The use of Ambroxol for the treatment of Gaucher disease: A systematic review

Abstract Gaucher disease (GD) is a heterogeneous condition requiring tailored treatment approaches. The aim of this systematic review was to synthesise and evaluate current evidence pertaining to the use of Ambroxol for the treatment of GD. Published and unpublished literature databases, conference proceedings and the reference lists of included studies were searched until 23 November 2023. A narrative synthesis was performed. Database search and risk of bias assessment were performed independently by two reviewers. Twenty‐one studies (182 patients) were included. The evidence was low in quality. Variable responses to Ambroxol were observed. Response rates were 36% and 55% in two studies reporting on type 1 GD. One study found a 22% response rate in type 2 GD, whereas another study found 29% of patients with type 3 GD reported neurological improvements. No severe adverse events were reported in the literature, with mild and reversible side effects reported. Varying response rates are to be expected (29%–100%) when treating neurological manifestations. Varying degrees of symptomatic improvement for the treatment of GD were noted in the literature. Multidisciplinary team input and clinical judgement are advised to provide personalized treatment of this complex and multi‐faceted condition.

progressive form and can lead to hydrops fetalis, congenital ichthyosis, and consequent childhood death (usually before 2 years of age) [3,5].Type 3 Gaucher disease (GD3) is chronic in nature, and its manifestations include visceral involvement, ataxia and myoclonic epilepsy [3].
Given the clinically heterogeneous nature of GD, therapeutic approaches must be tailored to individual patient presentations [2].
Enzyme replacement therapies (ERTs) and substrate reduction therapies (SRTs) are the main therapeutic regimes used to treat GD.The former is the standard of care for symptomatic GD1 [6,7].Examples include alglucerase, velaglucerase alfa, taliglucerase alfa and imiglucerase [2,8].SRTs such as miglustat and eliglustat reduce the accumulation of glucosylceramide [9] and are also recommended for the treatment of GD1 [2].There is no widely accepted therapy for the neurologic manifestations of GD2 and GD3 [10], with highly variable responses to treatment observed in clinical practice [2].Enzyme replacement therapy does not cross the blood-brain barrier but is indicated for the systemic manifestations of GD3.
Pharmacological chaperone therapies for the treatment of GD were first investigated in the 1990s [11,12].Of more than 860 known GBA1 variant alleles, many are translated into misfolded glucocerebrosidase (GCase) protein.The protein is designated for endoplasmic reticulum-associated degradation in the proteasome.The proteasome is overwhelmed, resulting in the aggregation of GCase [13].Ambroxol, an oral mucolytic available over the counter, was found to act as a pharmacological chaperone for mutant GCase when taken at a high dose in 2009 [14], binding to the misfolded GCase and facilitating its correct folding and functional recovery [5,13].
Numerous case reports have described satisfactory outcomes in patients with GD following treatment with Ambroxol [15][16][17][18][19][20].However, no systematic reviews aimed at evaluating Ambroxol as a therapeutic alternative for the treatment of GD have been performed.
A synthesis of current evidence may inform decisions regarding the treatment of patients with GD.Therefore, the aim of this systematic review was to synthesise and evaluate current evidence pertaining to the use of Ambroxol for the treatment of GD.

MATERIALS AND METHODS
The PRISMA 2020 checklist was satisfied in the reporting of this systematic review [21].The protocol for this systematic review was prospectively registered (PROSPERO Registration: CRD42023426572).

Study eligibility
Studies were eligible if they reported on the use of Ambroxol for the treatment of patients with a confirmed diagnosis of GD (including paediatric and adult populations).Both full-text and abstract were included.Eligible study designs were randomized controlled trials, cohort, case-control and cross-sectional studies, as well as case series.
Both retrospective and prospective studies were eligible.Papers not reporting original data such as literature or systematic reviews were excluded, along with letters to the editor, laboratory studies and papers which did not specify the number of patients receiving Ambroxol.
There was no constraint based on language, publication status or patient demographics.The eligibility assessment was performed independently by two reviewers (Diego Agustín Abelleyra Lastoria and Simranjeet Grewal).Disagreements regarding study eligibility were solved through discussion.

