UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 4 )*

Penwest Pharmaceuticals Co.

(Name of Issuer)

Common Stock, par value $0.001

(Title of Class of Securities)

709754105

(CUSIP Number)

Kevin C. Tang

Tang Capital Management, LLC

4401 Eastgate Mall

San Diego, CA 92121

(858) 200-3830

(Name, Address and Telephone Number of Person Authorized to

Receive Notices and Communications)

March 12 , 2009

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  x

Note:  Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7 for other parties to whom copies are to be sent.

* 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 disclosures provided in a prior cover page.

The information required on 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).

(Continued on following pages)

Page 1 of 9

CUSIP NO. 709754105 13D/A Page 2 of 9

1.	Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only). Tang Capital Partners, LP
2.	Check the Appropriate Box if a Member of a Group (a) x (b) o
3.	SEC Use Only
4.	Source of Funds WC
5.	Check If Disclosure of Legal Proceeding Is Required Pursuant to Items 2(d) or 2(e) ¨
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 6,396,598
		9.	Sole Dispositive Power 0
		10.	Shared Dispositive Power   6,396,598
11.	Aggregate Amount Beneficially Owned by Each Reporting Person   6,396,598
12.	Check if the Aggregate Amount in Row (11) Excludes Certain Shares         ¨
13.	Percent of Class Represented by Amount in Row (11)   20.2%
14	Type of Reporting Person PN

Page 2 of 9

CUSIP NO. 709754105 13D/A Page 3 of 9

1.	Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only). Tang Capital Management, LLC
2.	Check the Appropriate Box if a Member of a Group (a) x (b) o
3.	SEC Use Only
4.	Source of Funds WC
5.	Check If Disclosure of Legal Proceeding Is Required Pursuant to Items 2(d) or 2(e) ¨
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   6,396,598
		9.	Sole Dispositive Power  0
		10.	Shared Dispositive Power   6,396,598
11.	Aggregate Amount Beneficially Owned by Each Reporting Person   6,396,598
12.	Check if the Aggregate Amount in Row (11) Excludes Certain Shares         ¨
13.	Percent of Class Represented by Amount in Row (11)   20.2%
14	Type of Reporting Person OO

Page 3 of 9

CUSIP NO. 709754105 13D/A Page 4 of 9

1.	Names of Reporting Persons. I.R.S. Identification Nos. of above persons (entities only). Kevin C. Tang
2.	Check the Appropriate Box if a Member of a Group (a) x (b) o
3.	SEC Use Only
4.	Source of Funds PF, WC, OO
5.	Check If Disclosure of Legal Proceeding Is Required Pursuant to Items 2(d) or 2(e) ¨
6.	Citizenship or Place of Organization United States
Number of Shares Beneficially Owned by Each Reporting Person With		7.	Sole Voting Power  77,500
		8.	Shared Voting Power   6,544,098
		9.	Sole Dispositive Power  77,500
		10.	Shared Dispositive Power   6,618,098
11.	Aggregate Amount Beneficially Owned by Each Reporting Person   6,695,598
12.	Check if the Aggregate Amount in Row (11) Excludes Certain Shares         ¨
13.	Percent of Class Represented by Amount in Row (11)  21.1%
14	Type of Reporting Person IN

Page 4 of 9

Explanatory Note: This Amendment No.  4 relates to and amends the Statement of Beneficial Ownership on Schedule 13D of Tang Capital Partners, LP, a Delaware partnership, Tang Capital Management, LLC, a Delaware limited liability company and Kevin C. Tang, a United States citizen (each, a “Reporting Person” and collectively, the “Reporting Persons”), initially filed jointly by the Reporting Persons with the Securities and Exchange Commission on January 12, 2009 and amended February 19, 2009 , March 3, 2009 and March 10, 2009 (as so amended the “Statement”), with respect to the Common Stock, $0.001 par value (the “Common Stock”), of Penwest Pharmaceuticals Co., a Washington corporation (the “Issuer”).

Items 3, 4 and 5 of the Statement are hereby amended to the extent hereinafter expressly set forth.  All capitalized terms used and not expressly defined herein have the respective meanings ascribed to such terms in the Statement.

