Failure of Alglucerase Infused into Gaucher Disease Patients to Localize in Marrow Macrophages

Restoration of β-GC trafficking improves the lysosome function in Gaucher disease

Lysosomes function as a primary site for catabolism and cellular signaling. These organelles digest a variety of substrates received through endocytosis, secretion and autophagy with the help of resident acid hydrolases. Lysosomal enzymes are folded in the endoplasmic reticulum (ER) and trafficked to lysosomes via Golgi and endocytic routes. The inability of hydrolase trafficking due to mutations or mutations in its receptor or cofactor leads to cargo accumulation (storage) in lysosomes, resulting in lysosome storage disorder (LSD). In Gaucher disease (GD), the lysosomes accumulate glucosylceramide because of low β-glucocerebrosidase (β-GC) activity that causes lysosome enlargement/dysfunction. We hypothesize that improving the trafficking of mutant β-GC to lysosomes may improve the lysosome function in GD. RNAi screen using high throughput based β-GC activity assay followed by reporter trafficking assay utilizing β-GC-mCherry led to the identification of nine potential phosphatases. Depletion of these phosphatases in HeLa cells enhanced the β-GC activity by increasing the folding and trafficking of Gaucher mutants to the lysosomes. Consistently, the lysosomes in primary fibroblasts from GD patients restored their β-GC activity upon the knockdown of these phosphatases. Thus, these studies provide evidence that altering phosphatome activity is an alternative therapeutic strategy to restore the lysosome function in GD.

Incremental biomarker and clinical outcomes after switch from enzyme therapy to eliglustat substrate reduction therapy in Gaucher disease

In Gaucher disease (GD), genetic deficiency of acid β-glucosidase leads to accumulation of its substrate glucosylceramide (GlcCer) and glucosylsphingosine (GlcSph). Lipid-laden cells, most prominently seen as macrophages engorged with GlcCer and GlcSph-laden lysosomes, trigger chronic metabolic inflammation and multisystemic phenotypes. Among the pleiotropic effects of inflammatory cascades, the induction of glucosylceramide synthase accentuates the primary metabolic defect. First-line therapies for adults with GD type 1 include Enzyme Replacement Therapy (ERT) and eliglustat Substrate Reduction Therapy (SRT). The ENCORE phase 3 clinical trial of eliglustat demonstrated non-inferiority compared to ERT. It is not known whether switching stable patients from long-term ERT to SRT results in the incremental reversal of the disease phenotype and its surrogate indicators. Herein, we report real-world experience from a single tertiary referral center of 38 adult GD type 1 patients, stable on long-term ERT (mean 13.3 years), who switched to eliglustat SRT (mean 3.1 years). After switch to SRT, there was significant reduction in spleen volume (P = 0.003) while liver volume, which was normal at baseline, remained unchanged. Platelet counts increased significantly (P = 0.026). Concomitantly, there was reduction of three validated biomarkers of Gaucher disease activity: plasma GlcSph decreased from 63.7 ng/ml (95% CI, 37.6-89.8) to 26.1 ng/ml (95% CI, 15.7-36.6) (P < 0.0001); chitotriosidase fell from 1136.6 nmol/ml/h (95% CI, 144.7-2128.6) to 466.9 nmol/ml/h (95% CI, 209.9-723.9) (P = 0.002); and glycoprotein non-metastatic melanoma B (gpNMB) decreased from 59.3 ng/ml (95% CI, 39.7-78.9) to 43.6 ng/ml (95% CI, 30.7-56.6) (P = 0.0006). There were no episodes of avascular necrosis or fractures in patients on SRT. Patients reported favorable experiences of switching from alternate week infusions to oral therapy. Collectively, these results demonstrate that the switch to eliglustat SRT from ERT leads to incremental response, even in stable patients after long-term ERT.

