Antibody engagement with amyloid-beta does not inhibit [11C]PiB binding for PET imaging.

The elimination of amyloid-beta (Aß) plaques in Alzheimer's disease patients after treatment with anti-Aß antibodies such as lecanemab and aducanumab is supported by a substantially decreased signal in amyloid positron emission tomography (PET) imaging. However, this decreased PET signal has not been matched by a similar substantial effect on cognitive function. There may be several reasons for this, including short treatment duration and advanced disease stages among the patients. However, one aspect that has not been investigated, and the subject of this study, is whether antibody engagement with amyloid plaques inhibits the binding of amyloid-PET ligands, leading to a false impression of Aß removal from the brain. In the present study, tg-ArcSwe mice received three injections of RmAb158, the murine version of lecanemab or phosphate-buffered saline (PBS) before the administration of the amyloid-PET radioligand [11C]PiB, followed by isolation of brain tissue. Autoradiography showed that RmAb158- and PBS-treated mice displayed similar [11C]PiB binding. Moreover, the total Aß1-40 levels, representing the major Aß species of plaques in the tg-ArcSwe model, as well as soluble triggering receptor on myeloid cells 2 (sTREM2) levels, were similar in both groups. Interestingly, the concentration of soluble Aß aggregates was decreased in the RmAb158-treated group, along with a small but significant decrease in the total Aß1-42 levels. In conclusion, this study indicates that the binding of [11C]PiB to Aß accurately mirrors the load of Aß plaques in the brain, aligning with how amyloid-PET is interpreted in clinical studies of anti-Aß antibodies. However, early treatment effects on soluble Aß aggregates and Aß1-42 levels were not detected.

Comparing in vitro affinity measurements of antibodies to TfR1: Surface plasmon resonance versus on-cell affinity.

Despite years of utilizing the transferrin receptor 1 (TfR1) to transport large biomolecules into the brain, there is no consensus on how to optimally measure affinity to it. The aim of this study was to compare different methods for measuring the affinities of anti-TfR1 antibodies. Antibodies 15G11, OX26 and 8D3 are known to successfully carry large biologics across the blood-brain barrier in humans, rats, and mice, respectively. The affinity to their respective species of TfR1 was measured with different surface plasmon resonance setups in Biacore and an on-cell assay. When the antibody was captured and TfR1 was the analyte, the dissociation in Biacore was very slow. The dissociation was faster when the antibody was the analyte and TfR1 was the ligand. The Biacore setup with capture of N-terminal FLAG-tag TfR1 yielded the most similar apparent affinities as the cell assay. In conclusion, it is important to evaluate assay parameters including assay orientation, surface capture method, and antibody-format when comparing binding kinetics for TfR1 antibodies. Although it seems possible to determine relative affinities of TfR1 antibodies using the methods described here, both the FLAG-tag TfR1 capture setup and cell assays likely yield apparent affinities that are most translatable in vivo.

Synaptic density in aging mice measured by [18F]SynVesT-1 PET.

Synaptic alterations in certain brain structures are related to cognitive decline in neurodegeneration and in aging. Synaptic loss in many neurodegenerative diseases can be visualized by positron emission tomography (PET) imaging of synaptic vesicle glycoprotein 2A (SV2A). However, the use of SV2A PET for studying synaptic changes during aging is not particularly explored. Thus, in the present study, PET ligand [18F]SynVesT-1, which binds to SV2A, was used to investigate synaptic density at different ages in healthy mice. Wild type C57BL/6 mice divided into three age groups (4-5 months (n = 7), 12-14 months (n = 11), 17-19 months (n = 7)) were PET scanned with [18F]SynVesT-1. Brain retention of [18F]SynVesT-1 expressed as the volume of distribution (VIDIF) was calculated using an image-derived input function. Estimates of VIDIF were derived using either a one-tissue compartment model (1TCM), a two-tissue compartment model (2TCM), or the Logan plot with blood input to find the best-fit model for [18F]SynVesT-1. After the PET scans, tissue sections were immunostained for the detection of SV2A and neuronal markers. We found that [18F]SynVesT-1 data acquired 60 min post intravenously injection and analyzed with 1TCM described the brain pharmacokinetics of the radioligand in mice well. [18F]SynVesT-1 brain retention was lower in the oldest group of mice, indicating a decrease in synaptic density in this age group. However, no gradual age-dependent decrease in synaptic density at a region-specific level was observed. Immunostaining indicated that SV2A expression and neuron numbers were similar across all three age groups. In general, these data obtained in healthy aging mice are consistent with previous findings in humans where synaptic density appeared stable during aging up to a certain age, after which a small decrease is observed.

