Microelectrometric titration measurement of the pKa's and partition and drug distribution coefficients of narcotics and narcotic antagonists and their pH and temperature dependence.

The pKa's, partition coefficients, and drug distribution coefficients (apparent partition coefficients) have been investigated for a number of narcotics and, where possible, for their congener narcotic antagonists. These studies were carried out by a microelectrometric titration technique as a function of temperature and pH. This method enables one to determine not only the dissociation constants to deconvolute overlapping pKa's, but also to determine the solubilities of oil-water distribution of these various drugs. The drug distribution coefficients displayed marked sensitivity to pH at values which span the range of attainable human physiological pH values. This has significant pharmacological implications for proper choice and scaling of drug dosages under various clinical situations. The partition coefficients and drug distribution coefficients were noticeably different at 20 degrees (where such measurements are customarily made) than at 37 degrees (body temperature). Furthermore, various drugs exhibit very nonequivalent increases in drug distribution coefficients with increasing temperature, ranginf from 21% for morephine to 200% for naltrexone. This nonregularity indicates that it will not be valid to extrapolate by any constant factor the measurements made at lower temperatures. Even the true partition coefficients increase with temperature from 20 degrees to 37 degrees. There is more of a difference in the drug distribution coefficients for naloxone and naltrexone than might have been expected from the similarities in their structures with naltrexone being significantly less lipophilic than naloxone. This would imply that this would lead to naloxone having a more rapid onset for antagonist activity and likewise a shorter duration of action than naltrexone.     

Ionization Constants and Distribution Coefficients of Phenothiazines and Calcium Channel Antagonists Determined by a pH-Metric Method and Correlation with Calculated Partition Coefficients

The pH–metric technique was used to determine the ionization constants and distribution coefficients of 10 phenothiazines and five ionizable calcium channel antagonists. Because the studied compounds were poorly water soluble and quite lipophilic with partition coefficients in the range of 3.5 to 5.5, organic cosolvents had to be added for the determination of the ionization constants to avoid precipitation of the free bases. The effect of the cosolvents dioxane and methanol on the extrapolation to pure water was compared. For both cosolvents a very good agreement with accessible published ionization constants was obtained, however the slope of the regression line was much smaller for dioxane, yielding more reliable estimates according to the standard deviation of the extrapolated values. Thus, dioxane might be preferable to methanol as a cosolvent for the determination of ionization constants of sparingly water soluble bases. Also the n‐octanol/water partition coefficients were determined and compared with published data and values calculated with the ClogP, ACD, and HINT programs. Although the obtained values were approximate in conformity with the published data, the calculated partition coefficients differed from the experimental ones considerably for the majority of the investigated compounds. Furthermore, the ion pair partitioning and the distribution coefficients at physiological pH 7.4 were determined. The pH‐dependent distribution profiles showed the strong influence of the ionization constants and of the distribution of the ion pairs on the overall distribution. This result strongly suggests that greater use should be made of measured distribution coefficients in quantitative structure–activity relationship studies. The potentiometric method is a convenient way to determine the distribution properties of drug molecules at pH values relevant for the biological system under investigation.     

Relationship between human milk lipid-ultrafiltrate and octanol-water partition coefficients

The distribution into human milk lipid of 16 drugs with widely varying lipophilicity is reported. There is a high degree of correlation between milk lipid-ultrafiltrate and octanol-water partition coefficients, measured at 37 degrees C, provided digoxin and prednisolone are excluded. These steroid-based molecules may interact with free fatty acids in milk, forming micelles and thus perturbing the milk lipid-ultrafiltrate relationship. Equations are described which enable estimation of drug distribution into milk for lipid-soluble drugs, based on drug pKa and octanol-water partition coefficients.     

