Synthetic Ionizable Colloidal Drug Aggregates Enable Endosomal Disruption

Colloidal drug aggregates enable the design of drug-rich nanoparticles; however, the efficacy of stabilized colloidal drug aggregates is limited by entrapment in the endo-lysosomal pathway. Although ionizable drugs are used to elicit lysosomal escape, this approach is hindered by toxicity associated with phospholipidosis. It is hypothesized that tuning the pK_a of the drug would enable endosomal disruption while avoiding phospholipidosis and minimizing toxicity. To test this idea, 12 analogs of the nonionizable colloidal drug fulvestrant are synthesized with ionizable groups to enable pH-dependent endosomal disruption while maintaining bioactivity. Lipid-stabilized fulvestrant analog colloids are endocytosed by cancer cells, and the pK_a of these ionizable colloids influenced the mechanism of endosomal and lysosomal disruption. Four fulvestrant analogs-those with pK_a values between 5.1 and 5.7-disrupted endo-lysosomes without measurable phospholipidosis. Thus, by manipulating the pK_a of colloid-forming drugs, a tunable and generalizable strategy for endosomal disruption is established.

Triggered Release Enhances the Cytotoxicity of Stable Colloidal Drug Aggregates

Chemotherapeutics that self-assemble into colloids have limited efficacy above their critical aggregation concentration due to their inability to penetrate intact plasma membranes. Even when colloid uptake is promoted, issues with colloid escape from the endolysosomal pathway persist. By stabilizing acid-responsive lapatinib colloids through coaggregation with fulvestrant, and inclusion of transferrin, we demonstrate colloid internalization by cancer cells, where subsequent lapatinib ionization leads to endosomal leakage and increased cytotoxicity. These results demonstrate a strategy for triggered drug release from stable colloidal aggregates.

Colloidal aggregation: from screening nuisance to formulation nuance

It is well known that small molecule colloidal aggregation is a leading cause of false positives in early drug discovery. Colloid-formers are diverse and well represented among corporate and academic screening decks, and even among approved drugs. Less appreciated is how colloid formation by drug-like compounds fits into the wider understanding of colloid physical chemistry. Here we introduce the impact that colloidal aggregation has had on early drug discovery, and then turn to the physical and thermodynamic driving forces for small molecule colloidal aggregation, including the particulate nature of the colloids, their critical aggregation concentration-governed formation, their mechanism of protein adsorption and subsequent inhibition, and their sensitivity to detergent. We describe methods that have been used extensively to both identify aggregate-formers and to study and control their physical chemistry. While colloidal aggregation is widely recognized as a problem in early drug discovery, we highlight the opportunities for exploiting this phenomenon in biological milieus and for drug formulation.