corporation (hereinafter called the
"Corporation"), up to 150,000 shares (subject to adjustment as hereinafter
provided) of fully paid and non-assessable Common Stock of the Corporation (the
"Common Stock"), subject to the provisions and upon the terms and conditions
hereinafter set forth at the price of $0.01 per share (such price as from time
to time to be adjusted as provided herein is called the "Warrant Price"), at or
prior to the earlier of (i) 5:00 p.m. Pacific time on July 17, 2000, or (ii) the
effective date of the registration statement for the Corporation's initial
public offering of equity securities (the "Exercise Period"). This Warrant and
any Warrant subsequently issued upon exchange or transfer hereof are hereinafter
collectively called the "Warrant."

        Section 1. Exercise of Warrant. The rights represented by this Warrant
may be exercised by the Holder, in whole or in part (but not as to fractional
shares) at any time or from time to time during the Exercise Period by the
completion of the purchase form attached hereto and by the surrender of this
Warrant (properly endorsed) at the office of the Corporation as it may designate
by notice in writing to the Holder hereof at the address of the Holder appearing
on the books of the Corporation, and by payment to the Corporation of the
Warrant Price in cash or by certified or official bank check, for each share
being purchased. (In addition, see Section 2 below for net issuance provisions.)
In the event of any exercise of the rights represented by this Warrant, a
certificate or certificates for the shares of Common Stock so purchased,
registered in the name of the Holder, or its nominee or other party designated
in the purchase form by the


<PAGE>   2

Holder hereof, shall be delivered to the Holder within thirty (30) business days
after the date in which the rights represented by this Warrant shall have been
so exercised; and, unless this Warrant has expired or has been exercised in
full, a new Warrant representing the number of shares (except a remaining
fractional share), if any, with respect to which this Warrant shall not then
have been exercised shall also be issued to the Holder within such time. The
person in whose name any certificate for shares of Common Stock is issued upon
exercise of this Warrant shall for all purposes be deemed to have become the
holder of record of such shares on the date on which this Warrant was
surrendered and payment of the Warrant Price, except that, if the date of such
surrender and payment is a date on which the stock transfer books of the
Corporation are closed, such person shall be deemed to have become the holder of
such shares at the close of business on the next succeeding date on which the
stock transfer books are open. No fractional shares shall be issued upon
exercise of this Warrant and no payment or adjustment shall be made upon any
exercise on account of any cash dividends on the Common Stock issued upon such
exercise. If any fractional interest in a share of Common Stock would, except
for the provision of this Section 1, be delivered upon such exercise, the
Corporation, in lieu of delivery of a fractional share thereof, shall pay to the
Holder an amount in cash equal to the current market price of such fractional
share as determined in good faith by the Board of Directors of the Corporation
(the "Board").

        Section 2. Net Issuance.

               (a) Right to Convert. In addition to and without limiting the
rights of the Holder under the terms of this Warrant, the Holder shall have the
right to convert this Warrant or any portion thereof (the "Conversion Right")
into shares of Common Stock as provided in this Section 2 at any time or from
time to time during the Exercise Period. Upon exercise of the Conversion Right
with respect to a particular number of shares subject to the Warrant (the
"Converted Warrant Shares"), the Corporation shall deliver to the Holder
(without payment by the Holder of any exercise price or any cash or other
consideration) that number of shares of fully paid and nonassessable Common
Stock computed using the following formula:


        X = Y (A - B)
            ---------
            A


        Where  X = the number of shares of Common Stock to be delivered to the
                   holder

               Y = the number of Converted Warrant Shares

               A = the fair market value of one share of the Corporation's
                   Common Stock on the Conversion Date (as defined below)

               B = the per share exercise price of the Warrant (as adjusted to
                   the Conversion Date)

The Conversion Right may only be exercised with respect to a whole number of
shares subject to the Warrant. No fractional shares shall be issuable upon
exercise of the Conversion Right, and if the number of shares to be issued
determined in accordance with the foregoing formula is other



                                      -2-
<PAGE>   3

than a whole number, the Corporation shall pay to the Holder an amount in cash
equal to the fair market value of the resulting fractional share on the
Conversion Date (as defined below). Shares issued pursuant to the Conversion
Right shall be treated as if they were issued upon the exercise of the Warrant.

