Design-Expert® Optimization of Tamoxifen-Loaded Transethosomal Gels: A Promising Transdermal System with Cytotoxicity and Stability Validation

Background: This study evaluates transdermal delivery of tamoxifen (TXN) as an alternative to the oral route of administration in treating breast cancer, which is the leading cause of cancer-related death in women globally. Oral TXN, a Class II drug, is associated with first-pass metabolism and serious side effects, including secondary cancers. Objectives: To enhance transdermal delivery, lipid-based transethosomes (TRS) were formulated using three different 24 factorial designs with various non-ionic surfactants, including Tween 20®, Span 20®, and Span 80®. Methods: Optimized TRS formulations were incorporated into HPMC-based gels and characterized for morphology, drug content, pH, viscosity, spreadability, ex vivo skin penetration, and deposition. Additionally, cytotoxicity and stability were assessed. Results: All TXN-TRS gels were suitable for transdermal use; however, Span 20®-based TRS gel demonstrated the highest skin penetration (40.3 ± 1.5 µg/cm2), representing a 127-fold enhancement rate compared with the non-ethosomal TXN gel. In line with the enhanced penetration profile, cellular studies on MCF-7 cells showed concentration-dependent cytotoxicity, reaching 91.24 ± 1.01% inhibition at 2% w/w after 72 h, with an IC50 value of 0.85 ± 0.02% w/w. Stability testing showed all formulations were more stable under refrigeration than at dry room temperature storage, supporting their potential as preclinical transdermal tamoxifen delivery platforms. Conclusions: Span 20®-based TXN transethosomal gel markedly enhanced skin penetration while maintaining potent cytotoxic activity, supporting its further preclinical evaluation as a promising transdermal alternative to oral tamoxifen.