Tumor priming enhances siRNA delivery and transfection in intraperitoneal tumors.

Cancers originating from the digestive system account for 290,000 or ~20% of all new cancer cases annually in the US. We previously developed paclitaxel-loaded tumor-penetrating microparticles (TPM) for intraperitoneal (IP) treatment of peritoneal tumors (Lu et al., 2008; Tsai et al., 2007; Tsai et al., 2013). TPM is undergoing NIH-supported IND-enabling studies for clinical evaluation. The present study evaluated the hypothesis that TPM, via inducing apoptosis and expanding the interstitial space, promotes the delivery and transfection of lipid vectors containing siRNA. The in vivo model was the metastatic human Hs766T pancreatic tumor that, upon IP injection, produced widely distributed solid tumors and ascites in the peritoneal cavity in 100% of animals. The target gene was survivin, an anti-apoptotic protein induced by chemotherapy and associated with metastases and poor prognosis of patients with gastric and colorectal cancers. The siRNA carrier was pegylated liposomes comprising cationic and neutral lipids plus a fusogenic lipid (PCat). PCat-loaded with survivin siRNA (PCat-siSurvivin) was active in cultured cells (decreased survivin mRNA and protein levels, reduced cell clonogenicity, enhanced paclitaxel activity), but lost its activity in vivo; this difference is consistent with the well-known problem of inadequate delivery and transfection of siRNA in vivo. In comparison, single agent TPM prolonged animal survival and, as expected, induced survivin expression in tumors. Addition of PCat-siSurvivin reversed the TPM-induced survivin expression and enhanced the antitumor activity of TPM. The finding that in vivo survivin knockdown by PCat-siSurvivin was successful only when it was given in combination with TPM provides the proof-of-concept that tumor priming promotes the delivery and transfection of liposomal siRNA. The data further suggest the TPM/PCat-siSurvivin combination as a potentially useful chemo-gene therapy for peritoneal cancer.