Fluorescence quantification of nanoparticle diffusion from smart brachytherapy spacers in vivo

New implantable smart nanoparticle (NP) coated brachytherapy spacers have recently been developed which allows controlled, long-term release of chemotherapeutic drugs into tumors. However, kinetics of NP (drug) diffusion over time is still poorly understood, and new methods for controlling and optimizing this release are needed. In this work, we developed and validated a novel broad-field transmission fluorescence imaging system to observe NP diffusion in bulk biological tissue. This allowed us to systematically quantify diffusion both in phantoms in vitro and in mice in vivo. We performed in vitro studies with free dye and NPs of different sizes (30 nm and 200 nm) in agar gel phantoms and analyzed the diffusion over time. It was found that the free dye diffusion coefficient was orders of magnitude larger than both NP types verifying that diffusion could be controlled based on particle size. We also performed preliminary studies in mice, where functionalized brachytherapy spacers were implanted into the hind flanks of nude mice. Continuous diffusion of free dye and NPs was observed, with maximal diffusion areas observed on day 4 and day 6 for 30 nm and 200 nm NPs, respectively. Our fluorescence imaging system was capable of robustly quantifying this size-dependent diffusion. In the future, we will use it to optimize and control drug delivery from smart nanoparticles over time.