Gemcitabine: Future prospects of single-agent and combination studies

Gemcitabine (2',2'-difluorodeoxycytidine, Gemzar) is a deoxycytidine analog with excellent antitumor activity against a number of solid tumors. Gemcitabine needs to be activated by deoxycytidine kinase and other kinases to its triphosphate, gemcitabine triphosphate, which can be incorporated into RNA and DNA. The latter effect is considered to be responsible for its antitumor effect and causes masked chain termination and inhibition of DNA repair. This effect may be of importance for combination with DNA interacting agents. In phase I trials daily, twice weekly, weekly and every two weeks schedules have been evaluated. At the weekly schedule of 1,000-1,250 mg/m² significant antitumor activity was observed in bladder, breast, ovary, and pancreatic cancer, non-small cell lung cancer (NSCLC), and small cell lung cancer of 31%, 33%, 22%, 11%, 22% and 27% total response rates, respectively. Gemcitabine also showed considerable improvement in clinical symptoms, while toxicity was not severe with mild myelosuppression. Due to its ability to inhibit DNA replication, combination studies were initiated with DNA damaging agents. For the various combinations with cisplatin in phase II studies on NSCLC, response rates varied from 42%-54%, with a median survival of generally more than 12 months. Also, combinations with taxanes, etoposide, doxorubicin and vindesin seem promising. Gemcitabine is an important agent for the management of several relatively chemoresistant cancer types, both with respect to antitumor activity and clinical benefit. Future research on combination studies deserves high priority considering the high response rates in NSCLC and bladder cancer.