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HomeCommentaryBlogsIn the PipelineClick Chemotherapy
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CLICK CHEMOTHERAPY

 * 28 Oct 2020
 * By Derek Lowe
 * 3 min read
 * Comments
   

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So here's an ambitious idea that's about to get a hearing in human clinical
trials. A startup called Shasqi is using click chemistry as a drug delivery
method, and they have a new manuscript on the idea here at ChemRxiv.





The idea is this: you produce a modified version of a hyaluronate biopolymer,
decorated with aryl-tetrazine functional groups. The tetrazine/cyclooctene
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 cyclooctene group - and in this system, that
modification is through an ester/carbonate/carbamate linkage to a cyclooctenol
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.





Update: there's a company in the Netherlands (Tagworks) that is working on a
click delivery idea of their own, using antibody-drug conjugates. More on this
as it moves into the clinic itself!





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 are prepared to give it a try from several directions. 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!



ABOUT THE AUTHOR

DEREK LOWE

emailTwitter

Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in
organic chemistry from Duke before spending time in Germany on a Humboldt
Fellowship on his post-doc. He’s worked for several major pharmaceutical
companies since 1989 on drug discovery projects against schizophrenia,
Alzheimer’s, diabetes, osteoporosis and other diseases.



--------------------------------------------------------------------------------


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IN THE PIPELINE

Derek Lowe’s commentary on drug discovery and the pharma industry. An
editorially independent blog, all content is Derek’s own, and he does not in any
way speak for his employer.

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