Bone is a dynamic organ constantly undergoing remodelling, which consists of two coordinated processes: Bone resorption by osteoclasts and bone formation by osteoblasts. The coupling imbalance between bone remodelling, indicated by low bone-forming osteoblast activity and high bone-resorbing osteoclast activity, is the basic mechanism of osteoporosis. Osteoporosis is a bone disorder in which bones become weak, brittle and fragile, leading to increased susceptibility to fracture (1). A direct association of autoimmune diseases [such as rheumatoid arthritis and systemic lupus erythematosus (SLE)] and inflammatory diseases (such as systemic sclerosis and sarcoidosis) with impaired bone health has been reported, which can be explained by inflammation, immune dysregulation and the use of medications (2–4). Chronic inflammatory responses promote osteoclastic activity and accelerate bone breakdown, resulting in bone loss (5). T helper 17 (Th17) cells that produce IL-17 are overactive in various autoimmune and inflammatory disorders, which induces the receptor activator of nuclear factor κ-Β ligand (RANKL) and inhibits osteoprotegerin (OPG) production by osteoblasts (6). The long-term use of glucocorticoids and methotrexate adversely affect bone metabolism and increase the risk of fractures (7,8).

Chloroquine (CQ) and hydroxychloroquine (HCQ) are medications used to prevent and treat malaria caused by Plasmodium parasites. CQ and HCQ interfere with haemoglobin proteolysis in the parasite causing the accumulation of toxic byproducts to kill the parasite (9). Apart from their anti-plasmodial property, CQ and HCQ have diverse pharmacological effects on other medical conditions, including autoimmune diseases (10), cancer (11), viral infections (12), metabolic syndrome (9) and skeletal disorders (13,14), because of their anti-inflammatory and immunomodulatory properties. Chronic therapy with CQ or HCQ is necessary to manage autoimmune diseases (10). Therefore, investigating the effects and underlying molecular mechanism of CQ or HCQ on the skeleton could yield valuable information to prevent osteoporosis with minimal side effects.

The present review aims to summarize the role of CQ and HCQ in protecting the bones from becoming porous. It also highlights the underlying molecular mechanisms that govern the action of CQ and HCQ. The present review may aid researchers and interprofessional healthcare teams in recognizing the skeletal-protecting potential of CQ and HCQ, and their mechanisms of action.

The research question set for the present review was as follows: ‘What are the effects of CQ and HCQ on bone health and their underlying mechanism of actions?’. To identify relevant studies to be included in the present review, a comprehensive literature search was caried out using two electronic databases [PubMed (https://pubmed.ncbi.nlm.nih.gov/) and Web of Science (https://www.webofknowledge.com/)] in June 2024 using the search string ‘(chloroquine OR hydroxychloroquine) AND (bone OR osteoporosis OR fracture OR osteoblast OR osteoclast OR osteocyte)’. All available primary studies examining the effects of CQ or HCQ on bone health from the inception of the databases were considered. Articles without relevant results or primary data (including reviews, letters to the editor, perspectives, books and book chapters) were not considered. Conference abstracts and proceedings were excluded because of the preliminary nature of the data and the potential of data duplication from full-text articles. Articles not written in English were also excluded.

The literature search using the two electronic databases identified 1,383 records (PubMed, 755; Web of Science, 628). Duplicates (n=442) were removed. Preliminary screening of the title and abstract identified reviews (n=115), commentary (n=1), preprint (articles under review; n=1), conference proceedings (n=2), non-English articles (n=56), irrelevant articles (n=705) and retracted articles (n=2) to be excluded. Full-text articles were screened for eligibility based on the inclusion and exclusion criteria. All original research articles reporting bone-related changes and understanding of the mechanisms resulting from the monotherapy of CQ and HCQ or combined therapy with other drugs were included. Three articles were excluded after screening of full-text articles because they did not report the direct effects of CQ and HCQ on bone parameters as study outcomes. A total of 56 articles adhering to the inclusion criteria were included in the present review (Fig. 1).

