SP #8: Antibody Derived PET Ligands for Tau Pathology

Antibody Derived PET Ligands for Tau Pathology

Significance

The Sigurdsson laboratory pioneered the field of tau immunotherapy with the first report showing its efficacy for clearing tau pathology [1], followed by various efficacy and mechanistic studies, clearly showing that tau antibodies and their fragments get into the brain and into neurons to bind to pathological tau proteins [2-25]. On the diagnostic front, antibody detection of abnormal tau in cerebrospinal fluid has shown some promise [26,27]. In particular, smaller antibody fragments that bind to tau are attractive as ligands for in vivo imaging to detect tau lesions in patients with Alzheimer’s disease (AD) or other tauopathies. For in vivo studies, rapid clear­ance renders antibody fragments more attractive than unmodified antibodies. Several single chain variable frag­ments (scFv’s) are already being developed as imaging probes against various other targets [28]. In vivo imaging of Aβ plaques using compounds that bind well to β-sheets is in clinical use but only a few such tau-selective ligands have been identified in preclinical studies [29-38], and some have failed in clinical trials [39].  Antibody-based probes such as those proposed here are likely to provide greater specificity for detecting tau lesions.

Aims:

In this study, we will assess the diagnostic utility of various tau antibody fragments that we have generated using phage display technology. Towards this end, transgenic tauopathy mouse models will be used with μPET imaging as well as tissue from AD patients and related tauopathies.

A key advantage of scFv’s is that they may have better access than antibodies to tau aggregates, and we hypothesize that scFv’s will be more efficacious than antibodies as diagnostic markers for tau pathol­ogy. We have already identified several frag­ments that: 1) Have high affinity towards tau in ELISA and Biacore; 2) Bind to pathological tau in AD brain tissue, and; 3) When injected into the carotid artery or intravenously in tauopathy mice, enter the brain and neurons, and bind there to pathological tau [40], analogous to our published findings with tau antibodies [2-5]. Studies of these lead candidates and other scFv’s we have already generated may lead to novel diagnostic markers for AD and related tauopathies. We are not aware of other studies on tau antibody fragments as potential imaging ligands. These experiments may also lead to the development of specific tau aggregation inhibitors and/or diagnostic probes for pathological tau derived from these fragments.

Approach:

Following detailed characterization of the scFv’s in various binding assays as outlined in funded R01 AG032611, the best candidates will be advanced for in vivo imaging. a) IVIS Imaging: Based on the binding assays, neuronal uptake and co-localization with pathological tau within the mouse brain, IVIS imaging (multiple time points up to 24 h post-injection, intracarotid (i.c.) vs. intravenous (i.v.)) on three of the most promising scFv’s, labeled with a fluorescent tag. Up to forty Tg mice with advanced tau pathology (JNPL3 or htau/PS1 >12 months) and 15 age-matched wt mice will be used (at least 3 mice per group) with the majority of the mice receiving an i.c. injection but 8 receiving an i.v. injection. b) μPET Imaging: The scFv’s need to be modified to allow their detection by PET. 18-fluorine (18F; half-life 110 min) is commonly used for clinical brain PET imaging such as for assessing the distribution of fluorodeoxyglucose (FDG) and in Aβ ligands for quantifying Aβ burden [41-45]. Two of the scFv’s (with the highest IVIS signal and favorable rate of clearance) will then be labeled with non-radioactive fluorine followed by verification of their binding characteristics. These will then be labeled with 18F, and their binding properties verified again. After confirmation that the radiolabeling does not affect the binding properties of the fragments, the diagnostic utility of the 18F-scFv’s will be assessed with μPET imaging, using a similar design as for our preliminary studies on the fluorescently-tagged scFv’s [40]. Dose and route of administration will be based on the IVIS studies. Note that we have not observed any adverse reactions in preliminary IVIS studies and we anticipate similar lack of toxicity in the planned PET studies.  Five Tg mice with advanced tau pathology (JNPL3 or htau/PS1 >12 months) and five age-matched wild-type mice will undergo PET imaging with each of the two ligands (3-4 h dynamic scan following injection). Clearance will be verified in four additional animals at 8 h post-injection. In future studies, we will clarify how early tau pathology can be detected with this approach and if it can be used to evaluate treatment efficacy.

BTRC Resources Utilitized:

TR&D #3: Dr. Sigurdsson and his group are already in regular contact with Dr. Ding, who serves as a coinvestigator on NIH R01 AG032611.  The radiochemistry and μPET resources of the BTRC will clearly be essential in carrying forward the research plan of this SP.  In addition, tracer kinetic analysis of the PET dynamic data will be paramount for the quantification of tracer uptake and the correlation of tracer accumulation with tau burden in the tissue. The improved arterial input function quantification techniques discussed in TR&D #3 will be used in conjunction with direct arterial sampling on the animals (using the Swisstrace device to be procured through this proposal). This assessment will facilitate the development of clinically feasible protocols (i.e. without arterial sampling) for the tracers to be evaluated in this project. 

Principal Investigator: 

Sponsors

Latest Updates

Philanthropic Support

We gratefully acknowledge generous support for radiology research at NYU Langone Medical Center from:
 
• The Big George Foundation
• Raymond and Beverly Sackler
• Bernard and Irene Schwartz

Go to top