Chloroquine and hydroxychloroquine: Immunomodulatory effects in autoimmune diseases

Abstract

Chloroquine (CQ) and hydroxychloroquine (HCQ), originally developed as antimalarial drugs, have found a new purpose in treating various autoimmune diseases due to their immunomodulatory properties. These drugs work through multiple mechanisms, including inhibiting Toll-like receptor signaling, suppressing antigen presentation, and modulating autophagy. This review article provides a comprehensive analysis of the immunomodulatory effects of CQ and HCQ in several autoimmune diseases such as systemic lupus erythematosus, rheumatoid arthritis, systemic sclerosis, and others. We delve into the intricate mechanisms of action, highlighting the key immune cells involved and discussing the clinical implications of these drugs in managing autoimmune conditions. Our review covers the latest research and clinical trials, offering a comprehensive understanding of the therapeutic potential of CQ and HCQ in autoimmune diseases. We also discuss the challenges and controversies surrounding the use of these drugs, such as their long-term side effects and the need for personalized treatment approaches. By synthesizing current knowledge and identifying areas for future research, this review aims to provide a valuable resource for healthcare professionals and researchers involved in the management of autoimmune diseases.

Key Words: Chloroquine; Hydroxychloroquine; Autoimmune diseases; Immunomodulation; Toll-like receptor; Autophagy

Core Tip: Chloroquine (CQ) and hydroxychloroquine (HCQ) are antimalarial drugs repurposed for autoimmune diseases. CQ and HCQ work through multiple mechanisms, including inhibition of Toll-like receptor signaling, suppression of antigen presentation, and modulation of autophagy. These drugs have shown clinical efficacy in autoimmune diseases, including systemic lupus erythematosus, rheumatoid arthritis, and others. CQ and HCQ are generally well-tolerated with a favorable safety profile compared to many conventional immunosuppressants. Long-term use of CQ and HCQ can lead to ocular toxicity, necessitating regular ophthalmologic evaluations. Further research is needed to fully elucidate the therapeutic potential of CQ and HCQ in autoimmune diseases.

INTRODUCTION

Autoimmune diseases arise from immune system dysregulation, resulting in the loss of self-tolerance and immune-mediated tissue damage. This aberrant immune response results in inflammation, tissue damage, and a wide spectrum of clinical manifestations depending on the organ systems involved[1]. The global burden of autoimmune diseases is substantial, affecting millions worldwide and posing significant challenges to healthcare systems. These conditions often lead to diminished quality of life, increased morbidity, and even mortality, placing a considerable strain on individuals and society[2]. While the exact etiology of most autoimmune diseases remains elusive, a complex interplay of genetic predisposition, environmental triggers, and immune system dysregulation is believed to contribute to their development[3]. Current therapeutic strategies for autoimmune diseases primarily focus on suppressing the immune response to alleviate symptoms and prevent further tissue damage[4]. However, these treatments are often associated with significant side effects, highlighting the ongoing need for safer and more effective therapeutic interventions.

Chloroquine (CQ) and hydroxychloroquine (HCQ) are widely used in autoimmune diseases because of their favorable safety profile and broad immunomodulatory actions[5]. Originally developed as antimalarial agents[6], CQ and HCQ have been serendipitously repurposed for the treatment of a variety of autoimmune conditions[7], including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), Sjögren’s syndrome (SS), and others[8-11]. Their efficacy in these diseases, despite not being initially designed for autoimmune conditions, underscores the intricate connection between the immune system and parasitic infections, suggesting that modulating pathways involved in combating parasites can also impact the aberrant immune responses seen in autoimmunity[12]. CQ and HCQ are generally well-tolerated, but they can cause side effects such as nausea, vomiting, diarrhea, and skin rash. Although several studies have investigated the effect of HCQ on mortality risk in SLE patients[13], in rare cases, they can also cause more serious side effects, such as retinal toxicity and heart problems[14,15]. Regular monitoring is essential to ensure safe and effective use of these medications.

While both CQ and HCQ share similar mechanisms of action, they exhibit some differences in their medicinal chemistry profiles[16]. HCQ has a hydroxyl group, and this structural variation leads to a more favorable safety profile compared to CQ, with reduced toxicity, particularly regarding cardiac and ocular side effects[17]. HCQ also exhibits a longer half-life and higher bioavailability, allowing for less frequent dosing and potentially improving patient compliance. In terms of efficacy, both drugs have demonstrated clinical benefits across various autoimmune diseases; however, HCQ is more commonly preferred in long-term management due to its improved tolerability[18].

