title: Differential toxicity of camptothecin, topotecan and 9-aminocamptothecin to human, canine, and murine myeloid progenitors (CFU-GM) in vitro.
abstract: OBJECTIVE
  20(S)-Camptothecin (CAM), topotecan (TPT, active ingredient in Hycamtin) and 9-amino-20(S)-camptothecin (9AC) are topoisomerase I inhibitors that cause similar dose-limiting toxicities to rapidly renewing tissues, such as hematopoietic tissues, in humans, mice, and dogs. However, dose-limiting toxicity occurs at tenfold lower doses in humans than in mice. The purpose of the current study was to determine whether hematopoietic progenitors of the myeloid lineage from humans, mice, and dogs exhibit the differential sensitivity to these compounds that is evident in vivo.
  
  
  METHODS
  Drug-induced inhibition of in vitro colony formation by a myeloid progenitor in human, murine, and canine marrow colony-forming unit-granulocyte/macrophage (CFU-GM) provided the basis for interspecies comparisons at concentrations which inhibited colony formation by 50% (IC50) and 90% (IC90).
  
  
  RESULTS
  Murine IC90 values were 2.6-, 2.3-, 10-, 21-, 5.9-, and 11-fold higher than human values for CAM lactone (NSC-94600) and sodium salt (NSC-100880), TPT (NSC-609699), and racemic (NSC-629971), semisynthetic and synthetic preparations (NSC-603071) of 9AC, respectively. In contrast, canine IC90 values were the same as, or lower than, the human IC90 values for all six compounds.
  
  
  CONCLUSIONS
  The greater susceptibility of humans and dogs to the myelotoxicity of camptothecins, compared to mice, was evident in vitro at the cellular level. Differential sensitivity between murine and human myeloid progenitors explains why the curative doses of TPT and 9AC in mice with human tumor xenografts are not achievable in patients. Realizing the curative potential of these compounds in humans will require the development of therapies to increase drug tolerance of human CFU-GM at least to a level equal to that of murine CFU-GM. Because these interspecies differences are complicated by species-specific effects of plasma proteins on drug stability, not all in vitro assay conditions will yield results which can contribute to the development of such therapies.
authors.affiliation: Hipple Cancer Research Center, Dayton, OH 45439, USA.
authors.forename: C L
authors.initials: CL
authors.lastname: Erickson-Miller
authors.forename: R D
authors.initials: RD
authors.lastname: May
authors.forename: J
authors.initials: J
authors.lastname: Tomaszewski
authors.forename: B
authors.initials: B
authors.lastname: Osborn
authors.forename: M J
authors.initials: MJ
authors.lastname: Murphy
authors.forename: J G
authors.initials: JG
authors.lastname: Page
authors.forename: R E
authors.initials: RE
authors.lastname: Parchment
mesh_terms: D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000633:toxicity; D001854:Bone Marrow Cells; D002166:Camptothecin / Q000031:analogs & derivatives* / Q000633:toxicity*; D002478:Cells, Cultured; D003114:Colony-Forming Units Assay; D004285:Dogs; D016178:Granulocyte-Macrophage Colony-Stimulating Factor / Q000494:pharmacology; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000187:drug effects*; D006801:Humans; D007377:Interleukin-3 / Q000494:pharmacology; D051379:Mice; D011994:Recombinant Proteins / Q000494:pharmacology; D059005:Topoisomerase II Inhibitors; D019772:Topotecan
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D007377:Interleukin-3; D011994:Recombinant Proteins; D059005:Topoisomerase II Inhibitors; C080125:9-aminocamptothecin; D019772:Topotecan; D016178:Granulocyte-Macrophage Colony-Stimulating Factor; D002166:Camptothecin