Ultrasound-responsive doxorubicin microbubbles engineered by QbD: enhanced in vitro anti-breast-cancer efficacy with attenuated cardiac cell toxicity

Ultrasound-responsive microbubbles provide a powerful means for externally regulated drug delivery, yet their translation is often constrained by empirical formulation approaches and limited therapeutic selectivity. In this study, a Quality by Design (QbD) strategy was implemented to rationally engineer doxorubicin-loaded chitosan microbubbles with predictable physicochemical and biological performance. A Box–Behnken experimental design enabled systematic optimization of critical formulation variables, yielding a statistically validated design space governing microbubble size (≈ 3.2 µm), encapsulation efficiency (≈ 70–75%), and ultrasound-triggered burst release (> 80% within 40 s). The optimized formulation demonstrated minimal baseline drug leakage under non-activated conditions, while brief ultrasound exposure induced rapid, on-demand release. In vitro evaluation revealed a marked enhancement of anticancer efficacy in MCF-7 breast cancer cells following ultrasound activation, with a dramatic reduction in IC₅₀ to 0.0013 µg/mL compared with free doxorubicin (0.62 µg/mL). In contrast, cardiotoxicity in H9c2 cardiomyoblast cells remained attenuated under non-triggered conditions (IC₅₀ ≈ 8.72 µg/mL), resulting in a > 10³-fold improvement in therapeutic selectivity upon ultrasound activation. Mechanistic analysis indicated that intact microbubbles restrict passive drug diffusion, whereas ultrasound-induced cavitation and transient membrane permeabilization enable efficient intracellular delivery at the target site. Overall, this work demonstrates that integrating QbD-driven formulation control with spatially confined ultrasound activation can substantially expand the therapeutic window of doxorubicin, offering a transferable framework for precision, stimulus-responsive drug delivery without chemical modification of the parent drug.