Clinical and experimental evidence of Bcl-2 involvement in the response to photodynamic therapy

PURPOSE

The role of apoptosis related proteins in the response of human malignancies to photodynamic therapy (PDT) is under investigation. The aim of the study was to examine the role of p53 and of bcl-2 protein expression in the response to PDT.

MATERIALS AND METHODS

Paraffin-embedded material from 37 patients with early esophageal cancer treated with PDT (argon dye laser after intravenous injection of hematoporphyrine derivative) was studied immunohistochemically for p53 protein nuclear accumulation and bcl-2 cytoplasmic expression. Patients with residual disease after two rounds of PDT received definitive radiotherapy. In a subsequent in vitro study, W138 human lung fibroblasts and W138-SV-40 virus transformed were assessed for their sensitivity to PDT. The constitutive bcl-2 overexpression of the transformed cells vs. normal cells (assessed with RT-PCR) was 16-fold.

RESULTS

Positive bcl-2 and p53 expression was noted in 10 out of 36 (27%) and 14 out of 36 (39%) patients, respectively. Seven out of 11 tumors (63%) with bcl-2 expression responded completely to PDT vs. 6 out of 26 (23%) of cases with no bcl-2 expression (p = 0.02). No association of p53, T-stage and of histology grade with response to PDT or PDT/RT was noted. The sensitivity to PDT of transformed human fibroblasts compared to normal ones was 4 times more at a fluence of 4.3 J/cm2 (4% vs. 1% cell kill) as well as at a fluence of 5.4 J/cm2 (8% vs. 2% cell kill).

CONCLUSION

Bcl-2 protein expression is associated with favorable response to PDT and can be used as a predictor of cancer response to PDT. This finding can be explained by experimental studies showing that PDT induces selective degradation of the bcl-2 protein, leading to apoptosis by decreasing the bcl-2/bax ratio. Studies on PDT combination with agents targeting bcl-2 (i.e. taxanes) are on going to eventually assess a super-additive effect.

Expression kinetics of the (proto) oncogenes c-myc and bcl-2 following photodynamic treatment of normal and transformed human fibroblasts with 5-aminolaevulinic acid-stimulated endogenous protoporphyrin IX

In the same way as common tumour therapies can cause secondary tumour induction, photodynamic tumour therapy also shows a moderate mutagenicity. The oncogenes responsible for it can be distinguished from their proto-oncogenic precursors by an irreversible increase in their constitutive expression. Transient changes of the expression level of (proto) oncogenes can indicate the beginning of disturbances in the cell homeostasis: many of these genes have a normal function in proliferation or play a role in apoptosis. In this study, therefore, quantitative determination of the expression of the (proto) oncogenes c-myc and bcl-2 in normal and transformed human fibroblasts at different times following photodynamic treatment with 5-aminolaevulinic acid-stimulated endogenous protoporphyrin IX and low-dose irradiation has been carried out by quantitative reverse transcriptase polymerase chain reaction (RT-PCR). The aim is to investigate if irreversibly increased (proto) oncogene expression can be found, and if expression changes are involved in cell-cycle alterations (detected in a parallel study) and in initiation of apoptotic processes. The results show: (1) no mutagenic risk, since the over-expression of c-myc and bcl-2 is transient; (2) an interaction of bcl-2 and c-myc associated with an increase of the proliferative activity of the cell cycle of transformed cells; (3) a possible role of bcl-2 in counteracting processes that could be at least precursors for apoptosis induction; and (4) higher constitutive expression of both genes in transformed than in normal fibroblasts.

Does the in-vitro efficiency of meso-tetrahydroxy-phenyl-chlorin depend on pre-treatment of sensitizer?

The efficiency of a new photosensitizer of the second generation, meso-tetra-hydroxyphenyl-chlorin (mTHPC), which has a strong absorption at 652 nm, was investigated by oxygen consumption measurements and membrane integrity testing. The experiments proved a great increase in the efficiency of mTHPC after preincubation at 37 degrees C for 24 hours. From these findings it can be assumed that tumor cells can be treated in an optimal way with PDT after a longer delay following drug administration.

A new approach to cancer therapy due to appropriate uptake and retention kinetics of meta-tetrahydroxy-phenylchlorin in a human fibroblast cell line

Studies have shown that meta-tetrahydroxy-phenylchlorin is an efficient tumor targeting agent for laser photodynamic therapy. The effectiveness of this approach for cancer treatment depends on drug concentration, incubation time and extracellular protein. We studied uptake and retention kinetics of mTHPC in a human fibroblast cell line. Our results clearly demonstrate a difference in the amount of extracellular mTHPC at an incubation temperature of 37 degrees C compared to 20 degrees C and 4 degrees C. pH-values were always constant and not responsible for the increase. Furthermore, both absorption and fluorescence of mTHPC increase when incubated at normal human body temperature. Incubation of human fibroblast cells with mTHPC (10 micg/mL) showed that intracellular mTHPC increases in a linear manner reaching saturation after 24 hours and declining until 48 hours with concommitant increase of supernatant mTHPC. Therefore, we believe that tumor cells can be treated optimally with PDT following a delay > 24 hours after drug administration with a minimum of damage to surrounding normal tissues.