Noninvasive Molecular Imaging of MYC mRNA Expression in Human Breast Cancer Xenografts with a [<SUP>99m</SUP>Tc]Peptide−Peptide Nucleic Acid−Peptide Chimera

Human estrogen receptor-positive breast cancer cells typically display elevated levels of Myc protein due to overexpression of MYC mRNA, and elevated insulin-like growth factor 1 receptor (IGF1R) due to overexpression of IGF1R mRNA. We hypothesized that scintigraphic detection of MYC peptide nucleic acid (PNA) probes with an IGF1 peptide loop on the C-terminus, and a [^99mTc]chelator peptide on the N-terminus, could measure levels of MYC mRNA noninvasively in human IGF1R-overexpressing MCF7 breast cancer xenografts in nude mice. We prepared the chelator-MYC PNA-IGF1 peptide, as well as a 4-nt mismatch PNA control, by solid-phase synthesis. We imaged MCF7 xenografts scintigraphically and measured the distribution of [^99mTc]probes by scintillation counting of dissected tissues. MCF7 xenografts in nude mice were visualized at 4 and 24 h after tail vein administration of the [^99mTc]PNA probe specific for MYC mRNA, but not with the mismatch control. The [^99mTc]probes distributed normally to the kidneys, livers, tumors, and other tissues. Molecular imaging of oncogene mRNAs in solid tumors with radiolabel-PNA−peptide chimeras might provide additional genetic characterization of preinvasive and invasive breast cancers.