The idea is this: you produce a modified version of a hyaluronate biopolymer, decorated with aryl-tetrazine functional groups. The tetrazine/cyclooctyne reaction has been exploited many times (here’s an example) since the group was introduced by Joseph Fox’s group at Delaware some years ago. Under the banner of click chemistry, as introduced by Barry Sharpless, the idea is that the two components have little or no reactivity except for each other, but will react if they merely come into proximity under the right conditions.
So you take the tetrazine-laced biopolymer and inject it into a tumor site, where it is expected to largely sit there. You then inject a chemotherapy agent that has been modified to contain a cyclooctyne group – and in this system, that modification is through an ester/carbonate/carbamate linkage to a cyclooctynol group. That does several things for you, ideally: if optimized, it makes the chemotherapy agent less effective until that ester is cleaved, so larger doses of it can be given with a better safety profile. And when it encounters the tetrazine-containing polymer, it does the cycloaddition click reaction which then cleaves the ester and releases the free drug at the site of the tumor.
It’s a pretty slick idea. Does it work? The team published a proof-of-concept in a mouse model in 2014 (open access link) using a radioligand, and it really did seem to localize the agent at the site of injection when it was delivered a few hours after the hyaluronate polymer. In the latest paper, cyclooctynol versions of the well-known chemotherapy drugs doxorubicin, paclitaxel, etoposide, and gemcitabine. This was not trivial chemistry, as a look through the paper will show, but they were able to make example of all four modifications.
Then came the evaluations. The modified versions of doxorubicin and etoposide were indeed less cytotoxic than the parent compounds, but not the other two, unfortunately. Helpfully, the modified doxorubicin also had better solubility than the parent compound. It also had better plasma stability than the modified etoposide, so that made it the obvious choice to proceed with. And it does indeed react with the tetrazine hyaluronate polymer in vitro, releasing doxorubicin itself.
What about in vivo, though? In a mouse model, the modified dox compound could be dosed up to 10x the maximum tolerated dose of doxorubicin itself, so that part checked out. And the pharmacokinetics checked out as well: when mice were injected with the modified doxorubicin and then given an injection of the biopolymer, blood levels of the former compound dropped quickly (by over 2000-fold), with an increase in free doxorubicin at the same time. This phenomenon repeated over multiple daily injections of the modified doxorubicin, albeit with slight decreases in capture over time. These are presumably due to gradually diminished tetrazine sites on the polymer and/or its degradation in vivo, but the effects were still significant all the way through. Exposure to plain dox was significantly lower in peripheral tissue compared to a normal dosing protocol, as were its adverse effects in sensitive tissues such as cardiac muscle.
And here is another preprint in which this system is extended to mouse xenograft models, with effects both on the injected tumor and distal ones. So the idea, up to this point, appears worth trying out. The company has started dosing patients in a Phase I trial in people with various solid tumors who are ineligible for standard-of-care treatment. That’s a tough population to show good effects in, but that will make it all the more interesting if they can deliver. There are several places where things could go wrong, of course. The modified polymer and/or the modified doxorubicin could turn out not to be well tolerated in patients. The pharmacokinetics of the click-capture mechanism could be looser or less dramatic than they were in the mouse model (and this could vary from patient to patient and across different sorts of malignancies). And finally, there’s going to be only so much that doxorubicin itself can do for some of these cases. It’s not an infallible tumor destroyer – we don’t have too many of those – and the current trial will (at best) get the most out of doxorubicin treatment that it has to give.
But if the idea is sound, there are plenty of other applications for it, with more labor on the med-chem synthetic front to produce more modified molecules. Different compounds, cocktail treatment regimes, combinations of click-labeled drugs, even – there will be a lot to investigate, if it looks worthwhile. Let’s see if it does!
