Effects of Enzyme Replacement Therapy and Antidrug Antibodies in Patients with Fabry Disease
Authors: Malte Lenders, Eva Brand
Journal: Journal of the American Society of Nephrology (2018)

Fabry disease (FD) is an X-linked lysosomal storage disorder (LSD) caused by mutations of the α-galactosidase A gene. The lysosomal enzyme α-galactosidase A (GLA) mediates the hydrolysis of the terminal α-galactosyl moiety from globotriaosylceramide (Gb3). An enzymatic defect leads to the lysosomal accumulation of mainly Gb3 in different cells and tissues, causing a multisystemic disease. The first formulation of recombinant enzyme replacement therapy (ERT) was approved in Europe in 2001. Although ERT has been demonstrated to improve patient outcomes and disease course, especially when initiated early in the course of the disease, the intravenous infusion of the recombinant proteins can trigger a humoral immune response, resulting in infusion-associated reactions. Moreover, recent studies provide evidence that neutralizing antidrug antibodies (ADAs) seem to attenuate therapy efficacy in affected male patients, mediating disease progression despite ERT. The deleterious effect of ERT-related ADAs is well known in other LSDs. This review focuses on the efficacy of ERT, especially on the humoral response to continuous infusion of recombinant proteins in patients with FD, and considers the evidence about the effect of ERT-related neutralizing ADAs during ERT infusions and their effect on disease progression. It also proposes a model to explain how ADAs might mitigate therapy efficiency in FD. Finally, it discusses how clinical knowledge derived from immune tolerance induction protocols used in other LSDs might translate to FD, providing an approach to prevent ADA formation in ERT-naïve patients and decrease or supersaturate existing antibody titers in affected patients, leading to an improved disease course. Fabry disease (FD) is an X-linked lysosomal storage disorder (LSD) on the basis of mutations within the α-galactosidase A (GLA; Xq21.3-q22) gene. The worldwide incidence of FD has been estimated at 1 in 40,000 to 1 in 117,000 live male births.1 However, FD is probably underestimated because symptoms are often nonspecific. More than 900 GLA mutations have been identified, according to the Human Gene Mutation Database (http://www.hgmd.cf.ac.uk/), resulting in a wide spectrum of clinical symptoms and manifestations.2 The reduction or loss of enzymatic GLA activity results in a progressive lysosomal accumulation, mainly of globotriaosylceramide (Gb3).3 The deacetylated and, therefore, soluble form of Gb3, lyso-Gb3, seems to be a reliable biomarker for both disease progression and therapy efficacy, and can be detected in blood plasma as well as urine.4–10 The systemic cellular accumulation of Gb3 leads to progressive renal failure, cardiomyopathy, and recurrent cerebrovascular events, significantly limiting life expectancy in affected patients.3 The most common cause of death is cardiovascular disease.11–13 Although FD is an X-linked condition, females can also be severely affected,14 likely due to a skewed X-inactivation.15 However, onset of first symptoms in females is generally 5–10 years later than in males.16 In male patients, a definitive diagnosis of FD involves demonstrating a GLA deficiency of <5% of wild-type activity in leukocytes and the presence of a GLA mutation.17,18 In females, because of the high residual GLA activity, the diagnostic gold standard requires genetic analyses; in patients with an uncertain diagnosis, biopsy of an affected organ that demonstrates pathognomonic FD-typical deposits (zebra bodies) can confirm FD.17,18 Current treatment options for FD include two different recombinant enzyme replacement therapies (ERTs) (agalsidase-α and agalsidase-β), as well as chaperone therapy (migalastat). The latter, used in patients with certain amenable mutations, facilitates cellular clearance of Gb3 and an overall improvement of disease burden.19–21 The annual costs of FD-specific treatment in Europe are about €€250,000 per year. However, ERT can lead to infusion-associated reactions (IARs) in females and males, as well as the formation of neutralizing