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Photostability of Drugs and Drug Formulations, Second Edition
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Double distilled water was used as the solvent. For irradiation experiments, a Ushio W xenon arc-lamp light source housed in a housing shell model A and powered by a power supply model LPS, was used. This set up was cooled by tap water circulation through a pipe system. The excitation beam was guided through an optical fibre to impinge from the top of the sample cuvette i. The set up lamp, lamp housing and monochromator was manufactured by Photon Technology International Corporation.
A diode array spectrophotometer Agilent was used to measure the various absorption spectra and kinetic profiles for the irradiation and calibration experiments. This spectrophotometer was equipped with a 1 cm cuvette sample holder and a Peltier system model Agilent for temperature control. The spectrophotometer was monitored by an Agilent Chemstation kinetics software. In order to carry out non-linear fittings and to determine bestfit curves, a Levenberg-Marquardt iterative program within the Origin 6.
A flow rate of 1.
The detector wavelength was set at nm. Retention times of 10 and 8. The flask was protected from light by aluminium foil wrapping and was kept in the fridge. The stock solution was diluted to prepare fresh analytical solutions ca. For actinometric studies, Fluvo solutions of the same concentrations ca. Even though ubiquitous, the treatment of photokinetics on the basis of the latter classical reaction orders is unreliable, for at least three main drawbacks inherently linked to this approach.
Firstly, the differential equations of photoreactions are generally different from and not possibly integrated in closed-forms as is the case for thermal reactions of 0th-, 1st- or 2nd-order. This means that using such classical order treatments for the quantitative investigation of photoreactions must be considered as a mere approximation. Secondly, literature data have reported that the classical approach may lead to a confusion about the reaction order that should be attributed to the photodegradation reaction at hand given that the kinetic data of a given reaction generally up to half-life time can well be fitted by the equations corresponding to two different reaction orders most commonly 0th- and 1st-order.
In this context, the approach adopted to develop the equations of 0-order kinetics offers a more robust mathematical framework to investigate photoreactions. The model equation, a logarithmic expression involving a time-dependent exponential term, for unimolecular photoreactions where only the initial species absorbs, has been derived through closed-form integration.
They have been optimised by studying simulated photo-reaction traces obtained by numerical Runge-Kutta integration methods. A unique set of general equations eqn 1a and 1b can be derived for photochemical reactions involving two species, the. Such systems are labelled as AB 10 and AB 20 , respectively.
Hence, if it is assumed that the concentration of the excited state is negligible during the progress of the photoreaction while the reaction medium is concomitantly maintained at a constant temperature, homogeneously stirred, and continuously irradiated with a monochromatic beam with the latter beam's non-isosbestic wavelength.
J are not necessarily equal for a given experiment, and the absorbance of the medium under the excitation conditions i. J may not be directly accessible during the experiment. The reaction medium is irradiated at a given irradiation wavelength and simultaneously monitored at either a different observation wavelength. The numerical value of eqn 5 , obtained graphically, corresponds to the theoretical expression derived from the differentiation of the reaction,11 as. This condition is easily met by reducing the values of either the initial concentration of species A or the optical path length for irradiation,.
The value of the initial velocity can be obtained graphically and compared to its theoretical expression eqn 9.
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The monoexponential form of eqn 7 and 8 indicates that isosbestic irradiation induces first-order kinetics for AB 20 reactions. In order to overcome this situation, we have recentIy proposed a simpIe eIucidation method for photoreversibIe reactions that can be impIemented in three steps. Firstly, the reaction quantum yields are determined for an isosbestic irradiation. The variation of the species concentrations during photodegradation, under monochromatic irradiation at an isosbestic point, is monitored by HPLC.
The arrows indicate the direction of the absorbance evolution during the photoreaction and the vertical lines cross the spectra at the isosbestic points , , nm. Both the species pss concentrations and the quantum yield values allow determining the equilibrium constant eqn 12 ,. This feature finds its importance in the fact that the HPLC experiment that served its determination is usually performed at initial concentrations of species A that are not suitable too concentrated for spectrophotometric analyses which are bound to be realised at lower concentrations, specifically, where F.
The reconstruction of the full spectrum of the photoisomer B , can then be performed at lower concentrations in the second step of the elucidation method. The native electronic absorption spectrum of E-Fluvo isomer Fig. This molecule, thus, absorbs mainly in the UVB region of the spec-. Therefore, irrespective of the wavelength selected to perform the irradiation, the number of unknowns will constantly be two in total, as the spectrum of the photoproduct eB is fully known.