Search strategy
We searched the following electronic databases via OVID: MEDLINE, Global Health, Embase, Web of Science, Scopus and ScienceDirect.
Currently, registered studies were reviewed using the databases: ISRCTN registry, the UK National Research Register Archive, the World Health Organization International Clinical Trials Registry Platform and the National Institute for Health Research Portfolio.Conference proceedings from the European Hematology Association and the American Society of Hematology were searched.The reference lists of included studies were also searched (backwards searching).
Papers citing the studies included were also reviewed for eligibility (forward-searching).
Database search and data extraction were conducted independently by two reviewers (Diego Agustín Abelleyra Lastoria and Simranjeet Grewal).Searches were conducted twice for quality assurance.

Data extraction
Data extracted included baseline characteristics including number of patients, patient sex, age, study location, follow-up duration and GD type.Outcomes extracted included baseline laboratory values/symptoms, outcomes of treatment, percentage of patients suffering from adverse events and type of adverse events reported.
Quantitative pooled analysis was prevented by heterogeneous study designs, and differing treatment regimens, study populations and outcomes evaluated.Therefore, a narrative synthesis was performed.

Methodological appraisal
The level of evidence of the studies presented was determined with the March 2009 Oxford Centre for Evidence-Based Medicine: Levels of Evidence (5 = lowest level of evidence, corresponding to case reports; 1a = highest level of evidence, corresponding to systematic reviews of randomised controlled trials) [22].The Institute of Health Economics case series studies quality appraisal checklist [23] was used to assess the risk of bias of case series, and the tool for evaluating the methodological quality of case reports was used to assess the risk of bias of case reports [24].The level of evidence and risk of bias of each included study were evaluated independently by two reviewers (Diego Agustín Abelleyra Lastoria and Simranjeet Grewal).

RESULTS
In total, 19,811 records were screened, of which 21 studies were eligible (Figure 1).Nine case series were included and evaluated 165 patients (

Study quality assessment
Nine studies identified were case series (level of evidence 4).There was one non-full text case series for which the risk of bias could not be assessed [25].No studies reported whether outcome assessors were blinded to patient characteristics or treatment regimens, which could potentially lead to bias in the assessment of results (Table 3).The remaining 12 studies were case reports.Of these, two were non-fulltext studies for which the risk of bias could not be assessed [26,27].
The report by Ramadža et al. [18] carried a low risk of bias, with the remaining nine case reports carrying a high risk of bias (Table 3).Overall, all studies included exhibited methodological limitations pertaining to low levels of evidence and concerns regarding the risk of bias.

The effect of Ambroxol on clinical outcomes
Outcomes following treatment with Ambroxol in the case series included are presented in Outcomes following treatment with Ambroxol in the case reports included are presented in Table 5.Of 12 reports, two did not report on clinical outcomes [20,31], two reported no change or deterioration in symptoms [32,33], and the remaining reported symptomatic improvements.These included resolution of respiratory compromise [34], normal neurological and cardiovascular function [19], decreased hepatosplenomegaly [26], ataxia improvement [15], analgesic effect [17] and complete symptom resolution [18].Three studies reported adverse events occurred.These included increased mucus production, high-pitched screaming and hyperextension in a prone position [19], soft stools [18] and mild abdominal discomfort [32].

The effect of Ambroxol on laboratory values
Kim et al. [1] reported on patients with GD and myoclonic epilepsy.
With a mean baseline of 5.1 ± 0.6% peripheral leucocytes' activity compared to normal, 21 and 27 mg/kg/day Ambroxol led to activities 9.8 ± 3.2% (p = 0.042) and 13.7 ± 4.4% (p = 0.02) of the mean normal activity, respectively.There were no significant changes in the haematological profile nor bone densitometry findings following 4.5 years of treatment with Ambroxol.
Zimran et al. [35] reported on a 6-month follow-up of 12 patients with type 1 GD treated with Ambroxol.This resulted in minimal in one case report [33].The remaining reports reported improvements in laboratory values (Table 5).

