<PAGE>

                                                                     EXHIBIT 4.3




                           REVOLVING CREDIT AGREEMENT



This Revolving Credit (the "Credit Agreement"), dated as of August 5, 2001 is
entered into by and between Richard D. Propper, MD ("the Lender") and Parentech,
Inc., a Delaware corporation ("Borrower").
WHEREAS, Borrower desires to obtain financing from Lender; and
WHEREAS, Borrower has requested that Lender enter into this Credit Agreement
under which Borrower may, from time to time, subject to Lender's approval in
each case, borrow amounts from Lender, up to an aggregate of $1,000,000;
NOW THEREFORE, in consideration of the agreements and the mutual covenants
contained herein, the parties to this Credit Agreement agree as follows:



<PAGE>

Credit Facility

         Revolving Loan Commitment. Subject to the terms and conditions of this
                  Credit Agreement, Lender agrees to make loans to Borrower on a
                  revolving basis from time to time on or before June 30, 2002,
                  provided that Lender shall have no obligation to make loans in
                  excess of $100,000 (net of repayments) in any one calendar
                  month and $1,000,000 (net of repayments) in the aggregate; and
                  further provided, that all unpaid principal and accrued
                  interest on such loans are due and payable on or before the
                  earlier of June 30, 2002 or the receipt by Parentech, Inc. of
                  $1,000,000 or more in debt or equity financing prior to
                  deduction for commissions and expenses of such financing (the
                  "Commitment").

         Revolving Credit Note. All loans made pursuant to this Credit Agreement
                  are herein referred to as "Revolving Loans." All Revolving
                  Loans shall be evidenced by a promissory note (herein referred
                  to as a "Revolving Credit Note," which, together with the
                  Credit Agreement are referred to herein as the "Credit
                  Documents") in the form set forth in Exhibit A, bearing
                  interest (including default interest) and maturity as provided
                  in Exhibit A. The date and amount of each Revolving Loan made
                  by Lender and each repayment of principal thereon received by
                  Lender shall be recorded by Lender on the schedule attached to
                  the Revolving Credit Note or a continuation thereof or
                  otherwise recorded by Lender in its internal record and
                  accounts; provided, however, that the failure of Lender to
                  make a notation or entry, or any error in such notation or
                  entry, shall not limit or otherwise affect the obligation of
                  the Borrower hereunder or under the Revolving Credit Note to
                  repay the principal amount of the Revolving Loans together
                  with all interest accrued thereon.

         Prepayment Rights and Obligations. Borrower may from time to time
                  prepay the principal amount of any Revolving Credit Loan in
                  whole or part without premium or penalty. Any such prepayment
                  shall include accrued interest thereon through the date of
                  such prepayment and shall be applied as set forth in the
                  Revolving Credit Note.

         Interest. So long as no Event of Default is continuing, Borrower shall
                  pay interest on the outstanding principal amount of the
                  Revolving Loans as provided in the Revolving Credit Note.

         Termination of Commitment. Notwithstanding anything to the contrary
                  contained in this Credit Agreement, Lender shall have no
                  obligation to fund any Revolving Loan, and Lender's Commitment
                  shall terminate upon no less than ten (10) days prior written
                  notice to Borrower. Following any such termination or Lender's
                  Commitment hereunder, Borrower shall remain obligated to keep
                  and observe all of its covenants and obligations under this
                  Credit Agreement and the Revolving Credit Note until such time
                  as all Revolving Loans (including accrued and unpaid interest
                  thereon) are repaid as otherwise required hereby.

