Bioequivalence clinical trials involve healthy volunteers, whose blood tests must be within the normal ranges, which is very stringent for bilirubin and liver enzymes. Gilbert’s syndrome (GS) is a benign genetic disorder related to the metabolism of bilirubin in liver (Düzenli et al., 2021). Bilirubin is the last product of heme catabolism that comes mostly from the breakdown of erythrocyte hemoglobin in the reticuloendothelial system (Memon et al., 2016). Bilirubin elimination is conducted by converting it into direct bilirubin by conjugation with glucuronic acid (Gil and Sąsiadek, 2012). Since patients with GS have reduced level of glucuronidation and unconjugated bilirubin is not water-soluble as conjugated bilirubin, it cannot be excreted in bile and patient suffers from unconjugated hyperbilirubinemia and mild intermittent icterus (Thoguluva Chandrasekar et al., 2022). In healthy people, the normal level of bilirubin ranges from 0.1 to 1.2 mg/dL. However, the levels in GS patients usually ranges from 1.2 to 5.3 mg/dL (Gil and Sąsiadek, 2012). Therefore, GS patients with elevated bilirubin levels are excluded from bioequivalence studies due to the suspicion of any liver disease, even if the change is clinically insignificant and despite that it is well known that there is no alteration in liver enzymes in patients with this syndrome (Moreno et al., 1984; Sidorenko et al., 2022; Vítek and Tiribelli, 2023).

GS patients carry variants in the gene encoding for the enzyme responsible of converting unconjugated bilirubin into conjugated bilirubin, the uridine diphosphoglucuronate-glucuronosyltransferase 1A1 (UGT1A1) (Thoguluva Chandrasekar et al., 2022). More specifically, it is related to the short tandem repeat (STR) variation in the promotor of this gene that consists in an addition of a dinucleotide sequence (TA) into the transcription initiation sequence A(TA)₇TAA, converting it into A(TA)₈TAA (Horsfall et al., 2011; Thoguluva Chandrasekar et al., 2022). This variation was called allele UGT1A1*28 (rs3064744) and it was previously annotated as rs34815109 or rs34983651 (Aronica et al., 2022). As a result, having this variant makes the enzyme have only 30% of the normal activity. In addition, this position in the genome also defines other alleles such as UGT1A1*36 when one dinucleotide sequence is deleted (A(TA)₆TAA) or UGT1A1*37 when two TAs are added (A(TA)₉TAA). The UGT1A1*36 appears to have higher transcript levels than UGT1A1*1, while UGT1A1*37 appears to have lower levels (Gammal et al., 2016). These variants are less common or may be absent depending on the geographic region of ancestry (Gammal et al., 2016).

Not every person that has the allele UGT1A1*28 finally develops visible symptoms as it depends on environmental factors like physical stress, prolonged fasting, poor diet, hemolytic reactions, febrile illnesses and menstruation (Düzenli et al., 2021). For example, a reduced caloric uptake to 400 kcal diary produces a 2 to 3-fold increase of bilirubin concentration in 48 h. GS usually appears during early adolescence and its more frequently diagnosed in males that females due to differences in sex steroids concentration and higher production of bilirubin in males (Thoguluva Chandrasekar et al., 2022).

The UGT1A1 rs887829 C>T variant (UGT1A1*80) was studied for having a possible relation to UGT1A1*28. It has been described to be in almost complete linkage disequilibrium with UGT1A1*28 (r² ≅ 0.99), although there are other variants in this STR that prevent complete disequilibrium (Gammal et al., 2016; Bravo-Gómez et al., 2022). It was reported that C allele was correlated with UGT1A1*1 and UGT1A1*36 and T allele with UGT1A1*28 and UGT1A1*37 (Gammal et al., 2016; Université Laval, 2022). Therefore, they describe that analysis of this allele could be used as an indicator of UGT1A1*80+*28 and thereby to infer the metabolic phenotype of UGT1A1, as it is a faster and more cost-effective genotyping technique than UGT1A1*28 genotyping (Bravo-Gómez et al., 2022).

Another allele was associated with the development of GS due to the decrease in enzyme activity such as UGT1A1*6 (rs4148323) (Han et al., 2006). It was identified as a predictor of bilirubin concentration in East Asian populations, where it accounts for about 5% of the variability (Gammal et al., 2016). Despite being worldwide widespread, differences depending on the ethnicity are found, being Sub-Saharan African the ones with highest prevalence (15%–25%), Europeans in the middle (5%–10%) and East Asian with the lowest prevalence (0%–5%) (Gil and Sąsiadek, 2012).

The main objective of this work was to investigate whether volunteers with UGT1A1 IM and PM phenotypes have a higher incidence of analytical changes in liver parameters. As a secondary objective, we sought to discover a possible relationship between UGT1A1 phenotype and the alteration of other biochemical parameters.

GS is a disorder of bilirubin metabolism that does not pose a health problem to the individual; in fact, its accumulation appears to have an antioxidant effect that may be protective. For this reason, GS has been associated with a lower prevalence of chronic diseases such as cardiovascular disease or type 2 diabetes, a lower incidence of ischemic heart disease, and a lower incidence of Hodgkin’s lymphoma and endometrial cancer compared to the general population (Wagner et al., 2018). However, the deficient hepatic metabolism of GS patients is an exclusion factor in clinical trials. This is due to the suspicion that other hepatic factors besides bilirubin metabolism may be affected, thus altering the results of bioequivalence clinical trials. For example, increased toxicity of the drug irinotecan has been described in patients with a PM phenotype for UGT1A1, such as those with GS (Lankisch et al., 2008).

