Factors that control the circulation time of nanoparticles in blood: challenges, solutions and future prospects.

Numerous types of nanoparticles are being designed for systemic and targeted drug delivery. However, keeping nanoparticles in blood for sufficiently long times so as to allow them to reach their therapeutic target is a major challenge. Upon administration into blood, nanoparticles are quickly opsonized and cleared by the macrophages, thereby limiting their circulation times. Surface-modification of nanoparticles by PEG was developed as the first strategy to prolong nanoparticles circulation. While PEGylation has helped prolong particle circulation, it has several limitations including transient nature of the effect and compromised particle-target interactions. Accordingly, several other approaches have been developed to prolong nanoparticle circulation in blood. These include modification with CD47, modulation of mechanical properties, engineering particle morphology and hitchhiking on red blood cells. In this review, we discuss the factors that affect nanoparticles circulation time and discuss recent progress in development of strategies to prolong circulation time.