Mucosal Uptake of Gabapentin (Neurontin) vs. Pregabalin in the Small Intestine

Purpose. To compare the mucosal membrane transport of gabapentin and pregabalin in animal small intestine. Methods. Uptake of the two drugs by brush-border membrane vesicles (BBMV) from rat and rabbit small intestine was studied as a function of temperature, uptake-medium sodium content, and intestinal region. Amino acid inhibition studies were conducted with pregabalin. Results. Gabapentin uptake by rat and rabbit jejunal BBMV was sodium independent, whereas pregabalin uptake was sodium dependent. Uptake of both drugs in rabbit small intestinal vesicles was greater at 25°C than at 4°C in the absence of sodium and an additional increase in uptake was observed for pregabalin at 25°C in the presence of sodium. Pregabalin uptake in rabbit duodenal, jejunal, and ileal BBMV was equivalent, whereas gabapentin uptake was greater in duodenal and ileal BBMV, compared with jejunal BBMV. Although inhibition is weak, a decrease in BBMV uptake of pregabalin is observed with coincubation of high concentrations of both neutral and basic amino acids. Conclusions. Amino acid carriers mediate the apical uptake of both drugs in the small intestine. Although gabapentin and pregabalin are structurally similar, their small intestinal mucosal uptake differs in sodium dependence and region dependence. Gabapentin uptake is likely mediated by system b^0,+, whereas pregabalin uptake is also mediated by B⁰ and/or B^0,+.     

<Emphasis Type="BoldItalic">In Vivo</Emphasis> Effects of Glycyl-Glutamate and Glycyl-Sarcosine on Gabapentin Oral Absorption in Rat

Purpose The objective of this study was to evaluate the in vivo consequences of glycyl-glutamate coadministration on gabapentin oral absorption. Methods Rats were administered gabapentin (10 mg/kg plus radiotracer) by gastric gavage, in the absence and presence of dipeptides, and by intravenous administration. Serial blood samples were obtained over 6 h and the pharmacokinetics of gabapentin were determined by noncompartmental analysis. Results Glycyl-glutamate coadministration increased the C _max of gabapentin by 86% as compared to gabapentin alone. In agreement, the oral absorption of gabapentin, relative to the intravenous dose, was 79% after glycyl-glutamate loading but only 47% when drug was administered alone. However, when glycyl-sarcosine was added to the orally administered admixture of gabapentin plus glycyl-glutamate, values for C _max and AUC_{0–6 h} reverted back to that of control. In contrast, the t _max and terminal half-life of gabapentin did not change after oral dosing for all treatments. Conclusions These findings are unique in demonstrating that under physiologic, in vivo conditions, the luminal presence of glycyl-glutamate could dramatically enhance the C_max and AUC_{0–6 h} of gabapentin. The results are consistent with previous in situ intestinal perfusion studies in rat, and establish a functional interaction between the activities of PEPT1 and amino acid exchangers. David Fleisher (deceased) was a co-author of this article.     

Dose-Dependent Intestinal Absorption and Significant Intestinal Excretion (Exsorption) of the β-Blocker Pafenolol in the Rat

The elimination of [³H]pafenolol and metabolites was investigated in fasted and fed rats. Separate groups received intravenous doses (0.3 and 3.0 µmol/kg) and oral doses (1 and 25 µmol/kg). After iv administration of pafenolol, the excretion of unchanged drug into urine and feces was about 50 and 25–30% of the given dose, respectively. The predominating mechanism for the excretion of pafenolol into feces was intestinal excretion (exsorption) directly from blood into gut lumen, since only about 3% of a given iv dose was recovered as pafenolol in the bile. When the oral dose was raised from 1 to 25 µmol/kg, the mean (±SD) bioavailability, calculated from urine data, increased from 14 ± 9 to 30 ± 11% (P < 0.05) in the starved rats and from 14 ± 3 to 16 ± 3% in the fed animals. In parallel, the fraction absorbed from the gut (f _a) increased from 19 ± 9 to 31 ± 10% in the starved rats and from 16 ± 4 to 19 ± 5% in the fed animals, respectively. This indicates that the low bioavailability is due primarily to poor intestinal uptake.     

