A Review of Wastewater-Based Epidemiology Studies for the Assessment of Over-the-Counter Medicines Used as Recreational Drugs: The Example of Dextromethorphan

Introduction

Along with increasing access to information sources via the Internet, the public awareness of self-medication is also growing. Nevertheless, it can be a source of social inequality in healthcare. Properly conducted self-treatment (eg, as part of pharmaceutical care) can help reduce the number of medical visits, hospitalizations, and visits to a hospital emergency department, and contribute to rapid elimination of disease before the development of full symptoms, in accordance with the principle of disease prevention [1]. Among the problems related to self-medication before a medical visit, there may be unequal access to information and a lack of ability to acquire professional medical knowledge about medicinal products, like difficulties distinguishing knowledge from advertising over-the-counter (OTC) products and the social conviction that OTC drugs are safer than prescription drugs [2]. Other issues that can lead to incorrect medication use by patients include potential drug interactions, combining several drugs from the same group of chemical substances or even containing the same substance but different trade names that can lead to overdose, and in extreme cases, the willingness to use drugs to induce narcotic or euphoric effects [1]. These issues entail the necessity of legal regulations on access to OTC drugs, and show that pharmacists are particularly important in proper self-medication.

In Poland, the law regulates substances with a psychoactive effect, including pseudoephedrine (pseudoephedrinum), codeine (codeinum), and dextromethorphan (DXM, dextromethorphanum), and the maximum content in a medicinal product, which is a limitation in the dispensing of medicinal products as part of a one-off sale [3]. Unfortunately, this does not exclude the possibility of access to products in several places by the patient for potential non-medicinal use. Nowadays, we can observe a ‘recreational use’ of some drugs, such as antitussive drugs containing DXM. ‘Robotripping’ is overdosing on DXM, which can cause symptoms such as a toxidrome of psychomotor agitation, hallucinations, and paranoia [4], and can cause death [5,6].

The development of modern, selective analytical techniques and more sensitive methods gives a chance to monitor the substances at trace levels, such as in wastewater. The information about the concentration might be further used to estimate the consumption of an analyte of interest, ensuring additional, evidence-based information complementary to epidemiological studies and population interviews or surveys. The estimation of consumption of drugs of abuse based on wastewater analysis is well established [7–9], and these methods could be successfully extended, including new analytes. Being aware of the numerous dangers and social harm of misuse of selected medicines, including DXM, this review aims to: (A) report available methods of determining the DXM and its transformation products (TPs) in wastewater, (B) discuss their usefulness as an effective tool for monitoring drugs and assessing drug consumption patterns, (C) take the first steps toward showing the scale of the problem, and (D) start a social discussion about proper health education and access to OTC drugs.

The Basics of Wastewater Analysis

In the 1970s, an environmental drug testing study by Garrison et al described the presence of clofibric acid in the environment [10]. However, it was only in the late 1990s that articles appeared that holistically discussed the complexity of problems that arise when trying to detect pharmaceuticals in the environment, at the same time pointing to drugs as an important factor environmental pollutant with biologically active substances [11,12]. In 1995 Zuccato et al performed a study that for the first time measured levels of an illicit drug in an aquatic environment [13].

The methodology is based on the assumption that a substance of interest enters the body (eg, eaten, inhaled, injected) and is metabolized and excreted. Knowing the metabolic pathways, the substance can be detected (unchanged or as metabolites) and measured in wastewater. An analytical sample is a very dilute sample of compounds excreted from the human body. Thus, collecting an average sample from a given period of time from a sewage treatment plant serving a given population of inhabitants makes it possible to perform appropriate back-calculations to estimate the amount and number of substances consumed by a given population [14].

The Main Methods of Wastewater Analysis

Since wastewater is a complex matrix, many approaches have been proposed for wastewater-based epidemiology (WBE) studies to search for markers of the presence of chemical compounds in water. The methods are strictly related to the chemical composition of the analyte of interest. Among the most common methods of analysis of drugs in sewage is the liquid chromatography-mass spectrometry (LC-MS) method. It combines the advantages of the selectivity of the LC method and the sensitivity and selectivity of the mass spectrometry method [15]. Thanks to the possibility of using different types of chromatographic column fillings, it allows the separation of a spectrum of compounds with different chemical properties, but it is most useful in the analysis of polar and non-volatile compounds. An extension of the LC-MS method is the LC-MS/MS (liquid chromatography – tandem mass spectrometry) method, which additionally provides even higher sensitivity and selectivity thanks to the possibility of selecting the precursor ion in the first analyzer and the fragment ion in the second one. The triple quadrupole tandem mass spectrometry (QqQ) method [16,17] and a hybrid quadrupole time-of-flight mass spectrometry (Q-TOF) method [18–21] are often the method of choice for determining markers of consumption of selected compounds in WBE studies. As an alternative to the LC-MS method, the gas chromatography–mass spectrometry (GC-MS) method can be used to search for selected analytes, providing similar selectivity, but it is intended for volatile and non-polar compounds. This makes it possible to use this method in WBE research, with satisfactory results. Various drugs, including dextromethorphan [22,23], narcotics [24], and xenobiotics [25], were determined in this way.

