CUSIP No. 040047 10 2

Page 2 of 8 Pages

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Biotechnology Value Fund, L.P.

2.

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

(a)

 ý

(b)

 o

3.

SEC Use Only

4.

Source of Funds (See Instructions)
WC

5.

Check if Disclosure of Legal Proceedings Is Required Pursuant to Items 2(d) or 2(e)     o

6.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

7.

Sole Voting Power
0

8.

Shared Voting Power
920,477

9.

Sole Dispositive Power
0

10.

Shared Dispositive Power
920,477

11.

Aggregate Amount Beneficially Owned by Each Reporting Person
920,477

12.

Check if the Aggregate Amount in Row (11) Excludes Certain Shares (See Instructions)   o

13.

Percent of Class Represented by Amount in Row (11)
3.6%

14.

Type of Reporting Person (See Instructions)
PN

CUSIP No. 040047 10 2

Page 3 of 8 Pages

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
Biotechnology Value Fund II, L.P.

2.

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

(a)

 ý

(b)

 o

3.

SEC Use Only

4.

Source of Funds (See Instructions)
WC

5.

Check if Disclosure of Legal Proceedings Is Required Pursuant to Items 2(d) or 2(e)     o

6.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

7.

Sole Voting Power
0

8.

Shared Voting Power
592,884

9.

Sole Dispositive Power
0

10.

Shared Dispositive Power
592,884

11.

Aggregate Amount Beneficially Owned by Each Reporting Person
592,884

12.

Check if the Aggregate Amount in Row (11) Excludes Certain Shares (See Instructions)   o

13.

Percent of Class Represented by Amount in Row (11)
2.3%

14.

Type of Reporting Person (See Instructions)
PN

CUSIP No. 040047 10 2

Page 4 of 8 Pages

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
BVF Investments, L.L.C.

2.

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

(a)

 ý

(b)

 o

3.

SEC Use Only

4.

Source of Funds (See Instructions)
WC

5.

Check if Disclosure of Legal Proceedings Is Required Pursuant to Items 2(d) or 2(e)     o

6.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

7.

Sole Voting Power
0

8.

Shared Voting Power
1,556,351

9.

Sole Dispositive Power
0

10.

Shared Dispositive Power
1,556,351

11.

Aggregate Amount Beneficially Owned by Each Reporting Person
1,556,351

12.

Check if the Aggregate Amount in Row (11) Excludes Certain Shares (See Instructions)   o

13.

Percent of Class Represented by Amount in Row (11)
6.1%

14.

Type of Reporting Person (See Instructions)
OO

CUSIP No. 040047 10 2

Page 5 of 8 Pages

1.

Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only)
BVF Partners L.P.

2.

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

(a)

 ý

(b)

 o

3.

SEC Use Only

4.

Source of Funds (See Instructions)
OO

5.

Check if Disclosure of Legal Proceedings Is Required Pursuant to Items 2(d) or 2(e)     o

6.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

7.

Sole Voting Power
0

8.

Shared Voting Power
3,130,912

9.

Sole Dispositive Power
0

10.

Shared Dispositive Power
3,130,912

11.

Aggregate Amount Beneficially Owned by Each Reporting Person
3,130,912

12.

Check if the Aggregate Amount in Row (11) Excludes Certain Shares (See Instructions)   o

13.

Percent of Class Represented by Amount in Row (11)
12.2%

14.

Type of Reporting Person (See Instructions)
PN

CUSIP No. 040047 10 2

Page 6 of 8 Pages

1.

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

2.

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

(a)

 ý

(b)

 o

3.

SEC Use Only

4.

Source of Funds (See Instructions)
OO

5.

Check if Disclosure of Legal Proceedings Is Required Pursuant to Items 2(d) or 2(e)     o

6.

Citizenship or Place of Organization
Delaware

Number of
Shares
Beneficially
Owned by
Each
Reporting
Person With

7.

Sole Voting Power
0

8.

