Photoreactivity of nifedipine in vitro and in vivo

Novel Far-Red Fluorescent 1,4-Dihydropyridines for L-Type Calcium Channel Imaging

Upregulation of L-type calcium channels (LTCCs) is implicated in a range of cardiovascular and neurological disorders. Therefore, the development of toolboxes that unlock fast imaging protocols in live cells is coveted. Herein, we report a library of first-in-class far-red small-molecule-based fluorescent ligands (FluoDiPines), able to target LTCCs. All fluorescent ligands were evaluated in whole-cell patch-clamp and live-cell Ca2+ imaging whereby FluoDiPine 6 was found to be the best candidate for live-cell fluorescence imaging. Low concentration of FluoDiPine 6 (50 nM) and a quick labeling protocol (5 min) are successfully applied to fixed and live cells to image LTCCs with good specificity.

In-situ monitoring of in vitro drug release processes in tablets using optical coherence tomography

Film-coated modified-release tablets are an important dosage form amenable to targeted, controlled, or delayed drug release in the specific region of the gastrointestinal (GI) tract. Depending on the film composition and interaction with the GI fluid, such coated products can modulate the local bioavailability, systemic absorption, protection as an enteric barrier, etc. Although the interaction of a dosage form with the surrounding dissolution medium is vital for the resulting release behavior, the underlying physicochemical phenomena at the film and core levels occurring during the drug release process have not yet been well described. In this work, we attempted to tackle this limitation by introducing a novel in vitro test based on optical coherence tomography (OCT) that allows an in-situ investigation of the sub-surface processes occurring during the drug release. Using a commercially available tablet based on osmotic-controlled release oral delivery systems (OROS), we demonstrated the performance of the presented prototype in terms of monitoring the membrane thickness and thickness variability, the surface roughness, the core swelling behavior, and the porosity of the core matrix throughout the in vitro drug release process from OROS. The superior spatial (micron scale) and temporal (less than 10 ms between the subsequent tomograms) resolution achieved in the proposed setup provides an improved understanding of the dynamics inside the microstructure at any given time during the dissolution procedure with the previously unattainable resolution, offering new opportunities for the design and testing of patient-centric dosage forms.

Comparative Bioequivalence and Food Effect of Two Formulations of 30-mg Nifedipine Controlled-Release Tablets in Healthy Chinese Adults

Nifedipine is a potent antihypertensive medication classified as a dihydropyridine calcium channel blocker. The objective of this trial was to assess the bioequivalence of a 30-mg nifedipine controlled-release tablet and a reference drug in a cohort of healthy Chinese individuals. Two independent open-label, randomized, single-dose, crossover studies were conducted, 1 under fasting conditions (N = 44, with 1 participant dropping out midway) and the other under fed conditions (N = 44, with 4 participants dropping out midway). Plasma concentrations of nifedipine were determined using liquid chromatography-mass spectrometry, and pharmacokinetic (PK) parameters were calculated using noncompartmental analysis with Phoenix WinNonlin 8.0 software. In both fasting and fed studies, reasonable bioequivalence was observed for the PK parameters of both the test product and the reference drug. A good safety profile was demonstrated for both the test product and reference drug, with no serious adverse events reported, and both were similarly well tolerated. An important observation with food coadministration was that systemic exposure to nifedipine (based on area under the curve, AUC0-∞) was reduced by approximately 12%. The bioequivalence of the test product and reference drug under fasting/fed conditions in healthy subjects in China was demonstrated by the study results.

Crushing tablets or sprinkling capsules: Implications for clinical strategy and study performance based on BE studies of rivaroxaban and deferasirox

Administration of oral medicinal products as crushed tablets or open capsules is an important delivery option for patients suffering from dysphagia. To obtain full interchangeability of generics with the original products, demonstration of bioequivalence (BE) between both products administered as crushed tablets/open capsules was required for poorly soluble product by European Medicines Agency (EMA) at the time of development of our rivaroxaban and deferasirox generic products. We present the results of two BE studies with modified administration of these products, which compared relative bioavailability between generic and reference products. In the rivaroxaban study, the test product was administered as a capsule sprinkled on and mixed with applesauce, whereas the reference tablet was crushed and administered with applesauce under fed conditions. In the deferasirox study, both treatments were administered as crushed tablets under fasting conditions. Both studies applied a two-way crossover design and were conducted after a single-dose in healthy volunteers. The 90% confidence interval of the geometric mean ratio area under the analyte concentration versus time curve, from time zero to the time of the last measurable analyte concentration and maximum measured analyte concentration over the sampling period of the test to reference ratio were 103.36-110.37% and 97.98-108.45% for rivaroxaban, respectively, and 96.69-107.29% and 94.19-109.45% for deferasirox, respectively. Thus, the BE criteria (80.00-125.00%) were met in both studies which demonstrated that bioavailability was not affected when the test and reference products were administered in the form of crushed tablet/open capsule. These results support the argument of redundancy of crushed product studies for poorly soluble drugs, which is in line with the currently revised position of the EMA on this topic.