Relationships between octanol-water partition data, chromatographic indices and their dependence on pH in a set of beta-adrenoceptor blocking agents

A thorough investigation on the dependence of octanol/water distribution coefficients (log D) of basic and amphiprotic beta-blocker drugs on pH and pKa values has been carried out. An attempt is made to clarify the factors which govern the chromatographic parameters (log k') in reversed-phase high performance liquid chromatography (RPLC) in terms of chemical structure and of distribution coefficients. The correlations between log D and log k' of different compounds show that the bulkiness and the branching of nitrogen substituent are controlling factors in RPLC measurements. Finally, an ion pair chromatographic system is considered for the prediction of octanol/water partition coefficients.     

Tumor physiology and charge dynamics of anticancer drugs: implications for camptothecin-based drug development

Charge is an important characteristic of drug molecules, since ionization sites determine the pKa at a particular pH. The pKa in turn can affect many parameters, including solubility, dissolution rate, reaction kinetics, formulation, cell permeability, tissue distribution, renal elimination, metabolism, protein binding and receptor interactions. The impact of charge dynamics is amplified in human solid tumors that exhibit the glycolytic phenotype and associated acidic extracellular microenvironment. This phenotype is driven by hypoxia and creates a pH gradient in tumors that favors uptake of weak acids and exclusion of weak bases. Established anticancer drugs exhibit a range of pKa's and thus variable ability to exploit the tumor pH gradient. The camptothecins are a prime example as they represent a diverse class of approved anticancer drugs and drug candidates whose charge distribution varies with pH. An in silico method was used to predict charge distribution of camptothecins at physiological versus acidic pH in both the lactone and carboxylate forms. A significant amount of uncharged carboxylate was predicted at acidic pH that could enter tumor cells and accumulate in mitochondria to inhibit mitochondrial topoisomerase I. A model is presented to describe the charge dynamics of a new camptothecin analog and the impact on nuclear and mitochondrial mechanism(s) of action. This example illustrates the importance of integrating tumor physiology and charge dynamics into anticancer drug development.     

高效液相色谱法测定去氧氟尿苷在不同介质中的平衡溶解度和表观油水分配系数

目的测定去氧氟尿苷在不同介质中的平衡溶解度以及在正辛醇-水和正辛醇-缓冲液体系中的表观油水分配系数。方法采用HPLC法测定去氧氟尿苷的浓度,采用摇瓶法测定去氧氟尿苷的表观油水分配系数。结果37℃,pH=7时去氧氟尿苷在水中的平衡溶解度为2.570 1 g.L-1,在pH〈5磷酸盐缓冲液中的平衡溶解度较低;去氧氟尿苷在正辛醇和水相中表观油水分配系数Papp为0.97;当pH〈7时,受pH影响不显著,表现为亲水性。结论去氧氟尿苷在水中的平衡溶解度及油水分配系数与介质的pH值有关,可以通过改变pH值,增加该药物新剂型的稳定性。     

Influence of drug partition coefficient and pH value of sink solution on permeation from porous thick-walled ethyl cellulose microcapsules

The permeation of barbitone sodium, benzoic acid, and salicylic acid from microcapsules into aqueous medium has been examined at different pH values. The apparent diffusion coefficients of drugs were linearly proportional to the ethyl cellulose/water partition coefficient of drugs, and the straight line parameters were dependent upon volume fractions of water-filled pores (i.e. capsule size), testifying to a previously proposed mechanism of drug permeation. The rate of drug permeation was also a function of the pH-value of the surrounding sink solution; the period of zero order release was longer at low pH because of the change of drug partition or solubility or both.     