Development of the First Tritiated Tetrazine: Facilitating Tritiation of Proteins.

Tetrazine (Tz)-trans-cyclooctene (TCO) ligation is an ultra-fast and highly selective reaction and it is particularly suited to label biomolecules under physiological conditions. As such, a 3 H-Tz based synthon would have wide applications for in vitro/ex vivo assays. In this study, we developed a 3 H-labeled Tz and characterized its potential for application to pretargeted autoradiography. Several strategies were explored to synthesize such a Tz. However, classical approaches such as reductive halogenation failed. For this reason, we designed a Tz containing an aldehyde and explored the possibility of reducing this group with NaBT4 . This approach was successful and resulted in [3 H]-(4-(6-(pyridin-2-yl)-1,2,4,5-tetrazin-3-yl)phenyl)methan-t-ol with a radiochemical yield of 22 %, a radiochemical purity of 96 % and a molar activity of 0.437 GBq/µmol (11.8 Ci/mmol). The compound was successfully applied to pretargeted autoradiography. Thus, we report the synthesis of the first 3 H-labeled Tz and its successful application as a labeling building block.

Functionalization of Radiolabeled Antibodies to Enhance Peripheral Clearance for High Contrast Brain Imaging.

Small molecule imaging agents such as [11C]PiB, which bind to the core of insoluble amyloid-ß (Aß) fibrils, are useful tools in Alzheimer's disease (AD) research, diagnostics, and drug development. However, the [11C]PiB PET signal saturates early in the disease progression and does not detect soluble or diffuse Aß pathology which are believed to play important roles in the disease progression. Antibodies, modified into a bispecific format to enter the brain via receptor-mediated transcytosis, could be a suitable alternative because of their diversity and high specificity for their target. However, the circulation time of these antibodies is long, resulting in an extended exposure to radiation and low imaging contrast. Here, we explore two alternative strategies to enhance imaging contrast by increasing clearance of the antibody ligand from blood. The bispecific Aß targeting antibody RmAb158-scFv8D3 and the monospecific RmAb158 were radiolabeled and functionalized with either α-d-mannopyranosylphenyl isothiocyanate (mannose) or with trans-cyclooctene (TCO). While mannose can directly mediate antibody clearance via the liver, TCO-modified antibody clearance was induced by injection of a tetrazine-functionalized, liver-targeting clearing agent (CA). In vivo experiments in wild type and AD transgenic mice demonstrated the ability of both strategies to drastically shorten the circulation time of RmAb158, while they had limited effect on the bispecific variant RmAb158-8D3. Furthermore, single photon emission computed tomography imaging with TCO-[125I]I-RmAb158 in AD mice showed higher contrast 1 day after injection of the tetrazine-functionalized CA. In conclusion, strategies to enhance the clearance of antibody-based imaging ligands could allow imaging at earlier time points and thereby open the possibility to combine antibodies with short-lived radionuclides such as fluorine-18.

Transferrin Receptor Binding BBB-Shuttle Facilitates Brain Delivery of Anti-Aß-Affibodies.

