UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                  SCHEDULE 13G
                    Under the Securities Exchange Act of 1934
                                (Amendment No. 1)

                                  Cytogen Corp.
                                (Name of Issuer)

                                  Common Stock
                         (Title of Class of Securities)

                                    232824300
                                 (CUSIP Number)

                                December 31, 2003

             (Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule
is filed:

[x] Rule 13d-1(b)

[ ] Rule 13d-1(c)

[ ] Rule 13d-1(d)

*The remainder of this cover page shall be filled out for a reporting person's
initial filing on this form with respect to the subject class of securities, and
for any subsequent amendment containing information which would alter the
disclosures provided in a prior cover page.


The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities Exchange Act of
1934 ("Act") or otherwise subject to the liabilities of that section of the Act
but shall be subject to all other provisions of the Act (however, see the
Notes).








CUSIP No. 232824300

       1.    Names of Reporting Persons.
             I.R.S. Identification Nos. of above persons (entities only).

             OrbiMed Advisors LLC

       2.    Check the Appropriate Box if a Member of a Group (See Instructions)

             [ ] (a)
             [ ] (b)

       3.    SEC Use Only

       4.    Citizenship or Place of Organization

             Delaware

                 5.  Sole Voting Power:  0
Number of
Shares           6.  Shared Voting Power:  293,073
Beneficially
Owned by         7.  Sole Dispositive Power:  0
Each Reporting
Person With      8.  Shared Dispositive Power: 293,073

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person:

              293,073

       10.   Check if the Aggregate Amount in Row (9) Excludes Certain Shares
             (See Instructions)

       11.   Percent of Class Represented by Amount in Row (9) 2.22%

       12.   Type of Reporting Person (See Instructions) IA







CUSIP No. 232824300

       1.    Names of Reporting Persons.
             I.R.S. Identification Nos. of above persons (entities only).

             OrbiMed Capital LLC

       2.    Check the Appropriate Box if a Member of a Group (See Instructions)

             [ ] (a)
             [ ] (b)

       3.    SEC Use Only

       4.    Citizenship or Place of Organization

             Delaware

               5.    Sole Voting Power:  0
Number of
Shares         6.    Shared Voting Power:  293,073
Beneficially
Owned by       7.    Sole Dispositive Power:  0
Each Reporting
Person With    8.    Shared Dispositive Power:  293,073

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person:
             293,073

       10.   Check if the Aggregate Amount in Row (9) Excludes Certain Shares
             (See Instructions)

       11.   Percent of Class Represented by Amount in Row (9) 2.22%

       12.   Type of Reporting Person (See Instructions) IA








CUSIP No.  232824300

       1.    Names of Reporting Persons.
             I.R.S. Identification Nos. of above persons (entities only).

             Samuel D. Isaly

       2.    Check the Appropriate Box if a Member of a Group (See Instructions)

             [ ] (a)
             [ ] (b)

       3.    SEC Use Only

       4.    Citizenship or Place of Organization

             United States

               5.    Sole Voting Power:   0
Number of
Shares         6.    Shared Voting Power:  293,073
Beneficially
Owned by       7.    Sole Dispositive Power:  0
Each Reporting
Person With    8.    Shared Dispositive Power:  293,073

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person:
             293,073

       10.   Check if the Aggregate Amount in Row (9) Excludes Certain Shares
             (See Instructions)

       11.   Percent of Class Represented by Amount in Row (9) 2.22%

       12.   Type of Reporting Person (See Instructions) HC







Item 1. (a)  Issuer: Cytogen Corp.

        (b)  Address:

              650 College Road East, CN 5308
              Princeton, NJ 08540

Item 2. (a)  Name of Person Filing:

              OrbiMed Advisors LLC
              OrbiMed Capital LLC
              Samuel D. Isaly

        (b)  Address of Principal Business Offices:

              767 Third Avenue, 30th Floor
              New York, New York 10017

        (c)  Citizenship:
             Please refer to Item 4 on each cover sheet for each filing person

        (d)  Title of Class of Securities
             Common stock

        (e)  CUSIP Number: 232824300

Item 3.  OrbiMed Advisors LLC and OrbiMed Capital LLC are investment advisors in
         accordance with ss.240.13d-1(b)(1)(ii)(E). Samuel D. Isaly is a control
         person in accordance with ss.240.13d-1(b)(1)(ii)(G).

