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. 2012 Aug 14;1(8):e37.
doi: 10.1038/mtna.2012.28.

Microfluidic Synthesis of Highly Potent Limit-size Lipid Nanoparticles for In Vivo Delivery of siRNA

Nathan M Belliveau  1 , Jens Huft, Paulo Jc Lin, Sam Chen, Alex Kk Leung, Timothy J Leaver, Andre W Wild, Justin B Lee, Robert J Taylor, Ying K Tam, Carl L Hansen, Pieter R Cullis
Affiliations

Microfluidic Synthesis of Highly Potent Limit-size Lipid Nanoparticles for In Vivo Delivery of siRNA

Nathan M Belliveau et al. Mol Ther Nucleic Acids. .

Abstract

Lipid nanoparticles (LNP) are the leading systems for in vivo delivery of small interfering RNA (siRNA) for therapeutic applications. Formulation of LNP siRNA systems requires rapid mixing of solutions containing cationic lipid with solutions containing siRNA. Current formulation procedures employ macroscopic mixing processes to produce systems 70-nm diameter or larger that have variable siRNA encapsulation efficiency, homogeneity, and reproducibility. Here, we show that microfluidic mixing techniques, which permit millisecond mixing at the nanoliter scale, can reproducibly generate limit size LNP siRNA systems 20 nm and larger with essentially complete encapsulation of siRNA over a wide range of conditions with polydispersity indexes as low as 0.02. Optimized LNP siRNA systems produced by microfluidic mixing achieved 50% target gene silencing in hepatocytes at a dose level of 10 µg/kg siRNA in mice. We anticipate that microfluidic mixing, a precisely controlled and readily scalable technique, will become the preferred method for formulation of LNP siRNA delivery systems.