Proton-Cotransport of Pravastatin Across Intestinal Brush-Border Membrane

Purpose. The purpose of the present study is to clarify the intestinal brush-border transport mechanism of a weak organic acid, pravastatin, an HMG-CoA reductase inhibitor. Methods. The transport of pravastatin was studied by using intestinal brush-border membrane vesicles prepared from rabbit jejunum, and uptake by the membrane vesicles was measured using rapid filtration technique. Results. The initial uptake of [¹⁴C]pravastatin was markedly increased with decreases in extravesicular pH and showed a clear overshoot phenomenon in the presence of a proton gradient (pH_in/out = 7.5/5.5). A protonophore, carbonylcyanide p-trifluoromethoxyphenylhydrazone, significantly reduced the uptake of [¹⁴C]pravastatin. In addition, an ionophore for sodium, potassium and proton, nigericin, stimulated the uptake of [¹⁴C]pravastatin in the presence of a potassium gradient ([K ⁺ ]_in/[K⁺ ]_out = 0/145 mM). On the other hand, neither the imposition of an inwardly directed sodium gradient nor an outwardly directed bicarbonate gradient stimulated the uptake of [¹⁴C]pravastatin. In the presence of a proton gradient (pH_in/out = 7.5/5.5), the initial uptake of pravastatin was saturable with the apparent K_t of 15.2 ± 3.2 mM and J_max of 10.6 ± 1.21 nmol/mg protein/10 sec. The uptake of pravastatin was significantly inhibited by monocarboxylic acid compounds such as acetic acid and nicotinic acid in a competitive manner but not by di- or tri-carboxylic acids, or acidic amino acid. Conclusions. It was concluded that a pH-dependent transport of pravastatin across the brush-border membrane occurs by a proton-gradient dependent carrier-mediated mechanism rather than by simple diffusion of its unionized form.     

The Pharmacokinetics and Absorption of Recombinant Human Relaxin in Nonpregnant Rabbits and Rhesus Monkeys After Intravenous and Intravaginal Administration

Recombinant human relaxin (rhRlx) is being developed as a potential cervical ripening agent to be applied intravaginally or intracervically prior to parturition. The pharmacokinetics and absorption of rhRlx were determined in nonpregnant female rabbits and rhesus monkeys after intravenous bolus (iv) and intravaginal administration of 0.1 mg/kg; additionally, rabbits were dosed with 0.5 mg/kg intravaginally. In rabbits (n = 6), mean (±SD) peak concentrations following iv bolus administration were 1554 ± 296 ng/mL. The weight-normalized clearance (CL/W) was 5.9 ± 0.4 mL/min/kg, initial volume of distribution (V ₁/W) was 57 ± 9 mL/kg, and volume of distribution at steady state (V _SS/W), assuming central compartment elimination, was 240 ± 20 mL/kg. V _ss/W could be as large as 2000 ± 400 mL/kg without this assumption. The estimated amounts of rhRlx absorbed in rabbits following intravaginal administration of 0.1 and 0.5 mg/kg (n = 5/dose) were 3.1 ± 1.4 and 0.7 ± 0.3%, respectively; peak concentrations were 600 ± 297 and 1066 ± 584 pg/mL, respectively. In rhesus monkeys (n = 5) after iv administration, peak concentrations were 971 ± 277 ng/mL; CL/W was 4.1 ± 0.6 mL/ min/kg, V ₁/W was 78 ± 25 mL/kg, and V _ss/W, assuming central compartment elimination, was 690 ± 220 mL/kg. The upper limit for V _ss/W was 1600 ± 200 mL/kg when no assumptions were made regarding site (compartment) of elimination. After intravaginal administration (n = 6), two monkeys had undetectable rhRlx concentrations throughout the 48-hr sampling interval; one monkey had only one sample containing measurable rhRlx (51 pg/mL) at 24 hr; and three monkeys absorbed <2% of the 0.1 mg/kg dose. Peak concentrations in these three animals ranged from 64 to 1475 pg/mL. The absorption of rhRlx was low and variable in both species, and similar results have been observed in women.     

