Solubility and Related Physicochemical Properties of Narcotic Analgesics

The physicochemical properties of select opioid and anilinopiperidine narcotic analgesics were investigated. The solubilities of the narcotics in hexane and water and, for morphine, in other organic solvents were determined. Regular solution theory seems to be applicable to the solubility behavior of morphine in solvents that lack strong dipoles and hydrogen bonds. A best-fit solubility parameter of 13.2 (cal/cm³) for morphine was determined from its solubilities in London solvents and its ideal solubility. Calculation of morphine's solubility parameter from its hexane solubility alone and its melting properties gave a corresponding ₂ value. These measured solubility parameters were appreciably larger than the solubility parameter estimated from molar attraction constants. Solubility parameters of hydromorphone, codeine, fentanyl, and sufentanil were also calculated from respective hexane solubilities, melting points, and heats effusion and were 11.7, 10.9, 9.8, and 9.7 (cal/cm³) . For these compounds, experimental solubility parameters agreed with solubility parameters estimated from molar attraction constants. Because meperidine, fentanyl, and sufentanil exhibit low levels of intracrystalline cohesion, as reflected in low melting points and relatively modest heats of fusion, theoretically projected ideal solubilities and actual solubilities in organic solvents measured for them were considerably higher than determined for morphine and its analogues. Consistent with the solubilities, the octanol–water partition coefficients of the two 4-anilinopiperidine analogues and of meperidine were several orders of magnitude larger than those of the opioids, evidencing the fact that meperidine, fentanyl, and sufentanil are substantially more lipophilic than the opioids.     

Transdermal Delivery of Narcotic Analgesics: Comparative Permeabilities of Narcotic Analgesics Through Human Cadaver Skin

Relationships between the in vitro permeation rates of select narcotic analgesics through human skin and their physicochemical properties were investigated by following the permeation kinetics of six representative compounds in small diffusion cells. The relative permeability coefficients of three phenylpiperidine analogues, meperidine, fentanyl, and sufentanil, all measured on a single piece of skin, were 3.7 × 10^–3, 5.6 × 10^–3, and 1.2 × 10^–2 cm/hr, respectively. Using membranes from the same skin section, the permeability coefficients of three opioid alkaloids, morphine, codeine, and hydromorphone, were considerably lower, at 9.3 × 10^–6, 4.9 × 10^–5, and 1.4 × 10^–5 cm/hr, respectively. The high permeability coefficients of the former compounds are due to their highly lipophilic nature as reflected in high octanol/water partition coefficients and low solubility parameters. Generally, the permeability coefficients of the narcotics increase as the lipophilicity increases. When viewed in literature perspective, the data suggest that aqueous tissue control of transport is approached in the case of the phenylpiperidine analogues, all of which have K _{octanol/water} values greater than 40. Permeability coefficients of fentanyl and sufentanil were also determined as a function of pH over the pH range 7.4 to 9.4, in this instance with membranes prepared from additional samples of skin. The permeability coefficients of each drug varied less than threefold over the pH range, a behavior consistent with the highly hydrophobic natures of the compounds. The low permeability coefficients of morphine, codeine, and hydromorphone coupled with their low potencies make these drugs poor transdermal candidates. It appears that fentanyl and sufentanil can be successfully transdermally delivered.     

Transdermal Delivery of Narcotic Analgesics: pH,Anatomical, and Subject Influences on Cutaneous Permeability of Fentanyl and Sufentanil