Complex Spectroscopy Studies of Nifedipine Photodegradation

The aim of this work is to highlight the influence of UV light on the hydrolysis reaction of nifedipine (NIF) in the presence of alkaline solutions. In this context, the photodegradation of NIF in the absence of alkaline solutions caused (a) a change in the ratio between the absorbances of three bands in the UV-VIS spectra localized at 224-240 nm, 272-276 nm and 310-340 nm, assigned to the electronic transitions of -COOCH3 groups, -NO2 groups and a heterocycle with six atoms; (b) a red-shift of the photoluminescence (PL) band from 458 nm to 477 nm, simultaneous with an increase in its intensity; (c) a decrease in the ratio of the Raman line intensities, which peaked at 1224 cm-1 and 1649 cm-1, associated with the vibrational modes of -C-C-O in the ester group and C=C stretching; and (d) a decrease in the ratio between the absorbances of the IR bands, which peaked at 1493 cm-1 and 1223 cm-1, associated with the vibrational modes of the -NO2 group and C-N stretching. These changes were explained considering the NIF photodegradation reaction, which leads to the generation of the compound 4-(2-nitrosophenyl)-2.6-dimethyl-3.5-dimethoxy carbonyl pyridine. The interaction of NIF with NaOH in the absence of UV light was demonstrated to induce changes in the vibrational mode of the -C-C-O bond in the ester group. The photodegradation of NIF after its reaction with NaOH induces significant changes highlighted in its (a) UV-VIS spectra, by the shift of the absorption band at 238 nm; (b) PL spectra, by the supraunitary value of the ratio between the emission band intensities at 394-396 nm and 450 nm; (c) Raman spectra, by the change in the ratio between the intensities of the lines that peaked at 1224 cm-1 and 1649 cm-1 from 0.61 to 0.49; and (d) FTIR spectra, by the lowered absorbance of the IR band at 1493 cm-1 assigned to the vibrational mode of the -NO2 group as a result of the generation of the nitroso compound. These changes were explained considering the hydrolysis reaction products of NIF, as the nitroso compound is converted to a lactam-type compound. The photodegradation reaction rate constants of NIF and NIF after interaction with NaOH were also reported. The decrease in thermal stability of NIF samples after interaction with NaOH, as well as of NIF after exposure to UV light compared to NIF prior to exposure to UV light, was demonstrated by thermogravimetry, and the key fragments were confirmed by mass spectrometry.

Photodegradation profiling of nitrendipine: evaluation of active pharmaceutical ingredient, tablets and its altered forms

Nitrendipine (NTR) is a dihydropyridine drug, which is well-known as a photodegradable pharmaceutical. However, the photochemical reaction of NTR has not been evaluated in detail from now. In this study, we perform the photodegradation profiling of NTR for the elucidation of its photochemical behavior. NTR amounts during ultraviolet light (UV) irradiation were monitored using high performance liquid chromatography (HPLC). NTR was photodegraded almost completely within 24 h along with the generation of some photoproducts. Structural determination of two NTR photoproducts were carried out by means of electrospray ionization liquid chromatography tandem mass spectrometry (LC-ESI-MS/MS). Obtained results from this study clarified one novel NTR photoproduct, a nitroso pyridine analogue, in addition to a pyridine analogue. Furthermore, photodegradation pathway of NTR was speculated based on chemical structures of NTR photoproducts to clarify its photochemical behavior. It was proposed that a singlet oxygen molecule might withdraw two hydrogen radicals resulting in the form of a pyridine analogue, and the following reduction of its nitro group might produce a nitroso pyridine analogue. Finally, we evaluated the photostability of NTR tablets and its altered forms, indicating that the change of the dosage form led to a decrease of the photostability of NTR tablets. The obtained results will be helpful for the additional research to evaluate the effect of NTR photodegradation on its own biological activities.

The evaluation of photochemical behavior of antihistaminic drug triprolidine in an aqueous media

Photodegradation is widely known as a changer of both the quality and the quantity of several chemical compounds. In this study, we sought to examine the photochemical behavior of triprolidine (TRP), which is a member of the first-generation antihistamine and utilized for a relief of an allergic conditions, in an aqueous media. There are no reports focused on its potential photoproducts and photodegradation pathways in detail to the best of our knowledge. TRP photodegradation induced by ultraviolet light (UV) irradiation was monitored utilizing high-performance liquid chromatography (HPLC), and structural elucidation of the TRP photoproduct was performed by electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) and nuclear magnetic resonance (NMR). Finally, the mechanism of TRP photodegradation was speculated based on the identified photoproduct. TRP was photodegraded dependent on the irradiation time of UV in proportion to the generation of one TRP photoproduct (TRP-P). Structural determination by LC-ESI-MS/MS and NMR clarified that TRP-P was the geometrical isomer of TRP, which was formed by the cis-trans conversion of double bond. UV irradiation experiment for TRP-P revealed the conversion from it to TRP. It is clarified that cis-trans conversion between TRP and TRP-P is photo-equilibrium reaction and TRP-P is predominant under the condition as this experiment. Toxicological potencies of TRP and TRP-P might not be observed by ProTox-II in silico toxicity evaluation. This is the first study evaluating the photochemical behavior of TRP.

