To enhance the accumulation and penetration of nanomedicines in tumor tissue, we developed and evaluated the biological properties of matrix metalloproteinase 2 (MMP-2)-responsive N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer drugs and tumor-penetrating peptide conjugates (P-DOX-PLGLAG-iRGD). Two different spacers were used in the design: a lysosomally (cathepsin B) cleavable tetrapeptide GFLG spacer conjugated doxorubicin (DOX) to HPMA copolymer, and an MMP-2-degradable linker (PLGLAG) connected tumor-homing and -penetrating cyclic peptide iRGD to HPMA copolymer. The accumulation of DOX in P-DOX-PLGLAG-iRGD-treated monolayer (2D) and multilayer (3D) DU-145 prostate cancer cells was higher than that of control groups (P-DOX and P-DOX + iRGD). The cell cycle arrest analysis and cytotoxicity data demonstrated that P-DOX-PLGLAG-iRGD produced a higher G2/M arrest and possessed stronger cytotoxicity against DU-145 cells than P-DOX + iRGD or P-DOX, which was consistent with the drug uptake results. Similarly, P-DOX-PLGLAG-iRGD demonstrated the highest penetration ability in 3D multicellular DU-145 tumor cell spheroids. The results indicate that covalent conjugation of iRGD via MMP-2-sensitive bonds enhances accumulation and penetration of nanomedicines into tumor cell monolayers and spheroids.