[I-123 I-125]-RO-43-0463, A SITE-SPECIFIC TRACER FOR MAO-B MAPPING WITH AUTORADIOGRAPHY AS WELL AS WITH SPET/

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 mon oamine oxidase type B (MAO-B) which it inhibits (IC50=210(-8) mol/l) time dependently and reversibly. Ro 43-0463 having an IC50 of 310(-8) mol/l was labelled with I-123 as well as with I-125 to get a tool for measuring the MAO-B distribution autoradiographically and in the huma n brain with SPET (Single Photon Emission Tomography). The halogen exc hange reaction of the bromo-precursor (Ro 18-4950) in the presence of CuSO4 and ascorbic acid was applied. The reaction conditions were opti mized, varying the parameters time (30 to 105 min), precursor concentr ation (1 to 3.5 mg) and temperature (130 to 200 degrees C). The purifi cation of [I-123/I-125]-Ro 43-0463 was performed on HPLC (Lichrosorb R P-18, 5 mu m, 250x8 mm) with 0.36 M H3PO4/EtOH 97/3 and 0.01 M (NH4)(2 )HPO4 (1.5 ml/min) as eluent. The labelling yield was found to range b etween 60 and 70%. The activity concentration ranged between 18.5 and 37 MBq/ml. Autoradiography with rat brain slices was performed using 5 nM [I-125]-Ro 43-0463 in TRIS-buffer (pH 7.4) for 90 min at 20 degree s C. It showed a radioactivity pattern corresponding to the known dist ribution of MAO-B in the rat brain and proved, after displacement with L-Deprenyl (1 mu M), the high specificity of binding Ro 43-0463.