1 INTRODUCTION

Anthraquinone (AQ) derivatives are a class of chemical substances naturally occurring in different botanical species and used in food to improve bowel function (Gordon, Macdonald, Parker, Akobeng, & Thomas, 2016), and for some of them there are contradictory data on a possible carcinogenic risk (van Gorkom, de Vries, Karrenbeld, & Kleibeuker, 1999).

The European Food Safety Authority (EFSA) reported that some AQ derivatives in food supplements represent a relevant health problem (Younes et al., 2018). The EFSA Panel on Food Additives and Nutrient Sources added to Food reviewed the available scientific evidence on a possible relationship between AQ derivatives exposure and carcinogenic effects. On the basis of the data currently available, the Panel noted that some derivatives (such as emodin, aloe-emodin, and the structurally related substance danthron) showed evidence of in vitro and in vivo genotoxicity. Notably, the Panel was unable to provide advice on a health-safe daily intake of AQ derivatives. However, EFSA discouraged the long-term use and high-dose consumption of AQ derivatives because of potential safety issues, in particular an increased risk for colorectal cancer (CRC) (Lombardi et al., 2020). Moreover, the use of AQ-containing laxatives has been associated to the development of melanosis coli (MC), a clinical condition characterized by a black or brown pigment deposited in the colorectal mucosa (Yang, Ruan, & Jin, 2020), which is not a pre-neoplastic lesion.

In March 2021, the European Commission confirmed the adoption of the regulation prohibiting the use of all preparations based on Aloe spp., as well as those containing emodin and aloe-emodin, through the amendment of Annex III of Regulation (EC) no. 1925/2006 relating to botanical species containing AQ derivatives (European Commission, 2021). Furthermore, as there is a possibility of harmful effects on health associated with the use of Rheum, Cassia, and Rhamnus and their preparations in food supplements, such substances were placed under Union scrutiny and therefore, were included in Part C of Annex III to Regulation (EC) no. 1925/2006 (European Commission, 2006).

Products containing AQ derivatives are known to be used primarily worldwide as oral laxatives and have various biological effects (Lombardi et al., 2020), also associated with an increased risk of serious adverse events (AEs). In contrast, there is no clear qualitative and quantitative evidence on the association between their use and the risk of CRC onset. In this context, we conducted a systematic review of the literature and meta-analysis to estimate the overall CRC risk associated with the use of oral laxatives-containing AQ derivatives.

2 METHODS

3 RESULTS

3.1 Study selection and characteristics

A total of 3,585 citations were identified through database searching. After removing duplicates (n = 1,263), 2,328 citations were screened, excluding 2,283 records by title and abstract. Forty-five citations met inclusion criteria for full-text review. Thirty-one studies were excluded due to the intervention and 6 due to the study design (Figure 1). Eight manuscripts were finally included in the systematic review for a total of 41,873 patients. Of them, 5 were case–control (Badiali et al., 1985; Boyd & Doll, 1954; Kune, 1993; Nascimbeni et al., 2002; Nusko, Schneider, Schneider, Wittekind, & Hahn, 2000), and 3 were cohort studies (Nusko, Schneider, Ernst, Wittekind, & Hahn, 1997; Nusko, Schneider, Muller, Kusche, & Hahn, 1993), of which one presented a case–control nested design (Charlton, Snowball, Bloomfield, & De Vries, 2013) (Table 1). Four studies considered both CRC and MC (Nascimbeni et al., 2002; Nusko et al., 1993, 1997, 2000), three studies considered CRC only (Boyd & Doll, 1954; Charlton et al., 2013; Kune, 1993), while one study MC only (Badiali et al., 1985). The percentage of patients with a history of laxatives use across studies ranged from 12 to 100%. Although definitions of CRC and/or MC were homogenous, information on laxative exposure (in particular, dosage and treatment length) was lacking among the majority of included studies. Information regarding AEs were not reported in the included studies.

