A Relative Permittivity Approach for Fast Drug Solubility Screening of Solvents and Excipients in Lipid-Based Delivery

Drug solubility screening in solvents and lipids is central for the development of lipid-based formulations (LBFs), and any guidance to reduce the experimental workload would be highly desirable. Solubility parameters are interesting as they can be predicted in silico for a drug but they are hardly predictable for complex lipids. This paper uses a new approach to convert an in silico drug solubility parameter to an estimated relative permittivity, ε_r. Diverse solvents and lipid-based excipients were then experimentally tested for ε_r and solubility using fenofibrate as model. The typical excipients and solvents used in LBFs showed an ε_r range of about 2-24, and good solubility of fenofibrate was indeed evidenced in vicinity of its estimated relative permittivity 13.2 ± 2.7. Mixtures of promising excipients were studied subsequently, and the obtained ε_r was predictable based on the known values of the individual components. The novel permittivity approach has demonstrated its usefulness, it has much potential in early development for ranking of suitable excipients, and it gives an initial orientation to design formulations. Future research may clarify further opportunities and limits of the novel approach for LBFs.     

Analysis of the enhanced oral bioavailability of fenofibrate lipid formulations in fasted humans using an in vitro–in silico–in vivo approach

Lipid-based formulations have established a significant role in the formulation of poorly soluble drugs for oral administration. In order to better understand their potential advantages over solid oral dosage forms, we studied the solubility and dissolution/precipitation characteristics of three self-microemulsifying drug delivery system (SMEDDS) formulations and one suspension of micronized fenofibrate in lipid excipients, for which pharmacokinetic studies had already been reported in the open literature. The in vitro dispersion/dissolution studies were carried out in biorelevant media using USP II apparatus. These were followed up by in silico simulations using STELLA® software, in which not only dispersion/dissolution, but also the precipitation and re-dissolution of fenofibrate was taken into account. While unformulated drug exhibited poor solubility (0.22 μg/mL in FaSSGF and 4.31 μg/mL in FaSSIF-V2(PO₄)) and dissolved less than 2% in dissolution tests, the solubility of fenofibrate in the presence of the lipid excipients increased dramatically (e.g., to 65.44 μg/mL in the presence of the Myritol 318/TPGS/Tween 80 SMEDDS) and there was an attendant increase in the dissolution (over 80% from capsules containing the Myritol 318/TPGS/Tween 80 SMEDDS and about 20% from the dispersion of fenofibrate in lipid excipients). For the four lipid-based fenofibrate formulations studied, combining in vitro data in biorelevant media with in silico simulation resulted in accurate prediction of the in vivo human plasma profiles. The point estimates of C_max and AUC ratio calculated from the in silico and in vivo plasma profiles fell within the 0.8–1.25 range for the SMEDDS solution and capsule formulations, suggesting an accurate simulation of the in vivo profiles. This similarity was confirmed by calculation of the respective f₂ factors. Sensitivity analysis of the simulation profiles revealed that the SMEDDS formulations had virtually removed any dependency of absorption on the dissolution rate in the small intestine, whereas for the dispersion in lipid excipients, this barrier remained. Such results pave the way to optimizing the performance of oral lipid-based formulations via an in vitro–in silico–in vivo approach.     

High throughput screening: an in silico solubility parameter approach for lipids and solvents in SLN preparations

Objective: The present paper describes an in silico solubility behavior of drug and lipids, an essential screening study in preparation of solid lipid nanoparticles (SLN).

Materials and methods: Ciprofloxacin HCl was selected as a model drug along with 11 lipids and 5 organic solvents. In silico miscibility study of drug/lipid/solvent was performed using Hansen solubility parameter approach calculated by group contribution method of Van Krevelen and Hoftyzer. Predicted solubility was validated by determining solubility of lipids in various solvent at different temperature range, while miscibility of drug in lipids was determined by apparent solubility study and partition experiment.

