Pharmacology
Drug-Receptor Binding Affinity Tool
Turns Kd, Ki or an IC50 into receptor occupancy, classifies the affinity, and plots the binding isotherm and its Scatchard transform.
Receptor occupancy
Enter one of Kd, Ki or IC50, with a ligand concentration and Bmax.
Binding parameters
Enter only one of Kd, Ki or IC50 — they are read in that order of priority.
Dissociation constant. Takes priority over Ki and IC50.
Inhibition constant. Used when Kd is blank.
Converted to Ki by Cheng-Prusoff. Used when Kd and Ki are blank.
Assay conditions
Free ligand concentration at the receptor.
Total receptor concentration. Scales the amount bound, not the occupancy.
Reference affinities
Where some familiar drugs sit on this scale.
| Drug (target) | Kd (nM) | Class |
|---|---|---|
| Fentanyl (μ-opioid) | 0.1 | Ultra-high |
| Propranolol (β-blocker) | 1.0 | High |
| Aspirin (COX-1) | 100 | Medium |
| Warfarin (VKOR) | 1000 | Low |
| Penicillin (PBPs) | 10000 | Very low |
Bands used here: ultra-high below 0.1 nM, high 0.1–1, medium 1–100, low 100–1000, very low above 1000 nM.
Single-site (Langmuir) binding. Fractional occupancy depends only on Kd and [L]:
occupancy = [L] / (Kd + [L])
The amount actually bound scales that by the receptor density:
Bound = Bmax × [L] / (Kd + [L])
When an IC50 from a competition assay is entered, it is converted to Ki with the Cheng-Prusoff equation, where [L*] is the radioligand concentration (taken here as the entered [L]) and Km is the radioligand's own Kd:
Ki = IC50 / (1 + [L*] / Km)
The Scatchard transform rearranges the isotherm to a straight line: plotting Bound/Free against Bound gives a slope of −1/Kd and an x-intercept of Bmax. It assumes free ligand is approximately equal to total ligand.
Common questions
No. Occupancy is a fraction and depends only on [L] and Kd. Bmax scales how much ligand is bound in absolute terms (nM), which is what the Scatchard plot and the 'bound' figure use.
Only when the radioligand concentration is far below its own Kd — then the Cheng-Prusoff correction factor approaches 1. Otherwise IC50 overstates Ki, and the assay conditions must be corrected for.
Kd, then Ki, then IC50. Clear the higher-priority field to use a lower one — the radioligand Kd field only appears when IC50 is the value being used.
Because the isotherm is hyperbolic: 50% occupancy needs [L] = Kd, but 90% needs 9 × Kd and 99% needs 99 × Kd. That flattening is why increasing a dose gives diminishing returns near the top of the curve.
Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy