151
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Lui CY, Oberle R, Fleisher D, Amidon GL. Application of a radiotelemetric system to evaluate the performance of enteric coated and plain aspirin tablets. J Pharm Sci 1986; 75:469-74. [PMID: 3735085 DOI: 10.1002/jps.2600750510] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
The bioavailability of enteric coated and plain aspirin tablets was studied in four beagle dogs. Blood sampling for enteric coated tablets was planned with the aid of a radiotelemetric system. The release of aspirin from its dosage form was detected by monitoring the change in intestinal pH. Aspirin and salicylic acid levels in plasma obtained from the enteric coated dosage form exhibited familiar concentration versus time absorption profiles. Variation in the plasma concentrations of these two compounds within each dog studied (four runs each) was relatively small when time zero was adjusted to the commencement of tablet dissolution. The plasma levels obtained from plain aspirin (three runs each), however, show atypical absorption. The estimated absolute bioavailability was 0.432 +/- 0.0213 and 0.527 +/- 0.0260 for enteric coated and plain aspirin, respectively. Other pharmacokinetic parameters for these two dosage forms such as the highest observed plasma concentration (Cmax) (10.9 +/- 0.535 microgram/mL versus 13.6 +/- 1.88 micrograms/mL) and the time to reach Cmax (tmax) (26.6 +/- 1.94 min versus 31.0 +/- 7.04 min) agree well. The mean values for gastric emptying time, in vivo coating dissolution time, and in vivo disintegration/dissolution time of the tablet core for enteric coated aspirin are 48.7 +/- 7.23 min, 44.3 +/- 3.80 min, and 34.7 +/- 2.04 min, respectively.
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152
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Lui CY, Amidon GL, Berardi RR, Fleisher D, Youngberg C, Dressman JB. Comparison of gastrointestinal pH in dogs and humans: implications on the use of the beagle dog as a model for oral absorption in humans. J Pharm Sci 1986; 75:271-4. [PMID: 3701609 DOI: 10.1002/jps.2600750313] [Citation(s) in RCA: 159] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
Abstract
Gastrointestinal pH as a function of time was recorded for 4 beagle dogs and 10 human subjects using radiotelemetric pH measuring equipment. Results indicated that in the quiescent phase, gastric pH in the dogs (mean = 1.8 +/- 0.07 SEM) was significantly (p less than 0.05) higher than in humans (1.1 +/- 0.15). No significant difference in the time for the pH monitoring device to empty from the stomach was noted for the two species (99.8 +/- 27.2 min for dogs, 59.7 +/- 14.8 min for humans, p greater than 0.05). The fasting intestinal pH in dogs was consistently higher than in humans, with an average canine intestinal pH of 7.3 +/- 0.09 versus 6.0 +/- 0.14 for humans. The implication of these observations for extrapolation of drug absorption data from dogs to humans are discussed.
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153
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Johnson DA, Amidon GL. The effect of enzymatic reaction on dissolution rate: theoretical analysis and experimental test. J Pharm Sci 1986; 75:195-203. [PMID: 3958933 DOI: 10.1002/jps.2600750221] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
The dissolution behavior of N-acetylphenylalanine ethyl ester (1) and N-benzoyltyrosine ethyl ester (2) from a rotating disk into aqueous solutions containing the enzyme alpha-chymotrypsin was investigated. The effect of the bulk enzymatic reaction on the dissolution rates is modeled using the continuity equation where the reaction term is considered a constant throughout the reaction zone. Dimensional analysis on the continuity equation defines the important parameter R* = KcatE0h2/(CsD) which is the ratio of the diffusion time to the reaction time. This parameter correctly predicted the fact that the enzymatic reaction had only a slight impact on the dissolution of the highly soluble 1 while the effect on the less soluble 2 was large. Also predicted by R* is the dissolution dependence on the catalytic rate constant. The variation of this rate constant with pH is consistent with the dependence on pH found for the dissolution rate of 2. It is further demonstrated that the decrease in dissolution rate with solubility can be significantly reduced when the dissolving compound is an enzyme substrate. For the two compounds used in this study the dissolution rate decreased with the square root of solubility, as predicted by the theoretical analysis in the presence of enzyme. Other experiments included the variation of the enzyme concentration and the rotational speed of the spinning disk. All experiments were designed to show how R* could correctly predict the relative importance of the convective, diffusive, and reactive processes.
