Soft drugs--XIV. Synthesis and anticholinergic activity of soft phenylsuccinic analogs of methatropine

Identification of Major Esterase Involved in Hydrolysis of Soft Anticholinergic (2R3'R-SGM) Designed From Glycopyrrolate in Human and Rat Tissues

The glycopyrrolate soft analog, SGM, designed to be easily hydrolyzed into the significantly less active zwitterionic metabolite, SGa, typifies soft drug that reduces systemic side effects (a problem often seen with traditional anticholinergics) following local administration. In this study, hydrolysis of 2R3'R-SGM, the highest pharmacologically active stereoisomer of SGM, was investigated in human and rat tissues. In both species, 2R3'R-SGM was metabolized to 2R3'R-SGa in plasma but was stable in liver and intestine. The half-life of 2R3'R-SGM was found to be 16.9 min and 9.8 min in human and rat plasma, respectively. The enzyme inhibition and stimulation experiments showed that plasma paraoxonase 1 (PON1) is responsible for the hydrolysis of 2R3'R-SGM in humans and rats. The PON1-mediated hydrolysis of 2R3'R-SGM was confirmed in the lipoprotein-rich fractions of human plasma. As PON1 is naturally attached to high-density lipoprotein, it might be absent in topical tissues where 2R3'R-SGM is applied, supporting its local stability and efficacy. The metabolic behavior of 2R3'R-SGM indicates that it is an ideal soft drug to be detoxified as soon as it moves into systemic circulation. Furthermore, the similarity of 2R3'R-SGM metabolism in humans and rats showed that the rat is a suitable animal for preclinical study.

Soft Quaternary Anticholinergics: Comprehensive Quantitative Structure−Activity Relationship (QSAR) with a Linearized Biexponential (LinBiExp) Model

A comprehensive quantitative structure-activity relationship (QSAR) study is presented for quaternary soft anticholinergics including two distinctly different classes designed on the basis of the soft analogue and the inactive metabolite approaches. Because of the clear biphasic (bilinear) nature of the activity data when all structures (n = 76) were considered as a function of molecular size (volume), a nonlinear model had to be used, and a linearized biexponential (LinBiExp) model proved very adequate. LinBiExp can fit activity data that show a maximum (or a minimum) around a given parameter value but tend to show linearity away from this turning point. Contrary to Hansch-type parabolic models, LinBiExp represents a natural extension of linear models, and a direct correspondence between its parameters and those obtained earlier by linear regression on compound subsets covering more limited parameter ranges could be easily established. Stereospecificity was confirmed as important, and the presence of an acid moiety was found to essentially eliminate activity. The consideration of bilinear behavior, which most likely results from size limitations at the binding site, can also explain the embarrassingly low activity found for a relatively large compound predicted as highly active by Lien, Ariëns, and co-workers based on their QSAR study.

General Linearized Biexponential Model for QSAR Data Showing Bilinear-Type Distribution

A major impediment of many QSAR-type analyses is that the data show a maximum or minimum and can no longer be adequately described by linear functions that provide unrivaled simplicity and usually give good description over more restricted ranges. Here, a general linearized biexponential (LinBiExp) model is proposed that can adequately describe data showing bilinear-type distribution as a function of not just often-employed lipophilicity descriptors (e.g., log P) but as a function of any descriptor (e.g., molecular volume). Contrary to Hansch-type parabolic models, LinBiExp allows the natural extension of linear models and fitting of asymmetrical data. It is also more general and intuitive than Kubinyi's model as it has a more natural functional form. It was obtained by a differential equation-based approach starting from very general assumptions that cover both static equilibria and first-order kinetic processes and that involve abstract processes through which the concentration of the compound of interest in an assumed "effect" compartment is connected to its "external" concentration. Physicochemical aspects placing LinBiExp within the framework of linear free energy relationship (LFER) approaches are presented together with illustrative applications in various fields such as toxicity, antimicrobial activity, anticholinergic activity, and glucocorticoid receptor binding.

Pharmacokinetic and Pharmacodynamic Evaluations of the Zwitterionic Metabolite of a New Series of N-Substituted Soft Anticholinergics

PURPOSE

This study was conducted to evaluate the zwitterionic common metabolite of a novel series of N-substituted soft analogs of glycopyrrolate both as racemates and as 2R isomers.

METHODS

Activities were assessed using both in vitro (receptor binding assay, guinea pig ileum pA2 assay) and in vivo techniques (rabbit mydriatic response, rat cardiac effects). Pharmacokinetic characterizations in rats were also performed.

