<p>Hypoxia is a key feature of solid tumors, contributing to therapeutic resistance and poor prognosis. Targeting hypoxic environments presents an opportunity to enhance drug delivery selectivity and improve treatment outcomes. Among nanocarriers, mesoporous silica nanoparticles (MSNs) have demonstrated great potential for drug delivery; however, achieving precise control over drug release remains a challenge. In this work, we develop an enzyme-responsive MSN system for targeted drug delivery in hypoxic tumors. MSNs were loaded with the chemotherapeutic drug doxorubicin (Dox) and capped with an azobenzene (Azo) gatekeeper. The bulky Azo group acts as a responsive molecular gate that remains closed under normoxic conditions but undergoes enzymatic cleavage by azoreductases, which are overexpressed in hypoxic tumor microenvironment. This cleavage triggers the uncapping of the pores, inducing the release of Dox specifically in the hypoxic region, minimizing premature drug leakage and off-target toxicity. In vitro studies with A549 cells (which overexpress azoreductase) and THP-1 cells (with low expression of azoreductases) under normoxic and hypoxic conditions, demonstrated a significant increase in Dox release and cytotoxicity in the A549 cells compared with the THP-1, which was more pronounced under hypoxia. These findings highlight the potential of enzyme-responsive MSNs as a promising strategy for selective drug delivery in hypoxic tumors.</p> Graphical abstract <p></p>