<PAGE>

                                                                     EXHIBIT 4.3

                                     TEKELEC

                       NONSTATUTORY STOCK OPTION AGREEMENT

         Tekelec, a California corporation (the "Company"), hereby enters into
this Nonstatutory Stock Option Agreement (this "Option Agreement") with TIMOTHY
MUTTER (the "Optionee") effective as of the 11th day of June, 2003, whereby the
Company grants to the Optionee the right and option to purchase an aggregate of
55,000 shares of Common Stock (the "Shares") of the Company in accordance with
the terms of that certain Employment Agreement dated April 30, 2003 between
Santera Systems Inc., a Delaware corporation and majority owned subsidiary of
the Company ("Santera"), and the Optionee (the "Employment Agreement"). The
grant of this Option fully discharges Santera's obligations with respect to the
grant to the Optionee of stock options to purchase Tekelec Common Stock under
Section 7.4 of the Employment Agreement.

         1.       NATURE OF THE OPTION. This Option is intended to be a
nonstatutory stock option and is not intended to be an incentive stock option
within the meaning of Section 422 of the Internal Revenue Code of 1986, as
amended (the "Code"), or to otherwise qualify for any special tax benefits to
the Optionee.

         2.       EXERCISE PRICE. The exercise price is $12.70 per Share, which
price is not less than 100% of the fair market value thereof on the date of the
grant.

         3.       METHOD OF PAYMENT. The consideration to be paid for the Shares
to be issued upon exercise of this Option shall consist entirely of cash or
check payable to the Company.

         4.       EXERCISE OF OPTION. This Option shall be exercisable during
its term only in accordance with the terms and provisions of this Option
Agreement as follows:

                  (a)      This Option shall vest and become exercisable
cumulatively in 16 equal quarterly installments of 3,437.5 shares each, with the
first of such installments vesting on September 30, 2003, and one additional
installment vesting on the last day of each calendar quarter thereafter, as long
as the Optionee continues to serve as an employee of Santera; provided, however,
that all unvested installments of this Option that would otherwise vest on or
before June 30, 2004 if the Optionee's employment with Santera had continued
through such date shall immediately vest upon the termination of the Optionee's
employment with Santera by Santera without Cause during the Term of the
Employment Agreement or upon the Optionee's Resignation with Good Reason during
the Term of the Employment Agreement. Subject to Section 7 hereof, the Optionee
may exercise the exercisable portion of this Option in whole or in part at any
time during (i) his employment provided he has been in continuous employment
with Santera since the grant of this Option or (ii) the applicable period set
forth in Section 6 hereof; provided, however, that the Option may not be
exercised for a fraction of a Share. In the event of the Optionee's termination
of employment with Santera or the Optionee's disability or death, the provisions
of Sections 6 and 7 below shall apply to the right of the Optionee to exercise
this Option.

<PAGE>

                  (b)      This Option shall be exercisable by written notice
which shall state the election to exercise this Option, the number of Shares in
respect to which this Option is being exercised and such other representations
and agreements as may be required by the Company. Such written notice shall be
signed by the Optionee and shall be delivered in person or by certified mail to
the Secretary of the Company or such other person as may be designated by the
Company. The written notice shall be accompanied by payment of the purchase
price and an executed Notice of Exercise of Stock Option in the form attached
hereto. As soon as practicable after any proper exercise of this Option in
accordance with the provisions hereof, the Company shall deliver to the Optionee
at the principal executive office of the Company or such other place as shall be
mutually agreed upon between the Company and the Optionee, a certificate or
certificates representing the Shares for which the Option shall have been
exercised. The certificate or certificates for the Shares as to which this
Option is exercised shall be registered in the name of the Optionee.

                  (c)      No rights of a shareholder shall exist with respect
to the Shares under this Option as a result of the mere grant of this Option or
the exercise of this Option. Such rights shall exist only after issuance of a
stock certificate in accordance with Section 4(b) hereof.