Imaging of non-neuronopathic Gaucher disease: recent advances in quantitative imaging and comprehensive assessment of disease involvement

Gaucher disease is an inherited metabolic disorder resulting in deficiency of lysosomal enzyme β-glucocerebrosidase causing the accumulation of abnormal macrophages (“Gaucher cells”) within multiple organs, most conspicuously affecting the liver, spleen, and bone marrow. As the most common glycolipid metabolism disorder, it is important for radiologists encountering these patients to be familiar with advances in imaging of organ and bone marrow involvement and understand the role of imaging in clinical decision-making. The recent advent of commercially available, reliable, and reproducible quantitative MRI acquisitions to measure fat fractions prompts revisiting the role of quantitative assessment of bone marrow involvement. This manuscript reviews the diverse imaging manifestations of Gaucher disease and discusses more optimal quantitative approaches to ascertain solid organ and bone marrow involvement with an emphasis on future applications of other quantitative methods including elastography.

Orthoester functionalized N-guanidino derivatives of 1,5-dideoxy-1,5-imino-d-xylitol as pH-responsive inhibitors of β-glucocerebrosidase

Alkylated guanidino derivatives of 1,5-dideoxy-1,5-imino-d-xylitol bearing an orthoester moiety were prepared using a concise synthetic protocol. Inhibition assays with a panel of glycosidases revealed that one of the compounds prepared displays potent inhibition against human β-glucocerebrosidase (GBA) at pH 7.0 with IC50 values in the low nanomolar range. Notably, a significant drop in inhibitory activity is observed when the same compound is tested at pH 5.2. This pH sensitive activity is due to degradation of the orthoester functionality at lower pH accompanied by loss of the alkyl group. This approach provides a degree of control in tuning enzyme inhibition based on the local pH. Compounds like those here described may serve as tools for studying various lysosomal storage disorders such as Gaucher disease. In this regard, the most active compound was also evaluated as a potential pharmacological chaperone by assessing its effect on GBA activity in an assay employing fibroblasts from Gaucher patients.

The chaperone activity and toxicity of ambroxol on Gaucher cells and normal mice

Gaucher disease (GD), caused by a defect of acid β-glucosidase (β-Glu), is one of the most common sphingolipidoses. Recently, ambroxol, an FDA-approved drug used to treat airway mucus hypersecretion and hyaline membrane disease in newborns, was identified as a chemical chaperone for GD. In the present study, we investigated the chaperone activity and toxicity of ambroxol on both cultured GD patient cells and normal mice. We found that ambroxol treatment significantly increased N370S, F213I, N188S/G193W and R120W mutant β-Glu activities in GD fibroblasts with low cytotoxicity. Additionally, we measured the β-Glu activity in the tissues of normal mice which received water containing increasing concentrations of ambroxol ad libitum for one week. No serious adverse effect was observed during this experiment. Ambroxol significantly increased the β-Glu activity in the spleen, heart and cerebellum of the mice. This result showed its oral availability and wide distribution and chaperone activity in the tissues, including the brain, and its lack of acute toxicity. These characteristics of ambroxol would make it a potential therapeutic chaperone in the treatment of GD with neurological manifestations.

Synthesis of N-substituted ε-hexonolactams as pharmacological chaperones for the treatment of N370S mutant Gaucher disease

A series of N-substituted ε-hexonolactams have been designed and prepared by a concise route with a tandem ring-expansion reaction as the key step. Some of the N-substituted ε-hexonolactams show better enhancements to N370S mutant β-glucocerebrosidase activity than NB-DNJ and NN-DNJ. Both the experimental results and computational studies highlight the importance of the carbonyl group for stabilizing protein folds in the mutant enzyme. The structure-activity relationships are also discussed. These novel N-alkylated iminosugars are promising pharmacological chaperones for the treatment of N370S mutant Gaucher disease.

