Nortriptyline hydrochloride skin absorption: development of a transdermal patch.

The influence of propylen glycol (PG), ethanol, and oleic acid (OA) on nortriptyline hydrochloride (NTH) penetration through human epidermis was studied in vitro at two different pH values (5.5 and 7.4). The influence of lactic acid and polysorbate 80 was studied for a pH of 5.5. Permeation studies through Heat Separated Epidermis, as well as the enhancing effect of the different vehicles, showed a pH dependency. A pH value of 5.5 in the donor solution decreases significantly the permeability coefficient (Kp) with respect to a pH value of 7.4 (0.011+/-0.004 x 10(-6) versus 0.36+/-0.04 x 10(-6)cm/s). The vehicles showed an increasing enhancement effect in the order: polysorbate 80>ethanol/PG/OA>PG>ethanol>ethanol/lactic acid>lactic acid at pH 5.5 while they reduced the permeation of NTH at pH 7.4. Considering the results obtained at pH 5.5, the maximum enhancement ratios were found for polysorbate 80 and the combination ethanol/PG/OA (10.72 and 3.90). Both vehicles were selected for designing a NTH transdermal delivery system (NTH-TDS) using (hydroxypropyl)methyl-cellulose as polymer. The NTH-TDS based on the combination of ethanol/PG/OA showed an enhancement ratio with respect to control of 2.09 and the addition of polysorbate 80 to the matrix, of 5.82.

Labetalol absorption kinetics: rat small intestine and colon studies.

Labetalol is a widely used drug for the management of hypertension, which is preferably administered by the oral route despite its low bioavailability. The objective of this study is to ascertain the mechanisms underlying its absorption as an approach to help in predicting the influence of dosage changes, possible drug-drug and drug-fruit juice interactions. Perfusion experiments have been performed in rats in two sites of absorption: the intestine and the colon. The nonlinearity of the process has been established by means of the assay of a wide range of concentrations (2-2000 microM). Fitting of the concentration versus time data allows the estimation of passive diffusion constant in the intestine (1.42 +/- 0.05/h) and the colon (1.13 +/- 0.06/h), V(m) and K(m) of the input process (9.85 +/- 4.98 microM/h, and 10.44 +/- 26.16 microM, respectively) and K(m) of an efflux system (0.53 +/- 1.16 microM) and V(m) in both intestinal segments (2.60 +/- 11.37 microM . /h in the intestine and 0.66 +/- 1.38 microM . /h in the colon). The efflux carrier implicated is identified by means of several inhibition experiments, whose inhibition ability is mathematically estimated. Results suggest the p-glycoprotein as responsible for the efflux of labetalol.

Mathematical modelling of in situ and in vitro efflux of ciprofloxacin and grepafloxacin.

The efflux process due to p-glycoprotein-like mechanisms of ciprofloxacin (CIP) and grepafloxacin (GRX) has been studied "in situ" in rats and "in vitro" in Caco-2 cells. The results were modelled by a curve fitting procedure which allowed the characterization of the passive (Pd) and carrier mediated parameters (Vm and Km) from the raw data without initial velocities estimation. CIP absorption in rat was characterized as a passive diffusion at the assayed concentrations. Although the involvement of an efflux transporter cannot be ruled out, its relevance in the transport of the fluoroquinolone is negligible. In GRX absorption, an efflux process is implicated and it is detected in both absorption models. GRX permeability depends on the intestinal segment, reflecting the previously reported different expression level of the efflux transporters along the gut in rat. A first attempt to correlate the "in vitro" and the "in situ" data has been done. The mathematical model has been constructed using very simplistic assumptions and it will require further refinement but, nevertheless, the results are promising and demonstrate that a good modelling approach helps to identify the system critical parameters and how the system behaviour change when the parameters are modified as it happens when we move from the "in vitro" to the "in situ" level. Predicted versus experimental permeability values show a good correlation, demonstrating that the relevance of the secretion process "in situ" in rat can be predicted from the "in vitro" cell results.

Nanocarriers for optimizing the balance between interfollicular permeation and follicular uptake of topically applied clobetasol to minimize adverse effects.

