Dynamic NMR study and theoretical calculations on the conformational exchange of valsartan and related compounds

Exploring the interaction of Valsartan and Valsartan-Zn(ll) complex with DNA by spectroscopic and in silico methods

Belonging to the Sartan family, antihypertensive drug - Valsartan (Val) had been found to possess antioxidant properties. Also, the zinc complex of Valsartan (VZn) has been recently recognized as inducing agents of the reductive stress effects thus possessing anticancer activity. Hence, in this work an attempt has been made to understand the interaction of Val and VZn with DNA using spectroscopic and in silico methods as DNA has been identified as the target for many anticancer drugs. VZn has been prepared in 2:1 M ratio and characterised by absorbance, FTIR, HRMS, NMR and Job's continuous variation method. VZn has been tested against human lungs cancer cell line which exhibited good anticancer activity (IC₅₀ = 89 µg/mL). Interaction of Val and VZn with ct-DNA under physiological conditions has been studied by spectroscopic techniques such as fluorescence, absorbance, FTIR, circular dichroism (CD) and in silico methods. Fluorescence quenching, DNA melting and viscometric studies confirmed that both ligand and complex bind to the grooves of the ct-DNA. The experimental results have revealed that VZn strongly bind with DNA compared to Val. Docking study suggested that, Val binds at major groove while VZn binds to both minor and major grooves of B-DNA.

Nuclear magnetic resonance free ligand conformations and atomic resolution dynamics

Knowledge of free ligand conformational preferences (energy minima) and conformational dynamics (rotational energy barriers) of small molecules in solution can guide drug design hypotheses and help rank ideas to bias syntheses towards more active compounds. Visualization of conformational exchange dynamics around torsion angles, by replica exchange with solute tempering molecular dynamics (REST-MD), gives results in agreement with high-resolution 1 H nuclear magnetic resonance (NMR) spectra and complements free ligand conformational analyses. Rotational energy barriers around individual bonds are comparable between calculated and experimental values, making the in-silico method relevant to ranking prospective design ideas in drug discovery programs, particularly across a series of analogs. Prioritizing design ideas, based on calculations and analysis of measurements across a series, efficiently guides rational discovery towards the "right molecules" for effective medicines.

Candesartan and valsartan Zn(ii) complexes as inducing agents of reductive stress: mitochondrial dysfunction and apoptosis

Candesartan and valsartan Zn(ii) complexes as inducing agents of reductive stress, including mitochondrial dysfunction and apoptosis.

Exploring the role of the membrane bilayer in the recognition of candesartan by its GPCR AT1 receptor

Cardiovascular diseases and hypertension in particular are major health risks worldwide and the improvement on their treatment will be beneficial for the human health. AT1R antagonists belong to the sartans family that targets the renin-angiotensin aldosterone system (RAAS) through blocking the hormone angiotensin II to exert its detrimental effects in pathological states. As a consequence, they are beneficial to treat hypertension, diabetes related kidney failure and hyperaemic episodes. Long unbiased Molecular Dynamics (MD) simulations are performed in order to explore candesartan's possible 2D and 3D diffusion mechanisms towards AT1R receptor. 3D diffusion mechanism is referred to the direct binding of the AT1 antagonist candesartan to the AT1R 3D structure (PDB ID: 4YAY). 2D diffusion mechanism involves first, the incorporation of candesartan in the bilayer core and then its localization on the AT1R binding cavity, through a diffusion mechanism. The obtained results indicate that membranes interact significantly with the neutral form of candesartan, which is indeed approaching the receptors' active site through diffusion via the lipids. On the other hand, the deprotonated form of the drug is interacting with AT1R's extracellular loop and fails to enter the membrane, pointing out the importance of the pH microenvironment around the receptor. To validate the calculated diffusion coefficients of the drug in the lipid bilayers 2D DOSY NMR experiments were recorded and they were in good agreement. Information on the impact that has the interaction of candesartan with the membrane is very important for the rationally design and development of potent ARBs. Thus, its conformational features as well as its localization in the membrane core have to be thoroughly explored.

