Binding potency of paroxetine analogues for the 5-hydroxytryptamine uptake complex

Area of Science:

• Medicinal Chemistry
• Neuroscience
• Radiopharmaceutical Chemistry

Background:

• Paroxetine is a potent inhibitor of serotonin (5-HT) reuptake.
• Understanding structure-affinity relationships is crucial for developing imaging agents.
• Radiolabeled compounds are essential for in-vivo imaging studies of neurotransmitter systems.

Purpose of the Study:

• To determine the structure-affinity relationship of paroxetine analogues.
• To identify positions on paroxetine amenable to derivatization for radiolabeling.
• To assess the potential of novel analogues for in-vivo imaging of the 5-HT uptake system.

Main Methods:

• In-vitro inhibition assays using [3H]paroxetine to determine inhibition constants (Ki).
• Synthesis and evaluation of methyl-substituted paroxetine analogues.
• Synthesis and evaluation of fluoromethyl and fluoroethyl paroxetine analogues.

Main Results:

• Potency of methyl-substituted analogues followed the order: 4'- ≈ 3'- ≈ 2''- > 2'- ≈ 1- > 5''- > 6''-methyl.
• Equipotent molar ratios (EPMR) for 4'-, 3'-, and 2''-methyl derivatives were 1.9, 2.2, and 2.2, respectively.
• EPMRs for 4'- and 2''-fluoromethyl analogues were 2.0 and 3.5; for 4'- and 2''-fluoroethyl analogues, 5.2 and 6.2, respectively. The 2''-fluoromethyl analogue showed instability.

Conclusions:

• Derivatization at the 4'- and 2''-positions of paroxetine can be achieved with moderate increases in affinity loss.
• Fluorine substitution, particularly fluoroethyl groups, significantly impacts affinity.
• The 4'-fluoroethyl analogue shows promise as a ligand for in-vivo imaging of the 5-HT uptake system, while the 2''-fluoromethyl analogue is unsuitable due to instability.