CPIC, in its guideline on UGT1A1 and atazanavir published in 2016, refers to a frequency of 0.79% for UGT1A1*6 and 31.42% for UGT1A1*80 for Europeans. Similarly, for Latin-Americans it refers a frequency of 1.16% for UGT1A1*6 and 38.27% for UGT1A1*80. The frequencies of UGT1A1*6 and UGT1A1*80 reflected in this study are very similar to CPIC (Gammal et al., 2016). The allele frequency of UGT1A1*6 reflected in the European population in our study is consistent with the frequency of this allele previously described in Spanish population (Cerezo-Arias et al., 2022). This study reflects the largest population genotyped for UGT1A1*80 to date.

Results of this study confirmed the existing sex differences in biochemical parameters that were already described in previous articles (Werner et al., 1970; Dufour et al., 2005; Rushton and Barth, 2010; O Leary et al., 2017). This is the case for total bilirubin, uric acid and ALP and GOT enzymes, for which was described the existence of higher serum levels of these parameters in males versus females after puberty possibly due to differences in sex hormone activity throughout life (Werner et al., 1970). Reference ranges for hemoglobin in women of reproductive age showed lower values than those of men of equivalent age; when in the absence of different biological needs, similar values would be expected. This may be because these ranges were obtained from large population-based studies that possibly included a significant number of women who spend a part of their lives iron deficient due to blood loss during menstruation (Rushton and Barth, 2010). As for creatinine, it was also known that women have lower serum creatinine values than men even though they have similar renal function, because men have greater muscle mass (O Leary et al., 2017). Finally, it was described that the reference ranges for liver enzymes GPT and GGT should be higher for men than for women, which corresponds to the differences found in this study (Dufour et al., 2005).

The existence of differences in the levels of some biochemical parameters depending on the biogeographic origin highlights the necessity of establishing specific reference intervals for each of them (Lim et al., 2015). Although the groups reflected in these articles are different from those included in this study, genetic variation between origins is proposed as the cause of these differences (Jones et al., 1998; DeBoer et al., 2012; Schneider et al., 2014; Beydoun et al., 2018; Mariño-Ramírez et al., 2022).

Our results confirmed that UGT1A1 phenotype allows differences in total bilirubin levels to be seen, showing a higher bilirubin in IMs, even higher in PMs (Lin et al., 2006; Chen et al., 2011; Bravo-Gómez et al., 2022). The small number of subjects who were UGT1A1*6 carriers may cause the results to show no significant increase in bilirubin levels, although it was shown to affect its disposition in previous researches (Barbarino et al., 2014; Gammal et al., 2016; Hanafusa et al., 2022).

To our knowledge, the UGT1A1 gene has not been described to be involved in hemoglobin metabolism. The level in the PM is not outside the normal range. Moreover, all phenotypes decreased their hemoglobin levels at follow-up which can be explained by the blood draws performed during the clinical trial. The difference of PM was not maintained in multivariate analysis, suggesting that it could be a false positive. For all these reasons, we could suspect that this is a low value due to the assay itself.

Additionally, a lower uric acid level in subjects with the PM phenotype for UGT1A1 was observed compared to NM. Information on a possible relationship between the UGT1A1 gene and plasma uric acid levels is scarce. Only one article mentions that patients with GS did not present alterations in uric acid levels, which is consistent with the results found in this article, since although slight differences are observed depending on the phenotype, in no case do they exceed the range of normality (Bulmer et al., 2008). More studies are needed to clarify whether there is a relationship between the two.

No difference in liver enzyme levels was found as a function of UGT1A1 phenotype. Previous articles examining GOT, GPT and GGT liver enzyme levels in GS patients also found the same results (Bulmer et al., 2008; Hsu et al., 2022). These data suggest that there was no underlying liver pathology causing elevated bilirubin levels, but that it was a consequence of the UGT1A1 phenotype. Therefore, we suggest that genotyping of this gene should be performed in those subjects with out-of-range bilirubin levels. NM individuals should be excluded because of suspected liver involvement due to other causes, but IM and PM volunteers could be included because their bilirubin levels are only due to the phenotype of the gene. It should also be considered that the phenotype of other genes that could alter the metabolism of the study drugs are not considered an exclusion criterion, e.g., PM for CYP2C19 or ultrarrapid metabolizers (UMs) for CYP2D6, but UGT1A1 as a cause of Gilbert’s syndrome is considered even though it is a benign disease. In today’s world we should try to include as much variability as possible to cover all individuals in society.

This study has two major limitations. First, as this was a clinical trial with inclusion and exclusion criteria, subjects with high bilirubin levels were excluded. Therefore, most subjects with genotype corresponding to GS could have been excluded, reducing the prevalence of GS and possible reducing the existing differences both in total bilirubin levels and in other parameters studied. Second, because not all subjects were genotyped for the UGT1A1*6 allele, statistical power was reduced for evaluation of the relevance of this allele.

This study reflects the largest population genotyped for UGT1A1*80 to date. Bilirubin levels were higher in volunteers with IM and PM phenotypes for UGT1A1. Given that there is no hepatic involvement in these subjects who are more likely to develop GS, this study suggests that there is no underlying liver pathology causing elevated bilirubin levels, but that it is a consequence of the UGT1A1 phenotype. Therefore, IM and PM individuals could be included in clinical trials because their bilirubin levels are only due to the gene phenotype and not to other pathologies.