Estimating the fraction dose absorbed from suspensions of poorly soluble compounds in humans: a mathematical model

A microscopic mass balance approach has been developed to predict the fraction dose absorbed of suspensions of poorly soluble compounds. The mathematical model includes four fundamental dimensionless parameters to estimate the fraction dose absorbed: initial saturation (Is), absorption number (An), dose number (Do), and dissolution number (Dn). The fraction dose absorbed (F) increases with increasing Is, An, and Dn and with decreasing Do. At higher Dn and lower Do, the fraction dose absorbed reaches the maximal F, which depends only on An. The dissolution number limit on F can appear at both lower Do and lower Dn. Likewise, at higher Do and Dn, the fraction dose absorbed reaches a Do limit. Initial saturation makes a significant difference in F at lower Do and Dn. It is shown that the extent of drug absorption is expected to be highly variable when Dn and Do are approximately one. Furthermore, by calculating these dimensionless groups for a given compound, a formulation scientist can estimate not only the extent of drug absorption but also the effect, if any, of particle size reduction on the extent of drug absorption.

Mass balance approaches for estimating the intestinal absorption and metabolism of peptides and analogues: theoretical development and applications

A theoretical analysis for estimating the extent of intestinal peptide and peptide analogue absorption was developed on the basis of a mass balance approach that incorporates convection, permeability, and reaction. The macroscopic mass balance analysis (MMBA) was extended to include chemical and enzymatic degradation. A microscopic mass balance analysis, a numerical approach, was also developed and the results compared to the MMBA. The mass balance equations for the fraction of a drug absorbed and reacted in the tube were derived from the general steady state mass balance in a tube: [formula: see text] where M is mass, z is the length of the tube, R is the tube radius, Pw is the intestinal wall permeability, kr is the reaction rate constant, C is the concentration of drug in the volume element over which the mass balance is taken, VL is the volume of the tube, and vz is the axial velocity of drug. The theory was first applied to the oral absorption of two tripeptide analogues, cefaclor (CCL) and cefatrizine (CZN), which degrade and dimerize in the intestine. Simulations using the mass balance equations, the experimental absorption parameters, and the literature stability rate constants yielded a mean estimated extent of CCL (250-mg dose) and CZN (1000-mg dose) absorption of 89 and 51%, respectively, which was similar to the mean extent of absorption reported in humans (90 and 50%). It was proposed previously that 15% of the CCL dose spontaneously degraded systematically; however, our simulations suggest that significant CCL degradation occurs (8 to 17%) presystemically in the intestinal lumen.(ABSTRACT TRUNCATED AT 250 WORDS)

Viscoelastic properties of polyacrylic acid gels in mixed solvents

The objective of this study is to investigate the viscoelastic properties of Carbopol 934P polymeric systems in a variety of mixtures of pharmaceutical solvents. Carbopol 934P neutralized with a 1:1 equivalent ratio of triethanolamine was dissolved in various binary or ternary solvent mixtures consisting of propylene glycol, glycerol formal, and water. Dynamic moduli G' and G'', complex viscosities, eta' and eta'', and loss tangent, tan delta, were examined over a frequency range of 10(-3) to 10 Hz using an oscillatory viscoelastic rheometer at 30 degrees C. The results indicated that for 0.5-1.5 wt% neutralized Carbopol in ternary mixtures, G' and G'' increased by 3-4 orders of magnitude and the phase angle decreased from 80 to 25 degrees when the water content in the solvent mixture increased from 10 to 80 wt%. These studies also indicated that the addition of water to nonaqueous Carbopol 934P polymer systems transforms them from low-viscosity solutions to gels with significant elastic behavior involving physical interaction and entanglement of polymer segments with solvents.

Characterization of the oral absorption of several aminopenicillins: determination of intrinsic membrane absorption parameters in the rat intestine in situ

The absorption mechanism of several penicillins was characterized using in situ single-pass intestinal perfusion in the rat. The intrinsic membrane parameters were determined using a modified boundary layer model (fitted value +/- S.E.): Jmax* = 11.78 +/- 1.88 mM, Km = 15.80 +/- 2.92 mM, Pm* = 0, Pc* = 0.75 +/- 0.04 for ampicillin; Jmax* = 0.044 +/- 0.018 mM, Km = 0.058 +/- 0.026 mM, Pm* = 0.558 +/- 0.051, Pc* = 0.757 +/- 0.088 for amoxicillin; and Jmax* = 16.30 +/- 3.40 mM, Km = 14.00 +/- 3.30 mM, Pm* = 0, Pc* = 1.14 +/- 0.05 for cyclacillin. All of the aminopenicillins studied demonstrated saturable absorption kinetics as indicated by their concentration-dependent wall permeabilities. Inhibition studies were performed to confirm the existence of a nonpassive absorption mechanism. The intrinsic wall permeability (Pw*) of 0.01 mM ampicillin was significantly lowered by 1 mM amoxicillin and the Pw* of 0.01 mM amoxicillin was reduced by 2 mM cephradine consistent with competitive inhibition.

Structural specificity of mucosal-cell transport and metabolism of peptide drugs: implication for oral peptide drug delivery

The brush border membrane of intestinal mucosal cells contains a peptide carrier system with rather broad substrate specificity and various endo- and exopeptidase activities. Small peptide (di-/tripeptide)-type drugs with or without an N-terminal alpha-amino group, including beta-lactam antibiotics and angiotensin-converting enzyme (ACE) inhibitors, are transported by the peptide transporter. Polypeptide drugs are hydrolyzed by brush border membrane proteolytic enzymes to di-/tripeptides and amino acids. Therefore, while the intestinal brush border membrane has a carrier system facilitating the absorption of di-/tripeptide drugs, it is a major barrier limiting oral availability of polypeptide drugs. In this paper, the specificity of peptide transport and metabolism in the intestinal brush border membrane is reviewed.

Equilibrium and kinetic factors influencing bile sequestrant efficacy

In vitro bile salt binding equilibria and kinetic studies were performed with cholestyramine to determine how these factors influence bile sequestrant efficacy in vivo. Chloride ion at physiologic concentrations caused more than a twofold reduction in glycocholate (GCH) binding, compared to binding in the absence of salt, over a range of GCH concentrations and was also observed to displace bound GCH. In addition, chloride ion displaced from cholestyramine as a result of bile salt binding was measured using a chloride selective electrode, and the results show that bile salt binding is due to ion exchange. Comparison of the results of the equilibrium binding experiments to human data shows that the effect of anion binding competition alone cannot account for the lack of efficacy of cholestyramine. Consideration of other effects, such as additional binding competition or poor availability for binding, based on data from the literature, shows that adequate bile salt binding potential exists and that these interferences are not major factors influencing resin efficacy. In kinetic studies, both binding uptake of GCH and displacement of GCH from cholestyramine by chloride ion were relatively rapid, indicating that cholestyramine should equilibrate rapidly with bile salts in the GI tract. Based on these findings, it is suggested that the low efficacy of cholestyramine is a result mainly of its relatively poor ability to prevent bile salt reabsorption in the ileum.