This review aims to provide a comprehensive and up-to-date overview of the immunomodulatory effects of CQ and HCQ in various autoimmune diseases. We will delve into the intricate molecular mechanisms by which these drugs interact with the immune system, focusing on their impact on key immune cells, signaling pathways, and processes such as antigen presentation and autophagy. We will explore how these mechanisms translate into clinical benefits in specific autoimmune conditions, discussing the evidence supporting their use in SLE, RA, SS, and other relevant diseases. Furthermore, we will address the safety concerns associated with CQ and HCQ, providing a balanced assessment of their potential side effects and outlining strategies for minimizing these risks.

IMMUNOMODULATORY MECHANISMS OF CHLOROQUINE AND HYDROXYCHLOROQUINE

CQ and HCQ have pleiotropic immunomodulatory effects, meaning they can influence the immune system through multiple pathways. This section delves into the intricate molecular mechanisms by which these drugs interact with the immune system, focusing on their impact on key immune cells, signaling pathways, and processes such as antigen presentation and autophagy.

Inhibition of toll-like receptor signaling

Toll-like receptors (TLRs) are a family of pattern recognition receptors that play a crucial role in the innate immune system. They recognize pathogen-associated molecular patterns derived from microbes and damage-associated molecular patterns released from damaged tissues[19]. Upon activation, TLRs initiate signaling cascades that lead to the production of pro-inflammatory cytokines and the recruitment of immune cells[20].

CQ and HCQ interfere with TLR signaling by accumulating within endosomes and lysosomes, the acidic organelles where TLRs are localized. This accumulation disrupts the acidification process necessary for TLR activation, thereby preventing the downstream signaling events that promote inflammation[21]. Specifically, they interfere with both MyD88-dependent pathways (utilized by TLR2, TLR4, TLR7, and TLR9) and TRIF-dependent pathways (utilized by TLR3 and TLR4), thereby blocking downstream activation of NF-κB and IRF3/7 transcription factors[22]. This inhibition prevents nuclear translocation of these factors and subsequent transcription of pro-inflammatory cytokine genes including tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, and IFN-α. The suppression of type I interferon responses is particularly relevant in systemic lupus erythematosus, where aberrant TLR7/9 signaling contributes to disease pathogenesis[5,23]. By dampening TLR signaling, CQ and HCQ can reduce the production of pro-inflammatory cytokines and mitigate the excessive immune activation seen in autoimmunity.

Suppression of antigen presentation

Antigen-presenting cells (APCs), such as dendritic cells and macrophages, play a critical role in initiating adaptive immune responses. APCs internalize antigens, process them into peptides, and present them to T cells via major histocompatibility complex (MHC) molecules[24]. This interaction triggers T cell activation and the subsequent immune response.

CQ and HCQ can interfere with antigen processing and presentation by inhibiting lysosomal function and autophagy[25]. Lysosomes are essential for degrading proteins into peptides for MHC loading, while autophagy is involved in the degradation and recycling of cellular components, including antigens[26]. By disrupting these processes, CQ and HCQ can reduce the efficiency of antigen presentation, potentially limiting the activation of autoreactive T cells that drive autoimmune responses. Costedoat-Chalumeau et al[27] highlighted that CQ and HCQ accumulate within lysosomes, leading to an increase in lysosomal pH. This alteration in pH disrupts the activity of lysosomal enzymes, affecting various cellular processes, including antigen processing and autophagy. Nirk et al[21] further elaborated on the impact of HCQ on lysosomal activity. They demonstrated that HCQ inhibits lysosomal acidification, preventing the proper functioning of acid-dependent hydrolases. This inhibition disrupts the degradation of proteins and other cellular components within lysosomes.

MODULATION OF AUTOPHAGY

Autophagy is a highly conserved cellular process involved in the degradation and recycling of cellular components, including damaged organelles, protein aggregates, and even invading pathogens. It plays a crucial role in maintaining cellular homeostasis and is implicated in various physiological and pathological processes, including immunity and inflammation[28].

CQ and HCQ are known to inhibit autophagy by interfering with lysosomal function. Lysosomes are essential for the final stages of autophagy, where cellular components are broken down into their building blocks for recycling. By disrupting lysosomal function, CQ and HCQ can prevent the completion of the autophagy process[26].

