Biochemical response to substrate reduction therapy versus enzyme replacement therapy in Gaucher disease type 1 patients

Gaucher disease type I (GD1, OMIM230800) results from a deficiency of glucocerebrosidase (GBA1), a lysosomal enzyme responsible for the degradation of glucocerebroside (GlcCer) and glucosylsphingosine (GlcSph) [1, 2]. Subsequently, GlcCer and GlcSph accumulate, leading to the characteristic lipid laden macrophages, also known as Gaucher cells. These are thought to play a vital role in GD1’s pathophysiology, causing symptoms of hepatosplenomegaly, cytopenia and debilitating bone complications. Enzyme replacement therapy (ERT), currently the standard treatment of GD1, targets correction of these macrophages by intravenous administration of modified glucocerebrosidase. Three recombinant glucocerebrosidase products are approved for the treatment of GD1: imiglucerase (Genzyme a Sanofi Company), velaglucerase alpha (Shire Human Genetic Therapies) and taliglucerase alpha (Protalix Biotherapeutics). All demonstrate remarkable improvement of cytopenias and hepatosplenomegaly [3‐8]. Nonetheless there are important limitations of ERT. Besides inconvenient infusions and exorbitant costs, there is increasing evidence that ERT cannot completely prevent bone complications [9]. In particular patients with bone disease at start of therapy can experience despite treatment additional bone complications, although the frequency of such events seems reduced [10]. It has been suggested that a systemic therapy targeting not only macrophages might be more beneficial, as aspects of GD’s symptomatology (malignancies, pulmonary hypertension, Parkinson’s disease and osteoporosis) are insufficiently explained by macrophage involvement solely [11].

Substrate reduction therapy (SRT), aiming to reduce accumulating glycosphingolipids by inhibiting their synthesis, might circumvent these disadvantages of ERT. Inhibitors of GlcCer synthesis to be used in SRT of GD1 are small compounds that can be taken orally and have the potential to rapidly diffuse into various tissues, including bones and the central nervous system. For GD1 presently inhibitors of the enzyme glucosylceramide synthase (GCS) have been developed. The first developed GCS inhibitor was miglustat (Actelion Pharmaceuticals Ltd.), which over a decade ago was approved for mild to moderately affected GD1 patients unsuitable to receive ERT. Significant improvements in hepatosplenomegaly and biochemical markers have been observed with miglustat treatment [12‐14]. Although direct comparison with ERT has never been properly studied, the effects on key clinical parameters are less robust. Side effects such as gastrointestinal complaints (up to 80 %) and tremors in many cases have led to discontinuation of treatment. These side effects limit the use of miglustat for patients with GD [12, 13]. Although miglustat is able to cross the blood brain barrier in mouse models, effects on neurological outcomes in type III GD are controversial [15, 16]. Eliglustat tartrate (abbreviated to eliglustat, Genzyme a Sanofi Company) is a new GCS inhibitor with a stronger inhibitory potency than miglustat (IC50 value of 0.024 μM vs. 5–50 μM). Eliglustat does not cross the blood brain barrier. In contrast to miglustat, eliglustat does not potently inhibit intestinal glycosidases, thus largely preventing the gastro-intestinal symptoms observed with miglustat [17]. Eliglustat has shown high promise as an oral treatment for GD1 given the observed clinically relevant effects on hematological and visceral symptoms [18‐22].