Utilization of peptide carrier system to improve intestinal absorption: targeting prolidase as a prodrug-converting enzyme

The feasibility of targeting prolidase as a peptide prodrug-converting enzyme has been examined. The enzymatic hydrolysis by prolidase of substrates for the peptide transporter L-alpha-methyldopa-pro and several dipeptide analogues without an N-terminal alpha-amino group (phenylpropionylproline, phenylacetylproline, N-benzoylproline, and N-acetylproline) was investigated. The Michaelis-Menten parameters Km and Vmax for L-alpha-methyldopa-pro are 0.09 +/- 0.02 mM and 3.98 +/- 0.25 mumol/min/mg protein, respectively. However, no hydrolysis of the dipeptide analogues without an N-terminal alpha-amino group is observed, suggesting that an N-terminal alpha-amino group is required for prolidase activity. These results demonstrate that prolidase may serve as a prodrug-converting enzyme for the dipeptide-type prodrugs, utilizing the peptide carrier for transport of prodrugs into the mucosal cells and prolidase, a cytosolic enzyme, to release the drug. However, a free alpha-amino group appears to be necessary for prolidase hydrolysis.

Intranasal absorption of flurazepam, midazolam, and triazolam in dogs

Intranasal delivery of flurazepam, midazolam, and triazolam was studied in a dog model as a possible alternate route of drug administration for treatment of insomnia. Four beagles received each hypnotic by both intranasal and oral routes on two separate occasions. Plasma concentrations for each hypnotic after dosing were measured by electron-capture gas-liquid chromatography. The mean intranasal absorption rates (tmax) of flurazepam, midazolam, and triazolam were 1.7, 2.0, and 2.6 times faster, respectively, compared with oral dosing. The mean dose-normalized peak concentrations (Cmax) after intranasal delivery were 16.4, 2.9, and 3.4 times higher, respectively, versus oral administration. The mean dose-normalized AUCs estimated for these compounds after nasal administration were 2.4-, 2.5-, and at least 2-fold larger than after oral administration for midazolam, triazolam, and flurazepam, respectively. If these observations can be extrapolated to humans, the faster absorption achieved by the intranasal route would appear to benefit insomniacs characterized by difficulty in falling asleep because of an anticipated faster sedative effect onset. The higher peak concentrations and larger amounts absorbed in the case of intranasal midazolam and triazolam delivery may lead to dose reduction.

Influence of physical aging on mechanical properties of polymer free films: the prediction of long-term aging effects on the water permeability and dissolution rate of polymer film-coated tablets

The effects of physical aging on the water permeation of cellulose acetate and ethylcellulose, the mechanical properties of ethylcellulose, and the dissolution property of hydroxypropyl methylcellulose phthalate were investigated. The water permeabilities of cellulose acetate and ethylcellulose and the dissolution rate of hydroxypropyl methylcellulose phthalate were found to decrease with physical aging time after being quenched from above the glass transition temperatures to sub-Tg temperatures. The gradual approach toward thermodynamic equilibrium during physical aging decreases the free volume of the polymers. This decrease in free volume is accompanied by a decrease in the transport mobility, with concomitant changes in those properties of the polymer that depend on it. The effects of long-term aging on the dissolution rate and water permeabilities of these polymers can be estimated from a linear double-logarithmic relationship between the mobility properties and physical aging time. The existence of the linear double-logarithmic relationship can be derived from the Williams-Landel-Ferry equation, the Doolittle equation, Struik's model, and Fujita's relationship between diffusion and free volume.

Stereoselective high-performance liquid chromatographic assay for pirmenol enantiomers in dog plasma

Pirmenol enantiomers in dog plasma were quantified using a stereospecific high-performance liquid chromatographic method with ultraviolet detection at 262 nm. Racemic pirmenol and internal standard, (+)-propranolol, were isolated from dog plasma by a three-step extraction procedure using toluene, 0.1 M hydrochloric acid and hexane, respectively. A chiral analytical column (Chiralcel OJ) was used with a mobile phase consisting of hexane-isopropanol-diethylamine (98.9:1.0:0.1). Linear calibration curves were obtained in the concentration range 0.0200-5.00 micrograms/ml for each enantiomer. Precision of the method, expressed as coefficient of variation for nine quality control samples, was 7.1% for (+)-pirmenol and 6.4% for (-)-pirmenol. Bias was +/- 2.2% for (+)-pirmenol and +/- 1.5% for (-)-pirmenol in quality control samples.

Mixture experimental design in the development of a mucoadhesive gel formulation

The objective of the present study is to apply response surface methodology to the design and analysis of composite experiments containing independent covariate(s). The approach is illustrated here by the study of viscosity characteristics of a polymeric mucoadhesive formulation in multicomponent solvent vehicles. The nonaqueous formulation will produce a gel network with significant rheological change when in contact with body fluids. The process of water inclusion will induce not only solvent compositional change of the mixture but also concomitant dilution of the polymer concentration. To study the viscosity change over the solvent compositions and polymeric concentrations of interest, an experimental design is utilized consisting of a 10-point simplex-centroid lattice augmented with three interior points at each polymeric concentration. The contour patterns are compared with the experimental data using the variance and lack of fit, starting with the Scheffe linear model and building up to the full cubic model including the covariate terms. The fitted model provides information needed to predict optimum formulations, i.e., initial viscosity of less than 100 cP, but yielding rheological profiles commensurate with high degrees of substantivity when diluted with water. For illustrative purposes, the Carbopol resins neutralized with a 1:1 molar equivalent ratio of triethanolamine in three primary solvents, propylene glycol, glycerol formal, and water, were chosen for this study.

Viscometric study of polyacrylic acid systems as mucoadhesive sustained-release gels

This report describes a novel nonaqueous polymeric formulation that exhibits low-viscosity fluid behavior for ease of spraying with conventional nebulizer, which when sprayed into the nasal cavity, transforms to a high-viscosity gel for efficient retention and drug absorption. The transformation occurs because of the rheological changes induced by a change in the solvent composition of the polymeric formulation in the moist nasal cavity. Such a rheological change would then facilitate enhanced residence time of the drug at the site of administration in order to avoid drainage losses. This study reports the results of the effects of a variety of factors such as solvent composition and polymer concentration on the rheological properties of a polyacrylic acid polymer. An attempt to correlate viscosity enhancement effects with enhanced and sustained-release behavior of propranolol, a drug that undergoes extensive first-pass effects, from such formulations via nasal administration in beagle dogs is also described.