TABLE 1. Full-text manuscripts included in the systematic review of colorectal cancer and/or melanosis coli and any laxatives use (qualitative evidence)

First author	Publication year	Years included	Study design	Country	Number of patients	Age	Treatments	Dosage	Treatment length	Outcome
Boyd & Doll	1954	1948–1949	Case–control	United Kingdom	1927	<45 years: 246 45–54 years: 563 55–64 years: 666 >65 years: 452	Any laxatives (n = 1,129)	Not reported	Continuous period of at least 5 years	Cancer of large bowel
							No laxatives (n = 798)
Badiali et al.	1985	Not reported	Case–control	Italy	114	Melanosis: Median 38 (10–73) years; No melanosis: Median 40 (5–78) years	Any laxatives (n = 54)	Not reported	Not reported	Melanosis coli
							No laxatives (n = 14)
Kune et al.	1993	1980–1981	Case–control	Australia	1,442	Mean 65 (SD +11) years	Any laxatives (n = 324)	Not reported	Not reported	Colorectal cancer
							No laxatives (n = 1,084)
Nusko et al.	1993	1988–1991	Cohort, retrospective	Germany	2,277	Mean 52.9 (SD ±16.5) years	Any laxatives (n = 271)	Not reported	Not reported	Carcinomas of the large bowel, melanosis coli
							No laxatives (n = 2006)
Nusko et al.	1997	1985–1992	Cohort, retrospective	Germany	2,229	Median 55 (range 30–80) years	Any laxatives (n = 372)	Not reported	Not reported	Colorectal carcinoma, melanosis coli
							No laxatives (n = 1857)
Nusko et al.	2000	1993–1996	Case–control	Germany	554	Mean 58.47 years	Any laxatives (n = 97)	Not reported	Carcinoma group: Mean duration of anthranoid laxative 12.9 years; Control group: Mean duration of anthranoid laxative 14.8 years	Colorectal neoplasm, melanosis coli
							No laxatives (n = 457)
Nascimbeni et al.	2002	1997–1999	Case–control	Italy	192	Sigmoid cancer: Mean 65.6 (range 39–95) years; Diverticular disease: Mean 63.2 (range 28–90) years; Controls: Mean 64.3 (range 33–92) years	Anthracene laxatives (n = 37)	Not reported	Not reported	Colorectal cancer, melanosis coli
							No laxatives (n = 155)
Charlton et al.	2013	2000–2009	Cohort with a case–control nested design	United Kingdom	33,138	Males: 73.0 (SD 10.9) years; Females: 74.9 (SD 12.2) years	Non-macrogol only (n = 27,748)	Not reported	Not reported	Colorectal cancer
							Macrogol after other (n = 2,562)
							Macrogol only (n = 1999)
							Macrogol before other (n = 829)

3.2 Risk of CRC in AQ laxatives users versus “other laxative” and “no laxative” users

Five studies (Boyd & Doll, 1954; Charlton et al., 2013; Kune, 1993; Nascimbeni et al., 2002; Nusko et al., 2000) were included in the meta-analysis (Table 2), while three studies (Badiali et al., 1985; Nusko et al., 1993, 1997) were excluded due to the lack of information on AQ laxative use. In the included studies, the percentage of patients with a diagnosis of CRC ranged from 9.9 to 54%. In total, 6,063 patients had CRC (624 AQ laxative users; 5,439 “other laxatives” and “no laxative” users) and 31,156 patients did not have CRC (2,489 AQ laxative users; 28,667 “other laxatives” and “no laxative” users). When analysing all 5 studies using a random effects model, a history of AQ laxative use (any dosage and any treatment length) was associated with a non-statistically significant increased risk of developing CRC (OR: 1.41; 95% CI: 0.94–2.11) compared to “other laxatives” and “no laxative” use (Figure 2). The funnel plot was examined visually and revealed major asymmetry (Figure 3). Based on visual analysis of the funnel plot, the study by Nusko et al. (2000) appeared to be an outlier. Removing this study did not significantly change risk estimates (OR: 1.51; 95% CI: 0.97–2.34) (data not shown). Only one study (Charlton et al., 2013) was judged at low risk of bias for each domain of the Newcastle-Ottawa Scale (Figure 4). Selection of cases and controls was judged at low or unclear risk of bias across almost all studies, while comparability represented the domain at high risk of bias in the majority of the included studies.