The impact of autophagy modulation by CQ and HCQ in autoimmune diseases is complex and can be both beneficial and detrimental, depending on the specific context and the role of autophagy in the disease process. On the one hand, inhibiting autophagy can reduce antigen presentation and inflammation, potentially mitigating autoimmune responses. On the other hand, it can also impair the clearance of pathogens or apoptotic cells, potentially exacerbating inflammation or increasing the risk of infections[25]. This occurs through several molecular interactions; sequestration of LC3-II prevents proper autophagosome maturation, while inhibition of acid hydrolases like cathepsins blocks protein degradation[29]. Furthermore, CQ and HCQ dysregulate the mTOR and AMPK pathways, which normally coordinate cellular responses to metabolic stress[30]. The resulting blockade of autophagy reduces the processing and presentation of self-antigens via MHC class II molecules, limiting activation of autoreactive T cells. However, this same mechanism may impair clearance of intracellular pathogens and apoptotic debris, potentially contributing to disease complications. Nirk et al[21] both noted that CQ and HCQ inhibit autophagy by interfering with lysosomal function. Zhao et al[31] provided further insights into the specific molecular targets of HCQ in autophagy modulation. Using quantitative proteomics, they identified several proteins, including NQO2, SEC23A, and LGALS8, as potential targets of HCQ. These proteins are involved in various steps of autophagy, and their dysregulation by HCQ contributes to the overall inhibition of autophagy.

Targeting cytokine production and restoring immune homeostasis

Cytokines are signaling molecules that play a crucial role in intercellular communication within the immune system. They regulate various immune functions, including cell activation, proliferation, differentiation, and effector functions. Dysregulated cytokine production is a hallmark of autoimmune diseases, contributing to chronic inflammation and tissue damage[32].

CQ and HCQ can modulate the production of several cytokines, both pro-inflammatory and anti-inflammatory. Studies have shown that these drugs can reduce the production of pro-inflammatory cytokines such as TNF-α, interleukin-1 beta (IL-1β), and IL-6. These cytokines are known to play a critical role in the pathogenesis of various autoimmune diseases. Additionally, CQ and HCQ may also enhance the production of anti-inflammatory cytokines, such as IL-10, which can help to suppress the immune response and promote immune homeostasis[33,34]. At the transcriptional level, they suppress NF-κB and AP-1 activity, leading to reduced production of pro-inflammatory cytokines including TNF-α, IL-1β, and IL-6[5]. Post-translationally, these drugs inhibit inflammasome assembly, particularly the NLRP3 inflammasome, and subsequent caspase-1 activation, thereby limiting IL-1β processing and release. Concurrently, they promote anti-inflammatory responses through STAT3-mediated upregulation of IL-10. These coordinated actions help restore the balance between pro-inflammatory Th17 cells and immunosuppressive Tregs, while mitigating the cytokine storms characteristic of autoimmune disease flares[35]. By modulating cytokine production, CQ and HCQ can help to restore the balance between pro-inflammatory and anti-inflammatory signals, potentially mitigating the inflammatory cascade in autoimmune diseases.

IMPACT OF CHLOROQUINE AND HYDROXYCHLOROQUINE ON KEY IMMUNE CELLS

The immunomodulatory effects of CQ and HCQ on key immune cells are summarized in Figure 1. These drugs disrupt DC function by inhibiting TLR signaling and antigen presentation, while macrophages exhibit reduced pro-inflammatory cytokine release and a shift toward M2 polarization. T cell subsets, particularly Th1 and Th17, are suppressed, whereas Treg activity may be enhanced. B cell autoantibody production is also attenuated. These effects restore immune balance, underpinning their therapeutic efficacy in autoimmune diseases.

Figure 1 Immunomodulatory effects of chloroquine and hydroxychloroquine on key immune cells. This figure illustrates the impact of chloroquine (CQ) and hydroxychloroquine (HCQ) on various immune cell types, including dendritic cells (DCs), macrophages, T cells, and B cells. CQ and HCQ modulate DCs by disruption of Toll-like receptor (TLR) signaling (TLR4, TLR9) and inhibition of antigen presentation via lysosomal interference, reducing pro-inflammatory cytokine production (tumor necrosis factor-alpha, interleukin-12). In Macrophages, they suppress TLR signaling and phagocytosis, and promote of M2 anti-inflammatory polarization, and modulation of cytokine release. In T cells, they inhibit autoreactive T cell activation via impaired antigen presentation, suppression of Th1/Th17 subsets, and potential enhancement of regulatory T cells (Tregs), and in B cells, the act via reduction in autoantibody production by interfering with B cell activation and plasma cell differentiation.