                  (d)      For purposes of this Agreement, the terms "Cause,"
"Good Reason," "Resignation with Good Reason" and "Term of the Employment
Agreement" shall have the same meanings as the terms "Cause," "Good Reason,"
"Resignation with Good Reason" and "Term of this Agreement," respectively, have
in Sections 8.2, 8.3, 8.1(c) and 6, respectively, of the Employment Agreement,
except that, notwithstanding the foregoing and solely for purposes of Section 6
hereof, "Term of the Employment Agreement" shall mean the period of time
commencing on June 10, 2003 and ending on the date set forth in Section 6(i) of
the Employment Agreement.

         5.       RESTRICTIONS ON EXERCISE. This Option may not be exercised if
the issuance of Shares upon the Optionee's exercise or the method of payment of
consideration for such Shares would constitute a violation of any applicable
Federal or state securities law or other applicable law or regulation. As a
condition to the exercise of this Option, the Company may require the Optionee
to make any representation and warranty to the Company as may be required by any
applicable law or regulation.

         6.       TERMINATION OF EMPLOYMENT; DEATH OR DISABILITY. Each
installment of this Option shall expire and terminate to the extent not
exercised upon the earlier to occur of (a) four years after the vesting date of
each Option installment; or (b) the date which occurs (as applicable) (i) 12
months after the date upon which the Optionee's employment with Santera is
terminated (A) by Santera without Cause during the Term of the Employment
Agreement pursuant to Section 8.1(b) thereof or without Cause at any time after
the Term of the Employment Agreement, (B) with Cause due to the Optionee's death
or disability during or after the Term of the Employment Agreement, or (C) by
the Optionee with Good Reason during the Term of the Employment Agreement
pursuant to Section 8.1(c) thereof; (ii) six months after the date upon which
Optionee's employment is terminated by the Optionee with or without Good Reason
after the Term of the Employment Agreement; or (iii) 30 days after the date upon
which

                                        2

<PAGE>

the Optionee's employment is terminated (A) by Santera with Cause, other than
due to death or disability of the Optionee, during the Term of the Employment
Agreement pursuant to Section 8.1(a) thereof, (B) by Santera with Cause, other
than due to death or disability of the Optionee, at any time after the Term of
the Employment Agreement, or (C) by the Optionee without Good Reason during the
Term of the Employment Agreement.

         7.       TERM OF OPTION. Subject to Section 6 above, each quarterly
installment of this Option will expire and terminate, and may not be exercised
more than, four years after the date on which it vests (e.g., the installment
vesting on September 30, 2003 shall expire at the close of business on September
30, 2007 unless terminated earlier in accordance with the terms hereof) and may
be exercised during such term only in accordance with the terms of this Option
Agreement. Notwithstanding any provision herein with respect to the
post-employment exercise of this Option, this Option may not be exercised after
the expiration of its term.

         8.       RESERVATION OF SHARES. The Company covenants and agrees that
all Shares will, upon issuance and payment in accordance herewith, be fully
paid, validly issued and nonassessable. The Company further covenants and agrees
that during the term of this Option, the Company will at all times have
authorized and reserved for the purpose of issuance upon exercise of this Option
at least the maximum number of Shares as are issuable upon such exercise.

         9.       DISSOLUTION; LIQUIDATION, CONSOLIDATION, MERGER OR
RECLASSIFICATION. In the event that while the Optionee is an employee of
Santera, the Company proposes to dissolve or liquidate or to sell all or
substantially all of its assets (other than in the ordinary course of business),
or to merge or consolidate with or into another corporation as a result of which
the Company is not the surviving and controlling corporation, the Board of
Directors shall (i) make provision for the assumption of this Option by the
successor corporation or (ii) declare that this Option shall terminate as of a
date fixed by the Board of Directors which is at least 30 days after the notice
thereof to the Optionee and shall give the Optionee the right to exercise this
Option as to all or any part of the Shares, including Shares as to which this
Option would not otherwise be exercisable provided such exercise does not
violate Section 7 hereof.