Results and discussion: The presence of oxygen and OH functionality increases the polarity and hydrogen bonding possibilities of the compound which has reflected the highest solubility parameter values for Geleol and Capmul MCM C8. Ethyl acetate, Geleol and Capmul MCM C8 was identified as suitable organic solvent, solid lipid and liquid lipid respectively based on a solubility parameter approach which was in agreement with the result of an apparent solubility study and partition coefficient.

Conclusion: These works demonstrate the validity of solubility parameter approach and provide a feasible predictor to the rational selection of excipients in designing SLN formulation.     

Solubility Enhancement of Nucleosides and Structurally Related Compounds by Complex Formation

Water-soluble vitamins, amino acids, and nontoxic pharmaceutical excipients were studied as solubilizing agents for poorly water-soluble adenine (nucleic acid base), guanosine (nucleoside), and structurally related drugs (acyclovir and triamterene). The apparent solubility of the substrates (adenine, guanosine, acyclovir, or triamterene) was appreciably increased by forming complexes with the ligands (vitamins, amino acids, or other ligand). Apparent association constants (K _a ) values were measured at 25°C in pH 7 phosphate buffer using phase solubility analysis. The effect of combination ligands on substrate solubility was also studied. Additive solubility enhancement was obtained for several ligand pairs.     

Preparation and characterization of emulsified solid dispersions containing docetaxel

An emulsified solid dispersion of docetaxel was prepared and characterized in vitro. In contrast to conventional solid dispersions, emulsifying pharmaceutical excipients and hydroxypropyl methylcellulose (HPMC) as a supersaturation promoter were introduced into the PEG6000-based solid dispersion to further improve its solubilizing capability. The solubility, dissolution in vitro and stability of the prepared emulsified solid dispersions were studied taking into consideration of the effects of different emulsifying excipients, preparation methods and the media. Results of the emulsified solid dispersion of docetaxel showed that the solubility and dissolution at 2 h were 34.2- and 12.7-fold higher than the crude powder. The type of emulsifying excipient used had a significant influence on the dissolution of the emulsified solid dispersion. The dissolution of the emulsified solid dispersion prepared by the solvent-melting method or the solvent method was higher than the melting method. There were no apparent differences among the dissolution media utilized. The status of the drug in the emulsified solid dispersion was observed in an amorphous or a molecular dispersion state by differential thermal analysis and powder Xray diffraction. In conclusion, the incorporation of emulsifying pharmaceutical excipients and HPMC with polymers into a solid dispersion could be a new and useful tool to greatly increase the solubility and dissolution of poorly water-soluble drugs.     

Enhancement of the Solubility and Efficacy of Poorly Water-Soluble Drugs by Hydrophobically-Modified Polysaccharide Derivatives

Purpose This work was intended to develop and evaluate a new polymeric system based on amphiphilic carboxymethylpullulans (CMP₄₉C₈ and CMP₁₂C₈) that can spontaneously self-assemble in aqueous solutions and efficiently solubilize hydrophobic drugs. Methods The self-assembling properties of CMP₄₉C₈ and CMP₁₂C₈ were characterized by fluorescence spectroscopy and surface tension measurements. The solubilization of benzophenone and docetaxel was assessed from surface tension measurements, UV spectrometry and HPLC assays. The in vitro cytoxicity of CMP₄₉C₈ solutions and the docetaxel commercial vehicle (Tween 80?/Ethanol–water) were evaluated in the absence and in the presence of docetaxel. Results Compared to CMP₁₂C₈, CMP₄₉C₈ in aqueous solutions appeared to self-organize into monomolecular aggregates containing hydrophobic nanodomains, and to significantly increase the apparent solubility of benzophenone. Docetaxel solubility could also be improved in the presence of CMP₄₉C₈ but to a lower extent due to the surface properties of the drug. Nevertheless, in vitro, the cytotoxicity studies revealed that against cancer cells, the CMP₄₉C₈-docetaxel formulation was equipotent to the commercial docetaxel one. Furthermore, in the absence of the drug, CMP₄₉C₈ appeared less cytotoxic against macrophages than the Tween? 80/Ethanol–water. Conclusions CMP₄₉C₈ is a good candidate for solubilizing hydrophobic drugs and could be applied to docetaxel formulations.     

