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).     

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

The buccal absorption characteristics and physicochemical properties of the beta-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 surfaces 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 deltapH/deltapH 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 (epsilon = 3-4).     

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.     

The effects of physiocochemical properties of pethidine and its basic metabolites on their buccal absorption and renal elimination.

The pKa values, RF values, partition coefficients between n-heptane and phosphate buffer (pH = 7.4), and buccal absorption tests of pethidine and its basic metabolites, norpethidine and pethidine N-oxide, have been determined. The amounts of these compounds recovered in the 48 h urine samples after intramuscular administration of pethidine (1.5 mg kg-1) to six healthy subjects varies with urinary pH values: the recovery from acidic urine (pH 5.0) is 15.2 to 52.0%, 6.7 to 12.9% and 0.2 to 2.3% of dose for pethidine, norpethidine and pethidine N-oxide respectively; in alkaline urine (pH 8.0) the values are 0.8 to 1.8%, 0.6 to 2.8% and 0.3 to 2.1% respectively. The physicochemical properties (pKa values, RF values, partition coefficients) and buccal absorption of pethidine, norpethidine and pethidine N-oxide are in good agreement with the pattern of their renal elimination in acidic and alkaline urine conditions. The contribution of the physiocochemical properties of pethidine and its metabolites to the drug's disposition in the body and the effect of urinary pH on its metabolism should be taken into account in pharmacokinetic studies and interpretation of intersubject variation in response to pethidine.     

Activated carbon as a biological model: comparison between activated carbon adsorption and oil-water partition coefficient for drug activity correlation

Octanol-water partition coefficients have been used as a principal parameter for the analysis of quantitative structure-activity relationships (QSAR). With ionized drugs, however, the correlation is not impressive. Although clinical local anesthetics exist mainly in the ionized form at physiological pH, the nerve-blocking potency is correlated with the oil-water partition coefficients of the un-ionized species. In the present report, the adsorption of 18 local anesthetics onto activated carbon surfaces was compared with oil-water partition coefficients for correlation with the minimum nerve-blocking (MBC) potency. The log octanol-water partition coefficients showed large differences between the charged and uncharged species (lidocaine: 0.49 and 1.82, procaine: -0.56 and 1.74, respectively), whereas the log carbon surface adsorbabilities showed much smaller differences (lidocaine: 3.00 and 3.04, and procaine 2.95 and 3.00, respectively). An excellent correlation was found between the log carbon surface adsorption and the log MBC values in both ionized and un-ionized drugs.     

Transport of model compounds across the peritoneal membrane in the rat.

The absorption into the systemic circulation of compounds administered intraperitoneally in large volumes was investigated in the rat. The influence on absorption of molecular weight, lipid-water partition coefficient (K), and dissociation constant (pKa) was studied. Nine neutral compounds ranging in molecular weight from 18 to 2 million demonstrated absorptions that decreased with increasing molecular weight. Five compounds were tested with variable lipid partition (K) values (0.001--3.3) and the absorptions increased from 57 to 96% as the K values increased. A series of nine acids and bases covering a wide range of pKa values )0.9--9.9) were investigated. For the acids, absorption increased with increasing pKa value, while for the bases absorption decreased with increasing pKa. For both groups of compounds absorption was directly related to the extent of ionization at physiologic pH. As has been documented for other biological membranes, the peritoneal membrane in the rat was found to behave in a lipoid manner. Unionized or lipid-soluble compounds are absorbed to a greater extent than ionized or lipid-insoluble compounds, and neutral compounds are absorbed in relation to their molecular weights.     