Effects of chloroquine and hydroxychloroquine on dendritic cells

Dendritic cells (DCs) are professional antigen-presenting cells that play a crucial role in initiating T cell-mediated immune responses. CQ and HCQ interfere with DC function through several mechanisms. CQ and HCQ accumulate in lysosomes and endosomes, disrupting the acidification process necessary for TLR activation[14]. This inhibition affects various TLRs expressed on DCs, such as TLR4 and TLR9, which are involved in recognizing lipopolysaccharide from gram-negative bacteria and CpG DNA, respectively[36]. By suppressing TLR signaling, CQ and HCQ reduce the production of pro-inflammatory cytokines like TNF-α and IL-12, which are essential for DC maturation and T cell activation[21]. CQ and HCQ disrupt antigen processing and presentation by DCs. These drugs interfere with lysosomal function and autophagy, which are crucial for degrading antigens into peptides and loading them onto MHC molecules for presentation to T cells[37]. This impairment can lead to reduced activation of autoreactive T cells, mitigating the autoimmune response.

The impacts of chloroquine and hydroxychloroquine on macrophages

Macrophages are phagocytic cells that play a vital role in both innate and adaptive immunity[38]. CQ and HCQ modulate macrophage function through multiple pathways. Similar to DCs, macrophages also express various TLRs, and CQ and HCQ's interference with TLR signaling significantly impacts macrophage activation and cytokine production[39]. This suppression leads to reduced release of pro-inflammatory cytokines like TNF-α and IL-1β, contributing to the overall anti-inflammatory effect of these drugs. Studies have shown that CQ and HCQ can inhibit macrophage phagocytosis, the process by which macrophages engulf and destroy pathogens or cellular debris[40]. While this effect might seem counterintuitive in the context of infection, it can be beneficial in autoimmune diseases where excessive phagocytosis and inflammation contribute to tissue damage. Macrophages can be polarized into different phenotypes, M1 (pro-inflammatory) and M2 (anti-inflammatory)[41]. Recent research suggests that CQ and HCQ can promote a shift from M1 to M2 polarization[40]. This shift favors tissue repair and resolution of inflammation, offering a potential therapeutic advantage in autoimmune conditions.

The modulation of T cells upon chloroquine and hydroxychloroquine treatments

T cells are central players in adaptive immunity, orchestrating immune responses through various subsets with distinct functions. CQ and HCQ impact T cell activation and differentiation. CQ and HCQ can suppress T cell activation by interfering with antigen presentation by DCs and APCs[42]. Additionally, these drugs can directly affect T cells by inhibiting signaling pathways involved in T cell receptor activation, reducing their proliferation and cytokine production. Different T cell subsets, such as Th1, Th2, Th17, and regulatory T cells (Tregs), play distinct roles in immune responses. Studies suggest that CQ and HCQ can influence the balance between these subsets[43]. For instance, they may suppress the activity of Th1 and Th17 cells, which are involved in promoting inflammation in autoimmune diseases, while potentially enhancing the function of Tregs, which are crucial for immune suppression and maintaining self-tolerance. Recent findings indicate that CQ and HCQ can affect T cell metabolism, which is essential for their activation and differentiation[36]. By interfering with metabolic pathways, these drugs can fine-tune T cell responses and potentially dampen the aberrant activation seen in autoimmunity.

CQ and HCQ can differentially affect various T cell subsets, including T helper 1 (Th1), T helper 2 (Th2), T helper 17 (Th17), and regulatory T cells (Tregs). Studies have suggested that these drugs can suppress the activation and proliferation of Th1 and Th17 cells, which are known to promote inflammation in autoimmune diseases[44]. They may also enhance the function of Tregs, which play a crucial role in suppressing the immune response and maintaining immune tolerance[45]. Long et al[46] utilized single-cell RNA-sequencing to investigate HCQ impact on T cells, revealing that HCQ reduces effector CD4+ T cells while upregulating inhibitory genes like CTLA4 and TNFAIP3. It significantly expands effector CD8+ T cells, enhancing cytotoxicity through upregulation of genes such as GZMA, GZMB, GZMH, KLRD1, NKG7, PRF1, and IFNG, and reduces the dysfunctional CD38+ CD8+ T cell subset. These findings suggested that HCQ improves T cell cytotoxicity, potentially explaining its protective effect against infections in autoimmune diseases. By influencing the balance between different T cell subsets, CQ and HCQ can contribute to the restoration of immune homeostasis and the resolution of autoimmune responses.

