Synthesis and in vivo evaluation of 11C BU99008 as a ligand for the imidazoline 12 binding site

Background: Imidazoline−2 binding sites (I2BS) are thought to be mainly located on glial cells, and have prompted interest in neurodegenerative and neuro-inflammatory conditions where glia function is altered (e.g. Alzheimer's disease [1], glial tumors [2, 3] and Parkinson's disease [4, 5]). In vitro studies showed that compound BU99008 has a high affinity and selectivity towards the I2BS (Ki = 1.4 ± 0.8 nM) making it a potential candidate for PET labelling and in vivo investigations. This abstract reports the synthesis and initial preclinical in vivo characterization of [11C]BU99008. Methods: BU99008 was synthesised from indole 2-carboxylic acid in three steps. Methyl ester formation using standard conditions, followed by Lewis acid-promoted reaction with ethylene diamine provided the desmethyl labelling precursor, BU99007. [11C]BU99008 was obtained by alkylation of BU99007 using [11C]methyl iodide. Purification by semi-preparative HPLC and reformulation gave the desired product in good yield (1.87 ± 0.4 GBq), specific activity (93 ± 61 GBq/µmol) and purity (> 99%). Three subsequent [11C]BU99008 productions were intravenously administered to a 40 kg pig anaesthetised with isofluorane as follows: scan 1) baseline; scan 2) 30 min post 0.3 mg/kg BU224 (a selective I2BS blocker); and scan 3) 30 min post 3 mg/kg BU224. CNS and arterial plasma radioactivity concentrations were measured from 0 to 90 min post i.v. tracer administration. Parent radiotracer and radiometabolites were analysed by HPLC. Results and conclusion: [11C]BU99008 rapidly entered the CNS, reaching peak concentrations at under 10 min post i.v. administration, accumulating in brain regions believed to be rich in I2BS (Fig. 1). The parent radiotracer represented 20–40% of plasma radioactivity at 60–90 min post tracer administration. The CNS distribution of the tracer was altered post increasing doses of blocker. The initial data indicates that further investigation of [11C]BU99008 as a potential I2BS PET ligand is warranted.