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Background: Matrix metalloproteinases (MMPs) are implied in blood-brain barrier degradation and haemorrhagic transformation following ischaemic stroke, but their local relevance in the hyperacute disease phase is unknown. We aimed to examine ultra-early MMP-9 and MMP-2 release into collateral blood vessels, and to assess its prognostic value before therapeutic recanalisation by endovascular thrombectomy (EVT)., Methods: We report a cross-sectional proof-of-concept study including patients undergoing EVT for large-vessel ischaemic stroke at the University Hospital Würzburg, Germany. We obtained liquid biopsies from the collateral circulation before recanalisation, and systemic control samples. Laboratory workup included quantification of MMP-9 and MMP-2 plasma concentrations by cytometric bead array, immunohistochemical analyses of cellular MMP-9 and MMP-2 expression, and detection of proteolytic activity by gelatine zymography. The clinical impact of MMP concentrations was assessed by stratification according to intracranial haemorrhagic lesions on postinterventional computed tomography (Heidelberg Bleeding Classification, HBC) and early functional outcome (modified Rankin Scale, mRS). We used multivariable logistic regression, receiver-operating-characteristic (ROC) curves, and fixed-level estimates of test accuracy measures to study the prognostic value of MMP-9 concentrations., Findings: Between August 3, 2018, and September 16, 2021, 264 matched samples from 132 patients (86 [65.2%] women, 46 [34.8%] men, aged 40-94 years) were obtained. Median (interquartile range, IQR) MMP-9 (279.7 [IQR 126.4-569.6] vs 441 [IQR 223.4-731.5] ng/ml, p < 0.0001) but not MMP-2 concentrations were increased within collateral blood vessels. The median MMP-9 expression level of invading neutrophils was elevated (fluorescence intensity, arbitrary unit: 2276 [IQR 1007-5086] vs 3078 [IQR 1108-7963], p = 0.0018). Gelatine zymography experiments indicated the locally confined proteolytic activity of MMP-9 but not of MMP-2. Pretherapeutic MMP-9 release into stroke-affected brain regions predicted the degree of intracerebral haemorrhages and clinical stroke severity after recanalisation, and independently increased the odds of space-occupying parenchymal haematomas (HBC1c-3a) by 1.54 times, and the odds of severe disability or death (mRS ≥5 at hospital discharge) by 2.33 times per 1000 ng/ml increase. Excessive concentrations of MMP-9 indicated impending parenchymal haematomas and severe disability or death with high specificity., Interpretation: Measurement of MMP-9 within collateral blood vessels is feasible and identifies patients with stroke at risk of major intracerebral haemorrhages and poor outcome before therapeutic recanalisation by EVT, thereby providing evidence of the concept validity of ultra-early local stroke biomarkers., Funding: This work was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) and the Interdisciplinary Centre for Clinical Research (IZKF) at the University of Würzburg., Competing Interests: Declaration of interests MP has received speaker honoraria from Bayer and Merck Serono; unrelated to the present study. KGH has received honoraria for acting as an advisor/speaker for Boehringer Ingelheim; unrelated to the present study. MKS declares a grant from CSL Behring, funding from the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG) project No. 424778381—CRC/TR 295, and is a co-holder of patent PCT/EP2020/068464; all unrelated to the present study. All other authors have nothing to disclose., (Copyright © 2024 The Author(s). Published by Elsevier B.V. All rights reserved.)