The role of chloroquine and hydroxychloroquine on B cells activation

B cells are responsible for producing antibodies, which play a critical role in humoral immunity. In autoimmune diseases, B cells can produce autoantibodies that target self-antigens, contributing to disease pathogenesis. CQ and HCQ impact B cell function. CQ and HCQ can reduce the production of autoantibodies by B cells[42]. The precise mechanisms are still being investigated, but it is believed that these drugs can interfere with B cell activation, proliferation, and differentiation into plasma cells, which are the antibody-producing factories. Similar to T cells, B cells also have different subsets with distinct functions. CQ and HCQ may influence the balance between these subsets, potentially suppressing the activation of autoreactive B cells while sparing those involved in protective immunity[47].

The effects of chloroquine and hydroxychloroquine on natural killer cells

Natural killer (NK)cells are part of the innate immune system and play a role in recognizing and eliminating infected or cancerous cells[48]. Recent evidence suggests that CQ and HCQ can also modulate NK cell function. Studies have shown that HCQ can increase the expression of killer cell lectin-like receptor G1 (KLRG1) on NK cells[49]. KLRG1 is an inhibitory receptor that can dampen NK cell activity, suggesting that HCQ might utilize this pathway to regulate NK cell responses and potentially prevent excessive inflammation.

CLINICAL APPLICATIONS OF CHLOROQUINE AND HYDROXYCHLOROQUINE FOR AUTOIMMUNE DISEASES

CQ and HCQ have demonstrated clinical efficacy in a range of autoimmune diseases, solidifying their role as valuable therapeutic options. Their pleiotropic mechanisms of action, impacting various aspects of the immune response, contribute to their effectiveness in managing diverse manifestations of these conditions. Figure 2 summarizes the clinical applications of CQ and HCQ across autoimmune diseases. In SLE, these drugs mitigate flares and organ damage; in RA, they alleviate symptoms and slow progression. SS patients benefit from reduced glandular dysfunction, while antiphospholipid syndrome (APS) management includes thrombosis prevention. Emerging roles in systemic sclerosis (SSc), myopathies, and vasculitis further demonstrate their versatility. These applications reflect the drugs’ ability to target diverse pathogenic pathways, as detailed in earlier sections. Current treatment guidelines provide specific recommendations for CQ/HCQ use across autoimmune diseases. For SLE, EULAR recommends HCQ at a daily dose not exceeding 5 mg/kg actual body weight as foundational therapy, to be continued indefinitely unless contraindicated by toxicity. In RA, ACR guidelines position HCQ as a first-line option for mild disease or as part of combination therapy with conventional DMARDs, particularly methotrexate. For SS, expert consensus suggests HCQ for predominant systemic manifestations, while in antiphospholipid syndrome, it is recommended for thromboprophylaxis, especially in obstetric cases[50].

Figure 2 Clinical applications of chloroquine and hydroxychloroquine for autoimmune diseases. This figure summarizes the clinical applications of chloroquine (CQ) and hydroxychloroquine (HCQ) across various autoimmune diseases, including Systemic Lupus Erythematosus (SLE), Rheumatoid Arthritis (RA), Sjögren's Syndrome (SS), antiphospholipid syndrome (APS), Systemic sclerosis, Inflammatory Myopathies (Dermatomyositis/Polymyositis), and Vasculitis. In SLE, CQ and HCQ reduce flares, cutaneous lesions, and lupus nephritis risk, in RA they are involved in pain relief, slowed joint damage, and synergy with methotrexate, in SS, they aid in the alleviation of sicca symptoms and extraglandular manifestations, and in APS, they prevent Thrombosis and improved pregnancy outcomes.

The effects of CQ and HCQ on SLE

SLE is a chronic autoimmune disease characterized by the production of autoantibodies and immune complex deposition, leading to inflammation and damage in multiple organs. HCQ has become a cornerstone of SLE management, demonstrating benefits across various disease manifestations. HCQ is effective in treating cutaneous lupus erythematosus (CLE), reducing skin lesions, photosensitivity, and improving overall skin health[13]. HCQ alleviates joint pain and swelling in lupus arthritis, improving joint function and reducing the need for other immunosuppressive agents[51]. HCQ can help manage serositis, including pleuritis and pericarditis, reducing inflammation and fluid accumulation[51]. HCQ has shown promise in preventing the development of lupus nephritis, a severe complication of SLE[52]. Its use may contribute to preserving renal function and reducing the risk of end-stage renal disease. While the evidence is less robust, HCQ may have a role in managing some neurological manifestations of SLE, such as headaches and cognitive dysfunction[53]. HCQ plays a crucial role in reducing the frequency and severity of lupus flares[54]. HCQ stabilizes immune function and prevents disease flares. Recent research suggests that HCQ may positively impact long-term outcomes in SLE patients, potentially reducing damage accrual and improving overall prognosis[55]. Beyond its immunomodulatory effects, HCQ may also offer cardiovascular benefits in SLE, potentially reducing the risk of cardiovascular events, a major concern in this patient population[56].