antidrug antibodies (ADAs) in about 40% of all ERT-treated males, leading to an attenuation of therapy efficacy.19,20,22–25 This review discusses the use of agalsidase-α and agalsidase-β ERT in FD, with a special focus on the humoral response to the infused recombinant proteins that leads to the formation of neutralizing ADAs in affected patients. It also presents immune tolerance induction protocols for other LSDs that might be translated to mitigating the development of ADAs in patients with FD who receive ERT. In addition, it discusses alternative therapeutic strategies, including chaperone therapy and substrate reduction therapy, and suggests areas for future research. FD-Specific Treatment ERT Depletion of accumulated Gb3 from affected cells and organs by ERT should lead to an improvement or at least a stabilization of effects in the kidney and other organs. In Europe, ERT has been in use since 2001, when agalsidase-α was approved. Two different formulations are currently available in Europe, agalsidase-α (Shire), which is produced from a modified human fibrosarcoma cell line, and agalsidase-β (Sanofi), which is produced in Chinese hamster ovary cells. The recommended dosage for agalsidase-α is 0.2 mg/kg intravenously every 2 weeks, and the advised dosage for agalsidase-β is 1.0 mg/kg intravenously every 2 weeks. In addition, two new plant-based products are currently in development: pegunigalsidase α, (Protalix Biotherapeutics),26 which is produced in BY2 tobacco cells, and moss-aGal (Greenovation),27 which is produced in moss. Both agalsidase-α and agalsidase-β have been demonstrated to benefit patients in the short term and, in observational studies, patients treated for up to 10 years showed stabilization of eGFR or a slowing of the progression of eGFR decline.19,20,28–36 The natural course of disease is associated with a rapid decline of renal function (eGFR decline up 12 ml/min per 1.83 m2 per year) and left ventricular hypertrophy.37Tables 1 to 5 provide a general overview of the effects of both ERTs on symptoms and manifestations in patients with FD. Table 1. - Effect of ERT in patients with FD Organ Background/Setting/Measures Effect on Renal Function Reference Kidney Agalsidase-α Randomized, placebo-controlled study, 26 males treated for 6 mo, eGFR improved by +2.1 ml/min per 1.73 m2 (P=0.02) Stabilization of eGFR 20 Retrospective FOS analysis, 201 patients treated for 24 mo, eGFR stabilized in 13 patients with CKD stages 2 and 3 during treatment (P>0.05) Stabilization of eGFR 28 Retrospective FOS analysis, 268 (135 males) patients treated for 5 yr, compared with untreated patients eGFRs in males with <60 and ≥60 ml/min per 1.73 m2 changed by −2.86 (95% CI, −3.90 to −1.83) and −1.68 (95% CI, −2.05 to −1.31) ml/min per 1.73 m2 per year, respectively, and in females with <60 ml and ≥60 ml/min per 1.73 m2 by 0.36 (95% CI, −0.47 to 1.19) and −0.43 (95% CI, −0.83 to −0.02) ml/min per 1.73 m2 per year, respectively Stabilization of eGFR 31 Observational single-center study, 45 (21 males) patients treated for 10 yr, eGFR analyzed as ml/min per 1.73 m2 per year reported to be stable Stabilization of eGFR 32 Observational single-center study, 12 male patients switched from biweekly to weekly infusions and observed for 10 yr, biweekly infusion resulted in yearly eGFR loss of −7.92±2.88 ml/min per 1.73 m2, weekly infusion resulted in an amelioration of −3.84±4.08 ml/min per 1.73 m2 per year (both P=0.01) Stabilization of eGFR 34 Retrospective FOS analysis, 188 patients treated for 12 mo, eGFR changes in patients with an eGFR 30 to 60 ml/min per 1.73 m2 as well as 60 to 90 ml/min per 1.73 m2 were NS Stabilization of eGFR 42 Single-center prospective open-label study, 24 male patients treated for up to 4.5 yr, eGFR changed by approximately −3 ml/min per 1.73 m2 per year (P=0.04) Slowing the progression of eGFR decline 43 Retrospective FOS analysis, 165 (115 males) patients treated for 3 yr, in males eGFR changed by −2.66±5.07 ml/min per 1.73 m2 per year (P<0.01) and in females by −1.20±3.28 ml/min per 1.73 m2 per year (P<0.01) Slowing the progression of eGFR decline 44 Prospective, randomized, placebo-controlled trial, 85 male patients treated for 