An evaluation of the mode of action framework for mutagenic carcinogens case study: Cyclophosphamide

Abstract

In response to the 2005 revised US Environmental Protection Agency (EPA) Cancer Guidelines, a Risk Assessment Forum's Technical Panel has devised a strategy in which genetic toxicology data combined with other information are assessed to determine whether a carcinogen operates through a mutagenic mode of action (MOA). This information is necessary for EPA to decide whether age-dependent adjustment factors (ADAFs) should be applied to the cancer risk assessment. A decision tree has been developed as a part of this approach and outlines the critical steps for analyzing a compound for carcinogenicity through a mutagenic MOA (e.g., data analysis, determination of mutagenicity in animals and in humans). Agents, showing mutagenicity in animals and humans, proceed through the Agency's framework analysis for MOAs. Cyclophosphamide (CP), an antineoplastic agent, which is carcinogenic in animals and humans and mutagenic in vitro and in vivo, was selected as a case study to illustrate how the framework analysis would be applied to prove that a carcinogen operates through a mutagenic MOA. Consistent positive results have been seen for mutagenic activity in numerous in vitro assays, in animals (mice, rats, and hamsters) and in humans. Accordingly, CP was processed through the framework analysis and key steps leading to tumor formation were identified as follows: metabolism of the parent compound to alkylating metabolites, DNA damage followed by induction of multiple adverse genetic events, cell proliferation, and bladder tumors. Genetic changes in rats (sister chromatid exchanges at 0.62 mg/kg) can commence within 30 min and in cancer patients, chromosome aberrations at 35 mg/kg are seen by 1 hr, well within the timeframe and tumorigenic dose range for early events. Supporting evidence is also found for cell proliferation, indicating that mutagenicity, associated with cytotoxicity, leads to a proliferative response, which occurs early (48 hr) in the process of tumor induction. Overall, the weight of evidence evaluation supports CP acting through a mutagenic MOA. In addition, no data were found that an alternative MOA might be operative. Therefore, the cancer guidelines recommend a linear extrapolation for the risk assessment. Additionally, data exist showing that CP induces mutagenicity in fetal blood and in the peripheral blood of pediatric patients; thus, the ADAFs would be applied.

Evaluation of Environmental Contaminations and Occupational Exposures Involved in Preparation of Chemotherapeutic Drugs

“…In internal exposure studies 6,8 12,14) and in external exposure studies, 9,13,22,23) CP was frequently used as a marker of occupational exposures because of its slight volatility, 16) human genotoxicity, 16,24) reproductive toxicity, 25) carcinogenity, 17) and ease of detection. 26,27) Among several reports on occupational CP exposures in healthcare workers, some showed the presence of CP in the urine samples of nurses and pharmacists, 6,8 14) while others did not.…”

Ratlarda Deneysel Siklofosfamid Toksikasyonunda Silimarin ve Curcuminin Koruyucu ve Antigenotoksik Etkileri

“…The most important concern regarding high CP doses is the drug's toxic effects on the tissues [27,28] . It has been reported that CP's toxic effect is mediated by the degradation of antioxidant defence systems by acrolein, which is formed by the metabolisation of CP, resulting in excessive production of free radicals [2,3] . In the present study, SLY and CUR, which have strong antioxidant characteristics, were chosen in order to make it possible to use CP in high doses.…”

“…CP is an alkylating agent widely used in cancer chemotherapy, and its cytotoxic effects are the result of chemically reactive metabolites that alkylate DNA and protein, producing cross-links [1,2] . The major limitation of using CP is injury to normal tissues, which gives rise to numerous side effects [25] .…”

“…Despite its therapeutic importance, a wide range of adverse effects, including nephrotoxicity [2] , hepatotoxicity [3] , cardiotoxicity [4] and male reproductive toxicity [5] , have been reported for the drug.…”

“…Cyclophosphamide (CP) (N,N-bis(2-chloroethyl tetrahydro-2H-1,3,2-oxphosphorin-2-amine, 2-oxide monohydrate), also known Cytoxan or Endoxan, is a pharmaceutical product used as an antineoplastic agent in the treatment of a wide range of cancers, including Hodgkin's disease, many types of leukaemia, multiple myeloma and neuroblastomas [1,2] . Despite its therapeutic importance, a wide range of adverse effects, including nephrotoxicity [2] , hepatotoxicity [3] , cardiotoxicity [4] and male reproductive toxicity [5] , have been reported for the drug.…”