Victor Voicu

Pharmacokinetics and pharmacodynamics of some oximes and associated therapeutic consequences: a critical review

Undoubtedly, the use of oximes represents real progress in counteracting intoxications with organophosphates (OP), through potentiating antidotal effects of atropine. The penetration extent of these compounds through the blood–brain barrier (BBB) to significantly reactivate phosphorylated or phosphonylated acetylcholinesterase (AChE) in the brain still remains a debatable issue. Penetration of biological barriers by oximes was investigated mainly through determination of several quantitative parameters characterizing digestive absorption and BBB penetration. A weak penetration of biological barriers could be concluded from the available experimental data. The functional parameters/therapeutic effects following the penetration of oximes through BBB, more precisely the antagonism of OP‐induced seizures and hypothermia, prevention of brain damage and respiratory center protection, leading to the final end‐point, the survival of intoxicated organisms, are of high interest. It seems obvious that oximes are weakly penetrating the BBB, with minimal brain AChE reactivation (<5%) in important functional areas, such as the ponto‐medullar. The cerebral protection achieved through administration of oximes is only partial, without major impact on the antagonism of OP‐induced seizures, hypothermia and respiratory center inhibition. The antidotal effects probably result from synergic effects of other PD properties, different from the brain AChE reactivation process. Oxime structures especially designed for enhanced BBB penetration, through potentiating the hydrophobic characteristics, more often produce neurotoxic effects. Certainly, obtaining oximes with broad action spectrum (active against all OP types) would make a sense, but certainly, such a target is not achievable only through the increase in their penetrability in the brain.

Hydrophobicity/hydrophilicity descriptors obtained from extrapolated chromatographic retention data as modeling tools for biological distribution: Application to some oxime-type acetylcholinesterase reactivators

Chromatographic retention data obtained from interactions between some oxime-type compounds and different stationary phases (involving hydrophobic interaction, ion pairing formation availability, pi-pi, H-bonding, dipole-dipole, ion-dipole, electrostatic interaction and glycoprotein binding ability) have been studied. The logarithms of the capacity factors extrapolated at 0% or 100% organic solvent, resulting from the functional dependencies between retention and mobile phase composition, were used for estimation of different kind of hydrophobicity or hydrophilicity descriptors (HHDs) of these compounds. The conditions of the chromatographic separation were chosen as close as possible to in-vivo conditions (the aqueous component of the mobile phase has a pH in the physiologic interval 6.8-7.2, 0.9% sodium chloride was added to reproduce ionic strength and isotonic character, and the temperature was set at 37 degrees C). These descriptors characterizing the partition between stationary/mobile phases through specific interactions may be directly used for correlation to biological distribution processes, such as penetration of the blood/brain barrier. Oxime-type compounds used as acetylcholinesterase (AChE, E.C.3.1.1.7) reactivators have been considered for the retention study. The choice is supported by their use in the therapy of acute intoxication with organophosphorus AChE inhibitors (OPIs, especially nerve agents and pesticides), a rather complicated chemistry in solution and a relative lack of data about computational molecular descriptors used for modeling biological partition/distribution. Some correlations between the determined descriptors and computational values have also been discussed.

Evaluation of in vitro absorption, decontamination and desorption of organophosphorous compounds from skin and synthetic membranes.

Chemical warfare agents, such as soman, and pesticides, such as chlorpyrifos, dichlorvos or malathion, are toxic organophosphorous compounds (OPCs) that are readily absorbed by the skin. Decontamination using solvents or surfactants may modify the cornified layer - the skins main barrier against xenobiotic penetration. Thus, effective skin decontamination with fewer side effects is desired. We determined the membrane absorption, decontamination and desorption of toxic OPCs using human skin and synthetic membrane (cuprophane, cellulose acetate, methyl ethyl cellulose, acetophane and nylon) models, and estimated the efficacy of adsorptive powders (bentonite and magnesium trisilicate) at inhibiting this transfer. Using validated flow-through and static diffusion cell and HPLC methods, we found that the transfer of OPCs depends on their membrane affinity. The chlorpyrifos transfer decreased with a decrease in the membrane hydrophilicity, and that of malathion across hydrophilic membranes was less than half of that across hydrophobic membranes. We reliably modeled the toxicant transfer through the skin and synthetic membranes as first-order kinetic and/or square root law transfer processes, suggesting a potential application of synthetic membranes for predicting percutaneous absorption of OPCs. All tested adsorptive powders, applied either alone or as mixtures, significantly reduced the toxicant amount transferred across all membrane models, suggesting a potential therapeutic application with fewer later undesired effects on intact skin.

