Characterisation of a synergistic interaction between a thymidylate synthase inhibitor, ZD1694, and a novel lipophilic topoisomerase I inhibitor karenitecin, BNP1100: mechanisms and clinical implications.
We developed a combination protocol for inhibitors of thymidylate synthase (TS) and DNA topoisomerase I (Topo I) that can exert highly lethal effects in vitro against HCT-8 human colorectal cancer cells. The specific schedule was constructed so that a TS inhibitor could induce not only primary DNA damage but also cellular conditions optimal for the efficient action of a Topo I inhibitor. The initial drug treatment consisted of a brief exposure to a quinazoline-based antifolate, ZD1694. After an interval of approximately one cell-doubling time, cells were exposed for 8-24 h to BNP1100, a Karenitecin-class 7-thiomethyl-camptothecin, in the presence of 1-10 microM thymidine; the latter acted as a crucial factor to promote the collision of moving replication forks with the drug-stabilised DNA-Topo I cleavable complexes even under continuous TS inhibition. Clonogenic analyses confirmed that these mechanistically distinct drugs at clinically achievable concentrations worked in a highly synergistic manner, with a maximum effect abolishing the viability of virtually all cancer cells (> 99.9%). The pretreatment with ZD1694 increased the amount of DNA-bound Topo I by up to 4-fold and the DNA-damaging capability of BNP1100 by up to 15-fold. The possibility of at least four DNA-damaging pathways is proposed which might have resulted from the individual actions of TS and Topo I inhibitors as well as their concerted actions. Taken together, the present findings provided a logically permissible explanation as to why TS and Topo I inhibitors in concerted interactions induced a highly lethal effect which was more than a simple additive effect. Since these drugs are effective specifically on actively proliferating cancer cells, but not on non-cycling G0/G1 cells, this mechanism-based protocol may warrant consideration for clinical verification.
Matsui
S
S
Department of Pharmacology and Therapeutics, Grace Cancer Drug Center, Roswell Park Cancer Institute, Buffalo, New York 14263, USA.
Endo
W
W
Wrzosek
C
C
Haridas
K
K
Seetharamulu
P
P
Hausheer
F H
FH
Rustum
Y M
YM
D000964:Antimetabolites, Antineoplastic / Q000627:therapeutic use*; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000627:therapeutic use; D002455:Cell Division; D002470:Cell Survival; D003110:Colonic Neoplasms / Q000188:drug therapy* / Q000473:pathology; D004249:DNA Damage; D004357:Drug Synergism; D004791:Enzyme Inhibitors / Q000627:therapeutic use*; D006801:Humans; D011799:Quinazolines / Q000627:therapeutic use*; D013876:Thiophenes / Q000627:therapeutic use*; D013940:Thymidylate Synthase / Q000037:antagonists & inhibitors*; D059004:Topoisomerase I Inhibitors; D014407:Tumor Cells, Cultured
C122131:7-thiomethyl-camptothecin; D000964:Antimetabolites, Antineoplastic; D004791:Enzyme Inhibitors; D011799:Quinazolines; D013876:Thiophenes; D059004:Topoisomerase I Inhibitors; D013940:Thymidylate Synthase; C068874:raltitrexed; D002166:Camptothecin