By Matthew Chin
In the treatment of cancer, chemotherapeutics are administered to kill cancer cells. Unfortunately, these drugs can also exert toxic effects on normal cells. To reduce these side effects, cancer cells may be selectively targeted by conjugating chemotherapeutics to a protein that specifically binds to a receptor (a cell-surface protein) that is present at a high concentration (overexpressed) on the surfaces of cancer cells relative to normal cells. Moreover, these toxic side effects can also be minimized by encapsulating the chemotherapeutics inside particles to reduce contact between the drugs and normal cells.
The laboratory of Daniel T. Kamei, assistant professor of bioengineering, has been conducting research to improve upon these general approaches. Scientifically, his research group is interested in the transport, or trafficking, of proteins and particles inside cells. With the knowledge gained from studying these trafficking pathways, his research group is striving to engineer new pathways that may improve the delivery and efficacy of chemotherapeutics.
In one study, published in the Journal of Controlled Release in 2007, Kamei and his students Bert Lao, Wen-Lin Tsai, Foad Mashayekhi and Edward Pham, along with his collaborator Anne Mason, focused on engineering the protein transferrin (Tf). Researchers have conjugated chemotherapeutics to Tf in the past, since its receptor is overexpressed on the surfaces of cancer cells. However, Tf only spends a short period of time inside the cell, and therefore has a limited timeframe in which to release the drug.
To increase this timeframe, Kamei and coworkers implemented a novel three-step approach. A mathematical model was first derived with a systems analysis of the intracellular trafficking processes. This model was used to identify a molecular design criterion by which to engineer a Tf variant. One variant that satisfied the design criterion was subsequently generated by replacing carbonate with oxalate as the salt ion that is associated with the protein. Lastly, this Tf variant was shown to associate with cancer cells for a greater period of time using radioactively labeled versions of the Tf variant and native Tf. To test if this increase in cellular association would correspond to an increase in cell killing, the drug diphtheria toxin was conjugated to both the Tf variant and native Tf. The Tf variant conjugate was shown to be two- to four-fold more potent than the native Tf conjugate. Dennis Yoon, a student in the Kamei research group, is currently extending this work by investigating Tf mutants generated with site-directed mutagenesis.
“We intend to send our best candidates to our collaborator at UC San Francisco to test our Tf-drug conjugates in mouse models for brain cancer,” Kamei said. “It is our hope that we can obtain very good preclinical data so that we may get closer to eventually using these conjugates in brain cancer patients.”
In another study, published in the journal Nature Materials in 2007, Kamei teamed up with Timothy J. Deming, chair and professor of bioengineering, to investigate vesicles (nano- to microscale encapsulants with an aqueous interior) for delivering drugs. Deming’s research group, which had pioneered a new method for synthesizing well-defined polypeptides (polymers of amino acids) on a large scale, recently demonstrated that polypeptides comprised of the amino acids lysine and leucine could self-assemble into vesicles with favorable properties. These vesicles were stable up to 80°C, could encapsulate polar molecules with negligible leakage, and could be prepared with controllable diameters ranging from 50 nm to 1 mm.
These vesicles, however, were unable to enter cells. Since previously reported studies have shown that short polymers of the amino acid arginine could transport cargo into cells, Deming, Kamei and two of their students, Eric Holowka and Victor Sun, replaced lysine with arginine in the polypeptides. The hypothesis was that this replacement would not disrupt the formation of the vesicles, since lysine and arginine have the same charge, while imparting to the vesicle the arginine property of entering cells.
The Deming and Kamei research groups were indeed able to “kill two birds with one stone”, as they demonstrated that these new polypeptides with arginine could easily form vesicles, and that they were able to transport hydrophilic cargo into both endothelial and epithelial cells. They are currently investigating the trafficking properties of these vesicles and their ability to deliver therapeutics to cells.
Reprinted from the Fall 2008 UCLA Engineer magazine.