Affibodies targeting amyloid-beta (Aß) could potentially be used as therapeutic and diagnostic agents in Alzheimer's disease (AD). Affibodies display suitable characteristics for imaging applications such as high stability and a short biological half-life. The aim of this study was to explore brain delivery and retention of Aß protofibril-targeted affibodies in wild-type (WT) and AD transgenic mice and to evaluate their potential as imaging agents. Two affibodies, Z5 and Z1, were fused with the blood-brain barrier (BBB) shuttle single-chain variable fragment scFv8D3. In vitro binding of 125I-labeled affibodies with and without scFv8D3 was evaluated by ELISA and autoradiography. Brain uptake and retention of the affibodies at 2 h and 24 h post injection was studied ex vivo in WT and transgenic (tg-Swe and tg-ArcSwe) mice. At 2 h post injection, [125I]I-Z5 and [125I]I-Z1 displayed brain concentrations of 0.37 ± 0.09% and 0.46 ± 0.08% ID/g brain, respectively. [125I]I-scFv8D3-Z5 and [125I]I-scFv8D3-Z1 showed increased brain concentrations of 0.53 ± 0.16% and 1.20 ± 0.35%ID/g brain. At 24 h post injection, brain retention of [125I]I-Z1 and [125I]I-Z5 was low, while [125I]I-scFv8D3-Z1 and [125I]I-scFv8D3-Z5 showed moderate brain retention, with a tendency towards higher retention of [125I]I-scFv8D3-Z5 in AD transgenic mice. Nuclear track emulsion autoradiography showed greater parenchymal distribution of [125I]I-scFv8D3-Z5 and [125I]I-scFv8D3-Z1 compared with the affibodies without scFv8D3, but could not confirm specific affibody accumulation around Aß deposits. Affibody-scFv8D3 fusions displayed increased brain and parenchymal delivery compared with the non-fused affibodies. However, fast brain washout and a suboptimal balance between Aß and mTfR1 affinity resulted in low intrabrain retention around Aß deposits.

PET Imaging in Preclinical Anti-Aß Drug Development.

Positron emission tomography (PET), a medical imaging technique allowing for studies of the living human brain, has gained an important role in clinical trials of novel drugs against Alzheimer's disease (AD). For example, PET data contributed to the conditional approval in 2021 of aducanumab, an antibody directed towards amyloid-beta (Aß) aggregates, by showing a dose-dependent reduction in brain amyloid after treatment. In parallel to clinical studies, preclinical studies in animal models of Aß pathology may also benefit from PET as a tool to detect target engagement and treatment effects of anti-Aß drug candidates. PET is associated with a high level of translatability between species as similar, non-invasive protocols allow for longitudinal rather than cross-sectional studies and can be used both in a preclinical and clinical setting. This review focuses on the use of preclinical PET imaging in genetically modified animals that express human Aß, and its present and potential future role in the development of drugs aimed at reducing brain Aß levels as a therapeutic strategy to halt disease progression in AD.

In vivo imaging of synaptic density with [11C]UCB-J PET in two mouse models of neurodegenerative disease.

The positron emission tomography (PET) radioligand [11C]UCB-J binds to synaptic vesicle protein 2A (SV2A) and is used to investigate synaptic density in the living brain. Clinical studies have indicated reduced [11C]UCB-J binding in Alzheimer's disease (AD) and Parkinson's disease (PD) brains compared to healthy controls. Still, it is unknown whether [11C]UCB-J PET can visualise synaptic loss in mouse models of these disorders. Such models are essential for understanding disease pathology and for evaluating the effects of novel disease-modifying drug candidates. In the present study, synaptic density in transgenic models of AD (ArcSwe) and PD (L61) was studied using [11C]UCB-J PET. Data were acquired during 60 min after injection, and time-activity curves (TACs) in different brain regions and the left ventricle of the heart were generated based on the dynamic PET images. The [11C]UCB-J brain concentrations were expressed as standardised uptake value (SUV) over time. The area under the SUV curve (AUC), the ratio of AUC in the brain to that in the heart (AUCbrain/blood), and the volume of distribution (VT) obtained by kinetic modelling using the heart TAC as input were compared between transgenic and age-matched wild type (WT) mice. The L61 mice displayed 11-13% lower AUCbrain/blood ratio and brain VT generated by kinetic modeling compared to the control WT mice. In general, also transgenic ArcSwe mice tended to show lower [11C]UCB-J brain exposure than age-matched WT controls, but variation within the different animal groups was high. Older WT mice (18-20 months) showed lower [11C]UCB-J brain exposure than younger WT mice (8-9 months). Together, these data imply that [11C]UCB-J PET reflects synaptic density in mouse models of neurodegeneration and that inter-subject variation is large. In addition, the study suggested that model-independent AUCbrain/blood ratio can be used to evaluate [11C]UCB-J binding as an alternative to full pharmacokinetic modelling.

