Symmetrical Tetravalent Aptamer-Protein Conjugate with Ultrahigh <i>In Vivo</i> Stability for Targeted Cancer Imaging and Therapy

The clinical application of nucleic acid aptamers is hindered by rapid degradation and poor targeting in physiological conditions. Multivalent assembly can improve their performance, yet key structural rules for stability are not fully clarified. Here, we fabricate a symmetric tetravalent aptamer-streptavidin conjugate (Tetra-XQ@SA) for cancer theranostics with greatly enhanced stability. Thymine bases, longer sequences, and double-stranded structures collectively strengthen its nuclease resistance, enabling stable storage for 96 h in DNase I and 72 h in serum. Compared with free aptamers, Tetra-XQ@SA shows 2-fold higher binding affinity to cancer cell CD71 receptors, 7-fold greater doxorubicin loading, and improved antitumor activity. It achieves prolonged tumor retention and an ∼73% tumor inhibition rate in vivo. This study reveals the structure-stability relationship of aptamer assemblies and offers a versatile platform for targeted cancer therapy.