Accumulating evidence has revealed heterogenous outcomes for the effects of CQ and HCQ on bone health, whereby positive, negative and negligible effects have been observed (Table I). The literature search using the search string identified an early small-scale study investigating the effects of CQ on serum bone markers in patients with rheumatoid arthritis and seronegative spondyloarthropathies. The level of serum osteocalcin (OCN) was increased, while the level of alkaline phosphatase (ALP) remained unchanged after CQ therapy at a dose of 250 mg/day (15). Previous studies involving humans have shifted to elucidate the effects of HCQ treatment on bone health. In a retrospective nationwide cohort study involving patients with rheumatoid arthritis with and without new-onset cardiovascular disease (n=2,534), who were aged ≥40 years, the adjusted hazard ratio (HR, 0.12; 95% CI, 0.03–0.47) of developing osteoporotic vertebral fracture was lower in patients receiving HCQ in a Taiwanese cohort (13). In Japan, a cross-sectional study was conducted on outpatients with SLE (n=246) to examine the association of HCQ use with T-score and vertebral fractures. HCQ use was associated with high lumbar (coefficient, 0.44; 95% CI, 0.077–0.81) and femoral (coefficient, 0.33; 95% CI, 0.020–0.63) T-scores. In addition, HCQ use was associated with a lower incidence of vertebral fractures (odds ratio, 0.098; 95% CI, 0.013–0.73) (14).

Both et al (16) recruited patients with rheumatoid arthritis (n=63) aged ≥18 years, who were receiving HCQ as treatment. The levels of a serum bone resorption marker [β C-terminal telopeptide (β-CTx)] and an inflammatory marker (CRP) at baseline and after 6 months were measured. Blood samples were collected at random moments throughout the day without fasting. HCQ treatment for 6 months decreased the levels of β-CTx after adjusted for the decrease in serum CRP, indicating that HCQ had a direct effect on bone resorption, and the decrease in β-CTx was independent of the inflammatory response reduction (16). Yu et al (17) studied the effects of combined therapy of disease-modifying antirheumatic drugs (iguratimod, methotrexate and HCQ) on bone mineral density (BMD) in patients with rheumatoid arthritis (n=76; age, 59.05±10.95 years). BMD at the lumbar spine (L1-L4), left femoral neck and left total hip increased after 48 weeks of treatment. The combined therapy also reduced rheumatoid factor, CRP, the erythrocyte sedimentation rate and anti-cyclic citrullinated peptide (17).

However, two human studies reported negative effects of HCQ on bone health. A recent study by Park et al (18) found that patients with juvenile-onset SLE and low BMD had a higher cumulative HCQ dose (10.10 g/kg), whereas those with non-low BMD had a lower cumulative HCQ dose (4.64 g/kg). In addition, the cumulative HCQ dose was negatively associated with the lumbar spine BMD Z-score in patients with juvenile-onset SLE (18). Heidari et al (19) revealed that the BMD of the femoral neck and lumbar spine in patients with rheumatoid arthritis (n=19; age, 54.5±7.7 years) decreased after treatment with low-dose methotrexate (≤15 mg/week) alone or in combination with HCQ or sulfasalazine and prednisolone (5 mg) (19).

One study by Carbone et al (20) revealed the negligible effects of HCQ on bone health. A total of 363 women with rheumatoid arthritis aged 50–79 years who were treated with HCQ alone or combination therapy of HCQ and methotrexate were enrolled. Fracture incidence following HCQ use was estimated over a median follow-up length of 6.46 years. No significant association was identified between HCQ use and fracture incidence (HR, 1.0; 95% CI, 0.7–1.5). A similar relationship was noted between HCQ and methotrexate use and fracture incidence (HR, 0.9; 95% CI, 0.5–1.6) (20).

Collectively, the findings from the majority of the aforementioned studies indicated the potential protective effects of CQ and HCQ in improving bone health (13–17). One study reported that HCQ alone had an inhibitory effect on bone resorption, which was not influenced by the reduction of inflammation (16). On the contrary, another study indicated that inflammatory indicators decreased in response to combination drug therapy consisting of iguratimod, methotrexate and HCQ (17). Therefore, the suppression of the inflammatory reaction observed by Yu et al (17) may be at least partially attributed to iguratimod and/or methotrexate. Several methodological limitations (such as single-arm studies, small-scale samples, short study duration and lack of standardisation) may lead to fallacious conclusions, as well as less accurate and reliable results. The dose of HCQ that was administered was also not mentioned in some of the aforementioned studies (13,14,16,20). Two studies reported negative outcomes (18,19), which may be attributed to the small sample size and lack of control group.