Some electrochemical detection methods have also been successfully applied for illicit drug detection [26]. These methods are usually based on an electrochemical reaction between the electrode and the compound of interest, which frequently occurs in ionic form and can cause an electrode reaction on the electrode surface. Such a simple mechanism of detection results in simplicity, rapid detection, high sensitivity, and low cost of application of electrochemical strategies in assessment of drug abuse [15].

In addition to methods combining separation techniques with mass spectrometry, optical methods also are used for WBE research. Among them, surface-enhanced Raman spectroscopy (SERS) can detect antibiotics, pigments, estrogens, organic compounds, heavy metal ions, pathogens, substituted aromatics, explosives, and pesticides, ensuring sensitivity, time efficiency, and related structural information [27]. This technique was also used to detect addictive substances (opioid drugs, fentanyl), which have been increasingly abused in recent years [28,29].

Most recently, it was proven that WBE can be used to assess circulation of viral pathogens in a population through the use of polymerase chain reaction (PCR) for genetic material analysis [30], reverse transcription-quantitative polymerase chain reaction (RT-qPCR) technology, reverse transcription droplet digital PCR (RT-ddPCR), and chip-based biosensors [31]. Among them, RT-qPCR was the most commonly used method to detect viral RNA in wastewater samples [32].

Previous Reviews and Studies from Other Countries on WBE

As mentioned earlier, the idea of testing drug content based on wastewater analysis goes back 50 years, as evidenced by the numerous research papers cited earlier. It has contributed to establishing WBE studies as a complementary, comprehensive, cost-effective, and rapid way to monitor analytes of interest in a general population [33].

Numerous studies have shown the reliability of this type of approach. Various countries around the world are becoming involved in conducting WBE research, including the United States [34], Canada [35], Sweden [36], the United Kingdom [37], Switzerland [38], Poland [7], China [39], Australia [40,41], and many more [42,43], and between-country studies have also been performed [44–47].

WBE-based research is in line with existing data on drug prevalence from international organizations such as the European Monitoring Center for Drugs and Drug Addiction (EMCDDA) and the United Nations Office on Drugs and Crime (UNODC) [48]. The significant role in forming social policy and addiction-preventing action is confirmed by the fact that since 2011, the Sewage Analysis CORe Group-Europe (SCORE) has coordinated a study in European cities; to date, over 100 European cities and towns have been involved in the study [49].

Dextromethorphan

Dextromethorphan (DXM) is an effective cough suppressant widely used in cough and cold medications. A drug is generally considered safe, especially when administered as indicated by medical personnel and in therapeutic doses [50,51]. The drug’s mode of action is complex, affecting a variety of channels and receptors. It is postulated that the antitussive effect is a consequence of its sigma-1 receptors (σ1R) stimulation and N-methyl-d-aspartate (NMDA) antagonism. However, in higher doses, it can cause serotonin release stimulation and its reuptake inhibition, and affects σ1 opioid, α3β4 nicotinic, and NMDA receptors. Thus, the complexity of the action may result in a dose-dependent change in the mechanism of action, as observed in cases of intoxication [52]. A stimulant effect can occur, followed by mild hallucinations. Increasing the administered dose can result dysphoria and increase sensory experience. Another increase in the dosage can result in visual hallucinations and losing the sense of distance. The highest doses can cause an out-of-body experience, strong anesthesia, and/or death [53,54].

Examples of OTC Drugs That Are Used Recreationally

Many medicines have a complex mechanism of action and can have effects different than therapeutic effects if overdosed. This, combined with easy access to information on the Internet, might have contributed to the widespread phenomenon of recreational use of the drug for nontherapeutic, psychoactive effects. The term “recreational drug use” refers to both legal and illegal substances that are administered without medical justification, but rather to alleviate stress and to enhance enjoyment and pleasure to induce a heightened, euphoric, or altered state of consciousness. Such non-medical use of drugs is related to the possibility of inducing narcotic-like effects using substances available for medicinal purposes (usually legal) [55]. It must be emphasized that such an action is drug misuse. The complexity of the issue of drug misuse is also associated with diversification in the availability of products (for prescription or OTC) in different countries. Nevertheless, in many countries, there is a phenomenon of increased recreational use of drugs, ignoring indications for administration of the drug [2,56,57]. This applies not only to DXM but also other drugs, including pseudoephedrine, codeine, and benzydamine [3,58]. However, DXM is the most reported misused drug [59].

