Cytotoxic activity of calcein acetoxymethyl ester (calcein/AM) on primary cultures of human haematological and solid tumours

Abstract

The aim of this study was to determine the in vitro cytotoxicity of calcein acetoxymethyl ester (Calcein/AM) on primary cultures derived from solid and haematological human tumours. Calcein/AM is a fluorescent dye that localises intracellularly after esterase-dependent cellular trapping and which has shown cytotoxic activity against various established human tumour cell lines at relatively low concentrations. The semi-automated fluorometric microculture cytotoxicity assay, based on the measurement of fluorescence generated from cellular hydrolysis of fluorescein diacetate to fluorescein, in microtitre plates was used for the evaluation of Calcein/AM activity in tumour cell suspensions from patients. The cytotoxicity was measured as a survival index (SI), defined as the fluorescence as a percentage of control cultures. A total of 163 evaluable samples from various tumours were tested with continuous drug exposure. The activity of Calcein/AM was compared with representatives of six major classes of standard chemotherapeutic drugs. Calcein/AM was found to induce concentration-dependent decreases in the SI of both haematological and solid tumour cells. The ratio of solid over haematological tumour activity increased at a rate that was concentration dependent. Although it was relatively less active than cisplatin against solid tumours, Calcein/AM showed higher solid tumour activity compared to leukaemic specific agents (cytarabine and amsacrine), vincristine and doxorubicin (Dox). Among the solid tumours tested, childhood tumours, non-small cell lung cancer and sarcomas were the most sensitive to Calcein/AM. The best correlation between SI values was seen between Calcein/AM and Dox, with weaker correlations to representatives of antimetabolites, platinum compounds, topoisomerase II inhibitors, tubulin interactive agents and alkylators. Non-cytotoxic concentrations of cyclosporin A significantly potentiated calcein-induced cytotoxicity. The results show that Calcein/AM is differentially active against haematological tumours, but with substantial activity against solid tumours. The drug may represent a new class of anticancer compound with a unique means of drug delivery.