Kinase drug discovery--what's next in the field?

Abstract

Over the past 15 years protein kinases have become the pharmaceutical industry's most important class of drug target in the field of cancer. Some 20 drugs that target kinases have been approved for clinical use over the past decade, and hundreds more are undergoing clinical trials. However, the recent approval of the first protein kinase inhibitors for the treatment of inflammatory diseases, coupled with an enhanced understanding of the signaling networks that control the immune system, suggests that there will be a surge of interest in this area over the next 10 years. In this connection, we discuss opportunities for targeting protein kinases in the MyD88 signaling network for the development of drugs to treat chronic inflammatory and autoimmune diseases. Activating mutations in protein kinases underlie many other diseases and conditions, and we also discuss why the protein kinases SPAK/OSR1 and LRRK2 have recently become interesting targets for the treatment of hypertension and Parkinson's disease, respectively, and the progress that has been made in developing LRRK2 inhibitors. Finally we suggest that more focus on the identification of inhibitors of kinase activation, rather than kinase activity, may pay dividends in identifying exquisitely specific inhibitors of signal transduction cascades, and we also highlight "pseudo-kinases" as an attractive and unexplored area for drug development that merits much more attention in the years to come.

Figure 1

WNK signaling network that activates the protein kinases SPAK and OSR1 and the NCC and NKCC2 ion co-transporters that control blood pressure. Hyperosmotic and hypotonic low chloride conditions induce the autoactivation of WNK isoforms, which then activate the SPAK and OSR1 protein kinases by phosphorylating their activation loops. The interaction of SPAK and OSR1 with the MO25 component is also needed to generate maximal activity. SPAK and OSR1 phosphorylate and activate the NCC and NKCC2 ion cotransporters that play a vital role in regulating salt reabsorption in the kidney and are the targets of the widely used thiazide and loop diuretic drugs. It should be noted that in some studies where overexpression systems were employed, WNK4 was reported to negatively regulate NCC. Whether this inhibitory effect is of physiological significance or mediated through SPAK or OSR1 requires further investigation.

Figure 2

Domain structure of LRRK2 and some reported inhibitors of this protein kinase. LRRK2 is a large 2527-residue protein kinase that, in addition to a protein kinase domain, contains an active GTPase domain. The figure shows the structures of the specific LRRK2 inhibitors that are being used to dissect the cellular functions of LRRK2. To date no direct physiological substrates of LRRK2 have been identified. However, LRRK2 induces its own phosphorylation at Ser910 and Ser935 by an indirect mechanism that remains to be identified, and this allows it to bind to 14-3-3 isoforms. Monitoring the state of phosphorylation of LRRK2 at Ser910 and Ser935 with phospho-specific antibodies is therefore widely employed to assess the in vivo efficacy of LRRK2 inhibitors that are being elaborated for the treatment of Parkinson’s disease.

Figure 3

Simplified outline of the MyD88-signaling pathway by which TLR agonists induce the production of inflammatory mediators. Reasons why the protein kinases highlighted in red may be particularly attractive drug targets are discussed in the text. The activation of TLRs in myeloid cells recruits MyD88 and protein kinases of the IRAK family to the receptor, which induce the E3 ligase TRAF6 to produce Lys63-linked polyubiquitin (K63-pUb) chains and the E3 ligase LUBAC to produce linear pUb chains. The binding of K63-pUb chains to the TAB2 and TAB3 components of TAK1 kinase complex and K63-pUb and/or linear-pUb chains to the NEMO component of the canonical IKK complex is thought to induce conformational changes that activate these protein kinases. The IKKs phosphorylate the inhibitory IκBα component of the transcription factor NFκB and the inhibitory NFκB1/p105 component of the protein kinase Tpl2, triggering the proteasomal degradation of these inhibitors and the activation of NFκB and Tpl2. TAK1 not only initiates the activation of the IKK complex but also activates the pathways that switch on the mitogen-activated protein kinases (MAPKs) termed p38 MAP kinase and c-Jun N-terminal kinases (JNKs), while Tpl2 switches on the signaling pathway that leads to the activation of extracellular signalregulated protein kinases 1 and 2 (ERK1, ERK2). The p38 MAP kinases, JNKs, and ERK1/2, collectively termed MAPKs in the figure, phosphorylate many proteins, which regulate the transcription, translation, processing, and secretion of inflammatory mediators, including the pro-inflammatory cytokines TNFα, IL-6, and IL-12. The protein kinase MK2, which is activated by p38α MAP kinase, stimulates post-transcriptional events required for the production of pro-inflammatory cytokines, such as TNFα and IL-6. The p38α MAP kinase and ERK1/2 also activate the protein kinases MSK1 and MSK2, which phosphorylate the transcription factor CREB, stimulating the transcription of genes encoding anti-inflammatory cytokines, such as IL-10 and IL-1ra. CREB transcriptional activity is greatly enhanced by the coactivator CRTC3 in macrophages. The phosphorylation of CRTC3, which is catalyzed by the SIK subfamily of protein kinases, prevents CRTC3 from activating CREB, restricting the production of IL-10, which can be overcome by inhibition of the SIKs. Following its secretion, IL-10 activates the IL-10 receptor by autocrine and paracrine mechanisms switching on members of the JAK family of protein kinases that phosphorylate and activate the transcription factor STAT3. These lead to the synthesis of proteins that suppress the production of pro-inflammatory cytokines and drive the conversion of classically activated M1 macrophages to regulatory M2b macrophages.

Figure 4

Structures of compounds that prevent the activation of one kinase by another. The compound PD98059 prevents the activation of MEK1 by Raf, while Akti1/2 and MK2206 prevent the activation of Akt1 by PDK1. There are 47 clinical trials of MK2206 in progress for the treatment of different cancers.