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154
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Dressman JB, Amidon GL, Fleisher D. Absorption potential: estimating the fraction absorbed for orally administered compounds. J Pharm Sci 1985; 74:588-9. [PMID: 4020642 DOI: 10.1002/jps.2600740523] [Citation(s) in RCA: 166] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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155
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Amidon GL, Dressman JB. Enteric coated aspirin circumventing gastric retention. J Rheumatol 1985; 12:387-8. [PMID: 4032418] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
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156
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157
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Fleisher D, Stewart BH, Amidon GL. Design of prodrugs for improved gastrointestinal absorption by intestinal enzyme targeting. Methods Enzymol 1985; 112:360-81. [PMID: 3900636 DOI: 10.1016/s0076-6879(85)12030-6] [Citation(s) in RCA: 31] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/07/2023]
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158
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Johnson K, Amidon GL, Pogany S. Solution kinetics of a water-soluble hydrocortisone prodrug: hydrocortisone-21-lysinate. J Pharm Sci 1985; 74:87-9. [PMID: 3981427 DOI: 10.1002/jps.2600740124] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
Hydrocortisone-21-lysinate was synthesized as an amino acid prodrug of hydrocortisone to serve as a substrate for brush border aminopeptidases. This strategy was developed to demonstrate that an improvement in oral absorption could be obtained through reconversion in vivo. The aqueous stability of hydrocortisone-21-lysinate was studied over the pH range 3-8 at 25 degrees C. Reversible acyl migration of the lysine group between the 21- and 17-position hydroxyl groups was observed as well as hydrolysis. The observed half-life for direct hydrolysis of hydrocortisone-21-lysinate is 40 d at pH 3 and 30 min at pH 7. The relative instability at pH 7 is probably due to electrostatic stabilization of the negatively charged tetrahedral intermediate by the protonated amino groups.
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159
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Abstract
A two-tank perfect-mixing tank model was used to stimulate GI absorption. The effect of drug parameters (pK alpha, solubility, and intrinsic wall permeability) and system parameters (pH profile, volume of intestinal contents, and intestinal flow rate) on drug absorption were studied by numerical data stimulation. When the dose did not exceed the solubility of the drug in the intestinal lumen, the fraction absorbed depended on the transit rate relative to the absorption rate and the pK alpha relative to the pH profile, but was independent of drug dose. Saturation of one or both tanks led to dose-dependent absorption. The model was used to simulate absorption of chlorothiazide. Good agreement between simulated and experimental data led to the conclusion that the physical characteristics of chlorothiazide, rather than a saturable transport mechanism at the intestinal wall, may be responsible for the dose-dependent absorption observed for this drug. The model was also used to simulate hydrochlorothiazide absorption. By applying the same system parameters used for chlorothiazide, the model simulation correctly predicted the dose proportionality of hydrochlorothiazide absorption. The lack of dose dependency in this case may be attributed to the higher solubility and pK alpha of hydrochlorothiazide compared with chlorothiazide.
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160
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Abstract
A radiotelemetric method for the in vivo evaluation of enteric coating performance is described, and its advantages and disadvantages are compared with those of other available methods. Hydroxypropyl methylcellulose phthalate was used as the test enteric coating. Four dogs were administered several batches of enteric-coated tablets containing buffers. Tablet disintegration was determined by radiotelemetric detection of the pH drop in the upper intestine due to release of the buffer. Premature rupture of the coating in the stomach was detected by a rise and then a fall in gastric pH prior to gastric emptying. The average gastric emptying time was 80 +/- 18 min (SEM), while the average time for a tablet to disintegrate in the upper intestine was 14.2 +/- 2 min. The average disintegration time was not affected by a change in the batch (for a given tablet core pH) or the dog used, suggesting that the method yielded readily reproducible results. Although there was little correlation with in vitro disintegration times, the method gave results similar to those reported in the literature for the same enteric coating in a human study. Of the formulations tested, it was concluded that buffering the core to pH 4 was most suitable for studying enteric coating performance.
