Injectable, Ultrasound-Responsive Biomaterials as Cancer Vaccines

Abstract

Cell-based cancer vaccines have the potential to eliminate tumors and prevent recurrence. However, their clinical success has been limited due to rapid loss of the transplanted cells and their effector function. Recently, an injectable, covalently crosslinked methacrylated (MA)-alginate cryogel vaccine has been developed that can recruit and activate host dendritic cells (DCs) in situ, and generate potent antitumor responses. However, this cryogel requires a large 16-gauge needle for delivery, and a portion of the gels break after injection. Additionally, 80% of the danger signal cytosine-phosphodiester-guanine-oligonucleotide (CpG-ODN) is released from the cryogel before peak DC recruitment. It is hypothesized that combining covalent and ionic crosslinking of alginate will lead to an injectable, tough cryogel platform capable of on-demand delivery of CpG-ODN via ultrasound to enhance cancer vaccination. Incorporating ionic crosslinking to the covalently crosslinked-only MA-alginate cryogels resulted in a tough cryogel with improved injectability. All tough cryogels could be injected through a smaller, 18-gauge needle without sustaining any damage both in vitro and in vivo, whereas all covalently crosslinked-only cryogels break after injection. The tough cryogel vaccine elicited strong antigen-specific cytotoxic T-lymphocyte (CTL) and humoral responses, and induced effective tumor-free protection against a murine breast cancer model. An ultrasound-triggered CpG-ODN release system was developed based on the tough cryogel. CpG-ODN was first condensed with polyethylenimine and then adsorbed onto tough cryogels. Adsorbed CpG-ODN was released extremely slowly in vitro. However, ultrasound stimulation enhanced CpG-ODN release in a sustained manner, with minimal burst release during ultrasound stimulation. In vivo, ultrasound stimulation four days after vaccination induced significantly higher antigen-specific CTL responses in mice than that in control mice; stimulation immediately or 8 days after vaccination induced little CTL responses. Stimulation on Day 4 also generated the highest IgG2a antibody response. This optimal timing of CpG-ODN release coincided with the peak DC recruitment to the tough cryogel. Ultrasound-responsive tough cryogel vaccines present a promising minimally invasive delivery platform for cancer vaccination. It provides a useful tool to study the optimal timing of the delivery of immunomodulatory agents for scaffold-based cancer vaccines, and more broadly, an external stimuli-responsive, minimal burst release drug delivery system.