- Research article
- Open Access
Photostability and toxicity of finasteride, diclofenac and naproxen under simulating sunlight exposure: evaluation of the toxicity trend and of the packaging photoprotection
© Sammartino et al.; licensee Chemistry Central Ltd. 2013
- Received: 26 June 2013
- Accepted: 17 November 2013
- Published: 10 December 2013
Drugs photostability plays two different opposite roles; a real advantage arises considering the longer expiration time of the drugs while the consequent persistence in the environment involves an obvious negative effect bound to their harmfulness.
On this basis we tested the photostability and toxicity of three pharmaceutical active principles: Finasteride, Diclofenac and Naproxen. The pure active principles, as well as commercial drugs containing them, were considered; for the last, the protective effect of the packaging was also evaluated. Samples were irradiated according to the ICH Guidelines for photostability testing (The International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use); a simulating sunlight source (a mercury-vapor lamp coupled to a tungsten filament one) was used to cover the wavelength range 300–2000 nm; Temperature, Relative Humidity, Irradiance and Illuminance were maintained constant during the photodegradation. The concentrations of the pharmaceutical active principles during the photodegradation were monitored by HPLC with UV/Vis detector. Toxicity tests were performed by means of an amperometric biosensor based on suspended yeast cells. Since the products obtained by the photodegradation process can result as toxic or more toxic than the original molecules, tests were performed first and after the photodegadation.
After 90 hours of exposure the concentration resulted lowered by 42.9%, 88.4% and 91% for Finasteride, Naproxen and Diclofenac respectively. Toxicity of the pure active principles follows the same order of the photostability. After photodegradation a contribute of the reaction products was evidenced.
The simple and cheap analytical procedure here proposed, allowed to obtain not only data on photostability and toxicity of the pure active principles but, even if roughly, also useful information on the reactions kinetic and toxicity of the photodegradation products.
From several years it is well known  that many compounds, including drugs, are photoreactive. In the past, the topic received scarce attention due to the incorrect belief that the packaging could protect the active principle so avoiding its photodegradation . This led to a limited knowledge of the phenomenon until relatively recent time. The European Pharmacopoeia  states that a drug must be opportunely protected from the exposure to the light in order to avoid a reduction of the active principle concentration with a consequent lost of efficiency. Photodegradation was then recognized as an important limiting factor of the shelf life of a drug that can be greatly influenced by its wrong storage and/or handling. Thus not only quality control laboratories, but also international organization (like WHO)  and pharmaceutical companies [5, 6] are interested in the development of a valid protection to preserve active principles from possible degradation. Degradation products could be more toxic than the starting reagents causing side effects on humans and producing unknown biological effects and new risks for ecosystems.
With the introduction of ICH guidelines  for carrying out photostability tests on drugs, researchers have deepened studies on the topic and several drugs were individually examined [8, 9]; on the contrary, comparative studies on different drugs are still scarce. Such comparison can allow to obtain a scale of stability/recalcitrance (environmental persistence) of similar or different pharmaceutical formulations useful for solving ecopharmacology problems  (studies of the interaction between the environmental compartments and drugs for human and animals, products for personal care, for hospital cleaning, disinfectants, antibacterials). Here, we present a research aiming to compare the photostability of three active principles chosen among the most consumed in Italy : Naproxen [12, 13], Diclofenac [14, 15] and Finasteride  (Additional file 1: Figure S1). The first two are well-known anti-inflammatory active principles while the third one is an inhibitor of 5-alpha-reductase (it inhibits the reaction of testosterone to dihydrotestosterone) and it is commonly used for the treatment of prostatic hypertrophy and androgenetic alopecia. Photodegradation tests were carried out using an irradiation source simulating sunlight (see Experimental paragraph). The photodegradation products were not determined because of their possible large number; we preferred to correlate their formation during the degradation with the toxicity trend. Really such data can be more useful since they take into account synergic effect of both the residual drug and all the photodegradation products. The active principles were determined by HPLC using UV/Vis detection. Absorption spectra of the pure molecules (Additional file 1: Figure S2) were recorded in order to set the detector for the chromatographic analysis at the most suitable wavelength. Active principles were subjected to photodegradation as pure molecules (mostly dissolved in water), and as solid pharmaceutical forms; in the last case, they were exposed inside the marketind pack as well as inside and outside the immediate pack, in order to evaluate eventual differences in the protection ability. Kinetic parameters for the photodegradation reaction were determined for the pure molecules dissolved in aqueous solution. The Integral Toxicity Index (before and after their exposure to the simulated sunlight) was obtained by means of an amperometric biosensor  based on suspended yeast cells.
