Table of Contents

    Improving the Kinase Inhibitor Development Process

    The current approval rate for lipid kinase inhibitors in development is extremely low. This issue demonstrates the need for validated screening assays that not only better define compound specificity and selectivity, but also assist in improving the kinase inhibitor development process as a whole. Of the 20 phosphoinositide kinase-3 (PI3K) small molecule inhibitors in clinical trials in 2009, Gilead’s Idelalisib (CAL-101) was the only one to be approved by the FDA five years later.2

    Idelalisib was the sole compound targeting the δ isoform of PI3K and its success has been attributed to this specificity. During development, basophil activation testing was utilized to demonstrate that the compound is 40- to 300-fold more selective for PI3Kδ relative to the other PI3K Class I enzymes. The InhibiScreen Kinase Inhibitor Assay is the only commercially validated basophil activation test that can be utilized at multiple stages of development and testing by both pharmaceutical companies and their functional service providers alike.

    Improving the Kinase Inhibitor Development Process with InhibiScreen

    Incorporating a validated cell-based assay into the standard drug development workflow may help cut costs and improve success rates for compounds in the clinic. The InhibiScreen Kinase Inhibitor Assay is a flow cytometric, basophil activation test providing a low or high throughput option for screening inhibitors of the δ and γ isoforms of PI3K as well as inhibitors of the BTK and SYK pathways. InhibiScreen can be used at multiple stages of development and testing by both pharmaceutical companies and functional service providers, therefore offering a method for improving the kinase inhibitor development process.

    Improving the Kinase Inhibitor Development Process

    Target Identification and Validation

    During the target identification and validation stage of the drug development process, researchers are looking for druggable targets and vetting those targets out for candidates that produce a clinical effect when engaged by a drug compound. For pharmaceutical companies with pipelines dedicated to the development of compounds directed against specific kinases (e.g. PI3K, BTK and SYK), the target has been predefined and validated as druggable.

    Lead Discovery and Optimization

    Once a druggable target has been identified, hundreds of thousands of compounds can be screened against the target to find compounds with a desired activity profile. Those candidates are then potentially subjected to additional modifications to improve potency and selectivity. Use of the InhibiScreen Kinase Inhibitor Assay in this stage of development can assist in the earlier elimination of non-specific compounds.

    During the development of CAL-101, Gilead employed several lipid kinase screening assays and methods including basophil activation testing.3 Basophil activation testing was used as a surrogate method to define specificity of the compound for the δ or γ PI3K isoform. The addition of CAL-101 resulted in a reduction of CD63 expression at a much lower IC50 via the IgE pathway (δ) than through the fMLP (γ) pathway. These findings demonstrated how the compound is more specific for the δ isoform.

    Pre-Clinical Development

    In the pre-clinical development phase, lab and animal testing is conducted to establish safety and toxicity profiles of lead compounds. The InhibiScreen Kinase Inhibitor Assay provides researchers with a phenotypically-relevant assay utilizing whole blood from donors (healthy or disease state), therefore potentially eliminating the frequency of failure once the compound reaches clinical testing.

    During the pre-clinical development of PWT143, Pathway Therapeutics incorporated basophil activation testing as a means to define specificity of their compound. PWT143 was found to be a novel selective inhibitor of PI3Kδ with low nanomolar potency and improved cellular activity when compared to CAL-101.4

    Clinical Development

    After hundreds of thousands of compounds are subjected to many rounds of testing, one compound makes it to the clinical development phase. It is in this phase that maximum tolerated doses, efficacy and safety are really put to the test. Utilizing InhibiScreen in the development stage allows for a better understanding of compound pharmacodynamics once administered to clinical study participants.

    During the phase 1 clinical trial of PRN1008, Principia Biopharma demonstrated continued suppression of IgE-stimulation of basophils four hours after administration of the compound. This provided insight into the pharmacodynamics associated with PRN1008 and supported further development for rheumatoid arthritis and other inflammatory and autoimmune diseases.5


    As demonstrated by Gilead through the approval of Idelalisib, specificity and selectivity appears to play a pivotal role in the success of PI3K inhibitors in the clinic. There is a need for lipid kinase inhibitor screening assays to provide insight into the specificity and selectivity of compounds in development. Basophil activation testing has been used by several pharmaceutical companies to confirm compound specificity through different stages of the drug development process. The InhibiScreen Kinase Inhibitor Assay is a validated tool to measure the potency and selectivity of kinase inhibitors, and is useful for improving the kinase inhibitor development process as a whole.


    1. DiMasi. (2014). Innovation in the Pharmaceutical Industry: New Estimates of R&D Costs. [Briefing]. Tufts Center for the Study of Drug Development.
    2. U.S. Food and Drug Administration. (2014). FDA Approves Zydelig for Three Types of Blood Cancers. [Press Release]. U.S. Food and Drug Administration.
    3. Lannutti et al. (2011). CAL-101, a p110δ phosphatidylinositol-3-kinase inhibitor for the treatment of B-cell malignancies, inhibits PI3K signaling and cellular viability. Blood, 117(2), 591-4.
    4. O’Farrell et al. (2012). Preclinical characterization of PWT143, a novel selective and potent phosphatidylinositol 3-kinase delta (PI3K delta) inhibitor with ex-vivo activity in hematologic malignancies. [ASH 2012 Scientific Poster]. Pathway Therapeutics.
    5. Smith et al. (2015). A phase 1 clinical trial of PRN1008, an oral, reversible, covalent BTK inhibitor demonstrates clinical safety and therapeutic levels of BTK occupancy without sustained systemic exposure. [EULAR 2015 Scientific Poster]. Principia Biopharma.