Two distinct conformers of SAG investigated by solution NMR and variable-temperature experiments

SAG (Smo agonist) is high specific agonist of Smo receptor, it is widely used as chemical probe to explore the therapeutic value of activating the Hh-signaling pathway. In the present work, the coexistence of two distinct conformers (A and B) of SAG are revealed by solution NMR, the 3D structural difference of the two conformers are elucidated by ROESY spectroscopy and MMFF94 program. It is discovered that both conformers are agonists of Smo receptor, and the minor conformer (conformer B) in D₂O solution has higher affinity to Smo receptor by molecular docking. The result showed the detail about two distinct conformers of SAG are involved in activation of Smo receptor, and also provides information for designing more effective agonist of Smo by mimicking B conformer of SAG. By exchange dynamics investigation using variable-temperature NMR experiments, the ratio of two conformers has been shown to be drastically solvent dependent, so, the menstruum type is another important influence factor of SAG bioactivity, when using SAG as chemical probe.

Characterization of Two Distinct Amorphous Forms of Valsartan by Solid-State NMR

Valsartan (VAL) is an antihypertensive drug marketed in an amorphous form. Amorphous materials can have different physicochemical properties depending on preparation method, thermal history, etc., but the nature of such materials is difficult to study by diffraction techniques. This study characterizes two different amorphous forms of valsartan (AR and AM) using solid-state NMR (SSNMR) as a primary investigation tool, supported by solution-state NMR, FT-IR, TMDSC, and dissolution tests. The two forms are found to be clearly distinct, with a significantly higher level of structural arrangement in the AR form, as observed in (13)C, (15)N, and (1)H SSNMR. (13)C and (15)N NMR indicates that the fully amorphous material (AM) contains an approximately equal ratio of cis-trans conformers about the amide bond, whereas the AR form exists mainly as one conformer, with minor conformational "defects". (1)H ultrafast MAS NMR shows significant differences in the hydrogen bonding involving the tetrazole and acid hydrogens between the two materials, while (15)N NMR shows that both forms exist as a 1,2,3,4-tetrazole tautomer. NMR relaxation times show subtle differences in local and bulk molecular mobility, which can be connected with the glass transition, the stability of the glassy material, and its response to aging. Counterintuitively the fully amorphous material is found to have a significantly lower dissolution rate than the apparently more ordered AR material.

Valsartan

Valsartan is an antihypertensive drug which selectively inhibits angiotensin receptor type II. Generally, valsartan is available as film-coated tablets. This review summarizes thermal analysis, spectroscopy characteristics (UV, IR, MS, and NMR), polymorphism forms, impurities, and related compounds of valsartan. The methods of analysis of valsartan in pharmaceutical dosage forms and in biological fluids using spectrophotometer, CE, TLC, and HPLC methods are discussed in details. Both official and nonofficial methods are described. It is recommended to use LC-MS method for analyzing valsartan in complex matrices such as biological fluids and herbal preparations; in this case, MRM is preferred than SIM method.

Design and evaluation of a novel trifluorinated imaging agent for assessment of bile acid transport using fluorine magnetic resonance imaging

Previously, we developed a trifluorinated bile acid, CA-lys-TFA, with the objective of noninvasively assessing bile acid transport in vivo using (19) F magnetic resonance imaging (MRI). CA-lys-TFA was successfully imaged in the mouse gallbladder, but was susceptible to deconjugation in vitro by choloylglycine hydrolase (CGH), a bacterial bile acid deconjugating enzyme found in the terminal ileum and colon. The objective of the present study was to develop a novel trifluorinated bile acid resistant to deconjugation by CGH. CA-sar-TFMA was designed, synthesized, and tested for in vitro transport properties, stability, imaging properties, and its ability to differentially accumulate in the gallbladders of normal mice, compared with mice with known impaired bile acid transport (deficient in the apical sodium-dependent bile acid transporter, ASBT). CA-sar-TFMA was a potent inhibitor and substrate of ASBT and the Na(+) /taurocholate cotransporting polypeptide. Stability was favorable in all conditions tested, including the presence of CGH. CA-sar-TFMA was successfully imaged and accumulated at 16.1-fold higher concentrations in gallbladders from wild-type mice compared with those from Asbt-deficient mice. Our results support the potential of using MRI with CA-sar-TFMA as a noninvasive method to assess bile acid transport in vivo.

Bisoprolol and bisoprolol-valsartan compatibility studied by differential scanning calorimetry, nuclear magnetic resonance and X-ray powder diffractometry

Purpose

The objective of this study was to evaluate the thermal behavior of crystalline and amorphous bisoprolol fumarate and its compatibility with amorphous valsartan. This pharmacologically relevant drug combination is a potential candidate for fixed-dose combination formulation.

