Membrane solubility parameter and in situ release of theophylline

Dendimer-mediated solubilization, formulation development and in vitro-in vivo assessment of piroxicam

The present investigation was aimed at exploring dendrimer-mediated solubilization and formulation development followed by in vitro, in vivo assessment of piroxicam (PXM) nanocomposite. For this, two dendrimer generations (3.0G and 4.0G) were synthesized and characterized by IR, (1)H NMR spectroscopic and electron microscopy techniques. The optimized formulations containing 0.2% w/v of PXM loaded PAMAM dendrimer at pH 7.4 referred to as 0.2-D(3)P(7.4) (3.0G) and 0.2-D(4)P(7.4) (4.0G) resulted in significant enhancements of PXM solubility approximately by 107- and 222-fold, respectively. The in vitro release behavior of PXM from the formulation in medium-I (PBS 7.4) and medium-II (PBS with 1% albumin) and stability studies were also favorable. Pharmacokinetic study showed higher area under curve (AUC(0-->t); microg/mL/h) of 293.78 +/- 2.04 and 321.54 +/- 2.37 with optimized 0.2-D(3)P(7.4) and 0.2-D(4)P(7.4) formulations, respectively, as opposed to 279.11 +/- 1.48 with plain PXM. The elimination half-life of the drug encapsulated in the formulation was significantly higher (0.2-D(3)P(7.4), 36.6 and 0.2-D(4)P(7.4), 41.1; h) than that of pure drug (33.7 h; p < 0.005), and the overall elimination rate constant of formulations was also less as compared to free drug (p < 0.005). Pharmacodynamic assessment by rat-paw model of 0.2-D(3)P(7.4) and 0.2-D(4)P(7.4) formulations displayed inhibition levels of 54.21 +/- 1.25% and 59.33 +/- 0.63%, respectively, which are higher than those of plain PXM (41.81 +/- 2.9) formulations, after the sixth hour of administration. The second, fourth and eighth hour organ distribution data showed significantly higher recovery of PXM in rat paw with dendrimer-based formulations in comparison to plain PXM. However, comparison of overall data suggested 4.0G-based formulations to be superior to 3.0G as well as pure PXM.

Scientific perspectives on extending the provision for waivers of in vivo bioavailability and bioequivalence studies for drug products containing high solubility-low permeability drugs (BCS-Class 3)

Recently, there has been increased interest in extending the provision for waivers of in vivo bioavailability and bioequivalence (BA-BE) studies that appeared in the guidance published by the Food and Drug Administration (FDA) (1) to pharmaceutical products containing Class 3 drugs (High solubility-Low Permeability). The extension of the Biopharmaceutics Classification System (BCS) to Class 3 drugs is meritorious because of its impact on public health policy considerations. The rate limiting step in the absorption of Class 3 drugs is the permeability through the intestinal membrane. This commentary will focus its attention on the scientific considerations which need to be examined to assess the risk and the benefit prior to granting a waiver of in vivo bioavailability and/or bioequivalence studies for Class 3 drugs. It will examine the forces affecting the interconnectivity of the neuronal, immunological and hormonal systems in the gastrointestinal tract that may affect its permeability and functionality. It will also challenge the assumption that in vitro dissolution and in vitro permeability studies in tissue cultures in the presence and absence of excipients are good predictors for in vivo dissolution and in vivo permeability which are at the heart of the BCS.

Solubility behaviour of haloperidol in individual solvents determination of partial solubility parameters

The solubility behaviour of haloperidol in individual solvents ranging from non-polar to highly polar solvents was studied. Extended Hansen's method was used to analyze the solubility data and obtain partial solubility parameters of haloperidol. Flory-Huggin's size connection term 'B' was found to further improve the prediction of solubility. A four parameter extended Hansen's approach involving proton-donor and proton-acceptor parameters was also used in fitting the solubility data to a theoretical model. The term Wh, used as an empirical measure of solute-solvent interaction due to hydrogen bonding was used in calculating B. Different approaches were thus used in fitting the experimental solubility data to obtain regression equations which aim to provide a reasonable prediction of solubility of haloperidol in untested solvents. Solubility parameter was calculated from the partial solubility parameter values obtained from the different methods of data analysis, and compared with the theoretically obtained values. Solubility parameter of haloperidol is fixed at 10.58 H.