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161
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Williams NA, Amidon GL. Excess free energy approach to the estimation of solubility in mixed solvent systems II: Ethanol-water mixtures. J Pharm Sci 1984; 73:14-8. [PMID: 6694074 DOI: 10.1002/jps.2600730105] [Citation(s) in RCA: 36] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The use of the reduced three-suffix solubility equation in characterizing solubility in ethanol-water mixtures is discussed. The equation states that ln xs2,m = z1 ln xs2,1 + z3 ln xs2,3 - A1-3z1z3(2z1 - 1)- (q2/q1) + A3-12z21z3(q2/q3) + C2z1z3, where xs2,1, xs2,3, and xs2,m are the mole fraction solubilities of the solute in ethanol (subscript 1), water (subscript 3), and in the mixture (m); A1-3 and A3-1 are solvent-solvent interaction terms; C2 is a solute-solvent interaction term; and the q- and z-values are molar volumes and solute-free volume fractions, respectively. The contributions of the various terms in the equation to solubility are examined, and the possible use of its derivative in indicating whether a maximum may exist in the solubility profile is discussed. Methods of obtaining the solvent-solvent interaction constant and the ternary constant C2 are described, and the general effectiveness of the equation in describing solubility is examined. The equation is shown to be applicable to 10 compounds with widely different physical properties and, thus, appears to combine both ease of use and general utility.
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162
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Williams NA, Amidon GL. Excess free energy approach to the estimation of solubility in mixed solvent systems I: Theory. J Pharm Sci 1984; 73:9-13. [PMID: 6694092 DOI: 10.1002/jps.2600730104] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
An approach is developed by which the solubility of an organic compound in mixed solvents may be estimated. In this approach, an expression for the excess Gibbs free energy of mixing for multicomponent solvent systems was used to obtain parameters characteristic of the interaction between the solvents. A fairly simple equation which predicts the solubility of a solute in a binary solvent system over the entire solvent composition range was then derived. The equation may be partitioned into terms that contain (a) pure solvent solubilities, (b) solvent-solvent interaction contributions, and (c) contributions from the solute-mixed solvent interactions. The required data are the molar volume of the solute, the pure solvent solubilities, and, theoretically, one experimentally determined solubility in a solvent mixture. The equation can be easily extended for systems with three or more solvents.
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163
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Williams NA, Amidon GL. Excess free energy approach to the estimation of solubility in mixed solvent systems III: Ethanol-propylene glycol-water mixtures. J Pharm Sci 1984; 73:18-23. [PMID: 6694075 DOI: 10.1002/jps.2600730106] [Citation(s) in RCA: 31] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The reduced three-suffix solubility equation derived from the Wohl excess free energy expression is used to describe the solubility of phenobarbital in propylene glycol-water, ethanol-propylene glycol, and ethanol-water-propylene glycol mixtures and the solubility of hydrocortisone in propylene glycol-water mixtures. Solvent-solvent interaction constants were obtained by fitting total vapor pressure versus composition data, obtained at 25 +/- 0.1 degrees C, to the Wohl excess free energy model for the solvents. The equation describes solubility in these systems satisfactorily except for phenobarbital in ethanol-propylene glycol, where the solubility is fairly high and assumptions involved in the derivation of the equation do not hold.
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164
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Van Campen L, Amidon GL, Zografi G. Moisture sorption kinetics for water-soluble substances. I: Theoretical considerations of heat transport control. J Pharm Sci 1983; 72:1381-8. [PMID: 6663470 DOI: 10.1002/jps.2600721204] [Citation(s) in RCA: 76] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
A model based on heat transport control was developed to describe the uptake of water on a deliquescent solid in an atmosphere of pure water vapor. The model assumes the presence of a saturated liquid film on the surface of the solid. The decrease in the vapor pressure of water over the surface, brought about by the colligative effect of solid dissolved in the liquid film, is effectively offset by the increase in temperature of the film (and the solid) caused by the heat released on condensation of the water vapor. The thermal transients die out quickly and a steady-state analysis is valid. At steady state the temperature of the liquid film (and solid) is that temperature at which the vapor pressure of water above the saturated solution is equal to the chamber pressure. Consequently, water uptake occurs at a rate that depends on the heat flux away from the surface. The water uptake rate, W'h, is constant at a given relative humidity and is described by an equation of the form W'h = (C + F) . ln (RHi/RHo), where C and F are conductive and radiative coefficients, RHi the chamber relative humidity, and RHo the relative humidity at and above which continuous water uptake (deliquescence) occurs. The model contains no adjustable parameters and can thus be directly tested against experimental results.