The treatment of various hair disorders has become a central focus of good dermatologic patient care as it affects men and women all over the world. For many inflammatory-based scalp diseases, glucocorticoids are an essential part of treatment, even though they are known to cause systemic as well as local adverse effects when applied topically. Therefore, efficient targeting and avoidance of these side effects are of utmost importance. Optimizing the balance between drug release, interfollicular permeation, and follicular uptake may allow minimizing these adverse events and simultaneously improve drug delivery, given that one succeeds in targeting a sustained release formulation to the hair follicle. To test this hypothesis, three types of polymeric nanocarriers (nanospheres, nanocapsules, lipid-core nanocapsules) for the potent glucocorticoid clobetasol propionate (CP) were prepared. They all exhibited a sustained release of drug, as was desired. The particles were formulated as a dispersion and hydrogel and (partially) labeled with Rhodamin B for quantification purposes. Follicular uptake was investigated using the Differential Stripping method and was found highest for nanocapsules in dispersion after application of massage. Moreover, the active ingredient (CP) as well as the nanocarrier (Rhodamin B labeled polymer) recovered in the hair follicle were measured simultaneously, revealing an equivalent uptake of both. In contrast, only negligible amounts of CP could be detected in the hair follicle when applied as free drug in solution or hydrogel, regardless of any massage. Skin permeation experiments using heat-separated human epidermis mounted in Franz Diffusion cells revealed equivalent reduced transdermal permeability for all nanocarriers in comparison to application of the free drug. Combining these results, nanocapsules formulated as an aqueous dispersion and applied by massage appeare to be a good candidate to maximize follicular targeting and minimize drug penetration into the interfollicular epidermis. We conclude that such nanotechnology-based formulations provide a viable strategy for more efficient drug delivery to the hair follicle. Moreover, they present a way to minimize adverse effects of potent glucocorticoids by releasing the drug in a controlled manner and simultaneously decreasing interfollicular permeation, offering an advantage over conventional formulations for inflammatory-based skin/scalp diseases.

In vitro percutaneous penetration of acyclovir from solvent systems and Carbopol 971-P hydrogels: influence of propylene glycol.

The mechanism underlying propylene glycol (PG) effects on acyclovir (ACV) penetration through human epidermis were studied. Solvent systems and Carbopol gels containing increasing percentage of PG (from 0% to 70%, w/w) were used. Viscosity studies of both vehicles were carried out to characterise the influence of rheological behaviour. In solvent systems skin permeation values of ACV increase as the concentration of PG increase yielding a maximum enhancement ratio (ER = 10) for 70% PG. The release rate of ACV from gels was determined. Higuchi's model was used to estimate the apparent diffusion coefficient of the drug. These values show a decrease as the content of PG in the vehicle increases; this effect could be attributed to the increase of the viscosity in the diffusional pathway. When gels are used skin permeation values of ACV were smaller than those of the solvent systems. This could be attributed to the network structure created by the polymer that increases the length of the diffusional pathway. The maximum ER (= 6.8) was for Carbopol gel containing 50% PG. Therefore, these gels can be considered candidates for further research to confirm their usefulness as delivery systems for ACV topical formulations.

Pharmacokinetics, bioavailability and absorption of flumequine in the rat.

The study demonstrates that the oral extent of bioavailability of flumequine in the rat, relative to the intravenous injection, is complete (0.94 +/- 0.04) and not significantly different from that found by the intraduodenal route (0.95 +/- 0.04). The rate of oral bioavailability, however, is slow (ka = 1.20 +/- 0.07 h-1; Tmax = 2.0 h), but enough to maintain plasma levels above the minimal inhibitory concentration of the most common pathogens for an extended period of time (about 10 h). The reason for the oral absorption slowness could be a slow gastric emptying, an adsorption to the gastric mucosae, a precipitation in the gastric medium or any other feature concerning the stomach as the intraduodenal administration is very quick (kid = 38.1 +/- 4.7 h-1; Tmax = 0.05 h). A possible precipitation of flumequine cannot be discarded as the solubility of flumequine is very low in the pH range of 3 to 6 (mean pH values for rat stomach and rat intestine, respectively; T.T. Kararli, Biopharm. Drug Dispos. 16 (1995) 351-380). Flumequine was shown to be not substantially excreted in bile (2-3% of the dose). Surprisingly, plasma levels and AUC values found for animals with interrupted bile flow always surpass those found for animals with enterohepatic circulation. This could be due to experimental model features, which might bias plasmatic flumequine concentrations if the homeostatic equilibrium of the animal is not completely restored due to the volume reduction induced by biliary extraction.