Hence, in the last step of the method, the quantum yields for each non-isosbestic irradiation wavelength and. Table 1 Overall rate-constant and equilibrium constant for the photodegradation of an aqueous Fluvo solution 1. The clearly defined isosbestic points at , and nm and the smooth evolution of the spectra indicate that the photoreac-tion is quantitative and proceeds without by-products. The concentration profiles of E- and Z-Fluvo were readily fitted by eqn 7 and 8 Fig. Based on eqn 13 , the spectrum of the medium at pss and the spectrum of the E-isomer, the electronic absorption spectrum as absorption coefficients' values of the Z-isomer can be fully reconstructed Fig.
The geometric shapes represent the experimental data while the continuous lines represent the model fitted traces using the appropriate model equation. In general, a smooth decrease in absorption over irradiation time was observed eventually reaching a plateau region Fig. This represents a typical behaviour of AB 20 systems, which in turn corroborates the mechanism of Fluvo photodegradation Scheme 1.
This is also confirmed by the good. Therefore, FIuvo photoconversion obeys 0-order kinetics. The kinetic parameters determined for FIuvo photodegradation TabIe 2 , indicate that the overaII rate-constant of photo-reaction increases with increasing irradiation waveIength Table 2.
Therefore, it is mandatory to define, in subsequent steps, the absoIute vaIues of the photoreaction quantum yieIds at the seIected wavelengths. The recommended hypothesis for this type of studies is that the quantum yields of drugs should a priori be supposed to be waveIength dependent and then the hypothesis shouId be tested experimentaIIy.
B increases with increasing wavelength and is always higher than the reverse quantum yield. The most pronounced variation of the quantum yield. Furthermore, the evolution of the forward quantum yield values with wavelength has a defined sigmoid pattern eqn 16 , Fig. This advantageously enables the determination of E-Fluvo quantum yield at any desired wavelength using the sigmoid equation eqn The reverse quantum yield, on the other hand, follows a lower pattern with irradiation wavelength Fig.
A similar behaviour has been observed for Montelukast. The more pronounced difference between the isomers' quantum yields that were recorded in the longest wavelength region indicates that the excited-sate of lowest energy is much more efficient for E- than for Z-Fluvo. Overall, such results may illustrate a case where not only the chemical nature, the geometry of the molecule but also the irradiation conditions impact the photochemical behaviour of the drugs.
In the case of Fluvo, this represents a significant decrease in dosage as Z-Fluvo is biologically inactive,20 but could be a major issue if for other drugs the photoproduct is toxic. These results stress on the usefulness and necessity of a full kinetics elucidation of drug photodegradation.
They also confirm that reliable conclusions about the photoreactivity of a compound can only be reached when using monochromatic irradiation coupled to a treatment using the 0-order kinetics. It is then reasonable to suggest that the ICH recommendations would benefit from introducing an element of photostability assessment of the drugs at low concentration in solution.
Such data would not only shed light on the photokinetic behaviour and photodegradation parameters of the drug in vitro but also may lay down a platform for an understanding of the behaviour of drugs in vivo. Indeed, the distribution of the administered drugs in the skin and eyes of the patients occurs mostly at low concentrations within biological fluids and tissues.
It has been shown that despite the absorption spectrum of Fluvo ends ca. In this context, the FDA, EMEA and ICH have issued guidelines on the evaluation of the photosafety of all new systemic and topical pharmaceuticals capable of absorbing within the UVB, UVA or visible regions with absorption coefficients above M-1 cm-1 as well as existing drugs when unaddressed photosafety concerns arise. In this context, testing low concentrated solutions of drugs in vitro may arguably benefit the evaluation of the potential and extent of photodegradation that might be undergone by the drug in similar situations in vivo.
The low concentration studies are also important because the equations of the 0-order kinetics eqn 3 show that the rate of photodegradation of drugs increases with decreasing concentration. Furthermore, the ICH currently recommends an irradiance dose of approximately 5 J cm-2 UVA doses for the in vitro 3T3 Neutral Red Uptake phototoxicity test 3T3 NRU PT to corroborate natural irradiation conditions comparable to those obtained during prolonged outdoor activities on summer days around noon time, in temperate zones and at sea levels.
Therefore, the conditions of the present study reflect well the situation of drugs in the body as small concentrations ca. Such studies might be thought of as a relevant initial platform, that provide reliable data and valuable information about the inherent photoreactivity of a molecule in solution, to feed the evaluation of drug photosafety and photodegradation in vivo.
