Tactics for preclinical validation of receptor-binding radiotracers

Abstract

Introduction Aspects of radiopharmaceutical development are illustrated through preclinical studies of [125I]-(E)-1-(2-(2,3-dihydrobenzofuran-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA- BF-PE-PIPZE), a radioligand for sigma-1 (σ1) receptors, coupled with examples from the recent literature. Findings are compared to those previously observed for [125I]-(E)-1-(2-(2,3-dimethoxy-5-yl)ethyl)-4-(iodoallyl)piperazine ([125I]-E-IA-DM-PE-PIPZE). Methods Syntheses of E-IA-BF-PE-PIPZE and [125I]-E-IA-BF-PE-PIPZE were accomplished by standard methods. In vitro receptor binding studies and autoradiography were performed, and binding potential was predicted. Measurements of lipophilicity and protein binding were obtained. In vivo studies were conducted in mice to evaluate radioligand stability, as well as specific binding to σ1 sites in brain, brain regions and peripheral organs in the presence and absence of potential blockers. Results E-IA-BF-PE-PIPZE exhibited high affinity and selectivity for σ1 receptors (Ki = 0.43 ± 0.03 nM, σ2 / σ1 = 173). [125I]-E-IA-BF-PE-PIPZE was prepared in good yield and purity, with high specific activity. Radioligand binding provided dissociation (koff) and association (kon) rate constants, along with a measured Kd of 0.24 ± 0.01 nM and Bmax of 472 ± 13 fmol / mg protein. The radioligand proved suitable for quantitative autoradiography in vitro using brain sections. Moderate lipophilicity, Log D7.4 2.69 ± 0.28, was determined, and protein binding was 71 ± 0.3%. In vivo, high initial whole brain uptake, > 6% injected dose / g, cleared slowly over 24 h. Specific binding represented 75% to 93% of total binding from 15 min to 24 h. Findings were confirmed and extended by regional brain biodistribution. Radiometabolites were not observed in brain (1%). Conclusions Substitution of dihydrobenzofuranylethyl for dimethoxyphenethyl increased radioligand affinity for σ1 receptors by 16-fold. While high specific binding to σ1 receptors was observed for both radioligands in vivo, [125I]-E-IA-BF-PE-PIPZE displayed much slower clearance kinetics than [125I]-E-IA-DM-PE-PIPZE. Thus, minor structural modifications of σ1 receptor radioligands lead to major differences in binding properties in vitro and in vivo.

Multiple Site Hydrogen Isotope Labelling of Pharmaceuticals

“…[1][2][3] In drug discovery and development, tritiated drug candidates are notably used:1 )t o characterize receptors using radioligand binding assays; [4] 2) to provide qualitative and quantitative information on drug distribution and pharmacokinetics;3 )t oe lucidate drugs metabolic profile.F or some of these important applications, the synthesis of complex tritiated molecules with ahigh molar activity (beyond 50 Ci mmol À1 )i sm andatory. [5][6][7] Methods to incorporate tritium atoms into ad esired molecule are numerous, [8] but among them Hydrogen Isotope Exchange (HIE) is clearly the most appealing one as it allows al ate-stage functionalization at non-labile positions,r educing thus costs and time spent for the synthesis of the labelled compounds. [9] Tr ansition metal catalysis for CÀHa ctivation has recently known tremendous developments,allowing new HIE reactions to emerge.Among them, one of the most used catalyst for the labelling of pharmaceuticals is the Crabtrees iridium one,which gave tritium atoms incorporation mainly at the ortho position of ad irecting group under mild reaction conditions (see Figure 1a).…”

Design, synthesis and evaluation of 111In labeled DOTA-conjugated tetrapeptides having high affinity and selectivity for mu opioid receptors

“…In vitro, however, [ 3 H] [Dmt 1 ]DALDA is not sensitive to sodium ions or Gpp(NH)p alone, and can bind to antagonist as well as agonist states of μ opioid receptors [31]. These are important considerations for potential imaging radioligands, since agonists typically bind only a small fraction of the receptor conformations available [54][55][56]. [Dmt 1 ]DALDA pharmacology is almost unique among μ opioid receptor agonists, but another prominent exception is [ 11 C]carfentanil, a full agonist of μ opioid receptors, which displays a low sodium effect that contributes to its suitability for brain imaging [57,58].…”

Insight into the Development of PET Radiopharmaceuticals for Oncology

“…Radiopharmaceuticals may bind to different plasma proteins in blood during systemic circulation. Binding to these proteins can affect a radiopharmaceutical's half-life and distribution in tissue [217]. If a radiopharmaceutical is lipophilic, it has a higher propensity to associate with plasma proteins [218].…”