Transintestinal secretion of ciprofloxacin, grepafloxacin and sparfloxacin: in vitro and in situ inhibition studies.

The influence of the secretion process on the absorption of ciprofloxacin, grepafloxacin and sparfloxacin has been evaluated by means of inhibition studies. Two well known P-glycoprotein inhibitors (cyclosporine, verapamil), a mixed inhibitor of P-glycoprotein and the organic cation transporter OCT1 (quinidine) and a well established MRP substrate (p-aminohipuric acid) have been selected in order to distinguish the possible carriers implicated. An in situ rat gut perfusion model and CACO-2 permeability studies are used. Both methods suggest the involvement of several types of efflux transporters for every fluoroquinolone. The relevance of the secretory pathway depends on the intrinsic permeability of the quinolone. The in vitro model seems to be more suitable for discriminating mechanisms underlying the absorption process, while in situ studies are less sensitive to inhibition studies.

Kinetic modeling of triamterene intestinal absorption and its inhibition by folic acid and methotrexate.

The aim of this work was to study the intestinal absorption process of triamterene in situ in rats in order to gain insight on its absorption mechanism. The study shows an example of application of a pharmacokinetic model for drug disappearance from a compartment by simultaneous first order and Michaelis-Menten processes to the intestinal drug disappearance by absorption in a non-steady state system. The parameter used to quantify absorption was the absorption rate constant, determined in situ by means of a loop perfusion technique, performed in colon and in whole small intestine of rat at three pHs (5.00, 7.00 and 8.00) and in the presence of folic acid and methotrexate. Different concentrations of the drug were perfused in each condition. The concentrations versus time data were modeled with different kinetic absorption and inhibition differential equations to obtain the passive and active transport components and to elucidate the inhibition mechanisms. The results obtained confirm that triamterene is absorbed by means of passive diffusion and by a transporter or transporters related to folates. Folic acid and methotrexate are able to inhibit triamterene absorption. The results do not allow the selection of a competitive or non-competitive model for inhibition and this fact could be due to the presence of different carriers. The possibility of the existence of a secretion process sensitive to verapamil is also discussed.

Biophysical models as an approach to study passive absorption in drug development: 6-fluoroquinolones.

A preliminary study attempting to assess and explain the intestinal absorption of a series of antibacterial 7-piperazinyl-6-fluoroquinolones is presented. The synthesis, n-octanol partition coefficients, intrinsic rat gut in situ absorption rate constants, and in vitro antibacterial activity data found for these homologous compounds are described. A fluorimetric, reverse-phase HPLC method was performed for the quantification of the quinolones in absorption and partition samples. Equations based on two classic biophysical absorption models are given for predicting the intrinsic absorption features of the series according to the partition data or merely single structural parameters. In situ absorption rate constants were found to increase by a factor of 9.7-13.5 for moderately lipophilic derivatives relative to the simplest compound, while antibacterial activity decreased only by a factor of 4. In vivo absorption tests with two representative members of the series were carried out and the results showed a good accordance with those found in situ. This makes these compounds or related ones with similar partition features excellent candidates for further pharmacokinetic and pharmacological testing. The study can serve as an example of how to prevent potential absorption problems associated with the development of new drugs.

Validation of a biophysical drug absorption model by the PATQSAR system.

Absorption rate constants (in situ rat gut technique) and in vitro antibacterial activities of twenty fluoroquinolones have been evaluated. A biophysical model that relates the absorption of the compounds with their lipophilicity was fitted. The model considers the absorption process from the intestinal lumen as the sum of two resistances in series: aqueous diffusional barrier and lipoidal membrane. Even if partitioning into the membrane and membrane diffusion are both enhanced for lipophilic compounds, the absorption rate constant is limited by the aqueous diffusion. To estimate the influence of structural modifications on each property and to establish the role of lipophilicity in controlling in situ absorption and in vitro antibacterial activity, the PATQSAR search system is used to construct structure-property relationships. The structural models, which explain 99% of the total variance of each physicochemical property and 96% of each in vitro biological activity, provide an explicit and precise interpretation of lipophilicity, absorption, and antimicrobial activity. The results confirm the important role of lipophilicity in controlling absorption, as pointed out by the biophysical model for the piperazinyl series, and suggest the introduction of electronic factors in order to extend the model to heterologues. They also justify the mechanism by which quinolones are assumed to induce antibacterial activity.