Relationships between the antidotal efficacy and structure, PK/PD parameters and bio-relevant molecular descriptors of AChE reactivating oximes: inclusion and integration to biopharmaceutical classification systems

Introduction: The therapeutic outcome of oximes used as reactivators of phosphorylated human acetylcholinesterase (AChE) is influenced, among other factors, by their biological distribution, their in vivo ability to achieve the nucleophilic attack and their affinity for the anionic center of the intact/inhibited AChE. Areas covered: An in silico evaluation of the molecular descriptors and biopharmaceutical properties of 454 set of oximes has been achieved. The available pharmacokinetic (PK) data was analyzed, in an attempt to illustrate their common characteristics and particularities. Based on the observed high water solubility and low permeability across biological barriers, we applied the officially adopted classification systems based on biopharmaceutical properties to identify the existing biopharmaceutical differences between the various oxime entities and to predict their in vivo fate. Expert opinion: The structural differences of the organophosphorus compounds (OP) and the available oximes reactivators of OP-inhibited AChE generate distinct toxicokinetic or PK profiles. The tissue compartment specific distribution is one of the key elements for assessment of reactivating efficiency. The distribution through highly specialized barriers, such as blood–brain barrier remains a considerable challenge. The high solubility – low permeability biopharmaceutical profile of oximes can be used to suggest the possible involvement of active transport systems.

Lipophilicity indices derived from the liquid chromatographic behavior observed under bimodal retention conditions (reversed phase/hydrophilic interaction): application to a representative set of pyridinium oximes.

The liquid chromatographic behavior observed under bimodal retention conditions (reversed phase and hydrophilic interaction) offers a new basis for the determination of some derived lipophilicity indices. The experiments were carried out on a representative group (30 compounds) of pyridinium oximes, therapeutically tested in acetylcholinesterase reactivation, covering a large range of lipophilic character. The chromatographic behavior was observed on a mixed mode acting stationary phase, resulting from covalent functionalization of high purity spherical silica with long chain alkyl groups terminated by a polar environment created through the vicinal diol substitution at the lasting carbon atoms (Acclaim Mixed Mode HILIC 1 column). Elution was achieved by combining different proportions of 5 mM ammonium formiate solutions in water and acetonitrile. The derived lipophilicity indices were compared with logP values resulting from different computational algorithms. The correlations between experimental and computed data sets are significant. To obtain a better insight on the transition from reversed phase to hydrophilic interaction retention mechanisms, the variation of the thermodynamic parameters determined through the van׳t Hoff approach was also discussed.

New insights on the consequences of biotransformation processes on the distribution and pharmacodynamic profiles of some neuropsychotropic drugs

The metabolic processes frequently trigger highly complex pharmacokinetic (PK) and pharmacodynamic (PD) characteristics for the coexisting entities, parent drug and its active or inactive metabolites. The interpretation of both individual and cumulative profiles, frequently used in the therapeutic drug monitoring procedures, must take into consideration the biological coherence of the changes of the molecular descriptors characterizing the metabolites versus the parent drugs, and further qualitative and quantitative consequences on permeability processes across highly specialized biological barriers (e.g. blood-brain barrier [BBB]). This paper analyzes the correlation of molecular descriptor differences and the PK/PD consequences for three representative psychotropic drugs (risperidone, clozapine and tramadol) and their active metabolites, underlying the safety and efficacy concerns of using the products of metabolic processes as potential new drugs. The minimal structural changes are correlated with the predicted or experimental penetrability across the biological membranes, with a special emphasis on BBB penetration, as the limiting phase for the effect at central nervous system level. The PD characteristics related to the active metabolites are compared to the ones reported for the parent drugs, concerning mainly the affinity for cerebral receptors and the type of activity at a specific level. For the neuropsychotropic substances, with BBB penetrability as a sine qua non condition, the comparative analysis of PK/PD properties for the parent drug and its metabolites generates a complete and highly complex image of the consequences of their coexistence, since these entities must be conceived and analyzed not separately, but by inclusion of usually complementary properties generating a unique therapeutic profile.

