Enhanced oral absorption of saquinavir mediated by PEGylated solid lipid nanoparticles

Abstract

PEGylated solid lipid nanoparticles containing saquinavir were prepared by a solvent diffusion method combined with ionic complexation, and exhibited a higher drug encapsulation efficiency, drug loading capacity and slowed drug release behavior. Moreover, the nanoparticles presented low cytotoxicity, higher transport ability across intestinal epithelial cells, and hence higher relative bioavailability which increased with enhancing of the polyethylene glycol content in the nanoparticles.

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1. Introduction

In recent years, human immunodeficiency virus (HIV) protease treatment of acquired immune deficiency syndrome (AIDS) has become an important potential target for antiretroviral therapy. Remarkable progress has been made with respect to establishing the HIV-encoded protease as a viable target for chemotherapeutic intervention and toward developing potent inhibitors.¹ HIV protease inhibitors are a kind of drug which were successfully designed based on the structure and reaction mechanism of the HIV protease,^2,3 therefore a variety of potent and specific inhibitors are now known.

As the first HIV protease inhibitor which has been approved for marketing by the U.S. Food and Drug Administration (FDA) for the treatment of AIDS, saquinavir is a specific and high efficiency inhibitor of HIV-1 and HIV-2 proteinases in vitro⁴ and shows bright prospects for development and marketing.⁵ But the deficiencies in metabolism, distribution, and oral bioavailability of these inhibitors, are still the chief remaining challenges. Saquinavir, with poor oral bioavailability, needs a high dose regimen, which might increase the toxicity and side-effects. Therefore saquinavir needs to be further studied and discussed to identify how to improve the oral bioavailability and transform it into an effective therapeutic agent for the market.

Recently, nanocarriers, due to their small size and target specific localization properties, are being actively investigated, and higher bioavailability and increased residence time of the drugs can be achieved. For instance, solid lipid nanoparticles (SLNs) have been focused on as a new developed type of colloidal drug delivery system,⁶ which can replace fat emulsions,⁷ liposomes⁸ and polymeric nanoparticles⁹ The particle size of these solid colloidal particles ranges from 10 to 1000 nm and natural or synthetic solid lipid can be used as a carrier^10,11 wherein the drug is surrounded or dispersed and then a solid particle structural system is formed. As a drug carrier, SLNs integrate the advantages of drug-containing microemulsions and liposomes, and can improve the bioavailability of drugs and the physiological compatibility.^12–14 The pharmacokinetics of a drug can be changed greatly when the drug is packaged into nanoparticles, owing to the controlled release of the drug from the nanoparticles and the alteration in body distribution of the drug.¹⁵ In brief, SLNs could effectively enhance the oral absorption and bioavailability as an oral drug carrier while reducing the dose to lower the toxicity and side-effects. However, the SLNs have some disadvantages such as low drug loading capacity (usually as low as 5%), and low drug encapsulation efficiency.^15–17

Phospholipid complexes have been studied for many years to improve the drug loading capacity and enhance the therapeutic efficacy of some drugs with poor oral absorption.^18,19 In this research, saquinavir was firstly complexed with a phospholipid by ionic interaction to improve the drug loading capacity based on the compatibility between the phospholipid and the lipid matrix. Then the solid lipid nanoparticles containing saquinavir²⁰ (SQV–SLNs) were prepared by a solvent diffusion method to enhance the oral absorption and bioavailability of the drug, and the effect of polyethylene glycol (PEG) modification on the physiochemical properties of the nanoparticles including particle size, cellular uptake and transport ability across the intestinal epithelial cell monolayer, in vivo residence time and relative bioavailability were investigated in detail.

2. Materials and methods

2.1. Materials

Monostearin was purchased from Chemical Reagent Co., Ltd. (Shanghai, China). Polyethylene glycol monostearate (PEG₂₀₀₀-SA, M_w = 2000) was supplied by Tci Development Co., Ltd. (Shanghai, China). Phospholipid (Lipoid®S 100; soybean lecithin) was supplied by Lipoid GmbH (Ludwigshafen, Germany). Octadecylamine (ODA) was purchased from Fluka, U.S.A. Hoechst 33342 and fluorescein isothiocyanate (FITC) were obtained from Sigma-Aldrich (St. Louis, MO, U.S.A.). MDCK cell lines were obtained from the Institute of Biochemistry and Cell Biology (Shanghai, China). Fetal bovine serum (FBS) was purchased from Gibco (Grand Island, NY, U.S.A.). Dulbecco’s modified Eagle’s medium (DMEM, 4.5 g L⁻¹ glucose) culture solution, trypsin and ethylenediaminetetraacetic acid (EDTA) were purchased from Gibco BRL (U.S.A.). N-2-Hydroxyethylpiperazine-N-2-ethanesulfonic acid (HEPES) was purchased from Sigma Saint Quentin Fallavier (France). All other chemical reagents were analytical grade or better.

