Engineering multi-stage nanovectors for controlled degradation and tunable release kinetics

Abstract

Nanovectors hold substantial promise in abating the off-target effects of therapeutics by providing a means to selectively accumulate payloads at the target lesion, resulting in an increase in the therapeutic index. A sophisticated understanding of the factors that govern the degradation and release dynamics of these nanovectors is imperative to achieve these ambitious goals. In this work, we elucidate the relationship that exists between variations in pore size and the impact on the degradation, loading, and release of multistage nanovectors. Larger pored vectors displayed faster degradation and higher loading of nanoparticles, while exhibiting the slowest release rate. The degradation of these particles was characterized to occur in a multi-step progression where they initially decreased in size leaving the porous core isolated, while the pores gradually increased in size. Empirical loading and release studies of nanoparticles along with diffusion modeling revealed that this prolonged release was modulated by the penetration within the porous core of the vectors regulated by their pore size.

Keywords: Degradation; Drug delivery; Nanoparticle; Nanovector; Porosity; Porous silicon.

Fig. 1

MSV fabrication and characterization. A) Schematic of MSV fabrication showing order of procedures from photolithography and electrochemical etch to release of MSV (black arrow) under sonication. B) Solid works model of MSV used to approximate mass. C) Low magnification SEM micrograph of MSV illustrating uniform size and shape, scale bar = 1 μm. D) High magnification SEM micrograph of MSV with different pore sizes, starting from top left and proceeding clock-wise: SP, MP, LP, XLP; scale bar = 50 nm. E–F) Bar graphs comparing pore size (G) and volume (F) on the left axis and porosity (G) and mass (F) on the right axis for the MSV investigated. F) Bar graph comparing the surface charges of MSV after oxidization (stripped) and APTES (solid).

Fig. 2

MSV degrade in distinct times after pore size adjustment. SEM micrographs of MSV at various times emphasizing the impact pore size have on the overall changes in shape and size of MSV, scale bar = 1 μm. Contour and 3D plots of FSC versus SSC (z-axis is counts) acquired from flow cytometry confirm the observed changes in MSV size and shape over time. Black arrows indicate the peak of intact, or non-degraded, MSV while red arrows are used to specify where the degradation by-products accumulate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Qualitative and quantitative validation of degradation impact on MSV. A) SEM micrographs depict the degradation experienced by MSV over time on both their frontside and backside. Images were split to show the effect on the particle's shape and size (left, scale bar = 1 μm) and on its pores (right, scale bar = 100 nm). B) High magnification SEM images of MSV pores comparing SP & XLP at different points during degradation, scale bar = 100 nm. C) ICP–AES quantification of the concentration of silicon deposited in solution for each MSV. Values were then fitted with an exponential equation resulting in distinct degradation rates. D) Values extrapolated from the exponential fit to determine 50% and 95% degradation were plotted as pore size versus time. A linear straight fit was then applied to give an equation relating the two variables at each percentage of degradation.

Fig. 4

Loading of QD into MSV of varying pore sizes. A) Low magnification of scanning transmission electron microscopy images of MSV loaded with QD. B) High magnification images of the inset within A (red box), confirming the accumulation of QD (red arrows) at the surface (MP) or penetration within the porous matrix (LP, XLP). C) QD loading quantification within the different MSV tested. Each MSV demonstrated a highly significantly different ability to load QD (***p < 0.001). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 5

Release of QD from MSV. A) The release of QD was investigated and plotted with an exponential fit producing distinct release rates for each MSV. B) Values extrapolated from the exponential fit to determine 50% and 95% release were plotted as pore size versus time. A linear straight fit was then applied to give an equation relating the two variables at each percentage.

Fig. 6

Mechanism of MSV degradation and modeling release based on payload penetration. A) Schematic illustrating the proposed degradation mechanism of MSV. B) A diagram depicting the parameters and assumptions used to develop the diffusion model. The diagram shows that QD (green spheres) penetrate deeper in XLP and that the concentration of QD decreases as they approach the pore opening. C) The resulting data from the model is plotted against the actual values corresponding to their pore size and pore penetration of 18%, 40% and 100% for MP (15 nm), LP (26 nm), and XLP (51 nm) respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)