              (b) Method of Exercise. The Conversion Right may be exercised by
the Holder by the surrender of the Warrant at the principal office of the
Corporation together with a written statement specifying that the Holder thereby
intends to exercise the Conversion Right and indicating the total number of
shares under the Warrant that the Holder is exercising through the Conversion
Right. Such conversion shall be effective upon receipt by the Corporation of the
Warrant together with the aforesaid written statement, or on such later date as
is specified therein (the "Conversion Date"). Certificates for the shares
issuable upon exercise of the Conversion Right and, if applicable, a new warrant
evidencing the balance of the shares remaining subject to the Warrant, shall be
issued as of the Conversion Date and shall be delivered to the Holder promptly
following the Conversion Date.

              (c) Determination of Fair Market Value. For purposes of this
Section 2, fair market value of a share of Common Stock on the Conversion Date
shall mean:

                      (i) If traded on a stock exchange, the fair market value
of the Common Stock shall be deemed to be the average of the closing selling
prices of the Common Stock on the stock exchange determined by the Board to be
the primary market for the Common Stock over the ten (10) trading day period (or
such shorter period immediately following the closing of an initial public
offering) ending on the date prior to the Conversion Date, as such prices are
officially quoted in the composite tape of transactions on such exchange;

                      (ii) If traded over-the-counter, the fair market value of
the Common Stock shall be deemed to be the average of the closing bid prices
(or, if such information is available, the closing selling prices) of the Common
Stock over the ten (10) trading day period (or such shorter period immediately
following the closing of an initial public offering) ending on the date prior to
the Conversion Date, as such prices are reported by the National Association of
Securities Dealers through its NASDAQ system or any successor system; and

                      (iii) If there is no public market for the Common Stock,
then the fair market value shall be determined by mutual agreement of the holder
of the Warrant and the Corporation, and if the holder and the Corporation are
unable to so agree, by an investment banker of national reputation selected by
the Corporation and reasonably acceptable to the holder of the Warrant.

        Section 3. Stock Splits, Consolidation, Merger and Sale. In the event
that before the issuance of the shares of Common Stock into which this Warrant
may be exercised the outstanding shares of Common Stock shall be split, combined
or consolidated, by dividend, reclassification or otherwise, into a greater or
lesser number of shares of Common Stock, the Warrant Price in effect immediately
prior to such combination or consolidation and the number of shares purchasable
under this Warrant shall, concurrently with the effectiveness of such
combination or consolidation, be proportionately adjusted. If there shall be
effected any consolidation or merger of the Corporation with another
corporation, or a sale of all or



                                      -3-
<PAGE>   4

substantially all of the Corporation's assets to another corporation, and if the
holders of Common Stock shall be entitled pursuant to the terms of any such
transaction to receive stock, securities or assets with respect to or in
exchange for Common Stock, then, as a condition of such consolidation, merger or
sale, lawful and adequate provisions shall be made whereby the Holder of this
Warrant shall thereafter have the right to receive, upon the basis and upon the
terms and conditions specified herein and in lieu of the shares of Common Stock
immediately theretofore receivable upon the exercise of such Warrant, such
shares of stock, securities or assets as may be issuable or payable with respect
to or in exchange for a number of outstanding shares of such Common Stock equal
to the number of shares of such Common Stock immediately theretofore so
receivable had such consolidation, merger or sale not taken place, and in any
such case appropriate provisions shall be made with respect to the rights and
interests of the Holder to the end that the provisions hereof shall thereafter
be applicable, as nearly as may be, in relation to any shares of stock,
securities or assets thereafter deliverable upon the exercise of this Warrant.