SD rats were purchased from the Shanghai Laboratory Animal Center. The animal studies were approved by the Ethical Committee of Zhejiang University, Hangzhou, China.

2.2. Preparation and characterization of the saquinavir–phospholipid complex

For preparation of the saquinavir–phospholipid ionic complex, 10 mg saquinavir and 15 mg phospholipid were weighed out and placed in a round-bottom flask and dissolved in 2 mL anhydrous ethanol. After the ethanol was completely evaporated under vacuum at 50 °C, a uniform and transparent film was formed. The differential scanning calorimetry (DSC) curve of the resultant saquinavir–phospholipid complex was recorded by using a Differential Scanning Calorimeter (DSC Q200, Universal V3.8A TA Instruments, USA).

2.3. Preparation and characterization of SLNs and pSLNs

The solid lipid nanoparticles loaded with saquinavir (SQV–SLNs) were then prepared by a solvent diffusion method in an aqueous system. 25 mg saquinavir–phospholipid ionic complex was dissolved in 1 mL ethanol, and 25 mg monostearin was then dissolved in this solution in a water bath at 50 °C. The resultant organic solution was quickly dispersed into 9 mL distilled water under mechanical agitation in a water bath at 50 °C for 5 min. The obtained pre-emulsion was allowed to cool at room temperature, until a dispersion of solid lipid nanoparticles loaded with saquinavir was obtained. For preparation of the PEGylated solid lipid nanoparticles loaded with saquinavir (pSLNs), 5 mg, 10 wt% PEG₂₀₀₀-SA relating to the mass of SQV–SLNs was used instead of the mass of monostearin. The SQV–SLNs containing 0%, 5% and 10% PEG₂₀₀₀-SA were termed as SLNs, pSLNs-5% and pSLNs-10%, respectively.

After the SQV–SLNs were prepared, the number average diameter of the obtained SQV–SLNs and pSLNs in suspension was determined with a Zetasizer (3000HS, Malvern Instruments Ltd., UK) after the suspension was diluted 50 times with distilled water. The morphologies of the SQV–SLNs and pSLNs were also observed by transmission electron microscopy (TEM) (JEM-1200EX, JEOL, Japan). The samples were diluted 100 times and placed on copper grids with films and then stained with 2% (w/v) phosphotungstic acid for viewing by TEM.

Drug encapsulation efficiency (EE%) and drug loading capacity (DL%) should be determined. The entrapment efficiency was determined by measuring the content of free drug in the dispersion medium. High Performance Liquid Chromatography (HPLC) was mainly used to determine the content of saquinavir. The saquinavir content diluted in the mobile phase (C₁, μg mL⁻¹) was determined. The entrapment efficiency (EE, %) and drug loading (DL, %) of saquinavir in the SQV–SLNs were then calculated from eqn (1) and (2), respectively.

where W_a, W_b and W₀ denote the charged amount of saquinavir (mg), monostearin and PEG₂₀₀₀-SA (mg) and weight of lipid (mg) added in the system respectively, and V represents the total volume of the mobile phase solution (mL).

In vitro drug release from the SLNs was further measured by the dialysis bag method.²¹ 1 mL of the original precipitate of SLNs in preparation was loaded into the dialysis membrane (MWCO: 14.0 kDa, Spectrum Laboratories, Laguna Hills, CA) and added into a 50 mL plastic tube containing 15 mL release medium (phosphate buffer, pH 7.4). The drug content was analyzed using the HPLC method described above. Using saquinavir solution as a control, the release profiles of the SLNs, pSLNs-5% and pSLNs-10% were recorded.

