Effects of configuration on the myocardial uptake of radioiodinated 3(R)-BMIPP and 3(S)-BMIPP in rats

Radioiodinated 3(R)-(+)- and 3(S)-(-)-1 5-(p-iodophenyl)-3-(R,S)-methylpentadecanoic acid (BMIPP) were prepared and evaluated in rats to investigate the effects of absolute configuration of the 3(β)-methyl group on myocardial uptake and release kinetics. Methods: The 3(R)-(+)-BMIPP analog was synthesized by initial acylation of a thiophene template with the acid chloride of ethyl 3(R)-methylglutarate. 3(S)-(-)-BMIPP was obtained by separation from the mixture of diastereomeric amides prepared from reaction of the acid chloride of racemic BMIPP with the S-(-)-α-methylbenzylamine. The amide of synthetic 3(R)-BMIPP prepared from S-(-)-α-methylbenzylamine was identical to the chromatographically more polar isomer. Free acids were obtained by acid hydrolysis of the amides, fully characterized and then converted to the radioiodinated BMIPP isomers. Results: Biodistribution studies in rats with the dual-labeled [ 131 I] 3(S)-BMIPP/[ 125 I]-3(R)-BMIPP mixture demonstrated greater myocardial uptake of 3(R)-BMIPP compared with the 3(S)-BMIPP isomer [60 min: 3(R)-BMIPP = 4.37 %ID/g; 3(S)-BMIPP = 3.44; p < 0.05; 180 min, 2.31 and 1.78 %ID/G, respectively, p < 0.01], although both isomers had similar myocardial washout curves (5-180 min). Percent ID/g values for other tissues which were examined (blood, lungs, thyroid) were similar. Conclusion: Higher myocardial uptake of the 3(R)-BMIPP isomer observed in these animal studies may suggest differences in carrier-mediated myocyte uptake of the two isomers. These studies suggest that [ 123 I]-3(R)-BMIPP is a candidate for clinical evaluation and may show greater myocardial uptake than the 3(S)-BMIPP isomer and may thus require reduced injected dose.