Carcinogenicity testing of drugs

An in-silico receptor-pharmacophore based multistep molecular docking and simulation study to evaluate the inhibitory potentials against NS1 of DENV-2

DENV-2 strain is the most fatal and infectious of the five dengue virus serotypes. The non-structural protein NS1 encoded by its genome is the most significant protein required for viral pathogenesis and replication inside the host body. Thus, targeting the NS1 protein and designing an inhibitor to limit its stability and secretion is a propitious attempt in our fight against dengue. Four novel inhibitors are designed to target the conserved cysteine residues (C55, C313, C316, and C329) and glycosylation sites (N130 and N207) of the NS1 protein in an attempt to halt the spread of the dengue infection in the host body altogether. Numerous computer-aided drug designing techniques including molecular docking, molecular dynamics simulation, virtual screening, principal component analysis, and dynamic cross-correlation matrix were employed to determine the structural and functional activity of the NS1-inhibitor complexes. From our analysis, it was evident that the extent of structural and atomic level fluctuations of the ligand-bound protein exhibited a declining trend in contrast to unbound protein which was prominently noticeable through the RMSD, RMSF, Rg, and SASA graphs. The ADMET analysis of the four ligands revealed a promising pharmacokinetics and pharmacodynamic profile, along with good bioavailability and toxicity properties. The proposed drugs when bound to the targeted cavities resulted in stable conformations in comparison to their unbound state, implying they have good affinity promising effective drug action. Thus, they can be tested in vitro and used as potential anti-dengue drugs.Communicated by Ramaswamy H. Sarma.

Safety assessment of pharmaceuticals: examples of inadequate assessments and a mechanistic approach to assuring adequate assessment

For a conventional organic new chemical entity (NCE) being developed as a pharmaceutical, standard regulatory safety assessment studies are required. Early in development, an NCE should undergo a safety/benefit analysis to justify further development. This analysis is made easier and more effective when comprehensive nonclinical data are available. One of the most important aspects of nonclinical toxicologic studies is to provide information on absence of potential carcinogenicity in humans. To avoid human exposures to potentially carcinogenic agents, even in early development of an NCE, the Decision Point Approach to carcinogen testing provides a useful guide to acquisition of mechanistically relevant data for risk assessment.

The significance of hepatic microsomal enzyme induction and altered thyroid function in rats: implications for thyroid gland neoplasia

Hepatic microsomal enzymes play an important role in thyroid hormone homeostasis. Glucuronidation of thyroxine is the rate limiting step in the biliary excretion of thyroxine in the rat; the monodeiodinases are important in the conversion of T4 to T3 and reverse T3 and subsequent deiodinations. Phenobarbital is known to affect thyroid function in rats due to an alteration of hormone disposition. We have further characterized these effects and have demonstrated that phenobarbital increases the biliary excretion of thyroxine glucuronide primarily as a result of an induction of hepatic thyroxine glucuronyltransferase. Studies on the mode of action for phenobarbital promotion of thyroid follicular neoplasia were conducted using an initiation-promotion model established by Hiasa et al (35). In this model, we demonstrated that supplemental administration of thyroxine blocked the promoting effect of phenobarbital and furthermore, using various dosages of thyroxine, we observed that the tumor promoting effect of phenobarbital was directly proportional to the level of plasma TSH. The results of these studies support the hypothesis that the tumor promoting effect of phenobarbital in the thyroid gland is mediated via increased secretion of pituitary TSH as a compensatory response to the known effects of phenobarbital on peripheral thyroid hormone disposition. Since a number of microsomal enzyme inducing agents have increased the incidence of thyroid follicular neoplasia in rat carcinogenicity studies, thyroid function should be assessed and a secondary mechanism of hormone imbalance should be considered in the interpretation of the significance of these findings in rodents.

Hepatotoxicity of chronic high dose administration of acetaminophen to mice. A critical review and implications for hazard assessment

To evaluate the degree of toxicity to the liver of chronic administration of acetaminophen, the slides from two independent previously reported experiments using different strains of mouse were reviewed and compared. One experiment was performed using B6C3F1 mice of both sexes which were fed 0.3, 0.6 or 1.25% acetaminophen in the diet for 41 weeks. The other was conducted with NIH general purpose mice of both sexes which were fed 1.1% acetaminophen in the diet for 48 weeks. In both experiments, at the levels of 1.1 or 1.25%, the drug produced severe liver injury characterized by centrilobular necrosis which was comparable in both males and females at the end of the experiments. The findings show that ingestion of dietary levels of greater than 1% leads to chronic hepatotoxicity to mice. The import of these findings in interpretation of carcinogenicity studies of acetaminophen is discussed.