The permeation of fentanyl and sufentanil through cadaver skin membranes was investigated using in vitro diffusion cell techniques. Neither drug influenced the permeation of the other when they were concurrently applied to the skin membrane. With respect to transdermal delivery, short diffusion lag times of less than 0.5 hr were observed for each compound. Their permeation rates through heat-isolated epidermis and dermatomed (200- to 250-µm) skin sections were essentially the same. However, when the stratum corneum was removed by tape stripping, the respective permeability coefficients were increased >30-fold, establishing the stratum corneum as the principal barrier to their skin permeation. Permeation was also studied as a function of pH. From pH 4 to pH 8 the permeability coefficients of both fentanyl and sufentanil, calculated from the total solution concentration regardless of ionization, increased exponentially. The free base is thus responsible for the relatively facile skin permeation of these drugs. Factoring of the independent permeability coefficients of the ionized and free-base forms was possible, with the latter being over two log orders larger than seen for the protonated species. Permeability coefficients of fentanyl and sufentanil through skin sections obtained from different cadavers varied four- to fivefold. Neither gender nor age was a flux-determining variable for either drug. The permeability coefficients of the drugs through skin sites as diverse as the sole of the foot, chest, thigh, and abdomen were remarkably similar. Their fluxes were sufficient for transdermal administration.     

石杉碱甲的解离常数、表观溶解度和表观油/水分配系数的测定

分别采用电位滴定法、平衡溶解度法和摇瓶法,测定了石杉碱甲的解离常数、表观溶解度和表观油/水分配系数.结果表明石杉碱甲为一元弱碱,解离常数pKTα为6.18±0.02.随溶液pH的增加,表观溶解度下降,而表观油/水分配系数增大.     

The Effect of Temperature and pH on the Solubility of Quinolone Compounds: Estimation of Heat of Fusion

Although many reports involving fluoroquinolone agents have been published in the past decades, only a few address preformulation studies. In this paper, we describe the effect of temperature and pH on the aqueous solubility of two typically used quinolones, ciprofloxacin and norfloxacin. We measured the aqueous solubilities over the pH range of 5.5 to 9.5 at temperature of 6, 25, 30, and 40°C. The intrinsic solubilities and the thermodynamic dissociation constants were determined from solubility data and the temperature dependence of the intrinsic solubility was evaluated using vant Hoff and Hildebrand plots. The heat effusion was determined from these two plots. When the heat effusion was compared to that measured from differential scanning calorimetry (DSC) studies, we found that the Hildebrand method overestimated, and the vant Hoff equation underestimated, the heat of fusion. From the absolute values of the relative errors, the Hildebrand plot produced the better results. DSC results show that the heat effusion is 15.41 kcal/mol for ciprofloxacin and 7.88 kcal/mol for norfloxacin.     

Preformulation Study of Etoposide: Identification of Physicochemical Characteristics Responsible for the Low and Erratic Oral Bioavailability of Etoposide

Preformulation studies of etoposide, including pH–solubility profile, partition coefficient, pH–stability profile, and in vitro dissolution kinetics, were conducted to identify the responsible factor(s) for the low and erratic oral bioavailability of etoposide. A stability-indicating high-performance liquid chromatographic (HPLC) assay was used for drug monitoring. The equilibrium aqueous solubility of etoposide at 37°C was low, 148.5–167.25 µg/ml, and did not vary over the pH range of 2 to 6. The pH–stability profile indicated rapid degradation of etoposide at pH 1.3 and 10, with degradation half-lives of 2.88 and 3.83 hr, respectively, at 25°C. The half-life at pH 7.30 was 27.72 days. Maximum stability at 25°C was reached at pH 5 to 6.15, with half-lives of 63 and 49.5 days, respectively. The intrinsic dissolution rate, determined on a Wood's apparatus, was slow, 0.0094 mg/min/cm², while the etoposide partition coefficient between n-octanol and water was 9.94. Therefore, etoposide absorption appears to be dissolution rate limited rather than permeation rate limited. The low equilibrium aqueous solubility, slow intrinsic dissolution rate, and chemical instability at pH 1.3 could account for the low oral bioavailability.     

Limiting Solubilities and lonization Constants of Sparingly Soluble Compounds: Determination from Aqueous Potentiometric Titration Data Only

A new method is described for the concomitant determination of limiting solubilities and ionization constants of sparingly soluble compounds, i.e., drugs. Aqueous potentiometric titration data were recorded both before and after precipitation of the compound and subjected to computer-assisted analysis. Limiting solubilities and ionization constants were obtained for nucleoside transport inhibitors, viz., dilazep, soluflazine, and hexobendine. The method was validated by comparison of titration results for known antidepressants with data from the literature. The procedure was found to be rapid and reliable for compounds with limiting solubilities as low as 30 µM, and it circumvents problems of direct methods for measuring limiting solubilities.     