Effect of formulation adjuvants on gastrointestinal absorption of leuprolide acetate

Leuprolide acetate, [D-Leu6-desGly10]LH-RH ethylamide, a highly potent superagonist of luteinizing hormone-releasing hormone (LH-RH), was administered by intraduodenal (ID) injection to male castrate rats in a saline solution. Absorption was low, approximately 0.01% and 0.08% by oral (PO) and ID administration respectively, compared with intravenous (i.v.) controls. An aqueous formulation and a water in oil emulsion of a lipophilic salt, a decane sulfonic acid derivative of [D-Leu6-desGly10]LH-RH ethylamide gave ID bioavailabilities of approximately 0.2% and 1%, respectively. Evaluation of formulation effects on the oral absorption of leuprolide showed that lipophilicity, surfactant and vehicle properties significantly affected ID absorption of leuprolide. Absolute bioavailability of the drug in typical emulsion systems ranged from approximately 3 to 10% and represent an improvement of about 100 fold in gastrointestinal bioavailability of this peptide. The implications of these findings relative to the effect of formula adjuvants on oral absorption of leuprolide and other peptides following ID administration are discussed.

Partial molal volumes and solubilities of physostigmine in isopropanol:isopropyl myristate solvents in relation to skin penetrability

Partial molal volumes (Vi infinity) of physostigmine, ranging from 232.9 to 239.8 cm3.mol-1, and its mole fraction solubilities (Xi), ranging from 0.051 to 0.009, were determined at 25 degrees C in solutions of isopropanol (IPA), isopropyl myristate (IPM), and their mixtures. An inverse relation was found between Vi infinity and Xi. At solubility----0, Vi infinity----240.6 (by extrapolation). The experimentally derived liquid molal volume in the standard state, Vi degrees (231.1), of physostigmine was lower than its lowest Vi infinity (value 232.9) in the series tested. Virtual cohesion parameters (lambda i) and excess free energies (delta EGi) of physostigmine in the various solutions were estimated from the partial molal volumes, assuming regular solution behavior. For each solution, the free energy (-RT In Xi) of the drug was estimated from its solubility. An increase in the virtual cohesion parameter and a decrease in the excess free energy and the free energy was found with an increase in volume fraction of IPA in the mixed solvent. The increase in lambda i over the invariant cohesion parameter, delta i (10.2), reflects a compensation effect needed to maintain the geometric mean assumption of Regular Solution Theory. Deviation from the theoretically expected linearity between -RT In Xi and delta EGi of physostigmine is ascribed to the existence of solvated molecules distinct from unsolvated molecules of physostigmine. The highest permeability coefficient of the delivery of physostigmine through excised human skin from IPA:IPM mixtures was seen from the mixture exhibiting the highest solvation effect, giving additional evidence that physostigmine penetrates through the skin, possibly in combination with IPA.

Predicting the solubility of sulfamethoxypyridazine in individual solvents. I: Calculating partial solubility parameters

Sulfamethoxypyridazine, a representative model of a drug molecule, is used to test the extended Hansen method for estimating partial solubility parameters of solid compounds. Solubilities are determined in polar and nonpolar solvents. The method provides reasonable partial parameters for the sulfonamide, and it may be useful in obtaining partial parameters for other drug molecules. A four-parameter extended Hansen approach involving proton donor and acceptor parameters is used in fitting the data to a theoretical model. A term, Wh, is introduced as an empirical measure of solute-solvent interactions due to hydrogen bonding. The use of the empirical term Wh allows the researcher to fit experimental solubilities and thus design regression models and equations which provide a reasonable prediction of solubilities of a polar drug in a number of very different solvents. A Flory-Huggins size correction term improves the prediction of sulfamethoxypyridazine solubilities in these irregular solutions.

