The role of metabolic biomarkers in drug toxicity studies

ABSTRACT Metabolic profiling is a technique that can potentially provide more sensitive and specific biomarkers of toxicity than the current clinical measures benefiting preclinical and clinical drug studies. Both nuclear magnetic resonance (NMR) and mass spectrometry (MS) platforms have been used for metabolic profiling studies of drug toxicity. Not only can both techniques provide novel biomarker(s) of toxicity but the combination of both techniques gives a broader range of metabolites evaluated. Changes in metabolic patterns can provide insight into mechanism(s) of toxicity and help to eliminate a potentially toxic new chemical entity earlier in the developmental process. Metabolic profiling offers numerous advantages in toxicological research and screening as sample collection and preparation are relatively simple. Further, sample throughput, reproducibility, and accuracy are high. The area of drug toxicity of therapeutic compounds has already been impacted by metabolic profiling studies and will continue to be impacted as new, more specific biomarker(s) are found. In order for a biomarker or pattern of biomarkers to be accepted, it must be shown that they originate from the target tissue of interest. Metabolic profiling studies are amenable to any biofluid or tissue sample making it possible to link the changes noted in urine for instance as originating from renal injury. Additionally, the ease of sample collection makes it possible to follow a single animal or subject over time in order to determine whether and when the toxicity resolves itself. This review focuses on the advantages of metabolic profiling for drug toxicity studies.

Alanine aminotransferase apoenzyme in dogs

Unusually low serum activity of alanine aminotransferase (ALT) was detected in a Rottweiler dog with gastric dilatation-volvulus. Activity of ALT in the same sample was found to be much higher (estimated increase of 14,225%) when measured by methods adding the cofactor pyridoxal-5'-phosphate (P5P), indicating that nearly all serum ALT was in the apoenzyme form. An investigation was undertaken to determine the frequency of high serum aminotransferase apoenzyme levels in dogs. Eighty canine serum samples submitted to the Clinical Pathology Laboratory at Cornell University were assayed for ALT and aspartate aminotransferase (AST) in the presence or absence of exogenous P5P. In 79 dogs, inclusion of P5P in the LAT assay resulted in a median decrease in AST activity of -6.3% (range -33.3% to 25.0%) in all 80 dogs. One dog had an increase of 336% in ALT activity after inclusion of P5P in the assay, but lacked a similar increase in AST activity. The reason for the high levels of ALT apoenzyme in the 2 dogs was not determined.

Serum transaminase elevations as indicators of hepatic injury following the administration of drugs

During the preclinical, early clinical, late-stage clinical, and postmarketing phases of the pharmaceutical discovery and development process, one important aspect of drug safety assessment involves monitoring for possible drug-induced hepatic injury. Hepatic injuries vary in nature from direct, intrinsic effects that are observed in most recipients and more than one species to rare idiosyncratic responses seen only in a few clinical subjects. Histological types of injuries vary from hepatocellular to hepatobiliary with multiple cellular effects characteristic of each type. Of the various clinical laboratory markers for hepatic injury, serum transaminases, especially alanine aminotransferase (ALT), are the most universally important indicators for studies ranging from early preclinical animal testing to postmarketing patient monitoring. This review examines the characteristics of hepatic toxicity that result in serum ALT changes, the differences in the etiology of hepatic responses which govern when liver injury is most likely to be detected during the four phases of the drug discovery and development process, and those modulating factors which affect the utility of ALT as a dependable marker of hepatic injury in clinical populations. The paper concludes with a summary of some ancillary methods for early preclinical screening such as in vitro metabolism and toxicity assays, gene and protein expression analysis, and some strategies for enhancing the probability for the early detection of idiosyncratic hepatotoxic responses which are infrequent but significant factors in the safety assessment process.