Solubility Behavior,Phase Transition,and Structure-Based Nucleation Inhibition of Etanidazole in Aqueous Solutions

Purpose. The solubility behavior, phase transition and inhibition of the nucleation process of etanidazole were characterized. Methods. Solubility measurements as a function of time permitted characterization of the solubility behavior and phase transition. The precipitate from saturated solutions was isolated and characterized by differential scanning calorimetry, polarized light microscopy, x-ray powder diffraction and coulometric analysis. The physical stability of metastable systems was examined in the presence of various structure-based nucleation inhibitors. Results. Etanidazole is soluble in water with an equilibrium solubility of 68.1 mg/mL, pH 6.5 with changes in pH having virtually no effect on the solubility. Etanidazole reaches concentrations in excess of 150 mg/mL within one hour. Etanidazole solutions prepared at 150 mg/ mL contained crystals after rotating for 24 hours. The crystals were isolated and characterized as etanidazole monohydrate. The solubility of etanidazole monohydrate in water increased with time reaching an equilibrium solubility of 68 mg/mL after 24 hours. Therefore, the solubility studies were actually determining the solubility of the more stable monohydrate form of etanidazole. Etanidazole solutions at concentrations of 50, 100 and 150 mg/mL were stabilized to varying degrees with structure-based nucleation inhibitors (imidazole, ethanolamine or diethanolamine). Conclusions. Anhydrous etanidazole undergoes a transition in aqueous solutions to the more stable monohydrate when the solubility of the monohydrate is exceeded. The physical stability of etanidazole solutions at 4°C is improved following autoclaving. The addition of structure-based nucleation inhibitors effectively stabilized the metastable systems.     

Stability of Gonadorelin and Triptorelin in Aqueous Solution

The influence of pH, temperature, various buffer species at different concentrations, and ionic strength on the stability of gonadorelin and triptorelin in aqueous solution has been studied using stability-indicating high-performance liquid chromatographic methods. The degradation behavior of both peptides is similar. The maximum stability of both peptides was shown to be at an approximate pH of 5.0. Acetate has the most favorable effect on stability, while phosphate causes higher degradation. Varying the concentration of acetate buffer does not affect the degradation behavior of the peptides. A higher phosphate concentration in buffer solutions causes higher degradation, however. The ionic strength of buffer solutions has no significant influence on stability. Solutions of gonadorelin and triptorelin, respectively, buffered with acetate (0.1 M, pH 5.0) with 3% (w/v) mannitol as an additive show a predicted t _90% of 9.0 years and 7.7 years at 20°C, respectively.     

The Estimation of Solubility in Binary Solvents: Application of the Reduced 3-Suffix Solubility Equation to Ethanol–Water Mixtures

The reduced 3-suffix solubility equation (R3SSE) is applied to the characterization of solubility in the ethanol–water system. The data needed are the solubility of the compound in each of the pure solvents and at one ethanol–water composition. This composition has been estimated from solubility data to be 0.56 volume fraction of ethanol. The solubility obtained at this volume fraction is used to estimate the ternary solute–solvent interaction constant, C ₂. The R3SSE, with the C ₂ thus obtained, predicts the mixed solvent solubilities of the compounds tested, as accurately as that obtained from several volume fractions. The superiority of the R3SSE over two related equations—a simple second-degree polynomial equation and a simplified form of the R3SSE which neglects contributions to solubility from the solvent mixture—is also demonstrated for a number of solutes.     

Forecasting the Oral Absorption Behavior of Poorly Soluble Weak Bases Using Solubility and Dissolution Studies in Biorelevant Media

Pharmaceutical Research -     

Evaluation of electrophoretic method versus chromatographic, potentiometric and absorptiometric methodologies for determing pKa values of quinolones in hydroorganic mixtures

The advantages of using capillary electrophoresis (CE) over other methodologies for determining pK_a values of drugs in hydroorganic media are discussed. The focus of the discussion based upon the pK_a values of a series of quinolones determined in acetonitrile (MeCN)–water mixtures by CE, liquid chromatography, potentiometric, and spectrophotometric methods.     

