Analysis of environmental biodegradability of cellulose-based pharmaceutical excipients in aqueous media

Pharmaceutical cellulosic polymers will inevitably reach natural water systems if they are not removed after entering wastewater. Biodegradation of organic chemicals in sewage or in the aquatic environment is an important removal mechanism. In this study, we investigated the environmental biodegradation of 14 cellulose derivatives commonly utilized as pharmaceutical excipients using three different test systems that are based on the closed bottle test (OECD 301D) and the manometric respirometry test (OECD 301F). For the different cellulose derivatives tested, we observed varying degrees of biodegradation ranging from 0 to 20.4 % chemical oxygen demand (COD). However, none met the criteria for classification as 'readily biodegradable'. In addition, 10 out of 14 cellulose derivatives and/or their possible transformation products formed during the experiments, may exhibit possible toxic inhibitory effects on the inoculum. This includes one or several derivatives of hydroxy propyl methyl cellulose, hydroxy propyl cellulose, methyl cellulose, ethyl cellulose, and hydroxy ethyl cellulose. Based on the results obtained, we have developed a graded classification score ('traffic light system') for excipient biodegradation. This could help streamline the assessment and classification of cellulose derivatives concerning risk of persistence and potential adverse environmental effects, thereby assisting in the prioritization of more favorable compounds. In the long term, however, excipients should be designed from the very beginning to be biodegradable and mineralizable in the environment ('benign by design').

Multi-laboratory evaluation of the reproducibility of polymer biodegradation assessments applying standardized and modified respirometry methods

This research evaluated the intra- and interlaboratory variability when applying OECD 301F and OECD 301B Ready Biodegradation respirometric test methods to quantify polymer biodegradation as well as the impact of method modifications including test duration, inoculum level and test substance concentration on results. This assessment synthesizes results of mineralization studies on 5 polymers of varying structural components, molecular weight, charge, and solubility, evaluated at 8 different laboratories in 4 different countries, providing significant geographic variation in inoculum source as well as lab to lab variations in test setup. Across all laboratories, intralaboratory variability was low (≤18 % absolute difference) indicating the reproducibility of results between replicates and uniformity of test setup in each laboratory. Interlaboratory variation was also low for all 5 polymers with extent of mineralization being comparable in all OECD 301F and 301B studies even when test methods were modified. Across all studies mean mineralization was 89 ± 5.5 % for polyethylene glycol 35,000, 85 ± 7.4 % for polyvinyl alcohol 18-88, 44 ± 13 % for carboxymethyl cellulose (DS 0.6), 48 ± 4.1 % for a modified guar gum, and 88 ± 6.2 % for microcrystalline cellulose (MCC) at study completion. Due to the lack of polymeric reference materials, MCC was evaluated and found to be a suitable reference material for polymers that biodegrade rapidly in screening studies. An additional respirometric study was conducted quantifying mineralization of the 5 polymers in river water to evaluate the relationship with OECD 301 results using activated sludge as the inoculum. A similar extent of mineralization was observed for all 5 polymers in the OECD 301 and river water studies but time to reach the maximum extent of mineralization was longer using river water as the inoculum source likely due to the lower microbial counts (106 CFU/L) in the test system.

Water soluble polymer biodegradation evaluation using standard and experimental methods

Multiple polyethylene glycol (PEG) polymers ranging in molecular weight (MW) from 4000 to 500,000 Da, polyvinyl alcohol (PVOH) polymers with degrees of hydrolysis (DH) of 79 % and 88 % and MW 10,000 to 130,000 Da, and carboxy methyl cellulose (CMC) polymers with degrees of substitution (DS) ranging from 0.6 to 1.2 were evaluated in standard screening biodegradation tests to assess method limitations, modification potential, and reproducibility. All PEGs and PVOHs mineralized completely in OECD 301B and 302B studies reaching >80 % biodegradation with negligible dissolved organic carbon remaining at study completion. For high MW PEOs, extension of test duration was needed to reach full extent of mineralization. CMC biodegradation was directly correlated to degree of substitution with CMC 0.6 biodegrading extensively, CMC 0.79 partially biodegrading, and CMC 1.2 not biodegrading significantly in OECD 301B and 302B studies. For all materials tested in both an OECD 301B and 302B, fewer days were necessary to reach 60 % biodegradation in the OECD 302B indicating increased rates of biodegradation with higher inoculum to test chemical ratios. In a series of investigative studies using respirometry as the analytical endpoint, significant variability in the presence of competent degraders in small volume grab samples of river water was observed. Research is needed to overcome this variability and develop a standardized reproducible test method to accurately assess polymer mineralization in river water. At study completion, residual dissolved organic carbon (DOC) data confirmed respirometry data, high levels of mineralization resulted in negligible residual DOC while low levels of mineralization resulted in significant residual DOC, up to dose concentrations. DOC measurements provided confirmation of complete biodegradation when biomass incorporation and test system set up resulted in variable carbon dioxide production or oxygen demand.