Recreational use of medicines can even cause death, including deaths related to abuse/overdose of commercially available chemical substances that can be obtained from medicines. There are reports describing a connection between death and a history of medicine abuse, including DXM [5,60].

Analytical Methods for Detecting Dextromethorphan and/or Its Transformation Products in Environmental Water Samples

THE ROLE OF QUALITATIVE AND QUANTITATIVE RESEARCH FOR WBE STUDIES:

To the best of our knowledge, the first work that reported detection of DXM or its TPs in surface water was presented in 2012 by Thurman et al [20]. Their comprehensive study demonstrated that applying the LC/Q-TOF-MS (liquid chromatography-hybrid quadrupole time-of-flight mass spectrometry) technique enables detection and identification of dextromethorphan and its major TPs in urine, water samples, and wastewater.

Compared to other drugs and substances used to induce intoxication, which have been widely discussed in the literature in terms of both assay methods in various biological matrices and their metabolism and appearance in environmental samples (WBE studies), only a few studies have focused on detection of DXM and/or its TPs in environmental samples (Table 1). In WBE studies, quantitative research is of particular importance. Still, qualitative research may be equally important for understanding the pathways of compound transformation and a full understanding of substance metabolism in the human body and in the environment.

Some of the studies also indicated potential pathways for the metabolism or transformation of DXM or its TPs under specific conditions [17,20,61]. Jewell et al found that cleavage of the glucuronide-conjugate or demethylation of the dextromethorphan can increase the concentration of dextrorphan [17]. These works are important for the reliable estimation of concentrations in real-world samples. Differences in human metabolism might result in slightly different transformations of DXM in the human body. The part of the population defined as poor metabolizers, the unchanged form of DXM will dominate in the system. Therefore, we emphasize the importance of testing the broadest possible spectrum of compounds, from the prodrug to their transformation products.

Our review presents the available studies on the determination of DXM and/or its transformation products in wastewater found in PubMed (Table 1) in both quantitative and qualitative approaches. In the 7 selected studies, the analytes were studied in a quantitative manner [16–20,22,23], at least for selected compounds. Most authors focused on determining concerntrations of the most important analytes DXM [16,18–20,22,23], and DXO [17–20]. The researchers assessed the transformation products of the DXM, mainly qualitatively (or semiquantitatively). The following compounds were studied: N-demethyldextrorphan [18–20], NO2-dextrorphan [17], dextrorphan glucuronide, and N-demethyldextrorphan glucuronide [20]. NO2-dextrorphan was determined only in 1 study as the transformation product that can be potentially formed under acidic conditions in the presence of nitrite. The remaining compounds are the major TPs of DXM, which are reported in the pharmaceutical literature.

Some of the authors raised the issue of lack of standards of selected analytes that enable fully qualitative results in case of, for example, transformation products of API [18,20]. The development of a synthesis procedure for those compounds is a new challenge for scientists. New analytical standards might allow replacing qualitative or semi-quantitative results with fully quantitative data.

Three reviewed studies determined DXM and/or its transformation products qualitatively [21,61,62]; these studies were not originally designed for DXM or DXO analysis, but rather were extensive screening studies of a wide spectrum of different compounds. Nevertheless, these studies can provide valuable information about the presence of DXO in environmental matrices. Madikizela et al reported the presence of DXM, DXO, and N-demethyldextrorphan among the detected TPs, probably originating from DXM, which is not registered by the South African Health Products Regulatory Authority [61]. This suggests the unknown origin of this substance in Africa, not excluding illegal distribution of DXM for ‘recreational use’ or for intoxication. This observation provides clear evidence of the need to conduct WBE research in various countries because the drug trade may be conducted, excluding pharmacies, as an illegal practice. Substances originally designed as medicines can be used as intoxicants. DXM is often taken in conjunction with other substances to induce enhanced intoxication [63]. The role of both targeted and untargeted studies on the widest possible spectrum of metabolites is of great importance in eliminating drug abuse and drug misuse through proper and reliable estimation of consumption and for fast screening in the population.

THE ROLE OF SAMPLE COLLECTION SITE FOR WBE STUDIES:

