Exhibit 10.1


                                 AMENDMENT NO. 4
                                       TO
                        LICENSE AND DEVELOPMENT AGREEMENT


         This  Amendment  No.  4 to  License  and  Development  Agreement  (this
"Amendment") is made as of the 21st day of May, 1999 (the  "Effective  Date") by
and between Aronex Pharmaceuticals, Inc., a Delaware corporation ("Aronex"), and
Genzyme Corporation, a Massachusetts corporation ("Genzyme").  Capitalized terms
used without  definition in this Amendment shall have the meanings given to such
terms in the Development Agreement (as defined below).

                                    RECITALS

         WHEREAS, Aronex (f/k/a Argus Pharmaceuticals, Inc.) and Genzyme entered
into  a  License  and  Development   Agreement  dated  September  10,  1993  (as
subsequently  amended by amendments dated September 8, 1995,  September 10, 1996
and March 27, 1997, the  "Development  Agreement")  relating to the development,
license,   manufacture,   marketing  and  sale  of  pharmaceutical  compositions
incorporating "AR-623" (also known as "Atragen(TM)");

         WHEREAS, Genzyme has elected to terminate its Option and relinquish its
rights thereunder, effective as of March 24, 1999 (the "Election");

         WHEREAS,  as a result of the  Election by Genzyme,  Aronex is obligated
pursuant to Section 2.4 of the  Development  Agreement to pay Genzyme the sum of
$2,000,000 (the "Repayment  Amount") on or before April 24, 1999, which has been
extended to the  Effective  Date,  plus an  additional  amount of $500,000  (the
"Minimum Royalty Amount") of minimum royalty payments by April 24, 2000; and

         WHEREAS,  Aronex and Genzyme desire to amend the Development  Agreement
(i) to restructure the Repayment  Amount and the Minimum Royalty Amount owing by
Aronex as a result of such Election under Section 2.4 thereof into a convertible
note to be issued to  Genzyme  by  Aronex,  (ii) to issue  Genzyme a warrant  to
purchase 50,000 shares of Aronex Common Stock, and (iii) to provide Genzyme with
registration  rights with  respect to shares it may receive upon  conversion  of
such convertible note as well as shares issuable upon exercise of the warrant;

         NOW,  THEREFORE,  in  consideration  of the  foregoing  and the  mutual
covenants contained herein, Aronex and Genzyme agree as follows:




                                       -4-





         1.  RESTRUCTURING OF REPAYMENT  AMOUNT AND MINIMUM ROYALTY AMOUNT.
The  parties  agree that the  Repayment  Amount and the Minimum  Royalty  Amount
required  to be paid by Aronex to Genzyme as a result of the  Election  shall be
due and payable  pursuant  to the terms of a  three-year  Convertible  Note from
Aronex in the form of Exhibit 2.4(a) hereto. Such Convertible Note shall provide
for  interest of 10% per annum,  to be payable in  semi-annual  payments and the
principal  amount  thereof  to be due on  the  expiration  of  three  years.  In
addition, part or all of the principal of the Convertible Note may be converted,
at Genzyme's election, into shares of Common Stock of Aronex at a purchase price
of $4.35 per share,  at any time and from time to time,  prior to the  repayment
thereof. As a result, Section 2.4 of the Development Agreement is hereby amended
by adding the following paragraph at the end thereof:

         "Notwithstanding   anything  contained  in  this  Section  2.4  to  the
         contrary,  the obligation of Aronex to pay (i) $2,000,000 within thirty
         days of the Option  Expiration  Date and (ii) $500,000  minimum royalty
         within the first twelve months following the due date of the $2,000,000
         amount,  is hereby canceled and terminated in exchange for the issuance
         by Aronex  and  delivery  to  Genzyme  of (i) a  Convertible  Note (the
         "Convertible  Note") in the principal  amount of $2,500,000 in the form
         attached  hereto as  Exhibit  2.4(a) and (ii) a Common  Stock  Purchase
         Warrant (the  "Warrant") to purchase  50,000 shares of the Common Stock
         of  Aronex,  par value  $.001  per share  (the  "Common  Stock"),  at a
         purchase  price of $4.00  per  share in the  form  attached  hereto  as
         Exhibit 2.4(b)."

