Antibody semorinemab reduces tau pathology in a transgenic mouse model and engages tau in patients with Alzheimer's disease.

Tau has become an attractive alternative target for passive immunotherapy efforts for Alzheimer's disease (AD). The anatomical distribution and extent of tau pathology correlate with disease course and severity better than other disease markers to date. We describe here the generation, preclinical characterization, and phase 1 clinical characterization of semorinemab, a humanized anti-tau monoclonal antibody with an immunoglobulin G4 (igG4) isotype backbone. Semorinemab binds all six human tau isoforms and protects neurons against tau oligomer neurotoxicity in cocultures of neurons and microglia. In addition, when administered intraperitoneally once weekly for 13 weeks, murine versions of semorinemab reduced the accumulation of tau pathology in a transgenic mouse model of tauopathy, independent of antibody effector function status. Semorinemab also showed clear evidence of target engagement in vivo, with increases in systemic tau concentrations observed in tau transgenic mice, nonhuman primates, and humans. Higher concentrations of systemic tau were observed after dosing in AD participants compared to healthy control participants. No concerning safety signals were observed in the phase 1 clinical trial at single doses up to 16,800 mg and multiple doses totaling 33,600 mg in a month.

Characterization of a sensitive mouse Aß40 PD biomarker assay for Alzheimer's disease drug development in wild-type mice.

AIM: Transgenic mice that overexpress human amyloid precursor protein with Swedish or London (APPswe or APPlon) mutations have been widely used for preclinical Alzheimer's disease (AD) drug development. AD patients, however, rarely possess these mutations or overexpress APP. RESULTS: We developed a sensitive ELISA that specifically and accurately measures low levels of endogenous Aß40 in mouse plasma, brain and CSF. In wild-type mice treated with a bispecific anti-TfR/BACE1 antibody, significant Aß reductions were observed in the periphery and the brain. APPlon transgenic mice showed a slightly less reduction, whereas APPswe mice did not have any decrease. CONCLUSION: This sensitive and well-characterized mouse Aß40 assay enables the use of wild-type mice for preclinical PK/PD and efficacy studies of potential AD therapeutics.

Discovery of Novel Blood-Brain Barrier Targets to Enhance Brain Uptake of Therapeutic Antibodies.

The blood-brain barrier (BBB) poses a major challenge for developing effective antibody therapies for neurological diseases. Using transcriptomic and proteomic profiling, we searched for proteins in mouse brain endothelial cells (BECs) that could potentially be exploited to transport antibodies across the BBB. Due to their limited protein abundance, neither antibodies against literature-identified targets nor BBB-enriched proteins identified by microarray facilitated significant antibody brain uptake. Using proteomic analysis of isolated mouse BECs, we identified multiple highly expressed proteins, including basigin, Glut1, and CD98hc. Antibodies to each of these targets were significantly enriched in the brain after administration in vivo. In particular, antibodies against CD98hc showed robust accumulation in brain after systemic dosing, and a significant pharmacodynamic response as measured by brain Aß reduction. The discovery of CD98hc as a robust receptor-mediated transcytosis pathway for antibody delivery to the brain expands the current approaches available for enhancing brain uptake of therapeutic antibodies.

Lack of Widespread BBB Disruption in Alzheimer's Disease Models: Focus on Therapeutic Antibodies.

The blood-brain barrier (BBB) limits brain uptake of therapeutic antibodies. It is believed that the BBB is disrupted in Alzheimer's disease (AD), potentially increasing drug permeability de facto. Here we compared active versus passive brain uptake of systemically dosed antibodies (anti-transferrin receptor [TfR] bispecific versus control antibody) in mouse models of AD. We first confirmed BBB disruption in a mouse model of multiple sclerosis as a positive control. Importantly, we found that BBB permeability was vastly spared in mouse models of AD, including PS2-APP, Tau transgenics, and APOE4 knockin mice. Brain levels of TfR in mouse models or in human cases of AD resembled controls, suggesting target engagement of TfR bispecific is not limited. Furthermore, infarcts from human AD brain showed similar occurrences compared to age-matched controls. These results question the widely held view that the BBB is largely disrupted in AD, raising concern about assumptions of drug permeability in disease.

Therapeutic bispecific antibodies cross the blood-brain barrier in nonhuman primates.

Using therapeutic antibodies that need to cross the blood-brain barrier (BBB) to treat neurological disease is a difficult challenge. We have shown that bispecific antibodies with optimized binding to the transferrin receptor (TfR) that target ß-secretase (BACE1) can cross the BBB and reduce brain amyloid-ß (Aß) in mice. Can TfR enhance antibody uptake in the primate brain? We describe two humanized TfR/BACE1 bispecific antibody variants. Using a human TfR knock-in mouse, we observed that anti-TfR/BACE1 antibodies could cross the BBB and reduce brain Aß in a TfR affinity-dependent fashion. Intravenous dosing of monkeys with anti-TfR/BACE1 antibodies also reduced Aß both in cerebral spinal fluid and in brain tissue, and the degree of reduction correlated with the brain concentration of anti-TfR/BACE1 antibody. These results demonstrate that the TfR bispecific antibody platform can robustly and safely deliver therapeutic antibody across the BBB in the primate brain.

PILRα negatively regulates mouse inflammatory arthritis.

