What's the best carrier to take some sort of therapeutic agent into the bloodstream? That's often a tricky question to work out in animal models or in the clinic - there are a lot of possibilities. But what about using red blood cells themselves?
That idea has been in the works for a few years now, but there's a recent paper in PNAS reporting on more progress (here's a press release). Many drug discovery scientists will have encountered the occasional compound that partitions into erythrocytes all by itself (those are usually spotted by their oddly long half-lives after in vivo dosing, mimicking the effect of plasma protein binding). One of the early ways that people have attempted to try this deliberately was forcing a compound into the cells, but this tends to damage them and make them quite a bit less useful. A potentially more controllable method would be to modify the surfaces of the RBCs themselves to serve as drug carriers, but that's quite a bit more complex, too. Antibodies have been tried for this, but with mixed success.
That's what this latest paper addresses. The authors (the Lodish and Ploegh groups at Whitehead/MIT) introduce modified surface proteins (such as glycophorin A) that are substrates for Ploegh's sortase technology (two recent overview papers), which allows for a wide variety of labeling.
Experiments using modified fetal cells in irradiated mice gave animals that had up to 50% of their RBCs modified in this way. Sortase modification of these was about 85% effective, so plenty of label can be introduced. The labeling process doesn't appear to affect the viability of the cells very much as compared to wild-type - the cells were shown to circulate for weeks, which certainly breaks the records held by the other modified-RBC methods.
The team attached either biotin tags and specific antibodies to both mouse and human RBCs, which would appear to clear the way for a variety of very interesting experiments. (They also showed that simultaneous C- and N-terminal labeling is feasible, to put on two different tags at once). Here's the "coming attractions" section of the paper:
he approach presented here has many other possible applications; the wide variety of possible payloads, ranging from proteins and peptides to synthetic compounds and fluorescent probes, may serve as a guide. We have conjugated a single-domain antibody to the RBC surface with full retention of binding specificity, thus enabling the modified RBCs to be targeted to a specific cell type. We envision that sortase-engineered cells could be combined with established protocols of small-molecule encapsulation. In this scenario, engineered RBCs loaded with a therapeutic agent in the cytosol and modified on the surface with a cell type-specific recognition module could be used to deliver payloads to a precise tissue or location in the body. We also have demonstrated the attachment of two different functional probes to the surface of RBCs, exploiting the subtly different recognition specificities of two distinct sortases. Therefore it should be possible to attach both a therapeutic moiety and a targeting module to the RBC surface and thus direct the engineered RBCs to tumors or other diseased cells. Conjugation of an imaging probe (i.e., a radioisotope), together with such a targeting moiety also could be used for diagnostic purposes.
This will be worth keeping an eye on, for sure, both as a new delivery method for small (and not-so-small) molecules, fof biologics, and for its application to all the immunological work going on now in oncology. This should keep everyone involved busy for some time to come!