Effects of gravity on gastric emptying, intestinal transit, and drug absorption

The effects of microgravity on the physiologic response of the human body, the physical properties of gastrointestinal contents, and the influence these responses have on drug absorption are becoming more and more critical as the duration of humans in the hostile space environment dramatically increases. In this environment, some conventional oral dosage forms may be severely limited as an effective drug regimen. To understand the effects of microgravity, one must first understand the basic forces acting on a particle moving through a walled-tube such as the small intestine: gravity (FG), buoyancy (FB), and drag (FD). These forces can be combined and rearranged into a dimensionless ratio of gravitational forces to viscous forces. This is the most important dimensionless group influencing the motion of a particle relative to the fluid. Gastric emptying is highly influenced by several factors: volume, calories, exercise, size, density, temperature, viscosity, osmolality as well as those factors associated with physiologic responses: splanchnic blood flow, body position, and electrolyte balance. This array of factors can lead to variability in drug plasma levels. In the absence of gravity, the factors of size and density would appear to be most directly altered due to their dependence on the force of gravity. Intestinal transit rate in a gravity environment is highly dependent on the motility state of the GI tract either fasted or fed partly due to the higher viscosities of chyme in the fed state. In space, the absence of gravity may tend to increase the transit rate along the small intestine by decreasing the dimensionless ratio of gravitational forces to viscous forces. In zero gravity, therefore, these alterations in GI emptying and intestinal transit rate could lead to erratic plasma levels and inefficient absorption.

Stereoselective systemic disposition of ibuprofen enantiomers in the dog

The pharmacokinetics of ibuprofen are complicated by the unidirectional metabolic inversion of the (-)-R- to (+)-S-enantiomer. Chiral inversion is of therapeutic significance since the drug's pharmacologic activity has been shown to depend upon the (+)-S-isomer. As a result, the present study was undertaken to determine if chiral inversion occurs systemically and to elucidate further the kinetics of the inversion process. Experiments were performed in the beagle dog after intravenous bolus injections of ibuprofen enantiomers separately [100 mg (-)-R, n = 4; 100 mg (+)-S, n = 4] and as admixtures of varying proportions [100 mg (-)-R + 100 mg (+)-S, n = 4; 100 mg (-)-R + 200 mg (+)-S, n = 2]. Plasma samples of (-)-R- and (+)-S-enantiomers were measured by a stereospecific HPLC assay after all drug administrations. Based on the area under the plasma concentration-time curves for (+)-S after administration of each enantiomer alone, chiral inversion was 70 to 75%. A progressive reduction in total plasma clearance of (-)-R-ibuprofen is also observed as increasing amounts of (+)-S-enantiomer are added to the system. The results demonstrate that chiral inversion occurs to a significant extent in the systemic circulation in dog and that R-to-S inversion of ibuprofen may be inhibited by its (+)-S-enantiomer.

Predicting fraction dose absorbed in humans using a macroscopic mass balance approach

A theoretical approach for estimating fraction dose absorbed in humans has been developed based on a macroscopic mass balance that incorporates membrane permeability and solubility considerations. The macroscopic mass balance approach (MMBA) is a flow model approach that utilizes fundamental mass transfer theory for estimating the extent of absorption for passively as well as nonpassively absorbed drugs. The mass balance on a tube with steady input and a wall flux of Jw = PwCb results in the following expression for fraction dose absorbed, F: [formula; see text] where the absorption number, An = L/R.Pw/(vz), L and R are the intestinal length and radius, Pw is the unbiased drug wall permeability, (vz) is the axial fluid velocity, C*b = Cb/Co and is the dimensionless bulk or lumen drug concentration, Cb and Co are the bulk and initial drug concentrations, respectively, and z* is the fractional intestinal length and is equal to z/L. Three theoretical cases are considered: (I) Co less than or equal to S, Cm less than or equal to S, (II) Co greater than S, Cm less than or equal to S, and (III) Co greater than S, Cm greater than S, where S is the drug solubility and Cm is the outlet drug concentration. Solving the general steady-state mass balance result for fraction dose absorbed using the mixing tank (MT) and complete radial mixing (CRM) models results in the expressions for the fraction dose absorbed in humans. Two previously published empirical correlations for estimating fraction dose absorbed in humans are discussed and shown to follow as special cases of this theoretical approach. The MMBA is also applied to amoxicillin, a commonly prescribed orally absorbed beta-lactam antibiotic for several doses. The parameters used in the correlation were determined from in situ or in vitro experiments along with a calculated system scaling parameter. The fraction dose absorbed calculated using the MMBA is compared to human amoxicillin pharmacokinetic results from the literature with initial doses approximated to be both above and below its solubility. The results of the MMBA correlation are discussed with respect to the nonpassive absorption mechanism and solubility limitation of amoxicillin. The MMBA is shown to be a fundamental, theoretically based model for estimating fraction dose absorbed in humans from in situ and in vitro parameters from which previously published empirical correlations follow as special cases.

Prediction of physical aging in controlled-release coatings: the application of the relaxation coupling model to glassy cellulose acetate

The effect of physical aging on both the water transport properties and the mechanical properties of glassy cellulose acetate was investigated. Results indicate a reduction in the mechanical rate of relaxation as well as a reduction in the water permeability as the glass ages. A model which describes the low-frequency relaxation behavior of condensed, amorphous systems is used to quantitate the mechanical relaxation data. Systematic changes in key parameters from this model signify alterations in the microscopic or short-range structure as the glass physically ages. Predictions from this model correlate quite closely with the observed water permeability reductions and thus indicate that the transport properties of glassy polymers are dependent on the structure of the glass. This approach may provide further insight into the effects of nonequilibrium behavior on pharmaceutically important properties and may serve as a basis for predicting aging and permeability changes in controlled-release dosage forms.

Some general influences of n-decylmethyl sulfoxide on the permeation of drugs across hairless mouse skin

The influences of n-decylmethyl sulfoxide (NDMS) on the permeation of phenyl propanolamine, propranolol, acyclovir, and hydrocortisone through hairless mouse skin were investigated. Permeation of these compounds increased systematically with increasing NDMS concentration up to a limiting value for the permeability coefficient of about 0.1 cm/h. Permeability coefficients obtained with stripped skin indicated that, where NDMS has its maximal effect, the compounds diffuse through the skin as if there were no stratum corneum. Diffusion lag times of the test compounds through skin sections exposed to NDMS were far shorter than lag times in control experiments. The kinetics of NDMS action were assessed by pretreating skin sections for various periods of time with the enhancer and then assessing permeability. When applied as a 10 mM aqueous solution, the full effect of NDMS was obtained in 3 h of exposure and the effect lasted for at least 24 h. Treating the skin with ethanol/water (2:1) to elute NDMS after the pretreatment did not diminish its effect. Propranolol and phenyl propanolamine solubilized NDMS, presumably through the formation of mixed micelles, which had a profound effect on the permeation rates of both drug and enhancer.