         Additional Loans. In connection with any negotiation that may occur
                  regarding Lender (i) making any loan or commitment to or on
                  behalf of Borrower in excess of the Commitment, (ii) making
                  any loan or commitment to or on behalf of Borrower following
                  the termination of Lender's Commitment to make Revolving Loans
                  pursuant to Section 2.6 hereof, or (iii) Lender otherwise
                  incurring or entering into any agreement to incur any
                  obligation, commitment, guarantee or other responsibility with
                  respect to the lending of money to or on behalf of Borrower,
                  the financing of any Borrower's operations or the payment or
                  satisfaction of any debt or other obligation of any Borrower,
                  Borrower and Lender shall also negotiate in good faith the
                  form and terms of additional consideration that Borrower shall
                  offer to Lender in order to induce Lender to make or undertake
                  such additional loans, commitments or other obligations. Any
                  such consideration shall be in addition to such interest,
                  default interest, and other customary terms and conditions
                  applicable to any such additional loans, commitments or other
                  obligations. Borrower and Lender agree that such consideration
                  may include, by way of example and without limitation, such
                  items as (i) the issuance of equity of Borrower or securities
                  convertible into or exchangeable for equity of Borrower, which
                  equity may have terms preferential to the terms of Borrower's
                  then existing equity, or (ii) preferential or exclusive rights
                  or terms with respect to the purchase, license, lease or other
                  use of Borrower's assets, products, goods or services.
                  Notwithstanding the foregoing, Lender does not event have any
                  obligation to provide such additional financing or to
                  undertake any such additional commitment or obligation, and
                  Lender may accept or reject the terms thereof or terminate the
                  negotiation thereof at any time in its sole and absolute
                  discretion.

         Federal  Income Tax Withholding. All interest due and payable under
                  this Credit Agreement shall be subject to applicable U.S.
                  federal income tax withholding requirements, unless Lender
                  delivers to Borrower an IRS Form W-8 BEN evidencing exemption
                  from such withholding requirements, or other proof of
                  exemption from withholding in form and substance reasonably
                  satisfactory to Borrower.


Representations and Warranties

In order to induce Lender to enter into this Credit Agreement and to make the
Revolving Loans, Borrower hereby represents and warrants to Lender as of the
date hereof and as of any Drawdown Date as follows: Borrower is duly organized,
validly existing, and in good standing under the laws of the State of Delaware;
Borrower has the power and authority to make, execute, deliver and perform each
of the Revolving Credit Note and this Credit Agreement; The Credit Agreement has
been duly executed and delivered by Borrower and constitutes a legal, valid and
binding obligation of Borrower, enforceable against Borrower in accordance with
its terms.
<PAGE>

Affirmative and Negative Covenants.

         Borrower agrees that until the termination of the Commitment and the
payment and satisfaction in full of all the Revolving Loans, Borrower will
comply with its obligations as set forth throughout this Credit Agreement and
will cooperate with Lender, take such action, execute such documents, and
provide such information as Lender may from time to time request in order
further to effect the transactions contemplated by and the purposes of the
Credit Documents.

Conditions

The obligation of Lender to make any loans under this Credit Agreement is
subject to the satisfaction, or waiver by Lender, of the following conditions
precedent:
         Note.    Lender shall have received the Revolving Credit Note,
                  conformed to the requirements hereof, and duly executed and
                  delivered to Lender.

         Representations and Warranties. Each of the representations and
                  warranties made by Borrower herein shall be true and correct
                  in all respects as of the date made or deemed made.

         Covenants. Each covenant to be complied with or performed by Borrower
                  pursuant to the terms hereof shall have been complied with or
                  performed in all material respects as of the date of this
                  Credit Agreement.

Events of Default; Remedies

         Events of Default. The occurrence or existence of any one or more of
                  the following events are referred to herein individually as an
                  "Event of Default," and collectively as "Events of Default":