         2.       REGISTRATION RIGHTS.

         The following  section shall be added to the  Development  Agreement as
Section 11 thereof:

         "11.     Registration Rights.

          (a)  Shelf Registration.  Within sixty days from the receipt of notice
               from  Genzyme of its  interest to either  exercise the Warrant or
               convert  a portion  of the  principal  amount of the  Convertible
               Note, Aronex shall use its reasonable best efforts to prepare for
               filing  with  the   Securities  and  Exchange   Commission   (the
               "Commission"),  and  cause to be  declared  effective,  a "shelf"
               registration  statement  (the "Shelf  Registration")  pursuant to
               Rule 415  under  the  Securities  Act of 1933,  as  amended  (the
               "Securities Act"),  providing for the sale by the Stockholder (as
               hereafter  defined)  of the  Registrable  Shares (as  hereinafter
               defined).  Aronex  agrees to use its  reasonable  best efforts to
               keep such Shelf Registration  continuously effective for a period
               ending  on the  earliest  of (a)  the  tenth  anniversary  of the
               effective date of such Shelf Registration,  (b) the date on which
               all Registrable Shares covered thereby have been sold thereunder,
               or (c) the date upon  which all  Registrable  Shares  are  freely
               transferable  without  restriction  under the Securities Act. For
               the purpose of this Section 11, "reasonable best efforts"

                                       -5-



               shall mean the best efforts of Aronex  consistent  with sound and
               reasonable business practices and judgment.  "Registrable Shares"
               means any shares of Common Stock issuable upon  conversion of the
               Convertible  Note,  (ii) any shares of Common Stock issuable upon
               exercise  of the  Warrant,  and (iii) any other  shares of Common
               Stock issued in respect of the shares  referred to in (i) or (ii)
               (because  of  stock  splits,   stock   dividends,   subdivisions,
               combinations,  reclassifications,  recapitalizations,  or similar
               events),  provided,  however, that any such shares shall cease to
               be Registrable Shares when such shares become eligible for resale
               under Rule  144(k) of the  Securities  Act by persons who are not
               affiliates of Aronex or upon (a) any transfer that results in the
               purchaser   of  such  shares   receiving   shares  that  are  not
               "restricted securities" within the meaning of Rule 144 or (b) any
               transfer to a person or entity that,  by virtue of Section  11(g)
               hereof is not  entitled  to the rights set forth in this  Section
               11.  "Stockholder"  means  Genzyme and any persons or entities to
               whom the rights granted under this Section 11 are  transferred in
               accordance with Section 11(g).

     (b)  Registration Procedures.  In connection with Aronex's obligations with
          respect to the Shelf  Registration,  Aronex  shall use its  reasonable
          best efforts to effect the  registration in furtherance of the sale of
          the  Registrable  Shares in  accordance  with the  intended  method or
          methods of distribution  thereof described in the Shelf  Registration.
          In  connection  therewith,   Aronex  shall,  as  promptly  as  may  be
          practicable:

                    (i)  prepare  and file with the  Commission  a  registration
               statement with respect to the Registrable  Shares on any form for
               which  Aronex then  qualifies  or which  counsel for Aronex shall
               deem  appropriate  and  which  form  shall be  available  for the
               disposition  of the  Registrable  Shares in  accordance  with the
               intended method or methods of disposition thereof;

                    (ii) prepare and file with the  Commission  such  amendments
               and supplements to such registration statement and the prospectus
               used in  connection  therewith  as may be  necessary to keep such
               registration   statement  effective  for  the  applicable  period
               specified  in  Section  11(a)  above  or  as  may  be  reasonably
               requested  by  Stockholder  in order to  incorporate  information
               concerning  Stockholder or its intended  method of  distribution,
               and otherwise to comply with the provisions of the Securities Act
               with respect to the disposition of all securities covered by such
               registration statement.