Pharmacokinetics and pharmacodynamics of nifedipine in children with bronchopulmonary dysplasia and pulmonary hypertension

The pharmacokinetics and associated pharmacodynamics of nifedipine were studied in nine children aged 5 to 68 mo with bronchopulmonary dysplasia and pulmonary artery hypertension after a single oral dose of 1.44 mumol/kg (0.5 mg/kg). In the cardiac catheterization laboratory, hemodynamic measurements were made in duplicate just before the nifedipine dose and at 5 min and 0.5 and 1.0 h after the dose. The plasma nifedipine concentration was measured by HPLC at each of the above times and at 2.5, 4.0, 6.0, and 8.0 h after the dose. The mean (+/- SD) maximum plasma concentration and the time to maximum plasma concentration were 243.4 +/- 194.5 nmol/L and 1.0 +/- 0.8 h, respectively. The mean area under the plasma concentration-time curve was 761 +/- 509 nmol.h/L. The mean elimination rate constant and t1/2 were 0.456 +/- 0.194 h-1 and 1.8 +/- 0.8 h, respectively. Nifedipine caused a significant (p less than or equal to 0.05) reduction in the mean pulmonary artery pressure by 5 min and in the mean pulmonary vascular resistance index and mean aortic pressure by 30 min, and these reductions remained significant through the 1-h measurement interval. The magnitude of acute hemodynamic response correlated closely with the plasma nifedipine concentrations. No significant change occurred in the mean arterial oxygen saturation or cardiac index during the study period. The percentage changes from baseline in the mean pulmonary artery pressure and mean pulmonary vascular resistance index were approximately double the percentage change in the mean aortic pressure, suggesting that nifedipine had some degree of selective impact on the pulmonary vascular bed.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural requirements for the intestinal mucosal-cell peptide transporter: the need for N-terminal alpha-amino group

The requirement for a free alpha-amino group for the intestinal peptide carrier-mediated transport was investigated. A series of dipeptide analogues without the N-terminal alpha-amino group [including phenylpropionylproline, phenylacetylproline, N-benzoylproline, phenylacetyl-alpha-methyldopa, and hippuric acid (N-benzoylglycine)] were studied in the perfused rat intestinal segment. The absorption of phenylpropionylproline, phenylacetyl-alpha-methyldopa, and N-benzoylproline was concentration dependent. The transport parameters (mean +/- SD) of phenylpropionylproline and N-benzoylproline were as follows: Jmax*, 0.037 (+/- 0.019) mM; Km, 0.045 (+/- 0.027) mM; Pc*, 0.830 (+/- 0.130); and Pm*, 0.673 +/- 0.049; and Jmax*, 1.34 (+/- 0.24) mM; Km, 1.31 (+/- 0.30) mM; Pc*, 1.02 (+/- 0.11); and Pm*, 0; respectively. The intestinal permeabilities of phenylpropionylproline, phenylacetylproline, N-benzoylproline, and hippuric acid (N-benzoylglycine) were significantly reduced by dipeptides and cephradine. These results strongly suggest that these dipeptide analogues, without an alpha-amino group, are transported by the peptide carrier and provide more direct evidence that a free alpha-amino group is not absolutely essential for the mucosal-cell peptide carrier-mediated transport.

Calculation of the aqueous diffusion layer resistance for absorption in a tube: application to intestinal membrane permeability determination

The single-pass intestinal perfusion technique has been used extensively to estimate the wall permeability in rats. The unbiased membrane parameters can be obtained only when the aqueous resistance is properly accounted for. This aqueous resistance was calculated numerically from a convective diffusive mass transfer model, including both passive and carrier-mediated transport at the intestinal wall. The aqueous diffusion layer resistance was shown to be best described by a function of the form, [formula: see text] where G zeta, P*m, P*c, Km, and Co are, respectively, Graetz number, passive permeability, carrier-mediated permeability, Michaelis constant, and the drug concentration entering the tube. Asterisked are dimensionless quantities obtained by multiplying the permeability constants with R/D, where R and D being radius and drug diffusivity, respectively. A, B, C, D and E were obtained by a least-squares nonlinear regression method, giving values of 1.05, 1.74, 1.27, 0.0659, and 0.377, respectively, over the range of 0.001 less than or equal to G zeta less than or equal to 0.5, 0.01 less than or equal to P*m less than or equal to 10, 0.01 less than or equal to P*c less than or equal to 10, and 0.01 less than or equal to Km/Co less than or equal to 100. This aqueous resistance was found to converge to those calculated from Levich's boundary layer solution in low Graetz range, indicating the correct theoretical limit. Using an iteration method, the equation was shown to be useful in extracting the intrinsic membrane permeability from the experimental data.

Oral absorption of peptides: influence of pH and inhibitors on the intestinal hydrolysis of leu-enkephalin and analogues

Leu-enkephalin (YGGFL) and several analogues were chosen as model peptides for the study of peptide absorption and hydrolysis in the rat jejunum. An HPLC assay was adapted to detect YGGFL or the analogues and metabolites. Peptide hydrolysis was studied in the rat jejunum using a single-pass perfusion method. Extensive hydrolysis of YGGFL was observed in the rat jejunum and approaches to reduce its metabolism were studied. The brush border enzymes are a major site of enkephalin hydrolysis. Lumenal peptidases were secondary to the brush border enzymes in hydrolyzing the enkephalins in this system. In the in situ perfusion system, YGGFL is hydrolyzed primarily to Tyr and GGFL by the brush border aminopeptidase and to YGG and FL by brush border endopeptidase. Lowering the jejunal pH below 5.0 significantly reduces aminopeptidase activity and, to a lesser extent, endopeptidase activity. An aminopeptidase inhibitor, amastatin, produced more pronounced inhibitory effects at higher pH and the endopeptidase inhibitors, tripeptides YGG and GGF, are effective even below pH 5.0. Coperfusion of YGGFL with a combination of aminopeptidase and endopeptidase inhibitors, e.g., amastatin and YGG, is more effective in inhibiting hydrolysis since both metabolic pathways are inhibited. Leu-D(Ala)2-enkephalin, while showing enhanced stability against aminopeptidase hydrolysis, is hydrolyzed at the Gly-Phe bond by the endopeptidase. Its hydrolysis is not affected by pH changes or amastatin but is decreased by YGG. The YGGFL wall permeability was estimated and is not a limiting factor for oral absorption.

The influence of the interdigestive migrating myoelectric complex on the gastric emptying of liquids

It is unknown how the interdigestive migrating motor complex influences the gastric emptying of liquids. Therefore, the gastric emptying rate of 50- and 200-mL volumes of phenol red solution were measured while monitoring contractile activity. Motor activity was recorded using a hydraulic manometric system and expressed as either the proximity of dosing time to time of appearance of phase III or as a motility index, defined as (contractile area)/(sampling interval time). After an initial lag period, emptying was log linear. With a 50-mL oral dose, the mean gastric emptying rate of the log-linear phase was successively faster during phase I (0.018 +/- 0.003 min-1), phase II (0.083 +/- 0.031 min-1), and late phase II/III (0.171 +/- 0.066 min-1) (P less than 0.05). Similarly, the mean lag time decreased successively with phases I, II, and late II/III (19.1 +/- 12.4, 7.6 +/- 5.6, and 3.8 +/- 2.8 minutes, respectively). At a 200-mL oral dose, there was no difference in the emptying rate between phase I and phase II (0.104 +/- 0.0014 vs. 0.110 +/- 0.041 min-1), but the emptying rate during late phase II/III was significantly greater (0.236 +/- 0.069 min-1); lag time was not dependent on phase. There was a statistical difference in the overall mean emptying rate between the 50- and 200-mL volumes. Also, during phase I, the emptying rate was faster for the 200-mL volume. This study shows a strong dependence of liquid gastric emptying rate and lag time on interdigestive antral motility, the emptying of small volumes being more dependent on motility phase than that of large volumes. Phase-related fluctuations in contractile activity can account for much of the reported variability in gastric emptying data. Furthermore, this study suggests that dose volume and interdigestive motor activity at the time of drug administration can affect absorption and onset of therapeutic response for some drugs.

