Coruscating currants in a cake
Malignant tumours induce the formation of blood vessels that are more permeable than those found in healthy tissues, and allow larger molecules to exit from the bloodstream. In order to treat such tumours effectively, researchers have developed carrier systems that allow the transport of active agents into the tumour itself. This approach makes it possible to attack the tumour directly and consequently reduce the incidence of harmful side-effects. A major factor in the development of efficient delivery systems is the time course of dissemination of the drug in the body. Above all, one wants to avoid rapid clearance of the agent by the liver or speedy excretion by the kidney. Ideally, the active agent should circulate in the bloodstream long enough for it to reach the tumour and should then become concentrated there.
A team made up of pharmacists, biologists and physicists led by Dr. Manfred Ogris of the Department of Pharmacy at LMU Munich, has recently found a way to monitor the distribution of therapeutic agents in the the body with high temporal resolution. Dr. Andrey Rogach and his collaborators at the Physics Department of LMU and the Munich Center for NanoScience (CeNS) have synthesized so-called quantum dots made of the semiconductor cadmium telluride. Quantum dots are tiny crystals with dimensions of between 2 and 8 nanometers (a nanometer is a millionth of a millimeter). When these crystals are exposed to light, they fluoresce in different colours, depending on their size. “In contrast to the quantum dots that have been produced so far, these crystals form in water and are therefore very small”, says Rogach. In addition, they fluoresce not only in the visible, but also in the near-infrared region of the spectrum. This makes them especially suitable for use in living tissue.”
The next step was to couple the quantum dots to gene vectors consisting of a particular fragment of DNA coated with a positively charged macromolecule. “After uptake into the tumour, the DNA fragment induces the production of a specific protein that selectively attacks the tumour cells”, explains Ogris. “The macromolecule, on the other hand, serves to bind the negatively charged quantum dots tightly and to package the DNA. The nanocrystals are incorporated into the complex rather like the currants in a cake”. The scientists call the resulting structures “quantoplexes”: Irradiation of these complexes with near-infrared light induces fluorescence, which can be used to follow the dissemination of the particles in the body in great detail.
Ogris und his colleagues first examined the distribution of free quantum dots, i.e. without attached DNA. They injected the crystals into anaesthesized mice and recorded the fluorescent signal from the whole body every 15 seconds. This enabled them to determine the typical pattern of distribution of the particles in the mouse. Shortly after injection, the particles were found in the bloodstream, the lymphatic system and the liver. After a few hours, most of the particles had been cleared via the liver. When the quantum dots were added to the gene vectors, which are themselves between 100 and 300 nanometers in diameter, the picture was very different. Most of the fluorescing particles were now found in the lung, with smaller amounts localizing to the liver.
“Of course, this distribution is not always what one wants”, notes Ogris. “So we performed further experiments in which we coated the surface of the DNA with a different macromolecule, polyethylene glycol or PEG.” Indeed, this modification reduced the interaction of the particles with blood components, so that significantly more molecules now reached the tumour. And although the vectors were transported from the bloodstream to the liver within minutes of being injected, a clear signal could still be observed in the tumours up to 15 minutes later – showing that the particles that had made it to the tumour could be retained there for some time.
“In future, fluorescent nanocrystals could provide a very useful tool for the study and optimization of carrier systems for the treatment of tumours”, remarks Ogris. At present such studies are only possible in animal models, because the cadmium telluride which we use is toxic in the long term, and very small amounts are retained in tissues. However, a central issue in the development of new drugs is that we must first understand their distribution and modes of action in the whole body”, emphasizes Ogris. The next step is to use quantum dots to investigate the behaviour of gene vectors that can recognize and dock onto tumour-specific receptors and thus have the potential to mount a truly targeted attack on the malignant growth. (ca)
“Drug Nanocarriers Labeled With Near-infraredemitting Quantum Dots (Quantoplexes): Imaging Fast Dynamics of Distribution in Living Animals”;
Arkadi Zintchenko, Andrei S. Susha, Massimo Concia, Jochen Feldmann, Ernst Wagner, Andrey L. Rogach, Manfred Ogris;
Molecular Therapy Online Edition, 25 August 2009;
Dr. Manfred Ogris
Department of Pharmacy – Center of Drug Research
Pharmaceutical Biology and Biotechnology
Phone: +49 (0) 89 / 2180-77842