CTI BioPharma (CTIC) S-3 Shelf registration

Exhibit 15.1

LETTER REGARDING UNAUDITED FINANCIAL INFORMATION

Cell Therapeutics, Inc.

501 Elliott Avenue West, Suite 400

Seattle, WA 98119

We have reviewed, in accordance with the standards of the Public Company Accounting Oversight Board (United States), the unaudited interim financial information of Cell Therapeutics, Inc. for the periods ended June 30, 2005 and 2004, as indicated in our report dated August 5, 2005; because we did not perform an audit, we expressed no opinion on that information.

We are aware that our report referred to above, which is included in your Quarterly Report on Form 10-Q for the quarter ended June 30, 2005 is incorporated by reference in this Registration Statement.

We also are aware that the aforementioned report, pursuant to Rule 436(c) under the Securities Act of 1933, is not considered a part of the Registration Statement prepared or certified by an accountant within the meaning of Sections 7 and 11 of that Act.

/s/ Grant Thornton LLP

Seattle, Washington

February 3, 2006