There is an evident lack in the literature of useful methods to quantify photostabilisation of drugs. The Q1b document7 does not propose any detailed procedures in this respect including the case of solutions. In this section, the photoprotection of Fluvo with excipient dyes was assessed by 0-order kinetics. Table 3 Dye absorbances, overall reaction rate-constants, photokinetic factors, and percentage reduction in reaction rates of Fluvo photodegradation in the presence of various concentrations of TRZ when irradiated at nm and observed at nm.
Its effect was evaluated on solutions of TRZ of various concentrations, which were each irradiated after the addition of the same amount of Fluvo. Under these conditions, the temporal evolution of the absorbance of the medium could be recorded without the spectral interference of the dye the latter however does absorb part of the excitation light. Indeed, eqn 1 - 6 apply except that the total absorbance of the medium at the irradiation wavelength in eqn 4 must take into account the presence of the third molecule of the light-absorption competitor, i.
The model equation eqn 2 fitted all the curves well irrespective of the concentration of TRZ present in solution though below the limit of its linearity range. Accordingly, the overall photoreaction rate-. This confirms that the presence of the excipient-dye does not alter the photodegradation pattern and or quantum yields of the photoreactions but only reduces the rate of photodegradation. A similar phenomenon should also be expected to occur for an increase of the initial concentration of the mother compound eqn 3 , and hence, the rate of the reaction is concentration-dependent.
This confirms that zero- and first-order reaction treatments and interpretation of photodegradation kinetics are neither suitable nor reliable approaches. The present 0-order kinetic equations offer however an easy and useful tool to evaluate photostabilisation of drugs in solution.
An additional interesting and useful aspect offered by the equations of 0-order kinetics is the development of new actin-ometers. The assessment of Fluvo potential for actinometry is set out by preparing solutions of approximately the same concentration and exposing each one to a monochromatic light of given radiant power values selected from a set for each. Firstly, a a fresh solution of Z-Fluvo 3 x M in water is subjected to a monochromatic irradiation. In the third step, c the corresponding vaIues for the P.
Finally, d the unknown radiant power of the source is determined from one of the following equations eqn The circles represent the experimental data while the lines represent the model fitted traces. The kinetic traces obtained at the observation wavelength of nm were then well fitted to the model eqn 2 Fig. A linear correlation was observed between the values of. The linear relationships found here confirm the usefulness of Fluvo for actinometry. In order to facilitate the actinometric method even more, the Vorcldbs values calculated using eqn 6 were compared to. A very good agreement has been found, indicating that the initial velocity values can be worked out from the data corresponding to the first 5 to 10 min of Fluvo irradiation.
Nevertheless, if the concentration or path-lengths used in the unknown light source experiment differ from the ones employed in this study i. As well as facilitating actinometry studies, can also serve to inform about the photoreaction rate much more reliably than the overall rate-constant or the quantum yields. This parameter is, therefore, an ideal tool for comparing the photoreaction rates between different experiments and we propose to label it as the 'psewdo-rate-constant".
Similarly, could be considered as the pseudo-initial velocity varying only with the terms given in eqn 6 but not with radiant power. However, if the wavelength range. The above study emphasises the new perspectives offered by the 0-order kinetic model for photoreversible systems in general. For the particular case of drugs, it sets out a framework for targeted, accurate and complete kinetic studies. It can serve in the development of new technological AB 20 devices in photomedicine, targeted drug delivery and photo-responsive drug nano-carrier systems.
Using Fluvo as an example of photoreversible reaction systems, the model i fitted its full kinetic traces; ii allowed the determination of the overall-rate constant; iii offered the pseudo-rate-constant beta factors as new and reliable kinetic parameters truly reflective of the intra- and inter-experiment rate of photoreactions; iv allowed the quantification of effects of photostabilising additives; and v presented Fluvo as an accurate and reliable actinometer for the nm irradiation range.
Tisdale and D. Miller, Hearthside Publishing, Bethesda, , pp. Ferguson, Investigation of drug-induced photosensitivity in man, Toxicology, , , Ferguson, Photodermatology, in Photodermatology, ed. Ferguson and J. Dover, Manon Publishing Ltd, London, Piechocki and K.
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Maafi and R. New Spectrokinetic methods for photochromes, Photochem. Maafi and M. Maafi and W. Fukui, Y. Suzuki, T. Sugai, J. Watanabe, S. Ono, N. Tsuneyama and T. Someya, Promoter variation in the catechol-O-methyltransferase gene is associated with remission of symptoms during fluvoxamine treatment for major depression, Psychiatry Res. Figgitt and K. McClellan, Fluvoxamine: An updated review of its use in the management of adults with anxiety disorders, Drugs, , 60, Benfield and A.
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