Considering the presented review of studies on DXM and/or its transformation products in environmental water samples, it must be highlighted that various sample collection places were studied: river water [18,61], river water in the vicinity of wastewater discharge [16,22], raw wastewater [17], WWTP influent [17,23,62] and effluents [17–19,23,62], sewage sludge from WWTP [21,22], surface water [19,20]. The location of sample collection has a crucial impact on the comparability of studies, as the transformation pathways of chemical compounds can be different in various conditions, and different transformations occur during metabolism in the human body. Even variations between poor (DXM) and fast (DXO) metabolizers can affect the identification of transformation products of DXM, and completely different products might be created under acidic conditions in the presence of nitrite (NO2-dextrorphan). Moreover, comprehensive studies covering different sample collection sites might be useful for understanding potential transformation pathways in the environment and observing the presence of various transformation product changes at various collection sites [18,61]. Conducting quantitative studies on appropriate chemical compounds at regular intervals over an extended period of time enables drawing conclusions about changes in levels in the environment. This information can be further used to estimate drug consumption, but it requires in-depth interpretation combined with analysis of the pharmaceutical market and data on drug sales in pharmacies to draw a reliable conclusion. An interesting example of a comprehensive study was shown by Thurman et al [20], who the studied transport of DXM and its transformation products in the South Platte River, with a comment about the potential sources of DXM in samples during the analysis of 4 seasons on a monthly basis taking into account recreational use.

SAMPLE PREPARATION FOR DXM AND ITS TRANSFORMATION PRODUCTS ASSAYS:

The sample preparation procedure should be discussed in the context of a potential combination of different analytical methods to create a universal multi-analyte panel of compounds for WBE environmental testing. It is obvious that the type of matrix to be analyzed is the crucial factor determining the sample preparation method. In reviewed scientific articles, despite various matrices, the solid phase extraction (SPE) technique was most frequently used (Table 1). Filtration through microfiber disposable filters in sample preparation was used in only 1 study [16]. SPE, as a sample preparation technique, allows the simultaneous enrichment and purification of compounds of interest on an adsorbent (solid phase) through adsorption from the studied sample (solution). This technique has several advantages over classic liquid–liquid extraction, such as more efficient extraction of the analytes, reduced consumption of organic solvents, easier fractions collection, enriching compounds at trace levels, particle removal, and easier automation. The automation potential might be important during development of high-throughput multi-residue analysis to ensure the best possible repeatability and method reproducibility [64].

HYPHENATED ANALYTICAL TECHNIQUES FOR DRUGS DETERMINATION:

We would like to emphasize the significant role of all mentioned studies in the context of an attempt to propose remedial actions for the problem of OTC drug abuse, including DXM. The review of available studies on DXM and their transformation products shows that there are available sufficiently sensitive and specific analytical methods to perform reliable environmental studies on the presence of not only drugs of abuse, narcotics, or designer drugs, but also OTC drugs that might be overused to induce intoxication. Table 1 shows that all the methods included are based on mass spectrometry. To ensure the selectivity and time for analyte detection, separation techniques were hyphenated to mass spectrometry detection. One study used Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) [62], but the rest of the research was carried out using chromatography techniques. Liquid chromatography (LC) was the method of choice for most of the reviewed studies [16–21,61]. Only 2 studies were based on gas chromatography (GC) methods [22,23]. This observation of the predominance of the LC over GC for separating pharmaceutical products in the water agrees with the literature [65]. Although the instruments used for the study might be quite different, all of them are used to analyze the mass-to-charge ratio (m/z). This fact allows us to expect that it will be possible to adapt the methods of determining DXM and its transformation products to multiple analytical methods used in environmental research of other drugs [7] and estimate the consumption of substances in a given population.

Future Perspectives

In our opinion, there is a chance to diminish the phenomenon of overusing OTC drugs and OTC drugs misuse. There is a need to start a social discussion about proper health education and access to OTC drugs. Diverse and multifaceted actions should be introduced, such as new laws limiting access to selected OTC drugs, expanding the role of pharmacists in pharmaceutical care, improving cooperation between pharmacists and physicians, and there is a need for a broadly conceived educational action established in accordance with evidence-based information obtained from assessing drug consumption.

WBE testing is one of the best methods for determining patterns of psychoactive substance use. Thus, implementing multianalyte methods for detecting chemical compounds (including OTS drugs, prescription drugs, and their metabolites) may contribute to a better understanding of trends in using specific substances. Conducting multi-center research and comparing results between cities and countries can contribute to prevention and education programs. These actions may help reduce drug abuse, thus improving patient compliance and the effectiveness of treatment. Such actions might contribute to more effective OTC medication by patients and reduce the socially harmful overuse of OTC drugs, providing better patient compliance, course of treatment, and outcomes.

Conclusions

Recreational use of OTC medicines is an unofficial activity, so traditional surveys assessing the use of these drugs are affected by the bias of underreporting and are thus unreliable. Therefore, we performed the present study to highlight the role of studies focused on evaluating the estimated drug consumption as the most reliable source of evidence-based information. To the best of our knowledge, this is the first review article summarizing the analytical methods for determining DXM and/or their transformation products in wastewater. This article has reviewed the methods of determination of DXM and/or their transformation products in urban wastewater, and shows the role of monitoring inappropriate drug use to determine regional trends. Findings from these studies have a significant public health role and provide evidence to support improved control of certain OTC medications, such as dextromethorphan, particularly for vulnerable age groups.