2.5 yr, eGFR changed by −2.9±8.7 ml/min per 1.73 m2 per year (P=0.002) (patients with hyperfiltration were excluded) Slowing the progression of eGFR decline 45 Retrospective FOS analysis, 208 (134 males) patients treated for 7.4 yr, in males eGFR changed by −2.2 ( 95% CI, −2.8 to −1.7) ml/min per 1.73 m2 per year (P=0.01) and in females by −0.7 (95% CI, −1.4 to 0.0) ml/min per 1.73 m2 per year (P=0.05) Slowing the progression of eGFR decline 46 Open-label, multicenter study, 11 (ten males) pediatric patients treated for 6.5 yr, eGFR changed by +0.22 (95% CI, −2.84 to 3.28) ml/min per 1.73 m2 per year Stabilization of eGFR 47 Agalsidase-β Retrospective Fabry Registry analysis, 52 (50 males) patients treated for 10 yr, in patients with LRI eGFR changed by −1.89 ml/min per 1.73 m2 per year (P=0.001) and in patients with HRI eGFR changed by −6.82 ml/min per 1.73 m2 per year (P<0.001) Slowing the progression of eGFR decline 33 Observational multicenter study, 24 (21 males) patients treated at least for 24 mo, eGFR changed by 0.13 (95% CI, −3.75 to 5.00) ml/min per 1.73 m2 per year (P>0.05) Stabilization of eGFR 35 Multicenter phase 3 trial, 55 patients, treated for up to 3 yr, serum creatinine levels reported to be stable over time Stabilization of eGFR 48 Prospective single-center open-label study, 25 (19 males) patients treated for up to 2 yr, in eight patients with a baseline mGFR<90 ml/min per 1.73 m2, mGFR declined by −11 ml/min per 1.73 m2 (P=0.04), in 24 patients with a baseline mGFR≥90 ml/min per 1.73 m2 mGFR remained stable (P>0.05) Slowing the progression of mGFR decline 49 Open-label phase 3 extension study, 52 (50 males) patients treated for 4.5 yr, eGFR changed by −0.4 ml/min per 1.73 m2 per year (P=0.68) Stabilization of eGFR 50 Retrospective Fabry Registry analysis, 213 (151 males) patients treated for 2 yr, the highest risk factor for renal disease progression was an averaged urinary protein-to-creatinine ratio ≥1 g/g (OR, 112; 95% CI, 4 to 3109; P=0.01) and a longer time from symptom onset to treatment (OR, 19; 95% CI, 2 to 184; P=0.01) Stabilization/preservation of renal function due to early treatment and adjustment of risk factors 51 Observational multicenter study, 38 (26 males) patients treated for 12 mo, eGFR changed by +2 ml/min per 1.73 m2 per year (P=0.49) Stabilization of eGFR 52 Prospective observational study, 24 (15 males) patients treated for 2 yr, eGFR changed in patients with controlled proteinuria by −3.6 (95% CI, −4.8 to −1.1) ml/min per 1.73 m2 per year (P=0.01) and in patients with uncontrolled proteinuria by −7 (95% CI, −5.6 to −2.0) ml/min per 1.73 m2 per year (P=0.02) Stabilization of eGFR due to controlled proteinuria 79 FOS, Fabry outcome survey; 95% CI, 95% confidence interval; LRI, low renal impairment; HRI, high renal impairment; mGFR, measured glomerular filtration rate; OR, odds ratio. Table 2. - Effect of ERT in patients with FD Organ Background/Setting/Measures Effect on Cardiac Function Reference Heart Agalsidase-α Retrospective FOS analysis, 164 (71 males) patients treated for 5 yr, LVMI slopes were compared with untreated patients as 0.33 (95% CI, 0.13 to 0.53) g/m2.7 per year in males and 0.48 (95% CI, 0.30 to 0.66) g/m2.7 per year in females Reduction of LVMI 31 Observational single-center study, 45 (21 males) patients treated for 10 yr, MWT decreased by −1.89 (95% CI, −2.58 to −1.19) mm (P<0.001) in males and by −0.48 (95% CI, −1.05 to 0.09) mm (P=0.10) in females Reduction of MWT 32 Retrospective FOS analysis, 52 patients treated up to 2 yr, in patients with MWT>11 mm at baseline MWT decreased (P<0.05) and in patients with LVM>50 g/m2.7 LVM also decreased (P<0.05) Reduction of MWT and LVM 42 Open-label multicenter study, 11 (ten males) pediatric patients treated for 6.5 yr, LVMI changed by −0.48 (95% CI, −1.33 to 0.38) g/m2.7 per year Stabilization of LVMI 47 Phase IIIB study, 15 female patients treated for 12 mo, LVMI changed after 27 wk by −23.0±5.78 g/m2 (P=0.003) and after 41 wk by −25.2±8.12 g/m2 (P=0.02) Reduction of LVMI 54 Randomized, double-blind, placebo-controlled study, 15 male patients treated for 6 mo, LVM changed by −11.5 g (treated) versus +21.8 g (placebo) (P=0.04) Reduction of LVM 55 