RA management, CQ and HCQ

RA is a chronic autoimmune disease primarily affecting the joints, leading to inflammation, pain, swelling, and progressive joint damage. CQ and HCQ have been used in RA management for their ability to modify the disease course and alleviate symptoms. HCQ can reduce pain, swelling, and stiffness in RA joints, improving joint function and quality of life[57]. While not as potent as some other DMARDs, HCQ can contribute to slowing the progression of joint damage in RA, particularly when used in combination with other medications[57]. HCQ's immunomodulatory effects contribute to reducing systemic inflammation in RA, impacting both joint and extra-articular manifestations[58]. HCQ is often used in combination with other DMARDs in RA management, enhancing therapeutic efficacy and targeting multiple pathogenic pathways.

The role of CQ and HCQ in SS

SS is an autoimmune disease characterized by dryness of the eyes and mouth due to lymphocytic infiltration of the lacrimal and salivary glands[59]. CQ and HCQ have a role in managing certain aspects of SS. HCQ may help alleviate some glandular manifestations of SS, such as sicca symptoms (dry eyes and mouth)[60]. HCQ may also be used to manage certain extraglandular manifestations of SS, such as joint pain and fatigue. Research is ongoing to identify potential biomarkers that can predict response to HCQ in SS patients, allowing for more personalized treatment approaches[60]. Studies suggest that HCQ may influence microbiota dysbiosis in SS, potentially contributing to its therapeutic effects[11]. Sandino-Bermúdez et al[61] suggested that, while HCQ remains a central SS treatment due to its favorable safety profile, data on long-term outcomes such as damage accrual, quality of life, mortality, and cardiovascular risk prevention are lacking.

Applications of CQ and HCQ in other autoimmune diseases

CQ and HCQ have also shown potential benefit in conditions such as systemic sclerosis, vasculitis, and antiphospholipid syndrome. HCQ plays a crucial role in preventing thrombosis and improving pregnancy outcomes in APS, an autoimmune disease associated with an increased risk of blood clots[62]. HCQ and CQ may offer some benefits in managing skin thickening and Raynaud's phenomenon in SSc, a complex autoimmune disease affecting the skin and internal organs[63]. CQ and HCQ have been used in the treatment of inflammatory myopathies, such as dermatomyositis and polymyositis, potentially helping to reduce muscle inflammation and improve muscle strength[64]. HCQ may have a role in managing certain types of vasculitis, autoimmune diseases characterized by inflammation of blood vessels[7]. Fierro et al[62] found that HCQ use was associated with a significant reduction in the risk of thrombotic events in APS patients compared to non-users. HCQ use was also associated with a significant reduction in the risk of pregnancy complications in APS patients.

VARIABILITY IN RESPONSE TO CQ/HCQ THERAPIES

Despite the well-documented efficacy of CQ and HCQ across above-mentioned autoimmune diseases, significant inter-individual variability in therapeutic response has been observed. This variability can be attributed to several factors. Genetic polymorphisms, particularly in cytochrome P450 enzymes such as CYP2D6 and CYP3A4, influence drug metabolism and plasma concentrations, which result in altering the efficacy and toxicity profiles[65,66]. Moreover, genetic variants in ABCB1, encoding P-glycoprotein, may affect intracellular drug accumulation[67]. Patients with systemic SLE may exhibit distinct immunological phenotypes, such as type I interferon signatures or predominant B-cell dysregulation, that affect response to immunomodulatory therapy[68]. Furthermore, concomitant medications such as corticosteroids, methotrexate, or biologics may synergize with or antagonize the effects of CQ/HCQ, influencing clinical outcomes[5]. Recognizing and integrating these individual-level factors into treatment planning could alter the precision and effectiveness of CQ/HCQ therapy in autoimmune conditions.

PREDICTIVE BIOMARKERS OF CQ/HCQ RESPONSE

Recent research has discovered and introduced several biomarkers to predict response to CQ and HCQ therapy in autoimmune diseases[69]. These include whole blood HCQ concentration, which correlates with disease activity control in SLE[70], and LAMP3 mRNA expression in salivary gland tissue, which may predict therapeutic benefit in SS. Other proposed biomarkers include anti-Ro/SSA and anti-dsDNA titers, complement levels (C3, C4), interferon gene signatures, and T cell subsets such as PD-1+ and KLRG1+ populations[71]. Moreover, microRNA profiles, IL-6 and TNF-α baseline levels, and expression of autophagy-related genes (such as LC3B, ATG5) are being explored as potential predictors[72]. Utilizing these biomarkers into clinical decision-making could facilitate a more personalized approach to CQ/HCQ therapy.

