Development of an ESR-Based Diagnostic Probe for Matrix Metalloproteinase Activity Using a Spin-Labeled Peptide-Conjugated Styrene–Maleic Acid Copolymer

Abstract

Matrix metalloproteinases (MMPs), specifically MMP-2 and MMP-9, play a significant role in tumor growth and malignancy. Therefore, measuring their activity in vivo could enhance the diagnosis and treatment of cancer. Given the medical and pharmaceutical applications of the in vivo ESR techniques in recent decades, we developed a probe to evaluate MMP activity based on ESR spectral changes that depend on the rotational correlation time of the nitroxyl radical. The probe was synthesized by conjugating the nitroxyl radical to a styrene–maleic acid copolymer via an MMP substrate peptide and polyetheramine. The ESR signal of the probe was broadened by complex formation with bovine serum albumin. When either MMP-2 or MMP-9 was added to the complex, the intensity of the sharp signal increased markedly over time. This increase was completely inhibited by specific inhibitors of MMP-2/MMP-9 and did not occur with a probe containing scrambled substrate peptides. The specific constant (k_cat/K_m) for degradation of the complex by MMP-2 was 4.7 × 10³ M⁻¹ s⁻¹, comparable to or 1–2 orders of magnitude lower than that of previously reported MMP-2 substrates designed for cancer therapy and diagnosis. This lower catalytic efficiency was attributed to a higher Michaelis–Menten constant relative to other MMP-2 substrates, suggesting a reduced substrate binding affinity. Despite the need for improved probe affinity, this study demonstrates a mechanism in which ESR signals increase in response to MMP-2 and MMP-9 activity, highlighting its potential for noninvasive in vivo assessment of MMP activity.