Characterization of the four isomers of 123I-CMICE-013: A potential SPECT myocardial perfusion imaging agent

Myocardial perfusion imaging (MPI) with single photon emission computed tomography (SPECT) is widely used in the assessment of coronary artery disease (CAD). We have developed 123I-CMICE-013 based on rotenone, a mitochondrial complex I (MC-1) inhibitor, as a promising new MPI agent. Our synthesis results in a mixture of four species of 123I-CMICE-013 A, B, C, D. In this study, we separated the four species and evaluated their biodistribution and imaging properties. The cold analogs 127I-CMICE-013 A, B, C, D were isolated and characterized and their chemical structures proposed. Methods:123I-CMICE-013 was synthesized by radiolabeling rotenone with Na123I in trifluoroacetic acid (TFA) with iodogen as the oxidizing agent at 60 °C for 45 min, and the four species were separated by RP-HPLC. The cold analogs 127I-CMICE-013 A, B, C and D were isolated with a similar procedure and characterized by NMR and mass spectrometry. Biodistribution and microSPECT imaging studies were carried out on normal rats. Results: We propose the mechanism of the rotenone iodination and the structures of the four species. First, I+ forms an intermediate three-membered ring with 6′ and 7′ carbons. Second, the lone electron pair of the water molecule attacks the 6′ or 7′-carbon, following by the formation of 6′-OH, and 7′-I bonds as in major products C and D, or 6′-I and 7′-OH bonds as in minor products A and B. The weaker 6′-I bond in the intermediate prompts the nucleophilic attachment of water at the favorable 6′-carbon to generate C and D. MicroSPECT images of 123I-CMICE-013 A, B, C, D in rats showed clear visualization of myocardium and little interference from lung and liver. The imaging time activity curves and biodistribution data showed complex profiles for the four isomers, which is not expected from the structure activity relationship theory. Conclusion:123/127I-CMICE-013 A and B are constitutional isomers with C and D, while A and C are diastereomers of B and D, respectively. Overall, the biological characteristics of the four species are not correlated perfectly with their molecular structures.