Buccal absorption of enalapril and lisinopril

Objective: The buccal absorption of captopril does not exhibit the classical pH/partition hypothesis, suggesting that mechanisms other than passive diffusion are involved in its absorption; animal studies have suggested that a peptide carrier-mediated transport system may be responsible for its absorption. The present study evaluated the effects of pH on octanol partitioning, and on the buccal absorption of enalapril and lisinopril, using in vitro techniques and buccal partitioning in human volunteer subjects. Methods: The partitioning of enalapril and lisinopril into n-octanol was examined over the pH range of 3–9 at room temperature. Results: Enalapril exhibited maximal partitioning into the organic phase at pH 4–5; minimal partitioning was recorded at pH values 8 and 9. The partitioning of lisinopril into n-octanol was found to be maximal at pH 9 and minimal at pH 3. Using the buccal absorption technique, the partitioning of enalapril and lisinopril (0.5 mg), was examined in six healthy male volunteers from buffered solutions (pH 3, 4, 5, 6, 7, 8 and 9). In the case of enalapril, lowest buccal partitioning occurred at pH 3, 8 and 9, while maximal partitioning occurred at pH 5; absorption of lisinopril was not extensive at any pH, but was greatest at pH 6. These results, in addition to the n-octanol partition coefficients, indicated that enalapril obeyed the normal lipid partition hypothesis with respect to buccal absorption. The buccal absorption of lisinopril also obeyed the lipid partition hypothesis over the pH range 3–7. These findings are in direct contrast to those for captopril. The buccal partitioning experiments were repeated at the maximal pH for absorption for each angiotensin converting enzyme (ACE) inhibitor, but with the addition of cephradine (0.05 mmol · l⁻¹). Conclusion: The data indicated that the presence of this peptide transport inhibitor had no effect on the buccal absorption of enalapril (0.06 mmol · l⁻¹) and lisinopril (0.057 mmol · l⁻¹), which suggests that both drugs do not share a common buccal absorption pathway with cephradine. Received: 2 January 1998 / Accepted in revised form: 31 May 1998     

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.     

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.     

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.     

Partition of Dopamine Antagonists into Synthetic Lipid Bilayers: the Effect of Membrane Structure and Composition

Abstract— Partition coefficients, K_p, of four dopamine antagonists (pimozide, fluspirilene, haloperidol and domperidone) between the aqueous phase and lipid bilayer vesicles were determined as a function of lipid chain length, unsaturation and temperature encompassing the range of the lipid phase transition. Model membranes of egg phosphatidylcholine (PC), dimyristoyl (DMPC)-, dipalmitoyl (DPPC)-, distearoyl (DSPC)- and dioleoyl (DOPC)-phosphatidylcholines were studied. K_p values of the drugs are different in the various membranes under study and depend on temperature, aliphatic carbon chain-length and on the presence of unsaturation in the aliphatic lipid chain. First-order transition of membrane lipids from the gel to the liquid crystalline state is accompanied by a sharp increase of the partition coefficient of pimozide and fluspirilene in DMPC, DPPC and DSPC bilayers. For domperidone, K_p values are maximal within the midpoint of phase transition of DMPC and DPPC, while for DSPC K_p values increase progressively with increasing temperature. Haloperidol K_p values display a maximum at the mid-point of phase transition of DMPC, while a progressive increase of K_p is observed in DPPC and DSPC. The four drugs are easily accommodated in bilayers of short aliphatic chain lipids (DMPC), the partition coefficients being 17137 for pimozide, 18 700 for fluspirilene, 686 for domperidone and 722 for haloperidol, at temperatures 10°C below the mid-point of the lipid phase transition. Except for haloperidol, the partition of the drugs in DOPC (18:1) is higher than that in DSPC (18:0) bilayers at a temperature above the phase transition temperature of both lipids. From our experiments we can conclude that artificial membranes are useful models to understand the physicochemical mechanisms involved in the interaction of dopamine antagonists with biological membranes.     

Correlation of partition coefficients in n-heptane-aqueous systems with buccal absorption data for a series of amines and acids

Partition coefficients in n-heptane-0·1n sodium hydroxide or 0·1n hydrochloric acid have been determined for a series of amines and acids; linear relations between chain-lengths and the logs of their partition coefficients were found. The plot of alkyl chain-length against buccal absorption of some amphetamines and fenfluramines, when they were 1% unionized, was linear. There was also a linear relation between the logs of the partition coefficients and buccal absorption of the amines and acids when these were 1 and 10% unionized. Those amines and acids having similar partition coefficients, when equally unionized, were absorbed to the same extent in the buccal test over the pH range 4 to 9. During the test the pH at the surface of the buccal membrane was shown to be the same as that of the solution in the mouth. n-Heptane is considered to be equivalent in solvent properties to the buccal lipid membrane for the compounds used in the present test.     

