Redetermination of rifampicin pentahydrate revealing a zwitterionic form of the antibiotic

Preparation and Characterization of a Rifampicin Coamorphous Material with Tromethamine Coformer: An Experimental-Theoretical Study

Rifampicin (RIF) is an antibiotic used to treat tuberculosis and leprosy. Even though RIF is a market-available drug, it has a low aqueous solubility, hindering its bioavailability. Among the strategies for bioavailability improvement of poorly soluble drugs, coamorphous systems have been revealed as an alternative in the increase of the aqueous solubility of drug systems and at the same time also increasing the amorphous state stability and dissolution rate when compared with the neat drug. In this work, a new coamorphous form from RIF and tromethamine (TRIS) was synthesized by slow evaporation. Structural, electronic, and thermodynamic properties and solvation effects, as well as drug-coformer intermolecular interactions, were studied through density functional theory (DFT) calculations. Powder X-ray diffraction (PXRD) data allowed us to verify the formation of a new coamorphous. In addition, the DFT study indicates a possible intermolecular interaction by hydrogen bonds between the available amino and carbonyl groups of RIF and the hydroxyl and amino groups of TRIS. The theoretical spectra obtained are in good agreement with the experimental data, suggesting the main interactions occurring in the formation of the coamorphous system. PXRD was used to study the physical stability of the coamorphous system under accelerated ICH conditions (40 °C and 75% RH), indicating that the material remained in an amorphous state up to 180 days. The thermogravimetry result of this material showed a good thermal stability up to 153 °C, and differential scanning calorimetry showed that the glass temperature (Tg) was at 70.0 °C. Solubility studies demonstrated an increase in the solubility of RIF by 5.5-fold when compared with its crystalline counterpart. Therefore, this new material presents critical parameters that can be considered in the development of new coamorphous formulations.

Physicochemical interaction of rifampicin and ritonavir-lopinavir solid dispersion: an in-vitro and ex-vivo investigation

OBJECTIVE

To investigate the in-situ physicochemical interaction of Rifampicin and Ritonavir - Lopinavir Solid dispersion administered for the treatment of comorbid conditions i.e. Tuberculosis and HIV/AIDS.

METHODS

pH-shift dissolution of Rifampicin (RIF) in presence of Ritonavir-Lopinavir solid dispersion (RL-SD) was carried out in USP phosphate buffer 6.8 and FaSSIF. Equilibrium and amorphous solubility were determined for the drugs. Pure drugs, their physical mixtures, and pH-shifted co-precipitated samples were characterized using DSC, PXRD, and FTIR. Fluorescence spectroscopy was used to investigate drug-rich and drug-lean phases. In-vitro and ex-vivo flux studies were also carried out.

RESULTS

The results showed significant differences in the solubility and dissolution profiles of RTV and LOP in the presence of RIF, while RIF profile remained unchanged. Amorphicity, intermolecular interaction and aggregate formation in pH-shifted samples were revealed in DSC, XRD and FTIR analysis. Fluorescence spectroscopy confirmed the formation of drug-rich phase upon pH-shift. In-vitro and ex-vivo flux studies revealed significant reduction in the flux of all the drugs when studied in presence of second drug.

CONCLUSION

RIF, RTV and LOP in presence of each other on pH-shift, results in co-precipitation in the amorphous form (miscible) which leads to reduction in the highest attainable degree of supersaturation. This reduction corresponds to the mole fraction of the RIF, RTV and LOP within the studied system. These findings suggest that the concomitant administration of these drugs may lead to physicochemical interactions and possible ineffective therapy.

Theoretical study of structures and charge distributions of 2-, 3- and 6-hydroxypropyl-β-cyclodextrin with different degrees of substitution