黄芩苷在不同pH值缓冲液中理化常数的测定

目的考察黄芩苷在不同pH值中的平衡溶解度与表观油水分配系数,为制剂研究奠定基础。方法采用摇瓶-紫外分光光度法测定温度为25和37℃时,黄芩苷在不同pH值磷酸盐缓冲溶液中的平衡溶解度及在正辛醇/缓冲液体系中的表观油水分配系数。结果 25℃时,黄芩苷在pH=2.0,3.0,4.0,5.0,6.0,6.8,7.4,8.0和9.0缓冲液中的平衡溶解度分别为0.032,0.034,0.119,0.873,3.329,12.96,11.49,4.605和11.87mg.mL-1,相应条件下表观油水分配系数(P)值分别为0.363,0.244,0.292,0.137,0.057,0.046,0.036,0.028和0.029。37℃时,黄芩苷在pH=2.0,3.0,4.0,5.0,6.0,6.8,7.4,8.0和9.0缓冲液中的平衡溶解度分别为0.028,0.048,0.095,0.950,4.881,14.15,26.65,14.48和17.89mg.mL-1,相应条件下表观油水分配系数(P)值分别为0.234,0.224,0.365,0.103,0.074,0.049,0.034,0.034和0.035。结论黄芩苷的平衡溶解度在酸性及中性条件下受温度影响很小,在碱性条件下随着温度升高而增加;黄芩苷在酸性条件下的P值比在碱性条件下大,Pmax=0.363(T=25℃,pH=2),Pmax=0.365(T=37℃,pH=4),随着碱性的增加P值变化不明显,且温度对黄芩苷的P值几乎无影响。     

盐酸苯环壬酯油水分配系数的测定及pH值对其的影响

目的： 测定盐酸苯环壬酯油水分配系数,研究不同pH值对药物脂水分配系数的影响,为盐酸苯环壬酯新剂型的处方设计提供理论参考。方法： 本实验采用摇瓶-高效液相色谱法测定盐酸苯环壬酯在不同pH值条件下正辛醇-水/缓冲液体系中的表观脂水分配系数。结果： 盐酸苯环壬酯的脂水分配系数lgP值为1.29±0.16;在pH为1.2,3.0,4.5,5.0,5.5,6.0,6.8,7.0,8.0,9.0的环境中,脂水分配系数lgP值分别为0.31,0.72,1.16,1.34,1.99,2.26,2.22,1.61,2.34,3.24。结论： 盐酸苯环壬酯的脂水分配系数与介质的pH值有关,在pH 4.5~8.0生理条件下,盐酸苯环壬酯的脂溶性较好,水溶性差。     

沙美特罗和布地奈德的脂质体/水分配系数测定及影响因素考察

目的测定沙美特罗和布地奈德的脂质体/水分配系数(P_L/w)，考察影响两种药物P_L/w的因素，揭示药物与磷脂双分子层的作用机制。方法用平衡透析法测定两种药物在不同脂质体组成和不同介质中的P_L/w。结果两种药物P_L/w随脂质体中胆固醇含量及磷脂饱和程度增加而减小；脂质体表面负电荷、介质pH值及离子强度增加，沙美特罗的P_L/w增加。脂质体表面电荷、介质pH值及离子强度对布地奈德的P_L/w影响较小。结论药物P_L/w受药物性质、脂质体磷脂种类、饱和程度、胆固醇含量和表面电荷以及介质pH和离子强度等因素影响，在测定时应选择与生物膜环境相近的P_L/w测定条件，以反映药物在生物膜中的分配。     

Partition coefficients of dopamine antagonists in brain membranes and liposomes

Partition coefficients, Kp of dopamine antagonists, spiperone, haloperidol, domperidone and pimozide were determined in caudate nucleus microsomal membranes and in liposomes from membrane lipids. Kp values were measured as a function of temperature and the thermodynamics parameters for the transfer of the drugs from the aqueous medium to the lipid bilayer were evaluated. Partition in native membranes or in liposomes formed from the membrane lipids is not strongly dependent on temperature over the range from 8 to 37 degrees. The Kp values for spiperone, haloperidol and domperidone in membrane are 32 +/- 6, 192 +/- 11 and 308 +/- 40 respectively, whereas the equivalent values in liposomes are much higher: 195 +/- 12, 558 +/- 16 and 316 +/- 16. In contrast, for pimozide, the Kp values in membranes are higher than in liposomes: 1097 +/- 11 for microsomes and 662 +/- 10 for liposomes. Partition values in natural membranes decrease sequentially as follows: pimozide greater than domperidone greater than haloperidol greater than spiperone. Membranes rich in cholesterol show lower partition coefficients for haloperidol. The interaction of the antagonists with the bilayer is associated with small enthalpy changes and large increases in entropy, as expected for hydrophobic interactions. We conclude that the partition coefficients of the drugs studied for membranes and membranes lipids are very different from those reported for octanol/water and the latter values should not be used to estimate drug partition into membranes.     

