Development and Optimization of Tacrolimus-Loaded Solid Lipid Nanoparticle Gels Using a Box–Behnken Design: Comparison of Three Lipid Matrices and In Vivo Evaluation

Background/Objectives: Tacrolimus has poor aqueous solubility and limited skin permeation, necessitating formulation strategies for effective topical therapy. This study aimed to develop tacrolimus-loaded solid lipid nanoparticles (SLNs) and an SLN-gel, comparing optimized SLNs prepared with three lipids—Compritol 888 ATO (C888), Compritol HD5 ATO (CHD5), and Stearic acid (St.a). Methods: Each SLN was optimized via Box–Behnken design (BBD) using factors X1 (lipid amount), X2 (surfactant amount), and X3 (homogenization speed). Morphological analysis was conducted using SEM and TEM, while the solid-state properties were evaluated by XRD and FT-IR. In vitro drug release was assessed using the dialysis bag method, and permeation was evaluated using Franz diffusion cells. The SLNs showing the highest cumulative permeation with a consistent release profile were tested in a 1-chloro-2,4-dinitrobenzene (DNCB)-induced atopic dermatitis mouse model. Results: Optimized SLNs obtained from the BBD demonstrated particle sizes of 275.8, 190.9, 308.9 nm, PDIs of 0.35, 0.26, 0.14, zeta potentials of −15.2, −12.9, −24.0 mV, and encapsulation efficiencies of 84.6, 91.8, 92.5% for C888, CHD5, and St.a, respectively. In dialysis-bag studies, all SLNs showed higher cumulative release than drug solution and Protopic® 0.1%. SEM/TEM revealed spherical morphology, while FT-IR/XRD confirmed drug incorporation within the lipid matrix. Franz diffusion showed higher permeation than API-gel and Protopic® 0.1%, ranked St.a > C888 > CHD5, with 48 h cumulative amounts of 127.2, 96.8, 87.5 μg/cm2. The St.a-SLN-gel significantly improved atopic dermatitis lesions in the DNCB-induced mouse model in vivo (score 14.5→1.75, n = 6, p < 0.001). Conclusions: Box–Behnken optimization produced an SLN-gel surpassing existing formulations in release, permeation, and in vivo efficacy, demonstrating tunable performance via lipid selection and offering a reproducible, translatable approach for topical tacrolimus.