Transdermal delivery of bioactive peptides: the effect of n-decylmethyl sulfoxide, pH, and inhibitors on enkephalin metabolism and transport

We investigated the effects of the nonionic surfactant, n-decylmethyl sulfoxide (NDMS), pH, and inhibitors on the metabolism and the permeation of amino acids, dipeptides, and the pentapeptide enkephalin, through hairless mouse skin. An HPLC gradient method was developed to identify the possible peptide and amino acid metabolites of leucine-enkephalin. NDMS increased the permeability of all amino acids and peptides tested. At neural pH, the enzyme activity within the skin was such that no flux of leucine-enkephalin (YGGFL) was observed and the donor cell concentration of YGGFL decreased rapidly. The major cleavage occurred at the Tyr-Gly bond. At pH 5.0 the metabolic activity was reduced significantly and a substantial flux of YGGFL was observed. Enzymatically stable YGGFL analogues, Tyr-D-Ala-Gly-Phe-Leu (YDAGFL) and its amide, exhibited significant fluxes even at neutral pH in the presence of NDMS, but with substantial metabolism. YDAGFL amide was more stable to metabolism than YDAGFL. The rates of metabolism of the peptides in the skin homogenates were in the order: FL much greater than YGGFL greater than GFL greater than GGFL much greater than YG, YGG much greater than YDAGFL amide. In the skin homogenates puromycin and amastatin showed the highest inhibitory effects, while FL and GFL were only slightly active. However, in the skin diffusion experiments, FL allowed the highest amount of intact parent compound to permeate, making it the most potent inhibitor. These results show that the complex proteolytic enzyme activities occurring during skin permeation are different from those in skin homogenates and that a combination of enhancer, pH adjustment, and inhibitors can increase the transdermal delivery of peptides.

Absorption of polyethylene glycols 600 through 2000: the molecular weight dependence of gastrointestinal and nasal absorption

Polyethylene glycols (PEGs) 600, 1000, and 2000 were used to study the molecular weight permeability dependence in the rat nasal and gastrointestinal mucosa. Absorption of the PEGs was measured by following their urinary excretion over a 6-hr collection period. HPLC methods were used to separate and quantitate the individual oligomeric species present in the PEG samples. The permeabilities of both the gastrointestinal and the nasal mucosae exhibited similar molecular weight dependencies. The steepest absorption dependence for both mucosae occurs with the oligomers of PEG 600, where the extent of absorption decreases from approximately 60% to near 30% over a molecular weight range of less than 300 daltons. Differences in the absorption characteristics between the two sites appear in the molecular weight range spanned by PEG 1000. For these oligomers, the mean absorption from the nasal cavity is approximately 14%, while that from the gastrointestinal tract is only 9%. For PEG 2000, mean absorption decreases to 4% following intranasal application and below 2% following gastrointestinal administration. Within the PEG 1000 and 2000 samples, however, very little molecular weight dependency is seen among the oligomers. In the range studied, a distinct molecular weight cutoff was not apparent at either site.

The molecular weight dependence of nasal absorption: the effect of absorption enhancers

A series of polyethylene glycols (PEGs) ranging in molecular weight from near 600 to over 2000 daltons was used to study the effects of three absorption enhancers (sodium glycocholate, sodium lauryl sulfate, and polyoxyethylene 9 lauryl ether) on the molecular weight permeability profile of the nasal mucosa of the rat. Molecular weight-permeability properties were studied both by following changes in the excretion of the polyethylene glycols as a function of their molecular size and by examining the nasal mucosa for morphologic changes following exposure to the PEG/enhancer mixtures. Each absorption enhancer was found to affect the mucosa and its permeability in a unique manner. At a 1% concentration, sodium glycocholate only slightly affects tissue morphology and does not significantly alter the molecular weight permeability profile of the mucosa. In contrast, 1% sodium lauryl sulfate causes severe alteration of the mucosa and also greatly increases the absorption of both the PEG 600 and the PEG 2000 oligomers. Polyoxyethylene 9 lauryl ether was found to exert its action in a concentration-dependent manner. At a concentration of 0.1%, few changes were seen in either mucosal integrity or permeability. At a 1% concentration, however, a significant alteration in the structure of the mucosal tissues as well as a profound increase in the permeability of the mucosa to the PEGs was observed. Correlation of mucosal integrity with the effectiveness of an enhancer indicates that some of these compounds appear to be acting by altering the structure of the mucosa.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of physical aging on the dissolution rate of anionic polyelectrolytes

The effect of physical aging on the dissolution and mechanical properties of hydroxypropyl methylcellulose phthalate (HPMCP) was investigated. Dissolution rate measurements were performed on films which, initially above the glass transition temperature, Tg, were quenched to a sub-Tg storage temperature, aged at that temperature for a period of time and then quenched again to 25 degrees C. Within the time scale of observation, reductions in the dissolution rate to a limiting value were observed. HPMCP was also found to age in the same storage temperature range as determined by a creep compliance technique. These mechanical results indicate a change in glass structure and show that a limiting density was approached. Parallel changes were observed in the dissolution rate studies suggest that dissolution rate is governed in part by glass density. Therefore, mechanical changes of glassy films can yield pharmaceutically relevant information about the extent of physical aging and serve as an indicator of the effect of aging on dissolution rate.

Gastric emptying of nondigestible solids in dogs: a hydrodynamic correlation

The influence of particle size, particle density, fluid viscosity, and fluid flow rate on the gastric emptying of nondigestible solids was investigated in five dogs with chronically placed fistulas. Six hundred and fifty particles of 13 different size and density combinations were administered simultaneously with 500 ml of either normal saline or low-, medium-, or high-viscosity polymer solutions. The canine stomach was found to discriminate between these solids on the basis of size and density at all levels of viscosity above saline. The observed patterns of emptying are consistent with the hypothesis that gastric emptying of nondigestible solids is governed in part by hydrodynamics and correlate well with the gastric-emptying coefficient (GEC), a dimensionless grouping of variables that takes the form GEC = (Dpy/Dp) [g(rho f - rho p)Dp2]/[eta (nu)] where [g(rho f - rho p)] is particle buoyancy consisting of fluid (rho f) and particle (rho p) densities and g, the gravitational constant; (Dp) is the particle diameter, (Dpy) the estimated pyloric diameter, eta the fluid viscosity, and (nu) the average linear velocity of fluid exiting the stomach.

