Protein engineering of selective MMP-9 inhibitors reveals a therapeutic strategy for triple-negative breast cancer

Matrix metalloproteinases (MMPs) are key regulators of tumor progression, invasion, and metastasis; however, clinical efforts to inhibit MMPs have largely failed due to a lack of selectivity and unintended suppression of protective family members. In triple-negative breast cancer (TNBC), MMP-9 (gelatinase B) is strongly associated with tumor aggressiveness, extracellular matrix remodeling, and metastatic dissemination. In contrast, MMP-8 has been shown to exert anti-tumorigenic effects in breast cancer by limiting inflammation and suppressing metastasis. This functional divergence underscores the importance of selectively targeting MMP-9 while preserving MMP-8 activity as a rational therapeutic strategy in TNBC. To address this challenge, we engineered variants of the endogenous MMP inhibitor tissue inhibitor of metalloproteinases-1 (TIMP-1) with enhanced selectivity for MMP-9 over MMP-8. A yeast surface display library of human TIMP-1 was constructed and screened using fluorescence-activated cell sorting through iterative rounds of positive selection for recombinant MMP-9, followed by competitive counter-selection against MMP-8 to eliminate off-target binders. This strategy enabled enrichment of TIMP-1 variants with improved specificity for MMP-9 compared with wild-type TIMP-1. Several TIMP-1 variants exhibited enhanced binding and inhibitory selectivity toward MMP-9, with TIMP-1-C6.2 emerging as the most promising candidate. TIMP-1-C6.2 preferentially inhibited MMP-9 with a Ki of 0.3 ± 0.04 nM, while demonstrating substantially weaker inhibition of MMP-8 (Ki = 6.8 ± 0.27 nM), corresponding to an approximately 23-fold selectivity for MMP-9. Importantly, functional evaluation in Matrigel invasion assays using MDA-MB-231 TNBC cells revealed that TIMP-1-C6.2 significantly enhanced inhibition of tumor cell invasion compared with wild-type TIMP-1 in a dose-dependent manner (p < 0.0001). This improved anti-invasive activity reflects refined target selectivity rather than a generalized increase in inhibitory potency. Collectively, these findings demonstrate that selective engineering of TIMP-based inhibitors can overcome a central limitation of MMP-targeted therapies by suppressing pro-tumorigenic MMP-9 while sparing anti-tumorigenic MMP-8. This work provides a strong foundation for the development of selective MMP-9 inhibitors with improved safety and therapeutic potential for TNBC and other MMP-driven cancers.