Use of real-time confocal laser scanning microscopy to study immediate effects of photodynamic activation on photosensitized erythrocytes

Real time confocal laser scanning microscopy: potential applications in space medicine and cell biology

Photodynamic therapy (PDT), in which tissues may be rendered fatally light-sensitive represents a relatively novel treatment for cancer and other disorders such as cardiovascular disease. It offers significant application to disease control in an isolated environment such as space flight. In studying PDT in the laboratory, low energy lasers such as HeNe lasers are used to activate the photosensitized cellular target. A major problem associated with these studies is that events occurring during actual exposure of the target cells to the system cannot be examined in real time. In this study HeLa cells were photosensitized and photodynamic activation was accomplished using the scanning microbeam from a confocal laser scanning microscope. This form of activation allowed for simultaneous photoactivation and observation and facilitated the recording of events at a microscopic level during photoactivation. Effects of photodynamic activation on the target cells were monitored using the fluorophores rhodamine 123 and ethidium homodimer-1. Potential applications of these forms of analyses to space medicine and cell biology are discussed.

Electric field-enhanced activation of hematoporphyrin derivative: effects on a human tumour cell line

In a recent report we described the effects of combined electroactivation and photoactivation of hematoporphyrin derivative (HPD) on human erythrocytes and established that activation-induced cell lysis was more pronounced when both modes of activation were sequentially applied to the system. Here we demonstrate that electric field-induced activation of HPD-treated HeLa cells results in cell death. This effect is shown to be dependant on both electric field strength and on HPD concentration. In addition, we demonstrate that exposure of HPD-treated cells to short and intense electric pulses prior to photoactivation, results in increased cell mortality. The results confirm our earlier suggestion that HPD may be activated in the presence of an applied electric field. The results further suggest that activation of photosensitizers using combined exposure to electric fields and light may play an important role in increasing the efficiency of photodynamic therapy (PDT) in the treatment of cancer.

Differential response of photosensitized young and old human erythrocytes to photodynamic activation

It has recently been proposed that photosensitized erythrocytes may play an important role in the delivery and targeting of agents such as photosensitizers and chemotherapeutics for use in cancer treatment. It has been suggested that loading of photosensitized erythrocytes with chemotherapeutic agents would provide an ideal means of combining both treatment modalities. The recent application of real-time confocal laser scanning microscopy to the study of immediate effects of photodynamic activation on photosensitized erythrocytes has enabled us, in this study, to distinguish between the differential susceptibility of age-density resolved sub-populations of human erythrocytes to photodynamic activation. In this study we demonstrate that younger (low age-density) sub-populations of photosensitized erythrocytes are less susceptible than older (high age-density) sub-populations to photodynamic activation. We also demonstrate that this phenomenon is exhibited by cells photosensitized using hematoporphyrin derivative and rose bengal as photosensitizers. In both cases no significant difference in uptake of photosensitizer by both populations could be observed using absorbance spectrophotometry. The study suggests that age-density resolution of erythrocytes prior to loading and photosensitization might provide a means of enhancing the release of loaded components from the photosensitized system and this would, in turn, enhance the potential use of photosensitized erythrocytes as delivery or targeting systems for use in combination cancer therapies.

Vascular effects of photodynamic therapy

Vascular damage and blood flow stasis are consequences of photodynamic therapy (PDT) of solid tumors using many photosensitizers. Microvascular stasis and resulting hypoxia are effective means to produce cytotoxicity and tumor regression. The observation of blood flow stasis after photodynamic therapy results from a combination of damage to sensitive sites within the microvasculature and the resulting physiological responses to this damage. A generalized hypothesis for the mechanisms leading to vessel stasis begins with perturbation and damage to endothelial cells during light treatment of photosensitized tissues. Endothelial cell damage leads to the establishment of thrombogenic sites within the vessel lumen and this initiates a physiological cascade of responses including platelet aggregation, the release of vasoactive molecules, leukocyte adhesion, increases in vascular permeability, and vessel constriction. These effects from damage combine to produce blood flow stasis.