EXHIBIT 10.1

                           FIRST AMENDMENT TO SUBLEASE
                           ---------------------------



         THIS FIRST AMENDMENT TO SUBLEASE ("First Amendment") is executed on the
10th day of February,  2004,  by and between HALE AND DORR LLP, a  Massachusetts
limited liability  partnership  having a principal place of business at 60 State
Street,  Boston, MA 02109  ("Sublandlord") and CYTOGEN  CORPORATION,  a Delaware
corporation  having a principal  place of  business  at 650  College  Road East,
Princeton, NJ 08540 ("Subtenant").


                                 R E C I T A L S
                                 ---------------

         WHEREAS,  pursuant to that certain Sublease, dated May 23, 2002, by and
between  Sublandlord  and  Subtenant  ("Sublease"),   Subtenant  subleased  from
Sublandlord  11,493  rentable  square  feet on the third  floor in the  building
located  at  650  College  Road  East,  Princeton,   NJ  08540  (the  "Subleased
Premises");

         WHEREAS, Sublandlord and Subtenant acknowledge and agree that Subtenant
is desirous of expanding its Subleased Premises to include  contiguous  premises
consisting of 4,610 rentable square feet ("Expansion Space");

         WHEREAS,  Sublandlord  and  Subtenant   are  desirous of  amending  the
Sublease;

         NOW THEREFORE,  in  consideration of Ten Dollars  ($10.00),  the mutual
covenants made herein, and other  consideration,  the receipt and sufficiency of
which are hereby  acknowledged  and agreed,  Sublandlord and Subtenant agree the
Sublease shall be amended to provide as follows:

     1.   EXHIBIT BA,  EXPANSION SPACE,  annexed hereto,  shall constitute a new
          and  replacement  Exhibit  B.  Commencing  with  the  Expansion  Space
          Commencement  Date, as defined below,  Subtenant's  existing space and
          the  Expansion  Space  shall   collectively  be  referred  to  as  the
          "Subleased Premises."

     2.   TERM.   The  Term  for  the  Expansion   Space  shall   commence  upon
          Sublandlord's  delivery of  possession to  Subtenant,  which  delivery
          shall be on or about  March 1,  2004  ("Expansion  Space  Commencement
          Date") and shall  terminate on October 31, 2007.  Concurrent  with the
          Expansion Space Commencement  Date, the Term for Subtenant's  existing
          space  shall  be  amended  to  reflect:  a) that  said  Term  shall be
          co-terminus  with  the  Term  for  the  Expansion  Space,  and b) that
          Subtenant  shall have a one time right to exercise an option to extend
          the term of the  Sublease  commencing  November  1, 2007,  through and
          including  October 24,  2009.  The monthly Base Rent for each month of
          the  renewal  term  shall  be   $28,180.25.   Subtenant  must  provide
          Sublandlord with written notice of Subtenant's exercise of the renewal
          option, which written notice must be delivered to Sublandlord no later
          than April 1, 2007, time being made  specifically and expressly hereby
          of the  essence  for the  delivery  of said  notice  by  Subtenant  to
          Sublandlord.  It is also a precondition to the exercise of the renewal

<PAGE>

          option that Subtenant be and remain current in payment and performance
          of all  obligations  of the Sublease,  both at the time of delivery of
          the notice to exercise the option period and at the commencement  date
          of the option period.

     3.   Paragraph 4, BASE RENT, of the  Sublease,  shall be amended to include
          that  commencing  August  1,  2004,  the Base  Rent per  month for the
          Subleased Premises shall be $28,180.25,  and the Subtenant's Base Year
          shall be changed to 2004.

     4.   Paragraph 5,  ADDITIONAL  RENT, of the Sublease,  is hereby amended to
          provide  that,  effective  August 1, 2004,  Subtenant's  Proportionate
          Share of  Sublandlord's  Additional Rent obligation of the Prime Lease
          is hereby fixed at 23.53%.

     5.   Paragraph 12, SECURITY DEPOSIT,  of the Sublease,  shall be amended to
          provide  that  Subtenant   shall  not  be  required  to  pay  over  to
          Sublandlord any additional  Security  Deposit for the Expansion Space.
          Subtenant's  existing  Security  Deposit  shall  be  deemed  to be the
          requisite  Security  Deposit for  Subtenant's  existing  space and the
          Expansion Space.

     6.   SUBTENANT'S   IMPROVEMENTS.   Commencing   with  the  Expansion  Space
          Commencement  Date,  Paragraph 3,  DELIVERY,  of the Sublease shall be
          amended to provide that Sublandlord  shall,  upon  satisfaction of the
          following  conditions,  contribute the total sum of $40,000.00 towards
          Subtenant's improvements to the Expansion Space:

          a.   Upon Subtenant  furnishing  Sublandlord with adequate documentary
               proof  that  improvements,   in  an  amount  equal  to  at  least
               $40,000.00, have been installed at the Subleased Premises; and

          b.   Subtenant    executing   and    delivering   to   Sublandlord   a
               Contractor's/Sub-contractor's   Final   Waiver  of  Lien  in  the
               reasonable,  mutually  agreeable  form  provided to  Subtenant by
               Sublandlord; and

          c.   Subtenant actually occupying the Subleased Premises; and

          d.   Subtenant  not being in default  of any of the terms,  covenants,
               conditions  and  provisions  of the  Sublease and said is in full
               force and effect; and

          e.   Sublandlord  shall  pay said  sum  directly  to the  contractors,
               subcontractors or suppliers.

     7.   Paragraph  15,  PARKING AND SIGNAGE,  shall be amended to include that
          Sublandlord  shall designate five (5) parking spaces for the exclusive
          use of Subtenant.  Sublandlord shall have no obligation  whatsoever to
          police,  secure,  monitor or enforce any of Subtenant's parking rights
          hereunder.

