Evaluation of BBB Permeable Nucleolipid (NL): A Di-C15-ketalised Palmitone Appended Uridine As Neuro-tracer for SPECT

Despite being in routine for onco-diagnostics for years, the applicability of nucleosidic molecular imaging probes is severely restricted in neurological applications due to their low permeability across blood-brain-barrier (BBB). For extending nucleoside tracers utility for neuro-onco early diagnostics, suitable modification which enhances their BBB permeation needs investigation. Among various modifications, lipidization of nucleosides has been reported to enhance cellular permeability. Extending the concept, the aim was to exemplify the possibility of lipidized nucleosides as potential brain tracer with capability to cross intact BBB and evaluate as metal based neuro-imaging SPECT agent. Uridine based non-lipidic (NS) and di-C15-ketal appended lipidic (NL) ligands were conjugated to chelator, DTPA (DTPA-NS and DTPA-NL) using multi-step chemistry. The ligands were evaluated in parallel for comparative physical and biological parameters. Additionally, effects of enhanced lipophilicity on UV-absorption, acid strength, fluorescence and non-specific protein binding were evaluated. Fluorescence quenching of BSA indicated appreciable interaction of DTPA-NL with protein only above 10 mM without inducing conformational changes. In addition, DTPA-NL was found to be haematocompatible and cytocompatible with low dose-dependent toxicity in HEK-cells. The chelator DTPA was used for Tc-complexation for SPECT imaging. Optimized Tc-radiolabeling parameters resulted in quantitative (≥97%) labeling with good stability parameters in in-vitro serum and cysteine challenge studies. We demonstrate that the nucleolipid radiotracer (Tc-DTPA-NL) was successfully able to permeate the BBB with brain uptake of 0.2% ID/g in normal mice as compared to 0.06% ID/g uptake of Tc-DTPA-NS at 5 min. Blood kinetics indicate biphasic profile and t1/2(distribution) 46 min for Tc-DTPA-NL. The preferential accumulation of Tc-DTPA-NL in brain tumor intracranial xenograft indicate the targeting capability of the nucleoside. We conclude that as first-of-its-kind, this work presents the potential of the biocompatible nucleolipidic system for brain targeting and early diagnostics.