Dipeptide Transport Across Rat Alveolar Epithelial Cell Monolayers

The transepithelial transport and metabolism of two model peptides, glycyl-D-phenylalanine (Gly-D-Phe) and glycyl-L-phenylalanine (Gly-L-Phe), across primary cultured monolayers of rat alveolar epithelial cells were studied. These tight monolayers (>2000 -cm²) exhibited type I pneumocyte morphological and phenotypic characteristics. A reverse-phase HPLC was used to monitor the appearance of parent dipeptides and their metabolites (D- or L-Phe) in the receiver fluid. The apparent permeability coefficient (P_app) for Gly-D-Phe was about 1.6 × 10^–7 cm/sec at both 1 and 10 mM and in both the apical-to-basolateral (AB) and the basolateral-to-apical (BA) directions. In contrast, the P _app of Gly-L-Phe at 1 mM was about two times higher than that at 10 mM in the AB direction. The P _app of Gly-L-Phe in the BA direction at either concentration was about the same (about 1.4 × 10^–7 cm/sec). Whereas no metabolite was detected during Gly-D-Phe transport, the proportions of a metabolite, L-Phe, observed at 4 hr in the basolateral receiver fluid for 1 and 10 mM apical donor Gly-L-Phe accounted for 83 and 77% of the estimated total Gly-L-Phe (i.e., L-Phe + Gly-L-Phe), respectively. The corresponding values in the BA direction were 40 and 19% of the estimated total Gly-L-Phe in the apical receiver reservoir. Metabolism of Gly-L-Phe was significantly reduced in the presence of 3 µM actinonin (an inhibitor relatively specific for aminopeptidase M) in the apical but not the basolateral fluid. Under all experimental conditions, the monolayers remained intact, as indicated by no appreciable changes in the bioelectric parameters of transepithelial potential difference and electrical resistance. The above data provide evidence for cellular metabolism of Gly-L-Phe as well as paracellular restricted diffusional transport of intact Gly-D-Phe and Gly-L-Phe and comparatively lower transcellular transport of Gly-L-Phe across the rat alveolar epithelial cell monolayer.     

Unbiased membrane permeability parameters for gabapentin using boundary layer approach

The present study was performed to determine the relative contribution of both passive and nonpassive transport processes in jejunal absorption of gabapentin. The oral absorption of gabapentin was studied using in situ single pass intestinal perfusion technique in fasted rats. Unbiased intrinsic membrane absorption parameters such as maximal flux, Michaelis constant, carrier permeability, and membrane permeability were calculated using a modified boundary layer model. Gabapentin intestinal perfusion results indicate that its jejunal absorption in rats occurs via a nonpassive process, with no significant passive absorption component, as demonstrated by saturable absorption kinetics and its concentration-dependent permeability. A good correlation (r2 = 0.88) between observed human absorption fraction and calculated (from in situ rat intestine) human absorption fraction was obtained.     

Drug Absorption Limited by P-Glycoprotein-Mediated Secretory Drug Transport in Human Intestinal Epithelial Caco-2 Cell Layers

The hypothesis was tested that the operation of an ATP-dependent export pump localized at the apical (brush border) surface of the intestinal epithelium may limit substrate absorption kinetics. Human intestinal Caco-2 cell-layers display saturable secretion of vinblastine from basal to apical surfaces (K _m, 18.99 ± 5.55 µM; V _max, 1285.9 ± 281.2 pmol cm^–2 hr^–1) that is inhibited by verapamil, consistent with the expression of the ATP-dependent P-glycoprotein drug efflux pump at the apical brush border membrane. Inhibition of P-glycoprotein by a variety of modulators (verapamil, 1,9-dideoxyforskolin, nifedipine, and taxotere) is associated with an increased vinblastine absorptive permeability. Vinblastine absorption displayed a nonlinear dependence upon luminal (apical) vinblastine concentration, and vinblastine absorption increased markedly at concentrations where vinblastine secretory flux was saturated (>20 µM). Upon inhibition of P-glycoprotein by verapamil and 1,9-dideoxyforskolin, vinblastine absorption increased and was linearly dependent on vinblastine concentration. The limitation of P-glycoprotein substrate absorption by active ATP-dependent export via P-glycoprotein is discussed, together with the possibility that other classes of substrate may be substrates for different ATP-dependent export pumps.     