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165
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Van Campen L, Amidon GL, Zografi G. Moisture sorption kinetics for water-soluble substances. II: Experimental verification of heat transport control. J Pharm Sci 1983; 72:1388-93. [PMID: 6663471 DOI: 10.1002/jps.2600721205] [Citation(s) in RCA: 36] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
The rates of water sorption as a function of relative humidity for water-soluble substances exhibiting deliquescence have been measured in an atmosphere of pure water vapor. The substances studied included a series of alkali halides, choline halides, and sugars. The results were compared with a theoretical model, previously described, which relates the rate of water uptake to the transport of heat produced during the process away from the surface. Taking into account the heat of water vapor condensation, heat of solution, and heat of hydration, when hydration occurs, the model allows excellent a priori prediction of water uptake rates as a function of relative humidity.
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166
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Van Campen L, Amidon GL, Zografi G. Moisture sorption kinetics for water-soluble substances. III: Theoretical and experimental studies in air. J Pharm Sci 1983; 72:1394-8. [PMID: 6663472 DOI: 10.1002/jps.2600721206] [Citation(s) in RCA: 29] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
As an extension of the model of heat transport control developed for the kinetics of water sorption by water-soluble substances from an atmosphere of pure water vapor, equations have been developed to account for limitations of diffusion on mass transport of water vapor when air is present. Although the inability to determine the vapor diffusion layer thickness prevents using these equations to predict sorption behavior a priori, minimum water sorption rates can be calculated by assuming a diffusion layer thickness equal to the sample chamber radius. Combining heat transport and mass transport produces equations which describe very well the observed sorption by three water-soluble salts in one atmosphere of air. As in the absence of air, sorption rates are predicted and observed to be constant at a given atmospheric relative humidity.
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167
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Amidon GL, Lee M, Lee H. Intestinal absorption of amino acid derivatives: structural requirements for membrane hydrolysis. J Pharm Sci 1983; 72:943-4. [PMID: 6413676 DOI: 10.1002/jps.2600720826] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/20/2023]
Abstract
The intestinal absorption of L-lysine-p-nitroanilide, L-alanine-p-nitroanilide, and glycine-p-nitroanilide was studied in perfused rat intestine in the presence of a variety of potential competitive inhibitors. The results indicate that the hydrolysis site(s) show side-chain specificity, and that inhibitors require a free amino group in the alpha-position and must be in the L-configuration to be effective. Glycyl-L-proline, a peptide transport inhibitor, had no effect on the absorption rate.
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168
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Selen A, Amidon GL, Welling PG. Pharmacokinetics of probenecid following oral doses to human volunteers. J Pharm Sci 1982; 71:1238-42. [PMID: 7175716 DOI: 10.1002/jps.2600711114] [Citation(s) in RCA: 71] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
The pharmacokinetics of probenecid were examined following single 0.5-, 1.0-, and 2.0-g oral doses to healthy male volunteers. Doses were administered following overnight fast, according to a randomized design. Plasma levels of probenecid were determined by high-pressure liquid chromatography (HPLC), using sulfamethazine as the internal standard. Mean peak probenecid levels of 35.3, 69.6, and 148.6 micrograms/ml were obtained at 3-4 hr following the 0.5-, 1.0-, and 2.0-g doses, respectively. Probenecid levels from the 0.5- and 1.0-g doses declined in apparent monoexponential fashion, with mean elimination half-lives of 4.2 and 4.9 hr. Interpretation of the 2.0-g data by a kinetic model incorporating first-order elimination resulted in a plasma drug half-life of 8.5 hr. When first-order elimination was replaced by a Michaelis-Menten-type function, the mean value of the resulting Vm/Km ratios was 0.20, equivalent to a plasma drug half-life [0.693/(Vm/Km)] of 3.8 hr. Plasma probenecid curves from all three dosages were successfully fitted to the saturable elimination model using nonlinear regression and numerical integration routines. The results suggest that probenecid elimination may be saturable at therapeutic dose levels.
