Top-down production of drug nanocrystals: nanosuspension stabilization, miniaturization and transformation into solid products

During the last 10-15 years, the formulation of drugs as nanocrystals has rapidly evolved into a mature drug delivery strategy, with currently five products on the market. The major characteristic of these systems is the rapid dissolution velocity, enabling bioavailability enhancement after oral administration. This mini-review focuses on recent advances with respect to three topics considering drug nanocrystals. The first topic is nanosuspension stabilization. A current literature status is provided and special attention is given to studies attempting to extend our physicochemical understanding of the underlying principles. The second part describes recent advances on miniaturization of nanosuspension production, to enable formulation screening during preclinical development. Finally, literature available on further nanosuspensions solidification is discussed, focussing on the maintenance of the preservation of the rapid dissolution properties of the nanocrystals after further downstream processing.     

Physicochemical characterization of solid dispersions of the antiviral agent UC-781 with polyethylene glycol 6000 and Gelucire 44/14

The purpose of this study was to prepare and characterize solid dispersions of the antiviral thiocarboxanilide UC-781 with PEG 6000 and Gelucire 44/14 with the intention of improving its dissolution properties. The solid dispersions were prepared by the fusion method. Evaluation of the properties of the dispersions was performed using dissolution studies, differential scanning calorimetry, Fourier-transform infrared spectroscopy and X-ray powder diffraction. To investigate the possible formation of solid solutions of the drug in the carriers, the lattice spacings [d] of PEG 6000 and Gelucire 44/14 were determined in different concentrations of UC-781. The results obtained showed that the rate of dissolution of UC-781 was considerably improved when formulated in solid dispersions with PEG 6000 and Gelucire 44/14 as compared to pure UC-781. From the phase diagrams of PEG 6000 and Gelucire 44/14 it could be noted that up to approximately 25% w/w of the drug was dissolved in the liquid phase in the case of PEG 6000 and Gelucire 44/14. The data from the X-ray diffraction showed that the drug was still detectable in the solid state below a concentration of 5% w/w in the presence of PEG 6000 and Gelucire 44/14, while no significant changes in the lattice spacings of PEG 6000 or Gelucire 44/14 were observed. Therefore, the possibility of UC-781 to form solid solutions with the carriers under investigation was ruled out. The results from infrared spectroscopy together with those from X-ray diffraction and differential scanning calorimetry showed the absence of well-defined drug–polymer interactions.     

In Vivo Assessment of Intestinal,Hepatic, and Pulmonary First Pass Metabolism of Propofol in the Rat

Purpose. The relative contribution of the intestinal mucosa, liver and lung to the in vivo disposition of propofol in the rat was investigated. Methods. Propofol (4.9–5.1 mg · kg^–l) was administered to groups of rats (n = 4) via the intra-arterial, intravenous, hepatic portal venous and oral routes. The AUC's of propofol were estimated and the fractions of the administered dose escaping first pass metabolism by the gut wall (f_G), liver (f_H) and lung (f_L) were calculated. In addition, transport experiments were carried out using Caco-2 cell monolayers to rule out the possibility that intestinal permeability is limiting the oral absorption of propofol. Results. Values for f_G, f_H and f_L were the following: 0.21 ± 0.07, 0.61 ± 0.13, and 0.82 ± 0.09, respectively. The apparent permeability coefficient of propofol across Caco-2 cell monolayers was 24.2 ± 0.3 × 10^–6 cm · sec^–1, which is similar to the apparent permeability coefficient obtained for propranolol (30.7 ± 1.7 × 10^–6 cm · sec^–1), a compound known to easily cross the intestinal epithelial membranes. The formation of propofol glucuronide, a major metabolite of propofol, could not be demonstrated during the flux experiments across the Caco-2 cell monolayers. Conclusions. The intestinal mucosa is the main site of first pass metabolism following oral administration of propofol in the rat. Intestinal metabolism could therefore also contribute to the systemic clearance of propofol.     

Metabolism of stevioside in pigs and intestinal absorption characteristics of stevioside, rebaudioside A and steviol.

Stevioside orally administered to pigs was completely converted into steviol by the bacteria of the colon. However, no stevioside or steviol could be detected in the blood of the animals, even not after converting steviol into the (7-methoxycoumarin-4-yl)methyl ester of steviol, a very sensitive fluorescent derivative with a detection limit of about 50 pg. The intestinal transport characteristics of stevioside, rebaudioside A and steviol were also studied in the Caco-2 system. Only a minor fraction of stevioside and rebaudioside A was transported through the Caco-2 cell layer giving a Papp value of 0.16x10(-6) and 0.11x10(-6) cm/s, respectively. The Papp value for the absorptive transport of steviol was about 38.6x10(-6) cm/s while the Papp value for the secretory transport of steviol was only about 5.32x10(-6) cm/s suggesting carrier-mediated transport. The discrepancy between the relatively high absorptive transport of steviol and the lack of steviol in the blood may be explained by the fact that in the Caco-2 study, steviol is applied as a solution facilitating the uptake, whereas in the colon steviol probably is adsorbed to the compounds present in the colon of which the contents is being concentrated by withdrawal of water.     