Carrier-mediated intestinal absorption of valacyclovir,the L-valyl ester prodrug of acyclovir. 1. Interactions with peptides,organic anions and organic cations in rats

The mechanism of intestinal transport of valacyclovir (VACV), the L-valyl ester prodrug of acyclovir, was investigated in rats using an in situ intestinal perfusion technique. VACV demonstrates an oral bioavailability that is three to five time greater than acyclovir, concentration dependent, and saturable in humans. Homogenate and perfused buffer stability results demonstrated that VACV was increasingly unstable with increasing pH. VACV was converted to ACV in a concentration dependent manner during a single pass through the intestinal segment. Perfusions were performed at 37°C, pH 6.5, and under iso-osmotic conditions (290±10 mOsm L⁻¹). Intestinal outlet concentrations were corrected for VACV that was converted to ACV during the perfusion. The effective dimensionless intestinal permeability (P_e*) of VACV was concentration dependent, saturable (intrinsic K_m = 1.2±0.7 mM), and significantly reduced (p <0.05) in the presence of peptide analogues (amoxicillin, ampicillin, cefadroxil, and cephradine), by the organic anion, p -amino hippuric acid and by the organic cation quinine. VACV transport was not inhibited by classical nucleoside competitive substrates or inhibitors or by valine. These results suggest that H⁺–oligopeptide, H⁺–organic cation, and organic anion transporters are involved in the small-intestinal uptake of VACV. The permeability of VACV in the colon was very low, indicating that VACV is predominantly absorbed from the small intestine. VACV P_e* was not altered in the presence of glucose-induced convective fluid flow, suggesting that carrier-mediated, transcellular uptake is the predominant absorption pathway of VACV in rat small intestine. Based on these results, the oral bioavailability of VACV appears to be significantly influenced by the preabsorptive conversion of VACV to the poorly absorbed ACV, by the involvement of multiple transporters in VACV small-intestinal uptake, and by the low permeability of VACV in the colon. © 1998 John Wiley & Sons, Ltd.     

Clinical development of a once-daily gastroretentive formulation of gabapentin for treatment of postherpetic neuralgia: an overview

Introduction: Gabapentin immediate-release formulations (G-IR) administered three times a day is an efficacious treatment for postherpetic neuralgia (PHN), but its potential benefits may not be fully realized due to tolerability issues as well as its pharmacokinetic (PK) properties such as its short half-life, and regional and saturable absorption in the proximal small intestine. The gastroretentive once-daily formulation of gabapentin (G-GR) allows for less frequent dosing while maintaining efficacy and may also reduce adverse events (AEs) associated with high plasma concentration of gabapentin occurring during the waking hours. G-GR slowly releases the drug from the tablet to the upper small intestine, where gabapentin is best absorbed, over approximately 10 h.

Area covered: This report reviews the development of the gastroretentive technology used in the once-daily formulation of gabapentin (G-GR), and describes the clinical development of G-GR from PK studies through the Phase III efficacy and safety studies, with comparisons made with G-IR.

Expert opinion: The technology takes advantage of the normal physiology of the stomach in the fed state to provide gastroretention, which in turn allows for gradual release of the active ingredient over several hours to the small intestine where gabapentin is best absorbed. The GR technology used in G-GR resulted in a decreased dosing frequency from three times per day for the IR product to once daily in the treatment of PHN, while maintaining the same efficacy with an apparent reduced incidence of AEs common to G-IR therapy.     

Peptide formation in the presence of metal ion protecting groups

A quantitative study of the degree of racemization induced by the [(NH₃)₅Co-(III)-] protecting group when bound to the C-terminal of the amino acids Leu, Phe, and His, as has been carried out. Racemization was determined by forming the diastereomeric cobalt dipeptides [(Leu)(AA)Co(III)(NH3)₅] where AA = L-Leu, L-Phe, and L-His; after cobalt removal (using NaBH₄), the peptide diastereomers were analyzed quantitatively using an amino acid analyzer. No racemization was observed within experimental error (0.3%) as a result of the substitution of the [(NH3)₅Co(III)-] group on the amino acids and peptides studied.     

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 gives the following equation for fraction dose absorbed: F = 1– e ^{–2 P*w}. The correlation between fraction dose absorbed in humans and P _w ^* determined from steady-state 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.     

