8-Point Dose-Response IC50 Determination of Kinase Inhibitors by ADP-Glo Luminescent Kinase Assay

Each compound is tested in an 8-point, 3-fold serial dilution series spanning 10,000 nM to 4.6 nM final, in technical triplicate per plate across 3 independent plate runs (biological replicates n=3). Plates are run with ATP fixed at the empirically determined Km,ATP (e.g., 25 µM). Compound wells are randomized across plate columns to distribute edge/positional effects; a staurosporine reference standard is included on every plate. High (no inhibitor) and low (no enzyme) controls occupy 16 wells each per plate. Total assay volume is 10 µL.

BSL-1. Staurosporine and many kinase inhibitors are cytotoxic/cytostatic and potential reproductive hazards — handle as hazardous and avoid skin contact. DMSO is a skin penetrant and can carry dissolved compounds through gloves; use nitrile gloves and change frequently. Wear lab coat and safety glasses. DTT and ATP are low hazard. Dispose of DMSO/compound waste as chemical hazardous waste per institutional EHS; do not pour down the drain. Luminescent reagents are non-radioactive.

Positive inhibition control: staurosporine at a saturating 1 µM concentration defines 100% inhibition (matches the no-enzyme low control). Negative/vehicle control: 1% DMSO without inhibitor defines 0% inhibition (maximal signal). No-enzyme control: full reaction lacking kinase establishes assay background and the low-signal floor. No-ATP and no-substrate controls confirm signal is ATP- and substrate-dependent. A reference inhibitor (staurosporine) full dose-response is run on every plate to verify assay performance plate-to-plate.

Well-behaved curves show a clear upper plateau (0% inhibition, high control RLU) and lower plateau (100% inhibition matching no-enzyme background), spanning a signal window of at least 5-fold (S:B). Z' factor should be >= 0.5 across high/low controls. Staurosporine IC50 typically falls in the low-nanomolar range (1-20 nM depending on kinase). Active test compounds yield complete sigmoidal curves with Hill slopes near 1.0 (0.7-1.5); slopes >2 or <0.5 suggest aggregation or non-specific effects.

• Recombinant human kinase, full-length or active domain, e.g. 10 ng/µL working stock in kinase buffer
• ADP-Glo Kinase Assay kit (ADP-Glo Reagent + Kinase Detection Reagent + ultra-pure ATP)
• Kinase reaction buffer: 40 mM Tris-HCl pH 7.5, 20 mM MgCl2, 0.1 mg/mL BSA, 50 µM DTT (add fresh)
• Peptide or protein substrate at 0.2 mg/mL (kinase-specific, e.g. myelin basic protein)
• Ultra-pure ATP, 10 mM stock, diluted to assay Km,ATP
• Staurosporine, 1 mM in DMSO (reference inhibitor)
• Test compounds, 10 mM stocks in 100% DMSO
• White, low-volume 384-well plates (solid bottom)
• DMSO, molecular biology grade

To quantify the inhibitory potency (IC50) of a test compound series against recombinant human kinase activity in a homogeneous, ATP-competitive biochemical assay. This protocol establishes a robust, miniaturized 384-well workflow that converts kinase-generated ADP into a luminescent signal, enabling accurate eight-point dose-response fitting and confident potency rank-ordering across a compound library for medicinal chemistry SAR decisions.

1. Prepare an 8-point 3-fold serial dilution of each compound in 100% DMSO at 100x final, then intermediate-dilute 1:25 into kinase buffer (4x plate concentration); final DMSO is 1%.
2. Dispense 2.5 µL of 4x compound (or 1% DMSO vehicle) into assay wells using an acoustic dispenser or calibrated pipette.
3. Add 5 µL of 2x enzyme/substrate mix (kinase + substrate in reaction buffer); for low controls add buffer only.
4. Centrifuge plate at 200 x g for 1 min and pre-incubate 15 min at room temperature to allow compound binding.
5. Initiate reaction with 2.5 µL of 4x ATP at Km,ATP; final volume 10 µL.
6. Incubate 60 min at room temperature with the plate sealed to prevent evaporation.
7. Add 10 µL ADP-Glo Reagent; incubate 40 min at RT to deplete unconsumed ATP.
8. Add 20 µL Kinase Detection Reagent; incubate 30 min at RT to convert ADP to ATP and generate luminescence.
9. Read luminescence on a plate reader (0.5 s integration per well).

Independent variable: test compound concentration (8 levels, 4.6 nM to 10,000 nM). Dependent variable: relative luminescence units (RLU) converted to percent kinase inhibition. Controlled variables: ATP concentration (fixed at Km,ATP), enzyme lot and concentration, substrate concentration, reaction time and temperature, DMSO concentration (1% across all wells), incubation times for each detection step, and plate reader integration time.

Compounds that engage the kinase ATP-binding pocket will produce a concentration-dependent reduction in kinase turnover, yielding a sigmoidal inhibition curve whose midpoint (IC50) reflects binding affinity under the chosen ATP concentration. We predict that IC50 values shift rightward as assay ATP is raised toward Km,ATP for ATP-competitive inhibitors, confirming a competitive mechanism, whereas allosteric inhibitors will show ATP-independent IC50 values.

Subtract the mean no-enzyme background from all wells. Normalize each well to percent inhibition: %inh = 100 x (1 - (RLUsample - RLUlow)/(RLUhigh - RLUlow)) using plate-specific high (DMSO) and low (staurosporine/no-enzyme) controls. Fit normalized data to a four-parameter logistic (4PL) model in GraphPad Prism or a Python lmfit script to extract IC50, Hill slope, top, and bottom. Report IC50 as geometric mean +/- 95% CI across the 3 independent runs. Flag fits with R2 < 0.95 or incomplete plateaus for retest.

1. Low signal window / poor Z': enzyme inactive or too dilute — titrate enzyme to give 10-20% ATP-to-ADP conversion (linear range), and use fresh DTT. 2. No dose-response for a known active: ATP set far above Km masks ATP-competitive inhibitors — confirm and use Km,ATP. 3. Steep Hill slopes (>2) and flat-bottomed curves: compound aggregation — add 0.01% Triton X-100 and re-test, or check colloidal aggregation by dynamic light scattering. 4. Edge-well drift: evaporation — seal plates and avoid outer rows or use humidified incubation. 5. High background in no-enzyme wells: ATP contamination in buffer — use kit ultra-pure ATP and fresh reagents.

IC50 values are log-transformed (pIC50) before averaging because potency is log-normally distributed. Compare compound pIC50 values by one-way ANOVA with Dunnett's or Tukey's multiple-comparison correction at alpha = 0.05. With n=3 independent plate replicates and a typical pIC50 SD of ~0.15 log units, the design detects a 3-fold potency difference with >80% power. Plate quality is gated by Z' >= 0.5; plates failing this are excluded and repeated.