123I-labeling and evaluation of Ro 43-0463, a SPET tracer for MAO-B imaging.

Using the copper assisted halogen exchange the MAO-B inhibitor Ro 43-0463, N-(2-aminoethyl)-5-iodo-2-pyridinecarboxamide, was labelled with 123I as well as with 125I to allow in vitro and in vivo investigations including SPET with healthy volunteers. Ro 43-0463 is known to inhibit reversibly and specifically MAO-B, having an IC50 of 3 x 10(-8) Mol/L. The labeling in the presence of CuSO4 and ascorbic acid was optimised, varying time (30 to 105 min), precursor concentration (1-3.5 mg) and temperature (130-200 degrees C). The labeling yield ranged between 60 and 70%. Purification was achieved with Lichrosorb RP-18 (5 micron, 250 x 8 mm) and 1.5 mL/min 0.36 M H3PO4/EtOH 97/3 [0.01 M (NH4)2HPO4]. After neutralisation and sterile filtration the final activity concentration ranged between 18.5 and 37 MBq/mL. Biodistribution studies showed a brain to blood ratio greater than 1 within 1 h p.i. The main radiation burden calculated from these animal data is to alimentary and excretory organs and the ovaries. Autoradiography was performed using rat brain slices and 5 nM [125I]Ro 43-0463 in TRIS-buffer pH 7.4 for 90 min at 20 degrees C. Its radioactivity pattern corresponds to the known distribution of MAO-B in the rat brain. By displacement with L-deprneyl the highly specific binding of R0 43-0463 was proven in vitro. SPECT studies with normal volunteers corresponded with the pattern found in autoradiography.

[123I/125I]-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 KD = 15.7 nmol/l to the enzyme monoamine oxidase type B (MAO-B) which it inhibits (IC50 = 2 *10(-8) mol/l) 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 degrees C). The purification of [123I/125I]-Ro 43-0463 was performed on HPLC (Lichrosorb RP-18, 5 mu m, 250 x 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 degrees 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 mu M), the high specificity of binding Ro 43-0463.

Biodistribution and estimated radiation burden for a new 123I-labelled monoamine oxidase-B inhibitor, Ro 43-0463: a preliminary report.

The selective, reversible inhibitor of monoamine oxidase B (MAO-B), Ro 19-6327 (Lazabamide) N-aminoethyl-5-chloro-picolinamide, inhibits the enzyme with an initial competitive phase, followed by a time-dependent inhibition of MAO-B; i.e. Ro 19-6327 is a substrate for MAO-B, and after its oxidation it is activated into an intermediate form which remains tightly bound to the enzyme's active site. Our radiopharmaceutical is a new 123I-labelled derivative of Ro 19-6327, N-aminoethyl-5-[123I]iodo-picolinamide, which seems to be a potentially useful SPECT tracer for the imaging of the MAO-B enzyme distributions. The first biodistribution of this compound was measured in rats at 6 different points in time (10, 25, 40, 60, 180 and 900 minutes post-injection). For each point the average from three animals was taken. In the brain there was an activity plateau over the first hour. In the first hour post-injection the brain-to-blood ratio was over 1, with a maximum ratio of 1.24 at 25 minutes post-injection. Because MAO-B is abundant in the ependyma, pineal and cerebellar Bergmann glia cells, this ratio of 1.24 over the whole brain is encouraging. At first, radioactivity was principally and rapidly accumulated in the liver. After 1 hour, about 37% of the injected activity is accumulated there. The elimination of the compound seemed to take place mainly through the hepatobiliary system (about 75%), but also via the kidneys (about 25%). Fifteen hours post-injection, only 4% (corrected for decay) of the injected radioactivity was left in the body.(ABSTRACT TRUNCATED AT 250 WORDS)