Borrower fails to pay when due any of the Obligations or fails to perform any of
the terms, covenants, conditions or provisions contained in this Agreement or
the Revolving Credit Note and such failure to pay or perform is not cured (i)
with respect to any failure to pay any of the Obligations, within three (3)
Business Days after such failure, and (ii) with respect to any failure of
performance (other than one of payment) within twenty (20) business days
following the earlier to occur of (i) receipt of notice from Lender of such
failure to perform or (ii) Borrower having actual knowledge of such failure to
perform; any representation, warranty or statement of fact made by Borrower to
Lender in this Agreement, or any other agreement, schedule, confirmatory
assignment or otherwise shall when made or deemed made be false or misleading in
any material respect; the Borrower dissolves or suspends or discontinues doing
business; the Borrower makes an assignment for the benefit of creditors, makes
or sends notice of a bulk transfer or calls a meeting of its creditors or
principal creditors; a case or proceeding under the bankruptcy laws of the
United States of America now or hereafter in effect or under any insolvency,
reorganization, receivership, readjustment of debt, dissolution or liquidation
law or statute of any jurisdiction now or hereafter in effect (whether at law or
in equity) is filed against either Borrower or all or any part of its properties
and such petition or application is not dismissed within sixty (60) days after
the date of its filing or either Borrower shall file any answer admitting or not
contesting such petition or application or indicates its consent to,
acquiescence in or approval of, any such action or proceeding or the relief
requested is granted sooner; or a case or proceeding under the bankruptcy laws
of the United States of America now or hereafter in effect or under any
insolvency, reorganization, receivership, readjustment of debt, dissolution or
liquidation law or statute of any jurisdiction now or hereafter in effect
(whether at a law or equity) is filed by either Borrower or for all or any part
of its respective property.

         Remedies.

At any time an Event of Default exists or has occurred and is continuing, Lender
shall have all rights and remedies provided in this Agreement, the Uniform
Commercial Code and other applicable law, all of which rights and remedies may
be exercised without notice to or consent by Borrower, except as such notice or
consent is expressly provided for hereunder or required by applicable law. All
rights, remedies and powers granted to Lender hereunder, the Uniform Commercial
Code or other applicable law, are cumulative, not exclusive and enforceable, in
Lender's discretion, alternatively, successively, or concurrently on any one or
more occasions, and shall include, without limitation, the right to apply to a
court of equity for an injunction to restrain a breach or threatened breach by
Borrower of this Agreement.
Without limiting the foregoing, upon the occurrence of an Event of Default or an
event which with notice or passage of time or both would constitute an Event of
Default, Lender may, at its option, without notice, (i) cease making Loans to
Borrower and/or (ii) terminate any provision of this Credit Agreement providing
for any future Loans to be made by Lender to Borrower.
<PAGE>

         Other Remedies. If any Event of Default shall be continuing, Lender
                  may enforce its rights by suit in equity, by action at law, or
                  by any other appropriate proceedings, whether for the specific
                  performance (to the extent permitted by law) of any covenant
                  or agreement contained in this Credit Agreement or in aid of
                  the exercise of any power granted in this Credit Agreement and
                  may enforce the payment of the Revolving Credit Note and any
                  of its other legal or equitable rights.

         Conduct No Waiver, Enforcement Costs. No course of dealing on the part
                  of Lender, nor any delay or failure on the part of Lender to
                  exercise any of its rights, shall operate as a waiver of such
                  right or otherwise prejudice any Lender's rights, powers and
                  remedies.

         Remedies Cumulative. No right or remedy conferred upon or reserved to
                  Lender under this Credit Agreement is intended to be exclusive
                  of any other rights or remedy, and every right and remedy
                  shall be cumulative and in addition to every other right and
                  remedy given hereunder or now or hereafter existing under
                  applicable law. Every right and remedy given by this Credit
                  Agreement or by applicable law to Lender may be exercised from
                  time to time and as often as may be deemed expedient by any
                  Lender, as the case may be.

Assignment

Borrower may not assign its rights or obligations under this Credit Agreement to
any person or entity without the express written consent of Lender. Lender may
assign its rights and obligations under this Credit Agreement to any person or
entity. This Credit Agreement shall be binding upon and inure to the benefit of
each party hereto and its successors and permitted assigns.