Autoradiography of l–Methyl–4–phenyl–l,2,3,6–tetrahydropyridine (MPTP): Uptake in the Monoaminergic Pathways and in Melanin Containing Tissues

Abstract: A recently discovered neurotoxic compound, l–methyl–4–phenyl–l,2,3,6–tetrahydropyridine, has been found to cause a parkinsonian–like syndrome in man and monkey, but not in laboratory animals such as rat, mouse and guinea pig. MPTP seems to selectively destroy the melanin containing dopaminergic cells in pars compacta of substantia nigra. Lower mammalian species do not have melanin in these cells, which indicates that the presence of neuromelanin may be of importance for the development of MPTP–induced lesions. By means of whole–body autoradiography of ³H–MPTP in mice, accumulation and retention was observed in the dopaminergic pathways, in locus caeruleus and in structures in the medulla oblongata and spinal cord. A high uptake was also seen in melanin–containing tissues such as in the eyes of pigmented mice. MPTP has earlier been found to have high affinity in vitro for dopamine melanin, which is similar to the pigment in substantia nigra. The typical features of the MPTP–induced neurotoxicity with destruction of pigmented nerve cells and development of parkinsonism may be due to accumulation and retention of MPTP and its metabolites in these cells. In species with pigmented nerve cells, such as man and monkey, the accumulation may be much more pronounced because of the melanin affinity of MPTP and its metabolites     

Solubility of Small-Molecule Crystals in Polymers: <Emphasis Type="SmallCaps">d</Emphasis>-Mannitol in PVP,Indomethacin in PVP/VA,and Nifedipine in PVP/VA

Objective  Amorphous pharmaceuticals, a viable approach to enhancing bioavailability, must be stable against crystallization. An amorphous drug can be stabilized by dispersing it in a polymer matrix. To implement this approach, it is desirable to know the drug’s solubility in the chosen polymer, which defines the maximal drug loading without risk of crystallization. Measuring the solubility of a crystalline drug in a polymer is difficult because the high viscosity of polymers makes achieving solubility equilibrium difficult. Method  Differential Scanning Calorimetry (DSC) was used to detect dissolution endpoints of solute/polymer mixtures prepared by cryomilling. This method was validated against other solubility-indicating methods. Results  The solubilities of several small-molecule crystals in polymers were measured for the first time near the glass transition temperature, including d-mannitol (β polymorph) in PVP, indomethacin (γ polymorph) in PVP/VA, and nifedipine (α polymorph) in PVP/VA. Conclusion  A DSC method was developed for measuring the solubility of crystalline drugs in polymers. Cryomilling the components prior to DSC analysis improved the uniformity of the mixtures and facilitated the determination of dissolution endpoints. This method has the potential of providing useful data for designing physically stable formulations of amorphous drugs.     

Covalent and Noncovalent Protein Binding of Drugs: Implications for Hepatic Clearance,Storage, and Cell-Specific Drug Delivery

This review deals with the mechanisms by which the liver disposes of drugs that are covalently or noncovalently associated with proteins. Many drugs bind to plasma proteins such as albumin (mainly anionic compounds) and ₁-acid glycoprotein (cationic compounds). Nevertheless, the liver is able to clear such drugs efficiently from the circulation because of intrahepatic dissociation of the drug-protein complex. This clearance may involve spontaneous dissociation because of progressive removal of the unbound drug during liver passage, a process that can be rate limiting in hepatic uptake. Alternatively, the porous endothelial lining of the hepatic sinusoids may allow extensive surface interactions of the drug–protein complexes with hepatocytes, leading to facilitation of drug dissociation. Binding to plasma proteins and intracellular proteins in the cytoplasm or cell organelles is an important factor determining the hepatic storage and elimination rate of drugs. Drugs noncovalently associated with glycosylated proteins, which can be endocytosed by various liver cells, are not co-endocytosed with such proteins. However, covalently bound drugs can be internalized by receptor-mediated endocytosis, which permits specific targeting to hepatocytes, endothelial cells, Kupffer cells, and lipocytes by coupling to different glycoproteins that are recognized on the basis of their terminal sugar. The endocytosed drug–carrier complex is routed into endosomes and lysosomes, where the active drug is liberated by cleavage of acid-sensitive linkages or proteolytic degradation of peptide linkers. This concept has been applied to antineoplastic, antiparasitic, and antiviral drugs.     