Effects of pH and Dose on Nasal Absorption of Scopolamine Hydrobromide in Human Subjects

Purpose. The present study was conducted to evaluate theeffects of formulation pH and dose on nasal absorption of scopolaminehydrobromide, the single most effective drug available for the prevention ofnausea and vomiting induced by motion sickness. Methods. Human subjects received scopolamine nasally at adose of 0.2 mg/0.05 mL or 0.4 mg/0.10 mL, blood samples were collected atdifferent time points, and plasma scopolamine concentrations were determinedby LC-MS/MS. Results. Following administration of a 0.2 mg dose, theaverage C_max values were found to be 262 ± 118, 419± 161, and 488 ± 331 pg/mL for pH 4.0, 7.0, and 9.0formulations, respectively. At the 0.4 mg dose the average C_maxvalues were found to be 503 ± 199, 933 ± 449, and 1,308± 473 pg/mL for pH 4.0, 7.0, and 9.0 formulations, respectively. At a0.2 mg dose, the AUC values were found to be 23,208 ± 6,824, 29,145± 9,225, and 25,721 ± 5,294 pg.min/mL for formulation pH 4.0,7.0, and 9.0, respectively. At a 0.4 mg dose, the average AUC value wasfound to be high for pH 9.0 formulation (70,740 ± 29,381 pg.min/mL)as compared to those of pH 4.0 (59,573 ± 13,700 pg.min/mL) and pH 7.0(55,298 ± 17,305 pg.min/mL) formulations. Both the C_maxand AUC values were almost doubled with doubling the dose. On the otherhand, the average T_max values decreased linearly with a decreasein formulation pH at both doses. For example, at a 0.4 mg dose, the averageT_max values were 26.7 ± 5.8, 15.0 ± 10.0, and 8.8± 2.5 minutes at formulation pH 4.0, 7.0, and 9.0, respectively. Conclusions. Nasal absorption of scopolamine hydrobromidein human subjects increased substantially with increases in formulation pHand dose.     

Characterization of the Oral Absorption of β-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), K _m = 16.1 (±3.6), P _m ^* = 0, and P _c ^*= 1.32 (±0.07); cefadroxil, J _max ^* = 8.4 (±0.8), K _m = 5.9 (±0.8), P _m ^* = 0, and P _c ^* = 1.43 (±0.10); cephalexin, J _max ^* = 9.1 (±1.2), K _m = 7.2 (±1.2), P _m ^* = 0, and P _c ^* = 1.30 (±0.10); cefatrizine, J _max ^* = 0.73 (±0.19), K _m = 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), K _m = 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, K _m (mM) is the Michaelis constant, P _m ^* is the passive membrane permeability, and P _c*is the carrier permeability. Aminocephalosporin perfusion results indicate that jejunal absorption in the rat occurs by a nonpassive process, with some of the compounds possessing a small but statistically significant passive component, while the colon permeability is low and follows a simple passive absorption mechanism.     

Mechanism of Corneal Permeation of L-Valyl Ester of Acyclovir: Targeting the Oligopeptide Transporter on the Rabbit Cornea

Purpose. To delineate mechanisms associated with the corneal transport of a L-valine prodrug of an antiviral agent, acyclovir. Method. The permeability and enzymatic hydrolysis of L-Val-ACV were evaluated using freshly excised rabbit cornea. Transport mechanism across rabbit cornea was investigated through a competitive inhibition study of L-Val-ACV with other substrates of human peptide transporter (hPepT1). Results. L-Valyl ester of Acyclovir (L-Val-ACV) was approximately threefold more permeable across the intact rabbit cornea than acyclovir (ACV). Dipeptides, -lactam antibiotics, and angiotensin converting enzyme (ACE) inhibitors, strongly inhibited the transport of L-Val-ACV indicating that a carrier mediated transport system specific for peptides is primarily responsible for the corneal permeation of L-Val-ACV. L-Val-ACV transport was found to be saturable (K _m = 2.26 ± 0.34 mM, J _max = 1.087 ± 0.05 nmoles cm^{– 2} min^{– 1}), energy and pH dependent. Conclusions. Functional evidence of an oligopeptide transport system present on the rabbit cornea has been established. The peptide transporter on the corneal epithelium may be targeted to improve the ocular bioavailability of poorly absorbed drugs.     

