Fragments in Australia

Last year we highlighted the first FBDD conference held in Australia. That meeting has now led to a dozen papers in the December issue of Aus. J. Chem. Many of the papers use the same fragment libraries, so this is a good opportunity to survey a variety of outcomes from different techniques and targets.

The collection of papers (essentially a symposium in print) starts with a clear, concise overview of fragment-based lead discovery by Ray Norton of Monash University. Ray also outlines the rest of the articles in the issue.

A well-designed fragment library is key to getting good hits, and the next two papers address this issue. Jamie Simpson, Martin Scanlon, and colleagues at Monash University discuss the design and construction of a library built for NMR screening. Compounds were selected using slightly relaxed rule-of-three criteria, and special care was taken to ensure that at least 10 analogs of each were commercially available to facilitate follow-up studies. Remarkably, of 1592 compounds purchased, only 1192 passed quality control and were soluble at 1 mM in phosphate buffer. The properties of the final library are compared with nearly two dozen other libraries reported in the literature; this is the most extensive summary I’ve seen on published fragment libraries. The paper also analyzes the results of 14 screens on various targets using saturation transfer difference (STD) NMR. As the researchers note, this technique is prone to false positives, and indeed the average hit rate of 22.5% is high, with only about 50% confirming in secondary assays. There is also a nice analysis of what features are common to hits, along with a list of the 24 compounds that hit in more than 90% of screens.

The other paper on library design, by Tom Peat, Jack Ryan and others (including Pete Kenny), discusses library design at CSIRO. The researchers started with 500 fragments commercially available from Maybridge and supplemented these with roughly the same number of fragments from a collection of small heterocycles that had been synthesized internally; additional “three-dimensional” fragments are also being constructed. At CSIRO the primary screening method appears to be surface plasmon resonance (SPR), in particular the ProteOn instrument that allows simultaneous analysis of six fragments against six targets. Eight of about ten targets have yielded confirmed hits. The researchers show examples of specific (good), nonspecific (probably bad) and ill-behaved (ugly) fragments.

Next up is an excellent discussion of PAINS by Jonathan Baell (at Monash) and collaborators. Although Practical Fragments has covered this topic repeatedly (here, here, here, here, here, and here) it is a sad fact that more examples appear in the literature every day, so there is always something new to write about.

Fragment-finding methods make up the next several papers, starting with a nice overview of native mass spectrometry by Sally-Ann Poulsen at Griffith University. This paper covers theory, practical issues, and recent examples. Roisin McMahon and Jennifer Martin at University of Queensland, along with Martin Scanlon, describe thermal shift assays. In addition to highlighting a number of published examples, the paper also delves into some of the technical challenges and issues with false positives and false negatives, concluding with a nuanced discussion of how to deal with conflicting data.

The subject of conflicting data is central to the work of Olan Dolezal and Tom Peat, both of CSIRO, and their collaborators. They screened the protein trypsin against 500 Maybridge fragments using SPR. Unfortunately they couldn’t go higher than 100 micromolar without running into problems of solubility and aggregation, but even at this relatively low concentration they found 18 hits. X-ray crystallography validated 9 of them, and isothermal titration calorimetry (ITC) also validated 9, with 7 confirmed by all three techniques. (Incidentally, there are lots of great experimental details here.) Four of the SPR hits could not be confirmed by either ITC or X-ray, and 3 turned out to be false positives when repurchased and tested; in one case this appeared to be due to cross-contamination with a more potent compound. In general, the more potent compounds tended to be the ones that reproduced best, and solubility seemed to be a limiting factor for ITC. Despite the imperfect agreement of biophysical techniques, these were still superior to computational approaches on the same target with the same library. As they conclude:

It is gratifying to know (at least for these authors) that experimental data are still of enormous value in the area of fragment-based ligand design and that the modelling community still has a way to go before the experimentalists are put out to pasture.

But experimentalists should not get too cocky: the next paper, by Jamie Simpson and collaborators at Monash University, describes some of the things that can go wrong. An STD NMR screen of the antimicrobial target ketopantoate reductase (KPR) using the same Maybridge library of 500 compounds revealed 196 hits! The 47 with the strongest STD signals were then tested in a ¹H/¹⁵N-HSQC NMR assay, leading to 14 hits, of which 4 gave measurable IC₅₀ values in an enzymatic assay. Unfortunately, follow-up SAR was disappointing, and subsequent experiments revealed that aggregation was to blame: when the biochemical experiments were rerun in the presence of 0.01% Tween-20, only a single fragment gave a measurable IC₅₀ value. The researchers redid their STD-NMR screen in the presence of detergent, resulting in 71 hits, all of which were tested in the biochemical screen. This led to the identification of a new (and fairly potent) hit that had previously been missed. This nicely illustrates the fact that false positives are not just a problem in terms of wasted resources, they can also overwhelm the signal from true positives. The moral? Always use detergent in your assay!

The question of whether structure is needed to prosecute fragments has come up before, and the next paper, by Stephen Headey, Steve Bottomley, and collaborators at Monash University, addresses this question directly. The target protein, a mutant form of α1-antitrypsin called Z-AAT, unfolds and polymerizes in vivo, causing a genetic disease. The researchers used an STD NMR fragment screen of 1137 fragments to identify several hundred hits, and focused on those that bound to the mutant form of the protein rather than the wild-type. They then used a technique called Carr-Purcell-Meiboom-Gill (CPMG) NMR (which relies on line broadening when fragments bind to a protein) to confirm 80 hits, the best of which had a dissociation constant of 330 micromolar. If you’ve stuck through this post thus far you’ll recall that the Monash library was designed for “SAR by catalog”, and 100 analogs of this fragment were purchased and tested, leading to several new hits, one with a dissociation constant of 49 micromolar. Although there is still a long way to go, metastable proteins are tough targets, so this is a nice start.

The next paper, by Ray Norton at Monash University and collaborators, describes a fragment screening cascade against the antimalarial target apical membrane antigen 1 (AMA1). An initial STD NMR assay of 1140 fragments produced 208 hits, but competition experiments with a peptide ligand whittled this number down to 57 that confirmed in both STD and CPMG NMR assays. Of these, 46 confirmed in an SPR assay, and although most are fairly weak, some SAR is starting to emerge as new analogs are synthesized.

Another antimicrobial target, 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (HPPK), is the subject of a paper by James Swarbrick at Monash and collaborators. An initial STD NMR screen gave an unnervingly high hit rate (notice any themes emerging?), so 2D ¹⁵N-HMQC experiments were performed on 750 Maybridge fragments, yielding 16 hits. Competition experiments using CPMG NMR and close analyses of the chemical shifts suggested that these fragments bind in the substrate binding site, and SPR confirmed binding for some of the fragments.

Finally, Martin Drysdale of the Beatson Institute highlights some of the success stories of FBDD, including clinical compounds, and ends with a call for shapelier fragments.

All in all this is a great collection of papers, particularly for those relatively new to the field. It will be fun to revisit some of these projects in a few years to see how they’ve progressed.