Miscellaneous

Borrower agrees to indemnify and hold harmless Lender against all claims,
liabilities, demands, obligations and losses of every kind arising out of or
with respect to the Credit Documents.
This Credit Agreement may not be amended or waived except by a written
instrument signed by Borrower and Lender, and any such amendment or waiver shall
be effective only for the specific purpose given.
The provisions of this Credit Agreement are severable and if any one provision
hereof shall be held invalid or unenforceable in whole or in part in any
jurisdiction, such invalidity or unenforceability shall affect only such
provision in such jurisdiction.
This Credit Agreement, together with all Exhibits hereto and agreements referred
to herein, expresses the entire understanding of the parties with respect to the
transactions contemplated hereby.
This Credit Agreement and any amendment hereby may be executed in several
counterparts, each of which shall be an original, and all of which shall
constitute one agreement. In proving this Credit Agreement, it shall not be
necessary to produce more than one such counterpart executed by the party to be
charged.
This Credit Agreement and the Revolving Credit Note shall be governed and
construed in accordance with the laws of the State of California without regard
to conflict of law principles. Borrower agree that any suit for the enforcement
of any of the Credit Agreement or the Revolving Credit Note may be brought in
the courts of the State of California, County of San Diego or any federal court
sitting therein. Borrower, as an inducement to Lender to enter into this Credit
Agreement, hereby waives its right to a jury trial with respect to any action
arising in connection with any Credit Document.
No delay on the part of Lender or the holder of the Revolving Credit Note in the
exercise of any right, power, or remedy shall operate as a waiver thereof, nor
shall any single or partial exercise by Lender of any right, power, or remedy
preclude other or further exercise thereof, or the exercise of any other right,
power, or remedy.
At the request of Lender at any time and from time to time, Borrower shall, at
its expense, duly execute and deliver, or cause to be duly executed and
delivered, such further agreements, documents and instruments, and do or cause
to be done such further acts as may be necessary or proper to evidence, perfect,
maintain and enforce the security interests and the priority thereof in the
Collateral and to otherwise effectuate the provisions or purposes of this
Agreement or any of the other Credit Documents.
         (i) All notices and other communications given or made pursuant hereto
shall be in writing and shall be deemed to have been duly given or made as of
the date delivered, mailed or transmitted, and shall be effective upon receipt,
if delivered personally, mailed by registered or certified mail (postage
prepaid, return receipt requested) to the parties at the following addresses (or
at such other address for a party as shall be specified by like changes of
address) or sent by electronic transmission (provided that a confirmation copy
is sent by another approved means) to the telecopier number specified below:
                  (i) If to Lender:


                           2890 Moonridge Drive
                           La Jolla, CA  92037

                           Attention:       Richard D. Propper, MD
                           Telephone:       858 453-7702



                  (ii) If to Borrower:


                           777 South Highway 101, Suite 215
                           Solana Beach, CA  92075

                           Attention:       Scott D. Landow, President
                           Telephone:       858 847-9000
                           Facsimile:       858 847-9090
<PAGE>

                  IN WITNESS WHEREOF, the parties hereto have caused this Credit
Agreement to be duly executed and delivered by their proper and duly authorized
officers as of the day and year first above written.


                                                 LENDER
                                                 RICHARD D. PROPPER, MD


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



                                                 BORROWER
                                                 PARENTECH, INC.


                                                 ------------------------------
                                                 By:  Scott D. Landow
                                                 Its:   President





<PAGE>

                                    EXHIBIT A
                              REVOLVING CREDIT NOTE



U.S. $                                                          Dated:




         FOR VALUE RECEIVED, the undersigned, PARENTECH, INC., a Delaware
corporation (the "Borrower"), HEREBY PROMISES TO PAY to the order of Richard D.
Propper, MD, a resident of the State of California (the "Lender") the principal
amount of Dollars (U.S.$) on the Maturity Date (as defined below). In addition,
the Borrower hereby promises to pay to the Lender interest on the unpaid
principal amount hereof from the date hereof until such principal amount is paid
in full at the rate of percent (%) per annum. The full amount of principal plus
accrued interest shall be paid in one payment on November 1, 2003 (the "Maturity
Date"), unless earlier prepaid at the Borrower's option pursuant to Section 1.1
below.


                                    ARTICLE I

                                TERMS OF PAYMENT

         SECTION 1.1.  Optional Prepayments.

         (a) At its option the Borrower may, from time to time, upon at least
one Business Day's notice to the Lender, prepay this Note in full or in part.

         (b) Any prepayment by the Borrower hereunder shall be applied first to
the payment of all interest then accrued and the balance, if any, to the
reduction of the principal amount of this Note then outstanding.

         SECTION 1.2. Payments and Computations.

         (a) The Borrower shall make all payments hereunder in U.S. dollars to
the Lender at the address referred to in Section 3.2.

