Smart Superstructures with Ultrahigh pH-Sensitivity for Targeting Acidic Tumor Microenvironment: Instantaneous Size Switching and Improved Tumor Penetration

The currently low delivery efficiency and limited tumor penetration of nanoparticles remain two major challenges of cancer nanomedicine. Here, we report a class of pH-responsive nanoparticle superstructures with ultrasensitive size switching in the acidic tumor microenvironment for improved tumor penetration and effective in vivo drug delivery. The superstructures were constructed from amphiphilic polymer directed assembly of platinum-prodrug conjugated polyamido-amine (PAMAM) dendrimers, in which the amphiphilic polymer contains ionizable tertiary amine groups for rapid pH-responsiveness. These superstructures had an initial size of similar to 80 nm at neutral pH (e.g., in blood circulation), but once deposited in the slightly acidic tumor microenvironment (pH similar to 6.5-7.0), they underwent a dramatic and sharp size transition within a very narrow range of acidity (less than 0.1-0.2 pH units) and dissociated instantaneously into the dendrimer building blocks (less than 10 nm in diameter). This rapid size-switching feature not only can facilitate nanoparticle extravasation and accumulation via the enhanced permeability and retention effect but also allows faster nanoparticle diffusion and more efficient tumor penetration. We have further carried out comparative studies of pH-sensitive and insensitive nanostructures with similar size, surface charge, and chemical composition in both multicellular spheroids and poorly permeable BxPC-3 pancreatic tumor models, whose results demonstrate that the pH-triggered size switching is a viable strategy for improving drug penetration and therapeutic efficacy.