Inaccuracy evaluated for the analysis of the three drugs
Declared concentration (mg)
Found concentration (mg) ± SD
Diclofenac sodium salt
51.8 ± 0.2
Naproxen sodium salt
225.5 ± 3.4
5.16 ± 0.07
In order to study the photodegradation kinetic and to optimize the chromatographic parameters we started with the analyses of the pure active principles. The choice of the initial concentration has followed the criterion to fall inside the linearity range of both the spectrophotometric and HPLC methods. On this base about 10-3 mol/l aqueous solution were used.
The retention time of Finasteride (Figure 1) is 5′ 25″. The chromatographic peak decreases enough slowly (about 50% at the end of the test) and no other detectable peaks at the fixed wavelength appear. This could means that the basic structure is destroyed during the photodegradation and the formed fragments, with absorption at different wavelength, are less recalcitrant so that they undergo photodegradation preferentially, compared to Finasteride.
In the case of Diclofenac (Figure 2), having a retention time of 6′ 30″, the chromatographic peak decreases enough quickly during the first 30 hours (about 60%) and then the process slows reaching a photodegradation percentage of about 85%. With a similar speed trend, peaks of the reaction products appear in the chromatogram at lower retention times (between 3 and 5 minutes). In particular the peak at 4′ 20″ increases up to reach the maximum after 30 hours of degradation, and then remain constant up the end of the experiment. This could indicates that, after 30 hours, this product also undergoes degradation and concentration remains almost constant as a consequence of a similar speed of formation from Diclofenac and photodegradation. More, the appearance of peaks with absorption at the same wavelength can mean that the basic structure remains intact in the first phase of the process, in agreement with its aromatic nature.
In comparison with the previous two, Naproxen (retention time of 4′ 45″) undergoes a very quick photodegradation (Figure 3); after 30 hours of irradiation its chromatographic peak becomes not detectable. A peak at retention time equal to 5′ 30″ quickly increases reaching its maximum after 15 hours of irradiation and then just as quickly decreases indicating that also its photodegradation starts. Other smaller peaks appears between the 2′ 50″ to 4′ 35″, also attributable to the formation of degradation products, they become significant after 20 hours and remained almost constant up the end of the test. The same considerations made for the Diclofenac may be valid for Naproxen, the largest number of peaks is consistent with its naphthalenic structure.
Since we chosen to use a simple and cheap analytical method to follow the photodegradation trend, we are unable to recognize the reaction products and to fully understand the kinetic pattern; anyway, taking into account the formula structures, the noticeable differences in the chromatograms allow us to make some assumptions.
Naproxen trend follows the same kinetic equations only during the first 30 hours of irradiation, probably, as above said, this must be ascribed to the simultaneous photodegradation of its degradation product. In the first part, the decay curve fits a first order equation with an enough good correlation coefficient (R2 = 0.9855); basing on it, the half-life time results equal to 10.0 hours.
Toxicity tests were carried out, first and after the photodegradation tests, using yeast cell and a Clark oxygen electrode (see Experimental paragraph).
Integral toxicity index of active pure principles before and after exposure to 90 h of simulated sunlight
Photodegradation time (h)
It = ΔI/ΔI0
3.26 ± 0.02
1.65 ± 0.02
3.46 ± 0.04
1.35 ± 0.03
3.45 ± 0.02
2.66 ± 0.01
4.77 ± 0.02
2.25 ± 0.02
3.06 ± 0.01
2.35 ± 0.02
4.06 ± 0.03
0.55 ± 0.01
Choice of the light source for the photodegradation
Using the PeakFit v4.12 software, the value 2.76.107 J/m2 was obtained for the area below the curve. It corresponds to the solar energy collected by the photovoltaic panel over 24 hours of an average day (completely sunny) of July and results very close to the one declared on the website of the General Directorate of the European Commission - Joint Research Centre (2.71.107 J/m2)  for a completely July sunny day in Rome. Once established that collected data are significant, the next step was the choice of the irradiance to be used for the tests . Basing on Figure 7, an energy equal to 600 W/m2 was chosen that corresponds to the minimum irradiation during the sunniest hours. In such conditions, at least 22 hours of irradiation are needed in order to comply with ICH guidelines (exposure to not less than 1.2.106 lux.h and 200 W.h/m2).