Hydroxypropyl-β-cyclodextrin (HPβCD) is a derivative of cyclodextrin extensively used in the pharmaceutical industry, since it improves the solubility of drugs, and widens their oral bioavailability and safety profile. Theoretical studies about HPβCD configurations are important so as to simulate by molecular mechanics and dynamics, the complex formation and enantiomeric separations of these cyclodextrins with other molecules. Twelve structures and charge distributions of 2-, 3- and 6-hydroxypropyl-β-cyclodextrin (2-, 3-, 6-HPβCD) with different degrees of substitution were obtained using ab initio methods. The atomic positions of glucose units, dihedral angles of hydroxypropyl groups (HPs), radius of gyration, and H-bonds formed were analysed in the structures. The 3-HPβCD configurations showed the greatest variations in their atomic positions, their HPs groups being slightly rotated towards the interior of the rim of cyclodextrin and hence narrowing its opening. The structures of 2-HPβCD are the least influenced by the degree of substitution and include fewer H-bonds. Different values for the partial atomic charges for each glucose unit in a molecule were obtained, as well as for all HPβCD configurations. This result, consequence of the ab initio methods used, is a new contribution that can be important to simulate processes in which small energy differences decisively influence the results. The electric potential due to the charge distribution of HPβCDs was calculated inside and outside the cavity. It was found that the changes at each position were mainly due to atomic configurations, since the differences in partial atomic charges are one or two orders of magnitude smaller than in atomic positions.Communicated by Ramaswamy H. Sarma.

Optimization of Benzoxazinorifamycins to Improve Mycobacterium tuberculosis RNA Polymerase Inhibition and Treatment of Tuberculosis

Rifampin (RMP), a very potent inhibitor of the Mycobacterium tuberculosis (MTB) RNA polymerase (RNAP), remains a keystone in the treatment of tuberculosis since its introduction in 1965. However, rifamycins suffer from serious drawbacks, including 3- to 9-month treatment times, Cyp450 induction (particularly problematic for HIV-MTB coinfection), and resistant mutations within RNAP that yield RIF-resistant (RIF^R) MTB strains. There is a clear and pressing need for improved TB therapies. We have utilized a structure-based drug design approach to synthesize and test novel benzoxazinorifamycins (bxRIF), congeners of the clinical candidate rifalazil. Our goal is to gain binding interactions that will compensate for the loss of RIF-binding affinity to the (RIF^R) MTB RNAP and couple those with substitutions that we have previously found that essentially eliminate Cyp450 induction. Herein, we report a systematic exploration of 42 substituted bxRIFs that have yielded an analogue (27a) that has an excellent in vitro activity (MTB RNAP inhibition, MIC, MBC), enhanced (∼30-fold > RMP) activity against RIF^R MTB RNAP, negligible hPXR activation, good mouse pharmacokinetics, and excellent activity with no observable adverse effects in an acute mouse TB model. In a time-kill study, 27a has a 7 day MBC that is ∼10-fold more potent than RMP. These results suggest that 27a may exhibit a faster kill rate than RMP, which could possibly reduce the clinical treatment time. Our synthetic protocol enabled the synthesis of ∼2 g of 27a at >95% purity in 3 months, demonstrating the feasibility of scale-up synthesis of bxRIFs for preclinical and clinical studies.

Modifications, biological origin and antibacterial activity of naphthalenoid ansamycins

Covering: 2011 to 2021Structural division of natural naphthalenoid ansamycins, regarding the type of the core and length of the ansa chain, and their biosynthetic pathways in microorganisms are discussed. The great biosynthetic plasticity of natural naphthalenoid ansamycins is reflected in their structural variety due to the alterations within ansa bridge or naphthalenoid core portions. A comparison between the biological potency of natural and semisynthetic naphthalenoid ansamycins was performed and discussed in relation to the molecular targets in cells. The antibacterial potency of naphthalenoid ansamycins seems to be dependent on the ansa chain length and conformational flexibility - the higher flexibility of the ansa chain the better biological outcome is noted.

Novel C-3-(N-alkyl-aryl)-aminomethyl rifamycin SV derivatives exhibit activity against rifampicin-resistant Mycobacterium tuberculosis RpoBS522L strain and display a different binding mode at the RNAP β-subunit site compared to rifampicin