         (b) All computations of interest shall be made by the Lender on the
basis of a year of 365 or 366 days (as the case may be), for the actual number
of days (including the first day but excluding the last day) occurring in the
period for which such interest is payable.




<PAGE>

         (c) Whenever any payment hereunder shall be stated to be due on a day
other than a day of the year when banks are not required or authorized to close
in New York City (a "Business Day"), such payment shall be made on the next
succeeding Business Day, and such extension of time shall in such case be
included in the computation of payment of interest.

         SECTION 1.5. Usury Savings Clause. Notwithstanding anything herein to
the contrary, under no circumstances shall the interest contracted for, charged,
or received hereunder or pursuant hereto exceed the maximum amount permitted
under applicable law. If the interest otherwise payable to the Lender would
exceed the maximum amount permitted under applicable law, the interest payable
shall be reduced to such maximum amount. If the Lender receives anything deemed
interest under applicable law in excess of such maximum amount, an amount equal
to such excess shall reduce the principal hereof and not be a payment of
interest. If such excess exceeds the unpaid principal balance hereof, the Lender
shall refund such excess to the Borrower. When determining whether any amounts
contracted for, charged, or received hereunder or pursuant hereto constitute
excessive interest under applicable law, to the extent permitted thereby the
Borrower and the Lender shall:

          (a) exclude voluntary prepayments and the effects thereof, and

          (b) allocate, amortize, prorate, and spread such amounts throughout
the full term hereof so that the interest rate hereunder is uniform throughout
such term.


                                   ARTICLE II

                                EVENTS OF DEFAULT


         SECTION 2.1. Events of Default. Upon the occurrence of any of the
following specified Events of Default (each an "Event of Default"):

         (a) failure by the Borrower to pay the principal amount of this Note
when due; or

         (b) the Borrower shall make a general assignment for the benefit of
creditors; or any proceeding shall be instituted by or against it seeking to
adjudicate it a bankrupt or insolvent, or seeking liquidation, winding up,
reorganization, arrangement, adjustment, protection, relief, or composition of
it or its debts under any law relating to bankruptcy, insolvency or
reorganization or relief of debtors (the "Bankruptcy Code"), or seeking the
entry of an order for relief or the appointment of a receiver, trustee,
custodian or other similar official for it or for any substantial part of its
property and, in the case of any such proceeding instituted against it (but not
instituted by it), either such proceeding shall remain undismissed or unstayed
for a period of 30 days, or any of the actions sought in such proceeding
(including, without limitation, the entry of an order of relief against, or the
appointment of a receiver, trustee, custodian or other similar official for, it
or for any substantial part of its property) shall occur; or the Borrower shall
take any action to authorize or to consent to any of the actions set forth above
in this subsection (b); or

         (c) any judgment or order for the payment of money in excess of
$100,000 shall be rendered against the Borrower and either (i) enforcement
proceedings shall have been commenced by any creditor upon such judgment or
order or (ii) there shall be any period of 10 consecutive days during which a
stay of enforcement of such judgment or order, by reason of a pending appeal or
otherwise, shall not be in effect;

then, and in any such event, and at any time thereafter, if any Event of Default
shall then be continuing, (i) for a period of 15 consecutive days for an event
described in paragraph (a) and (ii) for a period of 30 consecutive days for an
event described in paragraph (c), the Lender, by notice to the Borrower, may
declare the principal of and accrued interest on this Note and all other amounts
payable under this Note to be forthwith due and payable, whereupon the same
shall become forthwith due and payable without presentment, demand, protest or
other notice of any kind, all of which are hereby expressly waived by the
Borrower; provided, however, that in the event of an actual or deemed entry of
an order for relief with respect to the Borrower or any of its subsidiaries
under the Bankruptcy Code, this Note, all such interest and all such amounts
shall automatically become and be due and payable, without presentment, demand,
protest or any notice of any kind, all of which are hereby expressly waived by
the Borrower.

