Influence of premicellar and micellar association on the reactivity of methylprednisolone 21-hemiesters in aqueous solution

Effect of Temperature on the Supramolecular Behavior of Antidepressant Drug Amitriptyline Hydrochloride in the Presence of Nonionic Surfactants (Tweens and Brijs) as Pharmaceutical Excipients

The aggregational and adsorption behavior of an important antidepressant amphiphilic drug amitriptyline hydrochloride have been studied in the presence of nonionic surfactants at different temperatures by tensiometry. By using regular solution theory and other thermodynamic models, different physicochemical properties such as critical micellar concentration, micellar composition, interfacial adsorption, energetic parameters (free energy, enthalpy, entropy) of micellization are evaluated. The data clearly depict that there is strong interaction between the amitriptyline hydrochloride and nonionic surfactants. Steric factor appears to play an important role during interaction when surfactant molecular structure varies in size of the head group and chain length of the hydrophobic part. The temperature has been observed to be vital in controlling the dehydration and thermal solubility which, in turn, makes environment conducive for aggregational behavior of the drug-surfactant mixtures in aqueous solution.

Prodrug strategies to overcome poor water solubility

Drug design in recent years has attempted to explore new chemical spaces resulting in more complex, larger molecular weight molecules, often with limited water solubility. To deliver molecules with these properties, pharmaceutical scientists have explored many different techniques. An older but time-tested strategy is the design of bioreversible, more water-soluble derivatives of the problematic molecule, or prodrugs. This review explores the use of prodrugs to effect improved oral and parenteral delivery of poorly water-soluble problematic drugs, using both marketed as well as investigational prodrugs as examples. Prodrug interventions should be considered early in the drug discovery paradigm rather than as a technique of last resort. Their importance is supported by the increasing percentage of approved new drug entities that are, in fact, prodrugs.

Supramolecular Behavior of the Amphiphilic Drug (2R)-2-Ethylchromane-2-Carboxylic Acid Arginine Salt (a Novel PPARα/γ Dual Agonist)

PURPOSE

This study was conducted to evaluate the aggregation properties of an amphiphilic drug.

METHODS

Aggregation of the drug was studied by various methods including phase-contrast and polarized microscopy, spectrophotometry, surface tensiometry, atomic force microscopy, and dynamic light scattering. Lymph-cannulated rats were used to assess fractions of drug that were absorbed into lymphatics.

RESULTS

During the pharmaceutical development of an alpha/gamma dual PPAR agonist, a derivative of a chromane-2-carboxylic acid (compound 1), it was discovered that the compound was able to form various aggregates in aqueous media from pH 6.5 to 7.1, whereas aggregating predominantly into micelles at higher pH values. Critical micelle concentrations seemed to be quite low, about 0.25 mM (0.17 mg/mL) in deionized water as determined by spectrophotometric (dye) and surface tensiometry (du Nuoy) methods. Aggregation of compound 1 into large supramolecular aggregates was visualized via phase-contrast microscopy and atomic force microscopy. The observed aggregates ranged from 250 nm to greater than 10 microm in size. Formation of liquid crystalline phases was observed by polarized microscopy as the material was gradually hydrated with water. Lymph studies in rats indicated that up to 6.9% of the orally administered dose of compound 1 in pH 6.5 buffer appeared in lymph, suggesting that supramolecular aggregation may also occur in vivo leading to partitioning between the portal and the lymph routes.

CONCLUSIONS

The aforementioned supramolecular aggregation was found to have a profound effect on the pharmaceutical development of the drug and potentially on in vivo absorption of the drug.

Self-micellization of gemfibrozil 1-O-beta acyl glucuronide in aqueous solution

PURPOSE

Phase II metabolism involves the conjugation of a polar moiety, such as sulfate or glucuronic acid, to a (relatively) nonpolar xenobiotic. Although it might be expected that such conjugates may exhibit amphiphilic character (e.g., surface activity and potential to form micelles), no detailed study of the micellization characteristics of any drug-glucuronide conjugates has yet been reported. Therefore, the aim of this study was to investigate the solution behavior and amphiphilic characteristics of gemfibrozil 1-O-beta glucuronide (GG), a model drug-glucuronide conjugate.