The effects of CQ and HCQ on bone health have been examined using healthy animals and animal models, including osteoporotic, knockdown, arthritic, fractured and osteotomy models (Table II). An early study examined the quantitative distribution of CQ radioactivity in the bone tissues of animals. Adult and 8-day-old Sprague-Dawley rats, as well as 1-day-old leghorn chickens received intraperitoneal (i.p.) injections of [3-¹⁴C]CQ. The radioactivity of CQ recovered in the bone was 11.6%, indicating the high affinity of CQ for bone (21).

Dexamethasone is a glucocorticoid medication used to relieve inflammation (including swelling, heat, redness and pain) that causes secondary osteoporosis (22). The continuous administration of dexamethasone (2 mg/kg) for 35 days resulted in lower ALP activity and higher tartrate-resistant acid phosphatase (TRAP) activity in mice. The i.p. injection of CQ (50 mg/kg) for 7 days (between days 28 and 35) caused a reduction in TRAP activity without altering ALP activity (23). TRAP is a key enzyme produced by osteoclasts to break down bone matrix proteins during bone resorption. Thus, lower TRAP activity suggests reduced osteoclast activity and bone resorption. In another dexamethasone-induced osteoporotic mouse model, i.p. injection of dexamethasone (1 mg/kg) for 8 weeks reduced the bone volume/tissue volume (BV/TV), trabecular number (Tb.N), osteoblast number and osteoblast surface, and increased the osteoclast number (Oc.N). The injection of CQ (2 mg/kg; i.p.) every 2 days mitigated dexamethasone-challenged bone loss (increased BV/TV and Tb.N) by reducing Oc.N without affecting the parameters linked to osteoblasts and bone formation (24).

Bilateral ovariectomy removes the ovaries and eliminates the secretion of oestrogen in animals, representing a preclinical model for postmenopausal osteoporosis (25). The administration of CQ (10 mg/kg; i.p.) twice a week for 8 weeks reduced cross-linked C-telopeptide of type 1 collagen (CTX; a bone resorption marker), but did not change the level of propeptide of type I procollagen (P1NP; a bone formation marker) in ovariectomised rats (26). Lin et al (24) investigated the effects of CQ (2 mg/kg; i.p.) administered every 2 days for 8 weeks on bone parameters using ovariectomised mice. Treatment with CQ resulted in a reduction in Oc.N, but no change was observed in osteoblast counts in ovariectomy-induced bone loss (24). In another study, CQ prevented ovariectomy-induced bone loss as indicated by increased BV/TV and reduced Oc.N in C57BL/6 mice (27). However, a study by Liang et al (28) demonstrated different findings, where i.p. injection of CQ (50 mg/kg/day) for 1 month reduced BV/TV, Tb.N and trabecular thickness, and increased the trabecular pattern factor but caused no change in the structure model index of ovariectomised mice (28). Another animal study reported that the administration of CQ at 12.5 mg/kg for 60 days was insufficient to induce any changes in bone markers (ALP and TRAP) and bone microstructure [BV/TV, Tb.N, trabecular separation (Tb.Sp) and trabecular bone area] in ovariectomised rats (29).

Using the ovariectomised animal model, several studies have demonstrated that the presence of CQ might enhance or weaken the bone-protecting effects of other compounds. For example, CQ acted synergistically with curcumin (a compound isolated from turmeric root) in ameliorating bone loss by increasing ALP expression and decreasing TRAP expression, resulting in good bone microarchitecture in ovariectomised rats (29). On the contrary, the combination of CQ with quercetin (a polyphenol flavonoid found in several fruits, vegetables, leaves, seeds and grain) (26) and icariin (a natural flavonoid glycoside extracted from Herba epimedii with a bone-protecting property) (28) weakened their actions on attenuating bone loss in ovariectomised mice. The co-administration of CQ with quercetin (50 mg/kg; oral administration) reduced P1NP in ovariectomised rats compared with those treated with CQ only (26). When CQ was co-administered with icariin, CQ diminished the icariin-induced increase of bone mass in ovariectomised mice (28).

RANKL is a cytokine essential for osteoclast differentiation (30). A study looked at 8-week-old mice that were injected (i.p.) with RANKL (1 mg/kg) at 24 h intervals for 3 days to stimulate osteoclasts for bone resorption. The injection of CQ (50–200 mg/kg) was given 1 h before every RANKL injection. All bone microstructural parameters, including BV/TV, Tb.Th, Tb.N and Tb.Sp, were measured after the last injection of RANKL. The findings revealed marked increases in BV/TV and Tb.N but a reduction in Tb.Sp in RANKL-induced bone loss in mice (30).