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169
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Amidon GL, Chang M, Fleisher D, Allen R. Intestinal absorption of amino acid derivatives: importance of the free alpha-amino group. J Pharm Sci 1982; 71:1138-41. [PMID: 7143212 DOI: 10.1002/jps.2600711015] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
Abstract
The intestinal absorption of L-lysine-p-nitroanilide, L-alanine-p-nitroanilide, and glycine-p-nitroanilide was studied in the presence of competitive inhibitors in a perfused rat intestine. It ws observed that L-lysine-p-nitroanilide absorption was inhibited by L-lysine methyl ester and L-arginine-beta-naphthylamide but not by N alpha-acetyl-L-lysine methyl esters. L-Alanine-p-nitroanilide absorption was inhibited by L-alinine methyl ester but not by beta-alanine methyl ester. It was further observed that N alpha-benzoyl-L-arginine-p-nitroanilide and N alpha-succinyl-L-phenylalanine-p-nitroanilide were poorly absorbed. It was concluded that the peptidase in the brush border region that serves as the hydrolysis site requires a free alpha-amino group (an aminopeptidase), and that passive absorption of these compounds occurs only to a small extent.
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170
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Amidon GL, Buckner CK. A theoretical model for the use of functional antagonism to estimate dissociation constants for agonists. JOURNAL OF PHARMACOLOGICAL METHODS 1982; 7:173-8. [PMID: 7098500 DOI: 10.1016/0160-5402(82)90030-4] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/23/2023]
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171
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Banerjee PK, Amidon GL. Physicochemical property modification strategies based on enzyme substrate specificities II: alpha-chymotrypsin hydrolysis of aspirin derivatives. J Pharm Sci 1981; 70:1304-6. [PMID: 7320840 DOI: 10.1002/jps.2600701203] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
Three aspirin derivatives, aspirin phenylalanine ethyl ester, aspirin phenylalanine amide, and aspirin phenyllactic ethyl ester, were investigated with respect to their hydrolysis by alpha-chymotrypsin. Of the three compounds, aspirin phenylalanine ethyl ester was the best substrate, with kcat = 25 sec-1 and Km = 1.3 x 10-6 M at pH 8.0. The results for all substrates were in the range of expectation based on kinetic data for other substrates. The apparent latitude in the nature of the acrylamide substituent of alpha-chymotrypsin substrates makes this enzyme a good potential reconversion site for many drug derivatives.
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172
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Banerjee PK, Amidon GL. Physicochemical property modification strategies based on enzyme substrate specificities I: rationale, synthesis, and pharmaceutical properties of aspirin derivatives. J Pharm Sci 1981; 70:1299-303. [PMID: 7320839 DOI: 10.1002/jps.2600701202] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
A rationale is developed for drug physicochemical property modification based on making derivatives that are substrates for known enzymes. The approach requires knowledge of the enzyme-substrate specificities to select the appropriate derivative. As a class, the digestive enzymes represent possible reconversion sites. It is shown that by using only known specificities of these enzymes, the physicochemical properties of a drug may be modified in almost any manner desired by appropriate derivative choice, with enzymatic regeneration remaining effective. The strategy is applied to making a stable aspirin derivative that is activated in vivo. Of the derivatives made, aspirin phenylalanine ethyl ester was shown to be stable in suspension form for over 4 years. It was also shown that aspirin is regenerated form the derivative in the presence of the enzymes alpha-chymotrypsin and carboxypeptidase in vitro. This biochemical approach to drug physicochemical property modification offers a new and powerful rationale for improving drug product efficacy.
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173
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Banerjee PK, Amidon GL. Physicochemical property modification strategies based on enzyme substrate specificities III: carboxypeptidase A hydrolysis of aspirin derivatives. J Pharm Sci 1981; 70:1307-9. [PMID: 7320841 DOI: 10.1002/jps.2600701204] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
Abstract
The aspirin derivatives aspirin phenylalanine and aspirin phenyllactic acid were studied as substrates for carboxypeptidase A. The phenyllactic acid derivative (an ester) was the best substrate but showed considerable product inhibition. The kinetic parameters for both substrates were in the range expected on the basis of other known substrates. The results indicate that the acylamide substituent (drug) has only a small effect on the enzyme kinetic parameters. Consequently, carboxypeptidase A may serve as a reconversion site for many drug derivatives.