<PAGE>

                                   ARTICLE III

                                  MISCELLANEOUS


         SECTION 3.1. Amendments, Etc. No amendment or waiver of any provision
of this Note, nor consent to any departure by the Borrower herefrom, shall in
any event be effective unless the same shall be in writing and signed by the
Lender and then such waiver or consent shall be effective only in the specific
instance and for the specific purpose for which given.

         SECTION 3.2. Notices, Etc. All notices and other communications
provided for hereunder shall be in writing (including telecopier communication)
and mailed or telecopied, if to the Borrower, at its address at 777 South
Highway 101, Suite 215, Solana Beach, CA 92075; Facsimile: 858/847-9090 and if
to the Lender, at his address at 2890 Moonridge Drive, La Jolla, CA 92037
Attention: Dr. Richard Propper; Facsimile: 858/847-9090 or, as to each party, at
such other address as shall be designated by such party in a written notice to
the other party. All such notices and communications shall, when mailed or
telefaxed, be effective when deposited in the mails or telefaxed, respectively.

<PAGE>

         SECTION 3.3. No Waiver; Remedies. No failure on the part of the Lender
to exercise, and no delay in exercising, any right hereunder shall operate as a
waiver thereof; nor shall any single or partial exercise of any right hereunder
preclude any other or further exercise thereof or the exercise of any other
right. The remedies herein provided are cumulative and not exclusive of any
remedies provided by law.

         SECTION 3.4. Costs and Expenses. (a) The Borrower agrees to pay on
demand all losses, costs and expenses, if any (including fees and expenses of
counsel), in connection with the enforcement (whether through negotiations,
legal proceedings or otherwise) of this Note and any other instruments and
documents delivered in connection herewith or therewith, including, without
limitation, fees and expenses of counsel in connection with the enforcement of
rights under this Section 3.4(a).

         (b) The Borrower agrees to indemnify and hold harmless the Lender and
each of its affiliates and their officers, directors, employees, agents and
advisors (each, an "Indemnified Party") from and against any and all claims,
damages, losses, liabilities and expenses (including, without limitation, fees
and expenses of counsel) that may be incurred by or asserted or awarded against
any Indemnified Party, in each case arising out of or in connection with or by
reason of, or in connection with the preparation for a defense of, any
investigation, litigation or proceeding arising out of, related to or in
connection with this Note, whether or not such investigation, litigation or
proceeding is brought by the Borrower, its directors, shareholders or creditors
or an Indemnified Party or an Indemnified Party is otherwise a party thereto,
except to the extent such claim, damage, loss, liability or expense results from
such Indemnified Party's gross negligence or willful misconduct. The Borrower
also agrees that no Indemnified Party shall have any liability (whether direct
or indirect, in contract or tort or otherwise) to the Borrower or its direct or
indirect security holders or creditors arising out of, related to or in
connection with this Note, except to the extent that such liability results from
such Indemnified Party's gross negligence or

         SECTION 3.5. Binding Effect. This Note shall be binding upon and inure
to the benefit of the Borrower and the Lender and their respective successors
and assigns, except that the Borrower shall not have the right to assign its
rights hereunder or any interest herein without the prior written consent of the
Lender.

         SECTION 3.6. Governing Law. This Note shall be governed by, and
construed in accordance with, the laws of the State of New York.



         IN WITNESS WHEREOF, the Borrower has caused this Note to be executed as
of the date first above written.


                                      Parentech, Inc.



                                      By:____________________________________



                                      Name:__________________________________



                                      Title:_________________________________


<PAGE>



                                    EXHIBIT B

                                 FORM OF REQUEST



                              Parentech Letterhead




Date

Richard D. Propper, MD
2890 Moonridge Drive
La Jolla, CA  92037

Re:      Request to Draw Against Revolving Line of Credit

Dear Dr. Propper:

This is a request that you extend additional credit in the amount of $        to
Parentech, Inc. pursuant to the Credit Agreement between you and the Company
dated _______, 2001. The current principal balance outstanding under the Credit
Agreement is $. There is also interest accrued of $        . If you were to
extend the credit requested in this request, the total principal balance would
increase to $         . The total amount of credit available is $1,000,000.

Please advise me of your willingness to extend the credit requested hereby at
your earliest convenience.


Very truly yours,



Scott D. Landow
President