Successful low-risk hematopoietic cell therapy in a mouse model of type 1 Gaucher disease

Hematopoietic stem cell-based gene therapy offers the possibility of permanent correction for genetic disorders of the hematopoietic system. However, optimization of present protocols is required before gene therapy can be safely applied as general treatment of genetic diseases. In this study we have used a mouse model of type 1 Gaucher disease (GD) to demonstrate the feasibility of a low-risk conditioning regimen instead of standard radiation, which is associated with severe adverse effects. We first wanted to establish what level of engraftment and glucosylceramidase (GCase) activity is required to correct the pathology of the type 1 GD mouse. Our results demonstrate that a median wild-type (WT) cell engraftment of 7%, corresponding to GCase activity levels above 10 nmoles/hour and mg protein, was sufficient to reverse pathology in bone marrow and spleen in the GD mouse. Moreover, we applied nonmyeloablative doses of busulfan as a pretransplant conditioning regimen and show that even WT cell engraftment in the range of 1%-10% can confer a beneficial therapeutical outcome in this disease model. Taken together, our data provide encouraging evidence for the possibility of developing safe and efficient conditioning protocols for diseases that require only a low level of normal or gene-corrected cells for a permanent and beneficial therapeutic outcome.

The pharmacokinetics and tissue distribution of the glucosylceramide synthase inhibitor miglustat in the rat

Miglustat (Zavesca) is a reversible inhibitor of glucosylceramide synthase, which catalyses the first step in the glucosylceramide biosynthetic pathway, and is approved for therapy in patients with type 1 Gaucher disease. The present report describes the pharmacokinetic profile of miglustat in the rat with a focus on tissue distribution. Experiments were performed with radiolabeled miglustat itself and with a perbutyrated prodrug, the latter being readily converted to miglustat during gastrointestinal absorption and first pass metabolism. Miglustat was well absorbed and exhibited an oral bioavailability of 40-60%. Tissue distribution studies indicated the presence of miglustat in a number of organs and tissues that are considered of importance for the long-term therapeutic benefit, in particular the central nervous system, bone and lung. Miglustat was eliminated via renal clearance by a combination of glomerular filtration and active secretion. Hepatic clearance was negligible, as was the role of metabolism in the overall elimination process of miglustat in the rat.

Effect of miglustat on bone disease in adults with type 1 Gaucher disease: a pooled analysis of three multinational, open-label studies

BACKGROUND

Bone manifestations are a source of disability among patients with Gaucher disease (GD) and a focus of disease management. The effect of enzyme replacement therapy (ERT) on GD bone disease can be limited and may take up to 8 years to become manifest. Miglustat, a glucosylceramide synthase inhibitor, may have a positive influence on GD bone disease.

OBJECTIVES

The aim of this analysis was to evaluate the effects of miglustat on bone manifestations and bone mineral density (BMD) in patients with type 1 GD.

METHODS

This was a pooled analysis of data collected prospectively over an observation period of 2 years from patients who participated in 3 multinational, open-label clinical trials evaluating the efficacy and tolerability of miglustat 100 mg TID (the currently approved therapeutic dose). Bone manifestations were assessed qualitatively and in relation to treatment and spleen status. The effects of miglustat on BMD were assessed by dual-energy x-ray absorptiometry at the lumbar spine and/or femoral neck. Bone response was defined as a positive change in BMD, based on the change in BMD Z-score from baseline to months 6, 12, and 24. Changes in BMD were also analyzed according to spleen status and baseline severity of osteopenia.

RESULTS

The analysis involved 72 patients, including 41 (57%) who had received previous ERT and 20 (28%) who had undergone splenectomy. Patients' mean (SD) age was 41.2 (13.1) years. The most frequent bone-related manifestations at study entry were osteoporosis (43/63 [68%] patients) and bone pain (41/65 [63%] patients). At 2 years, 54/65 (83%) patients reported no bone pain. The reductions in bone pain were comparable among all subgroups, including high-risk patients (ie, splenectomized). No new cases of bone crisis, avascular necrosis, or pathologic fractures were reported. BMD Z-scores were improved from baseline at both the lumbar spine and femoral neck at each time point (months 6, 12, and 24) (P < 0.001). As early as 6 months after the initiation of miglustat monotherapy, significant increases from baseline in the BMD Z-score were observed at both the lumbar spine (mean, 0.15; P = 0.022) and femoral neck (0.23; P < 0.001); the increases remained significant at 12 months (0.19 [P = 0.012] and 0.21 [P = 0.017], respectively) and 24 months (0.21 [P = 0.015] and 0.18 [P = 0.039]). Significant increases in BMD Z-scores were observed at the femoral neck in splenectomized patients (P < 0.001) and at both sites in osteoporotic patients (lumbar spine: P < 0.001; femoral neck: P = 0.006).