2.4. Pharmacokinetic studies

The pharmacokinetic study was then performed using male SD rats (230 ± 10 g, Laboratory Animal Center, Zhejiang University, China). The rats were fasted 12 h before the experiments but had free access to water and were divided at random into four sets (five rats per set). The first group received saquinavir suspension (54 mg kg⁻¹) by oral gavage. The other groups received the SLNs, pSLNs-5% and pSLNs-10% (54 mg kg⁻¹) by oral gavage. After administration, 0.5 mL blood samples were withdrawn from the tail vein into a heparin-rinsed vial at different time intervals. The whole blood samples were centrifuged at 5000 rpm for 10 min to obtain serum which was frozen at −20 °C until analysis. Then, saquinavir concentrations in these samples were measured.

Saquinavir content was determined using HPLC. For analysis, the serum sample (80 μL) was vortex-mixed with 40 μL potassium dihydrogen phosphate solution (0.01 mol L⁻¹, pH 7.5) and 1 mL ether for 2 min. This mixture was extracted using 4 mL of ethyl ether with shaking for 20 min. The solution was centrifuged at 8000 rpm for 10 min, and the supernatant was evaporated. The residue in the tube was added to 100 μL of the mobile phase for the HPLC analysis.

2.5. Permeation tests across a MDCK cell monolayer

The transport experiments of the SLNs and pSLNs were conducted using Madin–Daby canine kidney (MDCK) cells as a model intestinal epithelial cell line. The cytoxicity of the SLNs and pSLNs against MDCK cells were performed by MTT assay. Briefly, 1 × 10⁴ cells per well were seeded in a 96-well plate (Nalge Nunc International, Naperville, IL, USA) and treated with serial concentrations of SLNs and pSLNs (125–625 μg mL⁻¹), cells were incubated for a further 48 hours, and then MTT was added. After removing the unreduced MTT and medium, dimethyl sulfoxide (DMSO) was added to dissolve the formazan crystals with 30 min of shaking before the absorption was measured.

The permeation tests of the SLNs and pSLNs were conducted using MDCK cell monolayers. Before and after the experiments, the integrity of the monolayers was assessed by measuring the transepithelial electrical resistance (TEER) values using a Millicell-ERS volt-ohmmeter (Millipore Co., USA). For permeation tests, 0.5 mL of the transport medium containing nanoparticles (20 μg mL⁻¹ of saquinavir equivalent) was applied to the apical side followed by addition of 1.5 mL blank transport medium to the basolateral side. At certain time intervals, the transport medium in the basolateral side was collected and rapidly replaced with equivalent fresh transport medium. The content of saquinavir was detected using HPLC. The apparent permeability coefficient (P_app) was calculated from the measurement of the transfer rate of saquinavir across MDCK cells from upper to lower compartments of the transwell diffusion cells.

2.6. Cellular uptake investigation

The cellular uptake assay of the SLNs, pSLNs-5% and pSLNs-10% were performed using MDCK cells as model cells. For this assay, the nanoparticles were fluorescence labeled by the chemical conjugate of octadecylamine and fluorescein isothiocyanate (ODA-FITC). After the MDCK cells were incubated with the SLNs, pSLNs-5% and pSLNs-10% for 1, 2 and 8 h, the cells were washed with PBS and observed using an inverted fluorescence microscope (Nikon Eclipse Ti; Technical Instruments, San Francisco, CA).

3. Results and discussion

3.1. Preparation and characterization of the saquinavir–phospholipid complex

The saquinavir–phospholipid complex was prepared by the ionic interaction between saquinavir and phospholipids. The DSC curve of the resultant saquinavir–phospholipid complex was recorded by using DSC. Fig. 1 shows the DSC curves of saquinavir, the phospholipids, a physical mixture of saquinavir and phospholipids, and the saquinavir–phospholipid ionic complex. No endothermic peak around 120 °C was found from the thermogram of the saquinavir–phospholipid ionic complex, which was observed in the curve of the physical mixture of saquinavir and phospholipids. The melting peak around 74 °C of the saquinavir–phospholipid ionic complex was found to be wider than that in the thermogram of the physical mixture of saquinavir and phospholipids, and the phase transition temperature of the saquinavir–phospholipid ionic complex was lower than the phase transition temperature of the physical mixture of saquinavir and phospholipids. The results suggested that ionic bonds were formed between the amino group of saquinavir and the phosphate group of the phospholipids, in other words, a saquinavir–phospholipid ionic complex was formed.

Fig. 1 DSC curves of saquinavir, the phospholipids, a physical mixture of phospholipids and saquinavir, and the saquinavir–phospholipid ionic complex.

