Stepwise Ultra-pH-Sensitive Micelles Overcome a pK_a Barrier for Systemic Lymph Node Delivery

While local nanoparticle delivery to lymph nodes is well studied, there are few design criteria for intravenous delivery to the entire lymph node repertoire. In this study, we investigated the effect of NP pH transition on lymph node targeting by employing a series of ultra-pH-sensitive (UPS) polymeric micelles. The UPS library responds to pH thresholds (pK_a 6.9, 6.2, and 5.3) over a range of physiological pH. We observed a dependence of intravenous lymph node targeting on micelle pH transition. UPS_6.9 (subscript indicates pK_a) shows poor lymph node delivery, while UPS_5.3 delivers efficiently to lymph node sets. We investigated targeting mechanisms of UPS_5.3, observing an accumulation among lymph node lymphatics and a dependence on lymph node-resident macrophages. To overcome the pH-threshold barrier, which limits UPS_6.9, we rationally designed a nanoparticle coassembly of UPS_6.9 with UPS_5.3, called HyUPS. The HyUPS micelle retains the constitutive pH transitions of each polymer, showing stepwise responses to discrete pH thresholds. We demonstrate that HyUPS improves UPS_6.9 delivery to lymph nodes, extending this platform for disease detection of lymph node metastasis.