Structure Determination of Felodipine Photoproducts in UV-Irradiated Medicines Using ESI-LC/MS/MS

Dihydropyridine drugs are well known as photodegradable pharmaceuticals. Herein, we evaluate the photostability of felodipine (FL) medicine (Splendil^® (SPL) tablets) and its altered forms (powders and suspensions). FL is a type of dihydropyridine drug, but its photochemical behavior is unknown. FL contents after ultraviolet light (UV) irradiation for 24 h were monitored using high-performance liquid chromatography (HPLC). Values of the residual amounts of FL in UV-irradiated SPL powders and suspensions were 32.76 ± 4.88% and 0.79 ± 0.74%, respectively, with the generation of two photoproducts (FL photoproduct 1 and 2). To identify the chemical structures of these photoproducts, electrospray ionization liquid chromatography mass spectrometry (ESI-LC/MS/MS) analysis was performed. Based on their mass-to-charge ratio values and fragment patterns, it was proposed that FL photoproduct 1 was a pyridine derivative and FL photoproduct 2 was an FL dimer. Interestingly, generation rates of FL photoproduct 1 and 2 were dependent on the presence of the aqueous media. The photodimerization of FL was induced in UV-irradiated SPL suspensions. This is the first report evaluating the photostability of SPL tablets and its altered forms and estimating FL photoproducts induced by UV irradiation in the formulation of SPL.

Selective Laser Sintering of a Photosensitive Drug: Impact of Processing and Formulation Parameters on Degradation, Solid State, and Quality of 3D-Printed Dosage Forms

This research study utilized a light-sensitive drug, nifedipine (NFD), to understand the impact of processing parameters and formulation composition on drug degradation, crystallinity, and quality attributes (dimensions, hardness, disintegration time) of selective laser sintering (SLS)-based three-dimensional (3D)-printed dosage forms. Visible lasers with a wavelength around 455 nm are one of the laser sources used for selective laser sintering (SLS) processes, and some drugs such as nifedipine tend to absorb radiation at varying intensities around this wavelength. This phenomenon may lead to chemical degradation and solid-state transformation, which was assessed for nifedipine in formulations with varying amounts of vinyl pyrrolidone-vinyl acetate copolymer (Kollidon VA 64) and potassium aluminum silicate-based pearlescent pigment (Candurin) processed under different SLS conditions in the presented work. After preliminary screening, Candurin, surface temperature (ST), and laser speed (LS) were identified as the significant independent variables. Further, using the identified independent variables, a 17-run, randomized, Box-Behnken design was developed to understand the correlation trends and quantify the impact on degradation (%), crystallinity, and quality attributes (dimensions, hardness, disintegration time) employing qualitative and quantitative analytical tools. The design of experiments (DoEs) and statistical analysis observed that LS and Candurin (wt %) had a strong negative correlation on drug degradation, hardness, and weight, whereas ST had a strong positive correlation with drug degradation, amorphous conversion, and hardness of the 3D-printed dosage form. From this study, it can be concluded that formulation and processing parameters have a critical impact on stability and performance; hence, these parameters should be evaluated and optimized before exposing light-sensitive drugs to the SLS processes.

Drug-Induced Photosensitivity-From Light and Chemistry to Biological Reactions and Clinical Symptoms

Photosensitivity is one of the most common cutaneous adverse drug reactions. There are two types of drug-induced photosensitivity: photoallergy and phototoxicity. Currently, the number of photosensitization cases is constantly increasing due to excessive exposure to sunlight, the aesthetic value of a tan, and the increasing number of photosensitizing substances in food, dietary supplements, and pharmaceutical and cosmetic products. The risk of photosensitivity reactions relates to several hundred externally and systemically administered drugs, including nonsteroidal anti-inflammatory, cardiovascular, psychotropic, antimicrobial, antihyperlipidemic, and antineoplastic drugs. Photosensitivity reactions often lead to hospitalization, additional treatment, medical management, decrease in patient's comfort, and the limitations of drug usage. Mechanisms of drug-induced photosensitivity are complex and are observed at a cellular, molecular, and biochemical level. Photoexcitation and photoconversion of drugs trigger multidirectional biological reactions, including oxidative stress, inflammation, and changes in melanin synthesis. These effects contribute to the appearance of the following symptoms: erythema, swelling, blisters, exudation, peeling, burning, itching, and hyperpigmentation of the skin. This article reviews in detail the chemical and biological basis of drug-induced photosensitivity. The following factors are considered: the chemical properties, the influence of individual ranges of sunlight, the presence of melanin biopolymers, and the defense mechanisms of particular types of tested cells.