              (a) Stock to Be Reserved. The Corporation will at all times
reserve and keep available out of its authorized Common Stock, solely for the
purpose of issue upon the exercise of this Warrant as herein provided, such
number of shares of Common Stock as shall then be issuable upon the exercise of
this Warrant. The Corporation shall from time to time in accordance with
applicable law increase the authorized amount of its Common Stock if at any time
the number of shares of Common Stock remaining unissued and available for
issuance shall not be sufficient to permit exercise of this Warrant. The
Corporation covenants that all shares of Common Stock which shall be so issued
shall be duly and validly issued and fully paid and nonassessable and free from
all taxes, liens and charges with respect to the issue thereof, and, without
limiting the generality of the foregoing, the Corporation will take all such
action as may be necessary to assure that all such shares of Common Stock may be
so issued without violation of any applicable law or regulation, or of any
requirements of any national securities exchange upon which shares of capital
stock of the Corporation may be listed.

              (b) Issue Tax. The issuance of certificates for shares of Common
Stock upon exercise of this Warrant shall be made without charge to the Holders
of this Warrant for any issuance tax in respect thereof provided that the
Corporation shall not be required to pay any tax which may be payable in respect
of any transfer involved in the issuance and delivery of any certificate in a
name other than that of the Holder of this Warrant.

              (c) Closing of Books. The Corporation will at no time close its
transfer books against the transfer of the shares of Common Stock issued or
issuable upon the exercise of this Warrant in any manner which interferes with
the timely exercise of this Warrant.

        Section 4. Notices of Record Dates. In the event of:

              (a) any taking by the Corporation of a record of the holders of
any class of securities for the purpose of determining the holders thereof who
are entitled to receive any dividend or other distribution (other than cash
dividends out of earned surplus), or any right to subscribe for, purchase or
otherwise acquire any shares of stock of any class or any other securities or
property, or to receive any other right, or



                                      -4-
<PAGE>   5

              (b) any capital reorganization of the Corporation, any
reclassification or recapitalization of the capital stock of the Corporation or
any transfer of all or substantially all the assets of the Corporation to or
consolidation or merger of the Corporation with or into any other corporation,
or

              (c) any voluntary or involuntary dissolution, liquidation or
winding-up of the Corporation,

then and in each such event the Corporation will give notice to the Holder of
this Warrant specifying (i) the date on which any such record is to be taken for
the purpose of such dividend, distribution or right and stating the amount and
character of such dividend, distribution or right, and (ii) the date on which
any such reorganization, reclassification, recapitalization, transfer,
consolidation, merger, dissolution, liquidation or winding-up is to take place,
and the time, if any is to be fixed, as of which the holders of record of Common
Stock will be entitled to exchange their shares of Common Stock for securities
or other property deliverable upon such reorganization, reclassification,
recapitalization, transfer, consolidation, merger, dissolution, liquidation or
winding-up. Such notice shall be given at least ten (10) days and not more than
ninety (90) days prior to the date therein specified, and such notice shall
state that the action in question or the record date is subject to the
effectiveness of a registration statement under the Securities Act of 1933, as
amended (the "Securities Act") or to a favorable vote of stockholders, if either
is required.

        Section 5. No Stockholder Rights or Liabilities. This Warrant shall not
entitle the Holder hereof to any voting rights or other rights as a stockholder
of the Corporation. No provision hereof, in the absence of affirmative action by
the Holder hereof to purchase shares of Common Stock, and no mere enumeration
hereon of the rights or privileges of the Holder hereof, shall give rise to any
liability of such Holder for the Warrant Price or as a stockholder of the
Corporation, whether such liability is asserted by the Corporation or by
creditors of the Corporation.

        Section 6. Representations of Holder. By acceptance of this Warrant, the
Holder hereby represents and acknowledges to the Corporation that:

              (a) this Warrant, the Common Stock issuable upon exercise of this
Warrant and any securities issued with respect to any of them by way of a stock
dividend or stock split or in connection with a recapitalization, merger,
consolidation or other reorganization will be "restricted securities" as such
term is used in the rules and regulations under the Securities Act and that such
securities have not been and will not be registered under the Securities Act or
any state securities law, and that such securities must be held indefinitely
unless registration is effected or transfer can be made pursuant to appropriate
exemptions;

              (b) the Holder has read, and fully understands, the terms of this
Warrant set forth on its face and the attachments hereto, including the
restrictions on transfer contained herein;