In GD1, well established plasma markers reflecting disease burden have been described. Chitotriosidase (CHIT1 gene) and pulmonary and activation regulated chemokine (CCL18/PARC), both produced and secreted by macrophages, reflect to some extent the total burden of Gaucher cells [23, 24]. Chitotriosidase and CCL18 correlate with several clinical parameters [25, 26] and failures in correction of high levels of chitotriosidase are associated with the incidence of long term complications [26]. The use of plasma chitotriosidase in monitoring of GD1 patients has limitations, because chitotriosidase activity is subject to genetic heterogeneity. Roughly 6 % of the population has no chitotriosidase activity due to a 24-bp duplication in the CHIT1 gene [27, 28]. More importantly a common G102S CHIT1 polymorphism renders misleading data of chitotriosidase protein levels when using the commercial substrate 4-methylumbelliferyl-chitotrioside as a substrate [29]. This can be prevented using the novel 4-methylumbelliferyl-deoxychitobiosidase substrate [29, 30]. Even with the optimized 4-methylumbelliferyl-deoxychitobioside substrate the use of internal standards of recombinant chitotriosidase is warranted. Data produced by laboratories not using such internal controls should be interpreted with caution. In chitotriosidase-deficient GD patients CCL18 is frequently used to monitor GD1 disease. Little is known about intra-individual variations due to polymorphisms in the CCL18 gene. A more recent improvement is the use of plasma GlcSph as a marker of Gaucher cell burden. The sphingoid base is on average 200 fold elevated in GD1 patients [31]. The main source of the elevated GlcSph in GD1are lipid-laden macrophages, but all GBA deficient cells in GD patients may produce GlcSph locally [31]. In contrast to CCL18 and chitotriosidase, GlcSph is directly related to the primary molecular defect in GD1 patients. There is some evidence that GlcSph in GD1 is largely formed from intralysosomal GlcCer by deacylation [31‐33]. Recent studies with conditional GD1 mouse models provide some evidence for the hypothesis that abnormalities in GlcSph contribute to GD1 symptomatology [11, 31, 32, 34, 35].

Until now a direct comparison of effects on biochemical markers reflecting disease burden between the aforementioned SRT and ERT treatment modalities has not been available. In this study, the effects on plasma markers of disease burden (chitotriosidase, CCL18, and GlcSph), plasma GlcCer associated to lipoproteins and clinical response (visceral, hematological and skeletal) are compared among eliglustat, miglustat and ERT treated patients.

Our investigation with a limited number of naïve GD1 patients suggests that the response to eliglustat treatment with respect to established biomarkers of disease burden (chitotriosidase and CCL18) is on a par with that of moderate doses of ERT. In contrast, the same biomarkers respond less favorably to miglustat treatment. These findings are in agreement with literature reports. Mistry et al demonstrated that after 9 months of eliglustat treatment in naïve GD1 patients chitotriosidase decreased with a mean of 44 % [21]. Lukina demonstrated a median decrease of CCL18 of 50 % [19], while in naïve GD1 patients treated with miglustat mean decreases of chitotriosidase after one year were less pronounced, ranging from 6 to 17 % [12, 41‐44], median 5–13 % (median values were not given, but could be calculated from references [12, 42]).

Next, our investigation showed that seventy percent of patients switching from ERT to miglustat demonstrated an increase of plasma chitotriosidase levels. The prevalence of such deterioration is higher than reported in literature, where increases are described in 4–43 % of cases [13, 44‐46]. The small number of patients investigated might explain this disparity. There may also be bias in studies due to high drop-out rates by AE’s and treatment failure. In addition, the outcome of the switch from ERT to miglustat may be influenced by specific reasons, for instance the severely affected patient no 19 in our investigation had to switch due to antibody development towards imiglucerase. Since increases of plasma chitotriosidase are not always directly accompanied by prominent clinical deterioration [45], the value of chitotriosidase measurements to monitor GD1 patients deserves discussion. Recently van Dussen et al demonstrated that patients with sustained chitotriosidase increases ≥ 30 % are at risk of having clinical deterioration with a relative risk of 6.3 (CI 95 % 2.2–17.8): chitotriosidase increases are accompanied by clinical deterioration in 50 % of patients, whereas when chitotriosidase is stable, 8 % develop clinical deterioration [26]. Sustained increases of chitotriosidase after switch from ERT to miglustat reflect an increase of Gaucher cell burden and risk for clinical deterioration or new complications, which should therefore be avoided. In addition, van Dussen showed that high residual chitotriosidase activity after two years of treatment correlates to long term complications. Although limited patients switching from ERT to eliglustat were available in this study, it is reassuring that even after ±3 months of therapy in a severely affected GD1 patient with high residual chitotriosidase activity after 20 years of ERT, eliglustat was able to decrease chitotriosidase and CCL18 levels. Even more promising is the noted transient reduction in plasma GlcSph in this patient. This anecdotal finding suggests that eliglustat might be capable in some GD1 patients to reach Gaucher cells that are not responsive to ERT. Our investigation documents for the first time that elevated plasma GlcSph is comparably decreased by eliglustat and ERT in naïve GD1 patients. The two investigated patients who switch from ERT to eliglustat treatment showed a further reduction of plasma GlcSph levels. On the other hand, miglustat treatment of naïve GD1 patients led to only minor reductions and switching patients from ERT to miglustat tended to be associated with stable or increasing plasma GlcSph. The clinical implications of the elevated plasma GlcSph are yet unclear, like those of elevated plasma GlcCer. From the single study that addressed associations with clinical symptoms, it can be concluded that it could be seen as a general marker for disease burden in GD1 [31], but does not correlate to specific symptoms. In contrast to chitotriosidase, which is not pathogenic but a mere reflection of alternatively activated macrophages, it has been hypothesized that GlcSph as a toxic compound is directly implicated in GD1 pathology. Studies of Orvinsky and Nilson revealed that GlcSph levels were normal in a brain of a GD I patient, but elevated in GD III and highest in brains of GD II patients with the most severe cerebral involvement [47, 48]. In vitro studies suggest neurotoxicity of GlcSph [49]. In addition, GlcSph has been shown to cause hemolysis and to inhibit protein kinase C, a pivotal kinase in signal transduction and cell behavior [31]. Extrapolation of such in vitro findings with high concentrations of GlcSph to pathophysiological action in GD patients warrants care. Further circumstantial evidence of a toxic effect of GlcSph in GD1 has been offered by Mistry et al demonstrating that high levels of GlcSph impair osteoblastogenesis in cultured osteoblasts of a conditional mouse GD1 model demonstrating very low bone mineral density [11]. More recently, Pavlova et al suggested that GlcSph is oncogenic, based on the association of high GlcSph levels in a conditional mouse model with a high incidence of B-cell lymphoma [34, 35]. If further research substantiates that GlcSph contributes to the pathogenesis of GD1, ERT treated patients with remaining high levels of GlcSph might potentially benefit from eliglustat treatment.

