Nanoparticulate siRNA systems and therapeutic complexes directed by targeting moieties are under intense studyLiposomes are becoming increasingly valuable tools in the development of cancer therapeutics. These lipid bilayer vesicles can be loaded with a variety of therapeutic payloads, ferrying their cargoes to tumor sites while circulating in the bloodstream.
The first cancer drugs to use liposome technologies were established chemotherapeutic agents, packaged in long-circulating lipid-based delivery systems to increase their efficacy and reduce side effects. More recently, liposomes containing short interfering RNA, designed to specifically inhibit the production of disease-causing proteins, are showing promise in early clinical trials. Some of these systems are equipped with targeting components that direct them specifically to tumor cells.
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FIGURE 1. Design of the liposome nanoparticle, with gene payload inside a liposome and the targeting moiety on the outside.
Dr. Cullis, who has been working with liposomes for drug delivery for about 30 years, said early research included devising the methodology for producing liposomes in the desired dimensions, approximately 100 nm or less, and loading them efficiently with drug.
Delivery of these chemotherapy-loaded liposomes depended on the phenomenon of enhanced penetration and retention, or EPR. In areas in and around tumors, the vasculature is leaky, and liposomes accumulate preferentially in those places, but not in healthy tissue vessels, Dr. Cullis said, thereby concentrating drug accumulation at the site of disease.
“With anticancer drugs, for example, you can get up to 50 times more drug at a tumor site than you would get by injecting the same amount of free, non-encapsulated drug, by putting it inside these nanoparticles,” he said.
The first liposome-based cancer drug to enter the market was Doxil (Johnson & Johnson), a liposomal formulation of doxorubicin. Doxil is approved in Europe and the United States for treatment of ovarian cancer.
Dr. Cullis and coworkers developed an alternative formulation of doxorubicin (Myocet, Enzon Pharmaceuticals) that has been approved in Canada and Europe for treatment of metastatic breast cancer. They also developed a liposomal version of vincristine sulfate (Marquibo, Hana Biosciences) that has been granted orphan drug status by the FDA and is being evaluated for the treatment of adult lymphoblastic leukemia. In phase 1 and 2 clinical trials, patients tolerated doses of liposomal vincristine that were approximately 100% greater than conventional vincristine doses.
“Reduced toxicity is probably the major benefit from those systems, allowing us to achieve higher doses and see enhanced efficacy,” Dr. Cullis said.
CASE STUDY: Tumor-Specific Delivery to Metastatic LesionsResearchers at Georgetown University Medical Center, along with collaborators, recently reported tumor-specific delivery of a systemically administered therapy to metastatic lesions for the first time in patients.1
A liposome complex containing a plasmid that encodes for the tumor suppressor gene p53, decorated with a targeting moiety, was administered to 11 patients with solid tumors who had exhausted all standard therapies. In this phase 1 dose-escalation trial, primarily grade 1 and 2 easily managed adverse events were observed. A single grade 3 (fatigue associated with massive tumor necrosis) and no grade 4 adverse events were seen. The patients were treated by John Nemunaitis, MD, and Neil Senzer, MD, at the Mary Crowley Cancer Research Centers in Dallas.
Although this was a safety trial, there were signs of efficacy, the researchers reported. Eight of the 11 patients showed stable disease, and two showed more than disease stability: One patient’s adenoid cystic carcinoma was reclassified from inoperable to operable, and in another patient with leiomyosarcoma with metastases in liver and lung, computed tomography showed necrosis in all metastases after the treatment.
This liposome therapeutic is designed to work in combination with standard cancer therapies, said Kathleen F. Pirollo, PhD, a research professor at Georgetown University Medical Center. “The idea is to make standard therapy, either radiation or chemotherapy, more effective in these cancers,” she said. The p53 tumor suppressor gene activates the cell death pathway in the cancer cells, “so when we expose the tumors to conventional chemo and radiation, they can now respond and die.”
Esther H. Chang, PhD, a professor at Georgetown, noted that this is a truly targeted therapy.
