Iodocyanopindolol

Iodocyanopindolol (), also known as ICYP, is a synthetic compound derived from pindolol, primarily used as a radioligand in pharmacological research. It functions as a non-selective β-adrenoceptor antagonist and a serotonin 5-HT1A and 5-HT1B receptor antagonist. Its 125I-radiolabelled derivative, 25Iiodocyanopindolol ( 25IICYP), is widely employed to map the distribution and density of β-adrenoceptors and serotonin receptors in tissues, particularly in the brain, heart, and other organs. Iodocyanopindolol is not used clinically but remains a critical tool in studying receptor pharmacology and signal transduction. Its application extends to the central nervous system, where it labels 5-HT1B receptors in regions like the brainstem, hippocampus, and cortex, aiding research on serotonin autoreceptor regulation.

Pharmacological properties

Iodocyanopindolol acts as a high-affinity antagonist at β1 and β2 adrenoceptors, with a lesser affinity for β3 adrenoceptors. It also antagonizes 5-HT1A and 5-HT1B serotonin receptors, making it a versatile radioligand for studying both adrenergic and serotonergic systems. The 25IICYP form binds with high specificity, allowing quantitative analysis of receptor density (Bmax) and affinity (Kd) through techniques like autoradiography and radioligand binding assays.

The binding of 25IICYP to β-adrenoceptors is modulated by G-protein coupling. For example, guanine nucleotides like GTP reduce its affinity for 5-HT1B receptors by disrupting receptor-G-protein interactions, as observed in rat brain studies. This property enables researchers to distinguish high-affinity (G-protein-coupled) and low-affinity receptor states.

Research applications

25IIodocyanopindolol is extensively used to map β-adrenoceptor distribution in tissues such as the human heart, rat lung, and urinary bladder. Early studies demonstrated its utility in identifying coexisting β1- and β2-adrenoceptors in the human right atrium, providing insights into cardiac receptor pharmacology. Its application extends to the central nervous system, where it labels 5-HT1B receptors in regions like the brainstem, hippocampus, and cortex, aiding research on serotonin autoreceptor regulation.

In competition binding experiments, 25IICYP has been used to assess the selectivity of β-adrenergic antagonists. For instance, studies in rat brain showed that β1-selective antagonists (e.g., atenolol, metoprolol) and β2-selective antagonists (e.g., ICI-118,551) exhibit distinct displacement profiles, confirming receptor subtype specificity. Recent research has explored its potential to label β3-adrenoceptors in rat urinary bladder, though challenges with non-specific binding limit its reliability for this subtype.

Mechanism and binding characteristics

The radiolabelled 25IICYP binds with high affinity to β-adrenoceptors (Kd ≈ 0.037–0.056 nM in rat brain) and 5-HT1B receptors, modulated by assay conditions like magnesium ions or guanine nucleotides. For example, 5 mM MgSO4 increases 25IICYP affinity for 5-HT1B sites, while GTP or Gpp(NH)p reduces it, reflecting G-protein-mediated effects. In competition assays, agonists like isoproterenol displace 25IICYP with lower potency in the presence of GTP, indicating a shift from high- to low-affinity receptor states.

Non-specific binding remains a challenge, particularly in tissues with low receptor density or when studying β3-adrenoceptors. Studies suggest that compounds like SR 59,230A may compete for non-specific 25IICYP sites, complicating data interpretation. Researchers often use selective antagonists or alternative radioligands (e.g., HCGP12177) to validate findings.

History

Iodocyanopindolol was first characterized in the early 1980s as a radioligand for β-adrenoceptors, with its 125I derivative introduced by Brodde et al. for cardiac receptor studies. Its dual affinity for β-adrenoceptors and 5-HT1B receptors was later exploited in neuroscience, particularly in the 1990s, to investigate serotonin receptor regulation in rat models. The compound’s development built on pindolol’s established pharmacology, enhancing its utility through radiolabeling for precise receptor mapping.

Despite its research prominence, iodocyanopindolol has not been developed for clinical use due to its lack of therapeutic specificity and the availability of more targeted β-blockers like atenolol or metoprolol. Its radiolabelled form remains a niche tool, valued for its high sensitivity in experimental settings.

See also

* Pindolol
* Cyanopindolol
* Beta-adrenergic receptor
* 5-HT1A receptor
* 5-HT1B receptor
* Radioligand

References

External links

ChEBI – Iodocyanopindolol

IUPHAR/BPS Guide to Pharmacology – Iodocyanopindolol

{{Serotonin receptor modulators

5-HT1A antagonists
Beta blockers
Indoles
Iodoarenes
N-tert-butyl-phenoxypropanolamines
Nitriles