Strategies for triggered drug release from tumor targeted liposomes.

INTRODUCTION: Long circulating liposomal drug carriers are widely used in experimental cancer therapy because they avoid excretion and benefit from the enhanced permeability and retention-effect to accumulate at the tumor site while simultaneously limiting systemic exposure to the cytotoxic drug due to their high stability. A drawback of the stability of the formulation is that the unloading of the drug at the target site is very poor. This opens up a new challenge to trigger drug release at the target site, while still retaining most of the drug inside the carrier while it resides in the bloodstream. AREAS COVERED: A short introduction is given about lipid polymorphism and phase behavior. To illustrate how this can be used to design triggered release systems, the development of delivery systems that are activated by tumor environment, UV or visible light and mild heat are discussed. The most recent triggered release systems have evolved even further, creating a need for more sophisticated triggers, which are as non-invasive and patient friendly as possible. EXPERT OPINION: Currently the most promising triggered release systems that have advanced furthest are thermosensitive liposomal delivery systems. As mild hyperthermia also increases tissue permeability it appears a suitable trigger for drug release while it also assists in drug accumulation. Combined with an advanced imaging system in the MR-high intensity focused ultrasound, this could be the combination of delivery system and trigger that can achieve clinical success.