         10.      ADJUSTMENT OF EXERCISE PRICE AND NUMBER OF SHARES.

                  (a)      The number of Shares subject to this Option, as well
as the exercise price per Share hereunder shall be proportionately adjusted for
any increase or decrease in the number of issued shares of the Company's Common
Stock resulting from a stock split or combination or the payment of a stock
dividend (but only on the Company's Common Stock) or any other increase or
decrease in the number of issued shares of the Company's Common Stock effected
without receipt of consideration by the Company (other than stock awards to
employees or directors of the Company); provided, however, that the conversion
of any convertible securities of the Company shall not be deemed to have been
effected without the receipt of consideration. Such adjustment shall be
automatic and the form of this Agreement need not be changed because of any such
adjustment in the exercise price or in the number of Shares purchasable upon
exercise of all or any portion of this Option. Except as expressly provided
herein, no issue by the

                                        3

<PAGE>

Company of shares of stock of any class, or securities convertible into shares
of stock of any class, shall affect, and no adjustment by reason thereof shall
be made with respect to, the number or price of Shares subject to this Option.

                  (b)      No fractional shares of Common Stock shall be
issuable on account of any action contemplated by Section 9 or Section 10(a)
hereof, and the aggregate number of shares into which Shares then covered by
this Option, when changed as the result of any such action, shall be reduced to
the largest number of whole shares resulting from such action, unless the
Company's Board of Directors, in its sole discretion, shall determine to issue
scrip certificates in respect to any fractional shares, which scrip certificates
shall be in a form and have such terms and conditions as the Board of Directors
in its discretion shall prescribe.

         12.      WITHHOLDING UPON EXERCISE OF OPTION. The Company reserves the
right to withhold, in accordance with any applicable laws, from any
consideration payable to the Optionee any taxes required to be withheld by
Federal, state or local law as a result of the grant or exercise of this Option
or the sale or other disposition of the Shares issued upon exercise of this
Option. If the amount of any consideration payable to the Optionee is
insufficient to pay such taxes or if no consideration is payable to the
Optionee, upon the request of the Company, the Optionee shall pay to the Company
in cash an amount sufficient for the Company to satisfy any Federal, state or
local tax withholding requirements it may incur as a result of the grant or
exercise of this Option or the sale or other disposition of the Shares issued
upon the exercise of this Option.

         13.      NONTRANSFERABILITY OF OPTION. This Option may not be sold,
pledged, assigned, hypothecated, gifted, transferred or disposed of in any
manner either voluntarily or involuntarily by operation of law, other than by
will or by the laws of descent or distribution or transfer between spouses
incident to a divorce. Subject to the foregoing, the terms of this Option shall
be binding upon the executors, administrators, heirs, successors and assigns of
the Optionee.

         14.      NO RIGHT OF EMPLOYMENT. This Option shall not confer upon the
Optionee any right to continue in the employment of Santera or limit in any
respect the right of Santera to discharge the Optionee at any time, with or
without cause and with or without notice.

         15.      MISCELLANEOUS.

                  (a)      Successors and Assigns. This Option Agreement shall
bind and inure only to the benefit of the parties to this Option Agreement (the
"Parties") and their respective successors and assigns.

                  (b)      No Third-Party Beneficiaries. Nothing in this Option
Agreement is intended to confer any rights or remedies on any persons other than
the Parties and their respective successors or assigns. Nothing in this Option
Agreement is intended to relieve or discharge the obligation or liability of
third persons to any Party. No provision of this Option Agreement shall give any
third person any right of subrogation or action over or against any Party.

                                        4

<PAGE>

                  (c)      Amendments.

                           (i)      The Company reserves the right to amend the
terms and provisions of this Option without the Optionee's consent to comply
with any Federal or state securities law.

                           (ii)     Except as specifically provided in
subsection (i) above, this Option Agreement shall not be changed or modified, in
whole or in part, except by supplemental agreement signed by the Parties. Either
Party may waive compliance by the other Party with any of the covenants or
conditions of this Option Agreement, but no waiver shall be binding unless
executed in writing by the Party making the waiver. No waiver or any provision
of this Option Agreement shall be deemed, or shall constitute, a waiver of any
other provision, whether or not similar, nor shall any waiver constitute a
continuing waiver. Any consent under this Option Agreement shall be in writing
and shall be effective only to the extent specifically set forth in such
writing. For the protection of the Parties, amendments, waivers and consents
that are not in writing and executed by the Party to be bound may be enforced
only if they are detrimentally relied upon and proved by clear and convincing
evidence. Such evidence shall not include any alleged reliance.