Brain delivery of biologics using a cross-species reactive transferrin receptor 1 VNAR shuttle.

Transferrin receptor 1 (TfR1) mediated transcytosis is an attractive strategy to enhance brain uptake of protein drugs, but translation remains a challenge. Here, a single domain shark antibody VNAR fragment (TXB2) with similar affinity to murine and human TfR1 was used to shuttle protein cargo into the brain. TXB2 was fused to a human IgG1 Fc domain (hFc) or to the amyloid-ß (Aß) antibody bapineuzumab (Bapi). TXB2-hFc displayed 20-fold higher brain concentrations compared with a control VNAR-hFc at 18 hours post-injection in wt mice. At the same time point, brain concentrations of Bapi-TXB2 was threefold higher than Bapi. In transgenic mice overexpressing human Aß, the brain-to-blood concentration ratio increased with time due to interaction with intracerebral Aß deposits. The relatively stable threefold difference between Bapi-TXB2 and Bapi was observed for up to 6 days after injection. PET imaging and ex vivo autoradiography revealed more parenchymal distribution of Bapi-TXB2 compared with Bapi. In conclusion, the TXB2 VNAR shuttle markedly increased brain uptake of protein cargo and increased brain concentrations of the Aß binding antibody Bapi.

Pyroglutamation of amyloid-ßx-42 (Aßx-42) followed by Aß1-40 deposition underlies plaque polymorphism in progressing Alzheimer's disease pathology.

Amyloid-ß (Aß) pathology in Alzheimer's disease (AD) is characterized by the formation of polymorphic deposits comprising diffuse and cored plaques. Because diffuse plaques are predominantly observed in cognitively unaffected, amyloid-positive (CU-AP) individuals, pathogenic conversion into cored plaques appears to be critical to AD pathogenesis. Herein, we identified the distinct Aß species associated with amyloid polymorphism in brain tissue from individuals with sporadic AD (s-AD) and CU-AP. To this end, we interrogated Aß polymorphism with amyloid conformation-sensitive dyes and a novel in situ MS paradigm for chemical characterization of hyperspectrally delineated plaque morphotypes. We found that maturation of diffuse into cored plaques correlated with increased Aß1-40 deposition. Using spatial in situ delineation with imaging MS (IMS), we show that Aß1-40 aggregates at the core structure of mature plaques, whereas Aß1-42 localizes to diffuse amyloid aggregates. Moreover, we observed that diffuse plaques have increased pyroglutamated Aßx-42 levels in s-AD but not CU-AP, suggesting an AD pathology-related, hydrophobic functionalization of diffuse plaques facilitating Aß1-40 deposition. Experiments in tgAPPSwe mice verified that, similar to what has been observed in human brain pathology, diffuse deposits display higher levels of Aß1-42 and that Aß plaque maturation over time is associated with increases in Aß1-40. Finally, we found that Aß1-40 deposition is characteristic for cerebral amyloid angiopathy deposition and maturation in both humans and mice. These results indicate that N-terminal Aßx-42 pyroglutamation and Aß1-40 deposition are critical events in priming and maturation of pathogenic Aß from diffuse into cored plaques, underlying neurotoxic plaque development in AD.

Chemical imaging of evolving amyloid plaque pathology and associated Aß peptide aggregation in a transgenic mouse model of Alzheimer's disease.

One of the major hallmarks of Alzheimer's disease (AD) pathology is the formation of extracellular amyloid ß (Aß) plaques. While Aß has been suggested to be critical in inducing and, potentially, driving the disease, the molecular basis of AD pathogenesis is still under debate. Extracellular Aß plaque pathology manifests itself upon aggregation of distinct Aß peptides, resulting in morphologically different plaque morphotypes, including mainly diffuse and cored senile plaques. As plaque pathology precipitates long before any clinical symptoms occur, targeting the Aß aggregation processes provides a promising target for early interventions. However, the chain of events of when, where and what Aß species aggregate and form plaques remains unclear. The aim of this study was to investigate the potential of matrix-assisted laser desorption/ionization imaging mass spectrometry as a tool to study the evolving pathology in transgenic mouse models for AD. To that end, we used an emerging, chemical imaging modality - matrix-assisted laser desorption/ionization imaging mass spectrometry - that allows for delineating Aß aggregation with specificity at the single plaque level. We identified that plaque formation occurs first in cortical regions and that these younger plaques contain higher levels of 42 amino acid-long Aß (Aß1-42). Plaque maturation was found to be characterized by a relative increase in deposition of Aß1-40, which was associated with the appearance of a cored morphology for those plaques. Finally, other C-terminally truncated Aß species (Aß1-38 and Aß1-39) exhibited a similar aggregation pattern as Aß1-40, suggesting that these species have similar aggregation characteristics. These results suggest that initial plaque formation is seeded by Aß1-42; a process that is followed by plaque maturation upon deposition of Aß1-40 as well as deposition of other C-terminally modified Aß species.

