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Research tools

AM251

The reference CB1 antagonist/inverse agonist in the laboratory. 300× selectivity for CB1 over CB2.

Unscheduled
Level of detail

C₂₂H₂₁Cl₂IN₄O

CAS 183232-66-8

Family
ECS pharmacological toolAntagonist or inverse agonist
Origin
Pharmaceutical research (Makriyannis)
First described
1996
Molar mass
555.24 g/mol

Updated on

In plain terms

AM251 is used in the laboratory to block the CB1 receptor, and so the effect of cannabinoids that act through it. It is an experimental tool, not a medicine.

Receptors and activity

Target network

CB1Inverse agonistKi = 7.5 nM[3H]CP55940 or [3H]HU-243 displacement
  • Activates
  • Blocks or inhibits
  • Binds or modulates

The lower the cited value, the closer the target sits to AM251: one ring per factor of 100. A target with no measured value stays on the edge.

Where these targets live

Where each target has been found in humans, according to the cited studies. The map shows the target, not an effect of AM251.

Brain, side view
Body, front view
Blood

CB1CB1 found in 7 regions of the body

  1. Basal gangliadenseMost dense in basal ganglia outflow nuclei1
  2. CerebellumdenseVery dense in the cerebellum1
  3. HippocampusdenseVery high concentrations in the hippocampus2
  4. AmygdaladenseHigh concentrations in the amygdala2
  5. CortexpresentPresent throughout the neocortex2
  6. Adipose tissuepresentExpressed in adipose tissue3
  7. GutpresentColonic epithelium, smooth muscle and plexus4

Map sources

  1. Herkenham M et al. Cannabinoid receptor localization in brain. Proc Natl Acad Sci U S A, 1990 (new tab)
  2. Glass M et al. Cannabinoid receptors in the human brain: a detailed anatomical and quantitative autoradiographic study in the fetal, neonatal and adult human brain. Neuroscience, 1997 (new tab)
  3. Engeli S et al. Activation of the peripheral endocannabinoid system in human obesity. Diabetes, 2005 (new tab)
  4. Wright K et al. Differential expression of cannabinoid receptors in the human colon: cannabinoids promote epithelial wound healing. Gastroenterology, 2005 (new tab)

Cited values

  1. CB1Inverse agonistKi = 7.5 nM69 out of 100

    Pertwee, Curr Med Chem 2010 (table 1) (new tab) · [3H]CP55940 or [3H]HU-243 displacement

  2. CB2Binding (function not established)Ki = 2.3 µM27 out of 100

    Pertwee, Curr Med Chem 2010 (table 1) (new tab) · [3H]CP55940 or [3H]HU-243 displacement

  3. GPR55Agonistno measured value cited
Bar length follows the measured value (Ki, Kd, EC50 or IC50), on one scale shared by every entry, from 100 µM to 0.1 nM. A target with no cited value has no bar.

Measured effects

Not characterised. No placebo-controlled human trial has measured the effects of AM251, so no level is shown.

Levels computed from placebo-controlled human trials: difference from placebo on 100, at the dose and route stated. Read the method

Pharmacology

AM251 is the iodinated analogue of rimonabant, from the Makriyannis group: in the structure-activity study by Lan et al. (J Med Chem 1999, PMID 10052983), the p-iodophenyl derivative is the most potent of the pyrazoles tested; its iodine-123-labelled version was described by Gatley et al. (Eur J Pharmacol 1996, PMID 8836622). It is a CB1 antagonist / inverse agonist, with a Ki of 7.49 nM at CB1 against 2,290 nM at CB2 (Pertwee 2010, PMID 20166927). It is used to test whether CB1 takes part in an observed effect, with one caveat: AM251 also activates GPR55 (Kapur et al. 2009, PMID 19723626), which complicates interpretation in studies involving that receptor.

Key sources.

Origin

Research tool

An iodinated analogue of rimonabant (iodine replaces the chlorine para on the phenyl at position 5 of the pyrazole); a synthetic molecule of the diarylpyrazole series, developed at the University of Connecticut.

Classification

Family, origin and source verification

Family

ECS pharmacological tool › Antagonist or inverse agonist

Verification

Classification, sources and binding values checked on 20 October 2026; text read against its sources and Wikipedia.

Wikipedia (English)

AM-251 (drug) (new tab)

Article paired by the molecule's structure (InChIKey), revision read on 19 October 2026. Read as a second opinion, never cited as a source.

References

  1. Lan et al., J Med Chem 1999: structure-activity relationships of pyrazole cannabinoid antagonists (new tab)PMID 10052983
  2. Gatley et al., Eur J Pharmacol 1996: AM251 labelled with iodine-123 (new tab)PMID 8836622
  3. Pertwee, Curr Med Chem 2010: targets of synthetic cannabinoid agonists and antagonists (Ki, GPR55) (new tab)PMID 20166927
  4. Kapur et al., J Biol Chem 2009: AM251 and rimonabant, GPR55 agonists (new tab)PMID 19723626

Identifiers

Names

IUPAC name
1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-piperidin-1-ylpyrazole-3-carboxamide
Other names
AM-251

Identifiers

Formula
C₂₂H₂₁Cl₂IN₄O
CAS
183232-66-8
PubChem CID
2125(new tab)
InChIKey
BUZAJRPLUGXRAB-UHFFFAOYSA-N
SMILES
CC1=C(N(N=C1C(=O)NN2CCCCC2)C3=C(C=C(C=C3)Cl)Cl)C4=CC=C(C=C4)I

Machine-readable entry (JSON)

LLM ingestion format: a structured superset of the entry (identifiers, binding, versioned legal status). Full corpus.