Hepatic fibrosis is the common pathway for most chronic liver diseases characterized by excessive accumulation of extracellular matrix (ECM) proteins [1,2]. If chronic injury and accumulation in ECM persists, it can progress into cirrhosis and hepatic carcinoma, leading to poor outcome and high mortality. Currently, there is no clinically proven therapy to reverse liver fibrosis other than liver transplantation for fibrosis that has progressed into cirrhosis, but which is limited by proper donors and post-transplantation immune responses [[3], [4], [5]]. Hydroxycamptothecin (HCPT), a topoisomerase I inhibitor, is a potential antifibrotic drug as proved by cell and animal experiments [6,7]. Although HCPT has been applied clinically in treating cancers, the side effects, such as toxicity to hearts and causing nausea, limit its application [[8], [9], [10]]. With consideration of poor solubility and stability of active form, nanosized liposomal vehicles have been adopted for the delivery of HCPT to reduce side effects, improve biodistribution and enhance therapeutic effects [[11], [12], [13]]. However, extracellular matrix (ECM) of liver fibrosis is special that the passage of nanosized liposomes may be hindered, thus reducing drug delivery efficiency.

With the progression of liver fibrosis, the accumulation of fibrotic proteins in ECM may increase to 3–10 times comparing to normal livers, accompanying with endothelial cells losing fenestration [2]. It has been shown by Edward A. Sykes et al. that the passage of gold nanoparticles larger than 60 nm was significantly hindered by fibrotic ECM [14]. To overcome the drug delivery barrier of liver fibrosis, researchers have developed nanosized vehicles decorated with enzymes degrading fibrotic proteins as nanodrill to assist passage through ECM and improve drug utilization [[15], [16], [17]]. However, these strategies relied on the activeness of enzymes, which require stringent storage conditions and have a limiting span of shelf-life.

Sonoporation is an attractive strategy as it assisted drug delivery through blood-brain barrier and tumor tissues with spatial control [[18], [19], [20], [21], [22]]. We investigated whether sonoporation can assist to bypass the barrier of fibrotic ECM and lead to synergistic effect in treating liver fibrosis. Even though sonoporation has been adopted for targeted delivery of gene therapy in treating liver fibrosis [[23], [24], [25]], the treatment scheme involved only one-time sonoporation. The effect of consecutive treatments of sonoporation and potential synergy were still unclear. In this study, HCPT was encapsulated in microbubbles and delivered to livers via sonoporation. Two other delivering strategies, i.e., injection solution of HCPT and HCPT in liposomes, were evaluated comparatively. The feasibility, biodistribution and therapeutic effect of consecutive sonoporation in treating liver fibrosis were investigated with in vitro and in vivo experiments.