Purdue University researchers have developed a first-in-class microRNA (miRNA) therapy that slows tumor growth by targeting multiple genes at once, giving it the potential outperform current single-gene targeted agents.
The therapy, tested in mouse models, combines a folate-based delivery system that targets cancer cells with a fully modified version of miRNA-34a (miR-34a), a molecule that acts “like the brakes on a car,” slowing or stopping cell division, says Andrea Kasinski, lead author and the William and Patty Miller associate professor of biological sciences at Purdue University.
She explains to Inside Precision Medicine that miR-34a is a direct downstream target of the tumor suppressor gene p53, which is commonly mutated in cancer. miR-34a is abundant in healthy cells but its presence is dramatically reduced in many cancer cells. Previous work has shown that restoring miR-34a could restore the tumor suppressive effects of p53 but when a previous miR-34a-based agent reached the clinic in 2013 it failed due to various challenges.
Kasinski and team therefore set out to overcome these difficulties, including poor stability, non-specific delivery, and delivery-associated toxicity, by creating a “fully modified” version of miR-34a.
Unlike previously tested partially modified versions of miR-34a that included just a few changes to the ribose sugar component of some of the RNA nucleotides, the current molecule contains changes at all the ribose sugars of all nucleotides and some backbone modifications that link the nucleic acids together.
“Collectively these modifications reduce recognition by the immune system and also increase the stability of the RNA, resulting in less frequent dosing,” says Kasinski.
The researchers initially tested the agent in cancer cell lines and found that it was “orders of magnitude more stable than a partially modified version” that had modifications similar to those found in commercially available miR-34a mimics. The improved stability resulted in a stronger, more sustained repression of a greater number of miR-34a targets, the authors report in Oncogene.
The targets included the genes MET, CD44, and AXL, which are known to drive cancer proliferation and resistance to other cancer therapies.
Kasinski and team then tested the fully modified miR-34a in mice bearing tumors derived from breast cancer cells. To do so they used a folate conjugate-based delivery strategy, which allows for specific delivery of miRNA to tumors that overexpress the folate receptor without the need to package the miRNA within a nanoparticle vehicle.
They chose this strategy in part because the folate receptor is overexpressed in breast cancer and because folate-conjugates have already been approved for use with imaging agents by the FDA and therefore have a known safety profile. Other benefits include tight binding to the receptor and small size which allows the therapeutic RNA to penetrate the dense tumor mass more deeply than can be achieved with nanoparticles.
At 5 days after treatment with a single 1.5 nmol dose of the folate-conjugated, fully modified miR-34a the researchers found that several of the miR-34a target genes were down regulated. Both MET and AXL were undetectable in all three tumors assessed, while CD44 levels were significantly reduced, which the authors say confirms “the ability of FM-miR-34a to effectively silence its biological targets in vivo.”
They note that the degree of gene silencing was similar to that commonly observed following treatment with single-gene short interfering RNA, another class of small RNA molecules that are being investigated as cancer therapeutics.
Kasinski and colleagues also measured the size of tumors 21 days after treatment and observed that they had either stayed the same size or shrunk, with one animal completely cured. By comparison, treatment with the partially modified miR-34a only delayed tumor growth.
The say that their results “have the ability to revitalize miR-34a as an anti-cancer agent, providing a strong rationale for clinical testing.”
Following the study, Kasinski disclosed the innovation to the Purdue Innovates Office of Technology Commercialization, which has now filed a patent application on the intellectual property.
The next steps will be to carry out more extensive efficacy, toxicity, and pharmacologic studies and test the folate-conjugated miR-34a in additional model systems, including those for other tumors that overexpress the folate receptor, such as lung, colon, and ovarian cancers.
“I am optimistic that we should have the resources and the experiments completed within 12 to 18 months,” notes Kasinski. At that point they could submit an Investigational New Drug (IND) application, with clinical trials to follow, pending IND approval.