                    (iii) furnish to  Stockholder,  prior to the filing  thereof
               with the Commission,  a copy of such  registration  statement and
               each  amendment  thereof  and  each  supplement,  if any,  to the
               prospectus included therein and Aronex shall use its




                                       -6-





                  reasonable best efforts to reflect in each such document, when
                  so filed with the Commission, such comments as Stockholder may
                  reasonably propose;

                           (iv)   furnish   to   Stockholder   a  copy  of  such
                  registration statement,  each amendment and supplement thereto
                  (in each case including all exhibits thereto but excluding all
                  documents    incorporated   by   reference    therein   unless
                  specifically so requested by Stockholder)  and such reasonable
                  number  of  copies  of  the   prospectus   included   in  such
                  registration statement (including each preliminary prospectus)
                  as Stockholder may reasonably request;

                           (v)  use  reasonable  best  efforts  to  register  or
                  qualify the  Registrable  Shares  under such other  securities
                  laws or blue sky  laws of such  jurisdictions  as  Stockholder
                  shall reasonably request, and take any and all such actions as
                  may be reasonably necessary or advisable to enable Stockholder
                  to consummate the  disposition in such  jurisdictions  of such
                  Registrable Shares;

                           (vi)   notify   Stockholder,   at  any  time  when  a
                  prospectus  relating thereto is required to be delivered under
                  the  Securities  Act within the period that Aronex is required
                  to keep the registration statement effective, of the happening
                  of any event as a result of which the  prospectus  included in
                  such  registration  statement (as then in effect)  contains an
                  untrue  statement  of a  material  fact or omits to state  any
                  material  fact  required to be stated  therein or necessary to
                  make  the  statements  therein  not  misleading  and  promptly
                  prepare,  file with the Commission  and furnish  Stockholder a
                  reasonable number of copies of a supplement to or an amendment
                  of such  prospectus  as may be necessary so that as thereafter
                  declared to the  purchasers  of such shares,  such  prospectus
                  shall not include an entire  statement  of a material  fact or
                  omit to state any material fact required to be stated  therein
                  or necessary to make the statements therein not misleading.

                           (vii) advise  Stockholder,  promptly after  receiving
                  notice thereof,  of any stop order issued or threatened by the
                  Commission  and use its  reasonable  best  efforts to take all
                  actions  required to prevent the entry of such stop order,  or
                  to remove it if entered; and

                           (viii) use its  reasonable  best efforts to cause all
                  Registrable Shares included in such registration  statement to
                  be listed, by the date of the first sale of Registrable Shares
                  pursuant to such  registration  statement,  on each securities
                  exchange or market on which the Common Stock of Aronex is then
                  listed or proposed to be listed.

                           (ix)  shall  (a)  make   reasonably   available   for
                  inspection by  Stockholder,  and any  attorney,  accountant or
                  other agent retained by Stockholder all relevant financial and
                  other records, pertinent corporate documents and properties of
                  Aronex and (b) cause Aronex's officers, directors,  employees,
                  accountants and auditors to supply all



                                       -7-





                  relevant  information  reasonably  requested by Stockholder or
                  any such attorney, accountant or agent in connection with such
                  registration  statement,  in each  case  as may be  reasonably
                  necessary  to enable  such  persons  to  conduct a  reasonable
                  investigation   within  the  meaning  of  Section  11  of  the
                  Securities Act.

                           (x) shall use its reasonable best efforts to take all
                  other  steps   necessary   effect  the   registration  of  the
                  Registrable Shares contemplated hereby.

         (c)      Expenses.  All expenses incident to Aronex's performance of or
                  compliance  with the  provisions  of this  Section  11 will be
                  borne by Aronex.  Notwithstanding  the foregoing,  Stockholder
                  shall  pay  any  and  all  underwriting  fees,   discounts  or
                  commissions attributable to the sale of Registrable Shares.

         (d)   Indemnification.