D-galactose is a monosaccharide sugar that forms lactose when combined with glucose. It causes pathological changes that resemble natural ageing, including reduced bone mass (31). The prolonged administration of D-galactose increases advanced glycation end products, receptors for advanced glycation end products and nicotinamide adenine dinucleotide phosphate oxidase, leading to the excessive accumulation of reactive oxygen species (ROS) and mitochondrial damage (32). In D-galactose-induced osteoporotic rats, treatment with 10 mg/kg CQ for 4 consecutive weeks increased tibial BMD, femoral BMD, serum phosphate levels and OPG, but decreased serum calcium, ALP, OCN, RANKL, cathepsin K (CTSK), TRAP and the percentage of bone loss (33).

In a study using female mice with autosomal dominant osteopetrosis type II mutation (ADO2) that developed osteopetrosis, the administration of CQ (1–10 mg/kg/day) via drinking water for 6 months increased osteoclast activity, as evidenced by increased TRAP expression. However, no change was observed in the improvement of the osteoporotic bone phenotype of ADO2 mice (34). Foxo1 is a transcription factor abundantly found in bone, indicating its role as a mediator for skeletal development. Foxo1 is highly expressed in bone tissues, and its silencing impairs skeleton formation (35,36). The effects of CQ (2 mg/kg given every 2 days) via injection (i.p.) on bone have been investigated in Foxo1-overexpressing mice. Foxo1-overexpressing mice treated with CQ exhibited lower BV/TV and Tb.N but higher Tb.Sp in the femur as compared to the wild-type controls. The presence of CQ also downregulated the expression levels of several osteogenic markers, including Runt-related transcription factor 2 (Runx-2), osterix and ALP. The findings of the study reiterated that CQ reversed the favourable osteogenic effects of Foxo1 overexpression (36).

Several studies have demonstrated the effects of HCQ on bones. In a mouse model of arthritis, HCQ was administered by oral gavage at a dose of 80 mg/kg daily for 15 days. Oc.N, osteoclast surface (Oc.S) and CTX in the arthritic mice treated with HCQ were decreased compared with those in mice treated with normal saline (37). However, HCQ has been reported to induce oxidative stress, thereby impairing fracture healing (38). A study by Topak et al (38) utilised male Wistar albino rats with osteotomy at the mid-distal region of the right femur, which were administered HCQ sulphate orally at various doses (0.45–3 mg) for 5 days, and revealed that this resulted in a decrease in the radiological score, ALP, callus/diaphysis ratio, callus development and BMD. Mechanistically, the circulating antioxidant enzyme activities [catalase (CAT), superoxide dismutase (SOD) and glutathione peroxidase (GPx)] were increased, which could be a response to neutralise the overwhelming oxidative stress (as lipid peroxidation was elevated) in rats treated with HCQ sulphate (38). In male rats subjected to open diaphyseal femur fracture, the oral administration of HCQ sulphate (160 mg/kg) increased malondialdehyde levels. In HCQ-treated animals, no improvement was observed in the histological healing scores, whereby the cartilage callus tissue disappeared and the area of immature bone tissue was increased. The total callus diameter at the femur was lower in the HCQ group compared with the control group. Levels of ALP were decreased, whereas OCN and osteopontin (OPN) levels were unchanged. Meanwhile, CTSK and TRAP were increased in the HCQ group relative to the control group (39). Another recent study utilised male rats with a closed fracture created at the proximal region of the femur. Treatment with HCQ (3–10 mg/kg) two times per day (once in the morning and once in the evening) for 15 days did not improve the histologic scoring compared with the negative control (40).

In conclusion, bone is an important storage organ for CQ. The aforementioned studies revealed that different doses, treatment frequency and duration affect the skeletal actions of CQ. Long-term daily administration adversely affects bone health. CQ has more prominent effects on osteoclastic activity and bone resorption compared with osteoblastic activity and bone formation. In addition, CQ may serve as a modulator to reduce bone resorption in osteoporotic and arthritic conditions whilst decreasing osteoblastic activity and increasing osteoclastic activity during osteopetrosis. Although CQ may appear to be a potential strategy in the treatment of osteoporosis and osteopetrosis, CQ showed no beneficial effect in inducing fracture healing.