METHODS

Crude GG was extracted from the urine of volunteers dosed with 600 mg of gemfibrozil, and this material was then purified by reversed-phase high-performance liquid chromatography to yield a white solid. The amphiphilic properties of GG within the bulk aqueous phase were studied by isothermal titration microcalorimetry and 1H-NMR spectrometry, whereas those at the aqueous/air interface were studied by surface tensiometry.

RESULTS

The results of each independent analytical technique were consistent with GG in aqueous solution exhibiting amphiphilic properties typical of a hydrophilic surfactant. The titration microcalorimetry and 1H-NMR spectrometry data were in excellent agreement with each other, yielding critical micellization concentrations (cmc) for GG in 0.1 M acetate buffer of 18.1 +/- 0.4 mM and 18.3 +/- 0.3 mM, respectively. The profile and results of the surface tension measurements were consistent with GG localizing at the aqueous/air interface.

CONCLUSIONS

These results confirm the hypothesis that a glucuronide conjugate of a relatively nonpolar xenobiotic, such as gemfibrozil, behaves as an amphiphile in aqueous solution. The implications of this observation include a likely basis for the previously observed concentration-dependence in the degradation rate of the acyl glucuronides of 2-phenylpropionic acid, as well as identifying a possible broader contributory effect to the structural dependencies in biliary choleresis of different glucuronide conjugates of xenobiotics.

Surface active drugs: self-association and interaction with membranes and surfactants. Physicochemical and biological aspects

Many pharmacologically active compounds are of amphiphilic (or hydrophobic) nature. As a result, they tend to self-associate and to interact with biological membranes. This review focuses on the self-aggregation properties of drugs, as well as on their interaction with membranes. It is seen that drug-membrane interactions are analogous to the interactions between membranes and classical detergents. Phenomena such as shape changes, vesiculation, membrane disruption, and solubilization have been observed. At the molecular level, these events seem to be modulated by lipid flip-flop and formation of non-bilayer phases. The modulation of physicochemical properties of drugs by self-association and membrane binding is discussed. Pathological consequences of drug-membrane interaction are described. The mechanisms of drug solubilization by surfactants are reviewed from the physicochemical point of view and in relation to drug carrying and absorption by the organism.

Use of cultured cerebral capillary endothelial cells in modeling the central nervous system availability of 2',3'-dideoxyinosine

The biochemical and physiological mechanisms responsible for the limited central nervous system (CNS) uptake of dideoxynucleoside reverse transcriptase inhibitors currently used to treat HIV-1 infection in humans are poorly understood. In vitro models of the blood-brain barrier (BBB) offer an attractive alternative to in vivo or in situ animal studies for understanding the role of the blood-brain barrier in regulating brain tissue concentrations of these agents. In the present study, the kinetics of 2', 3'-dideoxyinosine (ddI) uptake and purine nucleoside phosphorylase (PNP) mediated catabolism in primary cultures of bovine brain microvessel endothelial cells (BBMECs) were determined in order to ascertain the importance of both transport and metabolism governing the CNS availability of this purine dideoxynucleoside. Initial rates of ddI uptake as a function of ddI donor concentration suggest the involvement of both passive diffusion and carrier-mediated processes. These studies confirm earlier in vivo findings that transporters may play a role in regulating the CNS concentration of ddI. Analysis of ddI uptake and metabolite accumulation in BBMECs over longer time intervals (beyond the intial rate region) provide substantial in vitro evidence for an enzymatic BBB for ddI. Simulations of the CNS availability of ddI derived from in vitro estimates of parameters for passive diffusion, carrier-mediation, and metabolism indicate that the fraction of ddI entering the BBB cells which actually reaches the brain parenchyma may be quite low (< 2%) due to metabolism by PNP localized within the BBB, consistent with the low CNS delivery of ddI observed in vivo. Transporters and metabolic enzymes within the BBB may function in coordinated fashion to reduce the CNS concentrations of both rapidly metabolized and poorly metabolized dideoxynucleosides.