High detection sensitivity with antibody-based PET radioligand for amyloid beta in brain.

PET imaging of amyloid-beta (Aß) deposits in brain has become an important aid in Alzheimer's disease diagnosis, and an inclusion criterion for patient enrolment into clinical trials of new anti-Aß treatments. Available PET radioligands visualizing Aß bind to insoluble fibrils, i.e. Aß plaques. Levels of prefibrillar Aß forms, e.g. soluble oligomers and protofibrils, correlate better than plaques with disease severity and these soluble species are the neurotoxic form of Aß leading to neurodegeneration. The goal was to create an antibody-based radioligand, recognizing not only fibrillary Aß, but also smaller and still soluble aggregates. We designed and expressed a small recombinant bispecific antibody construct, di-scFv 3D6-8D3, targeting the Aß N-terminus and the transferrin receptor (TfR). Natively expressed at the blood-brain barrier (BBB), TfR could thus be used as a brain-blood shuttle. Di-scFv 3D6-8D3 bound to Aß1-40 with high affinity and to TfR with moderate affinity. Di-scFv [124I]3D6-8D3 was injected in two transgenic mouse models overexpressing human Aß and wild-type control mice and PET scanned at 14, 24 or 72 h after injection. Di-scFv [124I]3D6-8D3 was retained in brain of transgenic animals while it was cleared from wild-type lacking Aß. This difference was observed from 24 h onwards, and at 72 h, 18 months old transgenic animals, with high load of Aß pathology, displayed SUVR of 2.2-3.5 in brain while wild-type showed ratios close to unity. A subset of the mice were also scanned with [11C]PIB. Again wt mice displayed ratios of unity while transgenes showed slightly, non-significantly, elevated SUVR of 1.2, indicating improved sensitivity with novel di-scFv [124I]3D6-8D3 compared with [11C]PIB. Brain concentrations of di-scFv [124I]3D6-8D3 correlated with soluble Aß (p < 0.0001) but not with total Aß, i.e. plaque load (p = 0.34). We have successfully created a small bispecific antibody-based radioligand capable of crossing the BBB, subsequently binding to and visualizing intrabrain Aß in vivo. The radioligand displayed better sensitivity compared with [11C]PIB, and brain concentrations correlated with soluble neurotoxic Aß aggregates.

Engineered antibodies: new possibilities for brain PET?

Almost 50 million people worldwide are affected by Alzheimer's disease (AD), the most common neurodegenerative disorder. Development of disease-modifying therapies would benefit from reliable, non-invasive positron emission tomography (PET) biomarkers for early diagnosis, monitoring of disease progression, and assessment of therapeutic effects. Traditionally, PET ligands have been based on small molecules that, with the right properties, can penetrate the blood-brain barrier (BBB) and visualize targets in the brain. Recently a new class of PET ligands based on antibodies have emerged, mainly in applications related to cancer. While antibodies have advantages such as high specificity and affinity, their passage across the BBB is limited. Thus, to be used as brain PET ligands, antibodies need to be modified for active transport into the brain. Here, we review the development of radioligands based on antibodies for visualization of intrabrain targets. We focus on antibodies modified into a bispecific format, with the capacity to undergo transferrin receptor 1 (TfR1)-mediated transcytosis to enter the brain and access pathological proteins, e.g. amyloid-beta. A number of such antibody ligands have been developed, displaying differences in brain uptake, pharmacokinetics, and ability to bind and visualize the target in the brain of transgenic mice. Potential pathological changes related to neurodegeneration, e.g. misfolded proteins and neuroinflammation, are suggested as future targets for this novel type of radioligand. Challenges are also discussed, such as the temporal match of radionuclide half-life with the ligand's pharmacokinetic profile and translation to human use. In conclusion, brain PET imaging using bispecific antibodies, modified for receptor-mediated transcytosis across the BBB, is a promising method for specifically visualizing molecules in the brain that are difficult to target with traditional small molecule ligands.