In vitro studies have revealed that CQ and HCQ inhibit osteoblastic (28,42–48,52) and osteoclastic (16,23,24,27,30,34,49,53–58,60–64) activities simultaneously. The inhibition of osteoblast differentiation and mineralisation by CQ and HCQ is mediated through the suppression of autophagy, activation of oxidative stress and osteoblast apoptosis. Autophagy dysfunction increases ROS production and accelerates cell senescence, leading to increased osteoblast apoptosis and decreased numbers of osteocytes (83). In osteoclasts, CQ inhibits osteoclastogenesis via the suppression of the RANK/RANKL/OPG system (49,62,69,70), autophagy (53,54,57,58,64,80) and the inflammatory response (64). Although CQ treatment increases the expression levels of RANK and c-fms (the receptors for RANKL and M-CSF respectively), the OPG/RANKL ratio is increased (69,70), and the expression levels of c-Fos remain unchanged (55,68), thereby leading to the downstream inhibition of ERK and NFATc1 expression during osteoclastogenesis. Furthermore, in osteoclasts, the induction of autophagy contributes to pro-osteoclastogenesis. CQ inhibits autophagy by preventing TRAF3 degradation and limiting RANKL-induced osteoclast formation (27,56). CQ also increases the lysosomal pH, thereby causing the accumulation of mTOR and phosphorylated-mTOR protein to subsequently inhibit autophagy (57,61). However, the inhibition of autophagy by HCQ has not been observed in osteoclasts. In addition, CQ and HCQ enhance osteoclast apoptosis in mature osteoclasts (64).

The inhibition of bone formation and bone resorption by CQ and HCQ occur in parallel, but their net effect on the skeletal system requires further investigation with a study design focusing on the effects of CQ and HCQ on bone health. The selection of the treatment dose, frequency and duration of CQ and HCQ should be carefully considered. Based on the aforementioned studies, the intervention dose of CQ was 2–200 mg/kg in mice and 10 mg/kg in rats (23,24,26–30,33,34,36). Meanwhile, the effective HCQ dose in protecting the bone was 80 mg/kg (37). The calculated human equivalent dose ranged between 9.73 and 973 mg for CQ and was ~389 mg for HCQ in an adult weighing 60 kg. In humans, 200–250 mg CQ or HCQ was used, which was in line with the well-tolerated doses of CQ and HCQ for rheumatic diseases (200–400 mg/day) and novel coronavirus-19 (500 mg/day) (9).

In animals, the short-term continuous daily administration of CQ (<28 days) was revealed to be protective to bone (23,27), and the intermittent administration of CQ was recommended to exert beneficial effects on bone for long-term treatment (24,26). These synergistic effects may not be observed in combination therapy with CQ and other osteoprotective agents (28). Only a paucity of animal studies has investigated the skeletal effects of HCQ (37–40); thus, drawing a concrete conclusion is challenging. The outcomes obtained from preclinical studies were not reflected in the clinical setting. Firstly, the effectiveness of CQ on bone has been widely established in vitro and in vivo; however, the use of HCQ has been preferred in human studies. HCQ is a metabolite derived from CQ by adding a hydroxyl group, making it less toxic with fewer side effects and allowing it to dissolve more easily in the body (84). Thus, HCQ is a safer medication for patients (84).

Both CQ and HCQ have excellent oral absorption, good bioavailability, high volume of distribution (extensive drug sequestration by tissues), are hepatically metabolized, have a long half-life and their metabolites are excreted through urine and faeces (85). They can cross the placenta but toxicity in the foetus has not been reported (85). Toxicity develops rapidly (1–3 h after ingestion) in the case of overdose (85). The use of CQ and HCQ is associated with several adverse events such as gastrointestinal discomfort, allergic reactions, retinal damage, cardiomyopathy and skin hyperpigmentation (86).

Although CQ and HCQ have protective effects on inflammatory bone loss, the risk of using these agents should not be overlooked. Dose optimisation and regular monitoring are the key for the safe use of CQ and HCQ. Regular ophthalmologic monitoring is important for long-term CQ and HCQ use to prevent retinal toxicity, which can cause irreversible vision loss if not detected early (87). The patient history of ocular disease should be considered before prescribing these medications (87). In addition, regular cardiac monitoring (including electrocardiogram and echocardiography) could be beneficial in detecting cardiac complications early, ensuring the safe prolonged use of these drugs (88). Dose optimisation for CQ and HCQ focuses on balancing therapeutic benefits whilst minimising toxicity. To further minimise risks, intermittent or pulsed dosing strategies (given every other day) may be considered for long-term use. Further research is necessary to confirm the efficacy of intermittent or pulsed dosing in metabolic bone disorders.

