MS Binding Assays – An Alternative to Radioligand Binding

Receptor-Ligand Interaction Measured by Inductively Coupled Plasma Mass Spectrometry and Selenium Labeling

In the search for an alternative strategy to the radioactivity measurement conventionally performed to probe receptor-ligand interactions in pharmacological assays, we demonstrated that selenium labeling of the studied ligand combined with elemental mass spectrometry was as efficient and robust as the reference method but devoid of its environmental and health hazards. The proof-of-concept was illustrated on two GPCR receptors, vasopressin (V_1A) and cholecystokinin B (CCK-B), involving peptides as endogenous ligands. We proposed several methodologies to produce selenium-labeled ligands according to peptide sequences along with binding affinity constraints. A selection of selenopeptides that kept high affinities toward the targeted receptor were engaged in saturation and competitive binding experiments with subsequent sensitive RP-LC-ICP-MS measurements. Experimental values of affinity constant ( K_i) were perfectly correlated to literature data, illustrating the general great potency of replacing radioactive iodine by selenium for ligand labeling to further undergo unaffected pharmacology experiments efficiently monitored by elemental mass spectrometry.

Simultaneous Multiple MS Binding Assays Addressing D1 and D2 Dopamine Receptors

MS Binding Assays are a label-free alternative to radioligand binding assays. They provide basically the same capabilities as the latter, but use a non-labeled reporter ligand instead of a radioligand. In contrast to radioligand binding assays, MS Binding Assays offer-owing to the selectivity of mass spectrometric detection-the opportunity to monitor the binding of different reporter ligands at different targets simultaneously. The present study shows a proof of concept for this strategy as exemplified for MS Binding Assays selectively addressing D₁ and D₂ dopamine receptors in a single binding experiment. A highly sensitive, rapid and robust LC-ESI-MS/MS quantification method capable of quantifying both SCH23390 and raclopride, selectively addressing D₁ and D₂ receptors, respectively, was established and validated for this purpose. Based thereon, simultaneous saturation and competition experiments with SCH23390 and raclopride in the presence of both D₁ and D₂ receptors were performed and analyzed by LC-MS/MS within a single chromatographic cycle. The present study thus demonstrates the feasibility of this strategy and the high versatility of MS Binding Assays that appears to surpass that common for conventional radioligand binding assays.

MS Binding Assays for D1 and D5 Dopamine Receptors

MS Binding Assays are a label-free alternative to radioligand binding assays. They provide basically the same capabilities as the latter, but an unlabeled reporter ligand is used instead of a radioligand. The study presented herein describes the development of MS Binding Assays that address D1 and D5 dopamine receptors. A highly sensitive, rapid and robust LC-ESI-MS/MS quantification method for the selective D1 dopamine receptor antagonist SCH23390 ((5R)-8-chloro-3-methyl-5-phenyl-1,2,4,5-tetrahydro-3-benzazepin-7-ol) was established and validated, using its 8-bromo analogue SKF83566 as an internal standard. This quantification method proved to be suitable for the characterization of SCH23390 binding to human D1 and D5 receptors. Following the concept of MS Binding Assays, saturation experiments for D1 and D5 receptors were performed, as well as competition experiments for D1 receptors. The results obtained are in good agreement with results from radioligand binding assays and therefore indicate that the established MS Binding Assays addressing D1 and D5 receptors are well-suited substitutes for radioligand binding assays, the technique that has so far dominated affinity determinations toward these targets.

(S)- and (R)-fluoxetine as native markers in mass spectrometry (MS) binding assays addressing the serotonin transporter

A recently established and validated LC-ESI-MS/MS method for quantification of fluoxetine was used to implement MS binding assays for the human serotonin transporter (hSERT)-the primary target in the treatment of depression and emotional disorders. As a label-free screening technique, MS binding assays offer the opportunity to perform kinetic, saturation and competition assays using both (S)- and (R)-fluoxetine as native markers. In kinetic experiments, an association rate constant (k(+1) of 0.92±0.17×10(6) M(-1) s(-1) and a dissociation rate constant (k(-1)) of 0.0032±0.0002 s(-1) for (S)-fluoxetine binding to hSERT were determined. Saturation experiments provided K(d) values of 4.4±0.4 nM and 5.2±0.9 nM for (S)- and (R)-fluoxetine, respectively; statistical analysis revealed that the two enantiomers are equipotent. In competitive experiments with (S)-fluoxetine as a marker, K(i) values were obtained for various known inhibitors with a broad range of affinities for hSERT that correlate well with literature data obtained from radioligand binding experiments with [(3)H]imipramine. Additional competitive experiments using (R)-fluoxetine as a marker led to K(i) values for SERT inhibitors that deviate only marginally from those determined using the (S)-enantiomer. No changes in the rank order of affinities occurred, indicating that there is no difference in the binding characteristics of the two enantiomers.