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An efficient method for the C3-glucuronidation of bile acids is developed under flow conditions. A modular mesoreactor assisted flow set-up was combined with statistical design of experiments to speed up the optimization of the Koenigs-Knorr reaction in terms of yield, regioselectivity, costs, as well as technical and practical standpoints. Using the optimal conditions, selective glucuronidation of naturally occurring bile acids was successfully achieved offering a new, valuable route to C3-glucuronidated bile acids useful for biological, diagnostic and PK/ADMET investigations.

With the discovery of the bile acid (BA)-activated nuclear and membrane receptors, the role of BAs as signalling molecules in important paracrine and endocrine networks has been fully documented in the last decade. Besides regulating their own synthesis and transport, BAs have been demonstrated being involved in triggering the adaptive response to cholestasis and other insults to liver. More to the point, their recognized ability to control the general energy-related metabolism and inflammation processes has contributed to justify the renewed interest towards this class of amphiphilic steroidal compounds. All these evidences feed a continuing interest in the BA research aimed at designing and synthesizing new side chain- and body-modified derivatives endowed with improved biological and physico-chemical profiles, as well as with proper ADMET behaviour. In this context, the micellar aggregation of BAs, and the respective critical micellization concentration (CMC) value (determined on the BA sodium salt, BS), is considered a key parameter that needs to be determined in the preliminary phase of compound characterization, being implicated in cytotoxicity issues. An extraordinary variety of different analytical techniques and methods have been proposed along the years with the aim of better identifying the start of the self-aggregation process of BS monomers. The unicity of the physico-chemical nature of such class of compounds can be invoked to explain this unusual interest. Accordingly, a number of both invasive and non-invasive approaches have been developed along with a limited number of indirect chromatographic-based estimation strategies. Worth to be mentioned among the non-invasive determination methods are those based on potentiometry, freezing point depression, surface tension, nuclear magnetic resonance, viscosimetry, turbidimetry, microcalorimetry, refractometry, conductimetry, spectrophotometry, cholesterol solubilization, and monoglucuronide solubilization. Dye solubilization- and fluorescence-based methods deserve instead credit among the invasive methodological approaches. Indirect chromatographic methods based on capillary electrophoresis and high performance liquid chromatography analysis also demonstrated to be profitably exploited for the CMC estimation, especially when a small amount of sample is available. The collection of literature data reveals that the CMC value of a given BS is markedly related to the method selected for determining it as well as to the experimental conditions applied during the analysis., (Copyright © 2013 Elsevier B.V. All rights reserved.)

The employment of the flow N-acyl amidation of natural bile acids (BAs) required the in-line connection with suitable analytical tools enabling the determination of reaction yields as well as of the purity grade of the synthesized glyco- and tauro-conjugated derivatives. In this framework, a unique HPLC method was successfully established and validated for ursodeoxycholic (UDCA), chenodeoxycholic (CDCA), deoxycholic (DCA) and cholic (CA) acids, as well as the corresponding glyco- and tauro-conjugated forms. Because of the shared absence of relevant chromophoric moieties in the sample structure, an evaporative light scattering detector (ELSD) was profitably utilized for the analysis of such steroidal species. For each of the investigated compounds, all the runs were contemporarily carried out on the acidic free and the two relative conjugated variants. The different ELSD response of the free and the corresponding conjugated BAs, imposed to build-up separate calibration curves. In all the cases, very good precision (RSD% values ranging from 1.04 to 6.40% in the long-period) and accuracy (Recovery% values ranging from 96.03 to 111.14% in the long-period) values along with appreciably low LOD and LOQ values (the former being within the range 1-27 ng mL(-1) and the latter within the range 2-44 ng mL(-1)) turned out., (Copyright © 2012 Elsevier B.V. All rights reserved.)