Designing benign molecules: The influence of O-acetylated glucosamine-substituents on the environmental biodegradability of fluoroquinolones

Antibiotics are detected worldwide in the aquatic environment, with continuously rising concentrations. Antibiotics in the environment have the potential to damage ecosystems and contribute to the development of resistance. Whilst a few antibiotics, such as some β-lactams, are eliminated by effluent treatment, others, such as fluoroquinolones, are not or just partially removed and enter the environment. Therefore, approaches are needed to tackle those problems at the compound level. Benign by design (BbD), an important part of green pharmacy, has the goal to integrate environmental fate and end-of-use considerations at the very beginning, i.e., into the design of active pharmaceutical ingredients. Hence, pharmaceuticals should be designed to be sufficiently active and stable during storage and usage but should degrade after excretion into the environment, so that they cannot cause any adverse effects. Fluoroquinolones (FQs) are important broad-spectrum antibiotics. They are known to be persistent in the environment and to be neither inactivated nor degraded or even mineralized during sewage treatment. The addition of new substituents via amidation, like glucosamine moieties, at the carboxylic group of FQs, led to better antimicrobial activity compared to its parent compounds against various microorganisms. To investigate if the addition of sugar moieties could improve the overall environmental biodegradability of FQs, eight novel quinolone and fluoroquinolone analogs conjugated with 1,3,4,6-Tetra-O-acetyl-β-d-glucosamine and 2-deoxy-d-glucopyranose have been investigated regarding their ready biodegradability (OECD 301D/F) and their degradation pathways have been analyzed. According to the OECD 301D test, none of the substances could be classified as readily biodegradable. However, the O-acetyl analogs did undergo a partial degradation of the O-acetyl glucosamine moiety, via stepwise deacetylation and the degradation of the whole glucosamine moiety. The degradation resulted in Fluoroquinolone-3-carboxamide derivatives. Those insights could be further used as input for fragment-based design of benign APIs that will degrade once they reached the environment.

Classification and Regression Machine Learning Models for Predicting Aerobic Ready and Inherent Biodegradation of Organic Chemicals in Water

Machine learning (ML) is viewed as a promising tool for the prediction of aerobic biodegradation, one of the most important elimination pathways of organic chemicals from the environment. However, available models only have small datasets (<3200 records), make binary classification predictions, evaluate ready biodegradability, and do not incorporate experimental conditions (e.g., system setup and reaction time). This study addressed all these limitations by first compiling a large database of 12,750 records, considering both ready and inherent biodegradation under different conditions, and then developing regression and classification models using different chemical representations and ML algorithms. The best regression model (R² = 0.54 and root mean square error of 0.25) and classification model (the prediction accuracy from 85.1%) achieved very good performance. The model interpretation indicated that the models correctly captured the effects of chemical substructures, following the order of C═O > O═C-O > OH > CH₃ > halogen > branching > N > 6-member ring. The consideration of chemical speciation based on pK_a and α notations did not affect the regression model performance but significantly improved the classification model performance (the accuracy increased to 87.6%). The models also showed large applicability domains and provided reasonable predictions for more than 98% of over 850,000 environmentally relevant chemicals in the Distributed Structure-Searchable Toxicity database. These robust, trustable models were finally made widely accessible through two free online predictors with graphical user interface.

Enhanced ready biodegradability screening tests for the evaluation of potential PBT substances

In persistence assessment enhanced ready biodegradation tests (eRBT) are aimed to close the gap between screening tests and complex simulation tests. However, only few data from these tests are available and neither guidance on the design and interpretation of eRBTs, nor suitable validity criteria have been established so far. In a practical testing programme 5 compounds with controversial degradation data have been tested in 4 test series including prolongation to 60 days and use of different inocula (activated sludge, final effluent from a STP), flask sizes, and endpoints (CO₂, O₂, DOC). The drug ibuprofen and the intermediate 4-fluorophenol were biodegraded by >60% within 28 days within a 10-day-window and therefore are considered as readily biodegradable and in conclusion fulfilling the criteria for "not persistent". The mean mineralization of the pesticide synergist piperonylbutoxide and the antioxidant octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate achieved 20%-50% (="potentially P"). The mineralization of the cosmetic ingredient cis-13-docosenonamide (Erucamide) was between 36%-64% after 60 days with activated sludge and 21% with the effluent from the STP. Diethylene glycol reached the pass level of 60% mineralization within 28 days in all test series without always meeting the 10-day window, and thus proved to be a suitable reference substance for eRBTs. Based on the results of the study several recommendations for the test design, the evaluation and the interpretation of eRBTs are made. However, a broader data set is required and further enhancements such as the quality and amount of the inoculum should also be considered in future research.

