Background

Bruton’s tyrosine kinase (BTK) is a key signalling node downstream of the B‑cell receptor (BCR) in B cells and Fc receptors (FcRs) in myeloid cells. Selective BTK inhibition may be beneficial for the treatment of MS by preventing B-cell and myeloid cell activation without immune cell depletion. BIIB091 is an orally active, selective, reversible (noncovalent), small molecule inhibitor of BTK in Phase 1 clinical development for the treatment of MS.

Objectives

Assess the safety, tolerability, and pharmacokinetics (PK) of single and multiple doses of BIIB091 in healthy adult participants.

Methods

This was a Phase I, randomized, double-blind, placebo-controlled, single ascending dose (SAD) and a multiple ascending dose (MAD) study in healthy adult participants (NCT03943056). Admitted participants were randomly assigned to BIIB091 or placebo (6:2). Participants received study drug orally at single dose levels of 50, 150, 300, 600, or 1200 mg in the fasted state during the SAD part. Participants completing 300 mg fasted dosing continued into a second period and received single dose 300 mg with a high fat meal. A separate group of participants enrolled in the MAD part and received doses of 50, 150, or 300 mg twice daily (BID) for 13 days, and a single dose on the morning of day 14. Safety (adverse events [AEs], vital signs, electrocardiograms [ECGs] and laboratory abnormalities), tolerability, and PK measures were assessed. The trial was ongoing as of February 2020. Preliminary results summarized here are based on a blinded data cut-off as of 20 Dec 2019, and include completed SAD cohorts, and 2 of 3 planned MAD cohorts.

Results

56 participants enrolled in the study (n=40 in SAD, n=16 MAD) and contributed to this analysis as of the data cut-off date. No deaths, serious AEs, severe AEs, cardiac related AEs, or AEs leading to study discontinuation were observed. All AEs were mild in severity. No clinically relevant changes in vital signs were reported as of the data cut-off date. BIIB091 PK was approximately dose linear, with median time to maximum plasma concentration ranging from approximately 0.8 to 1.4 hours after fasted dosing in SAD cohorts, with modest accumulation after multiple dosing. Food reduced C_max variability and increased AUC.

Conclusions

BIIB091 was well tolerated at single doses up to 1200 mg. No dose limiting clinical AEs were identified in the MAD as of the data cut-off. Complete study results will be described in the poster. Preliminary safety and PK profile supports continued development of BIIB091 for the treatment of MS.

Supported by: Biogen

Background

Autoantibodies are a hallmark feature of numerous neurologic disorders, including multiple sclerosis (MS) and Neuromyelitis optica (NMO) even though the exact pathogenic role(s) and mechanism(s) associated with these autoantibodies are not fully understood. Within the CNS, antibodies can bind to both activating and inhibitory Fc-Receptors (FcRs) that are expressed on barrier-associated macrophages, microglia and other trafficking immune subsets. While well understood in peripheral myeloid cells, the pathophysiological significance of autoantibody-induced FcR signaling in microglia remains unknown, in part due to the lack of a robust in vivo model.

Objectives

Develop an in vivo model to assess Fc Receptor (FcR) and Bruton's tyrosine kinase (BTK) dependent antibody-induced microglia activation.

Methods

Anti-Myelin oligodendrocyte glycoprotein (MOG) monoclonal antibodies (mAbs) were generated and injected peripherally. Anti-MOG CNS target engagement and microglia activation were measured by immunohistochemistry and flow cytometry. Microglia transcriptomics were assessed by RNAseq.

Results

Here, we report that peripheral injection of anti-Myelin oligodendrocyte glycoprotein (MOG) monoclonal antibodies (mAbs) triggers a rapid and tightly regulated microglia Ki-67⁺ proliferative burst in both brain and spinal cord. This microglia activation was FcR-dependent as only Fc effector-competent but not Fc effectorless aglycosylated antibodies triggered the proliferative response. Accordingly, anti-MOG induced microglia proliferation was fully abrogated in FcR knockout mice. The anti-MOG driven microglia proliferative response was associated with a transient and tightly-regulated gene expression signature of largely proliferation-associated genes. Moreover, we determined that anti-MOG-induced microglia activation in vivo was dependent on BTK, a signaling node downstream of FcRs. Specifically, we found that anti-MOG microglia response was amplified in BTK^e41K knock-in mice that express a constitutively active form of BTK and was blunted in mice treated with ibrutinib, a CNS penetrant small molecule BTK inhibitor.

Conclusions

Together, these results demonstrate the first report of an in vivo physiological function for FcR and BTK signaling in microglia and we propose that this model provides a novel tool to further dissect the roles of microglia-specific FcR and BTK driven responses to antibodies in CNS homeostasis and disease.

Background

Microglia are a unique type of brain-resident immune cell that secretes essential neurotrophic factors, promotes myelinogenesis, provides immune defense, clears debris and phagocytoses apoptotic cells. While this wide array of functional properties is suggestive of microglia heterogeneity, few microglia subsets have been described in the healthy brain thus far.

Objectives

To identify novel microglia subsets present in the steady-state brain and to characterize their unique functional roles.

Methods

Microglia isolated from mice and non-human primates were characterized by flow cytometry, electron microscopy and proteomics.

Results

In microglia isolated from mice and non-human primates and then analyzed by flow cytometry, we serendipitously observed that cellular autofluorescence (AF) presented as a bimodally-distributed signal which identified two subsets of microglia: AF-positive (AF⁺) and AF-negative (AF^–). While these subsets were present across the brain and maintained at a roughly 2:1 ratio (AF⁺:AF^–) throughout most of adulthood, microglia AF increased linearly and exclusively within the AF⁺ subset, while the AF^– subset continued to remain free of AF. Electron microscopy of FACS-isolated AF⁺ microglia revealed large and frequent lysosomal storage bodies, which contained lipids and electron dense material that increased in size and complexity withaging. Proteomic analysis of AF subsets revealed an overrepresentation of endolysosomal, autophagic, catabolic, and mTOR-related proteins in AF⁺ microglia, pointing to a unique dependence of the AF⁺ subset on lysosomal function. Accordingly, genetic disruption of lysosomal or autophagic pathways increased and decreased, respectively, the accumulation of AF in the AF⁺ subset while the AF^– subset remained unaffected. Lastly, both aging and lysosomal disruption differentially impacted AF subsets, as demonstrated by the increased cellular ROS content and apoptotic rates in AF⁺ microglia, which also correlated with diminished cell numbers of AF⁺ microglia at advanced ages.

Conclusions

AF⁺ and AF^– subsets represent discrete populations of microglia, present in healthy brain and marked by distinct subcellular content likely reflective of unique functional roles, and in particular distinct CNS clearance functions in steady-state and aging. The increased accumulation of AF material, restricted to AF⁺ microglia, uniquely impacts their physiology as indicated by elevated cellular ROS and their decreased survival in aging, factors possibly contributing to age-related cognitive decline.