Visualizable, AKR1C3-Activated Small-Molecule Antitumor Drug Conjugates with Ultra-Low Systemic Toxicity.

Developing targeted antitumor agents with minimal systemic toxicity is highly desirable. Exatecan, a potent camptothecin derivative, is clinically restricted due to its toxicity and limited efficacy. Targeting the tumor-enriched enzyme AKR1C3 enables selective drug activation. We designed two novel AKR1C3-responsive Exatecan conjugates, EP-1 and EP-2, with a fluorescence-quenched module for real-time "turn-on" tracking of drug release. The conjugates exhibited potent cytotoxicity in AKR1C3-high cancer cells (IC50: 7.7 ± 0.2 nM for EP-1, 4.6 ± 0.4 nM for EP-2), similar to Exatecan, but markedly reduced toxicity in AKR1C3-low and normal cells, and overcame sorafenib resistance. Cellular and zebrafish imaging confirmed a targeted release. In a mouse model, EP-2 displayed potent efficacy with significantly reduced systemic toxicity (MTD > 60 mg/kg) compared to Exatecan. These results nominate EP-2 as an ideal candidate for selective and safe tumor therapy, providing a new paradigm for biomarker-driven antitumor drug design.