Maximizing therapeutic gain with gemcitabine and fractionated radiation

The nucleoside analogue gemcitabine inhibits cellular repair and repopulation, induces apoptosis, causes tumor growth delay, and enhances radiation-induced growth delay. After single doses of drug and radiation, maximum enhancement of tumor response was obtained when gemcitabine preceded radiation by at least 24 h. Conversely, the cellular radioresponse of the normal gastrointestinal epithelium was slightly protected when gemcitabine and radiation were separated by 24 h. This differential response created a time frame within which therapeutic gain could be maximized. In our present investigation, we sought to define the most therapeutically beneficial scheme of gemcitabine administration when combined with fractionated radiotherapy.

C3Hf/Kam mice were given identical drug and radiation schedules of administration, and both normal tissue (jejunal mucosa) and tumor (Sa-NH) responses were measured. Irradiation was given once per day for 5 days in normal tissue and tumor growth delay studies and twice per day for the tumor cure endpoint. A total dose of 25 mg/kg gemcitabine was given i.p. in 1 of 3 schedules: a single dose of 25 mg/kg 24 h before the start of fractionated irradiation, 12.5 mg/kg 24 h before the first and third radiation doses, or 24 h before each of 5 radiation doses. Groups of mice bearing 7- or 8-mm diameter tumors were treated with gemcitabine alone or in combination with fractionated irradiation under ambient or hypoxic conditions. The survival response of the jejunal mucosa was quantified by the microcolony assay and histologically by quantifying apoptosis, mitosis, S-phase fraction, and crypt cellularity.

For tumor growth delay, dose-modifying factors (DMFs) were similar (1.34-1.46) for all 3 schedules of drug administration. In contrast, the response of the jejunum was strongly dependent on the schedule of gemcitabine administration. A single dose of gemcitabine before the start of fractionated radiotherapy resulted in slight radioprotection (DMF 0.96). Two doses and 5 daily doses of gemcitabine enhanced radiation response by factors of 1.09 and 1.23, respectively. Major factors affecting the response of the jejunal mucosa were apoptotic death of S-phase cells exposed to gemcitabine and cell cycle synchrony of surviving cells. Tumor reoxygenation was found to be a major mechanism for tumor radioenhancement, in addition to those reported earlier.

All 3 schedules of drug administration produced therapeutic gain; however, when gemcitabine was given more than once in a 5-fraction radiation treatment schedule, normal tissue toxicity increased. The highest therapeutic gain (1.4) was achieved by giving a single dose of gemcitabine (25 mg/kg) 24 h before the start of fractionated radiotherapy.