CONCLUSION

This pooled analysis of 3 open-label studies of miglustat 100 mg TID suggests that miglustat monotherapy may reduce the incidence of bone pain and improve BMD in patients with type 1 GD, including those with a history of splenectomy and/or osteoporosis.

Isofagomine- and 2,5-anhydro-2,5-imino-D-glucitol-based glucocerebrosidase pharmacological chaperones for Gaucher disease intervention

Gaucher disease, resulting from deficient lysosomal glucocerebrosidase (GC) activity, is the most common lysosomal storage disorder. Clinically important GC mutant enzymes typically have reduced specific activity and reduced lysosomal concentration, the latter due to compromised folding and trafficking. We and others have demonstrated that pharmacological chaperones assist variant GC folding by binding to the active site, stabilizing the native conformation of GC in the neutral pH environment of the endoplasmic reticulum (ER), enabling its trafficking from the ER to the Golgi and on to the lysosome. The mutated GC fold is generally stable in the lysosome after pharmacological chaperone dissociation, owing to the low pH environment for which the fold was evolutionarily optimized and the high substrate concentration, enabling GC to hydrolyze glucosylceramide to glucose and ceramide. The hypothesis of this study was that we could combine GC pharmacological chaperone structure-activity relationships from distinct chemical series to afford potent novel chaperones comprising a carbohydrate-like substructure that binds in the active site with a hydrophobic substructure that binds in a nearby pocket. We combined isofagomine and 2,5-anhydro-2,5-imino-D-glucitol active site binding substructures with hydrophobic alkyl adamantyl amides to afford novel small molecules with enhanced ability to increase GC activity in patient-derived fibroblasts. The cellular activity of N370S and G202R GC in fibroblasts is increased by 2.5- and 7.2-fold with isofagmine-based pharmacological chaperones N-adamantanyl-4-((3R,4R,5R)-3,4-dihydroxy-5-(hydroxymethyl)piperidin-1-yl)-butanamide (3) and N-adamantanyl-4-((3R,4R,5R)-3,4-dihydroxy-5-(hydroxymethyl)piperidin-1-yl)pentanamide (4), respectively, the best enhancements observed to date.

The iminosugar isofagomine increases the activity of N370S mutant acid beta-glucosidase in Gaucher fibroblasts by several mechanisms

Gaucher disease is a lysosomal storage disorder caused by deficiency in lysosomal acid beta-glucosidase (GlcCerase), the enzyme responsible for the catabolism of glucosylceramide. One of the most prevalent disease-causing mutations, N370S, results in an enzyme with lower catalytic activity and impaired exit from the endoplasmic reticulum. Here, we report that the iminosugar isofagomine (IFG), an active-site inhibitor, increases GlcCerase activity 3.0 +/- 0.6-fold in N370S fibroblasts by several mechanisms. A major effect of IFG is to facilitate the folding and transport of newly synthesized GlcCerase in the endoplasmic reticulum, thereby increasing the lysosomal pool of the enzyme. In addition, N370S GlcCerase synthesized in the presence of IFG exhibits a shift in pH optimum from 6.4 to 5.2 and altered sensitivity to SDS. Although IFG fully inhibits GlcCerase in the lysosome in an in situ assay, washout of the drug leads to partial recovery of GlcCerase activity within 4 h and full recovery by 24 h. These findings provide support for the possible use of active-site inhibitors in the treatment of some forms of Gaucher disease.

Gaucher Disease-Associated Glucocerebrosidases Show Mutation-Dependent Chemical Chaperoning Profiles

Gaucher disease is a lysosomal storage disorder caused by deficient glucocerebrosidase activity. We have previously shown that the cellular activity of the most common Gaucher disease-associated glucocerebrosidase variant, N370S, is increased when patient-derived cells are cultured with the chemical chaperone N-nonyl-deoxynojirimycin. Chemical chaperones stabilize proteins against misfolding, enabling their trafficking from the endoplasmic reticulum. Herein, the generality of this therapeutic strategy is evaluated with other glucocerebrosidase variants and with additional candidate chemical chaperones. Improved chemical chaperones are identified for N370S glucocerebrosidase. Moreover, we demonstrate that G202R, a glucocerebrosidase variant that is known to be retained in the endoplasmic reticulum, is also amenable to chemical chaperoning. The L444P variant is not chaperoned by any of the active site-directed molecules tested, likely because this mutation destabilizes a domain distinct from the catalytic domain.