Effect of pH on in vitro permeation of ondansetron hydrochloride across porcine buccal mucosa

The influence of drug concentration, pH in donor chamber, and 1-octanol/buffer partition coefficient on transbuccal permeation of ondansetron hydrochloride (pKa, 7.4) across porcine buccal mucosa was studied by using an in-line Franz type diffusion cell at 37 degrees C. The pH was adjusted to several values and the solubility of the drug in different pH was measured. Solubility of ondansetron hydrochloride decreases with increasing pH. The permeability of the drug was evaluated at different donor pH and drug concentrations. Permeability of un-ionized (Pu) and ionized (Pi) species of drug was calculated by fitting the data to a mathematical model. The steady state flux increased linearly with the donor concentration (r2 = 0.9843) at pH 7.4. The permeability coefficient and the partition coefficient of the drug increased with increasing pH. The values of Pu and Pi were 4.86 x 10(-6) cm/sec and 7.18 x 10(-7) (c)m/sec, respectively. The observed permeability coefficients and the permeability coefficients calculated from the mathematical model at various pH showed good linearity (r2 = 0.9799). The total permeability coefficient increased with increasing the fraction of un-ionized form of the drug. The drug permeated through buccal mucosa by a passive diffusion process. The non-ionized species of drug penetrated well through buccal mucosa and the permeation was a function of pH. Transbuccal delivery is a potential route for the administration of ondansetron hydrochloride.     

Link between drug absorption solubility and permeability measurements in Caco-2 cells

The objective of this investigation was to establish a relationship between drug permeability and solubility in vitro and the extent of drug absorption in humans. We selected drugs with varying permeabilities and solubilities with the aim of establishing a relationship between permeability and solubility measurements in vitro and the extent of absorption in vivo. Effective permeability coefficients of the model drugs (naproxen, phenytoin, propranolol, diltiazem, salicylic acid, ephedrine, cimetidine, chlorothiazide, and furosemide) at 37 degrees C and pH 7.2 were estimated using the Caco-2 cell line. Saturation solubilities of the model drugs were estimated at pH 7.2 and at 37 degrees C. Data obtained from the permeability and solubility experiments were employed in classifying the drugs into high and low permeability-solubility groups. The permeability coefficients ranged from 1x10(-7) to 4x10(-5) cm/s, and a good correlation was observed between the permeability coefficients in Caco-2 cells and percent absorbed in humans. Drugs in the high permeability, high solubility class are completely absorbed (90% or higher). The study results indicate that there is a strong link between permeability measured in Caco-2 cells, solubility, and fraction of drug absorbed in humans.     

Prediction of drug distribution into human milk from physicochemical characteristics

Decisions about the safety of breast feeding during maternal ingestion of drugs require knowledge of the amount of drug which might be present in the milk. For many drugs this has not been studied, and mothers are usually advised against breast feeding. In many cases this is undoubtedly unnecessary, as the total dose to which the baby is exposed is often negligible. It would be very helpful, therefore, to be able to predict the approximate amount of drug which might be present in milk. Existing theory of pH partitioning enables estimation of the distribution of unbound drug, i.e. milk: plasma unbound ratios. However, these ratios are poor estimates of the concentration ratios for whole milk, because whole milk contains proteins and lipid in which drugs will distribute in amounts which depend on their particular physicochemical properties. To predict the milk: plasma concentration ratios for whole milk the amount of drug present in the protein and lipid phases must be considered along with the unbound drug distribution. A 'phase distribution model' has therefore been developed which permits estimation of whole milk: plasma concentration ratios. The model requires a knowledge of the unbound drug concentration ratio, the plasma and milk unbound fractions and the milk lipid: ultrafiltrate partition coefficient. Evaluation of the model by comparison of predicted whole milk ratio values with literature milk: plasma area under the curve (AUC) ratios indicated a trend to overprediction for acidic and neutral drugs and underprediction for basic drugs. Transformation of the phase distribution equation by taking logarithms results in a relationship which can be analysed by multiple linear regression to derive predictive equations for acidic and basic drugs which take into account the relative contributions of each component of the model. Regression of the logarithms of the literature milk: plasma AUC values against the independent variables resulted in good correlations for acidic and basic drugs. The independent variables explained 93.1% and 82.9% of the variance in the values for acidic and basic drugs, respectively, with random scatter of residuals. The equations, together with those to predict unbound fractions of drug in milk and milk lipid: ultrafiltrate partition coefficients, enable the ratio of the milk: plasma AUCs to be estimated for any acidic or basic drug for which the distribution into human milk is not known, using the pKa, octanol: water partition coefficient and plasma protein binding values of the drug. The data set for neutral drugs (n = 3) was too small to develop a correlation equation.(ABSTRACT TRUNCATED AT 400 WORDS)     