Carticaine: action of the local anesthetic on myelinated nerve fibres

Unlike other clinically used local anesthetics, carticaine contains a thiophene ring. Current voltage relations of peak sodium current INa and steady state potassium current IK were measured in voltage clamp experiments on the node of Ranvier of Rana esculenta. The dependence of the maximum sodium conductance on external concnetration of carticaine is described by an apparent dissociation constant KNa = 0.065 mM at pH = 7.3. An apparent dissociation constant KK = 0.147 mM at pH = 7.3 was calculated on the basis of the action of carticaine on the maximum potassium conductance. Similar to other amine local anesthetics, carticaine blocks sodium channels at a lower concentration than potassium channels, but lower concentrations of the thiophene derivative than of the benzene derivatives are needed to block the ionic channels. The partition coefficients in-octanol/Soerensen buffer at pH = 7.35 follow the sequence lidocaine (46) more than carticaine (17) more than procaine (2). Therefore, the action of these local anesthetics on sodium channels and potassium channels seems not to follow mere lipid solubility properties of the neutral drug. Increasing the pH from 6.3 to 8.3 favoured the neutral drug form and enhanced the block of both ionic channels but the relative reduction of the ionic currents was larger for potassium currents.     

Narcotic and narcotic antagonist pKa''s and partition coefficients and their significance in clinical practice

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 antagonist. These studies were carried out by a novel 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 and 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 among which are cited hyperventilation under a general anesthetic while concomitantly under a narcotic analgesic, obstetetrical analgesia, and medical and anti-abuse usage of narcotic antagonists. The partition coefficients and drug distribution coefficients were noticeably different at 20degreesC (where such measurements are customarily made) from those at 37degreesC (body temperature). Furthermore, various drugs exhibit very non-equivalent increases in drug distribution coefficients with increasing temperature, ranging from 21% for naltrexone. This non-regularity 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 20degrees to 37degreesC.     

Effect of local anesthetics on electrically excitable bilayer lipid membranes

The effect of three local anesthetics on the action potential of electrically excitable bilayer lipid membranes was studied. The synthetic action potential of bilayers was abolished at drug concentrations roughly identical to those abolishing the action potential in nerves. The threshold depolarizing constant current pulse triggering the synthetic action potential was increased by slightly lower concentrations of these local anesthetics. It is suggested that the anionic channels, which are responsible for the rising phase of the action potential in bilayers, are affected preferentially by these drugs.     

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.     

Pharmacological analysis of the responsiveness of guinea-pig lung parenchymal strip to dopamine.

The local anaesthetic action of a series of nerve blocking agents was examined at temperatures between 4 degrees C and 20 degrees C in isolated sciatic nerves from cold-adapted frogs. Cooling alone had little effect on the amplitude of the action potential but the conduction velocity was decreased and the duration increased. Cooling had little effect on the local anaesthetic action of the short chain alkanols but that of the long chain alkanols, benzyl alcohol, benzocaine and pentobarbitone was markedly enhanced. The partition of both short and long chain alkanols and of pentobarbitone into a liposome suspension of similar composition to axonal membrane lipids was reduced by cooling from 40 degrees C to 4 degrees C. The results are not compatible with the lipid hypotheses of anaesthetic action. The implications for the nature of the site of action are discussed.     

Spectrofluorimetric analysis and buccal absorption of medifoxamine

Medifoxamine is a new investigational antidepressant drug. Its buccal absorption at pH 5-9, which can be considered as an in-vivo model of passive drug transfer through a lipid membrane, was studied in six normal, healthy volunteers to predict its pharmacokinetic profile in man. Maximum absorption of medifoxamine occurred at pH 8, which is close to its pKa (8.5).     

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.     

Fast and direct method for measuring 1-octanol-water partition coefficients exemplified for six local anesthetics.

The 1-octanol-water partition coefficients (P(OW)) of six local anesthetics were determined by an improved direct method. In the experimental setup, dialysis tubing is used to separate the aqueous from the octanol phase containing the analyte. This procedure allows the application of ultrasonic agitation to shorten the equilibration time to 2 h. The measurements had a reproducibility of +/- 0.1 log units and showed an excellent agreement with the results obtained with the traditional shake-flask method. However, the correlation with the indirect determination using reversed-phase liquid chromatography was considerably poorer, showing that high-performance liquid chromatography determinations are not always very reliable. Because all the anesthetics used are nitrogen bases, the measurements were performed at two pH values. The method allows a fast and reliable direct determination of partition coefficients.