[Mechanisms of bacterial resistance to quinolones]. / Mecanismos de resistencia bacteriana a las quinolonas.

In order to produce their cytotoxic effect, quinolones must enter the cell through the bacterial membrane to reach their target, DNA-gyrase or topoisomerase IV, and induce cell death. The mechanisms of resistance to fluoroquinolones include: those mediated by gene mutations codifying for DNA gyrase and topoisomerase IV and leading to QRDR; those characterized by changes in the permeability of the outer membrane which decrease intracellular penetration of the drug; and those caused by active endogenous carriers responsible for drug efflux. These resistance mechanisms can occur alone or in combination; in fact, it is believed that high resistance levels to quinolones in vivo are associated with simultaneous mechanisms. This article reviews such resistance mechanisms and establishes, when possible, their relation to the structure of quinolones.

[Quinolones and Streptococcus pneumoniae. Mechanisms of action and resistance]. / Quinolonas y Streptococcus pneumoniae. Mecanismo de acción y resistencia.

Streptococcus pneumoniae is considered the most frequent bacterial cause of community-acquired pneumonia, and is involved in a significant number of cases of acute exacerbations of chronic bronchitis, acute otitis, sinusitis, meningitis and other infectious diseases. Fluoroquinolones have been extensively investigated in recent years in the search for new agents that has been prompted by the emergence of resistance in this microorganism. Furthermore, the study of resistance from a molecular biology standpoint has helped in elucidating almost all the biochemical mechanisms of resistance and the routes of dissemination of genetic information between bacteria. This short review is focused on the mechanism of action of quinolones and on the mechanisms responsible for resistance of S. pneumoniae to them, given their clinical and epidemiological relevance. S. pneumoniae is a case apart because bactericidal activity against this microorganism can be produced through gyrase, topoisomerase IV or both, depending on the quinolone structure, which shows that structure has an influence on the success of treatment. Knowledge of the resistance prototype is therefore important so that the appropriate antibiotic therapy can be recommended when indicated.

QSPR in oral bioavailability: specificity or integrality?

During the last decade the technological advances in drug discovery changed the absorption, distribution, metabolism, excretion and toxicity (ADMET) profiles of New Chemical Entities (NCEs). Among ADMET processes, absorption plays an important role in the research and development of more effective orally administered drugs. Although significant progress has been made in in vitro, in situ and in vivo experimental determinations of absorption, the development of in silico methodologies has emerged as a cheaper and fast alternative to predict them. Even though several in silico models have been described in the literature to predict oral bioavailability and related properties, the prediction accuracy and their potential use is still limited. The low precision and high variability of data, the lack of a complete experimental and theoretical validation of in silico approach, and above all, the multi-factorial nature of the oral absorption term, make the development of predictive in silico models a thorny task. The present review discusses several important advances regarding the QSPR approaches used in the development of predictive oral bioavailability models. The importance of fixing the problem associated with data resource, as well as improving the reliability of in silico results is highlighted. Optimization of individual properties along the absorption process must be integrated in a multi-objective scenario for studying oral bioavailability behavior in the early drug discovery and development.

Nortriptyline for smoking cessation: release and human skin diffusion from patches.

The objective of this work was to develop a simple and inexpensive transdermal formulation containing Nortriptyline Hydrochloride (NTH) for smoking cessation support therapy. Hydroxypropyl-methyl-cellulose was chosen as polymer and a mixture of transdermal enhancers (selected from previous research) was incorporated. The formulations were characterised in terms of appearance, thickness, uniformity of NTH content, release and skin permeation. Release studies demonstrated controlled release for four formulations. Diffusion studies were performed through human heat separated epidermis (HHSE) using Franz Diffusion Cells (FDC). Patches provided different fluxes varying from 20.39+/-7.09 microg/(cm(2) h) to 256.19+/-94.62 microg/(cm(2) h). The penetration profiles of NTH within the stratum corneum (SC) and deeper skin layers (DSL) were established after three administration periods (3 h, 6 h, and 24 h). Skin changes induced by the application of the patches were observed by confocal laser scanning microscopy (CLSM). The highest flux obtained would provide the recommended doses for smoke cessation support therapy (25-75 mg per day) with a 2 cm x 2 cm patch or a 3.5 cm x 3.5 cm patch, respectively, without skin damage evidence.