SAFETY AND SIDE EFFECTS OF CQ AND HCQ

CQ and HCQ are generally well-tolerated, but their long-term use can be associated with various side effects. Understanding these potential adverse events is crucial for safe and effective use. This section provides a detailed account of the safety concerns associated with CQ and HCQ, including ocular toxicity, cardiac issues, other side effects, and specific concerns related to pregnancy, lactation, and drug interactions.

CQ and HCQ share similar pharmacokinetic profiles characterized by high oral bioavailability (approximately 70%–80%) and rapid absorption, with peak plasma concentrations typically reached within 2-4 hours post-ingestion[73]. Both drugs exhibit extensive tissue distribution, particularly in melanin-containing tissues, the liver, lungs, kidneys, and spleen[74]. They have exceptionally long elimination half-lives, ranging from 40 to 50 days for HCQ, due to slow release from tissues and hepatic metabolism via cytochrome P450 enzymes, especially CYP3A4 and CYP2D6. Renal excretion accounts for a substantial portion of their clearance[75]. Pharmacodynamically, CQ and HCQ act by increasing lysosomal pH, which interferes with antigen processing, autophagy, and Toll-like receptor signaling pathways, thereby attenuating inflammatory responses[5].

While CQ and HCQ are generally well-tolerated, their long-term use requires careful consideration of the risk-benefit balance, particularly in chronic autoimmune conditions. The benefits of HCQ are well established, especially in SLE, where it has been shown to reduce disease flares, improve long-term survival, and lower the risk of thrombosis and cardiovascular events[53]. In RA, HCQ contributes to symptom control and can be used synergistically with other DMARDs to slow disease progression[76]. However, these therapeutic advantages must be assessed against potential adverse events, particularly ocular toxicity and cardiac complications. Ocular toxicity, particularly retinopathy, is a major concern with long-term CQ and HCQ use. The risk of retinopathy can reach 7.5% in patients using HCQ for over five years[36]. This risk is influenced by several factors, including daily dose, treatment duration, and individual susceptibility. The risk of retinopathy increases significantly after five years of continuous use, especially at doses exceeding 5 mg/kg/day of actual body weight[77]. Regular ophthalmologic screening is crucial for early detection and prevention of irreversible damage[12,36,78].

Cardiac issues, including QT prolongation, arrhythmias, and cardiomyopathy, have been reported with CQ and HCQ use. In many patients, especially those with high disease activity or multiple organ involvement, the clinical benefits of CQ/HCQ outweigh these risks when used judiciously[79]. Recent studies have highlighted the risk of QT prolongation and potential for serious cardiac events, especially in patients with additional risk factors[80,81]. In rare cases, HCQ-induced cardiomyopathy has been reported, emphasizing the need for careful monitoring of cardiac function in patients on long-term therapy[82]. Personalized treatment strategies that incorporate disease severity, duration of therapy, comorbidities, and regular safety monitoring can help optimize outcomes and mitigate long-term complications.

Other common side effects of CQ and HCQ include gastrointestinal issues (nausea, vomiting, diarrhea) and dermatological manifestations (rashes, hyperpigmentation). These side effects are usually mild and manageable. However, rare but serious side effects, such as neuromuscular toxicity, hematological abnormalities, and hypersensitivity reactions, can occur[83,84].

HCQ is generally considered safe during pregnancy, with studies showing no increased risk of congenital defects or adverse pregnancy outcomes[8]. However, recent data suggests potential widespread empirical use of HCQ in pregnancy, even without clear autoimmune diagnoses, raising concerns about overprescription and the need for standardized management protocols[85]. CQ and HCQ can interact with other medications, potentially leading to adverse effects. A significant interaction has been reported with proton pump inhibitors, which can reduce the efficacy of CQ and HCQ by interfering with their lysosomal accumulation[86].

CQ and HCQ are known to interact with several classes of medications, potentially altering therapeutic efficacy or increasing the risk of adverse events[87]. These interactions are primarily related to their effects on lysosomal pH, hepatic metabolism via cytochrome P450 enzymes, and cardiac electrophysiology[88]. Table 1 summarizes key drug interactions, associated mechanisms, and clinical recommendations for safe co-administration.

Table 1 Clinically relevant drug interactions with chloroquine and hydroxychloroquine.