TABLE 2. Full-text manuscripts included in the meta-analysis of colorectal cancer and anthracene laxatives use (quantitative evidence)

Study	Publication year	Mean age	Number of treated patients	Treatments	Outcome	Number of events (%)
Boyd and Doll	1954	Not reported	355	Anthracene laxatives	Cancer of large bowel	112 (31.5)
			774	Other laxatives		196 (25.3)
			798	No laxatives		79 (9.9)
Kune et al.	1993	65.0 years	197	Anthracene laxatives	Colorectal cancer	95 (48.2)
			127	Other laxatives		69 (54.3)
			1,084	No laxatives		521 (48.1)
Nusko et al.	2000	58.5 years	78	Anthracene laxatives	Colorectal neoplasm	29 (37.2)
			19	Other laxatives		7 (36.8)
			457	No laxatives		166 (36.3)
Nascimbeni et al.	2002	64.4 years	37	Anthracene laxatives	Sigmoid cancer	18 (48.6)
			155	No laxatives		37 (23.9)
Charlton et al.	2013	73.9 years	2,446	Dantron	Colorectal cancer	370 (15.1)
			30,692	Other laxatives		4,364 (14.2)

3.3 Subgroup analyses

Three studies compared AQ laxatives use to both “other laxatives” and “no laxative” use (Boyd & Doll, 1954; Kune, 1993; Nusko et al., 2000). Furthermore, one study compared AQ laxatives use versus “other laxatives” (Charlton et al., 2013), and one study versus “no laxative” use (Nascimbeni et al., 2002), respectively. There was a non-statistically significant increase in the risk of CRC seen when comparing AQ laxatives versus “other laxatives” use (OR: 1.09; 95% CI: 0.89–1.34) (Figure 5) and AQ laxatives versus “no laxatives” use (OR: 1.88; 95% CI: 0.90–3.94) (Figure 6). No other subgroup analyses could be performed, such as those based on the duration of use of the AQs laxative (>2 weeks), as none of the included studies reported complete data on the duration of treatment.

4 DISCUSSION

The European Commission recently prohibited the use of all preparations based on Aloe spp., as well as those containing emodin and aloe-emodin, and placed under Union scrutiny the use of Rheum, Cassia, and Rhamnus and their preparations in food supplements, following the risk of harmful effects on health associated with their use (European Commission, 2006). In this scenario, our systematic review and meta-analysis contributed to estimate the aforementioned overall risk (CRC and/or MC), although a small number of studies were included.

Considering MC, the role of AQ laxatives in its development could not be assessed. In fact, the association between AQ laxatives use and MC is mainly reported in the literature as anecdotal (i.e., case report, case series, endoscopy studies, etc.) description (Younes et al., 2018). Moreover, most of observational evidence incorrectly considers MC as a proxy of long-term use of laxatives, specific for AQ derivatives (Kassim et al., 2020; Siegers, Von Hertzberg-Lottin, Otte, & Schneider, 1993), but their use is only one of the factor involved in its aetiology (Yang et al., 2020). Actually, other causes such as irritable bowel syndrome, inflammatory bowel disease, colonic neoplasms, hyperplastic polyps, the use of nonsteroidal anti-inflammatory drugs, vitamin E deficiency, the intake of unsaturated fatty acids, environmental factors, family history, psoriasis and Rett syndrome, feces stasis, and obstruction of colon and rectum could also contribute to the onset of MC (Yang et al., 2020). In this context, our results emphasise the need to evaluate the association between AQ laxatives and MC through large studies, with a proper follow-up, a clear evaluation of comorbidities and other risk factors for MC onset, and a defined period of exposure. Noteworthy, AQ laxatives are known to be widely used as over-the-counter medications or self-prescribed herbal products (Vitalone et al., 2012), making the assessment of the effective exposure to these products very difficult.

As reported in EFSA scientific opinion (Younes et al., 2018), previous epidemiological evaluations suggested an increased risk for CRC associated with the general use of laxatives (Sonnenberg & Muller, 1993), some of which contain AQ derivatives (Nusko et al., 1993; Siegers et al., 1993). To the best of our knowledge, this is the first systematic review and meta-analysis investigating the role of AQ laxatives alone on the risk of gastrointestinal neoplasia. Focusing on studies only reporting AQ laxatives, our meta-analysis showed that their intake was associated with a trend towards the development of CRC compared to “other” and “no laxatives” use. Actually, all studies had methodological limitations such as the use of a retrospective design, small sample size, and lack of adjustment for potential confounding factors. Moreover, no studies reported information on dosage and length of treatment with AQ laxatives. Anyway, our results may not be inconclusive (Alderson, 2003; Guyatt et al., 2011), especially from a clinical point of view.