Basing on what above said, in order to simulate at the best the solar spectrum, the OSRAM ULTRA-VITALUX  Sun Lamp was chosen for the photostability tests. The lamp consists of a tungsten filament (IR-Vis range) and a mercury-vapor lamp (UV range), (Additional file 1: Figure S4). The distance of the source from the sample was optimized in order to provide a value of 600 W/m2 that was measured at each sampling time in order to control the lamp’s stability. Eventual sample degradation, due to heating caused by the lamp, was minimized by means of a cooling system providing a laminar air flow. In such conditions, temperature and relative humidity, both monitored by a Lascar Mod. EL-USB 2 data logger, remained almost constant during the tests with values equal to 20 ± 2°C and 42 ± 2% respectively. Experimental conditions adopted for the photostability tests are reported in Additional file 1: Table S1 of the EDF.
Integral toxicity test
An already widely applied method [22, 23] was used in order to determine the integral toxicity of both the considered active principles, and drugs containing them, first and after the photodegradation tests. The respiratory activity of Saccharomyces Cerevisiae suspended yeast cells was evaluated by a Clark oxygen sensor in presence and absence of the sample, an integral toxicity index was then calculated taking into account the difference.
Photostability tests were initially carried out on aqueous solutions of the three pure active principles in order to speed up the process and to obtain kinetic information; furthermore, in such conditions we can also have a rough evaluation of their persistence in natural water bodies where they can enter (really at concentration of several order lower than those we here tested) . Solutions were irradiated inside conical flasks of Pyrex England glass because its low absorption at wavelength higher than 320 nm (Additional file 1: Figure S5) complies with ICH guidelines for photostability testing. Irradiation was carried out for 90 hours and, at preset intervals, 0.5 ml aliquots were sampled and stored in the dark until the analysis.
20 tablets of each drug were used for the photostability test: 5 tablets were not radiated in order to use them as reference (blank), while 5 tablets were exposed inside the marketing pack (cardboard box), 5 tablets were exposed inside the primary pack, 5 tablets were exposed by removing all packaging, after a fine grinding and thin layering on glass plate.
Calculations for photostability and toxicity test
where Af and Ai are the final and initial areas of the chromatographic peaks registered for the active principle.
where Itf and Iti are the final and initial toxicity indexes, calculated as above said.
Even if simple and cheap, the proposed procedure can be a starting point to obtain useful information about the fate of drugs entering the environment as a consequence of the normal metabolic processes or bad wasting and so on; a more complex procedure (analyte enrichment ) is needed to take into account the very low concentration up today found there . On the contrary, the proposed procedure can be of help in optimizing the process conditions adopted in water treatment plants of pharmaceutical industries; in such case, the concentration is enough high and its decrease need to be speeded up using, as an example, heterogeneous photocatalysis through UV irradiation catalyzed by TiO2 that can be combined with visible and/or micro-waves irradiation [24, 25]. Also in these cases the coupling of the two analytical techniques, HPLC and toxicity tests, allows us to achieve the goal of obtaining data not only on the photostability but also on non-specific toxicity of the active ingredients and, though roughly, of the photodegradation products. A nonspecific toxicity test is surely more useful, at least for a first approach, than specific ones; in addition, the use of yeast cells respects ethical principles (no use of higher animals) and, especially, the toxic effects on these cells result very similar to those on human ones . The three considered pure active principles, in aqueous solutions, undergo to an enough quick photodegradation with estimated half-life times equal to about 10, 30 and 100 h for Naproxen, Diclofenac and Finasteride respectively. Regarding the drugs, considered that following the ICH guidelines, we posed in extreme conditions, data reveal that the excipients exert a high protective effect against the degradation of the active principle. Surely the marketing package ensure an almost complete protection while, for tests performed on the drug inside the primary pack, we cannot separate the contribute of the excipients from that of the blister but, anyway, especially for Naproxen, photodegradation occurs at a significant level. Such data can be surely useful to establish the drug’s “expiration date” but, as above said, our main goal was to also obtain toxicity data first and after photodegradation. The toxicity order for the three pure active principles results Diclofenac = Naproxen < Finasteride luckily inverse with respect to their photostability. For all the three considered drugs, the percentage toxicity reduction (ItR%) after the photodegradation resulted, at different extent, lower than the photodegradation percentage (PD%) so revealing a toxicity of the photodegradation products. In particular looking at the Figure 5, it could be stated that the toxicity of products coming from the photodegradation of Naproxen are less toxic than those from Finasteride that, in turn, are less toxic than those of Diclofenac but, really such order is not reliable since their different photostability surely plays a role in their toxicity.