Estimation of the Relative Contribution of the Transcellular and Paracellular Pathway to the Transport of Passively Absorbed Drugs in the Caco-2 Cell Culture Model

Purpose. The objective of this investigation was to determine, using the Caco-2 cell culture model, the extent to which the paracellular and transcellular routes contributed to the transport of passively absorbed drugs. An effort was also made to determine the controlling factors in this process. Methods. We selected a heterologous series of drugs with varying physicochemical parameters for the investigation. Effective permeability coefficients of the model drugs (naproxen, phenytoin, salicylic acid, chlorothiazide, furosemide, propranolol, diltiazem, ephedrine, and cimetidine), at pH 7.2 and pH 5.4, were estimated using confluent monolayers of Caco-2 cells. The biophysical model approach, based on molecular size restricted diffusion within an electrostatic field of force, used by Adson et al. (1 ,2), was employed to estimate the permeability coefficients of the ionized and unionized forms of the drugs for the paracellular and transcellular route. Results and Conclusions. The permeability coefficients of the acidic drugs was greater at pH 5.4, whereas that of the basic drugs was greater at pH 7.2 and the transcellular pathway was the favored pathway for most drugs, probably due to its larger accessible surface area. The paracellular permeability of the drugs was size and charge dependent. The permeability of the drugs through the tight junctions decreased with increasing molecular size. Further, the pathway also appeared to be cation-selective, with the positively charged cations of weak bases permeating the aqueous pores of the paracellular pathway at a faster rate than the negatively charged anions of weak acids. Thus, the extent to which the paracellular and transcellular routes are utilized in drug transport is influenced by the fraction of ionized and unionized species (which in turn depends upon the pKa of the drug and the pH of the solution), the intrinsic partition coefficient of the drug, the size of the molecule and its charge.     

Black lipid membranes as a model for intestinal absorption of drugs.

Black lipid membranes were generated in isotonic buffer (pH 4-5 and pH 6-5) from egg phosphatidylcholine and intestinal lipid, and the permeability to salicylamide, salicylic acid, p-aminobenzoic acid and tryptophan of these membranes was studied. Electrical resistance of intestinal lipid membranes was higher than that of phosphatidylcholine membranes. The presence of cholesterol produced an increase in the electrical resistance of black lipid membranes and a small decrease in the permeability of membranes to drugs. The permeability coefficient of salicylamide, an uncharged drug, was much larger than the coefficients of the charged drugs examined. The values for salicylic acid and p-aminobenzoic acid were much larger than comparable values predicted from their partition coefficients. Intestinal lipid membranes were more permeable to acidic drugs than phosphatidylcholine membranes. It is suggested that phospholipids and other lipid components of the small intestine may play an important role in the membrane permeability to acidic drugs. This method may be of interest in studying the complex processes of drug absorption from intestine.     

In vitro studies of intestinal drug absorption. Determination of partition and distribution coefficients with brush border membrane vesicles.