Inventory of biodegradation data of ionic liquids

Ionic liquids (ILs) are increasingly of interest for environmentally open applications. Therefore, completely mineralising ILs are highly desirable. We reviewed the current state of knowledge on ILs' environmental biodegradability and identified research needs. Literature data were evaluated as for applied standard methods (e.g. OECD, ISO, APHA) for biodegradation of ILs in order to get an overview on the validity of the test results received and ILs' biodegradability. 109 studies were evaluated. The ILs were categorised based on the cation's core structure. The biodegradation data was classified according to a traffic light system (red: 0-19% degradation, amber: 20-59% degradation, green: ≥ 60% degradation). Not all studies could be assessed for compliance with the test guidelines due to missing test parameters. Moreover, no study discussed all validation criteria as defined by the test guidelines. Consequently, the reliability and quality of the existing biodegradation data is restrained. With regard to the different cations classified for ≥ 60% biodegradability, phosphonium ILs are the least biodegradable, followed by imidazolium ones. The most ILs that were biodegradable are cholinium ILs. The results indicate the need for more and qualitatively better testing according to standard methods including application and reporting of all validation criteria in order to get reliable data that enables the comparison of the test data and a comprehensive understanding of ILs' biodegradability. Moreover, reliable data allows the selection of sufficiently environmentally biodegradable ILs if an introduction into the environment during use cannot be excluded.

Establishing a ready biodegradability test system using OxiTop® to evaluate chemical fate in a realistic environment

The purpose of this study is to propose the use of OxiTop® for measuring biochemical oxygen demand (BOD) under the Japanese Chemical Substances Control Law in order to properly evaluate chemical fate in a real environment. In our previous study, the biodegradation of test chemicals was accelerated by both adsorbing the chemical to silica gel with chloroform and increasing the medium volume from 300 to 3900 mL in the OECD 301F test using a coulometer. However, the biodegradability of these chemicals could not be evaluated based on BOD due to chloroform residue in the silica gel, or the medium volume could not be increased further due to the oven size of the coulometer. In this study, we established an evaluation system using OxiTop® based on BOD by increasing the medium volume to 9000 mL. Based on triplicate testing, increasing the medium volume accelerated biodegradation and decreased variation in BOD.

Investigation of OECD 301F ready biodegradability test to evaluate chemical fate in a realistic environment

The OECD 301F ready biodegradability test has been approved for use under the Japanese Chemical Substances Control Law since 2018. This test uses activated sludge obtained from a sewage treatment plant instead of the standard activated sludge used for the 301C test. In addition, the test is allowed to add an inert support or emulsifying agent, and/or to change the volume of the test medium. In this study, we first confirmed that the standard sludge had lower biodegradation activities than the sludge taken from a sewage treatment plant. Second, we showed that biodegradation percentages were increased by adding suitable amounts of silica gel or Tween 80. Third, we found that the biodegradations were accelerated by only increasing the medium volume under the conditions that concentrations of chemical, silica gel, and sludge were held constant. These findings are expected to contribute to the appropriate evaluation of chemical fate in a realistic environment.

Assessing the suitability of a manometric test system for determining the biodegradability of volatile hydrocarbons

Manometric test systems, adapted from those used to measure biochemical oxygen demand (BOD), and the OxiTop^® test system in particular, are being increasingly used to determine the biodegradability of chemicals in accordance to OECD 301F guidelines. In this study, the suitability of the OxiTop^® test system for determining the biodegradability of volatile hydrophobic substances has been explored. Experiments in biotic and abiotic systems were conducted with readily biodegradable complex aliphatic hydrocarbons covering a range of volatilities. Results indicated that abiotic losses of test substances were occurring due to sorption of the test substance to plastic components used in the OxiTop^® system. A further 'plastic-free' biodegradation test system was designed using PreSens optical dissolved oxygen (DO) sensors. This significantly improved the measured biodegradation due to reduced abiotic losses and better retention of the test substance. These results highlight the importance of considering the physico-chemical properties of test substances when selecting test methods and equipment. They also highlight the value of incorporating chemical analysis and abiotic controls to improve the interpretation of biodegradation studies.