Enantiomers of cyclophosphamide and iphosphamide

This article presents an overview of the methods of bioanalysis of oxazaphosphorines, in particular, cyclophosphamide, ifosfamide, and trofosfamide as well as their metabolites. The metabolism of oxazaphosphorines is complex and leads to a large variety of metabolites and therefore the spectrum of methods used is relatively broad. The various methods used are shown in a table and the particularly important assays are described.

The enantiomers of the cytostatic drug ifosfamide and the two metabolites 2- and 3-dechloroethylifosfamide were isolated from plasma and urine by liquid-liquid extraction with ethyl acetate, resolved on a Chirasil-l-val gas chromatographic column and detected by a nitrogen-phosphorus-selective flame ionisation detector. Resolution of the racemic compounds for identification purposes was also accomplished with high-performance liquid chromatography on a chiral column. The validated gas chromatographic method was suitable to determine the total concentrations and the enantiomeric composition of ifosfamide and its dechloroethylated metabolites in plasma and urine samples from treated patients. Some metabolic preferences in the metabolism of ifosfamide were found.

Cyclophosphamide (CP) is used to treat a wide range of neoplastic diseases as well as some non-malignant ones such as rheumatoid arthritis. It is also used as an immunosuppressive agent prior to organ transplantation. CP is, however, a known carcinogen in humans and produces secondary tumors. There is little absorption either orally or intravenously and 10% of the drug is excreted unchanged. CP is activated by hepatic mixed function oxidases and metabolites are delivered to neoplastic cells via the bloodstream. Phosphoramide mustard is thought to be the major anti-neoplastic metabolite of CP while acrolein, which is highly toxic and is produced in equimolar amounts, is thought to be responsible for most of the toxic side effects.

DNA adducts have been formed after CP treatment in a variety of in vitro systems as well as in rats and mice using ³H-labeled CP. ³²P-postlabeling techniques have also been used in mice. However, monitoring of adducts in humans has not yet been carried out. CP has also been shown to induce unscheduled DNA synthesis in a human cell line.

CP has produced mutations in base-pair substituting strains of Salmonella typhimurium in the presence of metabolic activation, but it has been shown to be negative in the E. coli chromotest. It has also been shown to be positive in Saccharomyces cerevisiae in D7 strain for many endpoints but negative in D62.M for aneuploidy/malsegregation. It has produced positive responses in Drosophila melanogaster for various endpoints and in Anopheles stephensi.

In somatic cells, CP has been shown to produce gene mutations, chromosome aberrations, micronuclei and sister chromatid exchanges in a variety of cultured cells in the presence of metabolic activation as well as sister chromatid exchanges without metabolic activation. It has also produced chromosome damage and micronuclei in rats, mice and Chinese hamsters, and gene mutations in the mouse spot test and in the transgenic lacZ construct of Muta^™Mouse.

Increases in chromosome damage and gene mutations have been found in the peripheral blood lymphocytes of nurses, pharmacists and female workers occupationally exposured to CP during its production or distribution. Chromosome aberrations, sister chromatid exchanges and gene mutations have been observed in somatic cells of patients treated therapeutically with CP. In general, there is a maximum dose and an optimum time for the detection of genetic effects because the toxicity associated with high doses of CP will affect cell division.

In germ cells, CP has been shown to induce genetic damage in mice, rats and hamsters although the vast majority of such studies have used male mice. In males, the germ cell stages determined to be most sensitive to the induction of damage are the post-meiotic stages, which are sampled in the mouse by matings 1–3 weeks after treatment, and particularly the mid- to late-step spermatids sampled by matings 2–3 weeks after treatment. Tests by which CP has been shown to induce germ cell damage through assessment of effects in F₁ progeny include those for dominant lethality, heritable translocations, specific locus mutations and malformations. Positive responses have also been reported in tests analyzing for structural chromosome damage in both spermatogonial and meiotic metaphases following exposures of various meiotic and pre-meiotic germ cell stages. CP exposures of meiotic cells have been shown to produce structural aberrations of the synaptonemal complex and to increase the frequency of spermatid micronuclei. Further, CP has been shown to induce structural, as well as some numerical, chromosome damage through analyses of second meiotic metaphases and first cleavage metaphases as well as through karyotypic analysis of F₁ embryos. However, CP does not appear to be an effective aneugen. Other germ cell tests through which CP has been shown to produce genotoxic damage include those for sister chromatid exchanges and abnormal sperm head morphology. CP has also been shown to produce primary DNA damage during spermatogenesis as indicated by positive alkaline elution data from a study with rats, and by induction of unscheduled DNA synthesis in the testes of mice, rats and rabbits.

Genetic risk estimates for the male have been made for 1 mg/kg of exposure to CP under acute conditions, on the expectation that the additional risk for an individual's offspring could be obtained by multiplying the risk estimates by the total mg/kg to which an individual is exposed. The types of mutational data available following maternal exposures do not justify attempting even an approximation of the estimate of genetic risk for female germ cells. Examples are provided of how the risk estimates could be applied to practical situations based upon male exposures.

