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Pemetrexed is a novel antifolate with antitumor activity against a broad range of human malignancies. Since these tumor types are often treated using chemotherapy in combination with radiotherapy, it might be interesting to incorporate pemetrexed in chemoradiation protocols. Moreover, based on their different mechanisms of action and non-overlapping toxic side effects, the combination of pemetrexed with gemcitabine seems promising. In this study, the first results of an in vitro study on this combination are presented.

The human tumor cell lines examined are CAL-27 (a carcinoma cell line of the tongue), A549 (a lung carcinoma cell line) and PANC-1 (a pancreatic carcinoma cell line).

The schedule dependency of the potential radiosensitization by pemetrexed was investigated using different time intervals (24h, 8h, 4h, 0h) between 24h pemetrexed and irradiation (0 - 8 Gy, room temperature).

For the cytotoxicity experiments, cells were incubated with pemetrexed alone (0 - 1500 μM for 24h), gemcitabine alone (0 - 100 nM for 24h or 0 - 5 μM for 1h) or with a combination of both, where one drug was added at a fixed concentration, while a concentration range of the other drug was added. Three combination regimens were tested: (1) simultaneous exposure to pemetrexed and gemcitabine for 24 h; (2) gemcitabine for 1 or 24 h immediately followed by pemetrexed for 24 h; (3) pemetrexed for 24 h immediately followed by gemcitabine for 1 or 24 h.

These three treatment schedules were also tested in combination with radiotherapy to determine the interaction between pemetrexed, gemcitabine and irradiation.

Considering the different time intervals between 24h pemetrexed and irradiation, our results showed a maximal radiosensitization (DEF = 1.56) for a 4h interval between pemetrexed and irradiation in A549. In CAL-27, the highest DEF (1.49) was seen with a 24h interval before irradiation, while 24h pemetrexed immediately before or after radiation did not show any radiosensitization.

For the combination of 24h pemetrexed with 24h gemcitabine, cytotoxicity experiments showed, independent of the treatment schedule used, an additive to antagonistic effect in A549, CAL-27 and PANC-1 cells. Combined with irradiation, none of the combination schedules mediated an increase of radiosensitization in comparison with the radiosensitizing effect of pemetrexed or gemcitabine alone. In contrast, 24h pemetrexed followed by 1h gemcitabine resulted in a clear synergistic effect in both A549 and CAL-27. Experiments, examining this schedule in combination with irradiation, are currently ongoing.

So far, our results suggest that the in vitro interaction between pemetrexed, gemcitabine and irradiation depends on the cell line, the concentration of the drugs and the treatment schedule used.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA