May 16, 2006, Eindhoven, The Netherlands--Starting with a magnetic nanoparticle that also emits bright, fluorescent light, and adding a targeting molecule that recognizes a molecule found only on the inside of a cell's membrane, researchers have created a scale reporter of cell death that could provide real-time data on therapeutic efficacy. A report on this work appears in the journal Bioconjugate Chemistry.
Geralda van Tilborg and colleagues at Eindhoven University of Technology set out to create a nanoparticle that would detect apoptosis, the type of cell death triggered by anticancer drugs. Their starting point was a protein known as annexin A5, a molecule that binds tightly to a cell membrane component called phosphatidyl serine. Phosphatidyl serine is normally found only on the inside of the cell membrane, but one of the early events that occurs during apoptosis is that the cell membrane folds inside out, making phosphatidyl serine available for binding to annexin A5.
The investigators prepared two different "bimodal" nanoparticles, each of which was both magnetic and fluorescent, allowing them to be detected by both magnetic resonance imaging and optical imaging. To each of these nanoparticles, the researchers linked multiple copies of annexin A5 and then added a layer of polyethylene glycol to improve the particles' bioavailability and circulation times. One of the nanoparticles had an average diameter of approximately 100 nm, while the other was approximately 10 nm in diameter.
Tests with cells grown in culture and with tissue samples treated with apoptosis-triggering drugs showed that both nanoparticles were capable of detecting an increase in apoptosis from less than 1% of cells to greater than 30% in any given sample. At any given time in the body, less than 1% of cells are undergoing apoptosis, while the normal rate of apoptosis in untreated cancer cells is around 4%. Both nanoparticles, then, have the potential to spot changes in apoptosis triggered by anticancer drugs, which the investigators plan to assess in future experiments.
This work is detailed in a paper titled, "Annexin A5-functionalized bimodal lipid-based contrast agents for the detection of apoptosis."