Of note, our study confirms that eliglustat is a very potent inhibitor of GCS in humans. Whereas with miglustat plasma GlcCer normalized or increased, eliglustat dramatically decreased circulating GlcCer bound to lipoprotein in all eliglustat patients, even to levels 50 % below normal as determined for 20 healthy subjects. However, it is essential to analyze a much larger number of presumed normal subjects to establish the lowest limit of the normal population range.

Due to limited sample size our study is insufficient to draw definite conclusions whether ERT and eliglustat are on a par in clinical efficacy. Nonetheless, our data for the matched patients indicate that eliglustat is not inferior to moderate dose of ERT: an equal decline of liver and spleen volumes, and equal rise of platelets and fat fraction was observed. Extended comparative data on both treatment modalities are now essential to determine the optimal clinical strategy for naïve GD1 patients. Such data are not actively pursued. The presently conducted RCTs by Genzyme do not directly compare ERT to eliglustat treatment of naïve GD1 patients. Instead, eliglustat treatment is compared to placebo [21]. Safety of switching from ERT to eliglustat treatment is separately investigated [22]. The RCT comparing placebo to eliglustat, including mild to moderately affected patients (severe patients with bone disease and splenectomies were excluded), demonstrates clinically relevant responses on cytopenia and hepatosplenomegaly [21]. Results of the ENCORE trial [22] demonstrate that 85 % (84/99) of the eliglustat treated GD1 patients (pretreated with ERT) and 94 % (44/47) of imiglucerase treated patients maintained therapeutic goals. Eliglustat treatment was statistically non-inferior to imiglucerase treatment as the lower bound of the 95 % confidence interval difference was within pre-specified non-inferiority thresholds (-17.6 %).

Potential advantages of present and future SRT modalities are thought to be better prevention of bone complications due to drug delivery in the bone compartments. In our naïve GD patients, bone marrow infiltration (fat fraction) improved upon eliglustat therapy, and none developed bone complications during the study period. With the available limited clinical data is not possible to draw conclusions on superiority of eliglustat treatment over ERT regarding bone complications. It will be also of interest to learn how eliglustat treatment impacts the occurrence of long-term complications and associated conditions in GD1 such as cancer, pulmonary hypertension, metabolic alterations including insulin resistance as well as Parkinson’s disease [50‐52].

Our exploratory investigation with a small number of GD1 patients has indicated that response in biomarkers of disease burden is on a par in naïve patients receiving eliglustat treatment and those receiving moderate doses of ERT, the same biomarkers responded less to miglustat treatment in (very limited sample size). Clinical response in the compared ERT and eliglustat treated naive GD patients seemed also comparable. Our present biochemistry and clinical findings should be extended by studies with larger cohorts.