In Silico Modeling of Non-Linear Drug Absorption for the P-gp Substrate Talinolol and of Consequences for the Resulting Pharmacodynamic Effect

Purpose The aim of the present work was to demonstrate P-glycoprotein's involvement in the non-linear talinolol pharmacokinetics using an advanced compartment and transit model (ACAT) and to compare the results predicted from the model to the finding of a phase I dose escalation study with oral talinolol doses increasing from 25 to 400 mg.Materials and Methods Besides minimum input parameters for the compound (pKa(s), solubility at one or more pH's, P _eff, doses, formulation, diffusivity), physiological and pharmacokinetic properties, transporter data are included in these predictions. The simulations assumed higher expression levels in lower gastrointestinal regions, in particular in the colon, which is in accordance with the results of intestinal rat perfusion studies and intestinal distribution data from rats, catfishes, micropigs and humans reported in the literature. Optimized values for P-glycoprotein (P-gp) K _m and V _max were used for the final simulation results and for a stochastic virtual trial with 12 patients.Results Talinolol, a P-gp substrate, exhibits non-linear dose AUC relationship after administration of 25, 50, 100 and 400 mg immediate-release tablets. This dose dependency is due to a decrease of efflux transport caused by saturation of P-gp by talinolol. It was found that oral bioavailability increases after administration of higher doses of talinolol. The predicted bioavailability of the p.o. 25, 50, 100 and 400 mg doses of talinolol was 64, 76, 85, 94%, respectively. Pharmacokinetic parameters (AUC, C _max) from in silico simulations are within acceptable range comparing with data, observed in vivo. However, the in vitro value of K _m for talinolol's interactions with P-gp could not be used in the simulation and still reproduce the observed non-linear dose dependence. For each of the four doses, GastroPlus^? was used to model pharmacodynamic (PD) response and to optimize the values of CL_e, E _max, and EC₅₀ with the effect compartment linked indirectly to the central compartment. For all simulations, EC₅₀ was 114 nM and E₀ was 83 bpm.Conclusion Comparison between the results of the in vivo study and the in silico simulations determined the quality and reliability of the in silico predictions and demonstrate the simulation of dose dependent absorption. In contrast to previous simulation work for the non-linear dose dependence of interaction with intestinal transporters or enterocyte metabolism, optimized K _m and V _max values were required to reproduce the clinically observed non-linear dose dependence. The model developed may be useful in the prediction of absorption of other P-gp substrates including pharmacodynamic consequences.     

Dopamine depletion augments endogenous opioid-induced locomotion in the nucleus accumbens using bothμ1 andδ opioid receptors

The aim of this study is to analyze further the opioid receptor subtypes involved in the augmentation of behavioral activity after dopamine depletion in the nucleus accumbens of rats. Initially, the opioid receptors involved in the augmentation of locomotion produced by endogenous opioids were evaluated by microinjection of kelatorphan, an inhibitor of proteolytic enzymes that inactivates enkephalin, with or without specific antagonists forμ ₁ orδ-opioid receptors, naloxonazine or naltrindole, respectively. Kelatorphan produced a dose-dependent increase in horizontal photocell counts and vertical movements. At all doses examined the behavioral response was augmented in rats sustaining accumbal dopamine lesions. The augmentation in dopamine-depleted rats was partially blocked by naloxonazine or naltrindole. Since the motor stimulant response to intra-accumbens microinjection of theδ-opioid agonist, [d-penicillamine^2,5]-enkephalin, was not augmented in a previous study, we tested the behavioral response to a new endogenousδ-opioid agonist, [d-Ala²] deltorphin I. The locomotor response to deltorphin was slightly augmented in dopamine-depleted rats. These data suggest that the augmentation in the motor response elicited by endogenous opioids after dopamine lesions in the nucleus accumbens involves bothμ ₁ andδ-opioid receptors.     