                  (d)      Notice. Any notice, instruction or communication
required or permitted to be given under this Option Agreement to either Party
shall be in writing and shall be deemed given when actually received or, if
earlier, five days after deposit in the United States mail by certified or
express mail, return receipt requested, first class postage prepaid, addressed
to the principal office of such Party or to such other address as such Party may
request by written notice.

                  (e)      Governing Law. To the extent that Federal laws do not
otherwise control, all determinations made or actions taken pursuant hereto
shall be governed by the laws of the State of California, without regard to the
conflict of laws rules thereof.

                  (f)      Entire Agreement. This Option Agreement constitutes
the entire agreement between the Parties with regard to the subject matter
hereof. This Option Agreement supersedes all previous agreements between the
Parties, and there are now no agreements, representations, or warranties between
the Parties, other than those set forth herein.

                  (g)      Severability. If any provision of this Option
Agreement or the application of such provision to any person or circumstances is
held invalid or unenforceable, the remainder of this Option Agreement, or the
application of such provision to persons or circumstances other than those as to
which it is held invalid or unenforceable, shall not be affected thereby.

                                        5

<PAGE>

         IN WITNESS WHEREOF, this Option Agreement has been duly executed on
behalf of the Company by an authorized representative of the Company and by the
Optionee as of the date and year first written above.

DATE OF GRANT: June 11, 2003               TEKELEC

                                           By:    /s/ Frederick M. Lax
                                               ---------------------------------
                                               Frederick M. Lax
                                               President and Chief Executive
                                               Officer

                                           OPTIONEE

                                                  /s/ Timothy Mutter
                                           -------------------------------------
                                                         signature

                                        6

<PAGE>

                  THIS OPTION AND THE SECURITIES WHICH MAY BE PURCHASED UPON
EXECUTION OF THIS OPTION HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF
1933, AS AMENDED, OR QUALIFIED UNDER THE SECURITIES LAWS OF ANY STATE, AND HAVE
BEEN ACQUIRED FOR INVESTMENT AND NOT WITH A VIEW TO, OR IN CONNECTION WITH, THE
SALE, TRANSFER OR DISTRIBUTION THEREOF. NO SUCH SALE, TRANSFER OR DISTRIBUTION
MAY BE EFFECTED WITHOUT AN EFFECTIVE REGISTRATION STATEMENT RELATING THERETO OR
AN OPINION OF COUNSEL SATISFACTORY TO THE COMPANY THAT SUCH REGISTRATION IS NOT
REQUIRED.

<PAGE>

                                     TEKELEC

                 NOTICE OF EXERCISE OF NONSTATUTORY STOCK OPTION

         I, _______________ ("Optionee"), hereby agree, represent and warrant to
Tekelec (the "Company") as follows:

         1.       I was granted a Nonstatutory Stock Option (the "Option") on
                  June 11, 2003.

         2.       Pursuant to the Option, I was granted the right to purchase
                  ____________ shares of the Company's Common Stock (the
                  "Optioned Shares").

         3.       I am eligible to exercise the Option.

         4.       I hereby elect to exercise the Option to purchase
                  _________________ of such Optioned Shares (the "Shares") at
                  $12.70 per share, for an aggregate purchase price of $
                  ________________.

         5.       Payment of Purchase Price (please check applicable box):

                  [ ]      This Notice of Exercise is accompanied by a check
                           representing payment in full of the purchase price
                           for the Shares plus all applicable withholding taxes.

                           OR

                  [ ]      This exercise is a "cashless exercise" effected
                           through my broker. Payment in full for the Shares
                           (including all applicable withholding taxes) in the
                           form of a check will be transmitted by my broker to
                           the Company.

         6.       In connection with my exercise of the Option, I have received
                  a copy of any Prospectus of the Company's relating to the
                  shares of the Company's Common Stock issuable under the
                  Option.

Dated: _________________, 200__            OPTIONEE

________________________________           Signature: __________________________
Social Security Number

                                           Address: ____________________________
                                                    ____________________________
                                                    ____________________________

RECEIVED ON BEHALF OF TEKELEC ON ____________________, 200__.

                                           SIGNATURE: __________________________