Recombinant glucocerebrosidase uptake by Gaucher disease human osteoblast culture model

Bone lesions are a major cause of morbidity in Gaucher disease (GD) type I. Enzyme replacement therapy (ERT) has been successful in treating many symptoms of type I GD but skeletal response lags behind. Local exogenous glucocerebrosidase supplementation in bone lesions via a drug delivery system may overcome this limitation. Although local enzyme supplementation aims to target lipid-engorged macrophages (Gaucher Cells) in bone compartment, enzyme uptake by osteoblasts is not excluded. To investigate the ability of human osteoblasts to internalize recombinant glucocerebrosidase (rGCR), we have used an artificial GD human osteoblasts cell culture system. MG63 human osteoblasts were treated with conduritol B epoxide (CBE) to induce complete and prolonged inhibition of endogenous glucocerebrosidase activity of cells. rGCR uptake by glucocerebrosidase-inactivated osteoblasts was examined using (125)I-radiolabelling, Western blot analysis and measurement of glucocerebrosidase activity. Analysis of radiolabeled enzyme uptake by CBE treated osteoblasts showed 67.9% of internalized protein in cell extract. Enzyme internalization was also observed by Western blot analysis where the amount of mature form of glucocerebrosidase protein recognized by the glucocerebrosidase antibody was increased following the administrations of rGCR. Moreover, enzymatic activity measurement showed 23.9% of glucocerebrosidase activity of control cells. The rGCR internalization by MG63 osteoblast seems to be partially mediated by mannose receptors. These data provide evidence that MG63 human osteoblasts are able to internalize rGCR.

The role of the iminosugar N-butyldeoxynojirimycin (miglustat) in the management of type I (non-neuronopathic) Gaucher disease: a position statement

N-Butyldeoxynojirimycin (NB-DNJ, miglustat 'Zavesca') is an orally active iminosugar which inhibits the biosynthesis of macromolecular substrates that accumulate pathologically in glycosphingolipidoses. Clinical trials of NB-DNJ in patients with Gaucher's disease demonstrate the therapeutic potential of such substrate inhibitors in the glycolipid storage disorders. However, macrophage-targetted enzyme replacement using intravenous mannose-terminated human glucocerebrosidase (imiglucerase, Cerezyme) is highly effective in ameliorating many of the manifestations of Gaucher's disease and is a treatment in widespread use. Given that imiglucerase and miglustat are now both licensed for the treatment of Gaucher's disease, there is a need to review their therapeutic status. Here the treatment of type 1 (non-neuronopathic) Gaucher disease is evaluated with particular reference to the emerging role of oral N-butyldeoxynojirimycin (miglustat) as a substrate-reducing agent. This position statement represents the consensus viewpoint of an independent international advisory council to the European Working Group on Gaucher Disease.

Chemical chaperones increase the cellular activity of N370S beta -glucosidase: a therapeutic strategy for Gaucher disease