     8.   The holding over  provisions  of the Prime Lease,  Paragraph 18, shall
          apply to any holdover by Subtenant  subsequent to the  expiration,  or
          earlier termination, of the Sublease Term.


                                      -2-
<PAGE>

     9.   All  remaining  terms,  covenants,  conditions  and  provisions of the
          Sublease  shall remain in full force and effect except as  hereinabove
          modified.





                                SUBLANDLORD:

                                HALE AND DORR LLP, a Massachusetts limited
                                liability partnership


                                By:   /s/ John M. Wescott, Jr.
                                      -----------------------------------------
                                      John M. Westcott, Jr.
                                      Senior Partner


                                SUBTENANT:

                                CYTOGEN CORPORATION, a Delaware corporation


                                By:   /s/ Christopher P. Schnittker
                                      -----------------------------------------
                                      Christopher P. Schnittker
                                      Vice President and Chief Financial Officer




                            CONSENT OF PRIME LANDLORD


         The  undersigned  hereby  consents to the foregoing  First Amendment to
Sublease.

                                PRIME LANDLORD:

                                650 COLLEGE ROAD ASSOCIATES, L.P.,
                                a New Jersey limited partnership

                                By:   AEGIS FORRESTAL PARTNERS, INC.,
                                      a general partner


                                By:   ------------------------------------------
                                      James A. Kinzig
                                      Vice President



                                      -3-
<PAGE>



                                   EXHIBIT BA


                                 EXPANSION SPACE


                                 [Graphic Omitted]


                                      -4-
<PAGE>


                     CONSENT TO FIRST AMENDMENT TO SUBLEASE
                     --------------------------------------

         THIS CONSENT TO FIRST  AMENDMENT TO SUBLEASE  (the  "Consent")  is made
this 18th day of February,  2004,  by 650 COLLEGE ROAD  ASSOCIATES,  L.P., a New
Jersey limited partnership,  with an address of 50 West State Street, Suite 112,
Trenton,  New Jersey 08608  ("Landlord") and HALE AND DORR, LLP, a Massachusetts
limited  liability  partnership,  with an  address of 60 State  Street,  Boston,
Massachusetts 02107 ("Tenant") and CYTOGEN CORPORATION,  a Delaware corporation,
with an address of 650 College Road, Princeton, New Jersey 08540 ("Subtenant").

                                   BACKGROUND
                                   ----------

         WHEREAS,  pursuant to that certain Lease dated as of August 19, 1998 by
and between  650 COLLEGE  ROAD  ASSOCIATES,  L.P.,  as  Landlord,  and  Buchanan
Ingersoll Professional Corporation ("BIPC"), as Tenant, as amended by Amendments
Number One and Two to Lease  Agreement dated as of,  respectively,  May 23, 2000
and December 1, 2000  (collectively,  the  "Lease"),  BIPC leased from  Landlord
certain premises (the "Premises")  located on the third and fourth floors in the
building at 650 College Road East (the "Building"),  Plainsboro, NJ 08540, which
Premises  contain  approximately  68,437  rentable square feet of space, as more
fully described in the Lease; and

         WHEREAS,  the  Lease  has been  assigned  to and  assumed  by Tenant by
Assignment and Assumption Agreement dated February 15, 2001; and

         WHEREAS,  by Sublease  Agreement  dated May 23,  2002 (the  "Sublease")
between Tenant as sublandlord  and Subtenant as subtenant,  Subtenant has agreed
to sublease  from Tenant 11,493  rentable  square feet on the third floor of the
Building (the "Sublet Premises"); and

         WHEREAS,  Landlord has issued its Consent to Sublease  dated June 2002,
the "Sublease  Consent"),  by which  Sublease  Consent  Landlord has granted its
consent to the subletting by Tenant to Subtenant of the Sublet Premises; and

         WHEREAS,  by First  Amendment to Sublease dated February 10, 2004, (the
"Amendment"),  Tenant  desires to  sublease  4,610  rentable  square feet in the
Building, contiguous to the Sublet Premises (the "Expansion Space"), as the same
is depicted in the Amendment.

         NOW, THEREFORE, Landlord hereby grants its consent to the subletting by
Tenant to  Subtenant  of the  Expansion  Space  under  and  subject  to,  and in
consideration of Tenant's and Subtenant's acknowledgement and acceptance of, all
of the  terms  and  conditions  of the  Consent.  All of the  provisions  of the
Sublease Consent are incorporated herein by reference and all references therein
to the Sublease  shall refer  hereafter  referring to the Sublease as amended by
the Amendment.

         IN WITNESS WHEREOF,  Landlord,  Tenant and Subtenant have each executed
this Consent to First Amendment to Sublease as of the day and year first written
above.

                                      -5-
<PAGE>

                                SUBLANDLORD:

                                HALE AND DORR, LLP, a Massachusetts limited
                                liability partnership

                                By:   /s/ John M. Wescott, Jr.
                                      ------------------------------------------
                                      John M. Westcott, Jr.
                                      Senior Partner

                                SUBTENANT:

                                CYTOGEN CORPORATION, a Delaware
                                corporation

                                By:   /s/ Christopher P. Schnittker
                                      ------------------------------------------
                                      Christopher P. Schnittker
                                      Vice President and Chief Financial Officer

                                LANDLORD:

                                650 COLLEGE ROAD ASSOCIATES,
                                L.P., a New Jersey limited partnership

                                By:   AEGIS FORRESTAL PARTNERS,
                                      INC., a general partner

                                By:   /s/ James A. Kinzig
                                      ------------------------------------------
                                      JAMES A. KINZIG, Vice President



                                      -6-