Enantiomer–enantiomer interaction of ifosfamide in the rat

Ifosfamide - History, efficacy, toxicity and encephalopathy

In this review we trace the passage of fundamental ideas through 20^th century cancer research that began with observations on mustard gas toxicity in World War I. The transmutation of these ideas across scientific and national boundaries, was channeled from chemical carcinogenesis labs in London via Yale and Chicago, then ultimately to the pharmaceutical industry in Bielefeld, Germany. These first efforts to checkmate cancer with chemicals led eventually to the creation of one of the most successful groups of cancer chemotherapeutic drugs, the oxazaphosphorines, first cyclophosphamide (CP) in 1958 and soon thereafter its isomer ifosfamide (IFO). The giant contributions of Professor Sir Alexander Haddow, Dr. Alfred Z. Gilman & Dr. Louis S. Goodman, Dr. George Gomori and Dr. Norbert Brock step by step led to this breakthrough in cancer chemotherapy. A developing understanding of the metabolic disposition of ifosfamide directed efforts to ameliorate its side-effects, in particular, ifosfamide-induced encephalopathy (IIE). This has resulted in several candidates for the encephalopathic metabolite, including 2-chloroacetaldehyde, 2-chloroacetic acid, acrolein, 3-hydroxypropionic acid and S-carboxymethyl-L-cysteine. The pros and cons for each of these, together with other IFO metabolites, are discussed in detail. It is concluded that IFO produces encephalopathy in susceptible patients, but CP does not, by a "perfect storm," involving all of these five metabolites. Methylene blue (MB) administration appears to be generally effective in the prevention and treatment of IIE, in all probability by the inhibition of monoamine oxidase in brain potentiating serotonin levels that modulate the effects of IFO on GABAergic and glutamatergic systems. This review represents the authors' analysis of a large body of published research.

Worsening of the Toxic Effects of (±)Cis-4,4'-DMAR Following Its Co-Administration with (±)Trans-4,4'-DMAR: Neuro-Behavioural, Physiological, Immunohistochemical and Metabolic Studies in Mice

4,4’-Dimethylaminorex (4,4’-DMAR) is a new synthetic stimulant, and only a little information has been made available so far regarding its pharmaco-toxicological effects. The aim of this study was to investigate the effects of the systemic administration of both the single (±)cis (0.1–60 mg/kg) and (±)trans (30 and 60 mg/kg) stereoisomers and their co-administration (e.g., (±)cis at 1, 10 or 60 mg/kg + (±)trans at 30 mg/kg) in mice. Moreover, we investigated the effect of 4,4′-DMAR on the expression of markers of oxidative/nitrosative stress (8-OHdG, iNOS, NT and NOX2), apoptosis (Smac/DIABLO and NF-κB), and heat shock proteins (HSP27, HSP70, HSP90) in the cerebral cortex. Our study demonstrated that the (±)cis stereoisomer dose-dependently induced psychomotor agitation, sweating, salivation, hyperthermia, stimulated aggression, convulsions and death. Conversely, the (±)trans stereoisomer was ineffective whilst the stereoisomers’ co-administration resulted in a worsening of the toxic (±)cis stereoisomer effects. This trend of responses was confirmed by immunohistochemical analysis on the cortex. Finally, we investigated the potentially toxic effects of stereoisomer co-administration by studying urinary excretion. The excretion study showed that the (±)trans stereoisomer reduced the metabolism of the (±)cis form and increased its amount in the urine, possibly reflecting its increased plasma levels and, therefore, the worsening of its toxicity.

Stereochemical considerations in pharmacokinetic processes of representative antineoplastic agents

The vast majority of chemical drugs or drug candidates contain stereocenter(s) in their molecular structures. In these molecules, stereochemical properties are vital properties that influence or even determine their drug actions. Therefore, studying the stereochemical issues of drugs (or drug candidates) is necessary for rational drug use. These stereochemical issues are usually involved with the stereoselectivity in pharmacokinetic processes, especially in the metabolism process. Thus, the investigation of the stereochemical issues in drug metabolism process deserves great attention, especially in those chiral/prochiral antineoplastic agents exhibiting pharmacodynamics and toxicologic differences between stereoisomers. Published reviews concerning this certain issue are inspiring, however they were covering all drug types and only limited antineoplastic drugs were discussed. Here in this review, the research on stereochemical issues in pharmacokinetic processes of some representative antineoplastic agents were described, especially focusing on some newly developed compounds. We highlight the chemical transformations in pharmacokinetic processes of these chiral/prochiral compounds and discuss their different behaviors with metabolic enzymes or transporter proteins, to explicate the observed stereoselectivity intrinsically.

Species differences for stereoselective metabolism of ethofumesate and its enantiomers in vitro

1. The stereoselective metabolism of ethofumesate (ETO) and its enantiomers in rabbit and rat liver microsomes have been studied by chiral high-performance liquid chromatography (HPLC) method. Two metabolites were detected in both liver microsomes in the presence of beta-nicotinamide adenine dinucleotide phosphate (NADPH). 2. The T(1/2) of (+)-ETO and (-)-ETO in rabbit liver microsomes were 12.2 and 4.7 min of rac-ETO and 25.9 and 6.7 of ETO enantiomers. However, the T(1/2) of (+)-ETO and (-)-ETO in rat liver microsomes were 5.3 and 5.9 min of rac-ETO and 7.8 and 10.6 of ETO enantiomers. The stereoselective selectivity is similar to the in vivo study. 3. After incubation of ETO enantiomers, stereoselectivity was present in the formation of ETO-OH enantiomer in rabbit liver microsomes, but stereoselectivity was not evident in rat liver microsomes. 4. There was no chiral inversion from the (+)-ETO to (-)-ETO or inversion from (-)-ETO to (+)-ETO in both rabbit and rat liver microsomes.

Stereoselectivity in drug metabolism

Many chiral drugs are used as their racemic mixtures in clinical practice. Two enantiomers of a chiral drug generally differ in pharmacodynamic and/or pharmacokinetic properties as a consequence of the stereoselective interaction with optically active biological macromolecules. Thus, a stereospecific assay to discriminate between enantiomers is required in order to relate plasma concentrations to pharmacological effect of a chiral drug. Stereoselective metabolism of drugs is most commonly the major contributing factor to stereoselectivity in pharmacokinetics. Metabolizing enzymes often display a preference for one enantiomer of a chiral drug over the other, resulting in enantioselectivity. The structural characteristics of enzymes dictate the enantiomeric discrimination associated with the metabolism of chiral drugs. The stereoselectivity can, therefore, be viewed as the physical property characteristic that phenotypes the enzyme. This review provides a comprehensive appraisal of stereochemical aspects of drug metabolism (i.e., enantioselective metabolism and first-pass effect, enzyme-selective inhibition or induction and drug interaction, species differences and polymorphic metabolism).