Comparative in vitro cytotoxicity of cyclophosphamide, its major active metabolites and the new oxazaphosphorine ASTA Z 7557 (INN mafosfamide)

David S. Alberts, Janine G. Einspahr, Robert Struck, Gary Bignami, Laurie Young, Earl A. Surwit, Sydney E. Salmon

Research output: Contribution to journalArticlepeer-review

28 Scopus citations


Cyclophosphamide (CPA), the most commonly used alkylating agent in the treatment of a wide variety of hematologic and solid tumors, requires oxidation by hepatic microsomal enzymes to its active alkylating species. A number of alternative methods exist to simulate the in vitro cytotoxicity of CPA against animal and human tumors, including the co-incubation of CPA with the S-9 fraction of rat liver homogenates (S-9) and the use of either 4-hydroperoxy CPA (a stabilized form of a major blood-borne metabolite of CPA), phosphoramide mustard (PM, considered to be the ultimate intracellular alkylating metabolite of CPA), or ASTA Z 7557 [4-(2-sulfonatoethylthio)-CPA, a new oxazaphosphorine compound which after dissolution undergoes rapid spontaneous hydrolysis in vitro with liberation of 4-hydroxy-CPA]. Using a human tumor clonogenic assay (HTCA) we have quantitated the median molar inhibitory dose 50 (ID50) concentrations of S-9 activated-CPA, 4-hydroperoxy-CPA, PM, and ASTA Z 7557 against 107 previously untreated tumors, as well as determining the in vitro biological stability of the former three CPA metabolite preparations. 4-Hydroperoxy-CPA proved the most consistently cytotoxic (median molar ID50=5.77×10-5M) compound, followed by ASTA Z 7557, S-9 activated-CPA and PM in that order. Of additional interest S-9 activated CPA and PM proved relatively unstable biologically when frozen at -120°C, whereas 4-hydroperoxy-CPA lost none of its cytotoxicity over a 36 day period during freezing. On the basis of these data 4-hydroperoxy-CPA appears the compound of choice for use in vitro to evaluate the activity that CPA is likely to express clinically against solid tumors. Since 4-hydroperoxy-CPA is not available for clinical use, ASTA Z 7557, which was slightly less cytotoxic to ovarian cancers and a wide variety of other tumors in the HTCA, appears an attractive agent to develop further clinically, especially for regional chemotherapy (e.g., intraperitoneal and intra-arterial treatment) of solid tumors.

Original languageEnglish (US)
Pages (from-to)141-148
Number of pages8
JournalInvestigational New Drugs
Issue number2
StatePublished - Jun 1984


  • ASTA Z 7557
  • cyclophosphamide
  • oxazaphosphorine

ASJC Scopus subject areas

  • Oncology
  • Pharmacology
  • Pharmacology (medical)


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