               (i)  Upon the  registration  of  Registrable  Shares  pursuant to
                    Section 11 hereof,  Aronex  shall,  and it hereby agrees to,
                    indemnify and hold harmless, to the extent permitted by law,
                    Stockholder, its officers and directors, each underwriter of
                    such  Registrable  Shares,  if  any,  and  each  person  who
                    controls such person  (within the meaning of the  Securities
                    Act) against all losses,  claims,  damages,  liabilities and
                    expenses (including reasonable attorneys' fees and expenses)
                    to  which  Stockholder,   its  officers,   directors,   each
                    underwriter, or such controlling persons may become subject,
                    insofar as such losses,  claims,  damages,  liabilities  and
                    expenses (or actions in respect thereof) arise out of or are
                    based upon any untrue  statement or alleged untrue statement
                    of  material  fact   contained  in  any  such   registration
                    statement,   any   prospectus  or   preliminary   prospectus
                    contained therein or any amendment or supplement thereto, or
                    any omission or alleged omission to state therein a material
                    fact required to be stated  therein or necessary to make the
                    statements  therein  not  misleading,   and  will  reimburse
                    Stockholder, each such underwriter and each such controlling
                    person for any legal or other expenses  reasonably  incurred
                    by them in connection  with  investigating  or defending any
                    such loss, claim,  damage,  liability or action;  except (A)
                    insofar as the same arise out of or are based upon an untrue
                    statement or omission or alleged omission so made based upon
                    information   furnished  by   Stockholder,   underwriter  or
                    controlling  person in writing  specifically for use in such
                    registration  statement or  prospectus or (B) insofar as the
                    same  are  caused  by  Stockholder's  or such  underwriter's
                    failure to deliver a copy of such registration  statement or
                    prospectus or any  amendments or  supplements  thereto after
                    Aronex has furnished  Stockholder or such underwriter with a
                    sufficient number of copies of the same.


                                       -8-





               (ii) In connection  with any  registration  statement under which
                    Registrable  Shares are registered  under the Securities Act
                    and   pursuant  to  which   Stockholder   offers  and  sells
                    Registrable Shares,  Stockholder shall, and it hereby agrees
                    to, indemnify and hold harmless,  to the extent permitted by
                    law, each of Aronex,  its officers and  directors,  and each
                    person  who  controls  Aronex  (within  the  meaning  of the
                    Securities  Act) and,  if the  offering  is an  underwritten
                    offering,  the  underwriters,  against all  losses,  claims,
                    damages,  liabilities  and  expenses  (including  reasonable
                    attorneys' fees and expenses) to which Aronex,  its officers
                    and  directors,  underwriters,  or  controlling  persons may
                    become  subject,  insofar as such losses,  claims,  damages,
                    liabilities  and  expenses  (or actions in respect  thereof)
                    arise  out of or are  based  upon any  untrue  statement  or
                    alleged  untrue  statement of material fact contained in any
                    such registration  statement,  any prospectus or preliminary
                    prospectus  contained therein or any amendment or supplement
                    thereto,  or any  omission  or  alleged  omission  to  state
                    therein a material  fact  required  to be stated  therein or
                    necessary to make the statements  therein not misleading and
                    will  reimburse  Aronex  and each  such  officer,  director,
                    underwriter  and  controlling  person for any legal or other
                    expenses  reasonably  incurred  by them in  connection  with
                    investigating  or defending  any such loss,  claim,  damage,
                    liability or action, insofar as (A) the same arise out of or
                    are based upon any untrue  statement  or omission or alleged
                    omission  so  made  based  upon  information   furnished  by
                    Stockholder,  or an  underwriter  or  controlling  person of
                    Stockholder,   in  writing  specifically  for  use  in  such
                    registration  statement  or  prospectus  or (B) the same are
                    caused by  Stockholder's  or such  underwriter's  failure to
                    deliver a copy of such registration  statement or prospectus
                    or any  amendments or  supplements  thereto after Aronex has
                    furnished  Stockholder or such underwriter with a sufficient
                    number of copies of the same and provided, further, that the
                    liability  of  Stockholder  under  this  Section 11 shall be
                    limited to the proportion of any such loss,  claim,  damage,
                    liability or expense which is equal to the  proportion  that
                    the public  offering  price of  Registrable  Shares  sold by
                    Stockholder under such  registration  statement bears to the
                    total  public   offering  price  of  all   securities   sold
                    thereunder,  but not to exceed  the  amount of the  proceeds
                    received  by  Stockholder  from the sale of the  Registrable
                    Shares covered by such registration statement.