Verapamil hepatic clearance in four preclinical rat models: towards activity‐based scaling

The current study was designed to cross‐validate rat liver microsomes (RLM), suspended rat hepatocytes (SRH) and the isolated perfused rat liver (IPRL) model against in vivo pharmacokinetic data, using verapamil as a model drug. Michaelis‐Menten constants (K_m), for the metabolic disappearance kinetics of verapamil in RLM and SRH (freshly isolated and cryopreserved), were determined and corrected for non‐specific binding. The ‘unbound’ K_m determined with RLM (2.8 µ m ) was divided by the ‘unbound’ K_m determined with fresh and cryopreserved SRH (3.9 µ m and 2.1 µ m , respectively) to calculate the ratio of intracellular to extracellular unbound concentration (Kp_u,u). Kp_u,u was significantly different between freshly isolated (0.71) and cryopreserved (1.31) SRH, but intracellular capacity for verapamil metabolism was maintained after cryopreservation (200 vs. 191 µl/min/million cells). Direct comparison of intrinsic clearance values (Cl_int) in RLM versus SRH, yielded an activity‐based scaling factor (SF) of 0.28–0.30 mg microsomal protein/million cells (MPPMC). Merging the IPRL‐derived Cl_int with the MPPMC and SRH data, resulted in scaling factors for MPPGL (80 and 43 mg microsomal protein/g liver) and HPGL (269 and 153 million cells/g liver), respectively. Likewise, the hepatic blood flow (61 ml/min/kg b.wt) was calculated using IPRL Cl_int and the in vivo Cl. The scaling factors determined here are consistent with previously reported CYP450‐content based scaling factors. Overall, the results show that integrated interpretation of data obtained with multiple preclinical tools (i.e. RLM, SRH, IPRL) can contribute to more reliable estimates for scaling factors and ultimately to improved in vivo clearance predictions based on in vitro experimentation. Copyright © 2015 John Wiley & Sons, Ltd.     

New bis(SATE) prodrug of AZT 5'-monophosphate: in vitro anti-HIV activity, stability, and potential oral absorption

The in vitro anti-HIV activity, stability, and potential for oral absorption of a phosphotriester derivative of AZT (zidovudine; 3'-azido-2',3'-deoxythymidine) bearing a new esterase-labile S-acyl-2-thioethyl (SATE) group as transient phosphate protection are reported. The biolabile protection is characterized by the presence of a hydroxyl function in the acyl chain. In accordance with previously reported data in the bis(SATE) prodrug series, the present results demonstrate that the studied bis(hydroxytBuSATE)phosphotriester exerts its biological effects via intracellular delivery of the 5'-monophosphate of AZT. The hydroxyl function confers a high resistance against esterase hydrolysis, and the studied prodrug is able to cross the Caco-2 cell monolayers in intact form, suggesting that its further development as a possible anti-HIV pronucleotide candidate is warranted.     

Increasing the oral bioavailability of the poorly water soluble drug itraconazole with ordered mesoporous silica

This study aims to evaluate the in vivo performance of ordered mesoporous silica (OMS) as a carrier for poorly water soluble drugs. Itraconazole was selected as model compound. Physicochemical characterization was carried out by SEM, TEM, nitrogen adsorption, DSC, TGA and in vitro dissolution. After loading itraconazole into OMS, its oral bioavailability was compared with the crystalline drug and the marketed product Sporanox^® in rabbits and dogs. Plasma concentrations of itraconazole and OH–itraconazole were determined by HPLC-UV. After administration of crystalline itraconazole in dogs (20 mg), no systemic itraconazole could be detected. Using OMS as a carrier, the AUC_0–8 was boosted to 681 ± 566 nM h. In rabbits, the AUC_0–24 increased significantly from 521 ± 159 nM h after oral administration of crystalline itraconazole (8 mg) to 1069 ± 278 nM h when this dose was loaded into OMS. T_max decreased from 9.8 ± 1.8 to 4.2 ± 1.8 h. No significant differences (AUC, C_max, and T_max) could be determined when comparing OMS with Sporanox^® in both species. The oral bioavailability of itraconazole formulated with OMS as a carrier compares well with the marketed product Sporanox^®, in rabbits as well as in dogs. OMS can therefore be considered as a promising carrier to achieve enhanced oral bioavailability for drugs with extremely low water solubility.     

Early identification of availability issues for poorly water-soluble microbicide candidates in biorelevant media: a case study with saquinavir

In the search for a successful HIV microbicide, many poorly water-soluble antiviral agents are currently being investigated. Unfortunately, solubility and precipitation issues may limit intravaginal concentrations and thus availability of these agents upon application of an aqueous gel formulation. In the present study, we evaluated the in vitro precipitation behavior of the HIV protease inhibitor saquinavir in vaginal and seminal fluid simulants (VFS and SFS). Despite its limited solubility, the mesylate salt of saquinavir enables formulation of sufficiently high concentrations (2.5 mM, i.e. ca. 10⁵-fold in vitro IC₅₀ values) in a standard aqueous vehicle. While saquinavir stays in solution upon dilution with VFS, SFS induces precipitation of saquinavir, resulting in a 5-fold reduced availability and antiviral potency. Inclusion of the solubilizing excipients polyethylene glycol 1000 (12%) and hydroxypropyl-β-cyclodextrin (2.5%) was required to avoid saquinavir precipitation in SFS and to restore the antiviral potency of the formulation. This study illustrates the importance of identifying solubility and precipitation issues of microbicide candidates in biorelevant media and provides a simple in vitro procedure to implement this evaluation in early microbicide development.