Persistence of Imiglucerase in Cell Compartment Explains Dosing Frequency of Every Two Weeks in Type 1 Gaucher Disease

GD is a genetic lysosomal disease characterized by deficiency in glucocerebrosidase (Glc) particularly affecting macrophage-like cells where glucocerebroside accumulates in the lysosomes. Alglucerase then imiglucerase (IMI, Genzyme-Sanofi Corporation), a recombinant human glucocerebrosidase, improved or reversed many type 1 GD symptoms; Two new biosimilar agents are now available: velaglucerase-alfa (Shire Human Genetic Therapies) and taliglucerase-alfa (Biotherapeutics, Pfizer). All three recombinant enzymes are administered with identical therapeutic regimens, i.e. one infusion (30-60 IU/kg) every two weeks. However, disease responses remain variable and there are no pharmacokinetic explanations justifying this dosage regimen since plasma half-life is estimated to be less than 10 min. in murine models and in humans We compared endogenous glucocerebrosidase (Glc) activity evaluated by flow cytometry in the blood monocyte (Mo) compartment of 20 patients with 71 healthy donors, and we then followed residual intra-Mo Glc activity in patients starting imiglucerase (n=8).

By using the only technique able to identify blood leukocyte subsets, we confirmed the marked higher value in Mo (×43) as compared to lymphocytes and polymorphonuclear (PMN) cells. In the 20 type 1 GD patients, we confirmed that enzyme deficiency was clear in Mo (p = 10^-11 compared to HD), residual Glc activity in Mo from type 1 GD representing only 10% of normal Glc activity.

We then followed trough Glc activity in 8 patients starting imiglucerase treatment de novo (n=1) or restarting imiglucerase after shortage (n=7), at 40 (n=1) or 60 U (n=7) /Kg/2 weeks. We observed progressive uptake over 6 months and a significant cumulative persistence of enzyme activity in monocyte compartment even 14 days after infusion.

The estimated increase of residual intra-monocyte enzyme activity at M6 was 1.4 of endogenous level (p=.04) leading to a steady state value. The estimated rate constant of increase corresponded to an initial half-life of 2.8 days. These results explain for the first time the clinical efficacy of the routine infusion regimen, since intra-cellular pharmacokinetics are longer than plasma pharmacokinetics.

Furthermore, we can hypothesize that intra-cellular pharmacokinetics after infusion may vary between patients, as suggested by the different peaks of activity and variable residual levels of enzyme activity observed in this study. In this limited series, we observed several profiles: i) Four patients ( #1,2,3, 8) of whom one received 40 U/kg/2 weeks, showed progressive Glc uptake correlated with improvement of clinical and biological parameters; this corresponds to patients with hematological phenotype only (Figure S1 A). ii) Other patients showed variability of Glc uptake (Figure S1 B); these patients were more heterogenous, with bone disease (n=2), splenectomy (n=2), or BM disorders (myelodysplasia, n=1). Consequently intra-Mo imiglucerase uptake appears to be variable, suggesting for the first time a possible relation between Glc activity in cell compartments, phenotype and clinical efficacy.

In summary, the persistence of imiglucerase in monocyte compartments explains the efficacy of 2-week infusion rates and variability may be a factor of ERT efficiency