Modelling nifedipine photodegradation, photostability and actinometric properties

BACKGROUND

The photodegradation of drugs obeying unimolecular mechanisms such as that of nifedipine (NIF) were usually characterised in the literature by zero-, first- and second-order kinetics. This approach has been met with varying success. This paper addresses this issue and proposes a novel approach for unimolecular photodegradation kinetics. The photodegradation of the cardiovascular drug nifedipine is investigated within this framework.

METHODS

Experimental kinetic data of nifedipine photodegradation were obtained by continuous monochromatic irradiation and DAD analysis. Fourth-order Runge-Kutta calculated kinetic data served for the validation of the new semi-empirical integrated rate-law model proposed in this study.

RESULTS

A new model equation has been developed and proposed which faithfully describes the kinetic behaviour of NIF in solution for non-isosbestic irradiations at wavelengths where both NIF and its photoproduct absorb. NIF absolute quantum yield values were determined and found to increase with irradiation wavelength according to a defined sigmoid relationship. The effects of increasing NIF or excipients' concentrations on NIF kinetics were successfully modelled and found to improve NIF photostability. The potential of NIF for actinometry has been explored and evaluated. A new reaction order (the so-called Φ-order) has been identified and specifically proposed for unimolecular photodegradation reactions.

CONCLUSION

The semi-empirical and integrated rate-law models facilitated reliable kinetic studies of NIF photodegradation as an example of AB(1Φ) unimolecular reactions. It allowed filling a gap in kinetic studies of drugs since, thus far, thermal first-order or a combination of first- and zero- order kinetic equations were generally applied for drug photoreactions in the literature. Also, a new reaction order, the "Φ-order", has been evidenced and proposed as a specific alternative for photodegradation kinetics.

Antioxidant effects of photodegradation product of nifedipine

Recently, increasing evidence suggests that the antihypertensive drug nifedipine acts as a protective agent for endothelial cells, and that the activity is unrelated to its calcium channel blocking. Nifedipine is unstable under light and reportedly decomposes to a stable nitrosonifedipine (NO-NIF). NO-NIF has no antihypertensive effect, and it has been recognized as a contaminant of nifedipine. The present study for the first time demonstrated that NO-NIF changed to a NO-NIF radical in a time-dependent manner when it interacted with human umbilical vein endothelial cells (HUVECs). The electron paramagnetic resonance (EPR) signal of NO-NIF radicals in HUVECs showed an asymmetric pattern suggesting that the radicals were located in the membrane. The NO-NIF radicals had radical scavenging activity for 1,1-diphenyl-2-picrylhydrazyl, whereas neither NO-NIF nor nifedipine did. In addition, the NO-NIF radical more effectively quenched lipid peroxides than NO-NIF or nifedipine. Furthermore, NO-NIF attenuated the superoxide-derived free radicals in HUVECs stimulated with LY83583, and suppressed iron-nitrilotriacetic acid (Fe-NTA)-induced cytotoxicity in rat pheochromocytoma (PC12) cells. Our findings suggest that NO-NIF is a candidate for a new class of antioxidative drugs that protect cells against oxidative stress.

Protective effect of photodegradation product of nifedipine against tumor necrosis factor alpha-induced oxidative stress in human glomerular endothelial cells

Recently, increasing evidence suggests that the antihypertensive drug nifedipine acts as a protective agent for endothelial cells, and that the activity is unrelated to its calcium channel blocking. Nitrosonifedipine (NO-NIF) is metabolically and photochemically produced from nifedipine, and NO-NIF has been recognized as a contaminant of nifedipine because it has no antihypertensive effect. Treatment of tumor necrosis factor-α (TNF-α) suppressed the cell viability and facilitated the expression of Inter-Cellular Adhesion Molecule 1(ICAM-1) in human glomerular endothelial cells (HGECs) though, pretreatment of NO-NIF significantly recovered the TNF-α-induced cell damage to the same extent as Trolox-C did, and suppressed the ICAM-1 expression in a concentration dependent manner. In addition, NO-NIF inhibited the cell toxicity induced by cumene hydroperoxide, which hampers the integrity of cell membrane through oxidative stress, as effective as Trolox-c. These data suggest that NO-NIF is a candidate for a new class of antioxidative drug that protect cells against oxidative stress in glomerular endothelial cells.