3.2. Preparation and characterization of SLNs and pSLNs

The solid lipid nanoparticles containing saquinavir were then prepared by a solvent diffusion method using the saquinavir–phospholipid ionic complex. Physiochemical properties of the SQV–SLNs including particle size, morphology, drug encapsulation efficiency (EE%), drug loading capacity (DL%) and in vitro release characteristics were measured and evaluated. The size measurement results are shown in Table 1. The results indicated that the number average diameter of the SQV–SLNs was about 275 nm, and the incorporation of PEG₂₀₀₀-SA led to the formation of spherical nanoparticles with a smaller particle size. The number average diameter of the pSLNs-10% was about 158 nm. The TEM images of the SLNs and pSLNs are presented in Fig. 2. The photographs revealed that all of the nanoparticles of the SQV–SLNs, pSLNs-5% and pSLNs-10% had spherical shapes, and the size of the nanoparticles shown in Fig. 2 determined using TEM were found to be smaller than the size determined by the Zetasizer. This result could be explained by the theory that the nanoparticles were in an expanded state in solution when the size was determined by the Zetasizer while the particles would be dried when the TEM images were taken. In this case, the result that the size of the nanoparticles reduced with increasing the content of PEG₂₀₀₀-SA in the nanoparticles could be revealed by the results of the TEM which were in accordance with that of the Zetasizer determination.

Table 1 Physicochemical properties of the prepared SQV–SLNs containing different amounts of PEG₂₀₀₀-SA

PEG2000-SA content (wt%)	Size (nm)	PI	EE^a (%)	DL^a (%)
a Each value represents the mean ± SD (n = 3).
0	275 ± 34.90	0.509	99.19 ± 0.01	19.87 ± 0.01
5	220 ± 22.45	0.443	98.40 ± 0.15	19.74 ± 0.02
10	158 ± 15.95	0.449	95.23 ± 0.13	19.23 ± 0.02

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Fig. 2 TEM images of the SQV–SLNs with 0% PEG₂₀₀₀-SA content (A); pSLNs-5% (B); pSLNs-10% (C), scale bar = 0.1 μm.

Table 1 also indicates the drug encapsulation efficiencies (EE%) and drug loading capacities (DL%) of the prepared SQV–SLNs containing different amounts of PEG₂₀₀₀-SA. The higher drug encapsulation efficiencies were obtained with saquinavir, prepared using the combined ionic complex with the solvent diffusion method, which were above 95%. The SQV–SLNs had the highest drug encapsulation efficiency which was about 99.19%, and the drug encapsulation efficiency slightly decreased with increasing the content of PEG₂₀₀₀-SA. The drug encapsulation efficiency of the pSLNs-10% was about 95.23%, which was much higher than that of conventional SLNs.^16,17 Usually, the drug loading capacity of SLNs is lower than 5%. The formation of a saquinavir–phospholipid ionic complex could enhance the compatibility between saquinavir and the lipid matrix. As a result, the drug encapsulation efficiency and loading capacity were highly improved.

In vitro drug release from the SLNs was further measured by the dialysis bag method. Using saquinavir solution as a control, the release profiles of the SQV–SLNs, pSLNs-5% and pSLNs-10% are shown in Fig. 3. It was obvious that the saquinavir release behaviour from the SLNs, pSLNs-5% and pSLNs-10% was slower than that of the diffusion for saquinavir solution from the dialysis bag. A biphasic drug release pattern was found in the SLNs, pSLNs-5% and pSLNs-10%, which presented burst drug release at the initial stage and subsequently showed sustained release at the latter stage. The initial burst drug release might originate from the saquinavir adsorbed on the surface of nanoparticles or near the surface of nanoparticles. On the other hand, the PEG₂₀₀₀-SA modification of the SQV–SLNs could decelerate release of saquinavir from the SQV–SLNs in the initial stage, and no effect on the drug release rate was observed in the latter stage. In the initial stage, the released drugs were the drug located near the surface of the nanoparticles. The PEG₂₀₀₀-SA modification could cause a sterical hindering effect and the most structured interfacial membrane which could restrict saquinavir transport and slow down the degradation.^22,23 In the latter stage, the released drugs were the drug loaded in the core of the nanoparticles, which were released based on the corrosion of the lipid matrix in the SLNs, and no effect was observed for incorporation of PEG₂₀₀₀-SA in the SLNs.