Three types of risk estimates for human health effects were derived for first generation progeny following paternal exposures. The indirect method of genetic risk estimation was used to estimate the risk from gene mutations and small deficiencies on the basis of data from the mouse morphological specific locus test. Data from the heritable translocation assay were applied to estimate risk from the induction of reciprocal translocations. In addition, an overall estimate of genetic risk was made using a modification of the direct method applied to data on F₁ fetal malformations and growth retardation. These latter data are especially important because they show definitively that some mutations induced by CP actually cause serious organismic damage in first generation progeny. However, their use in estimating genetic risk warrants a special caution. Besides the unavoidable complication of making an interspecies extrapolation of phenotypic damage that is inherent in the direct method, the overall estimate of genetic risk has a further uncertainty in that the estimate of induced genetic effects manifest before the age of 25 in humans is based upon induced anomalies in the rat that are observed in late fetal stages.

The parallelogram approach was applied to our overall estimate of genetic risk to yield the estimate that, for every mg/kg of paternal exposure, there would be an increase of 600 individuals with serious induced genetic disorders by early adulthood per million liveborn offspring derived from sperm sampled within 5 weeks after treatment (i.e., for all post-meiotic stages). The available data provide no reason to expect any elevation in risk for conceptions more than 90 days after treatment if exposures are to CP alone; the risk between 5 weeks and 90 days after treatment would be relatively slight, and possibly zero. We emphasize that our risk estimates are far from precise. However, they are the best that can presently be given for hereditary damage from CP. The explanations provided for the steps needed to convert mutational information into risk estimates may lead to improvements in estimates of hereditary risk for CP and other chemicals.

There is a very extensive data base for CP which extends from the molecular level (except for adducts in humans) through the microbial and insect systems to somatic mammalian cells in various cell lines and primary cells in vitro, somatic cells in vivo, and germ cells in rodents. CP has been used as a positive control substance in many assay systems. In this review, extrapolation has been made from rodents to humans with risk estimates attempted at both individual and population levels.

The synthesis and biological evaluation of chiral, diastereomeric phosphorus mustards derived from natural and unnatural serine are reported herein.

Pharmacokinetic and pharmacodynamic properties of drugs and their ultimate therapeutic effects are often significantly influenced by interactions between the geometry of host receptors, host enzymes, and the three-dimensional structure of drugs. Drug molecules that are mirror images of each other are chiral stereoisomers, and such chiral isomer compounds are commonly used as therapeutic agents by rheumatologists either as racemates (mixtures of chiral isomers) or as pure stereoisomers. Understanding and using such stereoisomeric drugs may lead to lower risks of drug toxicity, better therapeutic indices, and newer approaches for the treatment of articular disorders. A review of the properties of these special isomers is presented, and their therapeutic advantages are discussed.

[R(+),S(−)]-Cyclophosphamide [(R,S)-CP] is an anticancer drug, containing a chiral phosphorous atom, which is prepared and used clinically as the racemic mixture. A new high-performance liquid Chromatographic assay suitable for pharmacokinetic studies of CP enantiomers in plasma has been reported recently by this laboratory (Reid et al., Anal Chem 61: 441–446, 1989). Briefly, the assay involves ethyl acetate extraction of CP enantiomers from plasma followed by derivatization to diastereomers in a two-step process utilizing chloral and (+)-naproxen acid chloride. Chromatographic analysis was performed on a reversed phase (ODS) column with detection at 232 nm. In the present study, preliminary results on the applicability of this assay to pharmacokinetic studies are presented. Several rabbits were used to compare the influence of i.p., i.v., and oral routes of administration on the stereoselective disposition of (R,S)-CP. Following i.p. administration, S-CP was cleared faster than R-CP. Following oral administration, only R-CP was detectable in plasma, while i.v. administration resulted in minor or no stereoselective disposition. These results indicated that there was a marked stereoselective metabolism of the S-CP enantiomer, with the i.p. and oral routes producing the greatest differences due to first-pass metabolism. Incubation of rabbit-liver microsomes with (R,S)-CP demonstrated that the monooxygenase system can exhibit marked stereoselectivity in its metabolism of CP. The ratio of R-CP to S-CP in the incubation medium increased during the incubation period from 1:1 initially to 4.5:1 after 60 min. The results from the experiments with rabbits indicate that the first-pass metabolism of this drug is highly stereoselective; in contrast, cancer patients who had received (R,S)-CP as an i.v. infusion showed no stereoselectivity in the elimination of the enantiomers. Pharmacokinetic studies with cancer patients, receiving (R,R)-CP as an oral dose, are in progress in order to determine if stereoselective first-pass metabolism of this drug also occurs in humans.