Gaucher disease is a lysosomal storage disorder caused by deficient lysosomal beta-glucosidase (beta-Glu) activity. A marked decrease in enzyme activity results in progressive accumulation of the substrate (glucosylceramide) in macrophages, leading to hepatosplenomegaly, anemia, skeletal lesions, and sometimes CNS involvement. Enzyme replacement therapy for Gaucher disease is costly and relatively ineffective for CNS involvement. Chemical chaperones have been shown to stabilize various proteins against misfolding, increasing proper trafficking from the endoplasmic reticulum. We report herein that the addition of subinhibitory concentrations (10 microM) of N-(n-nonyl)deoxynojirimycin (NN-DNJ) to a fibroblast culture medium for 9 days leads to a 2-fold increase in the activity of N370S beta-Glu, the most common mutation causing Gaucher disease. Moreover, the increased activity persists for at least 6 days after the withdrawal of the putative chaperone. The NN-DNJ chaperone also increases WT beta-Glu activity, but not that of L444P, a less prevalent Gaucher disease variant. Incubation of isolated soluble WT enzyme with NN-DNJ reveals that beta-Glu is stabilized against heat denaturation in a dose-dependent fashion. We propose that NN-DNJ chaperones beta-Glu folding at neutral pH, thus allowing the stabilized enzyme to transit from the endoplasmic reticulum to the Golgi, enabling proper trafficking to the lysosome. Clinical data suggest that a modest increase in beta-Glu activity may be sufficient to achieve a therapeutic effect.

Dixon quantitative chemical shift imaging is a sensitive tool for the evaluation of bone marrow responses to individualized doses of enzyme supplementation therapy in type 1 Gaucher disease

Type 1 Gaucher disease can be effectively treated with enzyme supplementation therapy. Bone disease is a debilitating feature of the disorder and results from infiltration of the bone marrow by Gaucher cells. The effect of treatment on bone marrow infiltration is difficult to measure, necessitating the development of sensitive techniques to allow adequate dosing. Dixon quantitative chemical shift imaging (Dixon-QCSI) is a MRI technique to measure displacement of fatty marrow by Gaucher cells. Low bone marrow fat fractions have been found in Gaucher disease. We studied the effect of individualized low doses of enzyme therapy on the fat fractions of the lumbar spine in 12 adult Gaucher disease patients before and during treatment and in 9 untreated Gaucher controls. Fat fractions were decreased in 9/12 patients (median 0.20, range 0.08-0.40) and equally low in the untreated Gaucher controls compared to age-matched healthy volunteers (normal values 0.27-0.43, P < 0.01). During treatment, fat fractions increased significantly already after 1 year in 11/12 patients (P = 0.007). After 4 to 5 years, fat fractions normalized in 11/12 patients. Fat fractions remained low in the untreated Gaucher controls (P = 0.5 and 0.6 at 1 and 2 years, respectively). Six of 11 patients had a dose increase, which did not clearly affect fat fractions. Dixon-QCSI is a sensitive tool for the measurement of the response of bone marrow to enzyme therapy.

T cell numbers relate to bone involvement in Gaucher disease

The major elements of bone pathology in Gaucher disease are a failure of osteoclast and osteoblast function, resulting in osteopenia and also osteonecrosis. T lymphocytes have recently been found to be involved in the regulation of osteoblast/osteoclast activity in vitro. In the present report the peripheral blood T major lymphocyte subsets were investigated in a group of genotyped type 1 Gaucher disease patients. A total of 31 patients were studied: 21 non-splenectomized (5 N370S homozygotes) and 10 splenectomized (of whom 1 was a N370S homozygote). The results show that non-splenectomized patients present a decrease in absolute numbers of peripheral blood T lymphocytes, specially the CD4+ T subset. However, when patients were analyzed with respect to the presence of bone disease, the number of CD8+ T lymphocytes was found to be statistically significantly lower in patients presenting bone involvement. Furthermore, lower numbers of CD8+ T lymphocytes were significantly correlated with higher levels of plasma tartrate resistant acid phosphatase (TRAP) activity, a putative marker of osteoclast cell activity. These in vivo findings are in agreement with the results reached in vitro by others. They provide an additional marker of disease severity in Gaucher disease. In the group of genotyped Gaucher disease patients, the majority of the N370S homozygous patients presented a clinically milder phenotype, including the absence of bone involvement, confirming earlier reports predicting that a number of these patients may remain undiagnosed. Collectively the homozygosity for the N370S mutation and normal T cell numbers may provide additional markers for the clinical heterogeneity of Gaucher disease.