               (iii)Any person  entitled to  indemnification  hereunder will (A)
                    give prompt  notice to the  indemnifying  party of any claim
                    with  respect  to which it  seeks  indemnification  (but the
                    failure  to give such  notice  will not  affect the right to
                    indemnification hereunder,  unless the indemnifying party is
                    materially  prejudiced  by such  failure)  and (B) unless in
                    such indemnified  party's reasonable  judgment a conflict of
                    interest may exist between such indemnified and indemnifying
                    parties with respect to such claim, permit such indemnifying
                    party to assume  the  defense  of such  claim  with  counsel
                    selected



                                       -9-





                    by the indemnifying party and reasonably satisfactory to the
                    indemnified  party.  If such  defense is not  assumed by the
                    indemnifying  party  or if  the  indemnifying  party  is not
                    permitted to assume such  defense  then (x) the  indemnified
                    party shall select counsel, which counsel must be reasonably
                    satisfactory   to  the   indemnifying   party  and  (y)  the
                    indemnifying  party will not be subject to any liability for
                    any settlement  made without its consent (which consent will
                    not be unreasonably  withheld).  No indemnifying  party will
                    consent  to  entry  of  any   judgment  or  enter  into  any
                    settlement which does not include as an  unconditional  term
                    thereof  the giving by the  claimant  or  plaintiff  to such
                    indemnified party of a release from all liability in respect
                    of such claim or litigation.  An  indemnifying  party who is
                    not entitled  to, or elects not to,  assume the defense of a
                    claim will not be  obligated to pay the fees and expenses of
                    more than one counsel for all  parties  indemnified  by such
                    indemnifying party with respect to such claim, unless in the
                    reasonably  judgment of any indemnified  party a conflict of
                    interest may exist  between such  indemnified  party and any
                    other  of such  indemnified  parties  with  respect  to such
                    claim,  in  which  case  the  indemnifying  party  shall  be
                    obligated  to pay the fees and  expenses  of one  additional
                    counsel,   who  must  be  reasonably   satisfactory  to  the
                    indemnifying party.

               (iv) Each  party  hereto  agrees  that,  if for  any  reason  the
                    indemnification  provisions contemplated by Section 11(d)(i)
                    or 11(d)(ii) are  unavailable  or are  insufficient  to hold
                    harmless  an  indemnified  party in respect  of any  losses,
                    claims,  damages,  liabilities  or  expenses  (or actions in
                    respect thereof) referred to therein, then each indemnifying
                    party shall contribute to the amount paid or payable by such
                    indemnified  party  as a  result  of  such  losses,  claims,
                    damages,  liabilities  or  expenses  (or  actions in respect
                    thereof) in such proportion as is appropriate to reflect the
                    relative fault of the indemnifying party and the indemnified
                    party   as   well   as   any   other   relevant    equitable
                    considerations.  The  relative  fault  of such  indemnifying
                    party and indemnified party shall be determined by reference
                    to, among other things, whether the untrue or alleged untrue
                    statement of a material fact or omission or alleged omission
                    to state a material fact relates to information  supplied by
                    such  indemnifying  party  or  indemnified  party,  and  the
                    parties' relative intent,  knowledge,  access to information
                    and  opportunity  to correct or prevent  such  statement  or
                    omission. The parties hereto agree that it would not be just
                    and  equitable  if  contribution  pursuant  to this  Section
                    11(d)(iv) were  determined by pro rata  allocation  (even if
                    Stockholder or any  underwriters or all of them were treated
                    as one entity for such  purpose)  or by any other  method of
                    allocation  which does not take into  account the  equitable
                    considerations  referred to in this  Section  11(d)(iv).  No
                    person  guilty of fraudulent  misrepresentation  (within the
                    meaning of Section 11(f) of the Securities Act)



                                      -10-





                    shall be  entitled to  contribution  from any person who was
                    not guilty of such fraudulent misrepresentation.