What above said highlights the need for further research to obtain complete information, i.e., the use of more advanced analytical techniques, a more detailed time trend for the toxicity and so on.
Surely the topic is of great environmental interest because the drugs are pollutants that tend to increase in concentration, due to both the aging of the population and increase of industrial pollutants [27–29]. Their photostability plays an important role in contrasting their own pollution; as a fact, a low photostability let to a low environmental persistence but a complete toxicity abatement can be attained only after their mineralization . In our tests, may be due to their not adequate irradiation time, the solar light resulted not completely suitable can be so useful in all the cases of unintentional leakage of drugs while inside water treatment plants the process is controlled and speeded up using, as an example, heterogeneous photocatalysis, by TiO2, combined with UV and micro-waves irradiation [24, 25]. Also in these cases the proposed procedure can be of help in optimizing the process conditions.
Chemicals and drugs
Acetonitrile (99.9%), water plus (distilled water filtered at 2 μm) and methyl alcohol (99.9%), all of HPLC grade, monobasic potassium phosphate (99.0%), orthophosphoric acid (98%) from Carlo Erba Reagenti (Milan, Italy).
Finasteride (N-tert-Butyl-3-oxo-4-aza-5α-androst-1-en-17β-carboxamide; ≥98%), Diclofenac Sodium Salt (2-[(2,6-Dichlorophenyl)amino]benzeneacetic acid sodium salt; ≥98%), Naproxen Sodium Salt ((S)-6-Methoxy-α-methyl-2-naphthaleneacetic acid sodium salt; ≥98%) from Sigma-Aldrich (Milan, Italy).
Momendol 220 tablets containing 220 mg of Naproxen Sodium Salt (Aziende Chimiche Riunite SpA Francesco Angelini, Italy), Prostide, tablets containing 5 mg of Finasteride (Sigma-Tau Industrie Farmaceutiche Riunite SpA, Italy) and Voltaren 50, tablets containing 50 mg of Diclofenac Sodium Salt (Novartis Farma SpA, Italy) were purchased from several pharmacies in Rome, Italy.
All the three drugs are sold in cardboard boxes. The immediate pack of Prostide and Momendol consists of opaque white plastic, while the one of Voltaren consists of transparent colourless plastic. All tablets are covered by a polysaccharide film. The complete compositions are listed in Additional file 1: Table S2 of the EDF.
All pure active principles were weighed by means of an analytical balance (0.01 mg resolution) and solutions were prepared using glass flasks of Class A grade. Diclofenac and Naproxen were dissolved in ultrapure water, while for Finasteride a 1:20 CH3CN/H2O mixture was used because not enough soluble in water.
Tablets of drugs were finely ground in a porcelain mortar and quantitatively racked in a centrifuge tube together with about 6 ml of solvent; the centrifuge tubes were sonicated for 5 minutes and then centrifuged at 10,000 rpm for 10 minutes in order to extract the active principle; supernatants collected from three successive extractions were brought to volume in a suitable flask and analysed by HPLC.
The HPLC equipment consists of a Kontron pump mod. 422, with head pump mod. 420, a Rheodyne valve injection mod. 7125, a 20 μl loop, a Kontron UV/VIS detector mod. 430 connected to a data logger (Pico Technology - Mod. Dr. Daq), a PC equipped with the softwares PicoLog v5.16.2 and PeakFit v4.12. An Alltech reversed phase column (mod. Alltima C8 250 x 4.6 mm I.D.) filled with particles of 5 μm of diameter was used as stationary phase while a mixture of Acetonitrile/Water 95:5 was used as eluent for Finasteride and a mixture of pH 3.3 phosphate buffer/Acetonitrile/Methyl alcohol 30:35:35 for Diclofenac and Naproxen.
A Perkin-Elmer mod. Lambda 16 spectrophotometer was used to select the wavelength to be set for the HPLC detector.
Temperature was maintained at 25 ± 1°C using a Alltech 330 Column Heater thermostatic bath in which the column was immersed.
A radiometer/luxmeter Gossen - model Mavolux Digital was used for the measures of irradiance, illuminance and radiant energy while temperature and humidity were monitored by a data logger Lascar - model EL-USB 3.
