[¹²³I/¹²⁵I]-RO 43-0463, A site specific tracer for MAO-B mapping with autoradiography as well as with spet

Abstract

The compound Ro 43-0463 [N-(2-aminoethyl)-5-iodo-2-pyridinecarboxamide)] is the iodo-analogue of Lazabemid (Ro 19-6327). The latter is well known to bind site specifically with K_D=15.7 nmol/l to the enzyme monoamine oxidase type B (MAO-B) which it inhibits (IC₅₀=2*10⁻⁸ mol/1) time dependently and reversibly. Ro 43-0463 having an IC₅₀ of 3*10⁻⁸ mol/l was labelled with ¹²³I as well as with ¹²⁵I to get a tool for measuring the MAO-B distribution autoradiographically and in the human brain with SPET (Single Photon Emission Tomography). The halogen exchange reaction of the bromo-precursor (Ro 18-4950) in the presence of CuSO₄ and ascorbic acid was applied. The reaction conditions were optimized, varying the parameters time (30 to 105 min), precursor concentration (1 to 3.5 mg) and temperature (130 to 200°C). The purification of [¹²³I/¹²⁵I]-Ro 43-0463 was performed on HPLC (Lichrosorb RP-18, 5μm, 250×8 mm) with 0.36 M H₃PO₄/EtOH 97/3 and 0.01 M (NH₄)₂HPO₄ (1.5 ml/min) as eluent. The labelling yield was found to range between 60 and 70%. The activity concentration ranged between 18.5 and 37 MBq/ml. Autoradiography with rat brain slices was performed using 5 nM [¹²⁵I]-Ro 43-0463 in TRIS-buffer (pH 7.4) for 90 min at 20°C. It showed a radioactivity pattern corresponding to the known distribution of MAO-B in the rat brain and proved, after displacement with L-Deprenyl (1 μM), the high specificity of binding Ro 43-0463.