Fig. 3 In vitro saquinavir release profiles of the SLNs, pSLNs-5% and pSLNs-10%, using saquinavir solution as a control (n = 3).

3.3. Pharmacokinetic studies

To further investigate the in vivo behavior of solid lipid nanoparticles loaded with saquinavir, the pharmacological effects of the SQV–SLNs were evaluated on male SD rats. The curves of plasma concentration of saquinavir after a single oral dose (54 mg kg⁻¹) are illustrated in Fig. 4. It was found that the plasma concentration and residence time of the SLNs, pSLNs-5% and pSLNs-10% were improved compared to that of the saquinavir suspension, and which were enhanced with the PEG₂₀₀₀-SA content in the SLNs. Interestingly, the curves for rats receiving the SLNs, pSLNs-5% and pSLNs-10% exhibited a concentration–time profile characterized by an early peak, followed by a smaller peak and a continuous decay in the plasma concentration, indicating a bimodal distribution. However, the curves for rats receiving saquinavir suspension only showed an early peak. These results indicated that SLNs were transported across the gastrointestinal tract into systematic circulation quickly, but also presented a hepatoenteral circulation.²⁴

Fig. 4 (A) Mean plasma concentration of saquinavir versus time curves after oral administration of saquinavir to rats at a dose of 54 mg kg⁻¹ from saquinavir suspension, the SLNs, pSLNs-5% and pSLNs-10%. Each value represents the mean ± SD (n = 5). (B) Enlarged figure of mean plasma concentration of saquinavir versus time curves.

Pharmacokinetic parameter calculations were then performed on each individual set of data, and the pharmacokinetic software DAS version 2.0.1 (Mathematical Pharmacology, Professional Committee of China, Shanghai, People’s Republic of China) was used to calculate the standard non-compartmental pharmacokinetic parameters (±SD). The key pharmacokinetic parameters obtained by a noncompartmental analysis after the oral administration of saquinavir suspension, the SLNs, pSLNs-5% and pSLNs-10% are given in Table 2. It is obvious that the C_max (peak plasma saquinavir concentration) and MRT (the mean residence time) are highly improved when the drug is encapsulated into the SLNs, which was significantly enhanced with increasing the PEG₂₀₀₀-SA content in the SLNs. Comparing with the saquinavir suspension, the C_max of the pSLNs-10% increased 24.52-fold, and the MRT_0–t increased from 4.62 h to 51.68 h. The AUC and relative bioavailability of saquinavir was enhanced nearly 1-fold, however, which were highly improved by the PEGylation of the SLNs. The relative bioavailability of saquinavir for the pSLNs-10% was 101.15-fold higher than that of the saquinavir suspension. The improved transport efficiency across the gastrointestinal tract and prolonged blood circulation times contributed by the higher PEG₂₀₀₀-SA content led to the longer residence time and the higher AUC and relative bioavailability.²⁵

Table 2 Pharmacokinetic parameters^a of saquinavir formulations (dose 54 mg kg⁻¹) after oral administration of saquinavir suspension, the SLNs, pSLNs-5% and pSLNs-10% to rats^b

PK	Control	SQV–SLNs
		SLNs	pSLNs-5%	pSLNs-10%
a PK: pharmacokinetics parameters. Tmax: the time when peak plasma saquinavir concentration was reached. Cmax: peak plasma saquinavir concentration. MRT: the mean residence time. Ke: the elimination rate constant of saquinavir in the systemic circulation. AUC0–t: area under the plasma saquinavir concentration time curve.b Each value represents the mean ± SD (n = 5).
AUC0–t (mg L^−1 h^−1)	4.43	8.89	273.59	448.11
MRT0–t (h)	4.62	11.88	36.21	51.68
Cmax (mg L^−1)	0.33	3.58	7.46	8.42
Tmax (h)	3.80	1.88	0.5	0.50
Ke (1/h)	0.06	0.08	0.02	0.01
Relative bioavailability	100%	200.68%	6175.85%	10115.35%

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3.4. Permeation tests across MDCK cell monolayers

