Vicente G. Casabó

Absorption-partition relationships for true homologous series of xenobiotics as a possible approach to study mechanisms of surfactants in absorption. II. Aromatic amines in rat small intestine

Abstract The previously proposed approaches in order to interpret the mechanisms elicited by the nonionic surfactant polysorbate 80 in the colonic absorption of 4-alkylanilines are extended here to the absorption of the same compounds in the rat small intestine, where compound behaviour seems to be complicated by the existence of a pore diffusion pathway, simultaneous with membrane absorption. Globally considered, the effects of surfactant in the intestinal absorption of anilines relative to the behaviour of these compounds in free solution are much less evident than in colon for low-molecular-weight hydrophilic compounds of the series, for which pore absorption is highly operative, but, as lipophilicity and molecular weight increase, the effects of surfactant become more and more significant, showing a close resemblance with those observed in colon. In the absence of surfactant, a bihyperbolic correlation can be established between intestinal absorption rate constants of anilines and partition constants. On this basis, the apparently complex absorption-partition correlations found in the presence of surfactant at the critical micelle concentration are explained here by assuming that pore absorption of the compounds is virtually unaffected by surfactant at this concentration, contrarily to that which occurs with membrane permeation, for which the same effects as found on bulk rate constants in colon are observed; a clearly linear and double-logarithmic correlation can be established between membrane absorption rate constants and partition constants, thus indicating that two main effects of the surfactant are elicited i.e. the removal of the limiting effect of the aqueous diffusion layer adjacent to the membrane, and an increased membrane polarity. At 5% surfactant concentration in the perfusion fluid, clearly supramicellar, correlations between membrane intestinal absorption rate constants and partition constants become, as occurred with bulk constants in colon, bilinear, due to the multiple-phase equilibrium arising from micellar solubilization of anilines; pore absorption constants are greatly reduced at this concentration, but correlate well with partition constants through an inverse hyperbolic equation as in the former cases. Biopharmaceutical implications of these observations are briefly discussed.

Computer simulations of bioequivalence trials: selection of design and analyte in BCS drugs with first-pass hepatic metabolism: Part II. Non-linear kinetics.

The objective of this work is to use a computer simulation approach to define the most sensitive analyte and study design of the in vivo bioequivalence study for all types of Biopharmaceutics Classification System (BCS) drugs undergoing first-pass hepatic metabolism under non-linear conditions. A semi-physiological model was developed in NONMEM VI to simulate bioequivalence trials. Eight classes from class I to IV BCS drugs (with high or low intrinsic clearance) in two variability scenarios (high-low) and in six drug products of decreasing quality were simulated in non-linear conditions to complete a total of 96 scenarios that were tested in single dose and steady state studies and compared with the previous results obtained under linear conditions. Parent drug in single dose is the most sensitive analyte and study design for bioequivalence trials in almost all the studied scenarios. However, this general rule has an exception not only in drugs with low permeability (class III and IV) and low intrinsic clearance, for which parent drug in steady state showed differences in the rate of exposure (Cmax) and also in some occasions in the extent of absorption (AUC), that are not reflected with the same sensitivity in the single dose scenario, but it could also be possible for Cmax in class III drugs with high intrinsic clearance. Metabolite data shows less sensitivity detect differences in biopharmaceutics quality in most of the scenarios or it gives the same information as the parent compound.

Studies on the reliability of a bihyperbolic functional absorption model. II. Phenylalkylamines.

Evidence is given that demonstrates the reliability of the bihyperbolic equation, proposed by Plá-Delfina and Moreno, in fitting the correlation between absorption rate constants (ka) found in the small intestine and in the colon of the living anesthetized rat, and partition constants (1/R.F−1), for a series of phenylalkylamines, a group of compounds which differ largely from others which have been tested. Emphasis is laid on the nonexistence of an optimum of lipophilicity for intestinal absorption/partition correlation: This feature makes inapplicable the probabilistic approaches to the reported data.

Innovative in vitro method to predict rate and extent of drug delivery to the brain across the blood-brain barrier.