Photophysics and Photochemical Studies of 1,4-Dihydropyridine Derivatives

The absorption and fluorescence properties of nifedipine (NPDHP), felodipine (CPDHP) and a series of structurally related 1,4-dihydropyridines were studied in aqueous solution and organic solvents of different properties. The absorption and fluorescence spectra were found to depend on the chemical nature of the substituents at the position 4 of the 1,4-dihydropyridine ring (DHP) and on solvent properties. In aqueous solution, the fluorescence spectra of 4-phenyl substituted compounds are blue-shifted with respect to the alkyl substituted compounds. The more fluorescent compound is CPDHP. Nifedipine is not fluorescent. All compounds, with the exception of CPDHP, present monoexponential fluorescence decay with very short lifetime (0.2-0.4 ns). CPDHP showed a biexponential emission decay with a long-lived component of 1.7 ns; this behavior is explained in terms of different conformers because of the hindered rotation of the phenyl group by the ortho-substitution. Analysis of the solvent effect on the maximum of the absorption spectrum by using the linear solvent-energy relation solvato-chromic equation indicates the redshifts are influenced by the polarizability, hydrogen bonding ability and the hydrogen bond acceptance of the solvent. Whereas, the fluorescence characteristics (spectra, quantum yields and lifetimes) are sensitive to the polarizabilty and hydrogen bond ability of the solvents. Photo-decomposition of nifedipine is dependent on the solvent properties. Faster decomposition rates were obtained in nonprotic solvents. The 4-carboxylic derivative goes to decarboxylation. Under similar conditions, the other DHP compounds did not show appreciable photodecomposition.

Intramolecular electron transfer in the photochemistry of some nitrophenyldihydropyridines

4-Phenyl-1,4-dihydropyridine-3,5-dicarboxylates contain two pi chromophores separated by an sp3 carbon. The lowest singlet is localized on the dihydropyridine moiety (1PyH2-Ph) and emits a blue fluorescence (with close to unitary efficiency in glass at 77 K). In 3-nitrophenyl derivatives (PyH2-PhNO2, some of which are photolabile drugs) the fluorescence is completely quenched. Reasonably, this is due to intramolecular electron transfer between the close-lying donor and acceptor moieties to give the charge-separated species (PyH2*+-PhNO2*-). In EPA glass at 77 K, back-electron transfer gives the dihydropyridine-localized triplet (3PyH2-PhNO2), which emits a yellow phosphorescence. In solution, deprotonation from the radical cation on the dihydropyridine moiety initiates rearomatization, finally giving Py-PhNO2 with low quantum yield (5 x 10(-4) to 5 x 10(-3), increasing up to 0.013 by irradiation at 254 nm, where direct excitation of the nitrophenyl chromophore contributes). In the presence of triethylamine, the reaction changes to neat reduction of the nitro group. When a tethered alkylamino group is present, oxidative degradation of that moiety occurs, again via an electron-transfer intramolecular process. This has been found with the drug nicardipine, where photodegration is more efficient (phi 0.02 to 0.1). Donor-acceptor dyads of this type, easily available through the Hantzsch synthesis, may be useful for building new photoinduced electron-transfer systems.

Why Escherichia coli alpha-hemolysin induces calcium oscillations in mammalian cells--the pore is on its own

Escherichia coli alpha-hemolysin (HlyA), archetype of a bacterial pore-forming toxin, has been reported to deregulate physiological Ca2+ channels, thus inducing periodic low-frequency Ca2+ oscillations that trigger transcriptional processes in mammalian cells. The present study was undertaken to delineate the mechanisms underlying the Ca2+ oscillations. Patch-clamp experiments were combined with single cell measurements of intracellular Ca2+ and with flowcytometric analyses. Application of HlyA at subcytocidal concentrations provoked Ca2+ oscillations in human renal and endothelial cells. However, contrary to the previous report, the phenomenon could not be inhibited by the Ca2+ channel blocker nifedipine and Ca2+ oscillations showed no constant periodicity at all. Ca2+ oscillations were dependent on the pore-forming activity of HlyA: application of a nonhemolytic but bindable toxin had no effect. Washout experiments revealed that Ca2+ oscillations could not be maintained in the absence of toxin in the medium. Analogously, propidium iodide flux into cells occurred in the presence of HlyA, but cells rapidly became impermeable toward the dye after toxin washout, indicating resealing or removal of the membrane lesions. Finally, patch-clamp experiments revealed temporal congruence between pore formation and Ca2+ influx. We conclude that the nonperiodic Ca2+ oscillations induced by HlyA are not due to deregulation of physiological Ca2+ channels but derive from pulsed influxes of Ca2+ as a consequence of formation and rapid closure of HlyA pores in mammalian cell membranes.