Phase I safety,pharmacokinetic and pharmacodynamic studies of 2-methoxyestradiol alone or in combination with docetaxel in patients with locally recurrent or metastatic breast cancer

Summary Purpose: We report the first phase I trials of 2-methoxyestradiol (2ME2, Panzem? Capsules, EntreMed, Rockville, MD), alone and in combination with docetaxel, in patients with metastatic breast cancer (MBC). Patients and methods: In Trial 001, 2ME2 monotherapy was administered orally once (200–1000 mg/d, cohorts 1–5) or twice daily (200–800 mg/q12h, cohorts 6–9) for 28 days followed by a 14-day observation period, continuously thereafter. In Trial 002, docetaxel 35 mg/m² was administered weekly for four of six weeks for a maximum six cycles; 2ME2 (200–1000 mg/d) was given orally once daily for 28 days followed by a 13-day observation period in cycle one, continuously thereafter. In both trials, responding or stable patients continued 2ME2 until progression. Results: Trial 001 enrolled 31 patients; there were no objective responses. Trial 002 enrolled 15 patients; ORR was 20% including one CR. There were no Grade IV toxicities; MTD was not reached in either study. When combined with docetaxel, three patients had significant transaminase elevations that returned to normal with continued treatment (in two of three patients). There was significant inter-patient variability and extensive metabolism to 2-methoxyestrone (2ME1). Steady-state AUC and trough concentrations of 2ME2 increased linearly up to 400–600 mg/d; doses above 400–600 mg/d did not increase 2ME2 levels. The target trough concentration (3–25 ng/mL) was not attained. Combined administration did not alter docetaxel or 2ME2 pharmacokinetics. Conclusion: 2ME2, alone or in combination with docetaxel, was well tolerated in patients with MBC but systemic exposure remained below the expected therapeutic range.     

In Vitro Human Epidermal and Polyethylene Membrane Penetration and Retention of the Sunscreen Benzophenone-3 from a Range of Solvents

Purpose. To study epidermal and polyethylene membrane penetration and retention of the sunscreen benzophenone-3 (BP) from a range of single solvent vehicles and evaluate solvent effects on permeability parameters. Methods. The solubility of BP was measured in a number of solvents. Penetration of BP across human epidermis and high density polyethylene (HDPE) membranes was studied from 50% saturated solutions in each solvent. Results. Maximal BP fluxes from the solvents across the two membranes varied widely. Highest fluxes were observed from 90% ethanol (EtOH) for epidermis and from isopropyl myristate (IPM) and C_12–15 benzoate alcohols (C_12–15 BA) for HDPE membrane. Both the flux and estimated permeability coefficient and skin-vehicle partitioning of BP appeared to be related to the vehicle solubility parameter (_v). The major effects of solvents on BP flux appear to be via changes in BP diffusivity through the membranes. Conclusions. Minimal penetration of sunscreens such as BP is best achieved by choosing vehicles with a _v substantially different to the solubility parameter of the membrane.     

Numerical validation and properties of a rapid binding approximation of a target-mediated drug disposition pharmacokinetic model

Target mediated drug disposition (TMDD) describes the phenomenon where high affinity binding of a drug to its pharmacological target (enzymes or receptors) significantly alters the pharmacokinetic profile of the drug. A rapid binding model replaces the often inestimable binding micro-constants (k _on and k _off) of TMDD models with the equilibrium dissociation constant (K _D) by assuming rapid binding of the drug to its target. The purpose of this study is to examine the validity of the rapid binding assumption and the pharmacokinetic properties of this model. Temporal profiles of free drug in plasma and a non-specific distribution site, free receptor, and the pharmacodynamically relevant, drug–receptor complex obtained from the rapid binding model compared favorably with the full TMDD model for small values of the parameter ɛ, which represents the ratio of the time required for drug–receptor binding relative to the time required for drug to be cleared from the system. The effect of escalating drug doses on the temporal characteristics and the comparison between the two models has been numerically investigated. A closer match between the full and rapid binding models is observed for high doses. Analysis for very large doses (Dose/V _c) relative to endogenous steady-state receptor concentration (R _ss), reveals that the rapid binding model reduces to a standard two compartmental model with a plasma compartment with linear drug elimination and a peripheral compartment. Decreasing clearance with increasing dose and decreasing R _ss indicates that for drugs exhibiting TMDD, the relative ratio of R _ss and dose is an important determinant of the pharmacokinetic properties rather than the individual parameters alone. An analytical solution derived for clearance shows that the primary elements of the apparent clearance of the drug are the linear clearance given by k _el V _c, the non-linear clearance due to drug–receptor complex internalization (k _int), and the ratio of AUC values of the receptor complex to that of free drug. Overall, simulations and analytical techniques applied here provide a better understanding of the validity of the rapid binding model and provide guidelines for its application. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Role of δ-opioid receptor subtypes in anxiety-related behaviors in the elevated plus-maze in rats