A multi-gram scale protocol for the N-acyl amidation of bile acids with glycine and taurine has been successfully developed under continuous flow processing conditions. Selecting ursodeoxycholic acid (UDCA) as the model compound and N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) as the condensing agent, a modular mesoreactor assisted flow set-up was employed to significantly speed up the optimization of the reaction conditions and the flow scale-up synthesis. The results in terms of yield, in line purification, analysis, and implemented flow set-up for the reaction optimization and large scale production are reported and discussed.

In a line of research focused on the design, synthesis and development of new bile acid-based compounds, the physico-chemical profile of the molecules must be thoroughly explored and analyzed. In this scenario, a fast and reliable information on the critical micellar concentration (CMC) of specific compounds through a profitable chromatographic parameter can be of aid to rationally direct the synthesis of new molecular entities, mainly during the early stages of the drug-discovery process. The derived 'chromatographic hydrophobicity index' (CHI), usually employed for a fast access to the log P/log D value of physico-chemically diverse compounds and obtained via RP-gradient elution, was for the first time engaged in the bile acid field. Accordingly, 14 unconjugated bile acids harboured with a different number, position and orientation of hydroxy groups, as well as other substituents onto the steroidal backbone and side chain, were selected to build up a calibration curve. Such a collection of compounds was rationally assembled in order to manage an almost continuous range of CMC values (spanning the spectrophotometrically obtained CMCs between 5 and 25 mM). A high degree of correlation between CMC and CHI values was obtained (R(2) and cross-validated R(xv)(2) of the pCMC vs CHI plot equal to 0.975 and 0.966, respectively). A selected new subset of five confidential research bile acids with experimental CMCs in the range 6-19 mM was finally recruited to validate the proposed method. The high statistical quality of the established mathematical model turned out into a very appreciable predictive power.

Reversed-phase high-performance liquid chromatography (RP-HPLC) is more and more adopted as a fruitful tool for the estimation of several physico-chemical properties of diverse classes of organic compounds. In this frame, derived chromatographic indices have been proposed as effective parameters to measure the lipophilicity (log P or log D) of compounds. Instead, a limited attention has been directed towards the chromatographic evaluation of the Critical Micellar Concentration (CMC), one of the most important parameters employed to study the bile acid physico-chemical profile. We have recently reported on the effectiveness of the derived chromatographic index phi(0) for the study of the self-aggregation process of bile acids. Here we show that this index is independent upon the adopted chromatographic environment so as to be instrumental for the evaluation of the hydrophobic/hydrophilic balance of bile acids. Molecular modelling studies have also been undertaken with the aim of rationalizing the experimental findings.

Owing to the re-flourished interest towards the bile acids (BAs) as versatile signalling hormones endowed with diverse endocrine functions, the development of reliable analytical protocols monitoring the synthesis of new BA-based receptor modulators, still represents a cogent concern. On this basis, for the first time, a HPLC study has been engaged with the aim to set up suitable chromatographic conditions for the analysis of three different epimeric couples of 23-methyl-substituted unconjugated BAs. Three different methods (one for each couple) have been successfully established and then validated. Good precision and accuracy (evaluated both in the short and long period) as well as appreciably low LOD and LOQ values have turned out. Moreover, the engagement of an evaporative light scattering detector (ELSD) has proven its high effectiveness for the analysis of such steroidal species.

The discovery that bile acids are involved in the modulation of nuclear steroid receptors has prompted renewed interest in this field of research. Due to the nature of research in this field, a technique that enables simple and effective assessment of the hydrophobic/hydrophilic balance, thus improving and speeding up evaluations of the biological profiles of these compounds, is greatly needed. In this context, both CMC value determination and RP-HPLC mobility evaluation were explored as possible approaches. While the CMC was calculated using the noninvasive conductimetric method, the RP-HPLC mobility was assessed by measuring the retention factor at several mobile phase compositions and extrapolating back to the pure aqueous mobile phase. The correlation of the CMC with the derived chromatographic hydrophobic index phi0 was satisfactory.