Our study has several strengths. This is the first systematic review and meta-analysis studying the association between AQ laxatives use and CRC. The strength of this analysis is bolstered by the large sample size of 41,873 patients. As can be seen from the risk of bias assessment, besides all of included studies are retrospective, they were judged at low risk of bias for cases and controls selection, and the length of follow-up was sufficient to assess with certainty the outcomes of interest. There are also some limitations. First, heterogeneity between studies was high. Second, baseline information regarding cumulative AQ laxatives dose and length of treatment was unavailable in all studies. Considering the included studies, it was not possible to evaluate other clinically relevant confounding variables, such as patients' alimentary behaviour and constipation severity. Moreover, the high risk of bias for comparability and exposure may have influenced the results of this analysis, particularly for case–control studies. Given these limitations, our study provides the best risk estimate currently available for counselling patients undergoing AQ laxatives use.

In response to the EFSA considerations (Younes et al., 2018), since a history of AQ laxative use may be associated with an increased risk of CRC, it should be made mandatory to insert specific warnings on the labelling of food supplements containing AQ derivatives used as laxative, for example: (1) “This product is only recommended for the short-term treatment of occasional constipation”; (2) “In case of previous diagnosis or family history of CRC the use of this product is not recommended”; (3) “Use this product only until intestinal function is restored. Otherwise, contact your doctor or pharmacist”; (4) “This product can induce the loss of electrolytes and therefore should avoided by patients receiving certain drugs (i.e., diuretics, digoxin, etc.)”; and (5) “This product increases intestinal motility, consequently can reduce the absorption of drugs administered concomitantly.” These warnings could be helpful especially for those individuals using AQ derivatives still on the market but currently under monitoring (i.e., Rheum, Cassia, and Rhamnus) (European Commission, 2021). The aforementioned warnings and contraindications could improve the appropriateness of both prescription and use of AQ derivatives, in particular those used as laxatives. Their use could also be applied to the labels of galenic preparations. For medicinal products containing AQ laxatives marketed in Italy, their summary of product characteristics already reports specific warnings and precautions, aimed to avoid an inappropriate use of these products.

5 CONCLUSIONS

In summary, qualitative and quantitative synthesis of observational studies does not show that AQ laxatives are associated with CRC development. Noteworthy, more extensive and high quality population studies are needed to collect information on the influence of AQ laxatives dosage and long-term use as risk factors for the onset of this relevant disease. The theoretical risk of CRC should be considered for patients with other known risk factors for CRC. To date, it is not possible to associate the use of AQ laxatives with the development of CRC. However, the trend towards an increased risk of CRC provides a strong indication for investigating this issue through pharmaco- and phytovigilance systems in the next future.

In conclusion, this study may contribute to enhancing public attention and to alert healthcare professionals (general practitioners and pharmacists) and customers regarding the potential risks associated with the misuse/abuse of AQ laxatives.

CONFLICT OF INTEREST

The authors declare no conflict of interest.

AUTHOR CONTRIBUTIONS

Niccolò Lombardi: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Writing – Original Draft. Giada Crescioli: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Writing – Original Draft. Valentina Maggini: Conceptualization, Methodology, Validation, Investigation, Formal analysis, Writing – Original Draft. Raffaele Bellezza: Data Curation, Visualization. Iacopo Landi: Data Curation, Visualization. Alessandra Bettiol: Formal analysis. Francesca Menniti-Ippolito: Validation, Writing – Review & Editing, Supervision. Ilaria Ippoliti: Data Curation, Visualization, Supervision. Gabriela Mazzanti: Writing – Review & Editing, Supervision. Annabella Vitalone: Writing – Review & Editing, Supervision. Eugenia Gallo: Data Curation. Francesco Sivelli: Data Curation. Francesco Sofi: Validation, Writing – Review & Editing, Supervision. Gian Franco Gensini: Validation, Writing – Review & Editing, Supervision. Alfredo Vannacci: Validation, Writing – Review & Editing, Supervision. Fabio Firenzuoli: Project administration.