Item 3. Source and Amount of Funds or Other Consideration

Item 3 of the Statement is hereby amended to add the following:

Since the date of the last filing on Schedule 13D/A, Tang Capital Partners, LP purchased a total of 25,100 shares of the Issuer’s common stock for an aggregate purchase price of $33,890.00 .

Tang Capital Partners, LP holds some of its shares in commingled margin accounts, which may extend margin credit to Tang Capital Partners, LP as and when required to open or carry positions in the margin accounts, subject to applicable federal margin regulations, stock exchange rules and credit policies. In such instances, the positions held in the margin accounts are pledged as collateral security for the repayment of debit balances in the accounts. The margin accounts may from time to time have debit balances. Since other securities are held in the margin accounts, it is not possible to determine the amounts, if any, of margin used to purchase the shares of common stock reported herein.

Item 4. Purpose of Transaction

Item 4 of the Statement is hereby amended to add the following:

On March 12, 2009, Tang Capital Partners, LP and the Perceptive Life Sciences Master Fund Ltd. sent to the corporate secretary of the Issuer a Demand to Inspect Corporate Books and Records pursuant to the Washington Business Corporation Act to, among other things: (a) investigate whether the Issuer’s board of directors has acted properly and in the best interest of the corporation; (b) determine what, if any, additional proposals Tang Capital Partners, LP and Perceptive Life Sciences Master Fund Ltd. should bring to a vote of shareholders at the Issuer’s 2009 annual meeting of shareholders; and (c) develop facts necessary to communicate with the Issuer’s shareholders, in compliance with applicable law, with respect to Tang Capital Partners, LP’s and Perceptive Life Sciences Master Fund Ltd.’s director nominations, additional proposals that they may bring to vote at the 2009 annual meeting of shareholders and other matters relating to their interests as shareholders, including, but not limited to, an investigation into possible mismanagement, waste of corporate assets and breach of fiduciary duties by the Issuer’s board of directors and officers.

On March 12, 2009, Tang Capital Partners, LP and the Perceptive Life Sciences Master Fund Ltd. filed a complaint for declaratory and injunctive relief against the Issuer in Superior Court in the State of Washington, Thurston County, requesting that the Court declare that plaintiffs satisfied the notice provisions set forth in the Issuer’s bylaws and requesting that the Court issue an order preventing the Issuer from seeking to disallow or otherwise prevent or not recognize the nominations by plaintiffs at the 2009 Annual Meeting of Shareholders of Joseph Edelman, Kevin C. Tang and Andrew D. Levin, M.D., Ph.D. as director candidates, or the casting of votes in favor of such candidates, on the basis that the plaintiffs have not complied with the provisions of the Issuer’s bylaws or applicable state law (the “Complaint”).  On March 13, 2009, Tang Capital Partners, LP and the Perceptive Life Sciences Master Fund Ltd. filed a motion for preliminary injunctive relief against the Issuer in Superior Court in the State of Washington, Thurston County, seeking a preliminary injunction to enjoin the Issuer from mailing any ballots to shareholders that contain director nominees and enjoining any shareholder vote on individuals nominated for the Issuer’s board of directors unless the three candidates submitted by plaintiffs are permitted to be nominated and votes are permitted to be cast in favor of such nominees, or a court resolves the merits of plaintiffs’ declaratory judgment action filed therewith.

Page 5 of 9

Item 5.  Interest in Securities of the Issuer.

Item 5 of the Statement is hereby amended and restated in its entirety as follows:

(a)	Amount beneficially owned and percentage of class:
	Tang Capital Partners, LP	6,396,598 shares, representing 20.2 % of the class
	Tang Capital Management, LLC	6,396,598 shares, representing 20.2 % of the class
	Kevin C. Tang	6,695,598 shares, representing 21.1% of the class

Tang Capital Partners, LP is the beneficial owner of 6,396,598 shares of the Issuer’s common stock.

            Tang Capital Management, LLC, as the general partner of Tang Capital Partners, LP, may be deemed to beneficially own the  6,396,598 shares beneficially owned by Tang Capital Partners, LP. Tang Capital Management, LLC shares voting and dispositive power over such shares with Tang Capital Partners, LP and Kevin C. Tang.