Multiple laboratory studies have described the mechanisms by which
Ambroxol may provide symptomatic relief in patients with GD.These include increasing GCase activity and reducing hexosylsphingosine concentration [36], increasing mutant β-glucosidase activity [37], and ameliorating the unfolded protein response [38].Though to varying degrees, all studies reported improvements in clinical parameters and/or laboratory values, with the exception of two case reports [32,33].However, the evidence was low in quality, with methodological limitations pertaining to the low level of evidence and concerns regarding the risk of bias.In addition, only 182 patients were included.Caution should therefore be placed when interpreting these findings.
A single type of GD comprised the entire cohort in seven case series, with the remaining two reporting multiple types (with only one reporting outcomes for these separately [28]).Three case series reported on type 1 GD [28,35,39].However, only two reported clinical outcomes.
These found 36% [39] and 55% [28] of patients reported symptomatic relief.The remaining study reported on changes in laboratory values rather than clinical parameters and did not state whether these were statistically significant.Further research evaluating clinical outcomes following Ambroxol for type 1 GD is required.Rates of adverse events in type 1 patients ranged from 8.5% to 35%.No severe adverse events were reported.In addition, three case reports pertaining to type 1 GD reported a lack of adverse events [15,16,20].This suggests Ambroxol could be a safe therapy for type 1 GD.
Of the two case series reporting on type 2 GD [3,25], one found a 22% treatment response rate, whereas the other stated mortality within 6 months was 40%.In addition, a case report found complete resolution of respiratory compromise, hepatomegaly, and cytopaenia following Ambroxol therapy in treatment with type 2 GD [34].These did not report on Ambroxol's safety profile, with further research required to ascertain this in type 2 GD.