Photochemistry of 4-(2-Nitrophenyl)-1,4-Dihydropyridines. Evidence for Electron Transfer and Formation of an Intermediate†

New evidence about the path followed in the photochemical reaction of 4-(2-nitrophenyl)-1,4-dihydropyridines such as the drugs nifedipine (Compound 1) and nisoldipine (Compound 2) to give the corresponding nitrosophenylpyridines has been found through determination of the steady-state photochemical parameters and a comparison of the photoreactions in solution and in matrix at 90 K. Additional support is given by comparison with the isomeric 4-(3-nitrophenyl)dihydropyridine as well as with simpler derivatives, such as the corresponding 4-methyldihydropyridine. In Compounds 1 and 2, the lowest lying singlet, localized on the dihydropyridine chromophore, is deactivated by (largely exothermic) electron transfer to the nitrobenzene moiety, as evidenced by the complete quenching of the blue fluorescence observed in analogues not containing the electron-accepting group. Intramolecular proton transfer ensues in the 2-nitrophenyl derivatives with a relatively medium-independent quantum yield of approximately 0.3 and leads to an aromatic zwitterion, which is detected in matrix at 90 K (photoionization of this intermediate takes place in 2-methyltetrahydrofuran secondary). The intermediate is smoothly converted into the end product upon melting the glass. The 3-nitrophenyl analog, for which such a path is not available, is less reactive by about three orders of magnitude at 366 nm, although the quantum yield arrives at approximately 0.01 by irradiation at 254 nm in MeOH, reasonably via the nitrophenyl localized triplet.

Instability of calcium channel antagonists during sample preparation for LC–MS–MS analysis of serum samples

1,4-Dihydropyridines calcium channel antagonists (1,4-DHP CCAs) are photolabile and the products of their photodecomposition have no pharmaceutical activity. In our previous work we have presented a screening procedure for eleven 1,4-DHPs in plasma by LC-MS-MS using multiple reaction motoring. The laboratory process includes preparation and storage of stock solutions, plasma storage, solid-phase extraction, reconstitution of extracts and storage time in an autosampler for LC-MS-MS analysis. Prior to validation of the analytical procedure, we have tested the stability of these compounds by exposure to light. Methanolic solutions have been exposed to laboratory and UV light and the stability of the compounds in plasma was tested by exposure of spiked plasma samples to laboratory light at room temperature. Stability during freeze-thaw cycles and stability during 2 month storage at -20 degrees C have been tested as well. Products of photodecomposition have been identified after forced degradation and the degree of degradation has been quantified using LC-UV-DAD and LC-MS-MS, respectively. A 96% degradation after only 2h has been observed when solutions of nifedipine or nisoldipine were exposed to laboratory light in clear glass vials. In plasma samples degradation was 25% in only 2h for both compounds. The main degradation product was produced by oxidation of the dihydropyridinic ring resulting in the pyridine analogue that has been described as the first metabolite in the metabolic pathway. Only minor degradation was found for the other tested compounds after 2h light exposure in methanolic solutions. Furthermore, lercanidipine and nicardipine were also degradated by esterhydrolysis. Several additional minor degradation products were found for the other tested 1,4-DHPs, however, some of them could not be identified. Preconditions for storage and handling of plasma samples prior to and during analysis for 1,4-DHP CCAs are suggested in order to avoid photodecomposition of the analytes.

Switch between charge transfer and local excited states in 4-aminophenyl-substituted Hantzsch 1,4-dihydropyridine induced by pH change and transition metal ions

The absorption and fluorescence spectra of a Hantzsch 1,4-dihydropyridine derivative bearing a N,N-dimethylaminophenyl group at 4-position (H(2)Py-PhN(CH(3))(2)) in aprotic solvents have been examined and compared to model compounds 4-phenyl- and 4-methyl-substituted Hantzsch 1,4-dihydropyridines (H(2)Py-Ph and H(2)Py-Me). While H(2)Py-Ph and H(2)Py-Me show fluorescence around 420 nm from the local excited state of the dihydropyridine chromophore, H(2)Py-PhN(CH(3))(2) exhibits fluorescence around 520 nm from the intramolecular charge transfer (ICT) state involving the aniline and dihydropyridine groups as donor and acceptor, respectively. Upon addition of an acid to the solution of H(2)Py-PhN(CH(3))(2), the amino group in the aniline is protonated. Thus, the photoinduced intramolecular charge transfer is prevented, and only the fluorescence from the local excited state of the dihydropyridine chromophore can be detected. These changes in the fluorescence behavior are fully reversible: subsequent addition of a base to the acidic solution leads to the recovery of the ICT fluorescence and the quenching of the local fluorescence. Transition metal ions also can switch the fluorescence of H(2)Py-PhN(CH(3))(2). Evidence for the interaction between transition metal ions and the amino group in the dimethylaniline have been provided by absorption and emission spectrum as well as NMR studies.