Spontaneous Self-Assembly of Polymeric Nanoparticles in Aqueous Media: New Insights From Microfluidics,In Situ Size Measurements,and Individual Particle Tracking

Supramolecular cyclodextrin-based nanoparticles (CD-NPs) mediated by host-guest interactions have gained increased popularity because of their “green” and simple preparation procedure, as well as their versatility in terms of inclusion of active molecules. Herein, we showed that original CD-NPs of around 100 nm are spontaneously formed in water, by mixing 2 aqueous solutions of (1) a CD polymer and (2) dextran grafted with benzophenone moieties. For the first time, CD-NPs were instantaneously produced in a microfluidic interaction chamber by mixing 2 aqueous solutions of neutral polymers, in the absence of organic solvents. Whatever the mixing conditions, CD-NPs with narrow size distributions were immediately formed upon contact of the 2 polymeric solutions. In situ size measurements showed that the CD-NPs were spontaneously formed. Nanoparticle tracking analysis was used to individually follow the CD-NPs in their Brownian motions, to gain insights on their size distribution, concentration, and stability on extreme dilution. Nanoparticle tracking analysis allowed to establish that despite their non-covalent nature, and the CD-NPs were remarkably stable in terms of concentration and size distribution, even on extreme dilution (concentrations as low as 100 ng/mL).     

Use of LC–MS–MS for the rapid,specific and sensitive quality control measurement of carrier in a PET radioligand: [18F]FECNT

In the production of radioligands for imaging low concentrations of target proteins (e.g. receptors or transporters) in human subjects with positron emission tomography, control of specific radioactivity is necessary for efficacy and safety. Such quality control requires a fast method to be available for measuring carrier (non‐radioactive ligand) in each batch of radioligand, preceding its release for administration. Measurement is usually achieved with HPLC equipped with an ultraviolet (UV) absorbance detector. However, this method is not easily applicable to radioligands that have low UV extinction coefficients and are produced at high specific radioactivity, such as [¹⁸F] (2β‐carbomethoxy‐3β‐(4‐chlorophenyl)‐8‐(2‐fluoroethyl)nortropane; [¹⁸F]FECNT). Here we describe a fast, specific and sensitive LC–MS–MS method for measuring carrier in [¹⁸F]FECNT preparations. Small samples of formulated [¹⁸F]FECNT plus an added internal standard (2β‐carbomethoxy‐3β‐(4‐chlorophenyl)‐8‐(n‐propyl)nortropane; INTSTD) are rapidly eluted from a short reverse phase HPLC column into an MS probe. Following electrospray ionization, the molecular ions ([MH]⁺) of FECNT (m/z=326) and INTSTD (m/z=322) are isolated and energized for collision‐induced dissociation. The product ions from FECNT (m/z=294) and INTSTD (m/z=290) are monitored selectively. The calibration curve for MS response is linear for FECNT concentrations in the range 2–20 pg/µl and suitable for reproducibly (RSD 5%) and rapidly (<3 min) measuring low concentrations of carrier in [¹⁸F]FECNT preparations. Copyright © 2005 John Wiley & Sons, Ltd.     