Kinetic modelling of passive transport and active efflux of a fluoroquinolone across Caco-2 cells using a compartmental approach in NONMEM.

The purpose was to develop a general mathematical model for estimating passive permeability and efflux transport parameters from in vitro cell culture experiments. The procedure is applicable for linear and non-linear transport of drug with time, <10 or >10% of drug transport, negligible or relevant back flow, and would allow the adequate correction in the case of relevant mass balance problems. A compartmental kinetic approach was used and the transport barriers were described quantitatively in terms of apical and basolateral clearances. The method can be applied when sink conditions are not achieved and it allows the evaluation of the location of the transporter and its binding site. In this work it was possible to demonstrate, from a functional point of view, the higher efflux capacity of the TC7 clone and to identify the apical membrane as the main resistance for the xenobiotic transport. This methodology can be extremely useful as a complementary tool for molecular biology approaches in order to establish meaningful hypotheses about transport mechanisms.

Dose-dependent absorption and elimination of cefadroxil in man.

The pharmacokinetic behaviour of cefadroxil was dose-dependent in healthy male volunteers following the oral administration of single doses of 5, 15, and 30 mg.kg-1. As the dose of cefadroxil increased from 5 to 15 and 30 mg.kg-1, the peak plasma concentrations, normalized to 5 mg.kg-1, decreased significantly from 15.1 to 10.7 and 7.6 mg.l-1, while the corresponding normalized areas under the plasma concentration-time curves from 0 to 2 h decreased significantly from 1258 to 946 and 801 min.mg.l-1. When the same subjects were given 5 mg.kg-1 of cefadroxil together with 45 mg.kg-1 of cephalexin, the absorption of cefadroxil was slowed to a similar or greater extent than with the high dose of cefadroxil. Although the absorption rate decreased as the dose increased, the systemic availability of cefadroxil was essentially complete at all doses, as judged by the 24 h urinary recoveries of the antibiotic. Kinetic analysis of the plasma concentration-time curves gave the best fit with a zero-order followed by a first-order absorption process, consistent with saturable intestinal absorption of cefadroxil. The elimination rate of cefadroxil was directly related to dose and plasma concentrations, and the clearance at the dose of 5 mg.kg-1 was significantly increased by the simultaneous administration of high-dose cephalexin. The renal clearance of cefadroxil ranged from 98 ml.min.l-1 at total plasma cephalosporin (cefadroxil + cephalexin) concentrations less than 2.5 mg.l-1 to 156 mg.l-1 at concentrations greater than 40 mg.l-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-absorption relationships of a series of 6-fluoroquinolones.

The physicochemical constants and some structural parameters (topological, steric, and electronic) of eight third-generation monofluorate quinolones (six uncommercialized and two used clinically [ciprofloxacin and enrofloxacin]) were determined: pKa, intrinsic solubility (S0), chromatographic capacity factor, partition coefficient (P), valency molecular connectivity, molecular volume, molecular surface area, dipolar moment, and charges associated with each atom of the molecule. The apparent intestinal absorption rate constants (K(abs)) in rat (in vivo perfusion) and the MICs at which 90% of the isolates are inhibited (MIC90s) against 100 Escherichia coli strains were also determined. We sought to establish simple nonlinear and multiple linear correlations between K(abs), on the one hand, and lipophilic parameters and other physicochemical and structural parameters estimated. Of the nonlinear functions examined, the hyperbolic had the best correlation between K(abs) and P, which was in accordance with the Wagner-Sedman (J. G. Wagner and A. J. Sedman, J. Pharmacokinet. Biopharm. 1:23-50, 1973) equation, whereas, after application of the stepwise multiple linear regression method, a multiple linear correlation with some predictive value could be established only between K(abs) as a dependent variable and log P and log S0 as independent variables. In conclusion, the K(abs) and MIC90 of the quinolone CNV 8902 suggest that it is a sufficiently interesting compound to warrant the investigation of its potential therapeutic use orally.