The effect of bulking agent on the solid-state stability of freeze-dried methylprednisolone sodium succinate

The rate of hydrolysis of methylprednisolone sodium succinate in the freeze dried solid state at 40 degrees C was determined in the presence of two common bulking agents--mannitol and lactose--at two different ratios of drug to excipient. Residual moisture levels were less than 1% in all samples tested, with no significant difference in residual moisture among different formulations. Rate of hydrolysis was significantly higher in mannitol-containing formulations versus lactose-containing formulations, and the rate of hydrolysis increases with increasing ratio of mannitol to drug. Thermal analysis and x-ray diffraction data are consistent with a composition-dependent rate of crystallization of mannitol in the formulation and its subsequent effect on distribution of water in the freeze-dried matrix. Increased water in the microenvironment of the drug decreases the glass transition temperature of the amorphous phase, resulting in an increased rate of reaction. The physical state of lactose remained constant throughout the duration of the study, and the rate of hydrolysis was not significantly different from the control formulation containing no excipient. Thermal analysis and x-ray diffraction data are consistent with formation of a liquid crystal phase in freeze-concentrated solutions of methylprednisolone sodium succinate containing no excipient.

Self-association of nicotinamide in aqueous solution: mass transport, freezing-point depression, and partition coefficient studies

The steady-state flux (SSF) of nicotinamide from an aqueous donor phase across a model Silastic membrane did not increase proportionally with increasing donor phase concentration. The suspected self-association of the drug in aqueous solution was evaluated by studying the concentration-dependent changes in (i) the molal osmotic coefficient of nicotinamide (freezing-point depression studies) and (ii) the partition coefficient between water and n-octanol. The freezing points of aqueous solutions of nicotinamide were measured and a plot of osmolality vs molality was nonlinear. The partition coefficient of nicotinamide, studied at 15, 25, and 32 degrees C, also decreased with increasing concentration of drug. Mathematical models describing dimerization and higher orders of association were applied to the data. The results indicated the involvement of higher orders of association and it was found that an isodesmic (step-association) model was an adequate description of the freezing-point depression and partition coefficient data. The association constant, K, ranged between 1.59 +/- 0.02 M-1 at the freezing point and 0.48 +/- 0.01 M-1 as estimated from the partition coefficient data at 32 degrees C. These models for the self-association of nicotinamide allowed estimation of the apparent concentration of "monomeric" nicotinamide in the donor phase solutions studied in the SSF experiments. When the SSF data were analyzed with regard to the concentration of monomeric nicotinamide in the donor phase, a relationship close to linearity was observed.

Self-association and solubility behaviors of a novel anticancer agent, brequinar sodium

To aid in the selection of appropriate excipients to formulate brequinar sodium [6-fluoro-2-(2'-fluoro-1,1'-biphenyl-4-yl)-3-methyl-4-quinolinecarboxyli c acid sodium salt; DuP 785], studies were initiated to characterize thoroughly its solubility behavior. The measured solubilities at RT (approximately 23 degrees C) agreed with the theoretical values in the pH range from 0.5 to 7.2, but became significantly greater than theoretical values at pH values above 7.2. This deviation was likely due to the vertical stacking-type self-association between brequinar molecules in water. The NMR and pH methods determined a critical association concentration of 15 mg/mL. Sodium salicylate, which has been proven to interfere with molecular self-association, reduced drug solubility from 116 to 10 mg/mL. But urea, another deaggregative agent, gave about a twofold increase rather than a decrease in solubility. Addition of sodium chloride caused a 226-fold decrease in solubility. The apparent solubility product did not remain constant but decreased as sodium chloride concentration increased, suggesting that the added salt decreased the degree of self-association between brequinar molecules. Among four surfactants examined (a bile salt with a rigid fused ring versus three ordinary surfactants with a flexible chain structure), only sodium cholate significantly increased the aqueous solubility of brequinar sodium.