              (c) the Holder has either a pre-existing personal or business
relationship with the Corporation or one of its officers, directors or
controlling persons;



                                      -5-
<PAGE>   6

              (d) the Holder is purchasing for investment for its own account
and not with a view to or for sale in connection with any distribution of this
Warrant or the Common Stock of the Corporation issuable upon exercise of this
Warrant and it has no intention of selling such securities in a public
distribution in violation of the federal securities laws or any applicable state
securities laws; provided that nothing contained herein will prevent Holder from
transferring such securities in compliance with the terms of this Warrant and
the applicable federal and state securities laws;

              (e) the Holder is an "accredited investor" within the meaning of
paragraph (a) of Rule 501 of Regulation D promulgated by the Securities and
Exchange Commission and an "excluded purchaser" within the meaning of Section
25102(f) of the California Corporate Securities Law of 1968; and

              (f) the Corporation may affix the following legend (in addition to
any other legend(s), if any, required by applicable state corporate and/or
securities laws) to certificates for shares of Common Stock (or other
securities) issued upon exercise of this Warrant ("Warrant Shares"):

              "These securities have not been registered under the Securities
              Act of 1933, as amended. They may not be sold, offered for sale,
              pledged or hypothecated in the absence of a registration statement
              in effect with respect to the securities under such Act or an
              opinion of counsel satisfactory to the Company that such
              registration is not required or unless sold pursuant to Rule 144
              of such Act."

        Section 7. Notice of Proposed Transfers. The Holder of this Warrant, by
acceptance hereof, agrees to comply in all respects with the provisions of this
Section 7. Prior to any proposed transfer of this Warrant or any Warrant Shares,
unless there is in effect a registration statement under the Securities Act
covering the proposed transfer, the Holder of such securities shall give written
notice to the Corporation of such Holder's intention to effect such transfer.
Each such notice shall describe the manner and circumstances of the proposed
transfer in sufficient detail, and shall be accompanied (except in transactions
in compliance with Rule 144) by either (i) a written opinion of legal counsel
who shall be reasonably satisfactory to the Corporation addressed to the
Corporation and reasonably satisfactory in form and substance to the
Corporation's counsel, to the effect that the proposed transfer of the Warrant
and/or Warrant Shares may be effected without registration under the Securities
Act, or (ii) a "no action" letter from the U.S. Securities and Exchange
Commission (the "Commission") to the effect that the transfer of such securities
without registration will not result in a recommendation by the staff of the
Commission that enforcement action be taken with respect thereto, whereupon the
Holder of such securities shall be entitled to transfer such securities in
accordance with the terms of the notice delivered by the Holder to the
Corporation. Each new certificate evidencing the Warrant and/or Warrant Shares
so transferred shall bear the appropriate restrictive legends set forth in
Section 6(f) above, except that such certificate shall not bear such restrictive
legend if, in the opinion of counsel for the Corporation, such legend is not
required in order to establish or assist in compliance with any provisions of
the Securities Act or any applicable state securities laws.



                                      -6-
<PAGE>   7
        Section 8. Lost, Stolen, Mutilated or Destroyed Warrant. If this Warrant
is lost, stolen, mutilated or destroyed, the Corporation may, on such terms as
to indemnity or otherwise as it may in its discretion reasonably impose (which
shall, in the case of a mutilated Warrant, include the surrender thereof), issue
a new Warrant of like denomination and tenor as the Warrant so lost, stolen,
mutilated or destroyed. Any such new Warrant shall constitute an original
contractual obligation of the Corporation, whether or not the allegedly lost,
stolen, mutilated or destroyed Warrant shall be at any time enforceable by
anyone.

        Section 9. Presentment. Prior to due presentment of this Warrant
together with a completed assignment form attached hereto for registration of
transfer, the Corporation may deem and treat the Holder as the absolute owner of
the Warrant, notwithstanding any notation of ownership or other writing thereon,
for the purpose of any exercise thereof and for all other purposes, and the
Corporation shall not be affected by any notice to the contrary.