The cytoxicity of the SLNs and pSLNs against MDCK cells was investigated by the MTT assay firstly. Fig. 5 shows the cytotoxicity of blank and saquinavir-loaded nanoparticles. The cell viabilities after incubation with blank nanoparticles for 48 h at the tested concentration were around 100%, which showed that the lipid matrix had no obvious cytotoxicity and were appropriate for use as oral drug carriers. After drug loading, the nanoparticles indicated slight cytotoxicity, and the pSLNs showed less toxicity than the SLNs. The 50% inhibition concentration (IC₅₀) of the SLNs could be calculated as 269.02 μg mL⁻¹, while the IC₅₀ of the pSLNs-5% and pSLNs-10% was 549.25 μg mL⁻¹ and 550.47 μg mL⁻¹, respectively. The cytotoxicity decreased with the increasing ratio of PEG in the SLNs. These results demonstrated that SLNs and pSLNs have low cytotoxicity, and the cell survival rate was approximately 100% when the concentration of the SLNs was 100 μg mL⁻¹, which was used for further experiments.

Fig. 5 Cytotoxic effect of the SLNs and pSLNs incubated with MDCK cells. Each value represents the mean ± SD (n = 3).

To investigate the transport efficacy of the nanoparticles across intestinal epithelial cells, the permeation properties of the drug-loaded SLNs were evaluated by comparing the SQV–SLNs, pSLNs-5% and pSLNs-10% with saquinavir solution in MDCK cell monolayers. The cell monolayers with TEER values above 180 Ω cm² were selected for the subsequent studies.²⁶ Before and after the experiment was performed, the integrity of the cell monolayers was assessed by measuring the TEER values. The TEER values before and after the experiment were around 250 Ω cm² which indicated that the monolayer was integrated and the results of the transport experiments were reliable. As shown in Fig. 6, comparing with saquinavir solution, the P_app value of the SLNs, pSLNs-5% and pSLNs-10% gradually increased. All of the SLNs, pSLNs-5% and pSLNs-10% showed significantly higher drug permeation compared with saquinavir solution. Furthermore, the drug transport ability across intestinal epithelial cell monolayers was enhanced with increasing PEG₂₀₀₀-SA content in the SLNs. Numerous studies highlighted the potential of nanoparticles in improving the gastrointestinal absorption and oral bioavailability in vivo.^9,27 It was also reported that the PEGylation of nanoparticles could significantly enhance the ability to convert nanoparticles from mucoadhesive to mucoinert.^28,29

Fig. 6 The apparent permeability coefficient (P_app) of the SLNs, pSLNs-5%, pSLNs-10% and saquinavir solution across MDCK cell monolayers (n = 3). ** p < 0.01 compared with saquinavir solution.

3.5. Cellular uptake investigation

To explore the enhanced transport mechanism of the SLNs and pSLNs across intestinal epithelial cell monolayers, the cellular uptake assays of the SQV–SLNs, pSLNs-5% and pSLNs-10% were performed using MDCK cells as model cells to explore the enhanced transport mechanism of the SLNs and pSLNs across intestinal epithelial cell monolayers. As shown in Fig. 7, the cellular uptake of the nanoparticles was time dependent, and the pSLNs had a faster cellular uptake ability. The cellular uptake ability increased with the increasing ratio of PEG possibly because the pSLNs had a smaller size. It has been proven that a small particle size contributes to endocytosis.³⁰ The results also demonstrated that the transport ability across intestinal epithelial cell monolayers of the nanoparticles increased by increasing the amount of PEG on the nanoparticle surface, which could improve the hydrophilicity of the SLNs.³¹

Fig. 7 Fluorescence images of MDCK cells incubated with the SLNs, pSLNs-5% and pSLNs-10% (drug content was 20 μg mL⁻¹) for 1 h, 2 h and 8 h.

4. Conclusions

Solid lipid nanoparticles loaded with saquinavir were successfully prepared using the combination of an ionic complex and solvent diffusion method in an aqueous system, which had high drug encapsulation efficiencies and drug loading capacities. The higher PEG₂₀₀₀-SA content in the SLNs led to a smaller particle size, slower drug release behavior in the initial stage, stronger cellular uptake and transport ability across intestinal epithelial cell monolayers, longer in vivo residence time, and higher relative bioavailability. These properties suggest that PEGylated solid lipid nanoparticles containing saquinavir are a new promising oral anti-HIV drug delivery system.

Acknowledgements

We are grateful for financial support from the technology and social development project of the Ningbo Science and Technology Bureau under contract 2010C50035 and medical and health research funding schemes of Zhejiang Province under contract 2012KYA064.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05283a

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