Photo-CIDNP Study of the Interaction of Tyrosine with Nifedipine. An Attempt to Model the Binding Between Calcium Receptor and Calcium Antagonist Nifedipine¶

This article proposes a new approach to the modeling of the molecular-level mechanism of ligand-receptor interaction for Ca2+ receptor binding site. Chemically induced dynamic nuclear polarization (CIDNP) technique has been used to unravel fine details of the reaction in the model system composed of one of the known Ca2+ antagonist drugs, nifedipine (NF), and isolated amino acid residuals (e.g. tyrosine [Tyr]) of Ca2+ receptor binding site. It has been conclusively demonstrated that the reaction between NF and Tyr resulting in the oxidation product--nitroso form of NF--obeys the radical mechanism. CIDNP data in combination with the results of mathematical modeling of the structures of ligand-receptor complexes have allowed to propose the mechanism of the interaction of NF with Ca2+ receptor binding site.

Photo-damage in Northern European renal transplant recipients is associated with use of calcium channel blockers

Photo-damage of the head and neck was assessed, by visual inspection, in 82 Northern European renal transplant recipients attending a dermatology clinic. Photo-damage was graded as absent, mild, moderate and severe and the presence or absence of skin telangiectasia, solar elastosis, erythema and pigmentation was assessed. The duration and type of immunosuppressant and calcium channel blocker medication was recorded. Ninety percent had photo-damaged skin with 41 (50%) having mild photo-damage, 20 (24%) moderate photo-damage and 13 (16%) severe photo-damage. Sixty-two (76%) had pigmentation, 43 (52%) telangiectasia, 65 (79%) papular changes and 51 (65%) erythema. Fifty-three patients (65%) had received a calcium channel blocker (49 nifedipine, four amlodipine). The grade of photo-damage was strongly associated with use of calcium channel blockers (P=0.001) as was the presence of telangiectasia (P=0.001) and solar elastosis (P=0.04). Photo-damage is frequent in this population of renal transplant patients. Telangiectasia in association with calcium channel blockers has been reported rarely but appears to be common in the transplant population.

Iron and transition metal transport into erythrocytes mediated by nifedipine degradation products and related compounds

The aim of this investigation was to determine the mechanism of action of the nitrosophenylpyridine derivative of nifedipine ("nitrosonifedipine", NN) on Fe(II) transport into erythrocytes. Nifedipine is rapidly degraded to NN by daylight. We used rabbit erythrocytes, NN, and several chemically related substances, and examined their effects on the transfer of iron and other transition metals (cadmium, cobalt, manganese, nickel, zinc) into and out of the cells. NN mediated the transfer of iron and zinc but not the other metals into the cell cytosol. The transfer of Fe(II) was not affected by changes in cell membrane potential and could not be ascribed to free radical production. Two hydroxamic acid compounds chemically related to NN also stimulated iron and zinc uptake, but no evidence was obtained for cell-induced transformation of NN to them. In vivo, NN is probably converted to a lactam derivative. This compound was found to have no effect on iron uptake by the cells. It is concluded that NN has a relatively high specificity for the transport of iron compared with other transition metals, and small changes in its structure markedly affect this action. Also, because the lactam to which NN is converted in vivo is inactive, it is unlikely that nifedipine will affect iron metabolism after therapeutic administration.

Spectroscopic and HPLC studies of photodegradation of nilvadipine

Photochemical decomposition of nilvadipine (NV), a derivative of 1,4-dihydropyridine (DHP), was studied. Photodegradation was carried out in the conditions recommended in the first version of the document issued by the International Conference on Harmonization (ICH), currently in force in the studies of photochemical stability of drugs and therapeutic substances. Methanol solutions of NV were irradiated with a high-pressure mercury arc lamp, type HBO 200 (300-400 nm). The maximum absorption of radiation at 365 nm was achieved by applying the interference filter and Wood's filter. The assessment of NV photodegradation was made on the basis of the UV spectrophotometric and high-performance liquid chromatographic (HPLC) methods. Quantitatively, the process was described with the calculated rate constants of decomposition k, time of decomposition of 50% of the compound to 5, and time of decomposition of 10% of the compound t(0.1). The two methods applied allowed a determination of the kinetic parameters of NV photodegradation from the relationship ln c = f(t). Using the Reinecke salt as a chemical actinometer, apparent quantum yields of photodegradation were obtained; after extrapolation to the time of irradiation zero, these gave the actual quantum yield (phi = 7.3 10(-5)). The quantum yield of fluorescence at lambda(exc) = 375 nm was about 9.3 x 10(-4) The methods used for evaluation of NV photodegradation were subjected to validation, and results of the analytical methods were statistically assessed by Snedecor F and Student t tests. The former test revealed no statistically significant difference between the variances obtained by the HPLC and UV spectrophotometric methods. Also, verification of the zero hypothesis of the Student t test on equality of means of the results obtained gave no significant diferences between the two methods.

