Calando recently proved systemic siRNA delivery and RNAi-mediated mRNA knockdown.

Arrowhead Research has increased its ownership of majority-owned subsidiary Calando Pharmaceuticals through the exchange of Calando Series A preferred stock for Arrowhead warrants. Arrowhead says that Calando recently achieved milestones in RNAi technology, including first proof of systemic siRNA delivery and first to show RNAi mediated knockdown of target mRNA and protein in humans.

Calando is developing targeted, siRNA-containing therapeutics using its three-part RNAi/oligonucleotide nanoparticle delivery (Rondel) technology, the foundation of which is the firm’s cyclodextrin-containing polymer. siRNA is protected from destruction due to nucleases in the bloodstream by the firm’s cyclodextrin-containing polymers. By encapsulating the siRNA, Rondel also protects the body from the immune reactions caused by naked siRNA.

CALAA-01 is Calando’s lead RNAi therapeutics program based on Rondel delivery. It comprises an siRNA targeting ribonucleotide reductase M2, an enzyme involved in nucleotide metabolism and required for DNA replication. RRM2 is considered a promising target for cell proliferative diseases, especially cancers.

An interim analysis of Phase I data from 15 solid tumor patients treated with the drug showed that it was well tolerated, including at the highest dose tested so far. The absence of significant, dose-limiting immune responses furthermore supports that Rondel, even with unmodified siRNAs, is relatively nonimmunogenic and does not function as an immune adjuvant, which is unlike a number of lipid-based delivery approaches, Calando points out. Overall, the safety profile suggests that the dose-limiting toxicities should only be observed at dosages significantly higher than the highest ~0.6mg/kg dose tested for the interim analysis.

Of the 15 patients enrolled at the time of the analysis, three with melanoma volunteered to have biopsies taken allowing for more detailed molecular analyses of the effects of CALAA-01.

The firm was able to confirm the dose-dependent accumulation of CALAA-01 in the melanoma cancer target tissues. This has not been established before for any RNAi therapeutics clinical candidate, Calando notes.

Most importantly, it says, and in yet another first for RNAi therapeutics, it was possible to isolate RNA nucleic acid material and thus confirm, through a modified form of PCR called 5’ RACE assay, that the siRNA not only accumulated in the target tissue but was also successful in directing mRNA targets for degradation through an RNAi mechanism of action. While the 5’ RACE assay is nonquantitative, it requires a robust level of cleaved mRNA to be successfully applied.

Calando was hence able to establish RRM2 mRNA knockdown by quantitative real-time PCR. Moreover, the knockdown was long lasting and could be detected after more than a month. In aggregate, the data provides first-ever direct proof of target mRNA knockdown through an RNAi mechanism of action in man, Calando reports.

“We see significant value in Calando and therefore are interested in maximizing our ownership position in the company,” says Christopher Anzalone, Arrowhead’s CEO. “Acquisition of this majority block of Calando Series A stock is a good transaction for us because it increases our ownership in Calando and enables us additional flexibility as we work to negotiate the right partnership or series of partnerships for Calando’s industry-leading RNAi delivery platform and exciting therapeutic candidate, CALAA-01.

“Calando is making continued progress toward completing the Phase I clinical trial of CALAA-01 and interfacing with potential strategic partners.” The open-label, dose-escalating study is being conducted in adults with solid tumors refractory to standard-of-care therapies. Patients will receive two 21-day cycles of CALAA-01. A cycle will consist of four infusions administered on days 1, 3, 8, and 10 followed by 11 days of rest. If safe, a second 21-day cycle will be administered consisting of infusions on days 22, 24, 29, and 31 followed by 11 days of rest.

Rondel binds to and self-assembles with siRNA to form uniform colloidal-sized particles. Analysis has shown that these particles are spherical and less than 100 nm in diameter, according to Calando, which allows for accumulation at the tumor site. The fully formulated polymer/siRNA particles exhibit a significant therapeutic window of safety in animals, even when repeated doses are used. Additionally, Rondel particles have been shown to be stable under physiological conditions.

Pursuant to the exchange agreement between Arrowhead and the Calando Series A holders, Arrowhead exchanged a warrant to purchase approximately 3.9 million shares of Arrowhead stock for Calando Series A preferred stock with a liquidation preference of approximately $3.9 million. The Calando preferred stock is convertible into 2.7 million shares of Calando common stock at the discretion of the holder. The warrants have an exercise price of $0.50 per share and are exercisable beginning March 17, 2011, with an expiration date of September 16, 2015.

Currently, Arrowhead has four subsidiaries, all of which are involved in nanotechnology. Its second majority-owned firm is Unidym, which provides bulk materials, carbon nanotube-enabled products, and intellectual property to a range of customers and partners.

Arrowhead has minority investments in two early-stage companies, Nanotope and Leonardo Biosystems. Nanotope is a regenerative medicine company developing a suite of products customized to regenerate specific tissues including neuronal, vascular, bone, myocardial, and cartilage. It expects to initiate clinical trials later this year.

Leonardo Biosystems has a multistage delivery platform that has been shown in animal models to be effective in targeting delivery of siRNA and small molecule drugs. It anticipates entering commercial development partnerships in 2010.

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