               (v)  The  indemnification  and contribution  obligations and each
                    other provision set forth in this Section 11(d) shall remain
                    in full force and  effect  regardless  of any  investigation
                    made by or on behalf of Aronex, Stockholder,  any officer or
                    employee  of Aronex or  Stockholder,  any  underwriter,  any
                    officer or employee of such underwriter,  or any controlling
                    person  of  any of  the  foregoing  and  shall  survive  the
                    transfer  and   registration   of   Registrable   Shares  by
                    Stockholder.

          (e)  Rule  144  Reporting.   With  a  view  to  making   available  to
               Stockholder   the  benefits  of  Rule  144   promulgated  by  the
               Commission  under the  Securities  Act,  Aronex agrees to use its
               reasonable best efforts to:

               (i)  make and  keep  adequate  current  public  information  with
                    respect to Aronex available, as those terms are used in Rule
                    144 under the Securities Act;

               (ii) file with the  Commission in a timely manner all reports and
                    other documents  required of Aronex under the Securities Act
                    and the  Securities  Exchange  Act of 1934,  as amended (the
                    "Exchange Act"); and

               (iii)furnish  to  Stockholder  promptly  upon  request  a written
                    statement by Aronex as to its compliance  with the reporting
                    requirements of Rule 144 and the Exchange Act, a copy of the
                    most recent annual or quarterly  report of Aronex,  and such
                    other  reports and  documents of Aronex as  Stockholder  may
                    reasonably  request in order to permit  Stockholder to avail
                    itself of any rule or regulation of the Commission  allowing
                    Stockholder   to  sell  its   Registrable   Shares   without
                    registration.

          (f)  Amendments  and Waivers.  Any provision of this Section 11 may be
               amended or waived  if, but only if, in the case of an  amendment,
               such  amendment  is in  writing  and  is  signed  by  Aronex  and
               Stockholder.  No  failure  or delay by Aronex or  Stockholder  in
               exercising any right, power or privilege  hereunder shall operate
               as a waiver  thereof,  nor shall any single or  partial  exercise
               thereof  preclude  any other or further  exercise  thereof or the
               exercise of any other right, power or privilege.

          (g)  Transfers of Certain Rights.

                  (i) The rights  granted to  Stockholder  under this Section 11
may be transferred or succeeded to only by (i) an affiliate of Stockholder, (ii)
a person or entity that acquires substantially all of the assets of Stockholder,
or  (iii)  any  other  person  or  entity  that  acquires  at  least  25% of the
Registrable Shares; provided, however, that Aronex is given written notice prior
to or



                                      -11-





promptly  following such transfer stating the name and address of the transferee
and  identifying  the  securities  with  respect to which such  rights are being
assigned.  Such notice shall include or be accompanied by a written  undertaking
by the transferee to comply with the obligations  imposed on Stockholders  under
this Section 11.

                  (ii) A transferee to whom rights are  transferred  pursuant to
this  Section  11(g) may not again  transfer  such rights to any other person or
entity, other than as provided in Section 11(g)(i) above."

     3.  NO  OTHER  AMENDMENTS.  Except  as  specifically  amended  hereby,  the
Development Agreement shall continue in full force and effect.

         IN WITNESS  WHEREOF the parties  hereto have executed this Amendment in
one or more copies effective as of the Effective Date.

                                        ARONEX PHARMACEUTICALS, INC.


                                        By:    /s/ Geoffrey F. Cox
                                               ---------------------------------
                                        Name:  Geoffrey F. Cox
                                               ---------------------------------
                                        Title: Chairman of the Board & Chief
                                                 Executive Officer
                                               ---------------------------------

                                        GENZYME CORPORATION

                                        By:    /s/ Richard Douglas
                                               ---------------------------------
                                        Name:  Richard Douglas
                                               ---------------------------------
                                        Title: Senior Vice President,
                                                 Corporate Development
                                               ---------------------------------





                                      -12-