1��gml?1) were studied in the MMR-deficient HCT116+ch2 and -proficient HCT116+ch3 cells by measuring the photosensitiser-mediated fluorescence intensity (counts per pixel, arbitrary Veliparib cost unit) as a function of the incubation time. Figure 3 shows that m-THPC-mediated fluorescence intensity markedly increased within 24h and reached a plateau with highest fluorescence intensity at 34h after incubation in MLH1-proficient as well as in MLH1-deficient cells. Thus, m-THPC uptake occurs within 24h and follows similar kinetics in both cell lines. Figure 3 Photosensitiser-mediated fluorescence intensity (counts per pixel, cpp) for MLH1-deficient and -proficient tumour cells shown as a function of incubation time. Each point represents the mean��s.d. of six experiments.

The subcellular distribution of m-THPC was analysed by confocal laser scanning microscopy. m-THPC-mediated fluorescence intensity in the cytoplasm of HCT116 cells increased within 24h after administration. At that time, only weak fluorescence intensity was associated with the nuclear membrane. Figure 4 represents a typical example of fluorescence distribution in the HCT116+ch3 (Figure 4A) and HCT116+ch2 (Figure 4B) cell lines after 24h of m-THPC administration, demonstrating that the fluorescence pattern of HCT116+ch3 was similar to that of HCT116+ch2 cells. Likewise, longer incubation with m-THPC resulted in increased fluorescence intensity associated with membranous structures in the cytoplasm, whereas the nucleus did not show detectable fluorescence at any time after photosensitisation.

Thus, drug uptake kinetics and subcellular distributions are very similar in MMR-proficient and -deficient cells. Figure 4 Bright field image of HCT116+ch3 (A) and HCT116+ch2 (B) after incubation with 0.1��gml?1m-THPC for 24h. After 24h of incubation there was no detectable fluorescence within the … DISCUSSION MLH1 or MSH2-deficient tumour cells have been reported to be resistant to a large number of anticancer drugs (Aebi et al, 1997; Fink et al, 1998b) and to radiotherapy (Fritzell et al, 1997; Xu et al, 2001), and as a result, MMR status may be an important resistance factor. Although loss of MMR results in relatively small degrees of resistance, there is evidence that this resistance is nevertheless of substantial clinical significance.

The strongest clinical evidence correlating chemotherapy responses with the in vitro data has been reported from Entinostat studies on ovarian and breast cancers. We and others (Brown et al, 1997; Fink et al, 1998a; Samimi et al, 2000) have shown that there is an increase in the number of ovarian tumour cells that score negative for MLH1 expression following platinum-based chemotherapy when compared with untreated tumours. Recently, studies have correlated tumour response and poor disease-free survival with loss of MLH1 expression in breast cancer tumours following anthracycline-based neoadjuvant chemotherapy (Mackay et al, 2000).