Rationale Recent studies have shown that endogenous opioid systems are associated with the regulation of emotional responses. In particular, it has been reported that δ-opioid receptors act naturally to inhibit stress and anxiety. Objective The present study was designed to examine the possible involvement of opioid δ-receptor subtypes in the anxiety-related behavior in the elevated-plus-maze test. Methods Six-week-old male Lewis rats were used. The total numbers of visits to the closed and open arms and the cumulative time spent and visits in the open arms were determined. Plasma corticosterone levels were measured by enzyme immunoassay. Results Naltrindole (NTI), a δ-opioid receptor antagonist (3 mg/kg s.c.), induced a significant decrease in the percentages of time spent and visits in the open arms. Naltriben (NTB), a δ₂-opioid receptor antagonist (3 mg/kg s.c.), but not 7-benzylidenenaltrexone, a δ₁-opioid receptor antagonist, produced similar anxiety-related behaviors in the elevated plus-maze. These effects of NTI and NTB were antagonized by pretreatment with (+)-4-[(aR)-a-((2S,5R)-4-allyl-2,5-dimethyl-1piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80), a δ-opioid receptor agonist. Furthermore, after exposure to the elevated plus-maze, the maximal increase in the plasma corticosterone level in NTI-treated rats was clearly higher than that in vehicle-treated rats. However, when NTI and SNC80 were coadministered, higher levels of plasma corticosterone were not seen after exposure to the elevated plus-maze. Conclusion These results suggest that endogenous δ₂-opioid-receptor-mediated systems are involved in the regulation of anxiety-related behaviors and might play a physiologically important role in the regulation of adrenocortical activity.     

Association study on GNB3 gene polymorphism with essential hypertension in Xinjiang Uygur group

The relationship between the tenth exon C825T of G-protein β3 subunit (GNB₃) genetic polymorphism and hypertension in the Uygur population of China was investigated. A nested case-control study (n = 738) was carried out. Polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) technique was used to genotype GNB₃ C825T polymorphism in 354 hypertensive (HT) and 384 normotensive (NT) Uygur subjects. The distributions of GNB₃ C825T genotypes were CC (27.2%), TT (42.9%), and CT (29.9%) in the hypertensive subjects and CC (27.7%), TT (42.4%), CT (29.9%) in the normotensive subjects. There were no significant differences in the genotype distributions between the two groups (χ ² = 0.0262 P = 0.99). The T allele was 51.4% in hypertensive subjects and 51.2% in normotensive subjects, which, between the two groups, was not a significant difference (χ ² = 0.0016 P = 0.97). Further analysis shows that there is no association between C825T genotypes and age, body mass index (BMI), Glucose (GLU), Triglyceride (TG), Cholesterol (CHO), systolic blood pressure (SBP) and diastolic blood pressure (DBP). No evidence was found to suggest an association between GNB₃ C825T polymorphism and hypertension in the Uygur population of China. Translated from Journal of Fudan University (Medical Sciences), 2006, 33(4): 433–436 [译自: 复旦大学学报 (医学版)]     

Vandetanib with docetaxel as second-line treatment for advanced breast cancer: a double-blind, placebo-controlled, randomized Phase II study