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174
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Amidon GL, Buckner CK. On the use of a dynamic approach to the estimation of dissociation constants for reversible competitive antagonists. J Pharmacol Exp Ther 1981; 216:352-6. [PMID: 6257889] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023] Open
Abstract
A dynamic approach to evaluating reversible competitive antagonists has been used in estimating the dissociation constant (apparent KB) for (+/-)-sotalol at beta adrenergic receptors which mediate a positive chronotropic response in rat atria. Cumulative dose-response curves to the beta agonists (-)-isoproterenol and (-)-soterenol were obtained after mixing each agonist with (+/-)-sotalol at fixed ratios (Q = [antagonist]/[agonist]). Experimental and theoretical dose-response curves obtained with the dynamic approach were in good agreement and the lowered maximum responses to the agonist-antagonist mixtures were dependent on Q. Also, apparent KB values for (+/-)-sotalol determined by three methods were independent of Q and agonist employed in the experiment. Apparent KB values for (+/-)-sotalol obtained by the dynamic method (6 x 10(-7)-2 x 10(-6) M) were similar to those obtained by the traditional equilibrium or static approach. As judged by the similarity of the apparent KB values derived by the two independent theoretical models, the dynamic approach may be considered an effective and alternative means to evaluate reversible competitive antagonists.
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175
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Amidon GL, Taylor J, Sorkness R. A convective diffusion model for estimating drug loss to tubing: sorption of vitamin A. JOURNAL OF PARENTERAL SCIENCE AND TECHNOLOGY : A PUBLICATION OF THE PARENTERAL DRUG ASSOCIATION 1981; 35:13-7. [PMID: 7229825] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
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176
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Elliott RL, Amidon GL, Lightfoot EN. A convective mass transfer model for determining intestinal wall permeabilities: laminar flow in a circular tube. J Theor Biol 1980; 87:757-71. [PMID: 7253676 DOI: 10.1016/0022-5193(80)90115-0] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/24/2023]
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177
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Amidon GL, Leesman GD, Elliott RL. Improving intestinal absorption of water-insoluble compounds: a membrane metabolism strategy. J Pharm Sci 1980; 69:1363-8. [PMID: 6780682 DOI: 10.1002/jps.2600691203] [Citation(s) in RCA: 58] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023]
Abstract
A strategy for improving the intestinal absorption of water-insoluble drugs was developed and tested. The strategy is based on making a soluble derivative of an insoluble compound which, in turn, is a substrate for enzymes in the surface coat of the brush border region of the microvillous membrane. Consequently, just prior to reaching the membrane, the physical properties of the diffusing species are changed from polar to nonpolar. The experimental test used two drug-drug derivative pairs, estrone-lysine estrone ester and p-nitroaniline-lysine p-nitroanilide. Wall permeabilities were determined using an external perfusion technique in the rat intestine and a laminar flow convective diffusion model for transport in the lumen. Analysis of the permeability results indicates that the derivatives have higher wall permeabilities than the parent compounds and that the microvillous surface coat may be a significant contributor to the intestinal wall resistance. Comparison of the absorption rates for estrone and the lysine estrone ester indicates that the absorption rate of the derivatives could be up to five orders of magnitude greater than that for the parent compound.
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178
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Amidon GL, Kou J, Elliott RL, Lightfoot EN. Analysis of models for determining intestinal wall permeabilities. J Pharm Sci 1980; 69:1369-73. [PMID: 7463318 DOI: 10.1002/jps.2600691204] [Citation(s) in RCA: 83] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
In determining intestinal wall permeabilities, several mass transport models may be applied to analyze the results from external perfusion experiments. The appropriateness of any given model depends on the applicability of the model assumptions to the experimental system. This report compares several mass transport models with respect to their assumptions and applicability to a particular experimental design. The models are shown to differ in their assumptions regarding convection and diffusion in the perfusing fluid. However, since the wall permeability is an unknown parameter in each model and is estimated from the data, all of the models fit the mass transfer results reasonably well, despite fundamentally different assumptions. However, the determined permeabilities differ. Residence time distribution analysis of the experimental system is more sensitive to the model assumptions. It is shown that, in a particular experimental system, laminar flow in a cylindrical tube is the most appropriate model. The model also has the advantage of implicitly accounting for the convection-diffusion problem in the perfusing fluid. Hence, the diffusion layer thickness is not estimated from the data. With the hydrodynamics defined, the relative permeabilities resulting from the application of the several models to the data can be interpreted. The wall permeability determined in the suggested manner provides an estimate of the limiting assistance under perfect mixing conditions.