Use of theoretical partition coefficients determined from solubility parameters to predict permeability coefficients for 5-fluorouracil

Values for experimental permeability coefficients of 5-fluorouracil (5-FU) in 7 single- and 4 two component vehicles were determined from flux measurements through hairless mouse skin in diffusion cells and from solubility data. Theoretical partition coefficients of 5-FU between vehicle and skin were determined from solubility parameters of drug, vehicle, and skin, and from this theoretical permeability coefficients were estimated. Comparison of theoretical with experimental values for the permeability coefficients showed a good correlation for vehicles with solubility parameters between 12-18 (cal/cm3)1/2. For vehicles or mixtures of vehicles with solubility parameters in the range of 8-12 (cal/cm3)1/2, increases in flux and permeability coefficients were seen compared with theoretical predictions, possibly due to the similarity in solubility parameters of the vehicles to that of skin (10 cal/cm3)1/2. There was an inverse relationship between 5-FU solubility in the vehicles and flux or permeability coefficient that corresponded approximately to the point where the solubility parameters of drug and vehicle were the same.

Use of solubility parameters of drug and vehicle to predict flux through skin

The solubilities of theophylline in, and fluxes through skin from, isopropyl myristate, octanol, dimethylformamide, propylene glycol, ethylene glycol, and formamide have been determined experimentally. Values for experimental permeability coefficients (Kp) corresponding to the respective fluxes were determined from, flux/solubility = Kp, which were then compared with values for the respective theoretical partition coefficients (PC) calculated from the known solubility parameters for the vehicles (delta v), theophylline (delta i) and skin (delta s). There was a good correlation for theoretical log PC - 2.52 = experimental log Kp for vehicles exhibiting solubility parameters in the range of delta v = 12-18 (cal/cm3)1/2. This allows relative fluxes to be determined from calculated theoretical partition coefficients and experimentally determined solubilities in that range. For vehicles or mixtures of vehicles exhibiting solubility parameters in the range of delta v = 8-12 (cal/cm3)1/2 large increases in fluxes and permeability coefficients, compared with those predicted from the results in the delta v = 12-18 (cal/cm3)1/2 range, were observed because of vehicle effects on the skin caused by the similarity in solubility parameters of those vehicles to that of skin. Qualitatively, fluxes and permeability coefficients were found to be inversely dependent on drug solubility in the vehicles with a minimum that corresponded approximately to the point where delta i = delta v.

Relationship between the solubility parameter and the binding of drugs by plasma proteins

An equation, based on regular solution theory, was used to relate the solubility parameter to the binding of drugs by plasma proteins. The equation was tested on a homologous series and a good correlation was found. Sulfonamides showed maximum binding when their solubility parameters were similar to the solubility parameters of the amino acids situated in a sequence with one tryptophan residue. This observation supports the assumption that this sequence is the primary binding site for the sulfonamides. Binding peaks were also found at solubility parameters in other drug series corresponding to the solubility parameters of human serum albumin (HSA) or bovine serum albumin (BSA) amino acids. It is suggested that the solubility parameter could be used to predict the binding of drugs to plasma proteins.

Dependence of solute solubility parameters on solvent polarity

In nonpolar solvents a solute may self-associate through polar interactions, exposing its nonpolar surface to a solvent with a low solubility parameter, delta 1. In polar solvents a solute is solvated, presumably, by the polar groups of the solvent. This "chameleonic" effect results in different solubility parameters for a solute, depending on the polarity of the solvent. This report presents data for solute solubility parameters in solvents of variable polarity and gives suggestions for dealing with the chameleonic effect associated with solute-solvent interaction.