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

                                  SCHEDULE 13G/A
                    Under the Securities Exchange Act of 1934

                           Alexion Pharmaceuticals, Inc.
                                (Name of Issuer)

                                  Common Stock
                         (Title of Class of Securities)

                                    0015351109
                                 (CUSIP Number)

                                February 7, 2000

             (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)

[x] 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. 0015351109

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

             OrbiMed Advisers Inc.

       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  614,300
Beneficially
Owned by         7.  Sole Dispositive Power  0
Each Reporting
Person With      8.  Shared Dispositive Power  614,300



       9.    Aggregate Amount Beneficially Owned by Each Reporting Person
             614,300

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

       11.   Percent of Class Represented by Amount in Row (9) 4.20%

       12.   Type of Reporting Person (See Instructions) IA








CUSIP No. 0015351109

       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  614,300
Beneficially
Owned by       7.    Sole Dispositive Power  0
Each Reporting
Person With    8.    Shared Dispositive Power  614,300

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person
             614,300

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

       11.   Percent of Class Represented by Amount in Row (9) 4.20%

       12.   Type of Reporting Person (See Instructions) CO







CUSIP No. 0015351109

       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  614,300
Beneficially
Owned by       7.    Sole Dispositive Power  0
Each Reporting
Person With    8.    Shared Dispositive Power  614,300

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person
             614,300

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

       11.   Percent of Class Represented by Amount in Row (9) 4.20%

       12.   Type of Reporting Person (See Instructions) HC







Item 1. (a)  Issuer: Alexion Pharmaceuticals, Inc.

        (b)  Address: 25 Science Park, Suite 360
             New Haven, CT  06511

Item 2. (a)  Name of Person Filing:
             OrbiMed Advisers Inc.
             OrbiMed Advisors LLC
             Samuel D. Isaly

        (b)  Address of Principal Business Offices:

             767 Third Avenue, 6th Floor
             New York, New York 10010

        (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: 0015351109

Item 3.  Not Applicable

Item 4.  Ownership

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

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

         If this statement is being filed to report the fact that as of the Date
         hereof the reporting person has ceased to be the beneficial Owner of
         more than five percent of a class of securities, check The following
         [X].


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

         Not Applicable


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: June 5, 2001

                                                         OrbiMed Advisers Inc.

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



         By signing below I certify that, to the best of my knowledge and
         belief, the securities referred to above were acquired and held in the
         ordinary course of business and 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: June 5, 2001

                                                            OrbiMed Advisors LLC

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

                                                                 Samuel D. Isaly

                                                  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/A, dated
June 5, 2001, (the "Schedule 13G"), with respect to the Common Stock, $.0001
par value, of Alexion Pharmaceuticals, Inc. 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/A . Each of the undersigned
agrees to be responsible for the timely filing of the Schedule 13G/A, 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 5th day of June, 2001.


                                                         OrbiMed Advisers Inc.

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

                                                            OrbiMed Advisors LLC

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


      EX-2.1 OTHERDOC
      Statement of Control Person

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

                             Statement of Control Person


The Statement on this Schedule 13G/A dated June 5, 2001 with respect to the
common stock $.0001 par value of Alexion Pharmaceuticals, Inc. is filed by
Samuel D. Isaly in accordance with the provisions of Rule 13d-1(c) and Rule
13d-1(k) respectively as a control person (HC) of Orbimed Advisors, LLC and
Orbimed Advisors, Inc.

Orbimed Advisors, Inc. files this statement on Schedule 13G/A in accordance with
the provisions of Rule 13d-1(b) and Rule 13d-1(k) respectively as an investment
advisor (IA) and Orbimed Advisors, LLC files this statement on Schedule 13G/A in
accordance with the provisions or Rule 13d-1(c) and 13d-1(k) respectively as a
corporation (CO).