Incurred sample reanalysis: different evaluation approaches on data obtained for spironolactone and its active metabolite canrenone.

BACKGROUND The inherent reproducibility of a bioanalytical approach is usually sustained through incurred sample reanalysis (ISR). Questions relating to the number of ISRs, the right moment for performing reanalysis, the way of performing an appropriate statistical refinement of experimental data and actions to be taken in the case of failure are frequently raised. RESULTS Data resulting from ISR following a bioequivalence study for spironolactone formulations are discussed. Reanalysis of samples was carried out twice: immediately after the end of the study and after a period that overcame the long-term stability study achieved during method validation. The Bland-Altman approach was used to assess experimental results. ISR was successful over the short reanalysis period for both compounds. Data produced through reanalysis after the long-term period indicated a systematic positive bias for the metabolite canrenone (although results supported reproducibility). The results obtained for spironolactone were affected by a strong negative systematic bias and failed to support reproducibility. The explanation deals with the continuous conversion of spironolactone to canrenone in plasma samples. However, reproducibility of the method may be sustained by comparing original and repeated differences between concentration values in samples by means of a paired t-test, Wilcoxon sign rank-sum test and linear regression. CONCLUSIONS Different statistical approaches for making data comparisons are discussed and may be successfully applied during reanalysis of samples from a bioequivalence study. Results of the evaluations may differ in accordance with the statistical procedure being applied, thus a definitive conclusion requires consideration of all specific experimental circumstances arising during production of the processed data.

Retention study of some cation-type compounds using bile acid sodium salts as ion pairing agents in liquid chromatography

The possibility of forming ion-pairs between bile acids (sodium taurocholate, sodium taurodeoxycholate and sodium taurochenodeoxycholate) and different compounds (pralidoxime, obidoxime and pyridostigmine) having a cationic character has been studied in reversed-phase liquid chromatography (RP-LC). This study can be useful in understanding the role of bile acids in the transport of ionic species through hydrophobic membrane. The present study focused on the influence of mobile phase composition on the retention parameters of chosen compounds (percentage of acetonitrile, pH of aqueous component or ionic strength). For constant concentration of bile acids in aqueous component of mobile phase the functional dependencies between the logarithm of the retention factor (k) and the methanol content in the mobile phase followed a binomial pattern (U-shaped), with a minimum positioned within the interval 70-85% methanol.

Unusual Retention Behavior of Some Cationic-Type Aldoximes Used as AChE Reactivators Under Ion-Pairing Liquid Chromatographic Mechanism

Retention of some cationic-type aldoximes used as AChE reactivators was studied under ion-pairing (IP) liquid chromatographic conditions, using alkyl sulphonates as IP agents. An unusual chromatographic retention behavior was observed: the functional dependencies between the logarithm of the capacity factor and the methanol content in the mobile phase followed a binomial pattern (U-shaped), with a minimum positioned within the interval 50–80% methanol. The tautomerism of the investigated structures and the stationary phase morphological modification caused by the adsorption of the ion-pairing agent may explain the experimental findings. The study focused on the influences brought by the pH, the alkyl chain length of the ion-pair agent, and the organic modifiers on the retention behavior of these compounds.

Combined approach to demonstrate acetylcholinesterase activity changes in the rat brain following tabun intoxication and its treatment

Reactivation effects of K203 and currently available oximes (obidoxime, HI-6) in combination with atropine on acetylcholinesterase activities in the brain parts of rats poisoned with tabun were studied. The activity was determined by quantitative histochemical and biochemical methods correlating between them very well. The tabun-induced changes in acetylcholinsterase activity as well as in reactivation potency of reactivators used were different in various parts of the brain. Pontomedullar area seems to be important for observed changes following tabun intoxication and its treatment. From the oximes studied, the reactivation effect of K203 was comparable with obidoxime; HI-6 was ineffective. Combination of bio- and histochemical methods allow fine differentiation among the action of different oximes following tabun poisoning.