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Figures

Figure 1
Figure 1
Schematic of lipid nanoparticle (LNP) small interfering RNA (siRNA) formulation strategy employing the staggered herringbone micromixer (SHM). (a) Lipid in ethanol and siRNA in aqueous solution is pumped into the two inlets of the microfluidic mixing device using a syringe pump. Herringbone structures induce chaotic advection of the laminar streams causing rapid mixing of the ethanol and aqueous phases and correspondingly rapid increases in the polarity experienced by the lipid solution. At a critical polarity precipitates form as LNP. (b) Parallelization of microfluidic mixers to enable formulation scale-up while maintaining identical production conditions. This is achieved through vertical (i = 1, 2,..) and horizontal (j = 1, 2,…) replication of the mixers with fluid handling through on-chip plumbing. Dimensions of the mixing channel were 200 µm × 79 µm, and the herringbone structures were 31-µm high and 50-µm thick.
Figure 2
Figure 2
Higher total flow rates through the staggered herringbone micromixer (SHM) reduce lipid nanoparticle (LNP) small interfering RNA (siRNA) polydispersity. Dependence of the polydispersity index (PDI) on the total flow rate and the concentration of lipid and siRNA in the ethanol and aqueous phases, respectively. The PDI was determined from the second order coefficient in the cumulants analysis provided by dynamic light scattering (DLS) (PDI = (σ/µ)2). The total flow rate was varied from 0.02 to 4 ml/min keeping the aqueous buffer to ethanol volumetric flow rate ratio constant at 3:1. The aqueous siRNA concentration was varied from 0.25 to 0.59 mg/ml while the lipid concentration was varied from ~4 to 10 mg/ml to keep the siRNA/total lipid ratio constant at 0.06 wt/wt. PDI values represent averages from 4 measurements. The lipid composition employed was DLinKC2-DMA/DSPC/cholesterol/PEG-c-DMA at mol ratios of 40:11.5:47.5:1. The aqueous buffer was 25 mmol/l acetate, pH 4. DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine.
Figure 3
Figure 3
Increasing PEG-c-DMA content produces progressively smaller lipid nanoparticle (LNP) small interfering RNA (siRNA) systems. (a) Influence of PEG-c-DMA content on LNP size as produced under rapid mixing conditions (4 ml/min total flow rate with an siRNA-buffer:lipid–ethanol volumetric flow rate ratio of 3:1). LNP were composed of DLinKC2-DMA/DSPC/cholesterol/PEG-c-DMA at mol ratios of 40:11.5:47.5:1, 40:11.5:46:2.5, and 40:11.5:43.5:5 for the 1, 2.5, and 5 mol% PEG-c-DMA, respectively. LNP were produced with an siRNA-total lipid ratio of 0.06 wt/wt. (b) Encapsulation efficiency as LNP size is decreased from 42 to 26 nm by increasing the PEG-c-DMA content from 1 to 5 mol%. LNP samples were dialyzed against phosphate-buffered saline (PBS) before measurement of encapsulation. Encapsulation refers to the percentage siRNA present in the LNP following removal of free siRNA using an anionic exchange spin column. (c) Polydispersity of LNP as the size was reduced from 54 to 28 nm by increasing the PEG-c-DMA content from 1 to 5 mol%. The polydispersity index (PDI) was determined as described in legend to Figure 2. (d) Size of empty LNP as a function of PEG-lipid content, which was varied from 0.25–5 mol%. LNP were composed of DLinKC2-DMA/DSPC/cholesterol/PEG-c-DMA, with DLinKC2-DMA and DSPC maintained at 40 and 11.5 mol%, respectively. Titration of PEG-c-DMA was compensated by adjustment of cholesterol. All LNP were produced at an initial lipid concentration of 20 mmol/l in the lipid-ethanol phase prior to mixing with 25 mmol/l acetate buffer, pH 4. Number-weighted mean diameters are shown for the LNP following dialysis against PBS to remove residual ethanol and increase the pH to 7.4. Error bars represent standard deviation from mean (n = 3). DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine.
Figure 4
Figure 4
Formulation of lipid nanoparticle (LNP) small interfering RNA (siRNA) employing microfluidic mixing results in highly efficient encapsulation over a wide range of siRNA-to-cationic charge ratios. LNP were composed of Dlin-KC2-DMA/DSPC/cholesterol/PEG-c-DMA at mol ratios of 40:11.5:47.5:1 employing an siRNA- total lipid ratio of 0.06 wt/wt. The total flow rate was maintained at 2 ml/min employing a 10 mmol/l lipid- in-ethanol phase mixed with aqueous buffer (25 mmol/l acetate, pH 4) containing siRNA. Encapsulation refers to the percentage siRNA present in the LNP following removal of free siRNA using an anionic exchange spin column. Error bars represent standard deviation of encapsulation as measured from three LNP formulations. DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine.
Figure 5
Figure 5
Cryo-transmission electron microscopy (cryo-TEM) micrographs of lipid nanoparticle (LNP) small interfering RNA (siRNA) systems containing 1 and 5 mol% PEG-c-DMA. (a) Cryo-TEM micrograph of LNP siRNA composed of DLinKC2-DMA/DSPC/cholesterol/PEG-c-DMA (mol ratios 40:11.5:47.5:1) and siRNA at an siRNA to total lipid ratio 0.06 (wt/wt). (b) Cryo-TEM micrograph of LNP siRNA composed of DLinKC2-DMA/DSPC/cholesterol/PEG-c-DMA (mol ratios 40:11.5:43.5:5) and siRNA at an siRNA-to-total lipid ratio 0.06 (wt/wt). LNP were imaged at 50K magnification. LNP formulation was performed under rapid mixing conditions (4 ml/min total flow rate with an siRNA-buffer:lipid–ethanol volumetric flow rate ratio of 3:1) with the staggered herringbone micromixer (SHM), with an ethanol phase containing 30 mmol/l lipid. The LNP siRNA dispersion was concentrated before imaging. The scale bar represents 100 nm. DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine.
Figure 6
Figure 6
Optimization of lipid nanoparticle (LNP) small interfering RNA (siRNA) gene silencing potency in a FVII mouse model as a function of cationic lipid content and siRNA/total lipid ratio. (a) Influence of LNP DLinKC2-DMA content on FVII gene silencing in the FVII mouse model. FVII expression was monitored 24 hours after intravenous injection of LNP siRNA systems containing between 40 and 60 mol% DLinKC2-DMA. The PEG-c-DMA content was held constant at 1 mol% and the addition of cationic lipid was compensated for by reduction in the DSPC and cholesterol content, holding the DSPC to cholesterol ratio constant at 0.25 (mol/mol). (b). Influence of variation of siRNA/total lipid ratio on FVII gene silencing in the FVII mouse model. The lipid composition used was DLinKC2-DMA/DSPC/cholesterol/PEG-c-DMA (mol ratios 60:7.5:31.5:1). The siRNA/total lipid ratio was varied from 0.01 to 0.35 (wt/wt), corresponding to a siRNA-to-cationic lipid charge ratio of 0.025, 0.25, 0.5, and 1, respectively. Systemic administration of LNP siRNA to mice was performed by tail vein injection (n = 3 per dose level). Blood was collected 24 hours postinjection and factor VII levels were determined by colorimetric assay. DSPC, 1,2-distearoyl-sn-glycero-3-phosphocholine.
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