Calando Pharmaceuticals' Lead siRNA Anti-Cancer Therapeutic Candidate Shown to Be Well Tolerated in Non-Human Primate Safety Study.

PASADENA, Calif. -- Calando Pharmaceuticals, a leading siRNA therapeutics company and a majority-owned subsidiary of Arrowhead Research Corporation (NASDAQ:ARWR), announced results of a pilot safety-study using Calando's lead siRNA therapeutic candidate in non-human primates. At the Second Annual Meeting of the Oligonucleotide Therapeutic Society, Dr. Jeremy Heidel, Chief Scientific Officer, will present data from the study showing that the therapeutic candidate is well tolerated at doses significantly higher than those shown to be efficacious in previous studies using a similar formulation.

The formulation investigated contains Calando's proprietary delivery technology with an siRNA duplex targeting the M2 subunit of ribonucleotide reductase, a well-established cancer target. This duplex, developed at Calando, demonstrates potent anti-proliferative activity across multiple types of cancer cells. The objective of this escalating-dose pilot study was to assess numerous safety-related parameters for this formulation in primates.

The data show that this formulation is well tolerated in non-human primates at doses well above doses of a similar formulation shown to have anti-tumor effects when tested in a metastatic mouse model of Ewing's sarcoma, a rare form of cancer affecting mainly children and young adults (Cancer Res 65 (19):8984-8892). One area of concern in translating any siRNA-containing formulation from animals to the clinic is the potential for immune responses to the siRNA duplex itself. Another concern is antibody generation to the formulation, including the targeting components. The Calando study examined both of these issues, and the results suggest that repetitive dosing in humans will be feasible with this formulation.

"These results keep us on track to be the first in the clinic with a targeted siRNA therapeutic," said John Petrovich, Calando's Chief Executive Officer. "We plan to initiate a Phase I clinical trial by the end of 2007 with this lead formulation to investigate its safety, tolerability and pharmacokinetics in patients with solid tumors of multiple types."

About Arrowhead Research Corporation

Arrowhead Research Corporation (www.arrowheadresearch.com) is a publicly-traded nanotechnology company commercializing new technologies in the areas of life sciences, electronics, and energy. Arrowhead is building value for shareholders through the progress of majority owned subsidiaries founded on nanotechnologies originally developed at universities. The company works closely with universities to source early stage deals and to generate rights to intellectual property covering promising new nanotechnologies. Currently, Arrowhead has four subsidiaries commercializing nanotech products and applications, including anti-cancer drugs, RNAi therapeutics, carbon-based electronics and compound semiconductor materials.

About Calando Pharmaceuticals Inc.

Calando Pharmaceuticals Inc. (www.calandopharma.com), a majority owned subsidiary of Arrowhead Research Corporation (NASDAQ: ARWR), is using its proprietary technologies in targeted polymeric delivery systems and siRNA design to design and create new, targeted siRNA therapeutics. The company is pursuing this goal through its internal research and development and also through collaborations and partnerships with pharmaceutical and biotechnology companies.

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