Intestinal absorption mechanism of dipeptide angiotensin converting enzyme inhibitors of the lysyl-proline type: lisinopril and SQ 29,852

The intestinal absorption mechanism of two nonsulfhydril lysyl-proline angiotensin converting enzyme (ACE) inhibitors, lisinopril (1) and SQ 29,852 (2; [(S)-1-[6-amino-2-[[hydroxy (4-phenylbutyl)-phosphinyl]oxy[-1-oxohexyl]-L-proline) were investigated in rats using a single-pass perfusion method. Compound 2 is well absorbed from rat jejunum, whereas lisinopril absorption is relatively low. The permeability of both ACE inhibitors is concentration dependent and is decreased by the dipeptide Tyr-Gly and by cephradine, indicating a nonpassive absorption mechanism via the peptide carrier-mediated transport system. Compound 2 is well absorbed by a nonpassive mechanism, in parallel with a small passive component. The estimated dimensionless carrier parameters for 2 are J*max = 0.16, Km = 0.08 mM, P*c = 2.0, and P*m = 0.25; for lisinopril, passive absorption is not significant and its absorption is nonpassive: J*max = 0.032, Km = 0.082 mM, and P*c = 0.39 (where J*max is the maximal flux, Km is the Michaelis constant, P*c is the carrier permeability, and P*m is the passive permeability). These results offer a mechanistic explanation for the prolonged ACE inhibition and the low oral bioavailability of lisinopril, and for the nonlinear pharmacokinetics of 2.

Passive and carrier-mediated intestinal absorption components of two angiotensin converting enzyme (ACE) inhibitor prodrugs in rats: enalapril and fosinopril

The intestinal absorption mechanism of two ACE inhibitor prodrugs, enalapril and fosinopril, was investigated in rats using a single-pass perfusion method. A modified boundary layer solution was applied to determine the apparent intestinal wall permeability. The prodrug enalapril is well absorbed from rat jejunum, whereas the parent drug, enalaprilat, is poorly absorbed. The permeability of enalapril is concentration dependent and is decreased by the dipeptide Tyr-Gly and by cephradine but not by the amino acids L-leucine or L-phenylalanine, indicating a nonpassive absorption mechanism via the small peptide carrier-mediated transport system. In contrast, fosinopril is readily absorbed by a concentration-independent mechanism without the involvement of the peptide carrier.

Characterization of the oral absorption of beta-lactam antibiotics. II. Competitive absorption and peptide carrier specificity

The beta-lactam antibiotic oral absorption pathway is studied using a single-pass perfusion technique in the rat small intestine. Beta-lactam antibiotic absorption in the presence of amino acids, small peptides, and other beta-lactams is modeled using a simple competitive inhibition boundary condition at the intestinal wall, with a corrected value for the intestinal wall concentration, Cw, derived from the modified boundary layer analysis. The model-predicted permeability in the presence of an inhibitor is used to characterize the beta-lactam antibiotic intestinal carrier system. Several concentrations of cephalexin, coperfused with a constant concentration of cefadroxil (equal to its Km), showed that the Km of cephalexin approximately doubled from 7.2 (+/- 1.1) to 18.8 (+/- 4.1) mM; Jmax remained unchanged at 9.2 (+/- 1.2) and 11.1 (+/- 2.1) mM; and the carrier permeability, Pc, was reduced by approximately 50% from 1.11 (+/- 0.10) to 0.59 (+/- 0.04), consistent with competitive absorption kinetics. The predicted in situ wall permeability, the mean value of P*w, of beta-lactams perfused in the presence of other beta-lactams was calculated and then compared with experimentally determined values. For cefadroxil, P*w = 0.27 (+/- 0.04), the mean value of P*w = 0.29; for cefatrizine, P*w = 0.67 (+/- 0.09), the mean value of P*w (+/- 0.09), the mean value of P*w = 0.59; and for cephalexin, P*w = 0.56 (+/- 0.05), the mean value of P*w = 0.59.(ABSTRACT TRUNCATED AT 250 WORDS)

Use of the peptide carrier system to improve the intestinal absorption of L-alpha-methyldopa: carrier kinetics, intestinal permeabilities, and in vitro hydrolysis of dipeptidyl derivatives of L-alpha-methyldopa

Intestinal permeabilities of five dipeptidyl derivatives of L-alpha-methyldopa (I) were studied by an in situ intestinal perfusion method. The dipeptides displayed a significant increase in their permeabilities compared to L-alpha-methyldopa. The increases ranged from 4 to 20 times. These results suggest that the peptide transport system is less structurally specific than the amino acid transport systems and can be used as an absorption pathway for peptide analogues. The kinetic advantage demonstrated by the dipeptide, L-alpha-methyldopa-L-phenylalanine, over the amino acid analogue, L-alpha-methyldopa, suggests that the peptide carrier would be a possible route for improving the intestinal absorption of pharmacologically active amino acid analogues. Furthermore, the preliminary results of in vitro hydrolysis studies of selected dipeptidyl derivatives indicate that the peptide carrier system could be used as a base for a prodrug strategy.

Passive and carrier-mediated intestinal absorption components of captopril

The intestinal absorption mechanism of captopril was investigated in fasted rats using a single-pass perfusion method. Captopril and captopril disulfide were analyzed by HPLC. A modified boundary-layer solution was applied to determine the apparent intestinal wall permeabilities (Pw*). The results indicated that captopril is very permeable in the small intestine but not in the colon, and the permeability in the small intestine is both pH and concentration dependent. The estimated parameters for the carrier are: Km*, 6 mM; Jmax*, 12 mM; and Pc*, 2. There is also a significant passive component to captopril absorption in the small intestine. Furthermore, the intestinal permeability of captopril was significantly decreased by the withdrawal of sodium from the perfusate (3 times), and the addition of 85 mM of gly-gly (2.5 times), 15 mM of gly-pro (4 times), dipeptide mixture (4.5 times), 0.5 mM of 2,4-dinitrophenol (4 times), and 10 mM of cephradine (6 times). This is the first demonstration that an angiotensin converting enzyme (ACE) inhibitor is at least in part transported by a carrier-mediated process in the intestine via the peptide carrier system. In addition, the results showed that the peptide carrier system can transport a substrate without a "N" terminal nitrogen atom.

Estimating human oral fraction dose absorbed: a correlation using rat intestinal membrane permeability for passive and carrier-mediated compounds

Based on a simple tube model for drug absorption, the key parameters controlling drug absorption are shown to be the dimensionless effective permeability, P*eff, and the Graetz number, Gz, when metabolism or solubility/dissolution is not rate controlling. Estimating the Graetz number in humans and assuming that P*aq is not rate controlling give the following equation for fraction dose absorbed: F = 1 - e-2P*w. The correlation between fraction dose absorbed in humans and P*w determined from steadystate perfused rat intestinal segments gives an excellent correlation. It is of particular significance that the correlation includes drugs that are absorbed by passive and carrier-mediated processes. This indicates that P*w is one of the key variables controlling oral drug absorption and that the correlation may be useful for estimating oral drug absorption in humans regardless of the mechanism of absorption.

