IF a bispecific nanobody construct — designated MedFib-RGD-Nb — comprising (i) a conformation-specific single-domain antibody (VHH) raised against the cross-β amyloid core of medin fibrils (residues 245–294 of MFG-E8, epitope accessible only in fibrillar configuration) fused via a flexible GGS linker to (ii) the isolated RGD-containing EGF-like domain of native MFG-E8 (residues ~1–70, sufficient for αvβ3/αvβ5 integrin engagement) is administered intravenously at 10–30 mg/kg biweekly to aged (18–22 month) C57BL/6J mice bearing endogenous aortic medin deposits,

THEN a ≥30% reduction in thioflavin-S-positive medin fibril burden in the aortic tunica media — measured by quantitative immunofluorescence with medin-specific antibody and confirmatory Congo red birefringence on cross-sectional aortic tissue — will be observed within 12 weeks of treatment initiation, accompanied by measurable reduction in aortic pulse wave velocity (PWV, marker of arterial stiffness) and reduced medin/Aβ co-deposits in basilar and middle cerebral arteries,

BECAUSE the following causal chain operates:

1. Medin fibrils expose a cross-β amyloid core epitope that is entirely cryptic in native, globular MFG-E8: The C2 domain (residues 245–294) is internally folded within soluble MFG-E8 and becomes surface-exposed only upon proteolytic release and subsequent β-sheet stacking into fibrils. The VHH arm of MedFib-RGD-Nb is selected against this fibril-specific conformational epitope — analogous to lecanemab's selectivity for Aβ protofibrils over monomers (van Dyck et al., NEJM, 2023, described in Evidence Set) — ensuring the construct does not bind the >97%-prevalent soluble MFG-E8 that mediates efferocytosis (Hanayama et al., Nature, 2002, Evidence Set). [Selectivity step; the fibril-specific VHH is the critical novelty enabling selectivity]

2. The fused MFG-E8 RGD-EGF domain co-opts the native opsonin-bridge mechanism of the precursor protein to recruit αvβ3/αvβ5-expressing phagocytes to fibril deposits: MFG-E8's canonical function is bridging phosphatidylserine on apoptotic cells to αvβ3/αvβ5 integrins on macrophages and VSMCs to facilitate engulfment (Hanayama et al., Nature, 2002, Evidence Set). MedFib-RGD-Nb inverts and repurposes this biology: the RGD domain now presents an integrin-binding signal at the surface of medin fibril deposits (to which the VHH is anchored), effectively displaying a synthetic "eat-me" flag recognizable by integrin-bearing adventitial macrophages and interstitial monocytes resident in the aortic media, without requiring Fc-gamma receptor engagement. [SPECULATIVE: that RGD display on a fibril surface is sufficient to trigger integrin-mediated phagocytic cup formation in VSM-adjacent macrophages — mechanistic precedent from Hanayama's bridging model supports this but direct fibril-targeted RGD opsonization has not been tested]

3. **Absence of an Fc region eliminates the primary risk mechanism of vascular amyloid immunotherapy:*...

SENS category: GlycoSENS