In response to the challenges of cancer chemotherapeutics, including poor physicochemical properties, low tumor targeting, insufficient tumor cell internalization/bioavailability, and side effects, we developed a unique tumor-targeted micellar drug-delivery platform. Using paclitaxel as a model therapeutic, a nanopreparation composed of a matrix metalloproteinase 2 (MMP2)-sensitive self-assembly PEG 2000-paclitaxel conjugate (as a prodrug and MMP 2-sensitive moiety), transactivating transcriptional activator peptide-PEG1000-phos-phoethanolamine (PE) (a cell-penetrating enhancer), and PEG1000-PE (a nanocarrier building block) was prepared. Several major drug delivery strategies, including self-assembly, PEGylation, the enhanced permeability and retention effect, stimulus sensitivity, a cell-penetrating moiety, and the concept of prodrug, were used in design of this nanoparticle in a collaborative manner. The nanopreparation allowed superior cell internalization, cytotoxicity, tumor targeting, and antitumor efficacy in vitro and in vivo over its nonsensitive counterpart , free paclitaxel and conventional micelles. This uniquely engineered nanoparticle has potential for effective intracellular delivery of drug into cancer cells. nanomedicine | polymer-drug conjugate | polymeric micelles | multifunctional | non-small cell lung cancer D rug-loaded nanocarriers such as liposomes, micelles, poly-meric and inorganic nanoparticles, and drug conjugates have demonstrated various advantages over free therapeutic molecules. These nanopreparations can be further engineered with functional moieties to improve their performance in terms of circulation longevity, targetability, cellular penetration, and stimulus sensitivity. The idea of a stimulus-sensitive drug delivery system is based on the abnormalities in the tumor microenvi-ronment, such as acidic pH (1), altered redox potential (2), and up-regulated proteins (3). These internal conditions and external stimuli such as hyperthermia (4), magnetic field (4), and ultra-sound (5) can be used to change the behavior of nanocarriers, resulting in an enhanced tumor targeting and antitumor effects. Matrix metalloproteinases (MMPs), especially MMP2, are known to be involved and overexpressed in many stages of human cancers (3, 6). Various MMP-sensitive substrates have been designed and showed stimulus responsiveness when used in drug delivery and imaging systems (3, 6). In our previous study, a synthetic octapeptide (GPLGIAGQ) was used as the MMP2-sensitive linker in a PEGy-lated liposomal nanocarrier that could trigger PEG deshielding and the resultant enhanced cell internalization (3). Although many targeted delivery strategies have shown drug disposition in the tumor, low cellular bioavailability of chemo-therapeutics due to insufficient cellular internalization could represent another barrier. To enhance the target cell internalization, cell-penetrating proteins/peptides (CPPs) such as transactivating transcriptional activator peptide (TATp) have been used to modify the nanocarriers/drugs (7). Paclitaxel (PTX) is one of the most effective antineoplastic agents. It inhibits cell proliferation by stabilization of micro-tubules and tubulin polymerization, resulting in cell apoptosis (8). However, its clinical application is complicated by its low water solubility, off-target toxicity, and acquired drug resistance. Among many attempts to deal with these issues, "core-shell" polymeric micelles have led to successes in delivery of PTX (9). However, the low drug loading (10), risk of premature drug release (11), and insufficient targetability (9) remain major problems. Because many drugs with high hydrophobicity result in poor solubility and bioavailability (9), the conjugation of a hydrophilic moiety (e.g., PEG) to a hydrophobic drug molecule (e.g., PTX) improves drug solubility as well as imparts amphiphilicity to the formed conjugates, so that the resultant amphiphilic molecules assemble into a core-shell structure. Although PEGylation provides many advantages (12), various studies have shown that the stable PEG corona is not always beneficial for drug delivery. Ideally, the protective PEG should be removed before cell in-ternalization and subsequent intracellular events (1, 3). Here, we synthesized a self-assembling drug-polymer conjugate/ prodrug, PEG2000-peptide-PTX, which contains the same MMP2-cleavable octapeptide between PEG and PTX. We hypothesized that the MMP2-sensitive and amphiphilic PEG2000-peptide-PTX would serve not only as a tumor environment-sensitive water-soluble PTX prodrug, but also as an MMP2-sensitive building block for the design of a PTX-containing nanopreparation. With this idea in mind, we prepared a unique MMP2-sensitive micellar nano-preparation composed of the PTX prodrug and two other easy-to-make polymers, TATp-PEG1000-phosphoethanolamine (PE) (a cell-penetrating enhancer) and PEG1000-PE (a nanocarrier building block) via their self-assembly in an aqueous environment Significance The clinical outcomes of anticancer drugs are compromised by their poor physicochemical properties, low tumor targeting, insufficient bioavailability, and side effects. Matrix metal-loproteinase 2 (MMP2) has been found overexpressed in most cancers and responsible for tumor cell proliferation and me-tastasis. In this study, a self-assembling MMP2-sensitive paclitaxel-containing micellar nanopreparation was developed. Several major drug delivery strategies, including self-assembly, PEGy-lation, the enhanced permeability and retention effect, stimulus sensitivity, a cell-penetrating moiety, and the concept of prodrug, were used in a collaborative fashion to design this nanoparticle. The nanopreparation showed superior tumor targeting, cell internalization, and antitumor efficacy over its nonsensitive counterpart, free paclitaxel and conventional micelles. This uniquely engineered nanoparticle has potential for effective intracellular delivery of drug into cancer cells.