Regional gastrointestinal absorption of ranitidine in the rat

Administration in fed state but not controlled release in the colon increases oral bioavailability of DF030263, a promising drug candidate for chronic lymphocytic leukemia

For treatment of chronic cancers, the oral administration route is preferred as it provides numerous advantages over other delivery routes. However, these benefits of oral chemotherapy can be limited due to unfavorable pharmacokinetics. Accordingly, pharmacokinetic development of chemotherapeutic agents is crucial to the improvement of cancer treatment. In this study, assessment and optimization of biopharmaceutical properties of a promising drug candidate for cyclin-dependent kinase 9 (CDK9) inhibitor (DF030263) was performed to promote oral delivery. Oral bioavailability of DF030263 in fasted rats was 23.8%, and a distinct double-peak phenomenon was observed. A two-site absorption windows mechanism was proposed as a possible explanation to the phenomenon. The two-site absorption window hypothesis was supported by in vitro solubility assays in biorelevant fluids with different pH levels, as well as by in silico simulation by GastroPlus™. Controlled release to the colon was conducted in rats in order to exploit the colonic absorption window but did not improve the oral bioavailability. On the other hand, oral administration at postprandial conditions in rats (performed based on the high in vitro solubility in fed state simulated fluid and reduced pH-dependency) resulted in an almost 3-fold increase in bioavailability to 63.6%. In conclusion, this study demonstrates an efficient in vitro-in vivo-in silico drug development approach for improving the oral bioavailability of DF030263, a promising candidate for the treatment of chronic lymphocytic leukemia.

Safety testing of metabolites: Expectations and outcomes

Metabolites arising from chemical entities, old or new, are often mediators of toxicity. Frequently, metabolites are investigated in test animals, with the expectation that the resultant toxicity or activity will mimic the exposure of their formed counterparts. This communication described observations that showed discrepant kinetics between formed and preformed metabolites in the liver, intestine, and kidney, major drug removal organs. Differences in the observed areas under the curve (AUCs) or the extraction ratios (Es) of formed and preformed metabolites in the liver had been attributed to zonal, enzyme heterogeneity, membrane barriers, or transporters. Preformed and formed metabolite also differed in their handling by the kidney; only the preformed and not the formed metabolite would be filtered. In the intestine, differences in the absorption of the precursor and the metabolite and the flow pattern in the intestine would bring about discrepancy in the time-courses of the formed vs. preformed metabolites. Analytical solutions of the AUCs of the metabolites and extraction ratios, based on physiological modeling of the liver, kidney, and intestine, showed that the AUC of the preformed, administered metabolite was dependent only on metabolite parameters, whereas the AUC of the formed metabolite was modulated additionally by the metabolic, secretory and intestinal absorptive intrinsic clearances of the precursor drug. Hence, administration of the synthetic metabolite would not reflect the toxicity associated with the metabolite formed via bioactivation. However, data on preformed metabolite may be used for simultaneous fitting by a combined model of drug and metabolite. Such a strategy is shown to be successful in risk assessment of environmental chemicals. Upon refinement of the resultant model with data on metabolite transport and handling by modeling and simulations, the resultant model would be more robust to provide improved predictions on metabolite toxicity pursuant to drug administration.

In situ kinetic modelling of intestinal efflux in rats: functional characterization of segmental differences and correlation within vitro results

The objective was to devise and apply a novel modelling approach to combine segmental in situ rat perfusion data and in vitro cell culture data, in order to elucidate the contribution of efflux in drug absorption kinetics. The fluoroquinolone CNV97100 was used as a model P-gp substrate. In situ intestinal perfusion was performed in rat duodenum, jejunum, ileum and colon to measure the influence of P-gp expression on efflux. Inhibition studies of CNV97100 were performed in the presence of verapamil, quinidine, cyclosporin A and p-aminohippuric acid. Absorption/efflux parameters were modelled simultaneously, using data from both in situ studies as well as in vitro studies. The maximal efflux velocity was modelled as a baseline value, corrected for each segment based on the expression level. CNV97100 passive diffusional permeability (P(diff)) and its affinity for the efflux carrier (K(m)) were assumed to be the same in all segments. The results indicate the new approach to combine in situ data and in vitro data succeed in yielding a unified, quantitative model for absorption/efflux. The model incorporated a quantitative relationship between P-gp expression level and the efflux functionality, both across in situ and in vitro systems, as well across different intestinal segments in the in situ studies. Permeability values decreased from duodenum to ileum in accordance with the increasing P-gp expression levels in rat intestine. The developed model reflects a strong correlation between in vitro and in situ results, including intrinsic differences in surface area. The successful application of a model approach to combine absorption data from two different experimental systems holds promise for future efforts to predict absorption results from one system to a second system.

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 Km 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.