PEPT1 Enhances the Uptake of Gabapentin via Trans-Stimulation of b<Superscript>0,+</Superscript> Exchange

Purpose The aims of this study were (1) to determine whether amino acid and dipeptide loading can improve the effective permeability of gabapentin and (2) to characterize the underlying mechanism that is responsible for this interaction. Materials and Methods An in situ single-pass rat intestinal perfusion model was used to assess the effective permeability of gabapentin in rat, in the absence and presence of cellular loading by amino acid and dipeptide mixtures. Results Compared to gabapentin alone, cellular loading with amino acid and dipeptide mixtures significantly improved the effective permeability of gabapentin by 46–79% in jejunum and by 67–72% in ileum (p ≤ 0.01). However, coperfusion of glycylsarcosine (i.e., PEPT1 substrate), methionine sulfoximine (i.e., glutamine synthase inhibitor), or lysine and arginine (i.e., b^0,+ substrates) with the amino acid and dipeptide mixtures compromised the intestinal uptake of gabapentin. Conclusions These findings demonstrate, for the first time, a direct relationship between the PEPT1-mediated uptake of a dipeptide and the trans-stimulated uptake of gabapentin (an amino acid-like drug) through the transport system b^0,+. This article is posthumous for David Fleisher.     

Saturable Absorptive Transport of the Hydrophilic Organic Cation Ranitidine in Caco-2 Cells: Role of pH-Dependent Organic Cation Uptake System and P-Glycoprotein

Purpose The purpose of this work was to investigate the involvement of carrier-mediated apical (AP) uptake and efflux mechanisms in the absorptive intestinal transport of the hydrophilic cationic drug ranitidine in Caco-2 cells. Methods Absorptive transport and AP uptake of ranitidine were determined in Caco-2 cells as a function of concentration. Permeability of ranitidine in the absorptive and secretory directions was assessed in the absence or presence of the P-glycoprotein (P-gp) inhibitor, GW918. Characterization of the uptake mechanism was performed with respect to inhibitor specificity, pH, energy, membrane potential, and Na⁺ dependence. Efflux from preloaded monolayers was evaluated over a range of concentrations and in the absence or presence of high extracellular ranitidine concentrations. Results Saturable absorptive transport and AP uptake of ranitidine were observed with K_m values of 0.27 and 0.45 mM, respectively. The ranitidine absorptive permeability increased and secretory permeability decreased upon inhibition of P-gp. AP ranitidine uptake was inhibited in a concentration-dependent fashion by a diverse set of organic cations including tetraethylammonium, 1-methyl-4-phenylpyridinium, famotidine, and quinidine. AP ranitidine uptake was pH and membrane potential dependent and reduced under conditions that deplete metabolic energy. Efflux of [³H]ranitidine across the basolateral membrane was neither saturable as a function of concentration nor trans stimulated by unlabeled ranitidine. Conclusions Saturable absorptive transport of ranitidine in Caco-2 cells is partially mediated via a pH-dependent uptake transporter for organic cations and is subject to attenuation by P-gp. Inhibition and driving force studies suggest the uptake carrier exhibits similar properties to cloned human organic cation transporters. The results also imply ranitidine transport is not solely restricted to the paracellular space.     

Modeling of the Saturable Time-Constrained Amoxicillin Absorption in Humans

Amoxicillin pharmacokinetics was modeled using a two-compartment disposition model and a saturable time-constrained absorption model with a storage compartment. The absorption model parameters estimated by the nonlinear regression are: a rate constant of the systemic input, k_sys, (median: 1.31 h^–1, range: 0.79–7.01 h^–1), a maximal absorption rate, V_ma, (median: 1407 mg/h, range: 703–4181 mg/h), an account corresponding to the half-maximal rate, K_ma, (median: 1077 mg, range: 235–4376 mg), time of the absorption cessation, T_abs, (median: 1.72 h, range: 0.82–4.53 h) and absorption lag time, T_lag, (median: 0.085 h, range: 0–0.123 h). It was shown, that the first-order absorption parallel to the saturable process is negligible in the dose range studied. The model described well the dependence of areas under concentration-time curves on the dose determined in several earlier studies. It was used also to predict the fraction of the amoxicillin dose absorbed for different doses. Simulations performed over a wide dose range (50–10000 mg) demonstrated that the fraction absorbed decreases nonlinearly from 90% at 50 mg to 22% at 10000 mg and strongly depends on the duration of the absorption period.     

