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)

                                December 31, 2002

             (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 Advisors 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: 797,500 Beneficially Owned by 7. Sole Dispositive Power: 0 Each Reporting Person With 8. Shared Dispositive Power: 797,500 9. Aggregate Amount Beneficially Owned by Each Reporting Person: 797,500 10. Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) 11. Percent of Class Represented by Amount in Row (9) 4.38% 12. Type of Reporting Person (See Instructions) IA 2

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: 797,500 Beneficially Owned by 7. Sole Dispositive Power: 0 Each Reporting Person With 8. Shared Dispositive Power: 797,500 9. Aggregate Amount Beneficially Owned by Each Reporting Person: 797,500 10. Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) 11. Percent of Class Represented by Amount in Row (9) 4.38% 12. Type of Reporting Person (See Instructions) IA 3

CUSIP No. 0015351109 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: 797,500 Beneficially Owned by 7. Sole Dispositive Power: 0 Each Reporting Person With 8. Shared Dispositive Power: 797,500 9. Aggregate Amount Beneficially Owned by Each Reporting Person: 797,500 10. Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) 11. Percent of Class Represented by Amount in Row (9) 4.38% 12. Type of Reporting Person (See Instructions) CO 4

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: 797,500 Beneficially Owned by 7. Sole Dispositive Power: 0 Each Reporting Person With 8. Shared Dispositive Power: 797,500 9. Aggregate Amount Beneficially Owned by Each Reporting Person: 797,500 10. Check if the Aggregate Amount in Row (9) Excludes Certain Shares (See Instructions) 11. Percent of Class Represented by Amount in Row (9) 4.38% 12. Type of Reporting Person (See Instructions) HC 5

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 Advisors Inc. 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: 0015351109 Item 3. OrbiMed Advisors Inc. and OrbiMed Advisors LLC are investment advisors In accordance with ss.240.13d-1(b)(1)(ii)(E). Samule D. Isaly is a control Person in accordance with ss.240.13d-1(b)(1)(ii)(G). Not applicable to OrbiMed Capital LLC. 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 Reporting persons are holding 4.38% 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 the proceeds from the sale of, such securities. 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 6

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 14, 2003 OrbiMed Advisors 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: February 14, 2003 OrbiMed Advisors LLC By: /s/ Samuel D. Isaly ------------------------------ Name: Samuel D. Isaly Title: President of Managing Member, OrbiMed Advisors Inc. OrbiMed Capital LLC By: /s/ Samuel D. Isaly ------------------------------ Name: Samuel D. Isaly Title: Managing Member By: /s/ Samuel D. Isaly ------------------------------ Name: Samuel D. Isaly 7

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

                             JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on this Schedule 13G/A, dated
Febraury 14, 2003, (the "Schedule 13G/A"), with respect to the Common Stock, par
value $0.0001 per share, 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 14th day of February, 2003.


                                                     OrbiMed Advisors 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: President of Managing
                                                   Member, OrbiMed Advisors Inc.


                                                     OrbiMed Capital LLC

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



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



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

                             STATEMENT OF CONTROL PERSON


The Statement on this Schedule 13G/A dated Febraury 14, 2003 with respect to the
common stock par value $0.0001 per share of Alexion Pharmaceuticals, Inc. is
filed by Samuel D. Isaly in accordance with the provisions of Rule 13d-1(b) and
Rule 13d-1(k), respectively, as a control person (HC) of OrbiMed Advisors LLC,
OrbiMed Advisors Inc. and OrbiMed Capital LLC.

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