Novel small-molecule inhibitors of hepatitis C virus entry block viral spread and promote viral clearance in cell culture.

Combinations of direct-acting anti-virals offer the potential to improve the efficacy, tolerability and duration of the current treatment regimen for hepatitis C virus (HCV) infection. Viral entry represents a distinct therapeutic target that has been validated clinically for a number of pathogenic viruses. To discover novel inhibitors of HCV entry, we conducted a high throughput screen of a proprietary small-molecule compound library using HCV pseudoviral particle (HCVpp) technology. We independently discovered and optimized a series of 1,3,5-triazine compounds that are potent, selective and non-cytotoxic inhibitors of HCV entry. Representative compounds fully suppress both cell-free virus and cell-to-cell spread of HCV in vitro. We demonstrate, for the first time, that long term treatment of an HCV cell culture with a potent entry inhibitor promotes sustained viral clearance in vitro. We have confirmed that a single amino acid variant, V719G, in the transmembrane domain of E2 is sufficient to confer resistance to multiple compounds from the triazine series. Resistance studies were extended by evaluating both the fusogenic properties and growth kinetics of drug-induced and natural amino acid variants in the HCVpp and HCV cell culture assays. Our results indicate that amino acid variations at position 719 incur a significant fitness penalty. Introduction of I719 into a genotype 1b envelope sequence did not affect HCV entry; however, the overall level of HCV replication was reduced compared to the parental genotype 1b/2a HCV strain. Consistent with these findings, I719 represents a significant fraction of the naturally occurring genotype 1b sequences. Importantly, I719, the most relevant natural polymorphism, did not significantly alter the susceptibility of HCV to the triazine compounds. The preclinical properties of these triazine compounds support further investigation of entry inhibitors as a potential novel therapy for HCV infection.

Chemical genetic transcriptional fingerprinting for selectivity profiling of kinase inhibitors.

The importance of protein kinases as a major class of drug targets across multiple diseases has generated a critical need for technologies that enable the identification of potent and selective kinase inhibitors. Bruton's tyrosine kinase (Btk) is a compelling drug target in multiple therapeutic areas, including systemic lupus erythematosus, asthma, rheumatoid arthritis, and B cell malignancies. We have combined potent, selective kinase inhibition through chemical genetics with gene expression profiling to identify a "fingerprint" of transcriptional changes associated with selective Btk kinase inhibition. The Btk transcriptional fingerprint shows remarkable relevance for Btk's biological roles and was used for functional selectivity profiling of two kinase inhibitor compounds. The fingerprint was able to rank the compounds by relative selectivity for Btk, and revealed broader off-target effects than observed in a broad panel of biochemical kinase cross screens. In addition to being useful for functional selectivity profiling, the fingerprint genes are themselves potential preclinical and clinical biomarkers for developing Btk-directed therapies.

Chemical genetic approaches to kinase drug discovery.

Chemical genetics is an important approach in biological research that utilizes small molecules to study protein function. In the context of kinase drug discovery, chemical genetics has broad applications in identifying and validating targets, demonstrating the druggability of a target and providing potential kinase inhibitor leads for further optimization. The successful application of this approach demands that the small-molecule kinase inhibitors used achieve a desired potency and selectivity. However, given the high number (> 518) and homology of kinases in the human genome, identifying potent and selective kinase inhibitors presents a major challenge. This article reviews recent advances in small-molecule kinase inhibitor design, with an emphasis on selectivity, and also discusses recent progress in the development of analog-sensitive kinase allele (ASKA)-based chemical genetics technology, which creates genetically engineered versions of protein kinases that are fully functional and can be selectively inhibited by a unique reference orthogonal inhibitor. Examples of how ASKA technology can be applied to kinase drug discovery is discussed.

The Ste20 kinase MST4 plays a role in prostate cancer progression.

MST4, a member of the Sterile 20 serine/threonine kinase family, was found to be expressed in prostate carcinoma tumor samples and cell lines. In addition, expression levels appeared to correlate with tumorigenicity and androgen receptor status of the cells. Ectopic expression of wild-type and kinase-inactive MST4 was used to alter cellular MST4 activity levels in three widely studied human prostate tumor cell lines: LNCaP, DU 145, and PC-3. Overexpression of wild-type MST4 induced anchorage-independent growth of the LNCaP cell line, and increased both in vitro proliferation and in vivo tumorigenesis of the DU 145 cell line. On the other hand, expression of a kinase-inactive form reverted the anchorage-independent growth phenotype and highly tumorigenic behavior of the PC-3 cell line. MST4 kinase activity was stimulated significantly by epidermal growth factor receptor ligands, which are known to promote growth of prostate cancer cells. Together, our studies suggest a potential role for MST4 in the signal transduction pathways involved in prostate cancer progression.