Murine MHC class II locus control region drives expression of human beta-glucocerebrosidase in antigen presenting cells of transgenic mice

Gaucher disease is the most prevalent lysosomal storage disorder in humans, resulting from an inherited deficiency of the enzyme glucocerebrosidase. Although the enzyme is ubiquitously expressed, cells of the reticuloendothelial system are particularly affected since they accumulate the undigested glucosylceramide substrate through their role in scavenging and breaking down cell debris. Gaucher disease is an attractive target for somatic gene therapy. To test the ability to express the enzyme in the affected cell types we have generated transgenic mice expressing human glucocerebrosidase under the control of the murine major histocompatibility complex (MHC) class II Ead locus control region (LCR). The four transgenic lines express the human enzyme in a copy number-dependent manner, independent of the integration site of the transgene. Over-expression of the human enzyme in mice did not result in any abnormal phenotype or pathology during the period of observation (> 2 years). The enzyme is expressed in B cells, monocytes, dendritic cells, thymic epithelial cells, and macrophages in various tissues: the peritoneal cavity, bone marrow, spleen, kidney, gastrointestinal tract, Kupffer cells in the liver and alveolar macrophages in lungs. Expression in the brain was limited to perivascular macrophages and was not seen in microglial cells. Therefore, the MHC class II LCR could potentially be of use in somatic gene therapy for type 1 Gaucher disease.

Enzyme replacement therapy for Gaucher's disease

Modified placental human glucocerebrosidase (alglucerase) and recombinant glucocerebrosidase (imiglucerase) are effective means of treating Type 1 Gaucher's disease. Amelioration of hepatosplenomegaly and of haematological manifestations is usually apparent within 6 months. Bone disease responds more slowly but within several years improvement is evident in most patients. Analysis of a large body of data demonstrates that the rate of response of all manifestations of Gaucher's disease is independent of dose over the range of 30 to 260 U/kg body weight per month. Even the response to 15 U/kg per month appears to be equivalent under most circumstances; treatment failures are the same in patients treated with 15, 30 and 130 U/kg per month. Patients with severe manifestations respond more rapidly than those with mild disease, and this, too, is true at all but the 15 U/kg per month dosage level. All available data thus support the administration of no more than 15 to 30 U of alglucerase or imiglucerase per kg/month. Frequent dosing, i.e. three times weekly, appears to be the most effective means of administration.

Therapeutic delivery of proteins to macrophages: implications for treatment of Gaucher's disease

BACKGROUND

The primary defect in Gaucher's disease, a lysosomal disorder affecting macrophages, is in the activity of glucocerebrosidase. Treatment with exogenous enzyme (modified to increase its affinity for macrophage glycoprotein receptors) aims to restore this activity. However, the fate of the exogenous enzyme in vivo is unknown. We used radiolabelled enzyme to assess macrophage receptor activity for mannosylated ligands in vivo.

METHODS

We examined the uptake and tissue distribution of radiolabelled enzyme molecules by gamma scintigraphy after bolus injection of iodine-123-labelled recombinant or placental enzyme (imiglucerase and alglucerase, respectively) in eight patients with type 1 Gaucher's disease, and in one healthy individual. The metabolism of the tracer enzyme was followed by scintigraphy and by analysis of blood, urine, and faeces.

RESULTS

The tracer enzyme was rapidly cleared from blood (half-life 4.7 min [SD 1.0]). Concomitantly, there was avid uptake by the liver (about 30% of the injected dose), the spleen (about 15%), and the bone marrow. 40-55% of the tracer was cleared rapidly from the viscera (half-life 1-2 h) and 45-60% was cleared slowly (half-life 34-42 h). The half-life in the bone marrow was 14.1 h. Infusion of alglucerase at dose of 5 U/kg bodyweight normalised acid beta-glucosidase activity of splenic Gaucher's cells in vivo. When the enzyme was administered at a seven-fold higher dose (35 U/kg over 1 h), the receptor-mediated uptake in vivo was saturated, as shown by the increase in blood-clearance half-life of tracer enzyme from 4.5 min to 12 min.

INTERPRETATION

Avid and saturable uptake of modified glucocerebrosidase was found, which indicates high-affinity targeting to the macrophage system in vivo. The rate of enzyme turnover suggests a rational basis for use of this therapy in treatment of Gaucher's disease.