The Aß protofibril selective antibody mAb158 prevents accumulation of Aß in astrocytes and rescues neurons from Aß-induced cell death.

BACKGROUND: Currently, several amyloid beta (Aß) antibodies, including the protofibril selective antibody BAN2401, are in clinical trials. The murine version of BAN2401, mAb158, has previously been shown to lower the levels of pathogenic Aß and prevent Aß deposition in animal models of Alzheimer's disease (AD). However, the cellular mechanisms of the antibody's action remain unknown. We have recently shown that astrocytes effectively engulf Aß42 protofibrils, but store rather than degrade the ingested Aß aggregates. In a co-culture set-up, the incomplete degradation of Aß42 protofibrils by astrocytes results in increased neuronal cell death, due to the release of extracellular vesicles, containing N-truncated, neurotoxic Aß. METHODS: The aim of the present study was to investigate if the accumulation of Aß in astrocytes can be affected by the Aß protofibril selective antibody mAb158. Co-cultures of astrocytes, neurons, and oligodendrocytes, derived from embryonic mouse cortex, were exposed to Aß42 protofibrils in the presence or absence of mAb158. RESULTS: Our results demonstrate that the presence of mAb158 almost abolished Aß accumulation in astrocytes. Consequently, mAb158 treatment rescued neurons from Aß-induced cell death. CONCLUSION: Based on these findings, we conclude that astrocytes may play a central mechanistic role in anti-Aß immunotherapy.

Secretion and Uptake of α-Synuclein Via Extracellular Vesicles in Cultured Cells.

In Parkinson's disease and other Lewy body disorders, the propagation of pathology has been accredited to the spreading of extracellular α-synuclein (α-syn). Although the pathogenic mechanisms are not fully understood, cell-to-cell transfer of α-syn via exosomes and other extracellular vesicles (EVs) has been reported. Here, we investigated whether altered molecular properties of α-syn can influence the distribution and secretion of α-syn in human neuroblastoma cells. Different α-syn variants, including α-syn:hemi-Venus and disease-causing mutants, were overexpressed and EVs were isolated from the conditioned medium. Of the secreted α-syn, 0.1-2% was associated with vesicles. The major part of EV α-syn was attached to the outer membrane of vesicles, whereas a smaller fraction was found in their lumen. For α-syn expressed with N-terminal hemi-Venus, the relative levels associated with EVs were higher than for WT α-syn. Moreover, such EV-associated α-syn:hemi-Venus species were internalized in recipient cells to a higher degree than the corresponding free-floating forms. Among the disease-causing mutants, A53T α-syn displayed an increased association with EVs. Taken together, our data suggest that α-syn species with presumably lost physiological functions or altered aggregation properties may shift the cellular processing towards vesicular secretion. Our findings thus lend further support to the tenet that EVs can mediate spreading of harmful α-syn species and thereby contribute to the pathology in α-synucleinopathies.

A bispecific Tribody PET radioligand for visualization of amyloid-beta protofibrils - a new concept for neuroimaging.

