Enhanced Therapeutic Effect of Liposomal Doxorubicin via Bio-Orthogonal Chemical Reactions in Tumors

Liposomes are highly biocompatible drug carriers in drug delivery systems (DDSs). Preferential accumulation ofliposomes and acceleration of drug release at target tumor sitesare essential for effective cancer therapy using liposomalformulations; however, conventional liposomes are unsuitable foron-demand drug release. We have previously reported that drugrelease can be acceleratedviaa bio-orthogonal inverse electrondemand Diels-Alder (IEDDA) reaction between amphiphilictetrazine (Tz)-containing liposomes and norbornene (NB)derivativesin vitro. In this study, we prepared HSTz-liposomescomposed of hydrogenated soybean phosphatidylcholine (HSPC) and Tz compound (2-hexadecyl-N-(6-(6-(pyridin-2-yl)-1,2,4,5-tetrazin-3-yl)pyridin-3-yl)octadecanamide) with particle sizes of 60-80 nm and zeta-potentials of-5 to 0 mV. Similar to our previousreport, the addition of 5-norbornene-2-carboxylic acid (NBCOOH) to HSTz-liposomes accelerated drug release from the liposomesin vitro. In the biodistribution study using colon26 tumor-bearing mice, the radiolabeled HSTz-liposomes were accumulated andretained in the tumor at 6-48 h post-injection, whereas the radioactivity in the blood almost disappeared at 48 h. Therefore, thetiming of the injection of NBCOOH was selected to be 48 h after the injection of the HSTz-liposome to avoid the IEDDA reactionin the bloodstream. We investigated thein vivodrug release by evaluating the intratumoral localization of doxorubicin (DOX)encapsulated in HSTz-liposomes labeled withfluorescent lipids. In the tumors treated with HSTz-liposomes and NBCOOH, DOXwas more widely dispersed in the tumor compared withfluorescent lipid, suggesting that the release of encapsulated drugs (DOX)from HSTz-liposomes was enhanced in the tumor tissueviathe bio-orthogonal IEDDA reaction. Furthermore, the combination ofDOX-encapsulated HSTz-liposomes with NBCOOH significantly suppressed tumor growth compared to conventional DOX-encapsulated liposomes. In conclusion, the bio-orthogonal IEDDA reactions in the liposomal membrane enabled the acceleration ofdrug release from HSTz-liposomesin vivo, suggesting a promising strategy for effective cancer therapy

Automated summary

Accelerated drug release from liposomes in tumor tissue was achieved using a bio-orthogonal IEDDA reaction. The combination of DOX-encapsulated HSTz-liposomes with NBCOOH showed significantly suppressed tumor growth compared to conventional DOX-encapsulated liposomes.