Retrospective FOS analysis, 57 patients treated up to 5 yr, after 5 yr LVMI changed in patients with LVH at baseline (n=32) by −7.3±15.3 g/m2.7 (P=0.01) and without LVH (n=25) by +1.8±10.1 g/m2.7 (P>0.05) Reduction of LVMI 56 Single-center open-label study of 37 females treated for 4 yr, independently of LVMI at baseline, LVMI decreased over time (P<0.01) Reduction of LVMI 57 Agalsidase-β Retrospective Fabry Registry analysis, 52 (50 males) patients treated for 10 yr, IVST changed in patients with LRI by +0.04 mm/yr (P=0.05) and in patients with HRI by +0.14 mm/yr (P=0.07) Stabilization of IVST 33 Single-center open-label study, 16 (14 males) patients treated for 12 mo, end-diastolic thickness of the left ventricular posterior wall, myocardial mass, peak systolic strain rate, and end-systolic strain of the posterior wall changed by −2 mm, −21 g, +0.9 s−1, +11% (all P<0.05) Reduction, stabilization of cardiac structures and improvement of cardiac function 58 Single-center open-label study, nine (seven males) patients treated for 12 mo, IVST, RWT, and LVMI were significantly reduced (all at least P<0.05) after 12 mo Reduction of IVST, RWT, and LVMI 59 Single-center open-label study, 11 (eight males) patients treated for 45 mo, LVM changed by −35 g and left ventricular wall thickness changed by −2 mm (both P<0.001) Reduction of LVM and LVWT 60 Single-center open-label study, 32 (27 males) patients stratified by fibrosis treated for 3 yr, LVM and septum thickness changed in patients without fibrosis by −36 g and −1.5 mm (both P<0.02) Reduction of LVM and septum thickness due to early treatment before fibrosis 61 Single-center open-label study, 23 (13 males) patients treated for 14 mo, LVM, phosphocreatine, and ATP changed by −30 g (P=0.01), +2.7 mmol/kg (P=0.003), and +1.0 mmol/kg (P<0.05) Reduction of LVM and amelioration of energetic depression 62 95% CI, 95% confidence interval; FOS, Fabry outcome survey; LRI, low renal impairment; HRI, high renal impairment; LVMI, left ventricular mass index; LVM, left ventricular mass; MWT, mean wall thickness; LVH, left ventricular hypertrophy; IVST, interventricular septum thickness; RWT, relative wall thickness; LVWT, left ventricular wall thickness. Table 3. - Effect of ERT in patients with FD Organ Background/Setting/Measures Effect on CNS Function/Symptoms Reference Central nervous system Agalsidase-α Placebo-controlled study, 26 male patients treated for 6 mo, resting global cerebral blood flow changed by −3.48±4.57 (treated, n=14) versus +1.25±5.11 (placebo, n=12) ml/min per 100 g of tissue (P=0.03) Improvement of cerebral blood flow 63 Open-label study, 63 male patients treated for 18 mo, Doppler blood flow parameters (peak flow velocity, mean flow velocity, end-diastolic velocity, flow acceleration) in certain vessels improved (all P<0.01) Improvement of cerebral blood flow 67 Agalsidase-β Post hoc analysis of phase 4 study, 31 patients treated for 12 mo, in patients≤50 yr of age, normalized WML diameter increased significantly in placebo group (n=13, P=0.03) and remained stable in the treated group (n=18, P>0.5) Stabilization of WMLs 68 CNS, central nervous system; WML, white matter lesion. Table 4. - Effect of ERT in patients with FD Organ Background/Setting/Measures Effect on Pain Reference Peripheral nervous system Agalsidase-α Randomized, placebo-controlled study, 26 males treated for 6 mo, BPI (worst) changed by −1.9 (P=0.02) Reduction of BPI (worst) 20 Retrospective FOS analysis, 25 patients treated for up to 2 yr, BPI (average) reported to decrease Reduction of BPI (average) 42 Retrospective FOS analysis, 53 patients treated for up to 5 yr, BPI (average) changed by −1.2±2.7 (n=53, P=0.002), BPI (worst) changed by −1.3±3.5 (n=49, P=0.01) Reduction of BPI (average and worst) 56 Single-center prospective open-label study, 36 females treated for 4 yr, BPI (worst) changed after 12 mo from 4.6±2.9 to 3.3±2.9 (P=0.001) and remained stable Reduction of BPI (worst) 57 Retrospective FOS analysis, 20 (16 males) patients treated for up to 2 yr, BPI (average, now, worst) decreased after the second year (all P<0.05) Reduction of BPI (average, now, worst) 69 Retrospective FOS analysis, 62 patients treated for 3 