Formulation Challenges in Dermal Drug Delivery Systems: A Comprehensive Review of Physicochemical Properties and Advanced Delivery Strategies

Dermal drug delivery systems (DDS) offer a promising route for localized and systemic therapy, bypassing gastrointestinal degradation and first-pass metabolism. However, formulating effective DDS poses significant challenges due to the complex barrier function of the skin, particularly the stratum corneum, and the critical influence of a drug’s physicochemical properties. This review provides a comprehensive analysis of formulation challenges in dermal drug delivery, focusing on several key physicochemical properties such as lipophilicity, molecular weight, solubility, pKa, and chemical stability that dictate drug absorption, bioavailability, and therapeutic efficacy. By synthesizing data from over 200 research papers published in the last five years, this review identifies key trends, innovations, and persistent challenges in optimizing DDS. Lipophilicity and molecular weight were found to significantly impact skin permeability, with moderately lipophilic drugs (logP 2-4) and molecules below 500 Da showing optimal absorption profiles. Poorly soluble drugs exhibited low bioavailability unless advanced formulation strategies like nanoemulsions, solid lipid nanoparticles (SLNs), and microneedles were employed to enhance solubility and penetration. Similarly, drugs with pKa values close to the skin’s pH demonstrated superior permeability, emphasizing the importance of pH-optimized formulations. Stability concerns, particularly for drugs prone to oxidation or crystallization, were effectively mitigated using lipid-based carriers and amorphous solid dispersions, which improved both solubility and shelf life. The review highlights the role of viscosity, surface tension, and thermal properties in determining drug release and penetration in various DDS formulations. Nanotechnological advancements, such as the use of nanostructured lipid carriers (NLCs) and microneedles, offer promising solutions for delivering larger or more challenging molecules, including peptides and proteins. Finally, the use of magnetically responsive nanoparticles presents new opportunities for controlled drug release but requires further research to address stability and reactivity concerns. This review concludes by outlining the current gaps in the understanding of the physicochemical factors influencing DDS and suggests future research directions aimed at improving drug permeability, stability, and bioavailability. Addressing these challenges through advanced formulation techniques will be critical for enhancing the clinical efficacy of dermal drug delivery systems.