Paired Ig-like type 2 receptor (PILR)α inhibitory receptor and its counterpart PILRß activating receptor are coexpressed on myeloid cells. In this article, we report that PILRα, but not PILRß, is elevated in human rheumatoid arthritis synovial tissue and correlates with inflammatory cell infiltration. Pilrα(-/-) mice produce more pathogenic cytokines during inflammation and are prone to enhanced autoimmune arthritis. Correspondingly, engaging PILRα with anti-PILRα mAb ameliorates inflammation in mouse arthritis models and suppresses the production of proinflammatory cytokines. Our studies suggest that PILRα mediates an important inhibitory pathway that can dampen inflammatory responses.

Transferrin receptor (TfR) trafficking determines brain uptake of TfR antibody affinity variants.

Antibodies to transferrin receptor (TfR) have potential use for therapeutic entry into the brain. We have shown that bispecific antibodies against TfR and ß-secretase (BACE1 [ß-amyloid cleaving enzyme-1]) traverse the blood-brain barrier (BBB) and effectively reduce brain amyloid ß levels. We found that optimizing anti-TfR affinity improves brain exposure and BACE1 inhibition. Here we probe the cellular basis of this improvement and explore whether TfR antibody affinity alters the intracellular trafficking of TfR. Comparing high- and low-affinity TfR bispecific antibodies in vivo, we found that high-affinity binding to TfR caused a dose-dependent reduction of brain TfR levels. In vitro live imaging and colocalization experiments revealed that high-affinity TfR bispecific antibodies facilitated the trafficking of TfR to lysosomes and thus induced the degradation of TfR, an observation which was further confirmed in vivo. Importantly, high-affinity anti-TfR dosing induced reductions in brain TfR levels, which significantly decreased brain exposure to a second dose of low-affinity anti-TfR bispecific. Thus, high-affinity anti-TfR alters TfR trafficking, which dramatically impacts the capacity for TfR to mediate BBB transcytosis.

Addressing safety liabilities of TfR bispecific antibodies that cross the blood-brain barrier.

Bispecific antibodies using the transferrin receptor (TfR) have shown promise for boosting antibody uptake in brain. Nevertheless, there are limited data on the therapeutic properties including safety liabilities that will enable successful development of TfR-based therapeutics. We evaluate TfR/BACE1 bispecific antibody variants in mouse and show that reducing TfR binding affinity improves not only brain uptake but also peripheral exposure and the safety profile of these antibodies. We identify and seek to address liabilities of targeting TfR with antibodies, namely, acute clinical signs and decreased circulating reticulocytes observed after dosing. By eliminating Fc effector function, we ameliorated the acute clinical signs and partially rescued a reduction in reticulocytes. Furthermore, we show that complement mediates a residual decrease in reticulocytes observed after Fc effector function is eliminated. These data raise important safety concerns and potential mitigation strategies for the development of TfR-based therapies that are designed to cross the blood-brain barrier.

A therapeutic antibody targeting BACE1 inhibits amyloid-ß production in vivo.

Reducing production of amyloid-ß (Aß) peptide by direct inhibition of the enzymes that process amyloid precursor protein (APP) is a central therapeutic strategy for treating Alzheimer's disease. However, small-molecule inhibitors of the ß-secretase (BACE1) and γ-secretase APP processing enzymes have shown a lack of target selectivity and poor penetrance of the blood-brain barrier (BBB). Here, we have developed a high-affinity, phage-derived human antibody that targets BACE1 (anti-BACE1) and is anti-amyloidogenic. Anti-BACE1 reduces endogenous BACE1 activity and Aß production in human cell lines expressing APP and in cultured primary neurons. Anti-BACE1 is highly selective and does not inhibit the related enzymes BACE2 or cathepsin D. Competitive binding assays and x-ray crystallography indicate that anti-BACE1 binds noncompetitively to an exosite on BACE1 and not to the catalytic site. Systemic dosing of mice and nonhuman primates with anti-BACE1 resulted in sustained reductions in peripheral Aß peptide concentrations. Anti-BACE1 also reduces central nervous system Aß concentrations in mouse and monkey, consistent with a measurable uptake of antibody across the BBB. Thus, BACE1 can be targeted in a highly selective manner through passive immunization with anti-BACE1, providing a potential approach for treating Alzheimer's disease. Nevertheless, therapeutic success with anti-BACE1 will depend on improving antibody uptake into the brain.

Boosting brain uptake of a therapeutic antibody by reducing its affinity for a transcytosis target.

Monoclonal antibodies have therapeutic potential for treating diseases of the central nervous system, but their accumulation in the brain is limited by the blood-brain barrier (BBB). Here, we show that reducing the affinity of an antibody for the transferrin receptor (TfR) enhances receptor-mediated transcytosis of the anti-TfR antibody across the BBB into the mouse brain where it reaches therapeutically relevant concentrations. Anti-TfR antibodies that bind with high affinity to TfR remain associated with the BBB, whereas lower-affinity anti-TfR antibody variants are released from the BBB into the brain and show a broad distribution 24 hours after dosing. We designed a bispecific antibody that binds with low affinity to TfR and with high affinity to the enzyme ß-secretase (BACE1), which processes amyloid precursor protein into amyloid-ß (Aß) peptides including those associated with Alzheimer's disease. Compared to monospecific anti-BACE1 antibody, the bispecific antibody accumulated in the mouse brain and led to a greater reduction in brain Aß after a single systemic dose. TfR-facilitated transcytosis of this bispecific antibody across the BBB may enhance its potency as an anti-BACE1 therapy for treating Alzheimer's disease.