“It was important for us to see whether in patients the p53 gene actually ends up in the tumor only and not in normal tissue,” she said. “We extracted DNA from the metastatic tumors, looking for the specific payload, the therapeutic gene. We found that even in patients treated with the lowest dose, we were able to see the specific unique presence of the gene we put in. There was also a dose response, with strong presence of the transgene at the highest dose administered. Also, when we look at the normal skin, it’s clean. We’re very proud of that—truly tumor-targeted delivery.”—TD
- Senzer N, Nemunaitis J, Nemunaitis D, et al. Results of a Phase I trial of SGT-53: a systemically administered, tumor-targeting immunoliposome nanocomplex incorporating a plasmid encoding wtp53. Paper presented at: American Society of Gene and Cell Therapy Annual Meeting; May 15-19, 2012; Philadelphia.
siRNA DeliveryIn their ongoing work, begun in the previous decade, Dr. Cullis and coworkers are principally focused on the use of liposomal nanoparticulate systems to deliver siRNA for therapeutic applications in the liver.
“siRNA could be a major therapeutic, as long as we can get it to the inside of target cells,” he said. “The issue is to protect it from degradation in circulation and get the material to the target tissue and then inside the target cells. These delivery systems have to be more sophisticated, with more components, than those that use the EPR effect.”
The Vancouver researchers take advantage of a naturally occurring phenomenon in hepatocytes (liver cells). Their liposomes accumulate the serum protein apolipoprotein-E, which is then taken up by so-called scavenging receptors on hepatocytes. Once the particles reach the endosome of the hepatocytes, they must then be taken into the cytosol to do their job, and this is accomplished by another component of the system, cationic lipids.
“Much of our work has been focused on getting very potent cationic lipids for delivery from the endosome to the inside of the cell. These systems are now very viable therapeutics with low toxicity,” Dr. Cullis said.
Three such therapeutics, in development by Alnylam Pharmaceuticals of Cambridge, Mass., are being investigated in humans, including one directed at liver cancer. A phase 1 clinical trial of that compound, ALN-VSP, showed that the drug was well tolerated and demonstrated evidence of antitumor activity in patients with advanced malignancies. Dr. Cullis said Alnylam is looking for partners to move the compound forward.
Researchers at Georgetown University have taken a different tack for targeting therapeutics to cancer cells. They have designed therapeutic complexes composed of cationic liposomes that can encapsulate multiple types of payloads and can be cancer directed by attaching a targeting moiety to the outside of the liposomeResearchers at Georgetown University in Washington, D.C., have taken a different tack for targeting therapeutics to cancer cells. They have designed therapeutic complexes composed of cationic liposomes that can encapsulate multiple types of payloads and can be cancer directed by attaching a targeting moiety to the outside of the liposome. The prototype of these targeted complexes has successfully completed a phase 1 clinical trial (see case study) to deliver the therapy to a range of solid tumors and is now in a phase 1b trial.
The therapeutic complex is a platform technology, said Kathleen F. Pirollo, PhD, a research professor in experimental therapeutics in the department of oncology of the Lombardi Comprehensive Cancer Center at Georgetown University Medical Center.
“We can mix and match,” she said. “We can switch out the targeting moiety, switch out the payload. It’s applicable for a number of molecular medicines and contrast agents. We have shown in preclinical studies that we can successfully deliver a number of payloads, including plasmid DNA, siRNAs, miRNAs, antisense oligonucleotides; we can even encapsulate chemotherapeutic agents to increase their efficacy and reduce their side effects because of the targeting nature of this platform technology.”
The targeting moiety that the group has explored most extensively is a single-chain antibody fragment that is designed to target cancer cells by binding to the transferrin receptor, she said.
“The transferrin receptor is a good target because most if not all cancer cells have elevated levels of expression of this receptor. Cancer cells grow so rapidly they need to bring iron in, so we take advantage of that to bind to these receptors and transport the complex into the cell,” Dr. Pirollo said.
In addition to the gene therapy work, the Georgetown researchers have also investigated the use of the liposome technology for delivery of chemotherapy agents in animal models, said Esther H. Chang, PhD, a professor of oncology and otolaryngology at Georgetown University Medical Center.
“We found that after encapsulation, the safety profiles look better than the original form, and we can edit or change the capacity or function of the chemotherapy drugs,” she said. “It turns out that when you encapsulate a conventional chemotherapeutic agent, it broadens its use and significantly increases its efficacy.”
In collaboration with SynerGene Therapeutics, of Potomac, Md., Dr. Chang and colleagues are developing a number of liposomal therapeutic entities that are in late translational stages.