Item 4.  Ownership

         Please see Items 5 - 9 and 11 on each cover sheet for each filing
         person

Item 5.  Ownership of Five Percent or Less of a Class

         Not Applicable.

Item 6.  Ownership of More than  Five  Percent  on  Behalf of  Another
         Person.

         Reporting persons are holding 2.22% of the securities on behalf of
         other persons who have the right to receive or the power to direct the
         receipt of dividends from, or proceeds from sale of, such securities.
         No one such other person's interest in the securities whose ownership
         is reported here relates to more than five percent of the class.

Item 7.  Identification and  Classification  of the  Subsidiary  Which
         Acquired the Security Being Reported on By the Parent Holding
         Company

         Not Applicable

Item 8.  Identification and Classification of Members of the Group

         Not Applicable

Item 9.  Notice of Dissolution of Group

         Not Applicable

Item 10. Certification

         By signing below I certify that, to the best of my knowledge and
         belief, the securities referred to above were not acquired and are not
         held for the purpose of or with the effect of changing or influencing








         the control of the issuer of the securities and were not acquired and
         are not held in connection with or as a participant in any transaction
         having that purpose or effect.



                                    SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify
that the information set forth in this statement is true, complete and correct.

Date: February 17, 2004

                                          OrbiMed Advisors LLC

                                          By:  /s/ Samuel D. Isaly
                                          ----------------------------
                                          Name:  Samuel D. Isaly
                                          Title: Managing Member


                                          OrbiMed Capital LLC

                                          By:  /s/ Samuel D. Isaly
                                          ----------------------------
                                          Name:  Samuel D. Isaly
                                          Title: Managing Member



                                          By:  /s/ Samuel D. Isaly
                                          ----------------------------
                                          Name:  Samuel D. Isaly








    EX-1.1 OTHERDOC
          2
          0002.txt
             JOINT FILING AGREEMENT



                                                                    Exhibit 1.1
                                                                    ----------

                             JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on this Schedule 13G, dated
February 17, 2004, (the "Schedule 13G"), with respect to the Common Stock, par
value $.01 per share, of Cytogen Corp. is filed on behalf of each of us pursuant
to and in accordance with the provisions of Rule 13d-1(k) under the Securities
and Exchange Act of 1934, as amended, and that this Agreement shall be included
as an Exhibit to this Schedule 13G. Each of the undersigned agrees to be
responsible for the timely filing of the Schedule 13G, and for the completeness
and accuracy of the information concerning itself contained therein. This
Agreement may be executed in any number of counterparts, all of which taken
together shall constitute one and the same instrument.

        IN WITNESS WHEREOF, the undersigned have executed this Agreement as of
the 17th day of February, 2004.


                                           OrbiMed Advisors LLC

                                           By:  /s/ Samuel D. Isaly
                                           ---------------------------
                                           Name:  Samuel D. Isaly
                                           Title: Managing Member


                                          OrbiMed Capital LLC

                                          By:  /s/ Samuel D. Isaly
                                          ----------------------------
                                          Name:  Samuel D. Isaly
                                          Title: Managing Member


                                          By:  /s/ Samuel D. Isaly
                                          ----------------------------
                                          Name:  Samuel D. Isaly












      EX-2.1 OTHERDOC
      Statement of Control Person

                                                                    Exhibit 2.1
                                                                    ----------

                           Statement of Control Person


The Statement on this Schedule 13G dated February 17, 2004 with respect to the
common stock, $.01 par value per share, of Cytogen Corp. is filed by Samuel D.
Isaly in accordance with the provisions of Rule 13d-1(b) and Rule 13d-1(k),
respectively, as control person (HC) of OrbiMed Advisors LLC and OrbiMed Capital
LLC.

OrbiMed Advisors LLC and OrbiMed Capital LLC file this statement on Schedule 13G
in accordance with the provisions of Rule 13d-1(b) and Rule 13d-1(k),
respectively, as investment advisors (IA).