Efficient Drug Release from Liposomes Introduced Tetrazine Derivatives with Pyrimidine Rings Using Click Chemistry

We developed a novel drug release method using a bioorthogonal inverse electron demand Diels-Alder reaction on liposomal membranes. Based on reports that replacing pyridine with pyrimidine in tetrazine derivatives improves the reaction rate with strained dienophiles, we investigated if liposomes with tetrazine derivatives containing pyrimidine rings efficiently release drugs via click chemistry. We synthesized and evaluated a tetrazine compound (Tz2) bearing a pyrimidine ring. The reaction rate constant of Tz2 with a norbornene (NB) derivative, 5-norbornenecarboxylic acid (NBCOOH), was higher than that of Tz1 with a pyridine ring. Liposomes containing the synthesized Tz2 (Tz2-liposomes) were prepared, and the reaction between Tz2 and NBCOOH on the liposomal membranes was confirmed using high-resolution mass spectrometry. We encapsulated indium-111-labeled diethylenetriaminepentaacetic acid ([¹¹¹In]In-DTPA) in liposomes as a model drug. The release of [¹¹¹In]In-DTPA from Tz2-liposomes was observed after the addition of NBCOOH, with release dependent on NBCOOH concentration. Moreover, release from Tz2-liposomes was significantly higher than that from Tz1-liposomes. These results suggested that tetrazine derivatives with pyrimidine rings efficiently released drugs, likely due to enhanced reaction rates. These findings would advance the development of controlled drug release methods using click chemistry.