        Section 10. Notice. Notice or demand pursuant to this Warrant shall be
sufficiently given or made, if sent by first-class mail, postage prepaid,
addressed, if to the Holder of this Warrant, to the Holder at its last known
address as it shall appear in the records of the Corporation, and if to the
Corporation, at 11588 Sorrento Valley Road, Suite 16, San Diego, California
92121, Attention: Secretary. The Corporation may alter the address to which
communications are to be sent by giving notice of such change of address in
conformity with the provisions of this Section 10 for the giving of notice.

        Section 11. Governing Law. The validity, interpretation and performance
of this Warrant shall be governed by the laws of the State of California without
regard to principles of conflicts of laws.

        Section 12. Successors, Assigns. Subject to the restrictions on transfer
by Holder set forth in Section 7 hereof, all the terms and provisions of the
Warrant shall be binding upon and inure to the benefit of and be enforceable by
the respective successors and assigns of the parties hereto.

        Section 13. Amendment. This Warrant may be modified, amended or
terminated by a writing signed by the Corporation and the Holder.

        Section 14. Severability. Should any part but not the whole of this
Warrant for any reason be declared invalid, such decision shall not affect the
validity of any remaining portion, which remaining portion shall remain in force
and effect as if this Warrant had been executed with the invalid portion thereof
eliminated, and it is hereby declared the intention of the parties hereto that
they would have executed the remaining portion of this Warrant without including
therein any such part which may, for any reason, be hereafter declared invalid.

                [REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]


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        IN WITNESS WHEREOF, the Corporation has caused this Warrant to be duly
executed and delivered on and as of the day and year first above written by one
of its officers thereunto duly authorized.

                                        IRORI, a California corporation


Dated: July 18, 1995                    By: /s/ Michael P. Nova
                                            -----------------------------------
                                                Michael P. Nova
                                        Title: President
                                               --------------------------------


        The undersigned Holder agrees and accepts this Warrant and acknowledges
that it has read and confirms each of the representations contained in Section
6.


                                        ENTERPRISE PARTNERS III, L.P.


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

                                        Its:

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

                                        Address:
                                                 ------------------------------

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

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


               [SIGNATURE PAGE TO COMMON STOCK PURCHASE WARRANT]
<PAGE>   9


                                  PURCHASE FORM


(To be executed by the Warrant Holder if he desires to exercise the Warrant in
whole or in part)

To: IRORI


        The undersigned, whose Social Security or other identifying number is
______________ , hereby irrevocably elects the right of purchase represented by
the within Warrant for, and to purchase thereunder,
_________________________________shares of Common Stock provided for therein and
tenders payment herewith to the order of

                                      IRORI
                                in the amount of


                              $___________________


The undersigned requests that certificates for such shares be issued as follows:

Name: __________________________________________________________________________

Address: _______________________________________________________________________

Deliver to: ____________________________________________________________________

Address: _______________________________________________________________________

and, if said number of shares shall not be all the shares purchasable hereunder,
that a new Warrant for the balance remaining of the shares purchasable under the
within Warrant be registered in the name of, and delivered to, the undersigned
at the address stated below

                                   Address: ____________________________________


Dated: ___________, 199__



                                        Signature ______________________________
                                        (Signature must conform in all respects
                                        to the name of the Warrant Holder as
                                        specified on the face of the Warrant,
                                        without alteration, enlargement or any
                                        change whatsoever)


<PAGE>   10


                                   ASSIGNMENT


(To be executed by the Warrant Holder if he desires to effect a transfer of the
Warrant)

FOR VALUE RECEIVED, the undersigned hereby sells, assigns and transfers unto
_________________________________________________________________ - whose Social
Security or other identification number is ________________ residing/located] at
___________________ the attached Warrant, and appoints _________________________
residing at
________________________________________________________________________________

________________________________________________________________________________
the undersigned's attorney-in-fact to transfer said Warrant on the books of the
Corporation, with full power of substitution in the premises.


Dated: ____________, 199__.

In the presence of:


____________________________            ________________________________________
                                        (Signature must conform in all respects
                                        to the name of the Warrant Holder as
                                        specified on the face of the Warrant,
                                        without alteration, enlargement or any
                                        change whatsoever).