Schedule and Dose Shape Efficacy and Selectivity of Drug Combinations

BUFFALO—New work on the role of schedule and dose in determining the efficacy of two classes of drugs—thymidylate synthase inhibitors and topoisomerase I inhibitors—was described by Youcef M. Rustum, PhD, at the Vanderbilt University Symposium. Dr. Rustum is senior vice president for scientific affairs and graduate education at Roswell Park Cancer Institute in Buffalo.

The thymidylate synthase inhibitors include fluorouracil, capecitabine (Xeloda), and raltitrexed (Tomudex). The topoisomerase I inhibitor singled out was irinotecan (Camptosar).

Hypotheses Being Tested

"The following hypotheses about efficacy are being tested," Dr. Rustum explained. "First, that optimal therapeutic synergy is drug dose and schedule dependent. Next, that recruitment of cells into S-phase is a key determinant for selection and curative therapy with irinotecan/fluorouracil combinations. Third, that overexpression of BAX will enhance cell susceptibility to drug-induced apoptosis."

The role of treatment schedule and dose was studied in rat colon cancer models using three schedules: concurrent irinotecan/fluorouracil, fluorouracil followed 24 hours later by irinotecan, and irinotecan followed 24 hours later by fluorouracil. "The highest cure rate is with irinotecan followed in 24 hours by fluorouracil," Dr. Rustum said. "There was significant, dose-dependent interaction between irinotecan and fluorouracil." A 50 mg/m² dose of irinotecan put cells into S-phase, but a 100 mg/m² dose put them into G2-phase.

In combinations of raltitrexed, fluorouracil, and capecitabine, the therapeutic effect occurred regardless of which drug followed irinotecan, Dr. Rustum noted. Irinotecan renders the tumor vulnerable to DNA inhibition by the second agent administered, said Dr. Rustum.

Therapeutic Synergy

A number of mechanisms are thought to contribute to the therapeutic synergy of irinotecan followed by thymidylate synthase (TS) inhibitors. A key step in this process is mediated by dihydropyrimidine dehydrogenase (DPD). "High levels of DPD are associated with resistance to fluoropyrimidine therapy. Under certain conditions, DPD is down-regulated by irinotecan," Dr. Rustum said.

DPD inhibition also mediates the therapeutic efficacy of capecitabine. "Inhibition of DPD makes resistant tumors susceptible to capecitabine and reverses resistance, but only in tumors with high levels of DPD," Dr. Rustum said.

With regard to the modulation of tumor markers by irinotecan, Dr. Rustum said that recruitment of cells into S-phase is concentration and time dependent. "Up-regulation of BAX and wild-type p53 protein expression by irinotecan occurs in tumors with pre-existing baseline levels of these markers as a time-dependent effect. Down-regulation of DPD and thymidylate synthase occurs only in tumors with higher levels of these enzymes, markers associated with resistence to fluorouracil. This process is also time and concentration dependent. Finally, we found that induction of tumor-associated apoptosis was significant when irinotecan preceded fluorouracil by 24 hours."

Future Investigation

Dr. Rustum outlined a number of questions for future investigation:

Is recruitment of cells into S-phase drug specific?

Is recruitment of cells into S-phase by irinotecan a necessary and sufficient mechanism predictive for the observed therapeutic synergy of the irinotecan/ fluorouracil sequential combination?

Are cells synchronized into S- phase more sensitive to antimetabolites? Can this be used to overcome resistance?