HCQ is commonly used as a first-line treatment for SLE and other rheumatic diseases. Its pharmacokinetic profile is complex, and blood levels vary widely between individuals. Blood concentrations of HCQ have been shown to correlate with clinical efficacy and can indicate patient compliance [6,7,8, 10]. In this study, we investigated potential factors that may affect HCQ and DHCQ concentrations by comparing patient characteristics between high and low concentration groups. Binary logistic regression analysis revealed that gender, age, weight, duration of HCQ use, SLEDAI, platelet count, and IgG levels were influencing factors for low blood HCQ concentrations. Duration of HCQ use, platelet count, SLEDAI and gender were influencing factors for low blood DHCQ concentrations.

Although several studies have investigated the relationship between HCQ concentrations and clinical factors in patients with rheumatic diseases, most have not examined DHCQ concentrations [5, 16, 21]. Our study was one of the few in which both HCQ and DHCQ concentrations were measured in the Chinese population with SLE. In addition, to avoid the confounding effects of dose, we only included patients taking a daily dose of 400 mg HCQ.

We found higher concentrations of HCQ and DHCQ in female patients. This gender difference in drug concentration may be attributed to differences in CYP-mediated metabolism, the influence of sex hormones on absorption, and the difference of fat percentage in body composition [22]. Estrogen has been shown to downregulate CYP3A4 expression [23].

HCQ has a large apparent volume of distribution (over 2000 l) due to poor plasma protein binding (about 50%) and high tissue binding (including blood cells) [7, 24]. Our study results were consistent with this finding. We observed higher HCQ blood levels in patients with low body weight, and the expected delay in reaching steady-state concentrations (3–4 months) due to the accumulation of HCQ in tissues may explain the positive correlation between HCQ and DHCQ concentrations and duration of HCQ use [25, 26].

HCQ is primarily metabolized by CYP450 enzymes in the liver to a variety of active metabolites, with approximately one-quarter of the prototype being cleared through the kidneys [27,28,29]. A previous study that included 111 patients with SLE receiving long-term HCQ therapy observed higher HCQ concentrations in patients with renal insufficiency [5]. In our study, we did not find an association between renal function and HCQ or DHCQ concentrations, as it was challenging to recruit patients with SLE on long-term HCQ due to their clinical severity and use of higher doses of immunosuppressive drugs and glucocorticoids. However, we did observe higher HCQ blood levels in older patients, which may be due to decreased renal function.

The SLEDAI score, which measures lupus disease activity, is strongly correlated with HCQ concentration [21]. The effective concentration threshold of HCQ in clinical practice is still controversial [30]. Costedoat-Chalumeau et al. [12] recommended a whole-blood HCQ target concentration of 1000 ng/ml for patients with SLE. In our study, only 19.49% of patients (n = 53) achieved this target. For the goal of 750 ng/ml in a meta-analysis by Garg et al. [30], 37.5% (n = 102) of patients achieved it. Our study showed that higher HCQ concentrations were associated with lower SLEDAI in the population with a mean HCQ concentration of 690.90 ng/ml. Thus, we found that higher HCQ concentrations were beneficial in reducing disease activity even in the range below previously recommended concentration. In addition, higher DHCQ concentrations were also found to be beneficial in reducing disease activity.

Our study also found a significant association between low platelet counts and low HCQ and DHCQ blood concentrations. We believe this finding may be related to the fact that we measured the whole blood rather than serum concentration. HCQ has been shown to bind to platelets and other blood cells, which can result in its retention in the blood and limit its distribution to eliminated organs [31]. This may explain why low platelet count was associated with lower HCQ and DHCQ blood concentrations in our study.

This study has some limitations. First, this study was limited by patient compliance, which may lead to bias. In our study, the mean HCQ concentration was significantly lower than that in other populations. Although we excluded patients with blood levels below 100 ng/ml, some patients may still be noncompliant with medication. One explanation for this could be that the study population were hospitalized patients, who tended to have more active SLE and were more likely to be nonadherent. Second, the sample size of this study was relatively small, and only hospital attenders were included, which may limit the generalizability of the findings to the broader population. Third, Lee et al. showed that HCQ and DHCQ blood concentrations may be influenced by genetic polymorphisms in CYP450 enzymes [32], but our study did not consider genetic factors. Future studies with larger sample sizes and more diverse populations, as well as consideration of genetic factors, may provide further insights into the factors that influence HCQ and DHCQ blood concentrations in patients with SLE.