A receptor-binding experiment can support a hypothesis about drug action without establishing how the drug works in patients. The distinction is important when a laboratory affinity estimate is used to predict a clinical effect or dose.
Philip Seeman’s 2016 paper examined cannabidiol (CBD) in rat striatal tissue. Its title proposed partial agonism at dopamine D2High receptors and a prediction of an antipsychotic dose. The evidence should be assessed at the level of the experiment actually performed.
What the assay measured
The study tested whether CBD competed with a radiolabeled compound, tritiated domperidone, for binding in rat brain-tissue preparations. Five independent experiments contributed to the reported competition results.
With that radioligand, the author observed a two-phase competition pattern and calculated different affinity estimates for high- and low-affinity components. The paper interpreted the pattern as similar to that of a known partial agonist.
This was a binding experiment in prepared tissue. It was not a trial administering CBD to patients, a human receptor-occupancy scan or a direct clinical assessment of symptom change.
The radioligand affected the observed pattern
The paper also reported that CBD competition against a different radioligand, tritiated raclopride, did not produce the same two-phase pattern. That result makes the assay conditions relevant to interpretation.
A binding constant is conditional on the experimental system and analysis. It should not be treated as a universal concentration threshold at which a particular patient experiences a defined drug effect.
Binding and functional response are separate questions
The proposed partial-agonist interpretation relied on the binding pattern and comparison with prior pharmacology. The experiment did not directly measure a full CBD-induced receptor-signaling response in these preparations.
That distinction limits the conclusion. Competition for binding can motivate functional testing, but the binding curve alone does not quantify a clinical response or establish that this receptor interaction causes a therapeutic effect.
Clinical dose extrapolation requires additional evidence
The author related laboratory affinity to clinical doses reported in other studies and made assumptions about available drug concentrations. Those extrapolations were not direct measurements of human brain occupancy in this experiment.
The paper therefore supports discussing a historical receptor hypothesis. It does not independently establish an effective treatment dose, demonstrate efficacy in a new patient population or justify changing medication.
The practical lesson is to preserve the chain of inference: tissue binding, receptor function, human exposure, target engagement and clinical outcome require distinct evidence. A result at the first step does not automatically establish the remaining steps.
Source: Seeman P. Cannabidiol is a partial agonist at dopamine D2High receptors, predicting its antipsychotic clinical dose. Translational Psychiatry. 2016;6:e920.
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