HSPiP, Computational, and Thermodynamic Model-Based Optimized Solvents for Subcutaneous Delivery of Tolterodine Tartrate and GastroPlus­Based In Vivo Prediction in Humans: Part II.

In part I, we reported Hansen solubility parameters (HSP, HSPiP program), experimental solubility at varied temperatures for TOTA delivery. Here, we studied dose volume selection, stability, pH, osmolality, dispersion, clarity, and viscosity of the explored combinations (I-VI). Ex vivo permeation and deposition studies were performed to observe relative diffusion rate from the injected site in rat skin. Confocal laser scanning microscopy (CLSM) study was conducted to support ex vivo findings. Moreover, GastroPlus predicted in vivo parameters in humans and the impact of various critical factors on pharmacokinetic parameters (PK). Immediate release product (IR) contained 60% of PEG400 whereas controlled release formulation (CR) contained PEG400 (60%), water (10%) and d-limonene (30%) to deliver 2 mg of TOTA. GastroPlus predicted the plasma drug concentration of weakly basic TOTA as function of pH (from pH 2.0 to 9). The cumulative drug permeation and drug deposition were found to be in the order as B-VI˃ C-VI˃A-VI across rat skin. This finding was further supported with CLSM. Moreover, IR and CR were predicted to achieve Cmax of 0.0038 µg/ mL and 0.00023 µg/mL, respectively, after sub-Q delivery. Added limonene in CR extended the plasma drug concentration over period of 12 h as predicted in GastroPlus. Parameters sensitivity analysis (PSA) assessment predicted that sub-Q blood flow rate is the only factor affecting PK parameters in IR formulation whereas this was insignificant for CR. Thus, sub-Q delivery CR would be promising alternative with ease of delivery to children and aged patient.

Self-microemulsifying Drug Delivery System for Improved Oral Delivery of Limonene: Preparation, Characterization, in vitro and in vivo Evaluation.

The current investigation aimed at formulating self-microemulsifying drug delivery system (SMEDDS) to ameliorate oral bioavailability of a hydrophobic functional ingredient, limonene. Solubility test, compatibility test, and pseudo-ternary phase diagrams (PTPD) were adopted to screen the optimal compositions of limonene-SMEDDS (L-SMEDDS). The characteristics of this system assessed in vitro, mainly included determination of particle size distribution, observation of morphology via transmission electron microscopy (TEM), testing of drug release in different dissolution media, and evaluation of stability. The oral bioavailability study in vivo of the formulated limonene was performed in rats with the free limonene as the reference. Compared with the free limonene, the distribution study of L-SMEDDS was conducted in Kunming mice after oral administration. The optimized SMEDDS (ethyl oleate, 14.2%; Cremophor EL, 28.6%; isopropanol, 28.6%; and loaded limonene, 28.6%) under the TEM (about 100 nm) was spherical with no significant variations in size/appearance for 30 days at 4, 25, and 60°C. In comparison with free limonene, higher than 89.0% of limonene was released from SMEDDS within 10 min in different dissolution media. An in vivo study showed a 3.71-fold improved oral bioavailability of the formulated limonene compared to the free limonene. The tissue distribution results showed that limonene predominantly accumulated in the various tissues for the L-SMEDDS compared with the free limonene. Hence, L-SMEDDS could remarkably improve the concentration of limonene in the various organs. These findings hinted that the oral bioavailability of limonene could be improved via an effectual delivery system like SMEDDS.

Determination of d-limonene in mice plasma and tissues by a new GC-MS/MS method: Comparison of the pharmacokinetics and tissue distribution by oral and inhalation administration in mice.

The aim of the present study was to develop a method based on gas chromatography-tandem mass spectrometry (GC-MS/MS) to determine and quantify the d-limonene in mouse plasma and tissue samples. This new method was validated for the quantification of d-limonene with the linearity ranges 1.0-1000.0 ng/mL (r2 > 0.9952) for plasma samples and 5.0-5000.0 ng/g (r2 > 0.9940) for tissue samples. The intra- and inter-day assay of precisions in plasma and tissues were <13.4% and the accuracies were within 91.1-105.8%. In the oral/inhalation administration pharmacokinetics and tissue distribution studies, the main pharmacokinetic parameters were the peak concentration = (97.150 ± 34.450)/(4336.415 ± 1142.418) ng/mL, the area under the curve = (162.828± 27.447)/(2085.721 ± 547.787) h ng/mL and the half-life = (3.196 ± 0.825)/(0.989 ± 0.095) h. The tissue distribution of d-limonene in mice after oral/inhalation administration demonstrated a decreasing tendency in different tissues (liver > kidney > heart > lung > spleen).

Review of toxicological assessment of d-limonene, a food and cosmetics additive.

R-(+)-limonene (d-limonene) is a commonly used flavor additive in food, beverages and fragrances for its pleasant lemon-like odor. Considering its increasing applications, it's necessary to understand toxicological effects and risk associated with its use. R-(+)-limonene is rapidly absorbed in experimental animals and human beings following oral administration. In humans, it gets distributed to liver, kidney, and blood resulting in the formation of metabolites like perillic acid, dihydroperillic acid, limonene-1,8-diol and limonene-1,2 diol. Important toxic effects primarily reported in rodents are severe hyaline droplet nephrotoxicity (only in male rats due to specific protein α2u-globulin; however, this effect isn't valid for humans), hepatotoxicity and neurotoxicity. R-(+)-limonene does not show genotoxic, immunotoxic and carcinogenic effects. Substantial data is available about limonene's stability after treatment with thermal and non-thermal food processing techniques; however, information about toxicity of metabolites formed and their safe scientific limits is not available. In addition, toxicity of limonene degradation products formed during storage of citrus juices isn't known. Based on all available toxicological considerations, R-(+)-limonene can be categorized as low toxic additive. More detailed studies are required to better understand interaction of limonene with modern food processing techniques as well as degradation products generated and toxicity arising from such products.