RADIOTRACER BINDING TO BRAIN MICROSOMES DETERMINED BY THIN-LAYER CHROMATOGRAPHY

A thin-layer chromatography (TLC) assay was developed to monitor the i nteraction of radiotracers with brain microsomes. Murine brain microso mes were coated onto a zone of a TLC strip, the unreacted sites blocke d with gelatin, and the radiotracers chromatographed over the microsom es. Radiotracers bound to the microsomes and were separated from the u nreacted materials which migrated at or near the solvent front. Up to 80% of the applied radioactivity bound to the brain microsomes when us ing Tc-99m-(d,l) hexamethyl-propyleneamine oxime (HMPAO) and thoxy-N-[ (1-ethyl-2-pyrrolidinyl)methyl]-benzamide (I-123-IBZM) as tracers. On the other hand, the presumptive negative control materials p-I-15-phen yl-pentadecanoic acid-I-123 (I-123-IPPA) and Tc-99m-mercapto-acetyl tr iglycine (MAG3) bound poorly (7% and 4%, respectively). Tc-99m-ethyl c ysteinate dimer (ECD) interacted poorly (9.9%), a result thought to be consistent with its known inability to be metabolized by nonprimate b rain tissue. Radiolabeled octreotide analogues (radiolabeled with In-1 11, I-131 or Tc-99m) also bound, and the binding could be reduced by e xcess unlabeled octreotide. Also, chemical modification by acylation o f Lys(5) in In-111-labeled octreotide led to decreased binding (approx imately 70%) compared to the original radiotracer. Chromatography of t he various radiotracers over TLC strips coated only with gelatin was u sed to monitor nonspecific binding and was low and frequently below 5% . This technique does not require wash steps or centrifugation, and as says are rapidly completed. The assay could be useful in monitoring th e interaction of radiotracers with brain microsomes and in evaluating and developing new radiotracers.