Characterization of the oral absorption of beta-lactam antibiotics. I. Cephalosporins: determination of intrinsic membrane absorption parameters in the rat intestine in situ

The oral absorption of five cephalosporin antibiotics, cefaclor, cefadroxil, cefatrizine, cephalexin, and cephradine, has been studied using a single-pass intestinal perfusion technique in rats. Intrinsic membrane absorption parameters, "unbiased" by the presence of an aqueous permeability (diffusion or stagnant layer), have been calculated utilizing a boundary layer mathematical model. The resultant intrinsic membrane absorption parameters are consistent with a significant carrier-mediated, Michaelis-Menten-type kinetic mechanism and a small passive component in the jejunum. Cefaclor colon permeability is low and does not exhibit concentration dependent behavior. The measured carrier parameters (+/- SD) for the jejunal perfusions are as follows: cefaclor, J*max = 21.3 (+/- 4.0), Km = 16.1 (+/- 3.6), P*m = 0, and P*c = 1.32 (+/- 0.07); cefadroxil, J*max = 8.4 (+/- 0.8), Km = 5.9 (+/- 0.8), P*m = 0, and P*c = 1.43 (+/- 0.10); cephalexin, J*max = 9.1 (+/- 1.2), Km = 7.2 (+/- 1.2), P*m = 0, and P*c = 1.30 (+/- 0.10); cefatrizine, J*max = 0.73 (+/- 0.19), Km = 0.58 (+/- 0.17), P*m = 0.17 (+/- 0.03), and P*c = 1.25 (+/- 0.10); and cephradine, J*max = 1.57 (+/- 0.84), Km = 1.48 (+/- 0.75), P*m = 0.25 (+/- 0.07), and P*c = 1.06 (+/- 0.08). The colon absorption parameter for cefaclor is P*m = 0.36 (+/- 0.06, where J*max (mM) is the maximal flux, Km (mM) is the Michaelis constant, P*m is the passive membrane permeability, and P*c is the carrier permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

pH-dependent swelling and solute diffusion characteristics of poly(hydroxyethyl methacrylate-co-methacrylic acid) hydrogels

Poly(hydroxyethyl methacrylate-co-methacrylic acid) hydrogels can swell extensively in a high-pH medium where the carboxyl groups are ionized. The swelling equilibrium is a strong function of the methacrylic acid composition of the polymer and pH of the medium. The nonionized gel structure was found to be rather insensitive to the amount of cross-linker, tetraethylene glycol dimethacrylate (TEGDMA), incorporated, within the range of 0.5 to 3%. This result is supportive of the existence of secondary interactions that shield the effect of covalent cross-links. Phenylpropanolamine (PPA) was used as a probe solute to study the diffusion characteristics of the poly(HEMA-co-MA) gels. Its diffusion coefficient in the swollen matrices of different methacrylic acid compositions at various pH's was measured via a desorption method. It is evident that these diffusion coefficients follow Yasuda's free volume theory, which expresses an exponential relationship between the solute diffusivity in a swollen polymer membrane and the reciprocal of the membrane hydration. Although interactions exist between PPA and the hydrogel matrix, these interactions are not significant enough to perturb the free volume relationship established. This observation can be explained by the high ionic strength of the system.

Reaction plane approach for estimating the effects of buffers on the dissolution rate of acidic drugs

A mass transfer model was developed to describe the dissolution and reaction of an acidic drug, naproxen [(+)-6-methoxy-alpha-methyl-2-naphthaleneacetic acid], from a rotating disk in buffered solutions. Dissolution in 0.0010 to 0.030 M acetate, citrate, and phosphate buffer solutions, with 0.1 M KCl added, over a pH range of 2 to 8 at 25 degrees C, was investigated. Features of the mass transfer model include: treatment of the mass transfer as a convective diffusion process; use of the buffer reactions and the acid ionization as a single flux boundary condition; the capacity to make accurate a priori predictions without fitting film thicknesses or diffusion coefficients; and the option of considering two limiting cases for the maximal increase in dissolution that could be expected in a reactive buffer and the increase in dissolution in a buffer at maximum buffer capacity.

Human postprandial gastric emptying of 1-3-millimeter spheres

Microspheres of pancreatin should empty from the stomachs of patients with pancreatic insufficiency as fast as food. The present study was undertaken in 26 healthy subjects to identify the size of spheres that would empty from the stomach with food and to determine whether different meals alter this size. Spheres of predefined sizes were labeled with 113mIn or 99mTc. Using a gamma-camera, we studied the concurrent gastric emptying of spheres labeled with 113mIn and of chicken liver labeled with 99mTc in 100-g, 154-kcal or 420-g, 919-kcal meals, or the concurrent emptying of 1-mm vs. larger spheres. One-millimeter spheres emptied consistently (p less than 0.01, paired t-test) faster than 2.4- or 3.2-mm spheres when ingested together with either the 420- or 100-g meals. Thus, in the 1-3-mm range of diameters, sphere size was a more important determinant of sphere emptying than meal size. Statistical analyses indicated that spheres 1.4 +/- 0.3 mm in diameter with a density of 1 empty at the same rate as 99mTc-liver. Our data indicate some commercially marketed microspheres of pancreatin will empty too slowly to be effective in digestion of food.

Determination of intrinsic membrane transport parameters from perfused intestine experiments: a boundary layer approach to estimating the aqueous and unbiased membrane permeabilities

A boundary layer approach is developed for estimating the aqueous resistance in a perfused rat intestine experiment. Knowing the aqueous resistance allows the membrane surface concentration to be calculated as a function of the perfusate inlet concentration and perfusional flow rate. Determination of membrane uptake as a function of the membrane surface concentration rather than the perfusate concentration gives the intrinsic, unbiased membrane parameters for the uptake mechanism of Michaelis-Menten-type kinetics in parallel with passive diffusion. The aqueous resistance derived in the analysis is verified by comparison with flux data for 1-leucine and progesterone measured at various flow rates and intestinal lengths. The approach allows for a direct estimate to be made of the unbiased membrane permeability parameters.

Membrane permeability parameters for some amino acids and beta-lactam antibiotics: application of the boundary layer approach

The boundary layer approach to analyzing the results of the perfused intestinal segment method of measuring membrane permeabilities is applied to the amino acids; leucine, valine, phenylalanine, lysine and aspartic acid and the beta-lactam antibiotics, cephalexin and penicillin V. The analysis indicates that in determining the membrane parameters, Pw vs. Cw data are preferable to using Jss = PwCw vs. Cw data. It is further shown that the carrier permeability, Pc* = Jmax*/Km, may be the most significant parameter to consider since luminal amino acid or drug concentration may generally be below the Km value. A comparison of P*c values for the beta-lactams with results for passively absorbed compounds indicates that the cephalosporins would be expected to be well absorbed orally based on the perfusion results. This suggests that this approach may be useful in estimating oral drug absorption for compounds that are absorbed passively as well as by a carrier-mediated mechanism.

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, C2. The R3SSE, with the C2 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.