Methods

DSC and TMDSC were used to examine thermal behavior of bisoprolol fumarate. SSNMR and XRPD were applied to probe the solid state forms. The thermal behavior of physical mixtures with different concentrations of bisoprolol and valsartan were examined by DSC and TMDSC, and the observed interactions were investigated by XRPD, solution- and solid-state NMR.

Results

The phase transitions from thermal methods and solid-state NMR spectra of crystalline and amorphous bisoprolol fumarate are reported. Strong interactions between bisoprolol fumarate and valsartan were observed above 60 C, resulting in the formation of a new amorphous material. Solution- and solid-state NMR provided insight into the molecular nature of the incompatibility.

Conclusions

A combined analysis of thermal methods, solution- and solid-state NMR and XRPD experiments allowed the investigation of the conformational and dynamic properties of bisoprolol fumarate. Since bisoprolol fumarate and valsartan react to form a new amorphous product, formulation of a fixed-dose combination would require separate reservoirs for bisoprolol and valsartan to prevent interactions. Similar problems might be expected with other excipients or APIs containing carboxylic groups.

Electronic supplementary material

The online version of this article (doi:10.1007/s11095-014-1471-7) contains supplementary material, which is available to authorized users.

Understanding the interaction between valsartan and detergents by NMR techniques and molecular dynamics simulation

Valsartan (VST) is one of the Angiotensin II receptor antagonists, which is widely used in clinical hypertension treatment. It is believed that VST incorporates into biological membranes before it binds to AT(1) receptor. Herein the interactions between VST and detergents, mimicking the membrane environment, were investigated by using nuclear magnetic resonance (NMR) techniques and molecular dynamics (MD) simulation. We observed that VST has two conformers (trans and cis) exchanging slowly in DPC (dodecyl-phosphocholine) micelles, a widely used detergent. The changes of chemical shifts, relaxation rates, and self-diffusion coefficients of VST protons indicate that both conformers have strong interactions with DPC. NOE cross peaks and MD simulation reveal that DPC interacts with VST not only through the hydrophobic lipid chain, but also the hydrophilic headgroup, locating VST at the charged headgroup and upper part of the micelles. Our results are in good agreement with the Raman spectroscopic studies of VST in the DPPC (dipalmitoyl-phosphatidylcholine) bilayers by Potamitis et al. (Biochim. Biophys. Acta. 2011). The concentration ratio of trans over cis conformers is 0.94, showing that two conformers have the same affinities with the detergent, which is significantly smaller than our previous results obtained in SDS (sodium dodecyl sulfate) micelles. MD simulation suggested that the cis conformer has slightly lower binding free energy than the trans conformer when interacting with DPC. The conformational change of VST was further investigated in two detergents, CTAB (hexadecyltrimethylammonium bromide) and Tween-20 (polysorbate 20). Ratios of conformer A and B in the presence of detergents are in the order of DPC, CTAB < Tween-20 < SDS, which is correlated with the charge characters of their head groups. NMR investigations and MD simulations indicate that the electrostatic interaction plays an essential role in the binding process of VST with detergents, and the hydrophobic interaction influences the packing of the drug in the micelles. These results may be of help in understanding delivery processes of sartan drugs in cell membranes.

Identification of adulterants in a Chinese herbal medicine by LC-HRMS and LC-MS-SPE/NMR and comparative in vivo study with standards in a hypertensive rat model

Based on anecdotal evidence of anti-hypertensive effect of Gold Nine Soft Capsules, an in vivo study of this complex Chinese "herbal-based" medicine was initiated. Dosage of the content of Gold Nine capsules in spontaneous hypertensive rats showed a remarkably good effect. This led to further investigation of the components of the preparation and eventual identification of three known anti-hypertensive drugs; amlodipine, indapamide and valsartan, which were not declared on the label. Compounds were rapidly identified using LC-HRMS and LC-MS-SPE/NMR, quantified by HPLC, and the in vivo activity of a combination of commercially purchased standards was shown to be equivalent to that of the capsule content. Adulteration of herbal remedies and dietary supplements with synthetic drugs is an increasing problem that may lead to serious adverse effects. LC-MS-SPE/NMR as a method for the rapid identification of such adulterants is highlighted in this case study.