First identification of a diuretic,hydrochlorothiazide, in hair: Application to a doping case and interpretation of the results

An athlete contested an adverse analytical finding involving hydrochlorothiazide, and requested our laboratory for testing his hair. As there is no reference in the literature about identification of hydrochlorothiazide in hair, several volunteers were first enrolled (4 after a single 25 mg administration and 10 with daily therapeutic treatment). A specific method was developed by ultra‐performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS. Hair samples were decontaminated with dichloromethane and 30 mg were incubated in buffer at pH 7.0 for 15 hours at 50°C. Then, 5 mL ethyl acetate was added for extraction. Linearity was observed for hydrochlorothiazide concentrations ranging from 5 to 2000 pg/mg. The limit of quantification was 5 pg/mg. The coefficients of variation (CVs) of repeatability and matrix effect were lower than 20%. Analysis of the 0–2‐cm segment of the 4 volunteers having received a single dose, collected 1 month after administration, was negative at the limit of quantification. The hair of the 10 patients (proximal 2 cm) on daily treatment was positive with concentrations ranging from 12 to 1845 pg/mg, with no correlation between daily dose and concentration. The athlete's hair tested positive for hydrochlorothiazide at 36 pg/mg in the segment corresponding to the period of the urinary control. Since a single exposure to hydrochlorothiazide is not detectable in hair and based on the results of the patients on daily treatment, the concentration found in the athlete has been interpreted as corresponding to repeated exposures, where it was not possible to establish the dosage and the frequency.     

New hydrazide–hydrazones of isonicotinic acid: synthesis,lipophilicity and in vitro antimicrobial screening

This study describes the synthesis, lipophilicity and in vitro antimicrobial assays of 15 new hydrazide–hydrazones of isonicotinic acid. New derivatives were obtained on the basis of the condensation reaction of isonicotinic acid hydrazide with different aromatic aldehydes. The chemical structure of synthesized compounds was confirmed by spectral methods. Experimental lipophilicity of new isonicotinic acid derivatives was determined using reversed‐phase thin‐layer chromatography. All synthesized compounds were subjected to in vitro antimicrobial assays against reference strains of Gram‐positive bacteria, Gram‐negative bacteria and fungi belonging to Candida spp. Some of the synthesized hydrazide–hydrazones proved to be significant antibacterial compounds and more potent than commonly used chemotherapeutic agents.     

Determination of DP‐VPA and its active metabolite,VPA, in human plasma,urine, and feces by UPLC–MS/MS: A clinical pharmacokinetics and excretion study

DP‐VPA is a phospholipid prodrug of valproic acid (VPA) that is developed as a potential treatment for epilepsy. To characterize the pharmacokinetics and excretion of DP‐VPA, four reliable ultra‐performance liquid chromatography–tandem mass spectrometry (UPLC–MS/MS) methods were validated for quantitation of DP‐VPA and its metabolite, VPA, in human plasma, urine, and feces. Protein precipitation and solid‐phase extraction (SPE) were used for extraction of C16, C18 homologs of DP‐VPA and VPA, respectively, from plasma. Urine and fecal homogenate involving the three analytes were efficiently prepared by methanol precipitation. The determinations of C16 DP‐VPA, C18 DP‐VPA, and VPA were performed using the positive multiple reaction monitoring (MRM) mode and the negative single ion monitoring (SIM) mode, respectively. The analytes were separated using gradient elution on C8 or phenyl column. Satisfactory results pertaining to selectivity, linearity, matrix effect, accuracy and precision, recovery, stability, dilution integrity, carryover, and incurred sample analysis (ISR) were obtained. The calibration ranges in human plasma were as follows: 0.00200–1.00 μg/mL for C16 DP‐VPA, 0.0100–5.00 μg/mL for C18 DP‐VPA, and 0.0500–20.0 μg/mL for VPA. The linear ranges in urine and fecal homogenate were 0.00500–2.00 μg/mL and 0.00200–0.800 μg/mL for C16 DP‐VPA, 0.00500–2.00 μg/mL and 0.0100–4.00 μg/mL for C18 DP‐VPA, and 0.200–80.0 μg/mL for VPA, respectively. The intra‐ and inter‐batch coefficients of variation in three matrices ranged from 1.7% to 12.4% while the accuracy values ranged from 85.4% to 111.7%. The developed methods were successfully applied to determine pharmacokinetics of DP‐VPA tablet after a single oral dose of 1200 mg in 12 healthy Chinese subjects under fed condition.