Fragments in the clinic: ASTX660

Three years ago we highlighted a paper from Astex describing the discovery of an extraordinarily weak fragment and its advancement to a dual inhibitor of the anti-cancer targets cIAP1 and XIAP. We ended that post by writing, “whether or not this leads to a drug, it does look like another candidate for a useful chemical probe.” As three papers now make clear, the program has indeed led to an experimental drug.

The first paper, by Emiliano Tamanini and colleagues, was published in J. Med. Chem. last year and describes the optimization of Compound 21, one of the best compounds from the 2015 report. The researchers noticed that compounds in the series were chemically unstable: the amide bond was subject to hydrolysis. Fortunately this was readily fixed by repositioning the pyridyl nitrogen.

Optimization of the benzyl group was complicated by the fact that it binds in the P4 pocket, which differs between cIAP1 and XIAP. In the end, adding a fluorine gave a slight potency improvement against both proteins. The bulk of the work was focused on elaborating the methoxy group of compound 21. Detailed modeling experiments were used to choose moieties that would fold back on the core of the molecules in solution, thus pre-orienting them for binding as well as shielding a critical hydrogen bond. These efforts led to AT-IAP, with low nanomolar cell activity against both proteins as well as activity in mouse xenograft models.

Although AT-IAP is orally bioavailable in mice and rats, the bioavailability is much lower in monkeys, and it also inhibits the hERG channel, which can lead to cardiac toxicity. Fixing these problems is the focus of a paper published last month in J. Med. Chem. by Christopher Johnson and colleagues.

Metabolite identification studies revealed that the morpholine ring of AT-IAP is cleaved by CYP enzymes, so this was one area the researchers tried to modify. Although somewhat successful, hERG was still a problem, and this correlated with lipophilicity. Knowing how the molecules bound allowed the researchers to introduce small hydrophilic substituents without disrupting critical interactions, ultimately leading to ASTX660. Not only did the added hydroxymethyl group decrease hERG binding, it also improved bioavailability – a reminder that decreasing lipophilicity can have useful effects even on distant parts of the molecule.

More characterization of ASTX660 is provided in a paper by George Ward and colleagues in Mol. Canc. Ther. This reports the crystal structure of the molecule bound to XIAP. As Johnson et al. note, the polar interactions made by the molecule are conserved from the original fragment – the additional protein interactions that improve affinity by more than a million-fold are all hydrophobic.

Ward et al. also provide more detailed mechanistic cell biology, pharmacokinetics, and xenograft data. In particular, ASTX660 is a much more potent antagonist of XIAP activity in vivo than other clinical-stage compounds, which will hopefully translate to better efficacy. The compound is currently in a phase 1-2 study.

Collectively these papers provide a valuable lesson in structure- and property-based drug design and illustrate just how much effort can be required to go from fragment to clinical compound. I’ll end this post with an echo of the original: whether or not this leads to an approved drug, it is a lovely story of perseverance combined with creative chemistry and biology. Practical Fragments wishes everyone involved the best of luck.