Antibodies are highly specific for their target molecules, but their poor brain penetrance has restricted their use as PET ligands for imaging of targets within the CNS. The aim of this study was to develop an antibody-based radioligand, using the TribodyTM format, for PET imaging of soluble amyloid-beta (Aß) protofibrils, which are suggested to cause neurodegeneration in Alzheimer's disease. Antibodies, even when expressed in smaller engineered formats, are large molecules that do not enter the brain in sufficient amounts for imaging purposes. Hence, their transport across the blood-brain barrier (BBB) needs to be facilitated, for example through interaction with the transferrin receptor (TfR). Thus, a Fab fragment of the TfR antibody 8D3 was fused with two single chain variable fragments (scFv) of the Aß protofibril selective antibody mAb158. Five TribodyTM proteins (A1-A5) were generated with different linkers between the Fab-8D3 and scFv-158. All proteins bound to TfR and Aß protofibrils in vitro. Three of the proteins (A1-A3) were radiolabeled with iodine-125 and studied ex vivo in wild-type (wt) and transgenic mice overexpressing human Aß. The systemic pharmacokinetics were similar with half-lives in blood of around 9h for all three ligands. Brain concentrations at 2h were around 1% of the injected dose per gram brain tissue, which is similar to what is observed for small molecular radioligands and at least 10-fold higher than antibodies in general. At 72h, transgenic mice showed higher concentrations of radioactivity in the brain than wt mice (12, 15- and 16-fold for A1, A2 and A3 respectively), except in the cerebellum, an area largely devoid of Aß pathology. A3 was then labelled with iodine-124 for in vivo positron emission tomography (PET) imaging. Brain concentrations were quantified in six different regions showing a clear distinction both quantitatively and visually between wt and transgenic mice and a good correlation with Aß pathology. We have thus produced a recombinant, bispecific protein, actively transported into the brain, for PET imaging within the CNS. In a longer perspective, this technique may enable imaging of other proteins involved in neurodegenerative diseases for which imaging agents are completely lacking today.

Cationization increases brain distribution of an amyloid-beta protofibril selective F(ab')2 fragment.

Antibodies and fragments thereof are, because of high selectivity for their targets, considered as potential therapeutics and biomarkers for several neurological disorders. However, due to their large molecular size, antibodies/fragments do not easily penetrate into the brain. The aim of the present study was to improve the brain distribution via adsorptive-mediated transcytosis of an amyloid-beta (Aß) protofibril selective F(ab')2 fragment (F(ab')2-h158). F(ab')2-h158 was cationized to different extents and the specific and unspecific binding was studied in vitro. Next, cationized F(ab')2-h158 was labelled with iodine-125 and its brain distribution and pharmacokinetics was studied in mice. Cationization did not alter the in vitro affinity to Aß protofibrils, but increased the unspecific binding somewhat. Ex vivo experiments revealed a doubling of brain concentrations compared with unmodified F(ab')2-h158 and in vivo imaging with single photon emission computed tomography (SPECT) showed that the cationized F(ab')2-h158, but not the unmodified F(ab')2-h158 could be visualized in the brain. To conclude, cationization is a means to increase brain concentrations of therapeutic antibodies or fragments and may facilitate the use of antibodies/fragments as imaging biomarkers in the brain.

Efficient and inexpensive transient expression of multispecific multivalent antibodies in Expi293 cells.

BACKGROUND: Immunotherapy is a very fast expanding field within drug discovery and, hence, rapid and inexpensive expression of antibodies would be extremely valuable. Antibodies are, however, difficult to express. Multifunctional antibodies with additional binding domains further complicate the expression. Only few protocols describe the production of tetravalent bispecific antibodies and all with limited expression levels.. METHODS: Here, we describe a protocol that can produce functional tetravalent, bispecific antibodies at around 22 mg protein/l to a low cost. The expression system is based on the Expi293 cells, which have been adapted to grow in denser cultures than HEK293 cells and gives higher expression yields. The new protocol transfects the Expi293 cells with PEI (which has a negligible cost). RESULTS: The protocol has been used to generate multiple variants of tetra- and hexavalent bispecific antibodies with yields of around 22 mg protein/l within 10 days. All materials are commercially available and the implementation of the protocol is inexpensive and straightforward. The bispecific antibodies generated in our lab were capable of binding to all antigens with similar affinity as the original antibody. Two of the bispecific antibodies have also been used in transgenic mice as positron emission tomography (PET) ligands to successfully detect amyloid-beta (Aß) aggregates in vivo. CONCLUSIONS: This protocol is the first describing transfection of the human Expi293 cells with PEI. It can be used to generate functional multi-specific antibodies in high amounts. The use of biological drugs, and in particular multispecific antibodies, is rapidly increasing, hence improved protocols such as the one presented here are highly valuable.