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179
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Amidon GL, Pearlman RS, Anik ST. The solvent contribution to the free energy of protein--ligand interactions. J Theor Biol 1979; 77:161-70. [PMID: 449367 DOI: 10.1016/0022-5193(79)90145-0] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
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180
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Welling PG, Lyons LL, Elliott R, Amidon GL. Pharmacokinetics of alcohol following single low doses to fasted and nonfasted subjects. J Clin Pharmacol 1977; 17:199-206. [PMID: 849994 DOI: 10.1177/009127007701700403] [Citation(s) in RCA: 45] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/24/2022]
Abstract
The absorption and elimination characteristics of alcohol have been studied in healthy fasted and nonfasted human volunteers using low single doses. In non-fasted subjects, carbohydrate reduced overall alcohol bioavailability by about 96 percent, compared to 90 per cent for fat and 75 per cent for protein. Inhibition of absorption in nonfasted subjects appeared to be due to less alcohol being available for absorption rather than a reduced absorption rate. Serum alcohol levels in fasted subjects were interpreted in terms of both first-order and zero-order absorption followed by first-order elimination. Of the two proposed models, that utilizing zero-order absorption provided a marginally better fit to observed data.
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181
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Yalkowsky SH, Valvani SC, Amidon GL. Solubility of nonelectrolytes in polar solvents IV: nonpolar drugs in mixed solvents. J Pharm Sci 1976; 65:1488-94. [PMID: 978408 DOI: 10.1002/jps.2600651018] [Citation(s) in RCA: 72] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
The molecular and group surface area approach to solubility is shown to be applicable to mixed aqueous solvent systems. An equation is derived which is consistent with the exponential increase in the aqueous solubility of nonpolar drugs that frequently accompanies the addition of a cosolvent. This equation predicts that: (a) the ability of a drug to be solubilized by a cosolvent is proportional to its hydrophobic surface area per molecule, and (b) the ability of a cosolvent to solubilize any drug is inversely proportional to its interfacial tension against a reference liquid hydrocarbon. These predictions are experimentally verified with solubility studies of several alkyl p-aminobenzoates in propylene glycol-water mixtures and of hexyl p-aminobenzoate in mixtures of water the ethanol, methanol, ethylene glycol, propylene glycol, glycerin, and formamide.
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182
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Amidon GL, Anik ST. Comparison of several molecular topological indexes with molecular surface area in aqueous solubility estimation. J Pharm Sci 1976; 65:801-6. [PMID: 932962 DOI: 10.1002/jps.2600650603] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
The molecular topological indexes proposed by Wiener (Wiener number), Hosoya (Z-value), and Randic (branching index) were corelated with computed molecular surface areas and aqueous solubilities for the monofunctional aliphatic alcohols, ethers, ketones, aldehydes, carboxylic acids, esters, and hydrocarbons. Comparison of the indexes with molecular surface area indicates that all three indexes (or simple modifications) correlate with molecular surface area and, although computed in different manners, reflect molecular topology. Comparison of the correlations of log solubility with the several indexes leads to the following conclusions: (a) the branching index works well for aliphatic, acyclic monofunctional compounds but not cyclic aliphatic compounds; (b) the square root of the Wiener number correlates less satisfactorily with log solubility than the other indexes but more correctly handles cyclic compounds (when generalized after Hosoya); (c) correlations of log solubility with the log Z-value are satisfactory, but the index is difficult to compute; and (d) the molecular surface are represents the single best parameter with which to correlate and estimate aqueous solubility due to its generality. However, for a restricted series of compounds, the branching index is perhaps the most useful index by virtue of its simplicity.
