A stereochemical investigation of the cytotoxicity of mianserin metabolites in vitro

Antidepressant pharmaceuticals in aquatic systems, individual-level ecotoxicological effects: growth, survival and behavior

The growing consumption of antidepressant pharmaceuticals has resulted in their widespread occurrence in the environment, particularly in waterways with a typical concentration range from ng L^-1 to μg L^-1. An increasing number of studies have confirmed the ecotoxic potency of antidepressants, not only at high concentrations but also at environmentally relevant levels. The present review covers literature from the last decade on the individual-level ecotoxicological effects of the most commonly used antidepressants, including their impact on behavior, growth, and survival. We focus on the relationship between antidepressants physico-chemical properties and dynamics in the environment. Furthermore, we discuss the advantages of considering behavioral changes as sensitive endpoints in ecotoxicology, as well as some current methodological shortcomings in the field, including low standardization, reproducibility and context-dependency.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

In vitro analysis of metabolic predisposition to drug hypersensitivity reactions

Idiosyncratic hypersensitivity reactions may account for up to 25% of all adverse reactions, and pose a constant problem to physicians because of their unpredictable nature, potentially fatal outcome and resemblance to other disease processes. Current understanding of how drug allergy arises is based largely on the hapten hypothesis: since most drugs are not chemically reactive per se, they must be activated metabolically to reactive species which may become immunogenic through interactions with cellular macromolecules. The role of drug metabolism is thus pivotal to the hapten hypothesis both in activation of the parent compound and detoxification of the reactive species. Although conjugation reactions may occasionally produce potential immunogens (for example, the generation of acylglucuronides from non-steroidal anti-inflammatory drugs such as diclofenac), bioactivation is catalysed most frequently by cytochrome P450 (P450) enzymes. The multifactorial nature of hypersensitivity reactions, particularly the role of often unidentified, reactive drug metabolites in antigen generation, has hampered the routine diagnosis of these disorders by classical immunological methods designed to detect circulating antibodies or sensitized T cells. Similarly, species differences in drug metabolism and immune system regulation have largely precluded the establishment of appropriate animal models with which to examine the immunopathological mechanisms of these toxicities. However, the combined use of in vitro toxicity assays incorporating human tissues and in vivo phenotyping (or, ultimately, in vitro genotyping) methods for drug detoxification pathways may provide the metabolic basis for hypersensitivity reactions to several drugs. This brief review highlights recent efforts to unravel the bases for hypersensitivity reactions to these therapeutic agents (which include anticonvulsants and sulphonamides) using drug metabolism and immunochemical approaches. In particular, examples are provided which illustrate breakthroughs in the identification of the chemical nature of the reactive metabolites which become bound to cellular macromolecules, the enzyme systems responsible for their generation and (possibly) detoxification, and the target proteins implicated in the subsequent immune response.

Elucidation of the structural requirements for the bioactivation of mianserin in-vitro

The aim of this study was to investigate structure-activity relationships among a series of compounds related to the antidepressant drug, mianserin, with respect to their ability to produce cytotoxic metabolites. Human peripheral lymphocytes were used as target cells and these were exposed to the individual compounds, in the presence or absence of a drug metabolizing system derived from human liver. The individual enantiomers of mianserin showed differences in their cytotoxicity profiles; the R-(-) isomer giving NADPH-dependent cytotoxicity while the S-(+) isomer showed direct cytotoxicity at high concentrations. Cytotoxicity was reduced by removal from mianserin of the nitrogen atom at the 5 position and by substitution of a methyl group for a hydrogen atom at position 14b. In contrast, insertion of an oxygen atom at position 10 of the drug molecule, precluding the formation of a carbonium ion, had little effect on cytotoxic metabolite formation. The data are consistent with the proposal that one or more iminium ions derived from mianserin are responsible for the cytotoxicity observed in this in-vitro system and that appropriate chemical modification may preclude bioactivation of mianserin by P450 enzymes.

Stereoisomers and drug toxicity. The value of single stereoisomer therapy

All drugs produce adverse effects, though the risk varies widely between different compounds. Many toxic reactions are an extension of the mechanism responsible for the therapeutic effect and can be avoided by careful dose adjustment. Other adverse events are not related to the beneficial action of the drug. Recent interest has focused on the role of the different properties of individual drug enantiomers in causing drug toxicity. For drugs with a single chiral centre, both enantiomers may be therapeutically active. However, if the main therapeutic benefit is in only 1 enantiomer, several possibilities exist for the other enantiomer--inactive, a qualitatively different effect, an antagonistic effect or greater toxicity.

Formation of active metabolites of psychotropic drugs. An updated review of their significance

Most of the currently available psychotropic drugs form 1 or more active metabolites during in vivo biotransformation in humans and/or animals. In some cases these metabolites are rapidly conjugated and excreted, but in others they attain blood and/or brain concentrations within the same range as, or even higher than, those of the parent drug, thus being potential biologically active compounds. The formation of metabolites with their own biological activity in addition to that of the parent compound may result in a complex situation where different chemical species participate in the final effects. These chemical species may have different pharmacokinetic properties of distribution and clearance. They may act by similar mechanisms, by different mechanisms or even antagonistically. The formation of active metabolites may be important not only for the therapeutic outcome but also for explaining the toxicity of particular drugs. The examples given, although limited, provide evidence that studies on drug metabolites are essential for an understanding of the mechanism of action of psychotropic drugs, and for extrapolating pharmacological and toxicological findings from animals to humans. The development of any new psychotropic agent requires knowledge of the pharmacology and toxicology of all active species as well as their pharmacokinetic profile, including the extent to which they reach the central nervous system.