Last year I mentioned an interesting paper that managed to do single-cell pharmacokinetics on olaparib, a poly(ADP) ribose polymerase 1 (PARP1) inhibitor. A fluorescently-tagged version of the drug could be spotted moving into cells and even accumulating in the nucleus. The usual warnings apply: adding a fluorescent tag can disturb the various molecular properties that you're trying to study in the first place. But the paper did a good set of control experiments to try to get around that problem, and this is still the only way known (for now) to get such data.
The authors are back with a follow-up paper that provides even more detail. They're using fluorescence polarization/fluorescence anisotropy microscopy. That can be a tricky technique, but done right, it provides a lot of information. The idea (as the assay-development people in the audience well know) is that when fluorescent molecules are excited by polarized light, their emission is affected by how fast they're rotating. If the rotation is slowed down to below the fluorescence lifetime of the molecules (as happens when they're bound to a protein), then you see more polarization in the emitted light, but if the molecules are tumbling around freely, that's mostly lost. There are numerous complications - you need to standardize each new system according to how much things change in increasingly viscous solutions, the fluorophores can't get too close together, you have to be careful with the field of view in your imaging system to avoid artifacts - but that's the short form.
In this case, they're using near-IR light to do the excitation, because those wavelengths are well known to penetrate living cells well. Their system also needs two photons to excite each molecule, which improves signal-to-noise and the two-photon dye is a BODIPY compound. These things have been used in fluorescence studies with wild abandon for the past few years - at one point, I was beginning to think that the acronym was a requirement to get a paper published in Chem. Comm. They have a lot of qualities (cell penetration, fluorescence lifetime, etc.) that make them excellent candidates for this kind of work.
This is the same olaparib/BODIPY hybrid used in the paper last year, and you see the results. The green fluorescence is nonspecific binding, while the red is localized to the nuclei, and doesn't wash out. If you soak the cells with unlabeled olaparib beforehand, though, you don't see this effect at all, which also argues for the PARP1-bound interpretation of these results. This paper takes things even further, though - after validating this in cultured cells, they moved on to live mice, using an implanted window chamber over a xenograft.
And they saw the same pattern: quick cellular uptake of the labeled drug on infusion into the mice, followed by rapid binding to nuclear PARP1. The intracellular fluorescence then cleared out over a half-hour period, but the nuclear-bound compound remained, and could be observed with good signal/noise. This is the first time I've seen an experiment like this. Although it's admittedly a special case (which takes advantage of a well-behaved fluorescently labeled drug conjugate, to name one big hurdle), it's a well-realized proof of concept. Anything that increases the chances of understanding what's going on with small molecules in real living systems is worth paying attention to. It's interesting to note, by the way, that the olaparib/PARP1 system was also studied in that recent whole-cell thermal shift assay technique, which does not need modified compounds. Bring on the comparisons! These two techniques can be used to validate each other, and we'll all be better off.