/s/ Arthur H. Reidel

Exhibit 10.53

June 16, 2003

VIA HAND DELIVERY

Shawn M. O’Connor

Pharsight Corporation

Dear Shawn:

As I have discussed with you, Pharsight Corporation (the “Company”) in conjunction with Execustaff, Inc., is pleased to offer you the additional stock option grant set forth in this letter.

Stock Options

On April 24, 2003, the Company’s Compensation Committee approved an additional stock option grant to you, of five hundred thousand (500,000) shares of the Company’s common stock with an exercise price equal to the fair market value of such shares on the date of grant, in accordance with the terms of the Company’s 2000 Equity Incentive Plan. Such options will vest over a four (4) year period as follows: 25% will vest on the first anniversary date of grant and the remainder will vest in equal monthly installments thereafter until fully vested (“Vesting Schedule”).  However, upon a Change of Control (as defined in the Company’s 2000 Equity Incentive Plan), the Vesting Schedule will accelerate, and any unvested shares shall accelerate one hundred percent (100%) and become fully vested (“Accelerated Vesting”).

Miscellaneous

This letter sets forth and forms the complete and exclusive statement concerning your additional stock option grant and this letter supersedes any other agreements or promises made to you by anyone, whether oral or written, concerning the subject matter set forth in this letter.

We are very pleased to offer you this additional stock option grant.

Sincerely,

PHARSIGHT CORPORATION

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