If you dispatch a suicide gene, you want to make sure that it bypasses healthy cells on its way to harmful cells, such as cancer cells. What’s more, you want to make sure that the suicide gene is sent via a delivery system that treads lightly—especially if the suicide gene is meant to treat pediatric patients, who have relatively fragile immune systems. Unfortunately, pediatric patients may have difficulty tolerating the most common gene delivery systems, which are derived from viruses.
To build a targetable and relatively innocuous delivery system, scientists based at the Johns Hopkins University School of Medicine turned to nanotechnology. Basically, they developed a library of poly(beta-amino ester) nanoparticles, or PBAEs. These tiny delivery vehicles consist of biodegradable, cationic polymers, and they self-assemble with nucleic acids.
According to a study (“Nonviral polymeric nanoparticles for gene therapy in pediatric CNS malignancies”) that appeared in the January issue of Nanomedicine: Nanotechnology, Biology and Medicine, the Johns Hopkins researchers used PBAEs to package and deliver the suicide gene herpes simplex virus type I thymidine kinase (HSVtk). Once HSVtk is delivered to a cancer cell, it makes an enzyme that helps restore the cell’s natural tumor suppression function, promoting apoptosis, the cell’s self-destruct mechanism.
To test their PBAEs, the Johns Hopkins researchers used them to transfect cells in mouse xenograft models of cancer, specifically, models that presented medulloblastoma (MB) and atypical teratoid/rhabdoid tumors (AT/RT). The researchers confirmed that their PBAE-delivered gene therapy made cancer cells more sensitive to ganciclovir, an antiviral drug.
Ganciclovir is commonly used in gene therapies that deliver an altered HSVtk gene to kill advanced melanoma cells and brain tumor cells. But such therapies typically rely on viral vectors.
“In vivo, PBAE-HSVtk treated groups had a greater median overall survival in mice implanted with AT/RT and MB,” the article’s authors wrote. “Our data provide proof of principle for using biodegradable PBAE nanoparticles as a safe and effective nanomedicine for treating pediatric CNS malignancies.”
The experiments showed that a combination of the suicide gene and ganciclovir delivered by intraperitoneal injection to mice killed more than 65% of the two types of pediatric brain tumor cells. The combination was deliberately transfected with the gene seven days after the nanoparticle therapy was used to deliver the genetic material.
Mice bearing an AT/RT-type tumor lived 20% longer after receiving the treatment—42 days, compared to 35 days for untreated mice. Those with a group 3 medulloblastoma-type tumor implanted in the brain lived 63% longer, surviving 31 days compared to 19 days for untreated mice.
The authors also indicated that PBAE nanoparticles formulated with plasmid DNA for intracellular gene delivery to pediatric brain cancer cells enabled >50% transfection in both cell lines tested. They also found that the nanoparticles preferentially targeted tumor cells over healthy cells.
The nanoparticles “can carry larger genes than what can be carried by a virus, and can carry combinations of genes,” emphasized Jordan Green, PhD, a co-corresponding author of the current study and an investigator at the Johns Hopkins Kimmel Cancer Center Bloomberg–Kimmel Institute for Cancer Immunotherapy. “It’s a platform that doesn’t have limitations on the cargo size being delivered, or limitations related to immunogenicity or toxicity. And, it is easier to manufacture than a virus.”
“It’s an exciting alternate way to be able to deliver gene therapy to a tumor in a selective fashion that targets only tumor cells,” added Eric Jackson, MD, another co-corresponding author and an associate professor of neurosurgery at the Johns Hopkins University School of Medicine. “Our idea now is to find other collaborators who may have a gene therapy that they think would work well to kill these tumors.”
Medulloblastoma and AT/RT are two of the most prevalent and deadly pediatric brain malignancies. Traditional treatments, including radiation, can harm healthy tissue as well as the tumor, and can produce long-lasting developmental side effects in growing children, making it critical to find new therapies, Jackson noted. Gene therapy that targets only cancer cells is a promising treatment avenue, but many gene therapy methods use a modified virus to deliver their therapeutic payloads of DNA, a method that may not be safe or suitable for pediatric use.
The mechanism that allows the PBAE nanoparticles to target tumor cells preferentially is still being investigated, but Green thinks “the chemical surface of the particle is likely interacting with proteins that are on the surface of certain types of cancer cells.”
“By making small chemical changes to the polymers that make up the nanoparticles, we can significantly change the cellular uptake into particular kinds of cancer cells, and the subsequent gene delivery to the cytosol, in a cell-specific way,” Green asserted.
“It should be noted that PBAEs are not restricted to delivering genes such as HSVtk,” the article’s authors added. “[These nanoparticles] have been validated as vectors for the delivery of other nucleic acids such as siRNA19 and miRNA24 as well.”
Jackson said that he hopes the nanoparticles can be used to deliver a variety of gene-based treatments—including therapies that alter the expression levels of genes, turn genes on and off altogether, or sensitize cells to other therapies—depending on the specifics of a patient’s tumor. “In some ways,” he remarked, “we’re still in the discovery phase of what genes to target” in medulloblastoma and AT/RT.