yr, BPI (average, least, worst) decreased (P<0.05) Reduction of BPI (average, least, worst) 70 Agalsidase-β Single-center prospective open-label study, 22 male patients treated for up to 23 mo, total symptom score for neuropathic pain changed from 1.76±1.97 to 0.83±1.53 (P=0.04), quantitative sensory testing revealed improvements for VDT (P<0.05) and HP (P<0.01) Reduction of pain, improvement of nerve function 71 BPI, brief pain inventory; FOS, Fabry outcome survey; VDT, thresholds of vibration; HP, heat-pain. Table 5. - Effect of ERT in patients with FD Organ Background/Setting/Measures Effect on GI Symptoms/Frequencies Reference Gastrointestinal system Agalsidase-α Single-center open-label study, 11 (nine males) patients treated for 6 mo, questionnaire for the presence of abdominal pain and the frequency of diarrhea revealed a significant reduction (both P<0.02) Reduction of abdominal pain and frequency of diarrhea 72 Retrospective FOS analysis, 58 (33 males) patients treated for 24 mo, frequency of abdominal pain decreased significantly (P<0.05) Reduction of abdominal pain 73 Agalsidase- β Single-center clinical trial, four male patients treated for 6–7 mo to 3 yr, patients reported decreased frequencies of abdominal pain and diarrhea and experienced weight gains of 3–8 kg Reduction of abdominal pain and frequency of diarrhea and weight gain 74 Retrospective Fabry Registry analysis, 168 female patients treated for up to 5.7 yr, presence of abdominal pain changed by −14% (P<0.01) and frequency of diarrhea by −12% (P<0.01) Reduction of abdominal pain and frequency of diarrhea 75 Table 6. - Overview of successful therapy strategies for prevention or elimination of ADAs in patients with LSDs Disease Target Drug Components Reference Immune tolerance induction in ERT-naïve patients FD Immunosuppression due to transplantations Methylprednisolone with azathioprine, calcineurin inhibitors, mycophenolate-mofetil/mycophenolate acid 115 PD B cell depletion Rituximab, MTX, IVIG 118 PD Immunosuppression, B cell depletion, mTOR inhibition Methylprednisolone, rituximab, rapamycin, IVIG 112 PD B cell depletion, immunomodulation Rituximab, MTX, IVIG 117 PD B cell depletion, immunomodulation Rituximab, MTX, IVIG 114 Acute reduction of neutralizing ADAs FD Short-term ADA reduction by immunosuppression due to transplantations Prednisolone, tacrolimus, mycophenolate-mofetil/mycophenolate acid 115 GD Acute ADA reduction, immunosuppression Plasma exchange (one plasma volume), cyclophosphamide, IVIG 116 PD B cell depletion Rituximab, MTX, IVIG 111 PD Immunosuppression proteasome inhibition, B cell/plasma cell depletion Cyclophosphamide, rituximab, bortezomib, MTX, IVIG 113 PD Acute ADA reduction, immunosuppression Cyclophosphamide, IVIG, plasma exchange 120 PD B cell depletion Rituximab, MTX, IVIG 119 PD, Pompe disease; IVIG, intravenous immunoglobulin; mTOR, mechanistic target of rapamycin; GD, Gaucher disease. In principal, most studies suggest that initiating early treatment is superior to late initiation, resulting in a more pronounced long-term improvement. In more advanced stages of the disease, however, patients benefit less from ERT. Early ERT has been demonstrated to lead to Gb3 depletion in renal cells in children and adults in a dosage- and frequency-dependent manner.38–41 Furthermore, ERT has been reported to stabilize renal function in terms of eGFR, whereas ERT's effect on proteinuria and albuminuria is inconsistent.26,29–37,42–52 If ERT is started before myocardial fibrosis has developed, a long-term improvement of myocardial morphology, function, and exercise capacity can be achieved.53–62 It also leads to a reduction of cerebrovascular and thromboembolic events63–65 and significant improvement in pain-related quality of life.67,69–71 Moreover, ERT is associated with a reduction of gastrointestinal symptoms such as abdominal pain and diarrhea72–75 and a reduction of lyso-Gb3 levels, a marker of disease load.9 Beneficial effects of ERT can be measured by different disease severity scores and tools, such as the Mainz Severity Score Index,76 the Disease Severity Scoring System,77 and the FAbry STabilization indEX.78 In addition to ERT, provision of concomitant