            Kevin C. Tang is the beneficial owner of 6,695,598 shares of the Issuer’s common stock, comprising 6,396,598 shares beneficially owned by Tang Capital Partners, LP, 147,500 shares beneficially owned by the Tang Family Trust, for which Kevin C. Tang serves as co-trustee, 77,500 shares beneficially owned by the Tang Advisors, LLC Profit Sharing Plan, for which Kevin C. Tang serves as trustee and is a participant, 37,500 shares beneficially owned by the Individual Retirement Account for the benefit of Chang L. Kong (the “Chang IRA”), and 36,500 shares beneficially owned by the Individual Retirement Account for the benefit of Chung W. Kong (the “Chung IRA”).

            Tang Capital Management, LLC, as the general partner of Tang Capital Partners, LP, and Kevin C. Tang, as the manager of Tang Capital Management, LLC, may also be deemed to beneficially own the shares beneficially owned by Tang Capital Partners, LP.  Kevin C. Tang is a beneficiary of the Tang Family Trust and shares voting and dispositive power over the shares held by the Tang Family Trust with his wife, Haeyoung K. Tang.  Chang L. and Chung W. Kong are Kevin C. Tang’s in-laws and Mr. Tang may be deemed to have shared dispositive power over the shares held in the Chang IRA and the Chung IRA.  Chang L. and Chung W. Kong are retired U.S. citizens.  Haeyoung K. Tang is a U.S. citizen.  The mailing address of all of the foregoing persons and entities is c/o Tang Capital Management, LLC, 4401 Eastgate Mall, San Diego, CA 92121. Kevin C. Tang disclaims beneficial ownership of all shares reported herein except to the extent of his pecuniary interest therein. 

Page 6 of 9

By virtue of the understanding reached between the Reporting Persons and Perceptive Advisors, LLC (“Perceptive”) described in Item 4, the Reporting Persons and Perceptive may be deemed to have formed a "group," within the meaning of Section 13(d)(3) of the Securities Act of 1934.  Collectively, the group may be deemed to have voting control over a combined 13,172,044 shares of Common Stock, or 41.6 % of the outstanding shares of Common Stock.  However, each of the Reporting Persons expressly disclaims beneficial ownership of the shares of Common Stock beneficially owned by Perceptive and its affiliates.

(b)	Voting and disposition powers:
	Sole power to vote or direct the vote:
	Tang Capital Partners, LP	0 shares
	Tang Capital Management, LLC	0 shares
	Kevin C. Tang	77,500 shares
	Shared power to vote or direct the vote:
	Tang Capital Partners, LP	6,396,598 shares
	Tang Capital Management, LLC	6,396,598 shares
	Kevin C. Tang	6,544,098 shares

Sole power to dispose or direct the disposition:
Tang Capital Partners, LP	0 shares
Tang Capital Management, LLC	0 shares
Kevin C. Tang	77,500 shares

Shared power to dispose or direct the disposition:
Tang Capital Partners, LP	6,396,598 shares
Tang Capital Management, LLC	6,396,598 shares
Kevin C. Tang	6,618,098 shares

Page 7 of 9

(c)           The Reporting Persons have engaged in the following transactions in the Issuer’s common stock since the filing of a Schedule 13D/A on March 10 , 2009:

Entity	Transaction	Trade Date	Shares	Price/Share
Tang Capital Partners, LP	Purchase	3/11/2009	21,000	1.36
Tang Capital Partners, LP	Purchase	3/11/2009	4,100	1.30

(d)           N/A.

(e)           N/A.

Page 8 of 9

SIGNATURES

After reasonable inquiry and to the best of his or its knowledge and belief, each of the following Reporting Persons certifies that the information set forth in this statement is true, complete and correct.

March 16 , 2009

Tang Capital Partners, LP
By:	Tang Capital Management, LLC
By:	/s/ Kevin C. Tang
	Kevin C. Tang, Manager
Tang Capital Management, LLC
By:	/s/ Kevin C. Tang
	Kevin C. Tang, Manager
/s/ Kevin C. Tang
Kevin C. Tang

Page 9 of 9