Antimicrobial resistance is a main concern in tuberculosis treatment and is often associated with the emergence of Mycobacterium tuberculosis strains resistant to rifampicin (RIF), which is one of the cornerstones of tuberculosis chemotherapy. In this study, aminoalkyl-aromatic ring tails were appended to the C3 position of rifamycin core to assess the role of C3 substitutions to the anti-mycobacterial activity of the rifamycin antibiotics. The typical hydrazone unit of RIF was replaced by an amino-alkyl linkage to connect the aromatic ring tails with the rifamycin naphthoquinone core. Eight novel C3-(N-alkyl-aryl)-aminoalkyl analogues of rifamycin SV were synthesised and screened in vitro against wild-type HR37Rv and "hypervirulent" HN-878 strains, and a panel of rifampicin-resistant M. tuberculosis clinical isolates carrying mutations at the 522, 531 and 455 positions of the rpoB gene (RpoB_S522L, RpoB_S531L and RpoB_H455D strains). The analogues exhibited anti-tubercular activity against H37Rv and HN-878 at submicromolar or nanomolar concentrations, and against clinical H37Rv isolates bearing the S522L mutations at low micromolar concentration. Benzylamine moiety-including analogue 8 was as active as rifampicin against HN-878 with a MIC₉₀ value of 0.02 μM, whereas 14 and 15, which included tryptamine and para-methyl-sulfonylbenzylamine C3-substituents, respectively, showed higher anti-tubercular activity (MIC₉₀ = 3 μM) compared to rifampicin against the S522L mutated H37Rv strain. Detailed in silico analysis of different RNAP molecular systems predicted a distinct, possibly novel, binding mode for the new rifamycin analogues. These were found to occupy a different space in the binding pockets of both wild type and mutated RNAP proteins compared to that of rifampicin. Moreover, the molecular modelling experiments investigated the ability of the novel analogues aromatic tails to establish key interactions at the RNAP binding site. These interesting findings might pave the way for generating rifamycin analogues that can overcome anti-microbial resistance in M. tuberculosis.

Managing Experimental 3D Structures in the Beyond-Rule-of-5 Chemical Space: The Case of Rifampicin

The beyond-Rule-of-5 (bRo5) chemical space is a source of new oral drugs and includes large and flexible compounds. Because of their size and conformational variability, bRo5 molecules assume different privileged conformations in the compartments of human body, i. e., they can exhibit chameleonic properties. The elucidation of the ensemble of 3D structures explored by such molecules under different conditions is therefore critical to check the role played by chameleonicity to modulate cell permeability. Here we characterized the conformational ensembles of rifampicin, a bRo5 drug, in polar and nonpolar solvents and in the solid state. We performed NMR experiments, analyzed their results with a novel algorithm and set-up a pool of ad hoc in silico strategies to investigate crystallographic structures retrieved from the CSD. Moreover, a polarity descriptor often related to permeability (SA-3D-PSA) was calculated for all the conformers and its variation with the environment analyzed. Results showed that the conformational behavior of rifampicin in solution and in the solid state is not superposable. The identification of dynamic intramolecular hydrogen bonds can be assessed by NMR spectroscopy but not by X-ray structures. Moreover, SA-3D-PSA revealed that dynamic IMHBs do not provide rifampicin with chameleonic properties. Overall, this study highlights that the peculiarity of rifampicin, which is cell permeable probably because of the presence of static IMHBs but is devoid of any chameleonic behavior, can be assessed by a proper analysis of experimental 3D structures.

Novel Ansa-Chain Conformation of a Semi-Synthetic Rifamycin Prepared Employing the Alder-Ene Reaction: Crystal Structure and Absolute Stereochemistry

Rifamycins are an extremely important class of antibacterial agents whose action results from the inhibition of DNA-dependent RNA synthesis. A special arrangement of unsubstituted hydroxy groups at C21 and C23, with oxygen atoms at C1 and C8 is essential for activity. Moreover, it is known that the antibacterial action of rifamycin is lost if either of the two former hydroxy groups undergo substitution and are no longer free to act in enzyme inhibition. In the present work, we describe the successful use of an Alder-Ene reaction between Rifamycin O, 1 and diethyl azodicarboxylate, yielding 2, which was a targeted introduction of a relatively bulky group close to C21 to protect its hydroxy group. Many related azo diesters were found to react analogously, giving one predominant product in each case. To determine unambiguously the stereochemistry of the Alder-Ene addition process, a crystalline zwitterionic derivative 3 of the diethyl azodicarboxylate adduct 2 was prepared by reductive amination at its spirocyclic centre C4. The adduct, as a mono chloroform solvate, crystallized in the non-centrosymmetric Sohnke orthorhombic space group, P212121. The unique conformation and absolute stereochemistry of 3 revealed through X-ray crystal structure analysis is described.

