Pre-clinical validation of a novel alpha-7 nicotinic receptor radiotracer, [(3)H]AZ11637326: target localization, biodistribution and ligand occupancy in the rat brain.

The alpha-7 neuronal nicotinic receptor is a novel pharmacological target for psychiatric and cognitive disorders. Selective radiotracer tools for pre-clinical receptor occupancy can facilitate the interpretation of the biological actions of small molecules at a target receptor. We discovered a high affinity nicotinic alpha-7 subtype-selective ligand, AZ11637326, with physical-chemical and pharmacokinetic properties suitable for an in vivo radioligand tool. [(3)H]AZ11637326 synthesis by tritiodehalogenation of the corresponding tribromide precursor yielded a high specific activity radiotracer with high affinity alpha-7 receptor binding in the rat hippocampus determined by autoradiography (Kd = 0.2 nM). When [(3)H]AZ11637326 was administered to rats by intravenous bolus, rapid uptake was measured in the brain followed by a 3-4 fold greater specific binding in regions containing the alpha-7 receptor (frontal cortex, hippocampus, hypothalamus and midbrain) when compared to non-target regions (striatum and cerebellum). Systemic administration of the high affinity alpha-7 receptor antagonist, methyllycaconitine (MLA), or pretreatment with alpha-7 selective agonists (AR-R17779, PyrQTC, DBCO-4-POM, and DBCO-3-POM) significantly blocked the alpha-7 specific binding of [(3)H]AZ11637326 in the rat brain. The rank order of ligand ED(50) values for in vivo alpha-7 receptor occupancy in rat hippocampus was: DBCO-4-POM > DBCO-3-POM âˆ¼ MLA > PyrQTC > AR-R17779. The occupancy affinity shift was consistent with in vitro binding affinity in autoradiography. Our studies established the optimal conditions for [(3)H]AZ11637326 in vivo specific binding in the rat brain and support the use of [(3)H]AZ11637326 as a pre-clinical tool for assessment of novel alpha-7 compounds in drug discovery.

Bioavailability of dietary doses of 3H-labelled tea antioxidants (+)-catechin and (-)-epicatechin in rat.

1. The bioavailability and pharmacokinetic characteristics of the tea antioxidants (+)-catechin and (-)-epicatechin were investigated in the rat following intake of dietary doses. 2. To achieve this objective, tritiated derivatives (tritium was incorporated at the 3-position of the heterocyclic ring) of these compounds were administered to rats orally and intravenously at dose levels equivalent to human dietary levels of intake. 3. Following intravenous administration of both compounds, about one-third of the dose was excreted in the urine and two-thirds in the faeces, indicating extensive biliary excretion. When the same doses were administered orally, only about 5% of the dose of each compound was recovered in the urine. 4. Comparison of the areas under the curve following oral and intravenous administration revealed that the bioavailability of both compounds was less than 5%. 5. Exchange of tritium with water in the blood occurred 3 h after oral, but not after intravenous, administration of the flavanols to rat. This is believed to represent microbial degradation of the compounds by the gut flora. 6. It was established that the bioavailability of the tea antioxidants (+)-catechin and (-)-epicatechin in the rat following intake of dietary doses was poor.