Effective bone health monitoring (including diagnostic assessments, regular follow-ups and lifestyle evaluations) is key for the prevention, early detection and management of bone disorders. Dual-energy X-ray absorptiometry is the gold standard for measuring BMD. The assessment is recommended every 1–2 years to monitor bone loss in women >65 years old, men >70 years old and younger individuals with risk factors such as oestrogen deficiency, smoking, low body mass, prior fractures or chronic glucocorticoid use (89). Micro-computed tomography, an advanced imaging technique that can generate three-dimensional images, enables the high-resolution evaluation of density, geometry and microarchitecture of mineralised tissues (90). The evaluation of bone turnover markers reflects the rate of bone remodelling, which may provide additional information on bone health and the effectiveness of osteoporosis treatments (91). Consistent follow-ups are essential to ensure medication adherence and adequate calcium and vitamin D intake, as well as to address lifestyle factors such as physical activity, smoking and alcohol consumption (92). All these approaches are non-invasive and suitable for human studies. Meanwhile, in vivo samples enable the use of bone histomorphometry and a universal testing machine to characterise bone cells, analyse bone microstructure, assess the bone remodelling rate and evaluate bone strength.

The currently available evidence has limitations that need to be addressed. Several of the included studies used CQ and HCQ as inhibitors for autophagy rather than directly investigating the skeletal effects of CQ and HCQ (29,53,66,80). In addition, in certain studies, a statistical test was not performed to compare the CQ or HCQ group and a control group because determining the effect of CQ or HCQ on bone was not the primary objective of the study (28,49,68). Clinical evidence supporting the use of CQ and HCQ for bone health remains limited. The majority of the available data are derived from observational studies in patients with autoimmune diseases such as rheumatoid arthritis and SLE (13–20). Notably, to the best of our knowledge, no large-scale randomised clinical trials have specifically assessed the impact of CQ or HCQ on osteoporosis, fracture prevention or long-term skeletal health. The absence of targeted research makes it difficult to determine their optimal dosing, duration and effectiveness in managing metabolic bone disorders.

The future development trends in the use of CQ and HCQ for bone health may focus on targeted delivery systems, the development of safer analogues and the integration of precision medicine approaches. Various nanotechnology platforms, including liposomes, polymeric nanoparticles, dendrimers, polyelectrolyte complexes, lipid-based nanoparticles and metallic nanoparticles, have been explored to enhance the reformulation of CQ and HCQ (93). These approaches aim to improve target tissue exposure while increasing overall safety and efficacy (93). In addition, developing novel CQ and HCQ analogues through molecular modifications can enhance their pharmacokinetic and pharmacodynamic properties, minimise undesirable side effects and reduce costs (94), while preserving their bone-protective properties. Researchers are also encouraged to explore precision medicine approaches by identifying genetic or metabolic markers to monitor treatment efficacy and determine the patients who would benefit from CQ- and HCQ-based therapies.

While CQ and HCQ show potential for modulating bone metabolism, several gaps in knowledge remain. Further in vitro studies are needed to elucidate dose-dependent effects on osteoblasts, osteoclasts and osteocytes, particularly from low to high concentrations of CQ and HCQ. Potential drug interactions and synergistic effects of CQ and HCQ with osteoporosis medications (such as bisphosphonates, denosumab, selective oestrogen receptor modulators, vitamin D and calcium) and natural anti-osteoporotic products should be investigated using cell culture models. Long-term animal studies are necessary to evaluate the effects of chronic CQ and HCQ use on bone microarchitecture, strength and turnover markers. Additionally, the efficacy and safety of optimal dosing regimens (continuous or intermittent administration) and targeted delivery formulations should be thoroughly investigated in animal models before clinical application. In clinical research, large-scale trials with patient stratification should be conducted to evaluate the efficacy and safety profile of CQ and HCQ, as well as to identify genetic and metabolic biomarkers. These trials should include a comparison between CQ and HCQ and standard osteoporosis treatments such as bisphosphonates, denosumab or teriparatide.

CQ and HCQ are potentially useful to strengthen the bone. However, the interaction of CQ or HCQ with other compounds may weaken their bone-conserving effects. Therefore, treatment strategies need to be carefully selected for patients with multiple comorbidities. CQ or HCQ are also contraindicated in patients with hepatic and renal insufficiency (85,86). Clinical trials are warranted to determine whether CQ and HCQ can be safely integrated into the management of osteoporosis.