Brush border membrane vesicles (BBMV) were isolated from rat small intestine and characterized in terms of relative enrichment of specific organelle marker enzymes (20-fold enrichment; 20% yield), contamination by other subcellular organelles (less than 1%) and functional integrity (Na(+)-dependent glucose uptake). Using these vesicles, techniques were developed for the determination of partition and distribution coefficients for the model solutes, nitrobenzene, toluene and benzoic acid. No gender, age or regional variation along the small intestine in partition coefficient (log P) values was detected. There was no temperature (10-40 degrees) or pH (4.5-8.0) dependence in partition coefficients of nitrobenzene and toluene. Fair agreement was obtained for log P and log D values for these two solutes determined with BBMV and those reported with octanol and propylene glycol dipelargonate. Selective removal of proteins, both ecto-brush border and micro-villus core proteins, did not alter the partition coefficients of the three model solutes. In contrast, depletion of the BBMV of non-esterified fatty acids significantly decreased the partition coefficients. Liposomes prepared from BBMV lipid extracts were also used for partition coefficient determinations and gave similar values to intact BBMV; addition of increasing amounts of cholesterol to the lipid extract caused small increases in the partition coefficients of the model solutes in the liposomes. It was concluded that the partition coefficients of the BBMV were related to the lipid and not to the protein composition of the vesicles. The method offers a rapid and reliable means of measuring the partition coefficient of non-protein bound drugs and nutrients in isolated intestinal BBMV and should assist in the subsequent modelling and prediction of intestinal absorption in vivo.     

A membrane model of the human oral mucosa as derived from buccal absorption performance and physicochemical properties of the β-blocking drugs atenolol and propranolol

The buccal absorption characteristics and physicochemical properties of the β-adrenoceptor blocking agents propranolol and atenolol have been investigated to evaluate their permeation properties across biological lipid membranes. The dissociation constants, solubilities of free base, and n-heptane partition coefficients show that propranolol in its unionized form is much more lipophilic than atenolol, both drugs being bases with a similar pKa. Buccal absorption was studied under conditions of varying drug concentration, contact time, and pH, and controlled through the use of a non-absorbable marker. The absorption findings are in general agreement with the pH-partition theory. A new compartmental diffusional model that includes membrane storage and a hypothetical ‘aqueous pH-buffering surface system’ allowed a more exhaustive interpretation to be made. A method for the estimation of the intrinsic pH and buffer capacity of the postulated surface system from drug pH-absorption data and partition coefficients alone is described. With human oral mucosa the intrinsic pH was near 6·7, and the buffering capacity of the system (expressed as the ratio ΔpH/ΔpH eff) about 2·86. The method was validated using published absorption data from the rat small intestine. Absorption of unionized drug through pores is shown to be negligible in the buccal absorption situation. The time course of absorption suggests membrane storage of lipophilic compounds; the in vivo partition coefficient of unionized propranolol relative to the mucous membrane could be calculated for the peusdo-steady state of absorption, i.e. the partition equilibrium between mouth content and membrane, to be approximately 776; this value is of the same order as the in vitro partition coefficient for the erythrocyte/plasma system. The lipid biophase of the buccal membrane is estimated semiquantitatively to be of intermediate polarity (? = 3–4).     

Rapid determination of liposome-water partition coefficients (Klw) using liposome electrokinetic chromatography (LEKC)

Liposome Electrokinetic Chromatography (LEKC) provides a simple and facile approach for determining liposome-water partition coefficients, Klw. LEKC is a Capillary Electrophoresis (CE) technique where liposomes are incorporated into a buffer solution and act as a pseudostationary phase providing sites of interaction for solutes. The retention factors of solutes in LEKC are directly proportional to Klw. This article describes how LEKC can be used to determine Klw for both neutral and charged solutes. The Klw values for a group of neutral aromatic compounds, beta-blockers, and other drugs are reported. In addition, the usefulness of two quantitative structure partition relationships (QSPR) for estimation of Klw is demonstrated. One is the logarithmic linear relationship between liposome water and octanol-water partition coefficients (Pow). The other is the Linear Solvation Energy Relationship (LSER). The calculated Klw values from the two QSPR agree nicely with the observed values and with one another.