TA B L E 5
Outcomes following treatment with Ambroxol in case reports.Kim et al. [1] reported on type 3 GD exclusively and found improvements in symptom scores.However, the statistical significance of these changes was not reported.Despite a 50% complication rate, no severe adverse events were reported.Istaiti et al. [28] noted that 29% of patients with type 3 GD reported neurological improvements.
Current evidence suggests Ambroxol may be used for the treatment of neurological manifestations of GD, though varying response rates are to be expected (29%-100%) [1,5,8].Lal et al. [3] reported statistically significant improvements in symptom scores in patients with PD, whereas Istaiti et al. [28] noted one in three patients with PD reported symptomatic improvement.Darling et al. [33] reported a patient with concomitant features of PD experienced worsening of neurological manifestation.Though this may suggest Ambroxol may not be useful for the treatment of GD-associated PD, further research is required.
Only a single case series and two case reports (comprising two patients each) reported Ambroxol's effects on epilepsy, with two of four patients with myoclonic epilepsy exhibiting reductions in number of seizures experienced [1], and each case report describing that only one of each of their two patients experienced a decrease in seizure frequency [27,32].Further research evaluating Ambroxol's ability to treat epilepsy is needed, given its impact on patients' quality of life and prognosis.ERTs such as imiglucerase do not cross the blood-brain barrier, which leads to neurologic manifestations of GD2 and GD3 being refractory to this treatment [40].SRTs such as miglustat do not appear to have significant benefits on the neurological manifestations of GD3 [41].Ambroxol could be considered for the treatment of patients with neurological symptoms of GD due to response rates ranging from 29% and 100%, and its availability over the counter.It may yield improvements in neurological symptoms in cases in which ERT and SRT are unable to achieve these due to their mechanism of action.In fact, Aries et al. [19] presented the first report of a patient with type 2 GD achieving developmental milestones at 3 years of age as a result of Ambroxol therapy (25 mg/kg/day).Though this consists of a single case, it may provide an incentive to conduct further work on Ambroxol as a means to foster neurological development in patients with GD.However, the low sample size and level of evidence of the studies included in this review must be taken into account when assessing the efficacy of Ambroxol for neurological manifestations of GD.Further work on larger sample sizes should be conducted to ascertain the validity of these findings.
Variable response rates to Ambroxol were reported, with different degrees of improvement observed.This could be attributed to the heterogeneous nature of Gaucher disease, with diverse presentations leading to varied responses.Treatment strategy must therefore be adapted according to patient presentation to optimize outcomes [2].Treatment guidelines recommend early multi-disciplinary team involvement for cases of GD [8].Clinical judgment is advised to guide decisions regarding the use of Ambroxol for the treatment of GD, taking patient presentation and previously unsuccessful therapies into account.Ambroxol was generally well tolerated, with rates of adverse events ranging from 8.5% to 60%.However, despite the high complication rate, adverse events were mild and reversible, with respiratory difficulty due to an increase in mucus production and uricaemia among the most severe complications.In addition, the study reporting a 60% complication rate consisted of five patients only [5], whereas the study reporting a 50% complication rate consisted of four patients [1].The low sample size of these studies could explain the high rate of adverse events observed.Overall response rates ranged from 22% to 61%, and improvements in laboratory values and symptom scores were noted.Neurological response rates ranged from 29% to 100%.
Therefore, clinicians may consider the counter-use of Ambroxol for symptomatic relief in patients with incurable forms of GD, balancing potential benefits and the effect of adverse events on patients' quality of life.Ambroxol may achieve more notable clinical effects during the presymptomatic or early disease period.Therefore, early intervention is desirable [5].Even though high-dose Ambroxol may provide symptomatic relief in patients with GD, not much is known about its long-term effects.Careful observation must take place to ensure continued patient safety [5], particularly because symptomatic improvement may not be achieved in the short term [1].Follow-up duration of the studies included in this review ranged from 6 months to 7 years, exemplifying the long treatment course.Informed patient discussions are required to maximize compliance with the long duration of Ambroxol therapy for GD.Clinicians may encounter barriers to its administration [28].First, there is likely to be a lack of reimbursement, since it is not a registered therapy.Second, despite Ambroxol being a relatively inexpensive drug, many families in developing countries may not be able to afford it.Third, doses required for symptomatic relief are much higher than the concentrations available over the counter.
Therefore, adults need to take a large number of capsules daily, and children need to drink a large amount of syrup, which may affect patient compliance.
Current evidence has limitations which must be improved to increase the understanding of Ambroxol as a therapeutic approach for GD.First, the studies identified carried a low level of evidence with concerns regarding their risk of bias, with a total of only 182 patients included.All studies were case series (three of which comprised less than five patients) or case reports.Though they are feasible study designs to evaluate rare diseases like GD, they are prone to publication and selection bias.Secondly, there was significant study heterogeneity pertaining to differing study designs, differing treatment regimens, patient characteristics and outcomes evaluated.This limits the ability to extrapolate the studies' results and hinders the performance of future meta-analyses on the subject.Thirdly, no studies comparing Ambroxol to other treatment regimens have been identified.Such work is required to determine whether Ambroxol provides an additional benefit over ERT and SRT for GD, considering the high costs and adverse events associated with these conventional therapies [13].
In addition, there have been no clinical trials conducted for the registration of Ambroxol as a treatment for GD.Considering Ambroxol is a relatively inexpensive over-the-counter drug, conducting welldesigned prospective clinical trials is unlikely; explaining why these have never been performed [13].International collaborations may help define the optimal doses and mode of administration of Ambroxol for the treatment of GD.Namely, the recently established global neuronopathic Gaucher disease registry (GARDIAN) [42] should encourage contributing clinicians to report on their experience with Ambroxol to make progress towards an international consensus.No studies performing cost-effectiveness analyses have been performed.Though Ambroxol can be purchased over the counter, other prescriptiononly medications may facilitate more cost-effective treatment of GD.
Fourthly, a single study treated type 1 GD patients with previous sub-optimal responses to ERT/SRT [39], and it reported on laboratory values rather than the effect on symptoms.Further research on Ambroxol for the treatment of type 1 GD with poor response to ERT/SRT is required.Finally, though three studies performed in-vitro analyses of responses to Ambroxol [1,5,29], no studies performed in-vitro tests to predict response to treatment.Such work could provide clinicians with tools to determine which patients would benefit from Ambroxol therapy.Regarding monitoring treatment response, biomarkers used should be specific to GD, be widely available and highly sensitive, and be directly involved in the pathological pathway of GD [43].

