Gold dust vs. the virus

A recent episode of South Park featured a story in which two of the main characters got infected with HIV and discovered that the cure for AIDS is an injection of about $200,000. As any viewer of the series would expect, the episode is crude and vulgar, and it wobbles to and fro over the line between humor and offensiveness. Yet the episode might also turn out to be oddly prescient, if research described in an upcoming JACS article bears further fruit. As it turns out, researchers from UNC, the University of Colorado, and NC State have had some success in inhibiting HIV activity using drug-coated nanoparticles, made out of gold.

The approach Bowman et al. use is based on the idea of multivalence, which is the operating principle of Velcro. A single hook-loop interaction between two pieces of fabric usually isn't enough to keep them fastened together. However, by having a large number of relatively weak interactions a strong connection can be made. Many biological systems make use of the same principle, using many weak interactions between repeating units to produce high overall affinity. The researchers set out to apply this idea to medicine, using many copies of a low-affinity drug attached to a nanoparticle.

The drug the authors used is based on a compound designated TAK-779, which is effective at preventing HIV virions from fusing with T cells, but also has some unpleasant properties for patients. The authors lopped off the part of the molecule that causes these problems, but doing so also removes most of its ability to fight HIV. So, they linked this new compound (called SDC-1721) to the gold particles at a ratio of about 12 molecules SDC-1721 per particle. In cultured cells, the nanoparticle-linked drug had an IC50 similar to TAK-779, even though SDC-1721 by itself was totally ineffective at preventing infection. Cutting the number of SDC-1721 molecules per particle to ~1 removed the inhibitory effect, proving that the multivalent approach was critical.

This is of course no demonstration of in vivo effectiveness, and there's no telling whether the nanoparticle will have side-effects that are better or worse than TAK-779. However, if this initial success is borne out by further trials this may be a promising angle on treatments. One of the advantages to this approach is that it has some ability to counter resistance built into it because of the multivalent binding. Even if a virus evolves a lower affinity for the drug, the weak binding of many ligands, and the increased effective local concentration of those ligands, may be enough to rescue inhibitory activity. Injecting yourself with money is no way to cure anything, but it is possible that in the future we will attack viruses by injecting patients with (drug-laden) gold.

1. Bowman, M., Ballard, T.E., Ackerson, C.J., Feldheim, D.L., Margolis, D.M., Melander, C. (2008). Inhibition of HIV Fusion with Multivalent Gold Nanoparticles. Journal of the American Chemical Society DOI: 10.1021/ja710321g

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An organizational problem

A clever little experiment appeared in this week's JACS preprints, from an Italian group at the University of Bologna. Their efforts concern fluorescent nanoparticles, specifically ones in which the fluorophores have been encapsulated in a silica substance. The advantage of this sort of construct is twofold: it concentrates a large number of fluorescent molecules in the same spot, and also it sequesters them from ions in the solution that might quench their fluorescence. The latter effect could be maximized if you synthesized a nanoparticle that had a fluorophore-doped center and a fluorophore-free shell. Rampazzo et al. have accomplished just this, and demonstrated that the system can be tuned to produce some interesting effects.

Using a pyrene derivative that was only weakly fluorescent in oxygenated water, Rampazzo et al. constructed a reaction to create a doped nanoparticle. In an initial case, they added the dye to about 0.1%, and then grew nanoparticles up to a size of about 90 nanometers. The dye was almost completely incorporated into the particles, and the quantum yield correspondingly increased in an almost linear fashion. Similarly, the results of light-scattering experiments indicated that the particles were growing with a rate equal to that of the increase in quantum yield. The interesting feature here is that although these rates are equal, the fluorescence plateaus significantly before the apparent particle radius reaches its maximum. This result indicates that the fluorescent dye is incorporated relatively early but that the particle continues to grow after this supply is exhausted, thus creating precisely the kind of shell we wanted (A, B, C below):


This success seems to be based entirely on a fortuitous choice of dye concentration. What if that concentration is changed? Rampazzo et al. changed the concentration by an order of magnitude and produced another interesting effect. It turns out that when there are a number of these dye molecules close to one another, they form excimers with an emission maximum at a wavelength of light ~100 nm longer than the monomer. When the dye constitutes 1% of the reaction mixture, there is an initial decrease in emission from the monomer that occurs with roughly the same rate as an increase in excimer emission and in particle radius. Later the monomer emissions recover, and all three processes plateau at approximately the same time. This suggests the formation of a heterogeneous particle as shown in D, E, F above.

This time the dye is not sequestered entirely from the solution, but local concentration at the core of the particle is so high that an unusual fluorescent property is observed. It's a conceptually simple little experiment, but it has an interesting result, and one that will have to be considered in future efforts to construct nanoparticles of this kind.

At the same time, if extreme local concentrations of some molecule have fortuitous or useful properties, this potential problem for nanoparticle construction might become an advantage. Consider if you have some molecule of interest that forms an excimer or exciplex with a known dye. Taking a sample containing traces of this molecule, one could use the encapsulation technique to construct a highly sensitive fluorescent detector.

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Automation for your nanodevice assembly

An interesting article appeared in JACS ASAP today from C.S. Hartley and J.S. Moore at UIUC on a clever way to direct the assembly of an asymmetric macrocycle. The approach relies on a simple idea to address a fundamental question about nanostructures, namely, how to make sure they assemble themselves as we would like. Given that protein design still isn't quite far enough along to create any product we desire, we have to rely on more conventional chemistry. The basics of the approach should be obvious from the image on the right (shamelessly stolen from the paper). Note the reactive groups on the lower side of these compounds; even without knowing the relevant chemistry it should be clear what mechanism for directed assembly is implied. Given that the smallest geometrically stable assembly consists of three units, and that entropy favors the creation of small assemblies and enthalpy dislikes dangling functional groups, it should be obvious that dumping all these into a reactive pot should mostly produce units that have a composition of 1-1-1 or 1-2-3.

This is in fact what the authors observe when they perform the experiment, though they find (perhaps surprisingly) that in mixtures of all three components the 1-2-3 macrocycle predominates. Glancing at the product it's clear that the macrocycles themselves will assemble into stacked arrays given the right conditions. If covalent chemistry is used to control this assembly, one has a rather simple method that could be used to construct fairly complex structures. Also, the yields from these reactions were encouragingly rich in the desired molecules, suggesting that only a little further optimization is necessary to produce effective scale-up.

Nothing in the paper is particularly earth-shattering, but then again, the cleverest answers often seem blazingly simple in hindsight. Hartley and Brown have come up with just such an answer here, in a commendably clear and readable paper.

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