Simultaneous determination of amoxicillin and ranitidine in rat plasma by high-performance liquid chromatography

A high-performance liquid chromatography method using ultraviolet detection at 230 nm for the simultaneous determination of amoxicillin and ranitidine in rat plasma has been validated. Plasma samples after pretreatment with acetonitrile to effect deproteinization were dried under N2 and reconstituted with water. The standard calibration curves for amoxicillin and ranitidine were linear (r2=0.9999) over the concentration range of 0.2-20 microg ml-1 and 0.03-6 microg ml-1 in rat plasma, respectively. The intra- and inter-day assay variability range for amoxicillin was 2.4-8.5% and 3.2-11.7%, and for ranitidine was 1.7-9.0% and 4.5-10.1%, respectively. This method has been successfully applied to a pharmacokinetic study after oral coadministration of amoxicillin and ranitidine to rats.

Prokinetic activity of an aqueous extract from dried immature fruit of Poncirus trifoliata (L.) Raf

Aqueous extracts from dried immature fruit of Poncirus trifoliata Raf. (Rutaceae) (PF-W) are used as a traditional Korean folk medicine for the treatment of digestive dysfunction. In the present study, PF-W exhibited no significant toxicity even at a dose of 5 g/kg when orally administered to mice. The effect of PF-W on gastrointestinal (GI) motor function was investigated by examining its effect on the serum concentration of orally administered ranitidine, a putative indicator of GI motility, in human subjects. The area under the serum concentration-time curve and the peak serum concentration of ranitidine following an oral administration (300 mg/individual) were decreased by one half as the result of a predose (10 g/individual) of PF-W, except for the time to reach peak serum concentration and the serum half-life at the terminal phase of ranitidine. In rat studies, PF-W had no effect on the apparent permeability of ranitidine across the jejunum or the gastric emptying rate (GER) of phenol red. However, the transit time for charcoal in the intestine was significantly increased by the PF-W pretreatment. The above results are consistent with the hypothesis that PF-W has a unique prokinetic activity, which accelerates the transit of intestinal contents, but has no effect on the GER.

Evidence for dissolution rate-limited absorption of COL-3, a matrix metalloproteinase inhibitor, leading to the irregular absorption profile in rats after oral administration

PURPOSE

This study was undertaken to elucidate the underlying mechanism of the irregular absorption profiles of COL-3, a matrix metalloproteinase inhibitor, with a double- or plateau-peak concentration after a single oral dose administration of COL-3 suspension to rats.

METHODS

The gastrointestinal absorption profiles of COL-3 in rats were assessed by comparing serum drug concentration curves after the following various modes of drug administration: oral and intraduodenal doses, oral doses of COL-3 in fine and coarse suspensions, intraduodenal dosing to the bile-duct intact and cannulated (BDC) rats, and oral doses with and without food. In addition, the biliary excretion of COL-3 in the BDC rats was examined.

RESULTS

Neither variable gastric emptying nor enterohepatic recycling was the source of the irregular gastrointestinal absorption of COL-3 in rats. Reduction in particle size, presence of food and endogenous bile emerged as the determinants of the oral absorption of COL-3 by enhancing the dissolution of the solid drug in the gastrointestinal fluids. Flip-flop of the absorption and elimination rate constants was noted only for COL-3 after intraduodenal administration of the coarse suspension to the BDC rats with the bile flow diverged out of the body.

CONCLUSIONS

Variability in dissolution rate-limited absorption was the main cause of the irregular absorption of COL-3 after oral administration of its solid dosage form.

Drug, meal and formulation interactions influencing drug absorption after oral administration. Clinical implications

Drug-drug, drug-formulation and drug-meal interactions are of clinical concern for orally administered drugs that possess a narrow therapeutic index. This review presents the current status of information regarding interactions which may influence the gastrointestinal (GI) absorption of orally administered drugs. Absorption interactions have been classified on the basis of rate-limiting processes. These processes are put in the context of drug and formulation physicochemical properties and oral input influences on variable GI physiology. Interaction categorisation makes use of a biopharmaceutical classification system based on drug aqueous solubility and membrane permeability and their contributions towards absorption variability. Overlaying this classification it is important to be aware of the effect that the magnitudes of drug dosage and volume of fluid administration can have on interactions involving a solubility rate limits. GI regional differences in membrane permeability are fundamental to the rational development of extended release dosage forms as well as to predicting interaction effects on absorption from immediate release dosage forms. The effect of meals on the regional-dependent intestinal elimination of drugs and their involvement in drug absorption interactions is also discussed. Although the clinical significance of such interactions is certainly dependent on the narrowness of the drug therapeutic index, clinical aspects of absorption delays and therapeutic failures resulting from various interactions are also important.