Sympathoinhibitory and depressor responses to long-term infusion of nifedipine in spontaneously hypertensive rats on high-salt diet

Short-term (by hour) intracerebroventricular (i.c.v.) or i.v. infusion of nifedipine at low rates evokes parallel decreases in renal sympathetic nerve activity (RSNA) and blood pressure (BP) in spontaneously hypertensive rats (SHR). In the present study, effects of long-term administration of nifedipine on BP and control of sympathetic tone were examined in SHR on a high-salt (8%) diet. From 6 to 8 weeks of age, for 2 weeks concomitant with taking a high-salt diet, rats were also treated with subcutaneous infusion of nifedipine at 10, 50, or 100 microg/kg/h or vehicle solvent as control using osmotic minipumps. At the end of the 2-week treatment period, mean arterial pressure (MAP), heart rate (HR), and RSNA at rest and in response to air-jet stress, i.c.v. injection of the alpha-adrenoceptor agonist guanabenz (25 microg), and i.v. injection of the ganglionic blocker hexamethonium were recorded in conscious rats. In rats on nifedipine 50 or 100 microg/kg/h, resting MAP was significantly lower (136+/-4 or 130+/-4 vs. 145+/-2 mm Hg in control rats, p < 0.05 for both), the sympathoinhibitory and depressor responses to i.c.v. guanabenz were significantly decreased, and the absolute decreases in MAP in response to i.v. injection of hexamethonium were significantly smaller. Sympathoexcitatory and pressor responses to air-jet stress, however, were not affected by nifedipine. Infusion of nifedipine at the three rates for 2 weeks caused concentrations of plasma nifedipine in a dose-related manner. Nifedipine was not detected in tissues of rats treated with 10 microg/kg/h nifedipine but was present in brain and other tissues of rats treated with nifedipine at the two higher rates. Thus in SHR on high-salt intake long-term treatment with nifedipine at 50 or 100 microg/kg/h decreased resting BP and the sympathetic component in BP control. In addition to possible peripheral effects, long-term administration of nifedipine may also act centrally to decrease sympathetic activity and BP, likely by increasing activity in central pathways involving sympathoinhibition, but not in pathways involving sympathoexcitation as evaluated by air-stress.

Identification of photodegradation products of nilvadipine using GC-MS

Nilvadipine (NV) photodegradation products have been analysed with gas chromatography-mass spectrometry (GC-MS). The photodegradation was carried out in the conditions recommended in the first version of the document issued by the International Conference on Harmonization (ICH), currently in force in the studies of photochemical stability of drugs and therapeutic substances. Methanol solutions of NV were irradiated with a high-pressure UV lamp - type HBO 200. The maximum intensity at the wavelength lambda = 365 nm was achieved by applying the interference filter and Wood's filter. Using the Reinecke salt as a chemical actinometer, apparent quantum yields of photodegradation were obtained, which after extrapolation to the zero time of irradiation gave the actual quantum yield ((phi = 7.58 x 10(-5). The structure of three nilvadipine photodegradation products was established, after mass spectra analysis of compounds registered during GC-MS carried out of irradiated nilvadipine solutions. The quantitative results of GC-MS analyses enabled to determination of the kinetic parameters of NV photodegradation, calculated from the dependence In c =f(t). Quantitatively the process was described with the calculated rate constant of decomposition (k), decomposition time of 50% of the compound (t0.5) and decomposition time of 10% of the compound (t0.1). The exposure of nilvadipine to UV light was found to lead to aromatization of the DHP ring and elimination of the HCN molecule.

Sympathoinhibition by central and peripheral infusion of nifedipine in spontaneously hypertensive rats

The present study assessed whether central mechanisms may contribute to the hypotensive effect of the calcium channel blocker nifedipine. In conscious, spontaneously hypertensive rats (SHR) on a high-salt diet, hemodynamic (mean arterial pressure [MAP] and heart rate) and sympathetic (renal sympathetic nerve activity) responses to low, central, intracerebroventricular infusion rates (25 microg. kg(-1). h(-1) for 2 hours) and peripheral intravenous rates (50 microg. kg(-1). h(-1) for 3 hours and then 100 microg. kg(-1). h(-1) for 2 hours) of nifedipine were evaluated. The distribution of nifedipine in the blood and tissues was assessed at the end of the infusions. Nifedipine significantly inhibited renal sympathetic nerve activity and lowered MAP in SHR beginning 30 minutes after the start of the intracerebroventricular infusion. The decrease of MAP by intravenous infusion began at 60 minutes and was more profound with 100 microg. kg(-1). h(-1). Inhibition of sympathetic activity preceded and then paralleled the decrease in blood pressure; it occurred earlier with central (15 to 30 minutes) than with peripheral (30 to 60 minutes) infusion. Intravenous infusion resulted in concentrations of nifedipine in brain structures (brain stem, midbrain, and cortex) that were 30% to 40% of those in the heart, kidneys, and liver. From the hemodynamic and sympathetic responses and the distribution of nifedipine into the central nervous system, we conclude that the peripheral infusion of nifedipine at relatively low rates may evoke a hypotensive response in SHR, not only via peripheral mechanisms, but also through central mechanisms, which will lead to an inhibition of sympathetic outflow and, therefore, a lowering of blood pressure.