Absolute Bioavailability and Absorption Profile of Cyanamide in Man

A pharmacokinetic study of cyanamide, an inhibitor of aldehyde dehydrogenase (EC1.2.1.3) used as an adjuvant in the aversive therapy of chronic alcoholism, has been carried out in healthy male volunteers following intravenous and oral administration. Cyanamide plasma levels were determined by a sensitive HPLC assay, specific for cyanamide. After intravenous administration cyanamide displayed a disposition profile according to a two-compartmental open model. Elimination half-life and total plasma clearance values ranged from 42.2 to 61.3 min and from 0.0123 to 0.0190 L · kg ^–1 · min^–1, respectively. After oral administration of 0.3, 1.0, and 1.5 mg/kg ± SEM values of C_max, t_max (median) and AUC were 0.18 ± 0.03, 0.91 ± 0.11, and 1.65 ± 0.27 g · ml ^–1 ; 13.5, 13.5, and 12 min; and 8.59 ± 1.32, 45.39 ± 1.62, and 77.86 ± 17.49 g · ml ^–1 · min, respectively. Absorption was not complete and the oral bioavailability, 45.55 ± 9.22, 70.12 ± 4.73, and 80.78 ± 8.19% for the 0.3, 1.0, and 1.5 mg/kg doses, respectively, increased with the dose administered. The models that consider a first-order absorption process alone (whether with a fixed or variable bioavailability value as a function of dose) or with loss of drug due to presystemic metabolism (with zero-order or Michaelis–Menten kinetics) were simultaneously fitted to plasma level data obtained following 1 mg/kg iv and 0.3, 1.0, and 1.5 mg/kg oral administrations. The model that best fit the data was that with a first-order absorption process plus a loss by presystemic metabolism with Michaelis–Menten kinetics, suggesting the presence of a saturable first-pass effect.     

Physiological mechanism-based analysis of dose-dependent gastrointestinal absorption of L-carnitine in rats

We evaluated the dose-dependent (saturable) gastrointestinal absorption of L -carnitine, a lipid-lowering agent, in rats by a physiological mechanism-based approach to clarify its absorption characteristics and to examine the in vitro (in situ)–in vivo correlation in intestinal transport. The intestinal absorption rate constant (k_a), which was estimated by the analysis of gastrointestinal disposition, decreased markedly from 0.1061 to 0.0042 min⁻¹ when the dose was increased from 0.05 μmol rat⁻¹ (low dose) to 100 μmol rat⁻¹ (high dose). The dose-dependence in k_a was attributable to the saturability of intestinal transport that, in the perfused intestine, was similar to the saturability in k_a. At the high dose, the apparent absorption rate constant (k_a′) of 0.0021 min⁻¹, which was estimated by the analysis of plasma concentrations after oral administration, was an order of magnitude smaller than the gastric emptying rate constant (k_g) of 0.059 min⁻¹ and comparable with the k_a of 0.0042 min⁻¹, suggesting that the gastrointestinal absorption of L -carnitine is absorption-limited in the intestine. At the low dose, where intestinal L -carnitine absorption was far more efficient, the k_a′ of 0.0172 min⁻¹ was smaller than the k_a of 0.1061 min⁻¹ and closer to the k_g of 0.072 min⁻¹, suggesting that apparent absorption was retarded by gastric emptying which is less efficient than intestinal absorption. This shift in the rate-determining process with an increase in dose explains the less marked dose dependence in k_a′ compared with k_a. The bioavailability decreased from 100 to 42% with an increase in dose. This could be accounted for quantitatively by a reduction in the fraction absorbed (F_a,oral) due to a reduction in k_a, assuming first-order absorption during the transit time of T_si through the small intestine (F_a,oral=1−exp(−k_a · T_si)). Thus, using L -carnitine as a model, this study has successfully demonstrated that the saturability in gastrointestinal absorption can be correlated with the intestinal transport in a quantitative and mechanism-based manner. This should be of help not only for developing more efficient oral L -carnitine delivery strategies, taking advantage of in vitro (in situ) information about the intestinal transport mechanism, but also for establishing a more generally applicable in vitro (in situ)–in vivo correlation in gastrointestinal absorption. © 1998 John Wiley & Sons, Ltd.