The equipment for the Toxicity test consists of a glass thermostated cell; a Clark oxygen electrode Orion 97–08 (Thermo Electron Corporation); an Orion Microprocessor Ionalyzer/901 potentiometer; an Amel Model 868 recorder. Saccharomyces cerevisie yeast cells were used as biological mediator.
The wavelength for the HPLC’s UV detector was chosen basing on the UV/Vis absorption spectra of the pure active principles (Additional file 1: Figure S2): 205 nm for Finasteride, 275 nm for Diclofenac and 262 nm for Naproxen. Spectra were acquired in the suitable solvents i.e. the one used as mobile phases for the HPLC analyses. Calibration curves were built not only to follow the photodegradation, but also to test the linear range for each active principle; basing on their lower value, we choose the initial concentration of the pure active principles to be subjected to photodegradation; the criterion was that, after a 90% of photodegradation, the residual concentration had to be appreciated. Additional file 1: Table S3 of EDF summarizes the experimental condition adopted for the analysis as well as the main analytical parameters of the calibration curves obtained for the three active principles.
- Pasteur L: Transformation of the two tartaric acids into racemic acid. Discovery of inactive tartaric acid. New method of separating racemic acid into dextro- and laevo- tartaric acid. Comp Rend. 1853, 37: 110-Google Scholar
- Albini A, Fasani E: Drugs Photochemistry and Photostability. 1998, Cambridge: The Royal Society of Chemistry, ISBN: 9780854047437Google Scholar
- European Directorate for the Quality of Medicines (EDQM): The European Pharmacopoeia. 2008, Strasbourg: BDC Ed., ISBN: 9789287160546, 6Google Scholar
- WHO: Pharmaceutical packaging – an overview including some considerations for paediatrics. http://apps.who.int/prequal/trainingresources/pq_pres/workshop_China2010/english/22/003-PharmaceuticalPackaging.pdf,
- Bhat J: Role of packaging material on Pharmaceutical product stability. 2009, http://www.ipapharma.org/events/stability/jbhat.pdf,Google Scholar
- Van Weeren R, Sashidharan A: Determining drug Stability and selecting packaging. Pharmaceut Med Packaging News. 2007, 15 (9):Google Scholar
- International Conference on Harmonisation of Technical Requirements for Registration of Pharmaceuticals for Human Use (ICH): Topic Q1B - Photostability testing of new active substances and medicinal products - CPMP/ICH/279/95. ICH Guidelines. 1996, http://www.ich.org/fileadmin/Public_Web_Site/ICH_Products/Guidelines/Quality/Q1B/Step4/Q1B_Guideline.pdf,Google Scholar
- Tønnesen HH: Photostability of Drugs and Drug Formulations. 2004, Boca Raton, Florida: CRC Press LLC, ISBN 0415303230, 2View ArticleGoogle Scholar
- Bajaj S, Singla D, Sakhuja N: Stability Testing of Pharmaceutical Products. J Appl Pharmaceut Sci. 2012, 02 (03): 129-138.Google Scholar
- Sammartino MP, Bellanti F, Castrucci M, Ruiu D, Visco G, Zoccarato T: Ecopharmacology: deliberated or casual dispersion of pharmaceutical principles, phytosanitary, personal health care and veterinary products in environment needs a multivariate analysis or expert systems for the control, the measure and the remediation. Microchem J. 2008, 88 (2): 201-209. 10.1016/j.microc.2007.11.021.View ArticleGoogle Scholar
- OsMed (Osservatorio Nazionale sull’Impiego dei Medicinali): L’uso dei Farmaci in Italia – Rapporto Nazionale anno 2005 (the drugs consume in Italy – national relationship for year 2005). 2006, Rome: Tipografia Quattroventi Editor, 83-89. ISBN 9788849001372Google Scholar
- Tixier C, Singer HP, Oellers S, Müller SR: Occurrence and fate of carbamazepine, clofibric acid, diclofenac, ibuprofen, ketoprofen, and naproxen in surface waters. Environ Sci Technol. 2003, 37 (6): 1061-1068. 10.1021/es025834r.View ArticleGoogle Scholar