NMR of Natural Products as Potential Drugs

This review outlines methods to investigate the structure of natural products with emphasis on intramolecular hydrogen bonding, tautomerism and ionic structures using NMR techniques. The focus is on 1H chemical shifts, isotope effects on chemical shifts and diffusion ordered spectroscopy. In addition, density functional theory calculations are performed to support NMR results. The review demonstrates how hydrogen bonding may lead to specific structures and how chemical equilibria, as well as tautomeric equilibria and ionic structures, can be detected. All these features are important for biological activity and a prerequisite for correct docking experiments and future use as drugs.

(Solvato-) polymorphism of formulations of rifampicin for pulmonary drug delivery prepared using a crystallization/spray drying process

Rifampicin is an antibiotic used in tuberculosis therapy showing extensive (solvato-) polymorphism. Per oral administration of high doses is recommended, but application as dry powder for inhalation at the site of infection being the lungs, is desirable. Recrystallization from ethanol and consecutive spray drying is reported to yield a rifampicin dihydrate with suitable aerosol performance and stability. Nevertheless, the origin of water in the crystal remained unclear and demanded further investigation so to clarify its solid state throughout manufacture and storage. The present study reports the relationship of (solvato-) polymorphs occurring during manufacture and storage of samples recrystallized and spray dried from ethanol, methanol and water and it was concluded that processes involving a recrystallization from EtOH and MeOH produce particles of a common isostructural group of channel solvates. Samples recrystallized and spray dried from water were identified as members of another isostructural group, which was already characterized in literature. As a second aim, aerosol performance and storage stability of the formulations were investigated and all samples showed stable aerosol performance. Chemical stability of samples spray dried from ethanol was found suitable over a period of six months, whereas samples spray dried from methanol or water showed significant degradation.

Enhanced Photoelectrochemical Detection of an Analyte Triggered by Its Concentration by a Singlet Oxygen-Generating Fluoro Photosensitizer

The use of a photocatalyst (photosensitizer) which produces singlet oxygen instead of enzymes for oxidizing analytes creates opportunities for designing cost-efficient and sensitive photoelectrochemical sensors. We report that perfluoroisopropyl-substituted zinc phthalocyanine (F₆₄PcZn) interacts specifically with a complex phenolic compound, the antibiotic rifampicin (RIF), but not with hydroquinone or another complex phenolic compound, the antibiotic doxycycline. The specificity is imparted by the selective preconcentration of RIF in the photocatalytic layer, as revealed by electrochemical and optical measurements, complemented by molecular modeling that confirms the important role of a hydrophobic cavity formed by the iso-perfluoropropyl groups of the photocatalyst. The preconcentration effect favorably enhances the RIF photoelectrochemical detection limit as well as sensitivity to nanomolar (ppb) concentrations, LOD = 7 nM (6 ppb) and 2.8 A·M^-1·cm^-2, respectively. The selectivity to RIF, retained in the photosensitizer layer, is further enhanced by the selective removal of all unretained phenols via simple washing of the electrodes with pure buffer. The utility of the sensor for analyzing municipal wastewater was demonstrated. This first demonstration of enhanced selectivity and sensitivity due to intrinsic interactions of a molecular photocatalyst (photosensitizer) with an analyte, without use of a biorecognition element, may allow the design of related, robust, simple, and viable sensors.

A study on polymorphic forms of rifampicin for inhaled high dose delivery in tuberculosis treatment

Rifampicin is a first-line, highly effective drug currently used orally as a part of a lengthy multi-drug regimen against tuberculosis (TB). Despite the potential of inhaled therapy as an effective approach for TB treatment, an inhalable formulation of rifampicin has not yet been developed for clinical use. In order to do so, it is necessary to evaluate its solid-state properties, which regulate important characteristics like solubility, dissolution, aerosolization, stability and bioavailability. In this study, a crystallization technique and spray drying were used to prepare inhalable rifampicin formulations. Spray drying yielded amorphous formulation of rifampicin while crystalline dihydrate and pentahydrate formulations were obtained by crystallization. The powders were evaluated for their solid-state properties, in vitro aerosolization and aerosolization stability for three months when stored at different relative humidity conditions. All formulations had a mean particle size smaller than 3.8 µm and had a fine particle fraction (FPF) higher than 58.0%. Amorphous and crystalline dihydrate formulations showed no change in aerosolization parameters (emitted dose and FPF) upon storage for three months. The amorphous and pentahydrate formulations were found to undergo oxidative degradation upon storage, and a decrease in their drug content was observed. Among the formulations prepared, rifampicin dihydrate formulation showed the least degradation over the three months period. The inhalable rifampicin formulations prepared in this study, being excipient free, have the potential to be delivered as inhaled dry powder high-dose rifampicin to the lungs for effective treatment of TB.