The relevant parameters for predicting rate and extent of access across the blood-brain barrier (BBB) are fu,plasma (unbound fraction in plasma), Vu,brain (distribution volume in brain) and Kp,uu,brain (ratio of free concentrations in plasma and brain). Their estimation still requires animal studies and in vitro low throughput experiments which make difficult the screening of new CNS candidates. The aim of the present work was to develop a new whole in vitro high throughput method to predict drug rate and extent of access across the BBB. The system permits estimation of fu,plasma, Vu,brain and Kp,uu,brain in a single experimental system, using in vitro cell monolayers in different conditions. From the ratios of the apparent permeability values (Papp) with the adequate mathematical analysis the relevant parameters can be estimated. Papp of ten model compounds has been obtained in MDCKII and MDCK-Mdr1cell monolayers in the absence and presence of albumin and brain homogenate. The ratio of Papp in the absence and presence of albumin allows estimation of in vitro fu,plasma. Papp in the presence of brain homogenate is used to estimate fu,brain and Vu,brain. Kp,uu,brain is estimated from the apical to basal versus basal to apical clearances. The BBB parameters obtained with the new method were predictive of the in vivo behavior of candidates. In vitro fu,plasma, Kp,uu,brain and Vu,brain (calculated with Papp from MDCKII cell line) presented a good correlation with in vivo fu,plasma, Kp,uu,CSF and Vu,brain published values (r=0.92; r=0.85; and r=0.99 respectively). Despite its simplicity the predictive performance is fairly good considering the reduced number of tested compounds with different physicochemical and transport properties. Further experimental modifications could be checked to optimize the method, but the present data support its feasibility. As other in vitro cell culture models, the system is suitable for miniaturization and robotization to allow high throughput screening of CNS candidates.

Computer simulations for bioequivalence trials: selection of analyte in BCS drugs with first-pass metabolism and two metabolic pathways.

The objective of this work is to use a computer simulation approach to define the most sensitive analyte for in vivo bioequivalence studies of all types of Biopharmaceutics Classification System (BCS) drugs undergoing first-pass hepatic metabolism with two metabolic pathways. A semi-physiological model was developed in NONMEM VI to simulate bioequivalence trials. Four BCS classes (from Class I to IV) of drugs, with three possible saturation scenarios (non-saturation, saturation and saturation of only the major route of metabolism), two (high or low) dose schemes, and six types of pharmaceutical quality for the drug products were simulated. The number of investigated scenarios was 144 (4 × 3 × 2 × 6). The parent drug is the most sensitive analyte for bioequivalence trials in all the studied scenarios. Metabolite data does not show sensitivity to detect differences in pharmaceutical quality or it gives the same information as the parent compound. An interesting point to notice is the case of class I drugs administered at a high dose when the principal metabolic route is saturated and the secondary one is not saturated. In this case a substantial reduction in dissolution rate (as it could occur in the case of a prolonged release formulation developed as a line extension of an immediate release formulation) leads to a considerable increase in the AUC of the major metabolite whose formation is saturated supporting the need to require pharmacokinetic and clinical data for new prolonged release medicinal products.

A Pharmacokinetic Model for Evaluating the Impact of Hepatic and Intestinal First-Pass Loss of Saquinavir in the Rat

The aim of this study was to quantify the intestinal and hepatic first-pass loss of saquinavir and to assess the effect of coadministration of ritonavir on this first-pass loss. Single doses of 12, 24, and 48 mg of saquinavir and a dose of 24 mg of saquinavir/6 mg of ritonavir were orally, intravenously, or intraperitoneally administered to 94 rats. Ten groups of animals were studied. A semiphysiological pharmacokinetic model incorporating a population pharmacokinetic analysis [nonlinear mixed-effects model (NONMEM)] was developed to analyze plasma concentration-time profiles after administration via each of the three above-mentioned routes. This model confirmed that saturable metabolism in hepatocytes and enterocytes and dose-dependent precipitation in the peritoneal cavity after intraperitoneal administration characterize the pharmacokinetics of SQV. It also demonstrated that low oral bioavailability of saquinavir is due mainly to intestinal rather than to hepatic first-pass metabolism. In addition, it was shown that ritonavir diminished saquinavir clearance through competitive inhibition. The present report presents a new pharmacokinetic model applied in rats to evaluate the impact of hepatic and intestinal first-pass loss on oral bioavailability.

Permutation Test (PT) and Tolerated Difference Test (TDT): two new, robust and powerful nonparametric tests for statistical comparison of dissolution profiles.