nephroprotective and cardioprotective medication (on the basis of angiotensin-converting enzyme and angiotensin II receptor blockers) is highly warranted to preserve renal79 and cardiac function.53 However, recent reviews reported only a small benefit of the use of ERT.66,80 One mechanism suspected to play a role in curbing ERT efficacy is the formation of ADAs against infused enzymes, which is a major focus of this review. Oral Therapy Another treatment option, oral chaperone therapy with migalastat (Amicus Therapeutics), was approved in Europe in May of 2016 for long-term treatment of FD in adults and adolescents 16 years or older with an amenable mutation. The recommended dose is one capsule (123 mg) every other day, and food should not be consumed for at least 2 hours before and 2 hours after taking it. The drug is not currently recommended for use in patients with an eGFR<30 ml/min per 1.73 m2. Migalastat (1-deoxygalactonojirimycin), an oral small molecule chaperone that stabilizes endogenous GLA enzyme and supports proper folding in the endoplasmic reticulum, leads to increased endogenous enzymatic GLA activity in the lysosomes of patients with an amenable mutation.21 These are missense mutations that result in reduced enzymatic activity, caused by misfolding of the enzyme, which the chaperone can correct. There are at least 359 amenable and 706 nonamenable mutations (on the basis of an in vitro test in human embryonic kidney cells), according to a list published by Amicus, which markets migalastat (http://www.galafoldamenabilitytable.com, update 16th May 2018).81 For patients with amenable mutations, migalastat offers promise as a first-in-class oral monotherapy alternative treatment for patients with FD.82 In addition, recent reports describe the first attempts toward oral substrate reduction therapy, which aims to reduce production of Gb3 by inhibiting glucosylceramide synthase.83 One study demonstrated that lucerastat markedly reduced plasma GSLs such as lyso-Gb3 within 12 weeks of treatment.84 Research is also under way for a gene therapy approach to achieve tissue-specific overexpression of GLA, resulting in a clinical trial (https://clinicaltrials.gov/ct2/show/NCT02800070?term=gene+therapy&cond=fabry&rank=3).85 Immune Response to ERT Because ERT is on the basis of the intravenous infusion of a mostly foreign recombinant protein, a humoral response is common,19,20,86 at least in males with classic FD, who lack endogenous enzyme. Such individuals are also classified as "crossreactive immunologic material" (CRIM) negative. CRIM-negative status means that there is not even a mutated or truncated form of endogenous GLA that can be recognized by the immune system as an innate molecule. In these patients, infused enzyme might be recognized as foreign, triggering an immune response. Several studies analyzing patients with LSDs demonstrate that CRIM-negative patients have a high risk for developing immune responses after ERT initiation.22,25,87,88 Because of this, it is important when treating patients with FD to distinguish an immune response caused by continuous exposure to ERT (leading to the formation of neutralizing ADAs) from IARs. IARs frequently occur in ERT-naïve patients with FD after ERT initiation, mainly in males with nonsense or null mutations (CRIM negative).19,20,89,90 Symptoms of IARs are mostly limited to fever and chills, necessitating premedication with antihistamines and steroids, as well as prolongation of infusion times (i.e., decreased infusion rates).90 However, severe life-threatening IARs also have been reported.91,92 According to the manufacturer's instructions, 24% of patients treated with agalsidase-α present with IARs.93 In contrast, 67% of patients treated with agalsidase-β experience this adverse effect.94 The nature of IARs is yet not completely understood but they are probably the result of anaphylactoid reactions (i.e., chemical compound–mediated) and not often anaphylactic (i.e., IgE-mediated type 1 hypersensitivity), because direct identification of IgE antibodies has been made in only a few male patients to date. In one report, IgEs were identified in one patient with severe IARs, including pain95; other