Evaluation of electrical conductivity-temperature curves using a mathematical model: temperature-dependent changes during thawing of frozen aqueous pharmaceuticals

The information contained in the electrical conductivity curves of pharmaceuticals measured as a function of temperature can be represented by a small set of parameters. This is achieved by approximating the electrical conductivity curve as a number of consecutive steps, using a suitable empirical model. The three parameters describing each step are: transition temperature, slope factor and step height. The validity of the calculated transition temperatures was established by applying the model to electrical conductivity curves measured on aqueous solutions of KCl, NaCl and on a KCl-NaCl mixture. It appears that the transition temperatures calculated for these inorganic salts are in good agreement with the respective eutectic temperatures reported in the literature. Subsequently, the method was applied to the corticosteroids prednisolone sodium succinate and prednisolone disodium phosphate. The mathematical model yields a satisfactory fit for both experimental conductivity curves. The actual consequences of freeze-drying an aqueous solution of prednisolone sodium succinate below and above the respective transition temperatures calculated are discussed in relation to the experimental conductivity data.

Strategies in the design of solution-stable, water-soluble prodrugs I: a physical-organic approach to pro-moiety selection for 21-esters of corticosteroids

The ideal water-soluble prodrug should exhibit sufficient aqueous solution stability to allow long-term storage of its solutions (i.e., 2 years at room temperature) and yet should be converted rapidly in vivo to the active parent drug--two severe and seemingly conflicting demands which limit the utility of many common solubilizing pro-moieties. For example, succinate esters, which are commonly utilized as water-soluble prodrugs, are unstable in solution and may undergo slow and incomplete bioconversion in vivo. In this study, the solution stability problems associated with 21-esters of corticosteroids are reviewed. It is concluded that the most important reaction limiting shelf life is ester hydrolysis. From a consideration of the influence of molecular structure on ester reactivity, a strategy for the design of solution-stable, water-soluble prodrugs of corticosteroids has been developed. Two key requirements for dilute solution stability are high solubility at the pH of optimum stability and appropriate design of the pH-rate profile. Several 21-esters of methylprednisolone have been synthesized, and the rates of their aqueous solution hydrolysis have been determined to test the strategy. Compounds exhibiting estimated shelf lives in dilute solution of greater than 2 years at 25 degrees C have been identified.

Strategies in the design of solution-stable, water-soluble prodrugs II: properties of micellar prodrugs of methylprednisolone

In a previous study, a physical-organic approach to the design of solution-stable, water-soluble prodrugs of the corticosteroid methylprednisolone was outlined, and several 21-esters were synthesized to test the approach. Compounds exhibiting dilute solution stabilities approaching 2 years at 25 degrees C were reported. A complicating factor in more concentrated aqueous solutions of water-soluble prodrugs, however, is the limited extent to which hydrolysis can occur before the solution becomes saturated with respect to the relatively insoluble parent drug. In this study the advantages of micellar prodrugs as water-soluble delivery systems for parenteral administration of relatively insoluble parent drugs are explored. Micellar prodrugs, besides being highly water soluble, have additional advantages in that their micelles solubilize poorly soluble degradation products which may otherwise precipitate and may act as a self-stabilizing influence due to protection of the hydrolytically labile prodrug linkage within the micelle interior. Two 21-esters of methylprednisolone previously identified as having promising dilute solution stability have now been shown to self-associate in aqueous solution at higher concentrations, as determined by solubility, kinetic, and light-scattering measurements. One consequence of self-association is that free methylprednisolone, the product of prodrug hydrolysis, is solubilized in concentrated prodrug formulations. In addition, acid- and base-catalyzed hydrolysis rate constants are altered in the micelles, resulting in further prolongation of shelf life in concentrated solutions. Due to the added benefits of self-micellization, the water-soluble 21-esters investigated exhibit shelf lives exceeding 2 years at 30 degrees C, the upper limit of the controlled room temperature range.