An injectable nanoparticle generator enhances delivery of cancer therapeutics

Abstract

The efficacy of cancer drugs is often limited because only a small fraction of the administered dose accumulates in tumors. Here we report an injectable nanoparticle generator (iNPG) that overcomes multiple biological barriers to cancer drug delivery. The iNPG is a discoidal micrometer-sized particle that can be loaded with chemotherapeutics. We conjugate doxorubicin to poly(L-glutamic acid) by means of a pH-sensitive cleavable linker, and load the polymeric drug (pDox) into iNPG to assemble iNPG-pDox. Once released from iNPG, pDox spontaneously forms nanometer-sized particles in aqueous solution. Intravenously injected iNPG-pDox accumulates at tumors due to natural tropism and enhanced vascular dynamics and releases pDox nanoparticles that are internalized by tumor cells. Intracellularly, pDox nanoparticles are transported to the perinuclear region and cleaved into Dox, thereby avoiding excretion by drug efflux pumps. Compared to its individual components or current therapeutic formulations, iNPG-pDox shows enhanced efficacy in MDA-MB-231 and 4T1 mouse models of metastatic breast cancer, including functional cures in 40-50% of treated mice.

Figure 1. iNPG-pDox characterization and pDox assembly and release from iNPG

(a) Schematic diagram depicting iNPG-pDox composition, pDox prodrug encapsulation, and pDox nanoparticle assembly and release from nanopores. (b) Z-series confocal microscopy imaging of the iNPG-pDox particles, highlighting the presence of pDox (red) within the nanopores of the silicon carrier particle (gray). Scale bar: 1 µm. (c) Three dimensional reconstruction following sagittal cross-sectioning of the iNPG-pDox particles, depicting pDox (red) within the nanopores of the silicon carrier particle (gray), as well as the presence of pDox nanoparticles (red) released from the microparticles. Scale bar: 1 µm. (d) AFM analysis of size distribution of pDox nanoparticles released from iNPG-pDox at pH 7.4. (e) Cryogenic TEM of pDox nanoparticles released from iNPG-pDox at pH 7.4. Scale bar: 150 nm. (f) Release of pDox or disassembled Dox from iNPG-pDox at pH 7.4 and pH 5.2 in 10% FBS. (g) Gel permeation chromatography (GPC) analysis on released pDox and disseminated Dox from iNPG-pDox. pDox was the predominant form at pH 7.4, whereas Dox was released from the polymer at pH 5.2. The inset represents release of Dox in different pH conditions following cleavage from pDox.

Figure 2. Accumulation of iNPG-pDox in metastatic MDA-MB-231 tumors

(a) Time-dependent tissue biodistribution based on Dox content in mice with MDA-MB-231 lung metastasis after administration of 6 mg/kg Dox, pDox NP, or iNPG-pDox. Differences and significance between iNPG-pDox and Dox or pDox NP were estimated by F-tests with Hommel’s adjustment of multiplicity. (b) iNPG-pDox accumulation in lung metastatic tumors by H&E staining (upper and middle panels). Presence of doxorubicin inside iNPG was visualized under fluorescence microscopy (bottom panel). Scale bar: 10 µm. (c) H&E histological evaluation of tumor nodules, demonstrating co-localization of iNPG-pDox with red blood cells and attachment on tumor microvessels. The red arrow points to an iNPG particle, and blue arrow points to red blood cells. The scale bar represents 5 µm. (d) CD31 staining for tumor microvessels (brown). The red arrow points to an iNPG particles. The scale bar represents 5 µm. (e) TEM analysis of iNPG attachment to microvessel walls in tumor-bearing lung tissues collected 24 h after iNPG-pDox treatment. The red arrow points to an iNPG particle, the yellow arrow points to a tumor cell, and the blue arrow points to a red blood cell inside the vessel. Scale bar: 2 µm. (f) Flow cytometry analysis of Dox distribution in CD31⁺ endothelial cells, HLA-ABC⁺ tumor cells, and HLA-ABC⁻/CD45⁻/CD31⁻ normal lung cells isolated from post-treatment mice. *: p<0.05; **: p<0.01

Figure 3. In vivo growth inhibition of lung metastatic MDA-MB-231 tumors, as well as a multidrug resistant cell line, following iNPG-pDox treatment

(a) Bioluminescence monitoring of MDA-MB-231 tumor metastasis in the lung. Nude mice were inoculated with MDA-MB-231 cells carrying a luciferase gene, divided into 6 treatment groups (n = 10 mice/group), and treated weekly with 3 mg/kg Dox or biweekly with 6 mg/kg Doxil, pDox, or iNPG-pDox for 6 weeks. Mice were maintained further thereafter to monitor survival. Images of 5 mice/group are shown. (b) Kaplan-Meier plot of animal survival with median survival time listed in the table. Differences in survival were evaluated by the log-rank test. A global test demonstrated a difference exists among the groups. Pairwise comparisons were performed to evaluate the advantage of iNPG-pDox formulation over the clinical formulations. (c) MTT cell viability assay of MDA-MB-231/MDR cells treated with Dox or pDox NP for 72 h. The inset represents Western blot analysis of P-gp expression in the parental MDA-MB-231 cells and cells transfected with a plasmid carrying the MDR1 gene. (d) Tumor growth in the lung, as measured based on bioluminescence and compared to that of the PBS control group, of mice inoculated with MDA-MB-231/MDR cells carrying a luciferase gene following biweekly administration of PBS, Dox, or iNPG-pDox at a dosage of 6 mg/kg. F-tests of the simple effects were applied to compare the effects of Dox and iNPG-pDox treatments at each time period. (e) Schematic diagram demonstrating the individual components of the iNPG-pDox construct, and the distinct biological barriers that each component is capable of overcoming following systemic administration.