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Harvard-led Team Reports Mouse Data on Directly Delivered RNAi Therapy for Breast Cancer

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A research team led by investigators from Harvard University this month published new mouse data showing that lipid nanoparticle-encapsulated siRNAs targeting the HoxA1 gene could stop the progression of breast cancer tumors when administered directly through the animals' nipples.

According to Harvard's Don Ingber, senior author of the study, the treatment approach has the potential to stop breast cancer before it occurs in patients genetically predisposed to the disease. "The idea would be [to] start giving it early on and sustain treatment throughout life to prevent cancer development or progression," he said in a statement.

As screening technologies advance, there has been an uptick in the detection of premalignant breast lesions, of which ductal carcinoma in situ (DCIS) is the most common, Ingber and colleagues wrote in Science Translational Medicine.

Characterized by the proliferation of abnormal epithelial cells within the mammary duct, DCIS does not always progress to breast cancer, but a lack of effective biomarkers leaves few options for intervention beyond watching and waiting or aggressive treatment such as mastectomy and radiation.

As such, "minimally invasive therapies that can be selectively targeted to the ductal epithelium to prevent progression of pre-malignant breast lesions without producing systemic toxicity [is] one of the highest priorities of translational breast cancer research," the team wrote.

Viewing RNAi as a promising solution to this problem but recognizing the importance of target selection and delivery to the gene-silencing technology, the scientists began by conducting a gene network interference analysis, evaluating gene expression changes in the context of the entire gene regulatory network.

Through this they pinpointed HoxA1 as a "critical mediator of mammary tumor progression" in humans, noting in Science Translational Medicine that other groups have previously identified a role for the gene in breast cancer development.

To examine the gene's functional role, the researchers downregulated its expression in cultured mammary epithelial tumor cells derived from transgenic mice, finding that its inhibition resulted in the reconstitution of normal tissue architecture and a significant reduction in cellular proliferation. Similar findings were obtained when HoxA1 was silenced in breast cancer cells obtained from patient tumors.

In order to test the therapeutic potential of HoxA1 suppression, the investigators formulated fluorescently tagged siRNAs against the gene with lipidoid nanoparticles, which were then delivered intraductally via injection through the nipples of mice in order to reach the transformed mammary epithelial cells that give rise to DCIS lesions.

They found that the nanoparticle solution filled the intact mammary tree, and that the nipple and mammary ducts remained intact. Imaging analysis showed that the nanoparticles were within mammary epithelial cells at 48 hours and one week after injection, but not in the stromal cells that surround the epithelium. Further, no nanoparticles were detected in in the liver, spleen, kidney, heart, lung, or peripheral blood up to one week after injection.

Next, virgin adult female transgenic mice were treated with either HoxA1 siRNAs or control siRNAs, both of which were formulated with the lipidoid nanoparticles.

Although all of the mice receiving control siRNAs developed macroscopic tumors by 21 weeks of age, only 25 percent of those receiving HoxA1 siRNAs exhibited tumor formation by the same time point. Additionally, tumor onset in the treated animals was delayed by about three weeks, while lesions that did develop failed to progress to invasive macroscopic tumors in glands treated with the HoxA1-targeting siRNAs.

RNAi treatment also reduced cellular proliferation, although there was no significant change in the rate of apoptosis, and prevented the loss of hormone receptor expression, the researchers noted. Treated animals showed no signs of local tissue damage, toxicity, or systemic side effects.

In their paper, the research group indicated that a number of questions need to be answered before this approach can be used clinically, including whether there are any untoward side effects associated with long-term treatment and if the delivery approach will be effective in humans whose breasts contain multiple ductal systems with separate openings on the nipples.

Pending positive results from these and other such studies, the strategy developed by the Harvard-led group has the potential to be used prophylactically in individuals at high risk for breast cancer even before the detection of DCIS or breast lesions.

"Alternatively, it could be administered with chemotherapy to women who have had tumors removed and who would normally receive chemotherapy or radiation therapy to prevent progression of remaining small undetected lesions," they concluded. "The feasibility, the absence of side effects over several months, and the overall prevention of tumor progression observed in [the published] study support the further investigation of intraductal delivery of patient-specific siRNA therapy for breast cancer prevention and treatment."

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