Effects of ethanol on intestinal absorption of drugs. I. In situ studies with ciprofloxacin analogs in normal and chronic alcohol-fed rats.

The effect of chronic alcohol intake on the intestinal absorption of seven compounds belonging to a homologous series (ciprofloxacin derivatives) was evaluated using an in situ rat gut technique that measures the intrinsic absorption rates of the compounds both in control and chronic alcohol-fed rats. For chronic alcohol treatment, the animals were fed a liquid diet containing ethanol (36% of calories), whereas an isocaloric diet was given to the pair-fed control animals. The biophysical absorption model, relating the intestinal absorption rate constants and partition indexes of the tested compounds, was then established either for control or alcohol-fed animals. Differences were analyzed and tentatively interpreted on the basis of general diffusion principles. Results revealed that, in chronic alcohol-fed animals, hydrophilic homologs are absorbed at a significantly faster rate than in control ones, whereas lipophilic homologs do not change their absorption rate relative to controls. Results demonstrate that the bulk polarity of the microvillous lipoidal membrane is enhanced by chronic ethanol intake, whereas basic features of the aqueous boundary layer are not altered. These observations suggest that the physicochemical properties of the compounds are an important factor in explaining the influence of chronic alcohol intake on passive intestinal absorption of xenobiotics. The possible practical implications of our results are discussed from a speculative view-point.

Studies on the reliability of a novel absorption-lipophilicity approach to interpret the effects of the synthetic surfactants on drug and xenobiotic absorption.

Some theoretical principles of the absorption/lipophilicity approach, which attempts to explain the effects of the synthetic surfactants on xenobiotic and drug intestinal absorption, are reviewed and experimentally checked by examining the correlations obtained between "in situ" absorption constants, ka, found in rat colon, and "in vitro" lipophilicity indexes, K', for two compound series (secondary aliphatic amines and phenylalkylamines) in the absence and in the presence of the nonionic surfactant Polysorbate 80, in the intestinal perfusion fluid. Evidence is given for the following actions of the synthetic surfactant: at its critical micelle concentration (CMC), it increases the polarity of the absorbing membrane and, at the same time, it disrupts the aqueous stagnant diffusion layer adjacent to the mucosal barrier. When a supramicellar concentration (SMC) is used, the above actions are almost totally masked by the micellar solubilization of the tested amines, which decreases their absorption constants relative to those found at CMC, as markedly as solute lipophilicity increases. As a consequence of these actions, the correlations between ka and K', which are clearly hyperbolic in free solution, become potential in the presence of the surfactant at its CMC, whereas at SMC a bilinear correlation is obtained. Absorption via lipophilic ionized species seems to take place for both compound series. Mathematical and physicochemical interpretations of this behaviour are outlined, and biopharmaceutical implications of these phenomena are discussed.

Effects of ethanol on intestinal absorption of drugs: in situ studies with ciprofloxacin analogs in acute and chronic alcohol-fed rats.

BACKGROUND: Previous work from our laboratory on the effect, in rats, of chronic ethanol intake on the intestinal absorption of ciprofloxacin analogs suggested an increased polarity of the lipoidal membrane constituents without effects on the aqueous environment. The aim of the present study was to investigate the influence of acute ethanol intake on the absorption of the same series of compounds. METHODS: The effects of in situ ethanol exposure on intestinal absorption were determined in rats fed either a standard liquid diet or a 5% (w/v) ethanol-containing liquid diet. Acute intestinal exposure to 5% (w/v) ethanol was performed in situ in each feeding group. The biophysical absorption model was used to establish correlations between the actual absorption rate constants, and the lipophilicity indexes, for each group of rats. RESULTS: Acute exposure to ethanol produces an increase only in the absorption of hydrophilic homologs in both control and chronic ethanol fed groups. This suggests the absence of homeoviscous adaptation of the intestinal membrane. The biophysical model used allows us to discriminate between the effects of acute and chronic ethanol treatment on the intestinal membrane. CONCLUSIONS: These results suggest that in contrast to previous reports chronic ethanol treatment increases membrane polarity and acute alcohol intake appears to modify membrane fluidity.