Synthesis and characterization of antibacterial drug loaded β-tricalcium phosphate powders for bone engineering applications

Powders of β-tricalcium phosphate [β-TCP, β-Ca₃(PO₄)₂] and composite powders of β-TCP and polyvinyl alcohol (PVA) were synthesized by using wet precipitation methods. First, the conditions for the preparation of single phase β-TCP have been delineated. In the co-precipitation procedure, calcium nitrate and diammonium hydrogen phosphate were used as calcium and phosphorous precursors, respectively. The pH of the system was varied in the range 7-11 by adding designed amounts of ammonia solution. The filtered cakes were desiccated at 80 °C and subsequently calcined at different temperatures in the range between 700-1100 °C. Later on, rifampicin form II was used to produce drug-loaded β-TCP and PVA/β-TCP powders. All the synthesized materials have been characterized from morphological (by scanning electron microscopy) and structural-chemical (by X-ray diffraction and Fourier transform infrared spectroscopy) point of view. The drug loading capacity of the selected pure β-TCP powder has been assessed. The biological performance (cytocompatibility in fibroblast cell culture and antibacterial efficacy against Escherichia coli and Staphylococcus aureus) has been tested with promising results. Application perspectives of the designed drug-bioceramic-polymer blends are advanced and discussed.

Ultrafast dynamics of the antibiotic Rifampicin in solution

Ultrafast time-resolved studies demonstrate that intra- and intermolecular H-bonds with water molecules act synergistically to stabilize the active zwitterionic form of Rifampicin, an effective antibiotic against mycobacterial infections.

Semi-synthetic zwitterionic rifamycins: a promising class of antibiotics; survey of their chemistry and biological activities

Rifamycins are an important group of macrocyclic antibiotics highly active against tuberculosis and various other Gram-positive pathogenic bacteria.

13C and 15N CP/MAS, 1H-15N SCT CP/MAS and FTIR spectroscopy as tools for qualitative detection of the presence of zwitterionic and non-ionic forms of ansa-macrolide 3-formylrifamycin SV and its derivatives in solid state

(13)C, (15)N CP/MAS, including (1)H-(13)C and (1)H-(15)N short contact time CP/MAS experiments, and FTIR methods were applied for detailed structural characterization of ansa-macrolides as 3-formylrifamycin SV (1) and its derivatives (2-6) in crystal and in powder forms. Although HPLC chromatograms for 2/CH3 OH and 2/CH3 CCl3 were the same for rifampicin crystals dissolved in respective solvents, the UV-vis data recorded for them were different in 300-375 nm region. Detailed solid state (13)C and (15)N CP/MAS NMR and FTIR studies revealed that rifampicin (2), in contrast to 3-formylrifamycin SV (1) and its amino derivatives (3-6), can occur in pure non-ionic or zwitterionic forms in crystal and in pure these forms or a mixture of them in a powder. Multinuclear CP/MAS and FTIR studies demonstrated also that 3-6 derivatives were present exclusively in pure zwitterionic forms, both in powder and in crystal. On the basis of the solid state NMR and FTIR studies, two conformers of 3-formylrifamycin SV were detected in powder form due to the different orientations of carbonyl group of amide moiety. The PM6 molecular modeling at the semi-empirical level of theory, allowed visualization the most energetically favorable non-ionic and zwitterionic forms of 1-6 antibiotics, strongly stabilized via intramolecular H-bonds. FTIR studies indicated that the originally adopted forms of these type antibiotics in crystal or in powder are stable in standard laboratory conditions in time. The results presented point to the fact that because of a possible presence of two forms of rifampicin (compound 2), quantification of the content of this antibiotic in relevant pharmaceuticals needs caution.

Structural characterization of form I of anhydrous rifampicin

Crystal structure determination of form I of anhydrous rifampicin by X-ray powder diffraction data. Crystal morphology prediction revealed a good agreement with the images of the crystals.