SN-38, A METABOLITE OF THE CAMPTOTHECIN DERIVATIVE CPT-11, POTENTIATES THE CYTOTOXIC EFFECT OF RADIATION IN HUMAN COLON ADENOCARCINOMA CELLS GROWN AS SPHEROIDS

Background and purpose: CPT-11 (7-ethyl-10-[4-(1-piperidino)-1-piperid ino] carbonyloxycamptothecin) is a new semisynthesized derivative of c amptothecin. SN-38 (7-ethyl-1O-hpdroxycamptothecin), a metabolite of C PT-11, plays a key role in the action of CPT-11. Materials and methods : To determine whether SN-38 potentiates the cytotoxic effect of radia tion, we investigated the interaction of SN-38 and radiation in vitro in monolayer cultures and multicellular spheroids of HT-29 human colon adenocarcinoma cells. Results: HT-29 spheroids were more resistant to both SN-38 and irradiation than monolayer cells. SN-38 at a concentra tion of 2.5 mu g/ml, which by itself was not cytotoxic, greatly increa sed the lethal effects of radiation in spheroids, but not in monolayer cultures. Exposure to SN-38 following irradiation inhibited the poten tially lethal damage repair (PLDR) in spheroids. It is suggested that the mechanism of the radiosensitization by SN-38 is due to the PLDR in hibition. Conclusions: These results indicate that CPT-11 may play a r ole as radiosensitizer and that a combination of CPT-11 and irradiatio n could prove to be a particularly effective strategy with which to tr eat human colon adenocarcinoma. (C) 1997 Elsevier Science Ireland Ltd.