Potential of microneedles in enhancing delivery of photosensitising agents for photodynamic therapy

• Photodynamic therapy is an effective treatment method for certain skin lesions.
• Topical photodynamic therapy is preferable but its applications are limited.
• Microneedles consist of micron-sized projections amassed on a base-plate.
• Microneedles enhance efficacy of photodynamic therapy.
• Deeper skin lesions can be treated with microneedles and photodynamic therapy.

SummaryPhotodynamic therapy can be used in the treatment of pre-malignant and malignant diseases. It offers advantages over other therapies currently used in the treatment of skin lesions including avoidance of damage to surrounding tissue and minimal or no scarring. Unfortunately, systemic delivery of photosensitising agents can result in adverse effects, such as prolonged cutaneous photosensitivity; while topical administration lacks efficacy in the clearance of deeper skin lesions and those with a thick overlying keratotic layer. Therefore, enhancement of conventional photosensitiser delivery is desired. However, the physicochemical properties of photosensitising agents, such as extreme hydrophilicity or lipophilicity and large molecular weights make this challenging. This paper reviews the potential of microneedles as a viable method to overcome these delivery-limiting physicochemical characteristics and discusses the current benefits and limitations of solid, dissolving and hydrogel-forming microneedles. Clinical studies in which microneedles have successfully improved photodynamic therapy are also discussed, along with benefits which microneedles offer, such as precise photosensitiser localisation, painless application and reduction in waiting times between photosensitiser administration and irradiation highlighted.