Use of the InteliSite capsule to study ranitidine absorption from various sites within the human intestinal tract

PURPOSE

The purpose of this study was to evaluate the extent of ranitidine absorption from an externally activated drug-delivery system in two distinct regions of the intestine (jejunum and ileum) in healthy human volunteers. This investigation also was designed to evaluate the utility of the InteliSite capsule for studying regional intestinal drug absorption in humans.

METHODS

The intestinal absorption of ranitidine from the jejunum and ileum was compared in eight, healthy volunteers in this open-label, two-way crossover study. In two of the eight volunteers, absorption from the colon also was studied. Subjects swallowed the capsule containing ranitidine solution (121 mg) and 100 microCi of 99mTc-DTPA. The endcap of the capsule contained 20 microCi of (111)In-DTPA. At the desired intestinal site, the capsule was activated by the application of an external RF magnetic signal (6.78 MHz operating frequency) and the ranitidine solution was released. Blood samples were collected from a forearm vein for 12 hours after capsule activation.

RESULTS

The capsule released the ranitidine solution when activated in the jejunum, ileum and colon (visualized by the gamma camera). There was no difference in the extent of ranitidine absorption or ranitidine pharmacokinetics when the capsule was activated in the jejunum or ileum.

CONCLUSIONS

This study demonstrates the utility of a novel, externally activated drug-delivery system to assess site-specific intestinal drug absorption in humans. Results indicate that use of the InteliSite capsule method to evaluate site-specific intestinal ranitidine absorption in humans yields data similar to that obtained previously by means of oral intubation studies.

Effect of pancreatico-biliary secretions and GI transit time on the absorption and pharmacokinetic profile of ranitidine in humans

PURPOSE

Ranitidine plasma concentration vs. time profiles and the extent of ranitidine absorption were examined in the presence and absence of pancreatico-biliary secretions in order to elucidate factors which may contribute to secondary peaks after oral ranitidine administration.

METHODS

Ranitidine solution (300 mg) was administered to 4 fasting healthy subjects via an indwelling small-bore oroenteric tube located approximately 16 cm distal to the pylorus On 3 consecutive days, subjects randomly received ranitidine alone (control), ranitidine 10 min after 0.04 micrograms/kg IV cholecystokinin (CCK) sufficient to cause gall bladder emptying into the duodenum, and ranitidine 30 min after inflation of an occlusive duodenal balloon located approximately 10 cm distal to the pylorus to prevent pancreatico-biliary secretions from reaching the dosing port or beyond. Small bowel transit time (SBTT; min) was measured by breath H2. Serial blood samples, obtained over 12 hours in each treatment, were analyzed by HPLC to determine ranitidine AUC0-12 (ng*h/mL), as well as Cmax (ng/mL) and Tmax (min) of the first and subsequent peaks, if subsequent peaks were observed.

RESULTS

Ranitidine AUC0-12 and Cmax were not altered significantly by treatments; treatment effects on SBTT varied. Secondary peaks were observed in subjects #1 and #3 during the control treatment and subjects #2 and #4 during the CCk treatment. No secondary peaks were observed in any subject during the balloon treatment, and Tmax1 was delayed.

CONCLUSIONS

Results support the hypothesis that pancreatico-biliary secretions (present in the intestinal lumen during control or CCK treatment) and gastrointestinal transit time may influence the occurrence of secondary peaks in ranitidine concentration vs. time profiles.

Gastrointestinal transit and distribution of ranitidine in the rat

PURPOSE

Ranitidine gastrointestinal distribution was examined in the rat small intestine after oral administration to determine whether intestinal transit or secretion (exsorption) may influence the appearance of secondary peaks in ranitidine serum concentration-time profiles.

METHODS

Male Sprague-Dawley rats received ranitidine (50 mg/kg) by oral gavage, and the mass of ranitidine recovered in all small intestinal segments (approximately 12 cm each) was determined 30, 60, 90, or 120 min after administration. In a separate group of anesthetized rats, the small intestine was divided into two segments of equal length that were perfused with normal saline in a single-pass manner. Rats received an escalating, zero-order IV infusion of ranitidine for 30 min, and venous blood and intestinal effluent were collected over 90 min to quantitate ranitidine exsorption.

RESULTS

Thirty min after oral administration, > 50% of the recovered ranitidine mass resided in the lower half of the small intestine in all rats. Ranitidine mass in 5 of 16 rats displayed a bimodal distribution with significant amounts of ranitidine recovered from the stomach 60 to 90 min after dosing. Ranitidine exsorption was more efficient from the lower jejunum and ileum than from the duodenum and upper jejunum. However, intestinal secretion of ranitidine was minor (5% of the IV dose).

CONCLUSIONS

Ranitidine absorption from the lower ileum contributes significantly to systemic ranitidine concentrations before and during the time of the first concentration maximum. Separation of the drug mass into multiple boluses may contribute to secondary peaks in ranitidine concentration-time profiles. Exsorption did not contribute significantly to ranitidine distribution in the gastrointestinal tract.