Injectable liposomal formulations of opiorphin as a new therapeutic strategy in pain management

BACKGROUND: Conventional and PEGylated liposomes were developed, aimed at improving the pain-killing effect of opiorphin. METHODS: The antinociceptive action of the formulations was investigated on rats (tail-flick test), and compared with that of opiorphin and morphine aqueous solutions (all at 5 mg/kg). RESULTS: Opiorphin loading in conventional liposomes enabled a 28% AUC increase with respect to free peptide. PEGylated liposomes provided AUC values 80, 60 and 40% higher than free peptide, morphine and opiorphin-loaded conventional liposomes, respectively. Moreover, opiorphin entrapment in PEGylated liposomes increased analgesic effect duration by more than 50%. These results were attributed to the greater effectiveness of PEGylated liposomes in protecting the drug and prolonging its circulation time. CONCLUSION: Opiorphin-loaded PEGylated-liposomes can represent a valid alternative to morphine in pain management.