The most popular way of comparing oral solid forms of drug formulations from different batches or manufacturers is through dissolution profile comparison. Usually, a similarity factor known as (f2) is employed; However, the level of confidence associated with this method is uncertain and its statistical power is low. In addition, f2 lacks the flexibility needed to perform in special scenarios. In this study two new statistical tests based on nonparametrical Permutation Test theory are described, the Permutation Test (PT), which is very restrictive to confer similarity, and the Tolerated Difference Test (TDT), which has flexible restrictedness to confer similarity, are described and compared to f2. The statistical power and robustness of the tests were analyzed by simulation using the Higuchi, Korsmayer, Peppas and Weibull dissolution models. Several batches of oral solid forms were simulated while varying the velocity of dissolution (from 30 min to 300 min to dissolve 85% of the total content) and the variability within each batch (CV 2-30%). For levels of variability below 10% the new tests exhibited better statistical power than f2 and equal or better robustness than f2. TDT can also be modified to distinguish different levels of similarity and can be employed to obtain customized comparisons for specific drugs. In conclusion, two new methods, more versatile and with a stronger statistical basis than f2, are described and proposed as viable alternatives to that method. Additionally, an optimized time sampling strategy and an experimental design-driven strategy for performing dissolution profile comparisons are described.

Effects of surfactants on amiodarone intestinal absorption. I. Sodium laurylsulfate.

Amiodarone is a widely used antiarrhythmic agent with high variability in therapeutic effects, which appears to be related, at least in part, to its pharmacokinetics, and in particular, gastrointestinal absorption. The drug exhibits physico-chemical properties highly suitable for diffusion across lipophilic absorbing membranes but its low aqueous solubility can act as the rate limiting step for absorption, making it erratic and variable. In studying the intestinal absorption mechanism of amiodarone, a series of experiments using a rat gut in situ preparation was performed in the presence of a synthetic anionic surfactant, as a drug solubilizer, i.e., sodium laurylsulfate, at variable supramicellar concentrations (from 2.6 to 104 mM). Absorption rate constants of amiodarone decreased as surfactant concentration increased, the absorption being unusually fast at lower surfactant concentrations. Equations were developed to evaluate the relationship between absorption rate constant and surfactant concentration in the intestinal luminal fluid.

Statistical Comparison of Dissolution Profiles to Predict the Bioequivalence of Extended Release Formulations

Appropriate setting of dissolution specification of extended release (ER) formulations should include precise definition of a multidimensional space of complex definition and interpretation, including limits in dissolution parameters, lag time (t-lag), variability, and goodness of fit. This study aimed to set dissolution specifications of ER by developing drug-specific dissolution profile comparison tests (DPC tests) that are able to detect differences in release profiles between ER formulations that represent a lack of bioequivalence (BE). Dissolution profiles of test formulations were simulated using the Weibull and Hill models. Differential equations based in vivo–in vitro correlation (IVIVC) models were used to simulate plasma concentrations. BE trial simulations were employed to find the formulations likely to be declared bioequivalent and nonbioequivalent (BE space). Customization of DPC tests was made by adjusting the delta of a recently described tolerated difference test (TDT) or the limits of rejection of f2. Drug ka (especially if ka is small), formulation lag time (t-lag), the number of subjects included in the BE studies, and the number of sampled time points in the DPC test were the factors that affected the most these setups of dissolution specifications. Another recently described DPC test, permutation test (PT), showed excellent statistical power. All the formulations declared as similar with PT were also bioequivalent. Similar case-specific studies may support the biowaiving of ER drug formulations based on customized DPC tests.

A new mathematical approach for the estimation of the AUC and its variability under different experimental designs in preclinical studies

The aim of the present work was to develop a new mathematical method for estimating the area under the curve (AUC) and its variability that could be applied in different preclinical experimental designs and amenable to be implemented in standard calculation worksheets. In order to assess the usefulness of the new approach, different experimental scenarios were studied and the results were compared with those obtained with commonly used software: WinNonlin® and Phoenix WinNonlin®. The results do not show statistical differences among the AUC values obtained by both procedures, but the new method appears to be a better estimator of the AUC standard error, measured as the coverage of 95% confidence interval. In this way, the new proposed method demonstrates to be as useful as WinNonlin® software when it was applicable.