reports described one patient with generalized urticaria during infusions96 and another with anaphylaxis.30 In addition, an agalsidase-β open-label safety study reported seven patients with IgEs or a positive skin test.97 Furthermore, Wilcox et al. reported three patients with IgEs identified by direct IgE measurement or a skin test.48 However, the risk of IARs seems to be higher in patients with an anti-agalsidase IgG antibody–positive status.25,90 The humoral response includes crosstalk between antigen-presenting cells, CD4+ T-helper cells, and B cells (Figure 1).98Figure 1.: Humoral response results in IgG formation. Schematic overview of a humoral immune response to recombinant GLA resulting in IgG antibody production. In a classic T cell–dependent immune response, the recombinant human (rh) GLA is processed by antigen-presenting cells (APC) and presented to CD4+ T cells, which become activated. Activated T cells, in turn, activate naïve B cells, which maturate into IgG4-secreting plasma cells.In patients with IgEs, the following general mechanisms might apply: After antigen presentation, an early anaphylactic response includes the formation of IgE antibodies, triggering mast cell activation and histamine release, which could explain the observed severe IARs. With ongoing antigen presentation this process is commonly followed by IgG4 isotype antibody production against similar epitopes.99 Because these antibodies compete with IgEs but do not lead to complement activation, immunization will be achieved and IARs may be attenuated or stop over time.99–101 Currently, the literature lacks comprehensive studies analyzing the presence of IgE and IgG antibodies in patients with FD. IgG antibodies seem to develop within 3–6 months of starting ERT,22,48,90,102 but frequency and severity of IARs in affected patients may be attenuated. Smid et al.90 reported that most IARs were observed during the first 13 infusions. Although awareness of ERT-related IARs emerged in the first ERT clinical trial, to date, no standardized assays or protocols have been developed to determine IgG levels.103 Most trial studies and reports analyzing an early IgG response were performed by the ERT-manufacturing companies using ELISAs and their own antibodies.104 However, Linthorst et al.22 demonstrated that IgG antibodies (measured by ELISA) mediate an easily measurable neutralizing activity that can be used to classify neutralizing antibody–positive and –negative patients. This neutralizing activity was confirmed by several other studies,24,25,90 and the isotype of drug-neutralizing antibodies in patients with infusion-related ADAs recently was identified as IgG4, mainly.102 Recent studies have found that, once neutralizing ADAs occur, their formation seems to be irreversible, and that the majority of affected patients stay neutralizing ADA–positive over 10 years.24,25,36 About 73% of patients treated with agalsidase-β and 24% of patients treated with agalsidase-α reportedly develop ADAs.103,105 This difference might be explained by the different dosages of both drugs and the different cell lines producing them, as well as the CRIM status of the treated patients, which has not been taken into account thus far. Recent data from Arends et al.36 confirmed an increased risk for formation of neutralizing ADAs in patients treated with agalsidase-β. Although the individual CRIM status of the treated patients was not determined, their study excluded patients with nonclassic FD. However, Rombach et al.24 and later Smid et al.90 reported no significant differences in ADA formation when using the same dosage (0.2 mg/kg every 2 weeks) for both drugs at ERT initiation. Of note, the CRIM status of patients in these studies was also not determined in detail. To analyze this topic adequately, head-to-head studies with comparable dosages and a predefined CRIM status of participating patients are needed. At this point, it seems that the dosage is the most important trigger for immune response. Although the presence of neutralizing ADAs was first described very early after ERT became available,22 the effect of ADAs on FD-specific biomarkers has been a topic of controversy. High ADA titers (measured by E