Vascular and cellular targeting for photodynamic therapy

Photodynamic therapy (PDT) involves the combination of photosensitizers (PS) with light as a treatment, and has been an established medical practice for about 10 years. Current primary applications of PDT are age-related macular degeneration (AMD) and several types of cancer and precancer. Tumor vasculature and parenchyma cells are both potential targets of PDT damage. The preference of vascular versus cellular targeting is highly dependent upon the relative distribution of photosensitizers in each compartment, which is governed by the photosensitizer pharmacokinetic properties and can be effectively manipulated by the photosensitizer drug administration and light illumination interval (drug-light interval) during PDT treatment, or by the modification of photosensitizer molecular structure. PDT using shorter PS-light intervals mainly targets tumor vasculature by confining photosensitizer localization within blood vessels, whereas if the sensitizer has a reasonably long pharmacokinetic lifetime, then PDT at longer PS-light intervals can induce more tumor cellular damage, because the photosensitizer has then distributed into the tumor cellular compartment. This passive targeting mechanism is regulated by the innate photosensitizer physicochemical properties. In addition to the passive targeting approach, active targeting of various tumor endothelial and cellular markers has been studied extensively. The tumor cellular markers that have been explored for active photodynamic targeting are mainly tumor surface markers, including growth factor receptors, low-density lipoprotein (LDL) receptors, transferrin receptors, folic acid receptors, glucose transporters, integrin receptors, and insulin receptors. In addition to tumor surface proteins, nuclear receptors are targeted, as well. A limited number of studies have been performed to actively target tumor endothelial markers (ED-B domain of fibronectin, VEGF receptor-2, and neuropilin-1). Intracellular targeting is a challenge due to the difficulty in achieving sufficient penetration into the target cell, but significant progress has been made in this area. In this review, we summarize current studies of vascular and cellular targeting of PDT after more than 30 years of intensive efforts.