RESULTS

The metabolite was highly water-soluble and very stable in buffer solutions as well as in rat biological media. Following i.v. administration in rats, it was very rapidly eliminated, mainly through renal excretion with a half-life of about 10 min. Receptor binding and guinea pig ileum assays indicated this metabolite as more than 1 order of magnitude less active than its parent soft drugs or glycopyrrolate. Moderate M3/M2 muscarinic receptor subtype selectivity was observed, further reducing the likelihood of cardiac side effects. The metabolite showed to some extent mydriatic effect and protective effect against carbachol-induced bradycardia, but of much shorter durations than glycopyrrolate; it had, however, no effect on resting heart rate.

CONCLUSIONS

N-Substituted zwitterionic metabolites retain some, but only considerably reduced activity of their parent quaternary ammonium ester soft anticholinergic drugs, and they are very rapidly eliminated from the systemic circulation. They are suitable for their assigned role within the framework of inactive metabolite-based soft anticholinergic design.

Receptor binding studies of soft anticholinergic agents

Receptor binding studies were performed on 24 soft anticholinergic agents and 5 conventional anticholinergic agents using 4 cloned human muscarinic receptor subtypes. The measured pK(i) values of the soft anticholinergic agents ranged from 6.5 to 9.5, with the majority being in the range of 7.5 to 8.5. Strong correlation was observed between the pK(i) s determined here and the pA 2 values measured earlier in guinea pig ileum contraction assays. The corresponding correlation coefficients (r2) were 0.80, 0.73, 0.81, and 0.78 for pK(i) (m1), pK(i) (m2), pK(i) (m3), and pK(i) (m4), respectively. Quantitative structure-activity relationship (QSAR) studies were also performed, and good characterization could be obtained for the soft anticholinergics containing at least 1 tropine moiety in their structure. For these compounds, the potency as measured by the pK i values was found to be related to geometric, electronic, and lipophilicity descriptors. A linear regression equation using ovality (O(e)), dipole moment (D), and a calculated log octanol-water partition coefficient (QLogP) gave reasonably good descriptions (r = 0.88) for the pK(i) (m3) values.

Soft drug design: general principles and recent applications

Soft drug design represents a new approach aimed to design safer drugs with an increased therapeutic index by integrating metabolism considerations into the drug design process. Soft drugs are new therapeutic agents that undergo predictable metabolism to inactive metabolites after exerting their therapeutic effect. Hence, they are obtained by building into the molecule, in addition to the activity, the most desired way in which the molecule is to be deactivated and detoxified. In an attempt to systematize and summarize the related work done in a number of laboratories, including ours, the present review presents an overview of the general soft drug design principles and provides a variety of specific examples to illustrate the concepts. A number of already marketed drugs, such as esmolol, remifentanil, or loteprednol etabonate, resulted from the successful application of such design principles. Many other promising drug candidates are currently under investigation in a variety of fields including possible soft antimicrobials, anticholinergics, corticosteroids, beta-blockers, analgetics, ACE inhibitors, antiarrhythmics, and others. Whenever possible, pharmacokinetic and pharmacodynamic properties are briefly summarized and compared to those of other compounds used in the same field.

Novel prodrug approach for tertiary amines: synthesis and preliminary evaluation of N-phosphonooxymethyl prodrugs

The synthesis and preliminary evaluation of a novel prodrug approach for improving the water solubility of drugs containing a tertiary amine group are reported. The prodrug synthesis involves a nucleophilic substitution reaction between the parent tertiary amine and a novel derivatizing reagent, di-tert-butyl chloromethyl phosphate, resulting in formation of the quaternary salt. The tertiary butyl groups are easily removed under acidic conditions with trifluoroacetic acid giving the N-phosphonooxymethyl prodrug in the free phosphoric acid form, which can subsequently be converted to the desired salt form. The synthesis was successfully applied to a model compound (quinuclidine) and to three tertiary amine-containing drugs (cinnarizine, loxapine, and amiodarone). The prodrugs were designed to undergo a two-step bioreversion process. The first step was an enzyme-catalyzed rate-determining dephosphorylation followed by spontaneous chemical breakdown of the N-hydroxymethyl intermediate to give the parent drug. Selected prodrugs were shown to be substrates for alkaline phosphatase in vitro. A preliminary in vivo study confirmed the ability of the cinnarizine prodrug to be rapidly and completely converted to cinnarizine in a beagle dog following iv administration.