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PROSPECTUS
SUPPLEMENT NO. 4
(TO
PROSPECTUS DATED MAY 12, 2006)
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Filed
pursuant to Rule 424(b)(3)
under
the Securities Act of 1933 in connection with
Registration
Statement No. 333-133786
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Common
Stock
This
Prospectus Supplement No. 4 supplements and amends the prospectus dated May
12,
2006 relating to the resale of up to 12,167,047 shares of our common stock
by
Kingsbridge Capital Limited (“Kingsbridge”).
This
prospectus supplement should be read in conjunction with the prospectus dated
May 12, 2006, Prospectus Supplement No. 1 dated May 31, 2006, Prospectus
Supplement No. 2 dated October 12, 2006, and Prospectus Supplement No. 3 dated
November 15, 2006 which are to be delivered with this prospectus supplement.
This prospectus supplement is not complete without, and may not be delivered
or
utilized except in connection with, the prospectus, including any supplements
or
amendments to it. All references in the prospectus to “this prospectus” are
hereby amended to read “this prospectus (as supplemented and
amended).”
We
are
filing this prospectus supplement to reflect a draw down by us pursuant to
the
Common Stock Purchase Agreement, dated April 17, 2006, between Kingsbridge
and
ourselves.
The
table
appearing under the caption “Selling Stockholder” on page 20 of the prospectus
is hereby further supplemented and amended by adding the following to the end
of
footnote (2) to that table, as previously supplemented and amended:
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“On
February 8, 2007, we delivered notice to Kingsbridge to effect a draw
down
of up to $2,000,000. The first trading day of the eight day pricing
period
for this draw down was February 9, 2007. In connection with this draw
down, we issued an aggregate of 942,949 shares of our common stock
to
Kingsbridge at an aggregate purchase price of $2,000,000. The settlement
dates for this drawdown were February 15, 2007 and February 22, 2007.”
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Investing
in our common stock involves significant risks. See “Risk Factors” beginning on
Page 7 of the prospectus, as well as the section entitled “Risk Factors”
included in our recent quarterly and annual reports filed with the Securities
and Exchange Commission.
Neither
the Securities and Exchange Commission nor any state securities commission
has
approved or disapproved of these securities or determined if this prospectus
is
truthful or complete. Any representation to the contrary is a criminal offense.
The
date
of this prospectus supplement is February 26, 2007.