Purpose The aim of this Phase II study was to assess the efficacy and safety of vandetanib in combination with docetaxel in patients with pretreated advanced breast cancer. Methods The primary study objective was to compare the number of progression events in patients receiving once-daily oral vandetanib (100 mg) in combination with docetaxel (100 mg/m² iv every 21 days) versus placebo plus docetaxel. Sixty-four patients were randomized to receive study treatment (n = 35, vandetanib; n = 29, placebo). Results A slightly greater number of patients had experienced a progression event by the data cut-off in the vandetanib group (24 [69%]) compared with the placebo group (18 [62%]); HR = 1.19, two-sided 80% CI: 0.79–1.81; two-sided P = 0.59), suggesting that the addition of vandetanib to docetaxel did not affect the risk of disease progression compared with placebo plus docetaxel. The safety and tolerability profile of the combination therapy reflected those of both drugs as monotherapy agents. Conclusions In patients with advanced breast cancer, vandetanib plus docetaxel was generally well tolerated. Clinical benefit was not different to that observed with placebo plus docetaxel. However, due to the small patient number it was not possible to yield robust results, further research is required to identify predictive factors for patient selection.     

Molecular Dynamics Simulation of Amorphous Indomethacin-Poly(Vinylpyrrolidone) Glasses: Solubility and Hydrogen Bonding Interactions

Amorphous drug dispersions are frequently employed to enhance solubility and dissolution of poorly water-soluble drugs and thereby increase their oral bioavailability. Because these systems are metastable, phase separation of the amorphous components and subsequent drug crystallization may occur during storage. Computational methods to determine the likelihood of these events would be very valuable, if their reliability could be validated. This study investigates amorphous systems of indomethacin (IMC) in poly(vinylpyrrolidone) (PVP) and their molecular interactions by means of molecular dynamics (MD) simulations. IMC and PVP molecules were constructed using X-ray diffraction data, and force-field parameters were assigned by analogy with similar groups in Amber-ff03. Five assemblies varying in PVP and IMC composition were equilibrated in their molten states then cooled at a rate of 0.03 K/ps to generate amorphous glasses. Prolonged aging dynamic runs (100 ns) at 298 K and 1 bar were then carried out, from which solubility parameters, the Flory-Huggins interaction parameter, and associated hydrogen bonding properties were obtained. Calculated glass transition temperature (T_g) values were higher than experimental results because of the faster cooling rates in MD simulations. Molecular mobility as characterized by atomic fluctuations was substantially reduced below the T_g with IMC–PVP systems exhibiting lower mobilities than that found in amorphous IMC, consistent with the antiplasticizing effect of PVP. The number of IMC–IMC hydrogen bonds (HBs) formed per IMC molecule was substantially lower in IMC–PVP mixtures, particularly the fractions of IMC molecules involved in two or three HBs with other IMC molecules that may be potential precursors for crystal growth. The loss of HBs between IMC molecules in the presence of PVP was largely compensated for by the formation of IMC–PVP HBs. The difference (6.5 MPa^1/2) between the solubility parameters in amorphous IMC (25.5 MPa^1/2) and PVP (19.0 MPa^1/2) suggests a small, positive free energy of mixing, although it is close to the criterion for miscibility (< 7 MPa^1/2). In contrast to the solubility-parameter method, the calculated Flory-Huggins interaction parameter (? 0.61 ± 0.25), which takes into account the IMC–PVP interaction energy, predicts complete miscibility at all PVP compositions, in agreement with experimental observations. These results from MD simulations were combined with experimental values for the crystalline γ-polymorph of IMC and amorphous IMC to estimate the solubility of IMC in amorphous PVP dispersions and the theoretical enhancement in the aqueous solubility of IMC molecularly dispersed in PVP at various volume fractions. © 2012Wiley Periodicals, Inc. and the American Pharmacists Association J Pharm Sci 102:876–891, 2013     

The in vitro pharmacology of the peripherally restricted opioid receptor antagonists,alvimopan, ADL 08-0011 and methylnaltrexone

