Synthesis and evaluation of a ligand targeting the μ and δ opioid receptors for drug delivery to lung cancer.

A well-established approach to developing new imaging agents and treatments for cancer begins with the recognition of receptors that are overexpressed in cancer cells. Ideally, these same receptors would also be absent, or minimally expressed, in healthy tissue. The mu (μ) and delta (δ) opioid receptors (MOR and DOR respectively) match these criteria, with expression in cancer cells that is higher than primary lung epithelial cells. Naltrexone is a drug approved by the U.S. Food and Drug Administration (FDA) for treatment of alcohol dependence or prevention of relapse from opioid addiction. Since naltrexone binds with high affinity to both MOR and DOR, it was selected as the platform for development of novel ligands capable of delivering a cytotoxic payload to non-small cell lung cancer (NSCLC). This study outlines the synthesis of two ligands, with peptide or PEG linkers that were synthesized from 6-amino-naltrexone and conjugated with rhodamine dye or 99mTc for in vitro imaging, binding affinity or in vivo imaging and biodistribution studies. Transfected HEK cells were used as a model system for over-expression of the μ-opioid receptor (MOR) or the δ-opioid receptor (DOR). Naltrexone and naltrindole were used as competition for MOR and DOR respectively during the binding affinity studies. Mice bearing a xenograft of HEK cells transfected with μ (HEK-mu) or δ (HEK-delta) opioid receptors were the animal model used for PET imaging and in vivo biodistribution studies. Although the binding affinity studies were encouraging, the biodistribution data for the selected conjugates lacked sufficient specificity. These conjugates were abandoned from further development but information about their synthesis may be valuable to other laboratories working in this field.