- DellaGreca M, Brigante M, Isidori M, Nardelli A, Previtera L, Rubino M, Temussi F: Phototransformation and ecotoxicity of the drug Naproxen-Na. Environ Chem Lett. 2003, 1 (4): 237-241. 10.1007/s10311-003-0045-4.View ArticleGoogle Scholar
- Eriksson J, Svanfelt J, Kronberg L: A photochemical study of diclofenac and its major transformation products. Photochem Photobiol. 2010, 86 (3): 528-532. 10.1111/j.1751-1097.2009.00703.x.View ArticleGoogle Scholar
- Salgado R, Pereira VJ, Carvalho G, Soeiro R, Gaffney V, Almeida C, Vale Cardoso V, Ferreira E, Benoliel MJ, Ternes TA, Oehmen A, Reis MA, Noronha JP: Photodegradation kinetics and transformation products of ketoprofen, diclofenac and atenolol in pure water and treated wastewater. J Hazard Mater. 2013, 244–245: 516-527.View ArticleGoogle Scholar
- Syed AA, Amshumali MK: LC determination of finasteride and its application to storage stability studies. J Pharm Biomed Anal. 2001, 25 (5–6): 1015-1019.View ArticleGoogle Scholar
- Coulet PR, Blum LJ: Biosensor principles and applications. 1991, New York: Marcel Dekker Inc, ISBN 9780824785468Google Scholar
- Directive. 2001, http://www.emea.europa.eu/docs/en_GB/document_library/Regulatory_and_procedural_guideline/2009/10/WC500004481.pdf, /83/ec of the European Parliament and of the council of 6 November 2001 on the Community code relating to medicinal products for human use,
- Photovoltaic geographical information system of general directorate of the european commission - Joint Research Centre: http://re.jrc.ec.europa.eu/pvgis/apps4/pvest.php,
- Piechocki JT: Selecting the right source for pharmaceutical photostability testing. Drugs: Photochemistry and Photostability. Edited by: Albini A, Fasani E. 1998, Cambridge: The Royal Society of Chemistry, 247-271. ISBN: 9780854047437View ArticleGoogle Scholar
- Osram Website, Osram ULTRA-VITALUX sunlight lamp technical data sheet. http://www.osram.ch/osram_ch/FR/Outils_et_Services/Documentation_a_telecharger/Magazines_clients/Newsletter/TRADE_Newsletter/pdf/Decembre_2010/105S001GB_DB_Ultra_Vitalux_UV1.pdf,
- Campanella L, Sammartino MP, Dragone R, Tromboni R: Test respirometrico e controllo dell’inquinamento. Inquinamento. 2005, 73: 64-67.Google Scholar
- Campanella L, Favero G, Tomassetti M: Immobilised yeast cells biosensor for total toxicity testing. Sci Total Environ. 1995, 171 (1–3): 227-234.View ArticleGoogle Scholar
- Campanella L, Cresti R, Sammartino MP, Visco G: Microwave assisted photodegradation of pollutants. Proc. SPIE 3534, Environmental Monitoring and Remediation Technologies (Boston, MA, 1998). 1999, Boston: The International Society for Optical Engineering, 105-113.Google Scholar
- Visco G, Bellanti F, Castrucci M, Costanza C, Campanella L: Heterogeneous photocatalysis degradation of wasted drugs in water matrix by combined UV/Vis/Microwave. 2005, Bologna-Rimini, Italy: X EuCheMS-DCE International Conference on Chemistry and the Environment, 4-7.Google Scholar
- Dragone R, Frazzoli C, Grappelli C, Campanella L: A new respirometric endpoint-based biosensor to assess the relative toxicity of chemicals on immobilized human cells. Ecotox Environ Safe. 2009, 72 (1): 273-279. 10.1016/j.ecoenv.2008.02.011.View ArticleGoogle Scholar
- Zuccato E, Calamari D, Natangelo M, Fanelli R: Presence of therapeutic drugs in the environment. Lancet. 2000, 355 (9217): 1789-1790. 10.1016/S0140-6736(00)02270-4.View ArticleGoogle Scholar
- Ternes TA: Occurrence of drugs in german sewage treatment plants and rivers. Water Res. 1998, 32 (11): 3245-3260. 10.1016/S0043-1354(98)00099-2.View ArticleGoogle Scholar
- Hignite C, Azarnoff DL: Drugs and drug metabolites as environmental contaminants: Chlorophenoxyisobutyrate and salicylic acid in sewage water effluent. Life Sci. 1997, 20 (2): 337-341.View ArticleGoogle Scholar
- Rittmann BE: Definition, objectives, and evaluation of natural attenuation. Biodegradation. 2004, 15 (6): 349-357.View ArticleGoogle Scholar
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