Indium-111 radiolabelling of a brain-penetrant Aß antibody for SPECT imaging.

Background: The development of bispecific antibodies that can traverse the blood-brain barrier has paved the way for brain-directed immunotherapy and when radiolabelled, immunoPET imaging. The objective of this study was to investigate how indium-111 (111In) radiolabelling with compatible chelators affects the brain delivery and peripheral biodistribution of the bispecific antibody RmAb158-scFv8D3, which binds to amyloid-beta (Aß) and the transferrin receptor (TfR), in Aß pathology-expressing tg-ArcSwe mice and aged-matched wild-type control mice. Methods: Bispecific RmAb158-scFv8D3 (biAb) was radiolabelled with 111In using CHX-A"-DTPA, DOTA, or DOTA-tetrazine (DOTA-Tz). Affinity toward TfR and Aß, as well as stability, was investigated in vitro. Mice were then intravenously administered with the three different radiolabelled biAb variants, and blood samples were collected for monitoring pharmacokinetics. Brain concentration was quantified after 2 and 72 h, and organ-specific retention was measured at 72 h by gamma counting. A subset of mice also underwent whole-body Single-photon emission computed tomography (SPECT) scanning at 72 h after injection. Following post-mortem isolation, the brains of tg-ArcSwe and WT mice were sectioned, and the spatial distribution of biAb was further investigated with autoradiography. Results: All three [111In]biAb variants displayed similar blood pharmacokinetics and brain uptake at 2 h after administration. Radiolabelling did not compromise affinity, and all variants showed good stability, especially the DOTA-Tz variant. Whole-body SPECT scanning indicated high liver, spleen, and bone accumulation of all [111In]biAb variants. Subsequent ex vivo measurement of organ retention confirmed SPECT data, with retention in the spleen, liver, and bone - with very high bone marrow retention. Ex vivo gamma measurement of brain tissue, isolated at 72 h post-injection, and ex vivo autoradiography showed that WT mice, despite the absence of Aß, exhibited comparable brain concentrations of [111In]biAb as those found in the tg-ArcSwe brain. Conclusions: The successful 111In-labelling of biAb with retained binding to TfR and Aß, and retained ability to enter the brain, demonstrated that 111In can be used to generate radioligands for brain imaging. A high degree of [111In]biAb in bone marrow and intracellular accumulation in brain tissue indicated some off-target interactions or potential interaction with intrabrain TfR resulting in a relatively high non-specific background signal.

Reducing neonatal Fc receptor binding enhances clearance and brain-to-blood ratio of TfR-delivered bispecific amyloid-ß antibody.

Recent development of amyloid-ß (Aß)-targeted immunotherapies for Alzheimer's disease (AD) have highlighted the need for accurate diagnostic methods. Antibody-based positron emission tomography (PET) ligands are well suited for this purpose as they can be directed toward the same target as the therapeutic antibody. Bispecific, brain-penetrating antibodies can achieve sufficient brain concentrations, but their slow blood clearance remains a challenge, since it prolongs the time required to achieve a target-specific PET signal. Here, two antibodies were designed based on the Aß antibody bapineuzumab (Bapi) - one monospecific IgG (Bapi) and one bispecific antibody with an antigen binding fragment (Fab) of the transferrin receptor (TfR) antibody 8D3 fused to one of the heavy chains (Bapi-Fab8D3) for active, TfR-mediated transport into the brain. A variant of each antibody was designed to harbor a mutation to the neonatal Fc receptor (FcRn) binding domain, to increase clearance. Blood and brain pharmacokinetics of radiolabeled antibodies were studied in wildtype (WT) and AD mice (AppNL-G-F). The FcRn mutation substantially reduced blood half-life of both Bapi and Bapi-Fab8D3. Bapi-Fab8D3 showed high brain uptake and the brain-to-blood ratio of its FcRn mutated form was significantly higher in AppNL-G-F mice than in WT mice 12 h after injection and increased further up to 168 h. Ex vivo autoradiography showed specific antibody retention in areas with abundant Aß pathology. Taken together, these results suggest that reducing FcRn binding of a full-sized bispecific antibody increases the systemic elimination and could thereby drastically reduce the time from injection to in vivo imaging.