One of the main routes that cancer cells can spread into the body is via the blood stream. If cancer cells are able to enter into the blood stream they can be passively transported to remote sites such that new tumor growth at that site is enhanced. This process is referred to as hematogenous metastasizing. Different types of tumors cells can be stained, detected, and destroyed in vivo by the use of appropriate dyes, called photosensitizers (PS), and light. The detection of diseased cells can be achieved using photodiagnosis (PD) while the destruction can be managed by photodynamic therapy (PDT). In both techniques the PS is administered to the body. After a defined post injection time the PS is mainly localized in the tumor tissue. With appropriate light the tumor can then either be visualized due to its fluorescence properties, PD, or it can be destroyed due to its photosensitizing properties, PDT. This leads to the question, whether or not these photosensitizers can also be used to detect and destroy tumor cells inside of blood without serious side effects on the healthy blood itself. A `yes` to this question would allow one to suppress or even stop the spread of tumor cells from a primary tumor site into remote sites of the body, if they propagate via the blood stream.

The results presented here show a first approach to the above mentioned problem. We tested and used in vitro methods to find suitable photosensitizers, and a suitable cell model. Furthermore, we achieved a method to count tumor cells in blood, and assembled a blood damage free irradiation system using special piping materials, a flow through suprasil cell and an argon - dye laser. Using these methods the following results could be achieved: Photofrin II (PII) and meso-tetra-(hydroxyphenyl)chlorin (mTHPC) were chosen using a home made photoactivity test based on uric acid. A colon carcinoma cell line (CX1) was chosen for the experiments, since colon tumor is known to spread hematogenous to a high percentage (up to 72%) into the liver. In order to assess the concept of hematogenous spread suppression using PDT, the colon carcinoma (CX1) cells were incubated with PII and mTHPC and added to fresh human blood. The mixture was irradiated while flowing through the irradiation system. The survival of CX1 cells was determined afterwards using an adapted plating efficiency assay. More than 99.98% of colon carcinoma cells (CX1) could be killed inside the blood without destruction of normal blood components. With the appearance of high specific photosensitizers (i.e. photosensitizers coupled to monoclonal antibodies), it becomes possible to use PDT as a method to suppress the hematogenous spread of cancer in vivo.

In this contribution emphasis is placed on determining possible changes and damage thresholds in blood components due to laser light irradiation using automatic blood counts, osmotic fragility and blood smears. Investigations were carried out with various fluence rates, two different wavelengths (630 nm and 653 nm), and a blood cooling device. Finally, we were also able to quantify the dark toxicity of both photosensitizers used on blood components and on CX1 cells.