CONCLUSION
Current evidence suggests Ambroxol may be considered for the treatment of neurological manifestations of GD.However, varying degrees of symptomatic improvement in the treatment of GD were noted.
This could be attributed to its heterogeneous nature, with diverse patient presentations leading to differing responses to therapy.Multidisciplinary team input and clinical judgement are advised to provide personalized treatment of this complex and multi-faceted condition.
changes in body weight, haemoglobin levels, liver volume and chitotriosidase activity.The mean platelet count increased from 84.4 to 92.4 × 10 3 /mm 3 and the mean spleen volume decreased from 1616 to 1357 cm 3 .Whether changes from baseline were statistically significant was not reported, with outcome measures globally unchanged overall after 6 months.Istaiti et al.[28] treated 40 type 1 GD patients with Ambroxol.Of these, 28 had 3 years of sub-optimal response to ERT or SRT (platelet count < 100 × 103/L, lumbar spine bone density T-score < −2.0 and/or LysoGb1 > 200 ng/mL), and 12 were naïve to treatment.Sixteen completed a 12-month treatment course.Reasons for withdrawal included adverse events (n = 12), coronavirus disease 2019 (n = 7), the inability to swallow Ambroxol capsules (n = 3), receiving a new diagnosis of PD,being advised to receive a higher dose of Ambroxol than that which was administered as part of the trial (n = 1), and participation in another clinical trial (n = 1).Five patients (31.2%) achieved greater than 20% improvement in platelet count and three patients (18.9%) had a 20% or greater reduction in Lyso-Gb1 at the end of treatment.Narita et al.[5] reported on five patients with GD.Glucocerebrosidase activity in lymphocytes increased by 171.1% (p = 0.03) and reached levels observed in carriers or control subjects.Cerebrospinal fluid glucosylsphingosine levels fell by 25.7% (p = 0.03) after therapy.Mullin et al.[29] administered Ambroxol to 18 patients with GD-associated PD.At 186 of follow-up, only improvements in cerebrospinal fluid GCase activity, α-Synuclein, GCase protein level and blood Tau were statistically significant.Zhan et al.[30] reported statistically significant improvements in haemoglobin concentration, spleen and liver volume, Chitotriosidase activity and Glucosylsphingosine level were noted in 28 patients following a mean of 2.6 years of treatment with Ambroxol.Changes in laboratory values were not reported

Table 4
Baseline characteristics of case series included in a systematic review.Baseline characteristics of case reports included in a systematic review.Dose -25 mg/kg/day in five divided doses Duration -6 years, Concomitant therapies-ERT with imiglucerase was started at the dose of 60 IU/kg every other week Dose -25 mg/kg/day divided into six doses, Duration -7 years, Concomitant therapies-ERT with imiglucerase was commenced at the age of 3 months, at 60 mg/kg every other week Results of risk of bias assessment.
125]ker-Alpan et al.[25]reported on 12 infants with Type 2 GD receiving Ambroxol 10 mM.Three distinct phenotypes were observed: early onset, classical (associated with facial dysmorphism) and nonclassical.Negative response (GCase TA B L E 1 quency.The frequency of generalised convulsive status epilepticus decreased following Ambroxol treatment in two patients.The constriction rate and latency of the pupillary light reflex improved in all patients.TA B L E 2Dose -20 mg/kg/day in 2 doses and then, after12months, at 25 mg/kg/day in 3 doses, Duration -42 months, Concomitant Therapies -ERT, sodium valproate, levetiracetam and clonazepam Dose-Maximum dose of 1300 mg/daily administered in three divided doses, Duration -1 year, Concomitant Dose -150 mg/day (initially) → 300 mg/day → 450 mg/day (withing 4 months of starting) → reduction to constant dose of Duration -approx. 2 years, Concomitant therapies -ERT, tramadol 200 mg/day, paracetamol 2 g/day, flupirtine 300 mg/day, local lidocaine blockades and physiotherapy Dose -15 mg/kg/day, Duration -9 months, Concomitant therapy-Imiglucerase, biweekly, at 60 units/kg increased to 120 units/kg ERT months Abbreviations: ERT, enzyme replacement therapy; NR, not reported.TA B L E 3

Tool for evaluating the methodological quality of case reports [24] risk of bias assessment questions (continued) Pawlinski et al., 2020 [17] Charkhand et al., 2019 [15]
Outcomes following treatment with Ambroxol in case series.