Volume 81, Number 34
CENEAR 81 34 p. 38
ISSN 0009-2347
If the cell you're targeting with a drug doesn't have a receptor for that drug, that's okay. Just add one yourself.
That's what Blake R. Peterson, an assistant chemistry professor at Pennsylvania State University, is doing with "synthetic receptor targeting" [Bioconjugate Chem.,14, 67 (2003)]. The team working on the synthetic receptors includes graduate students Stephen L. Hussey and Scott E. Martin.
"We've discovered over the past two years that it's possible to make synthetic molecules that stably associate with cellular plasma membranes," Peterson says. The compounds that he uses are derived from 3b-cholesterylamine.
"By linking protein-binding motifs to 3b-cholesterylamines, we're able to display from these nonnatural cell surface receptors protein-binding groups that can engage protein ligands," he says. When proteins bind to the receptors on the cell surface, the binding triggers endocytosis. The ligands are typically antibodies, but molecules such as streptavidin can also be used [J. Am. Chem. Soc.,124, 6265 (2002)].
"We think that we're organizing a piece of the cellular plasma membrane when the ligand binds the receptor. That organization is sensed by the cell, and the cell in turn internalizes the ligand," Peterson says. "We see enhancements of as much as 300-fold in terms of delivery. We can really deliver a tremendous amount of material into cells in this way."
Peterson believes that certain cell types are going to be more susceptible to delivery by this method than others. For example, cells with higher rates of endocytosis may be more susceptible. In addition, tumor cells need large amounts of cholesterol to grow, so they may be more likely to take up the cholesterol-mimicking receptors.
Peterson believes that the mechanism of uptake involves cross-linking the receptors. Therefore, molecules being delivered will probably have to form bivalent interactions with the receptors.
Peterson is working on the delivery of the anticancer drug methotrexate, which inhibits the enzyme dihydrofolate reductase. Methotrexate is not cell permeable. Instead, it's taken up by folate receptors on the cell surface. One way that cancer cells become resistant to methotrexate is by stopping production of the folate receptors.
Peterson is investigating whether these missing cell surface receptors can be replaced with synthetic ones. Methotrexate would be delivered by linking it to a protein that targeted the receptor. If Peterson's group can design a receptor that will dimerize when it interacts with methotrexate, the protein would be unnecessary.
In his original systems, both the receptor and the protein were internalized. Now, Peterson is working on a system in which the receptors are recycled.
"We have a new set of compounds that undergo recycling. They're even better mimics of naturally occurring receptors in the sense that the receptor goes into the cell via endocytosis, but it releases the protein and can return to the cell surface by plasma membrane recycling."
CYCLING In synthetic receptor targeting, the plasma membranes of living cells are loaded with synthetic compounds that function as nonnatural receptors. In this illustration, the light blue portion is the cell surface, which has been loaded with receptors that have protein-binding motifs. The darker blue portion is the intracellular region, and the boundary between them is the plasma membrane. A macromolecular ligand, in this case antifluorescein IgG, binds to the receptor, triggering the formation of lipid rafts and the uptake of the receptor-ligand complex via endocytosis. Nonnatural receptors capable of dissociating from the ligand are recycled to the cell surface.
COURTESY OF BLAKE PETERSON
