Casting a beady eye on kinases

A huge area of drug discovery is targeting protein kinases. There are about 500 protein kinase-like proteins in the human proteome. Some of these are probably not active (pseudokinases; review [paid]) and periodically there are claims of protein kinase activity in novel proteins, but that's the ballpark. An increasing number of drugs target these, with Gleevec as perhaps the best known but a large parade of others coming forward.

Most kinase-targeting drugs compete with ATP in the active site. The ATP-binding site of kinases shows a lot of conservation, and so cross-reactivity is a big topic in the field. What is desired for specificity depends on the target & disease & one's tolerance for risk. Gleevec was originally touted as being laser-focused on BCR-ABL, but it actually hits a number of kinases and many of these have yielded new markets, such as KIT for gastrointestinal stromal tumors. Being 'dirty' may be useful in oncology, where many kinases may be contributing to the tumor's growth & survival. On the other hand, in chronic diseases one probably wants a really focused drug (or at least can't tolerate one that isn't).

I got involved a little in kinase screening back at MLNM. The workhorse in the industry are in vitro assays using purified kinases. These are useful and can be run en masse, but everyone knows in vitro isn't always predictive of in vivo. Furthermore, despite diligent efforts by a number of vendors, not every kinase is available. So, the field is ripe for development of new approaches, especially ones which explore the compound in vivo.

In an ideal world, there would be a complete panel of biomarkers specific for each kinase which could be used to measure the impact of a compound on every kinase-regulated pathway in the cell. That's a long, long ways off -- only a few kinases really have good, reliable assays & many are essentially uncharacterized.

The latest Nature Biotech has a nice paper from CellZome on a proteomic approach to the problem & an accompanying News & Views item from a top mass spec person (either link prior requires Nat Biotech subscription, but Cellzome has the paper for free also).

The strategy is to derivatize beads with promiscuous kinase-binding compounds and use these to pull down bound kinases for identification by Mass Spec. Such has been described previously, particularly by the company which Cellzome acquired which had been doing this work. Big advances in this paper is to use iTRAQ labeling reagents to enable accurate quantification of the bound proteins and to use a competitive-binding format to assay test compounds.

The competitive binding angle is clever. Past efforts tried to derivatize the compound of interest to link it to the beads. This has many undesirable properties: the linkage may change the binding properties, each compound must be worked out separately. etc. The new work identified a set of standard promiscuous-binders which can be used to get a large fraction of the kinase repertoire. This same set can be used with just about any compound. By using these in a competitive format, where what is measured is how much the compound of interest disturbs the binding profile of the kinobeads, quantitative measurements can be made. Entire binding curves can be pulled out from a few experiments -- and binding curves for every kinase reliably pulled down by the beads. Slick!

Their coverage of the kinase world is quite good, though not perfect. In a set of experiments they pulled down 307 kinases. While that isn't everything, it's a lot -- and some of the rest may be pseudokinases or not expressed in any of the tissues they looked at. However, some probably just aren't bound well by the reference compound set. Whole small subtrees are missing from their Figure 2 -- examples of the missing include all the GPCR kinases (BARK, GRKs, etc), the two TLKS1, 2/4 polos, the WNKS, none of the 3 Akts, a number of miscellaneous cell cycle kinases (CDC7, BubR1&R2, etc. Some of those are mighty interesting, but of course the solution is to find more compounds to put on the beads.

What's also nice is that the assay isn't limited to kinases -- lots of other stuff comes down too. Initially it was apparently clogged with heat shock proteins (using a n ATP analog as the probe). But, in the current format they found a good sampling of non-kinases, and for Gleevec identified a candidate off-target with some known biology.

So what's the catch? Well, there are a few caveats. First, it is a binding assay, so hits need to be followed up to demonstrate actual inhibition. Second, it is going to be ATP-site specific. That covers most current kinase inhibitors, but there are probably many out there that work outside the ATP site, and this method will be blind to that (or to off-targets bound not by ATP-mimicry). Third, many of things being pulled down won't have much known about them -- do you worry about it or not?

As described in the M&M, the assay requires 5 mL of cell lysate -- not tiny, but not whopping. So this probably wouldn't be applied to every compound coming out of medicinal chemistry, but perhaps to either characterize representative members of particular lead families and to characterize compounds that are quite a ways down the med chem path.

One other interesting bit at the end: instead of adding the test compounds to lysates, they added the compounds & waited a few hours. As they note, many kinase inhibitors have slow off-rates -- they stick to their target rather fiercely. So, the assay can be run after a delay. The peptide mixtures could then be subjected to phosphopeptide enrichment & identification, enabling the phosphorylation state of downstream kinases to be probed & examined in relation to the concentration of compound used.