Abstract B115: Pharmacokinetics and biodistribution analysis using near-infrared emissive polymersomes.
Authors: Hofmann Christina L. , Qi Wei , Gill Taylor , Ghoroghchian Peter , Palmer Greg M. , Therien Michael J. , Dewhirst Mark W. 
Published: June 2012

Introduction: Near-infrared (NIR) emissive polymersomes are bi-layered vesicles that contain membrane-incorporated multi-porphyrin fluorophores for in vivo imaging. The high emission dipole strength NIR fluorescence of these particles allows for deeper tissue imaging relative to conventional fluorophores, creating a noninvasive technique for analysis of biodistribution and pharmacokinetics (PK) of nano-scale systems - a significant improvement over current methodology. Due to controllable membrane properties, both hydrophilic and hydrophobic compounds can be incorporated into polymersomes, offering application in tumor imaging and drug delivery.

Methods: Poly(ethylene oxide)-block-polycaprolactone (PEO-b-PCL) copolymers were used to form polymersomes through thin film hydration followed by sonication, freeze/thaw cycles, and membrane extrusion. An ethyne-bridged zinc porphyrin trimer (ex = 772; em = 806nm) was incorporated into these films prior to hydration. High MW PEO(2kDa)-b-PCL(14.6kDa) and low MW PEO(1kDa)-b-PCL(4kDa) formed vesicles of 250nm and 200nm diameters, respectively.

Results: Epithelial, endothelial, and liver cells were treated with PEO-b-PCL polymersomes to study the effects on proliferation and viability. These vesicles were found to be non-toxic to all three cell types.

In vivo studies were carried out on balb/c mice by i.v. injection of NIR emissive polymersomes. A LI-COR Pearl® Impulse was used for real-time imaging during and after treatment, demonstrating that polymersomes could be successfully imaged deep within the mouse, due to minimal NIR scattering and absorbance and strong emission of the multi-porphyrin within the polymersome membrane. Upon injection of the polymersomes, fluorescence was detected immediately within the vasculature of the mice. Over time, fluorescence localized within the spleen and liver, as expected due to the mononuclear phagocytic system. Ex vivo biodistribution results confirmed these in vivo imaging data, with all other organs having at least an order of magnitude lower intensity.

Fluorescence intensities in the spleen, liver, and femoral artery were quantified for at least 50 time points over 6 or 24 hours. Mathematical modeling was used to evaluate the intensity changes in these tissues and to estimate circulation half-lives from the change in fluorescence of the femoral artery. PK analysis of PEO(2k)-b-PCL(14.6k) 250nm polymersomes unexpectedly demonstrated circulation half-lives varying from 0.5–10 hours, perhaps due to particle aggregation. PEO(1k)-b-PCL(4k) 200nm polymersomes, on the other hand, provided consistent, yet much lower than anticipated, half-lives of 0.25 hours. This short circulation time may be due to the smaller PEO length, reducing the “stealth” characteristic of the particles.

Conclusions: NIR-emissive polymersomes were found to be non-toxic to various cell lines, and were successfully used to analyze vesicle biodistribution and PK in real-time in vivo. It is expected that once a tumor model is added, the polymersomes will accumulate within the tumor perivascular space due to the EPR effect. In addition to NIR imaging agents, both water soluble and insoluble compounds can be incorporated into these structures, thus providing a nano-scale technology for tumor imaging and drug delivery.

This work was supported by NIH training grant 5T32GM008555-18.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr B115.