This study characterized the pharmacology of the peripherally restricted opioid receptor antagonists, alvimopan, its metabolite, ADL 08-0011, and methylnaltrexone. The activities of the compounds were investigated with respect to human or guinea pig opioid receptor binding and function in recombinant cell lines and mechanical responsiveness of the guinea pig ileum. Alvimopan and ADL 08-0011 had higher binding affinity than methylnaltrexone at human μ opioid receptors (pK _i values of 9.6, 9.6, and 8.0, respectively). The compounds had different selectivities for the μ receptor over human δ and guinea pig κ opioid receptors. ADL 08-0011 had the highest μ receptor selectivity. With respect to their μ opioid receptor functional activity ([³⁵S]GTPγS incorporation), methylnaltrexone had a positive intrinsic activity, consistent with partial agonism, unlike alvimopan and ADL 08-0011, which had negative intrinsic activities. Alvimopan, ADL 08-0011, and methylnaltrexone antagonized inhibitory responses mediated by the μ opioid agonist, endomorphin-1 (pA ₂ values of 9.6, 9.4, and 7.6, respectively) and by U69593, a κ opioid agonist (pA ₂ values of 8.4, 7.2, and 6.7, respectively). In morphine-naive guinea pig ileum, methylnaltrexone reduced, while alvimopan and ADL 08-0011 increased, the amplitude of electrically evoked contractions and spontaneous mechanical activity. In tissue from morphine-dependent animals, alvimopan and ADL 08-0011 increased spontaneous activity to a greater degree than methylnaltrexone. The data suggested that alvimopan-induced contractions resulted predominantly from an interaction with κ opioid receptors. It is concluded that alvimopan, ADL 08-0011, and methylnaltrexone differ in their in vitro pharmacological properties, particularly with respect to opioid receptor subtype selectivity and intrinsic activity. The clinical significance of the data from this study remains to be determined.     

In vitro metabolic stability of moisture-sensitive rabeprazole in human liver microsomes and its modulation by pharmaceutical excipients

A reliable method to assess in vitro metabolic stability of rabeprazole and its modulation by Generally Recognized As Safe (GRAS)-listed pharmaceutical excipients was established in human liver microsomes. The metabolic stability of rabeprazole decreased as a function of incubation time, resulting in the formation of thioether rabeprazole via nonenzymatic degradation and enzymatic metabolism. Buffer type was also a determining factor for the degree of both nonenzymatic degradation and enzymatic metabolism. The net extent of enzymatic drug metabolism, obtained by calculating the difference in drug degradation between a microsome-present reaction system and a microsome-free solution, was about 9.20 ± 0.67% in phosphate buffer and 2.27 ± 1.76% in Tris buffer, respectively. Rabeprazole exhibited first-order kinetics in microsome-free solution but showed non-linear kinetics in the microsome-present reaction system. The maximal velocity, V_max, in phosphate buffer was 5.07 μg mL⁻¹ h⁻¹ and the Michaelis-Menten constant, K_m, was 10.39 μg mL⁻¹ by computer-fitting to the classical Michaelis-Menten equation for pattern of time-dependent change in the substrate concentration. The intact drug and its thioether form were well resolved and successfully identified by HPLC chromatography and liquid chromatography mass spectroscopy (LC/MS). The metabolic stability of rabeprazole was also modulated by the presence of pharmaceutical excipients. Among the five pharmaceutical excipients tested, poloxamer 188 and Gelucire 44/14 had potentially inhibitory effects on rabeprazole metabolism in human liver microsomes (p < 0.05). A greater understanding of metabolic stability and its modulation by pharmaceutical excipients would be useful for optimizing the bioavailability of rabeprazole at the early formulation stages.     