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183
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Birnbaum JE, Abel PW, Amidon GL, Buckner CK. Changes in mechanical events and adenosine 3',5'-monophosphate levels induced by enantiomers of isoproterenol in isolated rat atria and uteri. J Pharmacol Exp Ther 1975; 194:396-409. [PMID: 168352] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/13/2022] Open
Abstract
Beta adrenergic receptors of rat atria and uteri were examined with the use of enantiomers of isoproterenol as agonists and mechanical responses and adenosine 3',5'-monophosphate (cyclic AMP) levels as measured effects. Assuming that stereoselectivity reflects the unique asymmetry of receptors, potency differences between the enantiomers are expected to provide a sensitive indication of ligand binding. All effects in each tissue were investigated under similar experimental conditions. Both isomers produced the same maximum effect on all measured responses. Enantiomeric potency differences (in log units) for positive chronotropic and inotropic responses and increases in cyclic AMP levels in atria were 3.31, 3.51 and 3.48, respectively. In uteri, the values for reduction of spontaneous contractile amplitude and increases in cyclic AMP were 2.90 and 2.79 log units, respectively. Even though these absolute values varied slightly with the experimental conditions, they were consistently smaller in uteri than in atria. In both tissues, dose-response curves for production of mechanical effects were greater than 2 log units to the left of those for increases in cyclic AMP levels. Regardless of the interpretation of this phenomenon, the results show the following. 1) The stereoselectivity for isoproterenol-induced effects is different between the two tissues at both levels of response. Therefore, it is suggested that this reflects dissimilar beta adrenergic receptor types in rat atrium vs. rat uterus. 2) The stereochemical selectivity for isoproterenol-induced mechanical effects and increases in cyclic AMP is the same in rat atrium and in rat uterus. Therefore, the data support the postulate that cyclic AMP is formed from interaction of isoproterenol with a receptor that is similar to the one activated to produce a mechanical effect.
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184
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Yalkowsky SH, Amidon GL, Zografi G, Flynn GL. Solubility of nonelectrolytes in polar solvents III: Alkyl p-aminobenzoates in polar and mixed solvents. J Pharm Sci 1975; 64:48-52. [PMID: 1133705 DOI: 10.1002/jps.2600640109] [Citation(s) in RCA: 46] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
The relative solubilities of n-alkyl p-aminobenzoates in water, proplyene blycol-water mixtures, proplyene glycol, and several other pharmaceutically important solvents can be predicted on the basis of a theoretical equation. This equation relates the activity coefficient of the hydrophobic portion of the molecule to the product of its surface area and its interfacial tension [free energy per unit area of a hydrocarbon (tetradecane) against the polar or semipolar solvent of interest]. The assumptions, conclusions, and applicability of the theorectical relationship are compared to those of the Scatchard-Hildebrand approach.
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185
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Amidon GL, Yalkowsky SH, Leung S. Solubility of nonelectrolytes in polar solvents II: solubility of aliphatic alcohols in water. J Pharm Sci 1974; 63:1858-66. [PMID: 4449016 DOI: 10.1002/jps.2600631207] [Citation(s) in RCA: 114] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
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186
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187
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Amidon GL. Comparison of theoretical, absolute interaction energies with heats of complexation in carbon tetrachloride. J Pharm Sci 1974; 63:1514-9. [PMID: 4436779 DOI: 10.1002/jps.2600631005] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
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188
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Amidon GL. Structure and reactivity of theobrominate and theophyllinate complexes with methyl trans-cinnamate. J Pharm Sci 1974; 63:1524-6. [PMID: 4436781 DOI: 10.1002/jps.2600631007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/10/2023]
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189
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Amidon GL. Theoretical calculations on an acetic acid, 5-methylimidazole, methanol hydrogen bond network: a model charge relay system. J Theor Biol 1974; 46:101-9. [PMID: 4855179 DOI: 10.1016/0022-5193(74)90141-6] [Citation(s) in RCA: 23] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023]
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190
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Welling PG, Craig WA, Amidon GL, Kunin CM. Pharmacokinetics of trimethoprim and sulfamethoxazole in normal subjects and in patients with renal failure. J Infect Dis 1973; 128:Suppl:556-66 p. [PMID: 4758046 DOI: 10.1093/infdis/128.supplement_3.s556] [Citation(s) in RCA: 57] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/12/2023] Open
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191
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