Towards dual SPECT/optical bioimaging with a mitochondrial targeting, 99mTc(i) radiolabelled 1,8-naphthalimide conjugate.

A series of six different 1,8-naphthalimide conjugated dipicolylamine ligands (L1-6) have been synthesised and characterised. The ligands possess a range of different linker units between the napthalimide fluorophore and dipcolylamine chelator which allow the overall lipophilicity to be tuned. A corresponding series of Re(i) complexes have been synthesised of the form fac-[Re(CO)3(L1-6)]BF4. The absorption and luminescence properties of the ligands and Re(i) complexes were dominated by the intramolecular charge transfer character of the substituted fluorophore (typically absorption ca. 425 nm and emission ca. 520 nm). Photophysical assessments show that some of the variants are moderately bright. Radiolabelling experiments using a water soluble ligand variant (L5) were successfully undertaken and optimised with fac-[99mTc(CO)3(H2O)3]+. Confocal fluorescence microscopy showed that fac-[Re(CO)3(L5)]+ localises in the mitochondria of MCF-7 cells. SPECT/CT imaging experiments on naïve mice showed that fac-[99mTc(CO)3(L5)]+ has a relatively high stability in vivo but did not show any cardiac uptake, demonstrating rapid clearance, predominantly via the biliary system along with a moderate amount cleared renally.

Targeted cell imaging properties of a deep red luminescent iridium(iii) complex conjugated with a c-Myc signal peptide.

A nuclear localisation sequence (NLS) peptide, PAAKRVKLD, derived from the human c-Myc regulator gene, has been functionalised with a long wavelength (λ ex = 550 nm; λ em = 677 nm) cyclometalated organometallic iridium(iii) complex to give the conjugate Ir-CMYC. Confocal fluorescence microscopy studies on human fibroblast cells imaged after 18-24 h incubation show that Ir-CMYC concentrations of 80-100 µM promote good cell uptake and nuclear localisation, which was confirmed though co-localisation studies using Hoechst 33342. In comparison, a structurally related, photophysically analogous iridium(iii) complex lacking the peptide sequence, Ir-PYR, showed very different biological behaviour, with no evidence of nuclear, lysosomal or autophagic vesicle localisation and significantly increased toxicity to the cells at concentrations >10 µM that induced mitochondrial dysfunction. Supporting UV-visible and circular dichroism spectroscopic studies show that Ir-PYR and Ir-CMYC display similarly low affinities for DNA (ca. 103 M-1), consistent with electrostatic binding. Therefore the translocation and nuclear uptake properties of Ir-CMYC are attributed to the presence of the PAAKRVKLD nuclear localisation sequence in this complex.