Bicycloheptylamine-doxorubicin conjugate: Synthesis and anticancer activities in σ₂ receptor-expressing cell lines

Background: Novel bicycloheptylamines were designed and synthesized. These compounds were found to be selective for sigma-2 receptors. These receptors have been found to be up to 10 fold over-expressed in certain cancer cell lines, leading to investigation of possible uses as a biomarker in diagnosis and/or treatment especially in cancers with poor prognosis. Objectives: The aim was to conjugate a novel sigma-2 receptor ligand to doxorubicin to examine anticancer activities, with and without conjugation, and therefore possibilities in drug delivery. Methods: Conjugation was conducted using N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide HCl as a coupling agent. Affinity towards the sigma-2 receptor was tested using ligand-receptor binding studies. Anticancer activities against cancer cell lines were carried out using cell viability assays. Caspase dependency was tested using Z-VAD-FMK, a pan-caspase inhibitor, to begin to investigate mechanisms of action. Results: The target compound retained affinity towards the sigma-2 receptor and exhibited potent anticancer activities on cancer cell lines expressing the sigma-2 receptor. The potencies exceeded those of doxorubicin, the lead sigma-2 receptor ligand, as well as non-covalent combination of both drugs. The activity was also found to be caspase-dependent. Conclusion: The conjugation of target bicycloheptylamines with cytotoxic moieties may yield potent and selective molecules for detection and/or treatment of certain cancers.