Shared Voting Power
3,130,912

9.

Sole Dispositive Power
0

10.

Shared Dispositive Power
3,130,912

11.

Aggregate Amount Beneficially Owned by Each Reporting Person
3,130,912

12.

Check if the Aggregate Amount in Row (11) Excludes Certain Shares (See Instructions)   o

13.

Percent of Class Represented by Amount in Row (11)
12.2%

14.

Type of Reporting Person (See Instructions)
IA, CO

CUSIP No. 040047 10 2

Page 7 of 8 Pages

This Amendment No. 8 to the Statement on Schedule 13D (this “Amendment”) is being filed with the Securities and Exchange Commission on behalf of the Biotechnology Value Fund, L.P., a Delaware limited partnership (“BVF”), Biotechnology Value Fund II, L.P., a Delaware limited partnership (“BVF2”), BVF Investments L.L.C., a Delaware limited liability company (“Investments”), BVF Partners L.P., a Delaware limited partnership (“Partners”) and BVF Inc., a Delaware corporation (“BVF Inc.” and, together with BVF, BVF2, Investments, and Partners, the “Reporting Persons”) with respect to the Common Stock, par value $0.0001 per share (the “Stock”), of Arena Pharmaceuticals, Inc., a Delaware corporation (“Arena”).  The principal executive office of Arena is located at 6166 Nancy Ridge Road, San Diego, California 92121.

Item 5 is hereby amended to read in its entirety as follows:

Item 5.

Interest in Securities of the Issuer

(a)           BVF beneficially owns 920,477 shares of the Stock, BVF2 beneficially owns 592,884 shares of the Stock, Investments beneficially owns 1,556,351 shares of the Stock and each of Partners and BVF Inc. beneficially owns 3,130,912 shares of the Stock, representing approximately 3.6%, 2.3%, 6.1% and 12.2%, respectively, of the approximate aggregate number of shares of common stock outstanding as of October 31, 2003.

(b)           Each of BVF, BVF2 and Investments shares with Partners voting and dispositive power over the shares of the Stock each such entity beneficially owns. Partners and BVF Inc. share voting and dispositive power over the 3,130,912 shares of the Stock they beneficially own with, in addition to BVF, BVF2 and Investments, Investment 10, L.L.C., an Illinois limited liability company ("ILL10") and managed account on whose behalf Partners, as investment advisor, purchased such shares. ILL10 specializes in holding biotechnology stocks for investment purposes and its business address is 227 West Monroe Street, Suite 4800, Chicago, Illinois 60606.

(c)           On January 9, 2004, BVF sold 7,000 shares of the Stock, BVF2 sold 5,000 shares of the Stock, Investments sold 12,000 shares of the Stock and ILL10 sold 1,000 shares of the Stock.  Each such sale was made on the open market at a price of $6.30 per share.  On January 14, 2004, BVF sold 30,000 shares of the Stock, BVF2 sold 19,000 shares of the Stock, Investments sold 46,000 shares of the Stock and ILL10 sold 5,000 shares of the Stock.  Each such sale was made on the open market at a price of $6.05 per share.  On January 20, 2004, BVF sold 30,000 shares of the Stock, BVF2 sold 19,000 shares of the Stock, Investments sold 46,000 shares of the Stock and ILL10 sold 5,000 shares of the Stock.  Each such sale was made on the open market at a price of $6.00 per share.  On January 21, 2004, BVF sold 15,500 shares of the Stock, BVF2 sold 10,000 shares of the Stock, Investments sold 24,000 shares of the Stock and ILL10 sold 3,000 shares of the Stock.  Each such sale was made on the open market at a price of $6.00 per share.

(d)           ILL10 is entitled to receive dividends and any sale proceeds with respect to shares of the Stock in proportion to its respective ownership interest therein.

Item 7.

Material to Be Filed as Exhibits

Exhibit 1 – Agreement Regarding Joint Filing

CUSIP No. 040047 10 2

Page 8 of 8 Pages