Exhibit 10.1

August 21, 2007

Mr. Charles R. Eyler

3125 Collingswood Drive

El Dorado Hills, CA 95762

Dear Mr. Eyler:

This letter (the “Letter Amendment”) shall confirm our understanding as to the
amendment of certain terms of that employment letter agreement dated August 5,
2004 (the “Agreement”) between Cougar Biotechnology, Inc. (“Cougar”) and you
relating to your employment as Vice President of Finance of Cougar.

1.    Paragraph 1 of the Agreement shall be deleted in its entirety and replaced
with the following:

“You shall receive an annual base salary equal to One Hundred Eighty-Five
Thousand Dollars ($185,000), payable in accordance with Cougar’s normal payroll
practices, effective as of August 23, 2007.”

2.    Paragraph 2 of the Agreement shall be deleted in its entirety and replaced
with the following:

“At the sole discretion of Cougar’s Board of Directors (the “Board”), you shall
receive an additional annual bonus of up to 30% of your base salary (the
“Discretionary Bonus”), the amount of which shall also be determined solely by
the Board and shall be based upon your performance on behalf of Cougar during
the prior year. The Discretionary Bonus, if any, shall be payable either as a
lump-sum payment or in installments as determined by the Board in its sole
discretion.”

3.    Except as expressly amended herein, the terms of the Agreement shall
continue to be in full force and effect. Nothing herein shall be construed to
alter the agreement between you and Cougar that your employment relationship
with Cougar remains at-will.

If you find the terms of this Letter Amendment acceptable and believe that the
foregoing accurately summarizes our understanding, please kindly so indicate by
executing and dating the attached copy of this Letter Amendment in the space
provided and return it to me.

Very truly yours,

/s/ Alan H. Auerbach

Alan H. Auerbach

Chief Executive Officer

Agreed and accepted this 21st day of

August, 2007:

/s/ Charles R. Eyler                        

Charles R. Eyler