In vivo spatiotemporal fate of nanoparticle-incorporated dissolving microneedles: nanoparticle size effects

Integrating dissolving microneedles (DMNs) with nanocarrier (NC) provides an effective strategy to overcome the stratum corneum barrier, achieving controllable transdermal drug delivery through tuning physicochemical parameters. Among various physicochemical parameters, particle size of nanocarrier plays a pivotal role in governing transdermal diffusion dynamics and drug retention behavior of NC-loaded DMNs. Nevertheless, the spatiotemporal in vivo fate of NC-loaded DMNs, particularly the particle-size-dependent diffusion patterns, remains insufficiently elucidated. In this study, an aggregation-caused quenching (ACQ) fluorescent probe with accurate bioimaging potential, P4, was encapsulated into solid lipid nanoparticles (SLNs) to enable visualization of intact NC. P4-labeled SLNs with varying particle sizes (SLNs1-150 nm, SLNs2-250 nm, and SLNs3-380 nm) were prepared via formulation modulation and subsequently embedded into DMNs to investigate their transdermal diffusion and retention in murine dorsal skin. The fabricated P4 SLNs demonstrated excellent physicochemical stability after incorporation into DMNs, with negligible changes in morphology or fluorescence characteristics. Both in vivo and ex vivo fluorescence imaging revealed a clear size-dependent diffusion trend, where smaller nanoparticles exhibited faster transdermal transport and broader tissue distribution (SLNs1 > SLNs2 > SLNs3). Further quantitative pharmacokinetic analysis verified significant size-dependent differences in key in vivo kinetic parameters. Specifically, the AUC₀₋_t values of SLNs1, SLNs2, and SLNs3 were 1711.82 ± 147.80%·h, 2155.38 ± 128.58%·h, and 2367.02 ± 181.48%·h, respectively, while their corresponding elimination T_1/2 values were 19.66 ± 4.62 h, 20.82 ± 7.48 h, and 39.98 ± 19.57 h. The quantitative data demonstrated that the larger-sized SLNs3 possessed a significantly longer half-life than the smaller-sized SLNs. Collectively, these findings confirmed that the transdermal diffusion and retention profiles of NC-loaded DMNs were strongly size-dependent. This study provided new insights into the spatiotemporal fate of nanoparticle-embedded DMNs, offering valuable guidance for rational formulation design and precise modulation of drug delivery efficiency and distribution kinetics.