By Doug Macron
Preclinical data from Marina Biotech's bladder cancer program shows that the firm's proprietary siRNA constructs, encapsulated in liposomes and delivered directly to the bladders of mouse models of the disease, could knock down their targets and decrease tumors over a three-week period.
The study, which appears in the current issue of the journal Molecular Therapy, comes about a month after Marina exclusively licensed the program to Switzerland's Debiopharm, which is responsible for all related development and commercialization activities, for up to $25 million, plus royalties (GSN 2/3/2011).
In the paper, the Marina team focused on two bladder cancer-specific genes, survivin and polo-like kinase-1 — two key targets under investigation in the program licensed to Debiopharm — both of which are over-expressed in the disease and are “often considered prognostic biomarkers for many cancers,” they wrote.
Specifically, survivin plays a key role in apoptosis suppression and the regulation of cell division, according to the paper. At the same time, “selective over-expression of survivin has been documented in a wide variety of cancer cell types including bladder cancer.”
PLK-1, meanwhile, is a cell cycle-regulated kinase that is often dysregulated and over-expressed in tumor cells. RNAi-mediated inhibition of both genes, the authors added, can lead to cell cycle arrest, growth inhibition, and apoptosis in cancer cells.
The investigators developed survivin- and PLK1-targeting UsiRNAs — essentially duplex siRNAs modified with non-nucleotide acyclic monomers — and encapsulated them in liposomes. The liposomes were created using Marina's so-called DiLA2 platform, which contains a nor-arginine head group, cholesterol hemisuccinate, cholesterol, and 1, 2-dimyristoyl-phosphatidylethanolamine-polyethyleneglycol 2000.
The researchers then administered the agents locally to tumors via intravesical instillation in orthotopic bladder cancer mouse models at doses of either 0.5 mg/kg on days 2 and 4 or 1 mg/kg on days 7 and 9 following tumor implantation.
This route of administration, the researchers wrote, represents an “attractive delivery approach” for treating bladder cancer since the normal urothelium provides a relatively tight barrier between blood and urine that can “prevent or reduce the absorption of siRNAs from the bladder into systemic circulation, thereby limiting any potential toxicity.” Direct administration into the bladder cavity also provides “maximal exposure to tumors in the urothelium.”
The treatment with both the survivin- and PLK1-targeting UsiRNAs resulted in “significant reduction in tumor volumes that were sustained for at least 12 days after the last dose,” according to the paper. Tumor reduction was more pronounced with the PLK1-inhibiting molecules, which triggered a roughly 68-fold reduction versus a ten-fold reduction with survivin UsiRNAs, according to the paper.
The mRNA down-regulation, which was confirmed as an RNAi effect with 5' RACE analysis, was found to be greater with PLK1 UsiRNA in comparison to survivin UsiRNA treatment. Similarly, reduction of PLK1 protein expression was also “more pronounced compared to the modest decrease observed in survivin protein.”
The researchers noted that a report, published in the Proceedings of the National Academy of Sciences in 2009, suggests that the anti-tumor effect of a non-targeting siRNA on melanoma was the result of activation of the RIG-I pathway.
However, the Marina team wrote in their study that up-regulation of interferon-inducible genes IFIT1 and OAS1a was detected after treatment with the DiLA2-encapsulated UsiRNAs, further supporting the conclusion that “abrogation in mRNA expression and tumor growth in bladder tissue occurred via a target-specific RNAi-mediated pathway.”
Overall, “UsiRNA treatment resulted in target-specific down-regulation of survivin or PLK1 mRNA
expression and suppressed the growth of bladder tumor cells in a dose-dependent manner,” the investigators wrote.
But because survivin and PLK1 have been shown to regulate different cellular pathways, a cocktail of UsiRNAs against both genes could result in a “more pronounced additive or synergistic effect on tumor growth inhibition.”
Marina officials previously indicated that they were considering targeting multiple genes with the same therapeutic agent in the bladder cancer program.
“Furthermore, UsiRNAs could potentially be combined with existing chemotherapeutic agents to enhance the chemosensitivity of bladder cancer cells in refractory bladder carcinomas,” the Molecular Therapy paper concludes. “Future work includes screening additional genes to target critical cancer phenotypes for enhanced efficacy and prolonged therapeutic effect against transitional cell bladder carcinoma.”
"The publication of this preclinical data is a validation of our broad capabilities in identifying highly active UsiRNAs targeting cancer-specific genes and providing preclinical, in vivo proof of concept demonstrating safe and efficient delivery of those UsiRNAs with our DiLA2 delivery technology,” Marina CSO Barry Polisky said in a statement.
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