Interacting drug/class	Mechanism of interaction	Clinical implication	Recommendation
Proton pump inhibitors	Reduce lysosomal acidification, affecting HCQ accumulation	Decreased therapeutic efficacy	Consider dose separation or switching to H2 antagonists
Digoxin	Inhibition of P-glycoprotein leads to increased digoxin levels	Risk of digoxin toxicity	Monitor serum digoxin levels
Methotrexate	Overlapping immunosuppression, altered metabolism	Potential increased toxicity	Monitor hepatic and hematologic parameters
Tamoxifen	Additive risk of retinopathy	Increased ocular toxicity	Ophthalmologic monitoring if used long-term
Antacids (e.g., magnesium/aluminum)	Interfere with GI absorption of CQ/HCQ	Reduced drug bioavailability	Separate doses by at least 4 hours
QT-prolonging agents (e.g., azithromycin, amiodarone)	Additive risk of QT prolongation	Increased risk of arrhythmias	Baseline and follow-up ECG monitoring
Cytochrome P450 inhibitors (e.g., ketoconazole)	Impaired metabolism of CQ/HCQ	Elevated plasma drug concentrations	Dose adjustment and monitoring for toxicity
Insulin and antidiabetic agents	HCQ enhances insulin sensitivity and lowers blood glucose	Risk of hypoglycemia	Monitor blood glucose closely

CQ: Chloroquine; HCQ: Hydroxychloroquine; GI: Gastrointestinal; QT: QT interval; ECG: Electrocardiogram.

CONCLUSION

CQ and HCQ are valuable immunomodulatory agents in autoimmune disease management, with established efficacy but important safety considerations. These drugs work through multiple mechanisms, including the inhibition of Toll-like receptor signaling, suppression of antigen presentation, and modulation of autophagy. While CQ and HCQ have shown promise in the treatment of autoimmune diseases, their use is associated with several challenges and controversies. One major concern is the potential for long-term side effects, such as ocular toxicity and cardiac toxicity. Therefore, patients on long-term CQ or HCQ therapy require regular monitoring for potential adverse effects. Another challenge is the need for personalized treatment approaches. As each patient's condition is unique, treatment plans need to be tailored to the individual's specific needs and circumstances. This approach may involve combining CQ or HCQ with other immunosuppressive agents or using them in conjunction with other therapies.

In conclusion, CQ and HCQ have shown promise in the treatment of autoimmune diseases, but their use is associated with several challenges and controversies. While these drugs have a role in the management of autoimmune conditions, their use should be carefully considered in light of the potential risks and benefits. Further research is needed to fully elucidate the therapeutic potential of CQ and HCQ in autoimmune diseases, to optimize their use in clinical practice, and to develop safer and more effective therapies.

Future perspectives

Recent clinical trials have explored the use of HCQ in conjunction with methotrexate, azathioprine, and mycophenolate mofetil in SLE and RA, demonstrating enhanced disease control and reduced steroid dependency. In parallel, studies investigating biologic combinations, such as HCQ with belimumab or rituximab, are ongoing to assess synergistic effects in refractory or severe cases[89]. Accordingly, studies are evaluating the synergistic effects of HCQ with methotrexate in RA and with other immunosuppressants in SLE[90].

Furthermore, there is growing interest in exploring novel immunomodulatory mechanisms beyond those currently attributed to CQ and HCQ. Research is focused on identifying specific molecular targets and signaling pathways that can be modulated to achieve more precise and effective therapeutic interventions. Quantitative proteomics approaches, as demonstrated by Zhao et al[31], are providing new information about the potential targets and action mechanisms of hydroxychloroquine.

Future research should also focus on addressing the challenges and controversies surrounding the use of CQ and HCQ, such as long-term side effects and the need for safer and more effective therapies[18]. Future therapeutic strategies should also focus on minimizing toxicity while enhancing efficacy through precision medicine approaches. Advances in nanocarrier systems, such as liposome-encapsulated or polymeric nanoparticle formulations, are being investigated to improve tissue-specific delivery and reduce systemic exposure[91]. Moreover, identifying novel immunomodulatory targets, such as TLR7/9 antagonists, AIM2 inflammasome regulators, and lysosomal pH-modulating agents, could be useful in finding a way for CQ/HCQ derivatives with improved safety profiles[22]. Combination regimens pairing HCQ with biologics (such as belimumab, and rituximab) or targeted small molecules (such as JAK inhibitors, BTK inhibitors) are currently under evaluation in clinical trials and may offer additive or synergistic benefits in refractory cases[92]. These innovative approaches are essential to enhance therapeutic outcomes while mitigating long-term adverse effects associated with monotherapy.