Issue Date: March 28, 2011
Porphysomes Do It All
Liposome-like nanostructures that incorporate covalently bound porphyrin moieties within their bilayers can be used to image, deliver drugs to, and heat and destroy cancer cells, according to a new report (Nat. Mater., DOI: 10.1038/nmat2986). The nanovesicles, called porphysomes, could aid personalized medicine approaches to cancer treatment.
The “one for all” imaging and treatment package that these porphysomes offer, says Gang Zheng, a senior scientist at Canada’s Ontario Cancer Institute and the study’s leader, is a “critical paradigm shift” from previous nanomaterials: All these capabilities are intrinsic to the new nanovesicles, rather than having to be tacked on.
Unlike inorganic nanoparticles such as quantum dots, which are also used as bioimaging agents but might be toxic, porphysomes are enzymatically biodegradable and nontoxic, Zheng and his team show. The 100-nm-diameter nanovesicles self-assemble in buffer from lipid-porphyrin conjugates generated via an acylation reaction between the lipid lysophosphatidylcholine and the porphyrin pyropheophorbide, a chlorophyll-based molecule.
Porphysomes also have advantages over organic drug delivery nanomaterials such as liposomes, the researchers say, because the porphyrin-laden nanovesicles absorb sufficient near-infrared light to enable their use for in vivo through-tissue light activation and imaging.
To demonstrate application of the porphysomes as photothermal cancer therapy agents, the researchers injected the nanovesicles into mice with tumors. During a one-minute exposure to a 750-mW laser at 660 nm, the cancerous cells heated to 60 °C, compared with 40 °C for control cells. The porphyrin moieties converted light energy to local thermal energy to achieve this effect. Two weeks later, the tumors in the porphysome-administered mice completely disappeared, but those in the control group continued to grow rapidly.
Some of the thermal energy generated by the nanomaterials’ porphyrins creates acoustic waves, and the research team showed that porphysomes are also promising photoacoustic imaging agents. Fifteen minutes after their injection, the nanovesicles made lymph nodes and vessels in the tumor-bearing mice visible to an ultrasound transducer.
“This could have a clinical application in identification of sentinel lymph nodes,” says Michael S. Patterson, a photodynamic therapy researcher at McMaster University, in Ontario, referring to “the node that first drains a tumor and is most likely to harbor small numbers of metastatic cancer cells.” Porphysomes thus could be useful, he adds, in breast cancer surgery as a way to identify cancerous nodes without dissecting entire lymph node chains suspected of being cancerous.
Zheng believes that the porphysomes’ multimode photothermal therapy and imaging capabilities, combined with their ability to carry drugs, will make them an indispensible tool in theranostics, a field that integrates therapeutics and diagnostics for personalized medicine.
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