The influence of variable gastric emptying and intestinal transit rates on the plasma level curve of cimetidine; an explanation for the double peak phenomenon

A physiological flow model is presented to account for plasma level double peaks based on cyclical gastric emptying and intestinal motility in the fasted state. Central to the model is the assumption that gastric emptying and intestinal transit rates will vary directly with the strength of the contractile activity characteristic of the fasted state motility cycle. Simulated curves clearly indicate that variable gastric emptying rates can result in variable absorption rates from the gastrointestinal tract and double peaks in the plasma level curves of cimetidine. Vital to the occurrence of double peaks are (i) dosing time relative to phasic activity, (ii) variability in flow out of the stomach, and (iii) a small emptying rate constant Qs/Vs, for a period of time within the first hour after administration. Variability in intestinal flow rates alone does not cause a double peak in the plasma level curve. Results of the simulations, as well as experimental results, can be categorized according to the shapes of the plasma level curves into four types: type A, Cpmax (1) less than Cpmax (2); type B, single peak; type C, Cpmax (1) greater than Cpmax (2); type D, Cpmax(1) = Cpmax(2). Assuming that the experimental results were obtained from fasted subjects, with the time of dose administration being a random variable, the frequency of the experimental curves having shape A, B, C, or D correlates extremely well with theoretical predictions. It is concluded that variable gastric emptying rates due to the motility cycle can account for plasma level double peaks. Furthermore, variable gastric emptying rates combined with the short plasma elimination half-life and poor gastric absorption of cimetidine can be the cause of the frequently observed plasma level double peaks.

Comparison of gastrointestinal pH in cystic fibrosis and healthy subjects

The primary objective of this study was to define the pH conditions under which supplemental pancreatic enzyme preparations must function in the upper gastrointestinal tract. The hypothesis was that normal or greater acid output in patients with cystic fibrosis (CF), combined with low pancreatic bicarbonate output, results in an acidic duodenal pH, compromising both dosage-form performance and enzyme activity. Gastrointestinal pH profiles were obtained in 10 CF and 10 healthy volunteers under fasting and postprandial conditions. A radiotelemetric monitoring method, the Heidelberg capsule, was used to continuously monitor pH. Postprandial duodenal pH was lower in CF than in healthy subjects, especially in the first postprandial hour (mean time greater than pH 6 was 5 min in CF, 11 min in healthy subjects, P less than 0.05). Based on the dissolution pH profiles of current enteric-coated pancreatic enzyme products, the duodenal postprandial pH in CF subjects may be too acidic to permit rapid dissolution of current enteric-coated dosage forms. However, the pH was above 4 more than 90% of the time on the average, suggesting that irreversible lipase inactivation in the duodenum is not likely to be a significant limitation to enzyme efficacy. Overall results suggest that slow dissolution of pH-sensitive coatings, rather than enzyme inactivation, may contribute to the failure of enteric-coated enzyme supplements to normalize fat absorption.

Oral absorption of 21-corticosteroid esters: a function of aqueous stability and intestinal enzyme activity and distribution

The intestinal absorption of hydrocortisone and prednisolone are compared with three water-soluble derivatives (succinate, phosphate, and lysinate) in experiments at two levels of biological system complexity. Rates of absorption are compared by measuring permeabilities from rat intestinal perfusions of drugs and derivatives in solution. Extents of absorption are compared over a 10-fold dose range of parent steroid and with the steroid derivatives by measuring plasma levels from solid oral dosage in dogs. While the parent steroids are well absorbed over the entire length of the intestinal tract, variability in plasma levels is observed at higher doses. Limited solubility and resultant dissolution rate variability are likely to be playing a role in the early erratic blood level profiles found at higher doses. While the soluble prodrugs have a dissolution rate advantage which results in a greater concentration gradient, their absorption is limited by their aqueous luminal stability, their polarity and resultant passive membrane permeability, and the distribution and activity of enzyme reconversion sites in the intestinal tract. The unstable lysinate ester, targeted for aminopeptidase, has an absorption profile and permeability similar to that of the parent steroid. The absorption of the moderately stable succinate ester is limited by its polarity and the activity of intestinal esterases. The stable phosphate derivative is well absorbed in the upper intestine, where high levels of alkaline phosphatase exist, while the prodrug polarity and drop-off of enzyme activity limit its absorption from the lower gastrointestinal (GI) tract.

Uptake of prodrugs by rat intestinal mucosal cells: mechanism and pharmaceutical implications

The in vitro intestinal ring uptake of prednisolone (11 beta,17,21-trihydroxypregna-1,4-diene-3,20-dione), prednisolone 21-succinate, and prednisolone 21-phosphate was compared in rings prepared from rat jejunum and colon. An HPLC assay was developed to determine whether the drug or intact prodrug was taken up by the tissue. In jejunum and colon, the uptake of prednisolone was limited by its solubility (0.84 mM, 37 degrees C). The freely soluble succinate and phosphate esters were well absorbed in the jejunum. The only species detected in jejunal tissue after incubation with prednisolone 21-phosphate was prednisolone, indicating hydrolysis prior to absorption. This implication was verified by light microscopy. Incubation of tissue from the jejunum with prednisolone 21-succinate resulted in uptake of a mixture of prodrug and parent drug, with the latter form predominating. Prednisolone 21-succinate was also absorbed well in the colon, where the predominant species taken up by the tissue was the intact ester. The half-life of the succinate ester in the tissue was approximately 1 h postincubation, implicating enzyme mediation. Uptake of the phosphate ester by the colon was less than 20% of that observed in the jejunum, with the species absorbed still being primarily the parent alcohol. Light microscopy techniques confirmed that prednisolone 21-phosphate and hydrocortisone 21-phosphate are good substrates for brush border membrane alkaline phosphatase in the jejunum, and that lack of this enzyme in colon tissue was most likely responsible for poor uptake in the colon.

Dissolution of acidic and basic compounds from the rotating disk: influence of convective diffusion and reaction

A mass transfer model was developed to describe the dissolution and reaction of acidic and basic compounds from a rotating disk in unbuffered water. Dissolution of two carboxylic acids, 2-naphthoic acid (1) and naproxen [(+)-6-methoxy-alpha-methyl-2-naphthaleneacetic acid, 2], and the free base, papaverine (6,7-dimethoxy-1-veratrylisoquinoline, 3), in aqueous solutions (mu = 0.1 with KCI) at 25 degrees C were investigated. An automated dissolution apparatus, which consisted of microcomputer-controlled autoburets, was constructed to monitor and adjust the pH of the aqueous solutions during the experiments. Unique features of the mass transfer model include treatment of mass transfer as a convective diffusion process rather than a stagnant film diffusion only process; treatment of ionization and acid-base reactions as heterogeneous reactions; use of experimental diffusion coefficients for all species, particularly H+ and OH-; and application of boundary conditions that specify flux for surface ionization produced species. The model accurately predicted the dissolution rate assuming the solubility, pKa, and diffusion coefficient of the compound were independently known. The model also predicted pH at the solid-liquid surface, the flux of H+ from the surface, and the contribution of A- to the total acid flux as a function of bulk pH of the aqueous solution.

Concentration and pH dependency of alpha-methyldopa absorption in rat intestine

The intestinal absorption of alpha-methyldopa from perfused segments of rat intestine was determined. Jejunal and ileal segments were studied at pH 4.5, 6.0 and 7.4 at an alpha-methyldopa concentration of 0.01 mM, and at pH 6.0 at concentrations of 0.01, 0.10 and 10 mM. Intestinal absorption was found to depend on both concentration and pH but not on segment. A weak positive correlation (r = 0.5) was observed between net water absorption and alpha-methyldopa absorption, similar to that observed for amino acids. The low intestinal wall permeability of alpha-methyldopa under normal intestinal pH conditions (pH 7.4) is consistent with the incomplete oral bioavailability of this drug.