Comparison of the effects of clozapine and 8-hydroxy-2-(di-<Emphasis Type="Italic">n</Emphasis>-propylamino)tetralin (8-OH-DPAT) on progressive ratio schedule performance: evidence against the involvement of 5-HT<Subscript>1A</Subscript> receptors in the behavioural effects of clozapine

Rationale Performance on progressive ratio schedules has been proposed as a means of assessing the effects of drugs on the efficacy of reinforcers. A mathematical model (Killeen PR (1994) Mathematical principles of reinforcement. Behav Brain Sci 17:105–172) affords a basis for quantifying the effects of drugs on progressive ratio schedule performance. The model postulates a bitonic function relating response rate and ratio size. One parameter of the function, a, expresses the motivational effect of the reinforcer, whereas another parameter, δ, expresses the minimum time needed to execute a response, and is regarded as an index of ‘motor capacity’. Previously we found that the atypical antipsychotic clozapine increased a, indicating an increase in reinforcer efficacy; a similar effect was observed with the 5-hydroxytryptamine (5-HT)_1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). It has been suggested that some of clozapine’s behavioural effects are mediated by agonistic action at 5-HT_1A receptors. Objective This study was conducted to compare the effects of clozapine and 8-OH-DPAT on progressive ratio schedule performance. Methods Rats were trained under a time-constrained progressive ratio schedule (50-min sessions). In experiment 1, they received acute doses of clozapine (4 mg kg⁻¹) and 8-OH-DPAT (100 μg kg⁻¹), alone and in combination with the 5-HT_1A receptor antagonist N-[2-(4-[2-methoxyphenyl]-1-piperazinyl)ethyl]-N-2-yridinylcyclohexanecarboxamide (WAY-100635; 30 μg kg⁻¹). In experiment 2, the effects of clozapine (2, 4 and 8 mg kg⁻¹) and 8-OH-DPAT (25, 50 and 100 μg kg⁻¹) were compared between intact rats and rats whose 5-HTergic pathways had been ablated by 5,7-dihydroxytryptamine (5,7-DHT). Results In both experiments, clozapine and 8-OH-DPAT increased a and δ. In experiment 1, WAY-100635 abolished the effect of 8-OH-DPAT on a and δ, but did not alter clozapine’s effects on these parameters. In experiment 2, the effects of clozapine and 8-OH-DPAT did not differ between sham-lesioned and 5,7-DHT-lesioned rats. Conclusions The results confirm previous findings on the effects of clozapine and 8-OH-DPAT on progressive ratio schedule performance. 8-OH-DPAT’s effects are probably mediated by post-synaptic 5-HT_1A receptors; clozapine’s effects are mediated by a different mechanism, which does not appear to involve 5-HT_1A receptors and which does not depend upon an intact 5-HTergic pathway. Jonathan Francis Rickard (1977–2003), a gifted and dedicated Ph.D. student, made a major contribution to this work     

Effects of tesmilifene,a substrate of CYP3A and an inhibitor of P‐glycoprotein,on the pharmacokinetics of intravenous and oral docetaxel in rats

Objectives It has been reported that docetaxel is a P‐glycoprotein substrate and is metabolized via the cytochrome P450 (CYP) 3A subfamily in rats. Tesmilifene is a substrate of the CYP3A subfamily and is an inhibitor of P‐glycoprotein. Thus, the effects of various doses of tesmilifene on the pharmacokinetics of intravenous and orally administered docetaxel have been investigated in rats. Methods Docetaxel (20 mg/kg as base) was administered intravenously and orally without and with tesmilifene (5, 10, and 20 mg/kg) in rats. Key findings After intravenous administration of docetaxel with tesmilifene, the values of nonrenal clearance (CL_NR) and area under the plasma concentration–time (AUC) for docetaxel were comparable with those without tesmilifene. Tesmilifene did not increase the values of AUC or of absolute oral bioavailability (F) for docetaxel after oral administration of docetaxel with tesmilifene. Conclusions The inhibition for the metabolism of docetaxel via hepatic and intestinal CYP3A subfamily, and inhibition of P‐glycoprotein‐mediated efflux of docetaxel in the intestine by tesmilifene were almost negligible. The extremely low value of F for docetaxel was due to the incomplete absorption from the gastrointestinal tract and considerable first‐pass metabolism of docetaxel in rats.