Errors introduced in radioligand binding studies due to displaceable, cation dependent, [3H]prazosin binding to glass-fibre filters and glass surfaces

Binding of cationic cell-permeable peptides to plastic and glass

Cell-penetrating peptides derived from hydrophilic regions of the homeoprotein Antennapedia (Antp) or the transcription-regulating factor Tat have been used to transport several peptide and oligonucleotide cargoes into the interior of cells. Such vector peptides penetrate cells, in part, because they contain multiple lysine and arginine residues. Using radiolabeled peptide cargoes covalently linked to Antp- or Tat-related vectors, or to D-Arg heptamers, we found that a significant amount of the label remained tightly bound to plastic and glass surfaces. Binding of the labeled conjugates was due entirely to the cationic vector moieties. Under certain conditions, such non-specific binding could be mistaken for cellular penetration.

An in vivo binding assay to determine central alpha(1)-adrenoceptor occupancy using [(3)H]prazosin

An alpha(1) adrenoceptor (alpha(1)-AdR) assay using [(3)H]prazosin binding in mouse brain is described which allows in vivo determination of central alpha(1)-AdR occupancy for ligands with alpha(1)-AdR affinity. Binding of [3H]prazosin in rat and mouse brain membranes in vitro was used to characterise the pharmacological profile of alpha(1)-AdRs in order to determine any potential species variations. Saturation and displacement studies yielded comparable affinity and pharmacological profile for [(3)H]prazosin binding in mouse and rat brain homogenates. These studies confirmed the absence of species variation for ligands in central alpha(1)-AdR pharmacology which is in good agreement with previous studies in rat brain. Subsequently, in vivo binding of [(3)H]prazosin in mouse whole brain was used to measure the occupancy of a number of AdR ligands. Timecourse studies revealed that a [3H]prazosin (5 mu Ci/mouse) pretreatment time of at least 20 min following intravenous (i.v.) administration was required for optimal specific binding. Ligands were administered systemically 40 min prior to i.v. administration of radiolabel. The alpha(1)-adrenoceptor ligands prazosin (ED(50)=0.15 mg/kg i.p.), benoxathian (0.52 mg/kg i.p.) and phentolamine (51 mg/kg i.p.) were all able to block in vivo [(3)H]prazosin binding from mouse brain. In addition, receptor occupancy values for a number of compounds including haloperidol (ED(50)=0.83 mg/kg s.c.), clozapine (2.2 mg/kg s.c.) and MDL-100907 [R(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidine-methanol], (10 mg/kg s.c.)], which possess high to moderate affinity at alpha(1)-adrenoceptors, were also determined. These results suggest that in the mouse, [(3)H]prazosin binding can be used to measure in vivo receptor occupancy of ligands with affinity at central alpha(1)-adrenoceptors.

Synthesis of N-substituted 4-(4-hydroxyphenyl)piperidines, 4-(4-hydroxybenzyl)piperidines, and (+/-)-3-(4-hydroxyphenyl)pyrrolidines: selective antagonists at the 1A/2B NMDA receptor subtype

Antagonists at the 1A/2B subtype of the NMDA receptor (NR1A/2B) are typically small molecules that consist of a 4-benzyl- or a 4-phenylpiperidine with an omega-phenylalkyl substituent on the heterocyclic nitrogen. Many of these antagonists, for example ifenprodil (1), incorporate a 4-hydroxy substituent on the omega-phenyl group. In this study, the position of this 4-hydroxy substituent was transferred from the omega-phenyl group to the benzyl or phenyl group located on the 4-position of the piperidine ring. Analogues incorporating pyrrolidine in lieu of piperidine were also prepared. Electrical recordings using cloned receptors expressed in Xenopus oocytes show that high-potency antagonists at the NR1A/2B subtype are obtained employing N-(omega-phenylalkyl)-substituted 4-(4-hydroxyphenyl)piperidine, 4-(4-hydroxybenzyl)piperidine, and (+/-)-3-(4-hydroxyphenyl)pyrrolidine as exemplified by 21 (IC(50) = 0.022 microM), 33 (IC(50) = 0.059 microM), and 40 (IC(50) = 0.017 microM), respectively. These high-potency antagonists are >1000 times more potent at the NR1A/2B subtype than at either the NR1A/2A or NR1A/2C subtypes. The binding affinities of 21 at alpha(1)-adrenergic receptors ([(3)H]prazosin, IC(50) = 0.54 microM) and dopamine D2 receptors ([(3)H]raclopride, IC(50) = 1.2 microM) are reduced by incorporating a hydroxy group onto the 4-position of the piperidine ring and the beta-carbon of the N-alkyl spacer to give (+/-)-27: IC(50) NR1A/2B, 0.026; alpha(1), 14; D2, 105 microM. The high-potency phenolic antagonist 21 and its low-potency O-methylated analogue 18 are both potent anticonvulsants in a mouse maximal electroshock-induced seizure (MES) study (ED(50) (iv) = 0.23 and 0.56 mg/kg, respectively). These data indicate that such compounds penetrate the blood-brain barrier but their MES activity may not be related to NMDA receptor antagonism.