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

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

                           Acadia Pharmaceuticals Inc.
                                (Name of Issuer)

                                  Common Stock
                         (Title of Class of Securities)

                                    004225108
                                 (CUSIP Number)

                                 April 15, 2005

             (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. 004225108


       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: 621,606
Beneficially
Owned by         7.  Sole Dispositive Power:  0
Each Reporting
Person With      8.  Shared Dispositive Power: 621,606

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person
             621,606

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

       11.   Percent of Class Represented by Amount in Row (9) 2.65%

       12.   Type of Reporting Person (See Instructions) IA






                               CUSIP No. 004225108


       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: 863,906
Beneficially
Owned by         7.  Sole Dispositive Power: 0
Each Reporting
Person With      8.  Shared Dispositive Power:  863,906

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person
             863,906

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

       11.   Percent of Class Represented by Amount in Row (9) 3.69%

       12.   Type of Reporting Person (See Instructions) IA




                               CUSIP No. 004225108


       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: 1,485,512
Beneficially
Owned by         7.  Sole Dispositive Power: 0
Each Reporting
Person With      8.  Shared Dispositive Power: 1,485,512

       9.    Aggregate Amount Beneficially Owned by Each Reporting Person:
             1,485,512

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

       11.   Percent of Class Represented by Amount in Row (9) 6.34%

       12.   Type of Reporting Person (See Instructions) HC




Item 1. (a)  Issuer: Acadia Pharmaceuticals Inc.

    	 1.   Address:
              3911 Sorrento Valley Blvd.
              San Diego, CA 92121


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: 004225108

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 for each cover sheet for each filing
separately

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 6.34% (2.65% in the case of
         OrbiMed Advisors LLC and 3.69% in the case of OrbiMed Capital LLC) 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.

         OrbiMed Advisors LLC and OrbiMed Capital LLC hold shares on behalf
	 of Eaton Vance Worldwide Health Sciences (621,606 shares) and Finsbury
	 Worldwide Pharmaceutical Trust (774,006 shares and 89,900 warrants).





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 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: April 21, 2005
                                       OrbiMed Advisors LLC

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

                                       OrbiMed Capital LLC

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


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


			JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on this Schedule 13G/A, dated
April 21, 2005, (the "Schedule 13G/A"), with respect to the Common Stock, par
value $.001 per share, of Acadia 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 21st day of April 2005.


                                          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


				Statement of Control Person


The Statement on this Schedule 13G/A dated April 21, 2005 with respect to the
common stock, $.001 par value per share, of Acadia 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 control person (HC) of OrbiMed Advisors LLC
and OrbiMed Capital LLC.

OrbiMed Advisors LLC and OrbiMed Capital 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).