A novel B-ring modified homocamptothecin, 12-Cl-hCPT, showing antiproliferative and topoisomerase I inhibitory activities superior to SN-38

Anticancer Drug Des. 2001 Feb;16(1):27-36.


We report the synthesis and pharmacological evaluation of a novel homocamptothecin (hCPT) derivative, 12-Cl-hCPT, which contains a seven-membered beta-hydroxylactone in place of the conventional six-membered alpha-hydroxylactone found in camptothecin (CPT) and bears a chloro substituent at position 12. The capacity of 12-Cl-hCPT to inhibit DNA topoisomerase I was compared with that of SN-38, the active metabolite of the clinically used antitumour prodrug CPT-11. In the DNA relaxation assay, 12-Cl-hCPT proved to be slightly more potent than SN-38 at stimulating the formation of nicked plasmid DNA molecules. A series of radiolabelled DNA restriction fragments were employed to identify and compare the position of the DNA cleavage sites induced by topoisomerase I in the presence of 12-Cl-hCPT and SN-38. These sequencing studies confirm that both 12-Cl-hCPT and SN-38 strongly promote DNA cleavage by topoisomerase I and reveal that the majority of the cleavage sites are located at the same nucleotide positions for the two drugs. However, a certain number of DNA cleavage sites were found to be specific to 12-Cl-hCPT. These sites, previously characterized with unsubstituted hCPT, generally correspond to 5'-CG sites whereas the sites common to the 12-Cl-hCPT and SN-38 essentially correspond to 5'-TG sites. We also quantified the formation of drug-induced protein-DNA complexes formed in HT29 human colon carcinoma cells. Trapping of endogenous proteins onto DNA was found to be much more efficient with 12-Cl-hCPT than with SN-38. These data provide a molecular basis to account for the enhanced antiproliferative activity of 12-Cl-hCPT compared with that of SN-38. Biological evaluation on a panel of sensitive and drug-resistant cell lines revealed 12-Cl-hCPT to be more cytotoxic to tumour cells than SN-38. 12-Cl-hCPT proved 14- and 23-fold more active than SN-38 toward the K562adr and T24anp multidrug-resistant cell lines, respectively. The marked topoisomerase I inhibitory properties of 12-Cl-hCPT coupled with its interesting antiproliferative activity, in particular against cancer cells presenting multidrug resistance phenotype with overexpression of P-glycoprotein, makes 12-Cl-hCPT a valid candidate for subsequent preclinical evaluation. Collectively, the data strengthen homocamptothecin as an extremely promising template to generate novel and potent antitumour agents.

Publication types

  • Comparative Study
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Antibiotics, Antineoplastic / pharmacology
  • Antineoplastic Agents / isolation & purification
  • Antineoplastic Agents / pharmacology*
  • Antineoplastic Agents, Phytogenic / pharmacology*
  • Camptothecin / analogs & derivatives*
  • Camptothecin / isolation & purification
  • Camptothecin / pharmacology*
  • Cell Division / drug effects
  • Cell Survival / drug effects
  • DNA, Neoplasm / genetics
  • DNA, Neoplasm / isolation & purification
  • DNA, Superhelical / drug effects
  • Doxorubicin / pharmacology
  • Enzyme Inhibitors / pharmacology*
  • HT29 Cells
  • Humans
  • Irinotecan
  • Plasmids / genetics
  • Plasmids / isolation & purification
  • Topoisomerase I Inhibitors*


  • Antibiotics, Antineoplastic
  • Antineoplastic Agents
  • Antineoplastic Agents, Phytogenic
  • DNA, Neoplasm
  • DNA, Superhelical
  • Enzyme Inhibitors
  • Topoisomerase I Inhibitors
  • Irinotecan
  • Doxorubicin
  • homocamptothecin
  • Camptothecin