The number of publications since 1990 on small-molecule protein kinase inhibitors that have been evaluated in animal efficacy models and clinical trials for oncology indications was determined by a literature search with SciFinder Scholar. The number steadily increased until the recent plateau in 2007. The orange bar denotes the number of publications on central nervous system (CNS) disease indications in 2008. This activity level is comparable to that for the cancer field in 1999. The curve shows several small inflection points that correlate with milestones related to the kinase inhibitor imatinib (Gleevec; Novartis). For example, the first reports of in vivo efficacy of imatinib appeared in 1995 and 1996, and shortly thereafter the slope of the curve showed a small increase. An increased slope is also seen after the first clinical trials with imatinib were initiated in 1998 and after drug approval in 2001. Before 2001, only one kinase inhibitor was approved as a drug (fasudil; approved in Japan in 1995 for cerebral vasospasm). Since 2001, eight additional kinase inhibitors have received approval for oncology indications: gefitinib (Iressa; AstraZeneca) was approved in the United States in 2003; erlotinib (Tarceva; OSI/Genentech/Roche) was approved in the United States and Europe in 2004; sorafenib (Nexavar; Bayer/Onyx) was approved in the United States and Europe in 2005; sunitinib (Sutent; Pfizer) and dasatinib (Sprycel; Bristol–Myers Squibb) were approved in the United States and Europe in 2006; and temsirolimus (Torisel; Wyeth) was approved in the United States, and lapatinib (Tykerb; GlaxoSmithKline) and nilotinib (Tasigna; Novartis) were approved in the United States and Europe in 2007.

Glycogen synthase kinase 3 (GSK3) is an example of a widely distributed target that modulates diverse cellular functions. Death-associated protein kinase 1 (DAPK1) has a more restricted cellular role, but both are components of an integrated web of pathways. Selective biological responses are possible because such networks can transduce various cellular stimuli into an integrated response, through mechanisms that are incompletely understood. The target kinases represent network nodes, the quantitative importance of which is altered under conditions of deregulation. The activity of GSK3 is regulated by several mechanisms that allow selective phosphorylation of substrates in response to distinct input signals. The regulatory mechanisms include modulation of GSK3 activity by inhibitory serine phosphorylation and activating tyrosine phosphorylation of the enzyme, coordinated phosphorylation of its substrates, dynamic regulation of the subcellular distribution of GSK3 and the assembly of GSK3 into distinct multi-protein complexes for selective substrate phosphorylation. DAPK1 regulates signalling pathways that are important to cell survival and translational control through phosphorylations of a highly selective set of substrate proteins in response to its activation by distinct cellular stimuli, such as increased Ca²⁺ levels, which are sensed through its calmodulin (CAM) subunit. Examples of central nervous system functions that have been proposed for DAPK1 and GSK3 are shown in orange. Aβ, amyloid-β; BAX, BCL2-associated X protein; CAMK1, Ca²⁺–calmodulin-dependent protein kinase 1; CAMKK, CAMK kinase; C/EBP, CCAAT/enhancer-binding protein; CREB, cyclic AMP response element binding protein; ECM, extracellular matrix; EIF2B, eukaryotic initiation factor 2B; ERK, extracellular signal-regulated kinase; HSF1, heat shock factor 1; IRS1, insulin receptor substrate 1; JNK, Jun I-terminal kinase; MAP1B, microtubule-associated protein 1B; MAPK, mitogen-activated protein kinase; MLC, myosin light chain; MRPL13, mitochondrial ribosomal protein L13; MRPS6, mitochondrial ribosomal protein S6; MYC, myelocytomatosis viral oncogene homologue; NFAT, nuclear factor of activated T cells; NF-κB, nuclear factor κB; phospho–S, phosphorylated serine; phospho–Y, phosphorylated tyrosine; PI3K, phosphoinositide 3-kinase; PKA, protein kinase A; PKC, protein kinase C; PKD, protein kinase D; PYK2, proline-rich tyrosine kinase 2; TP53, tumour protein p53; RPS6KA1, ribosomal protein S6 kinase, 90kDa, polypeptide 1.

Protein kinases are characterized by a bi-lobe architecture consisting of an amino-terminal β-sheet lobe and a larger α-helical carboxy-terminal domain, with a canonical fold that contains the nucleotide-binding site located between the two domains. Other conserved protein kinase features include the P-loop (shown in magenta), which contains a glycine-rich motif (GXGXXG), and the activation loop (shown in purple), which is characterized by an initial DFG and terminal APE motif. Upon ATP binding, a reorientation of the C-helix (shown in light blue) and the glycine-rich loop is often observed. This structure shows SYK tyrosine kinase in complex with the therapeutic kinase inhibitor imatinib (Protein Data Bank identification number: 1XBB).

See TABLE 1 for further information.

Comparisons of mean values of three key molecular properties, lipophilicity (LogP) (a), molecular weight (MW) (b) and polar surface areas (PSA) (c), are shown for CNS-penetrant small molecules (CNS; see Supplementary information S3 (table)), approved and marketed drugs for any indication (MKT; data from Vieth121) and kinase inhibitor drugs for all indications (KID; see Supplementary information S4 (table)). One-way ANOVA was used to evaluate significant differences to CNS mean values. Mean values for LogP (d), MW (e) and PSA (f) were also calculated for compounds in the CNS dataset (see Supplementary information S3 (table)) that are P-glycoprotein substrates (PGP⁺) compared with compounds that have been experimentally validated as non-P-glycoprotein substrates (PGP⁻). Student’s t-test was used to compare mean values. *p < 0.05. **p < 0.001.

Focusing on small-molecule kinase inhibitors with key molecular property characteristics and promising profiles in early-stage pharmacological screens increases the potential for positive outcomes in the subsequent preclinical development of investigational new drugs. This substantially reduces early discovery demands on synthetic chemistry and biology efforts and the later attrition rate in development. Medicinal chemistry refinement takes into consideration molecular properties, suitability for central nervous system (CNS) penetration and ligand activity. Pharmacology-related screens are used to measure brain uptake, assess microsomal stability, determine whether the compound is a cytochrome P450 2D6 (CYP2D6) or CYP3A4 substrate and ensure that there are no adverse effects. HTS, high-throughput screening; IC₅₀, half-maximal inhibitory concentration; SAR, structure–activity relationship.