Species specificity in the blood cholesterol-lowering effect of YM-16638

1. The compound YM-16638, [[5-[[3-(4-acetyl-3-hydroxy-2-propylphenoxy)propyl] thio]-1,3,4-thiadiazol-2-yl]thio] acetic acid was developed in a series of in vitro and in vivo studies as a leukotriene D4 receptor antagonist. 2. In a clinical trial as a leukotriene antagonist drug, this compound was found to have a potent serum cholesterol lowering effect in normolipidaemic healthy male volunteers. 3. In the present study, we investigated the serum cholesterol lower effect of this compound in various species of experimental animals. 4. Administration of YM-16638 did not cause a significant decrease in serum total cholesterol (TC) in mice (up to 200 mg kg-1, body weight per day for 28 days), rats (200 mg kg-1 for 15 days) or rabbits (90 mg kg-1 for 18 days). In hamsters, administration of YM-16638 orally or by peritoneal injection at 50 mg kg-1 or more daily for 7 days caused a significant decrease in serum TC and the rate of body weight gain. In monkeys, serum TC did not change in YM-16638-administered squirrel monkeys (50 mg kg-1 daily for 3 weeks), but a significant decrease in serum TC was observed in cynomolgus monkeys (33% decrease at 30 mg kg-1 for 4 weeks) and rhesus monkeys (27% decrease at 30 mg kg-1 for 3 weeks) without any serious decrease in body weight. These results were consistent with those in a phase I study with human subjects. In contrast, serum alanine aminotransferase (ALT) level decreased in all animals after YM-16638 treatment. 5. From these results, we conclude that YM-16638 has a potent hypocholesterolaemic effect, but that this effect if species-specific and is only recognized clearly in human subjects and old-world monkeys.

Contrasting interspecies efficacy and toxicology of 1,2-diethyl-3-hydroxypyridin-4-one, CP94, relates to differing metabolism of the iron chelating site

In order to define a predictive animal model for the effects of hydroxypyridinone (HPO) iron chelators in humans, we have compared the 28 d oral efficacy and toxicology of the HPO, 1,2-diethyl-3-hydroxypyridin-4-one (CP94) in rats and guinea-pigs and related the results to the contrasting metabolism of this compound in the two species. CP94 was highly effective at mobilizing liver iron in rats but showed toxicity at higher doses, whereas in the guinea-pig the compound lacked toxicity but was ineffective at mobilizing liver iron. These differences can be explained by the contrasting metabolism of the drug between the two species. In rats, at the top dose of 300 mg/kg intragastrically, all animals died before the end of the study, with no deaths or weight loss at lower doses. At 100 mg/kg, rat liver non-haem iron concentrations were reduced by 53% and 44% in females and males respectively (P < 0.001). At this dose, adrenal medullary cell vacuolation, increased mammary secretory activity, vacuolation of corpora luteal cells and single cell hepatocyte necrosis were seen. There were no reductions in the white cell count. At 50 mg/kg rat liver non-haem iron concentrations were decreased by 50% and 34% in females and males respectively (P < 0.02). In female rats this was associated with increased mammary secretory activity. In iron-overloaded rats given 100 mg/kg by gavage for 28 d, liver non-haem iron concentration was reduced by 39% (P < 0.01) and serum ferritin by 71% (P < 0.001). Ovarian and mammary changes were not influenced by iron loading. In guinea-pigs, CP94 was evaluated at 50 mg/kg, 100 mg/kg or 200 mg/kg by oral insufflation for 28 d. No reduction in liver iron was seen and no systematic dose related histological, biochemical or haematological effects were observed. Whereas in guinea-pigs 99% of urinary recovery following an oral dose of CP94 (100 mg/kg) was as the inactive glucuronide metabolite, in the rat only 23% of the dose was excreted in the urine as the glucuronide with remainder as the free drug or an iron binding metabolite. The lack of both efficacy and toxicity in the guinea-pig may therefore be explained by the rapid inactivation of CP94 by glucuronidation. This metabolism of CP94 in the guinea-pig is closer to humans than the rat, suggesting that both the efficacy and toxicity of this compound in humans may also be limited by glucuronidation.

Gingival hyperplasia in dogs induced by oxodipine, a calcium channel blocking agent

Subchronic oral exposure of dogs to Oxodipine, a new calcium channel blocker of the dihydropyridine-type, resulted in dose-related gingival hyperplastic changes. The doses at which an effect was elicited were 24 and 73 times the intended therapeutic dose for man. The effects were first noted after 7 weeks of treatment, and were limited to the high and intermediate dose groups of both sexes. Macroscopically, a generalized enlargement of the maxillary and mandibular facial and lingual gingivae were noted. The histological changes were similar to those described in man for Nifedipine and hydantoin-related drugs. An increase in the activity of alkaline phosphatase and a decrease in alanine aminotransferase was demonstrated. This article is the first to describe gingival hyperplasia in dogs induced in a dose-dependent manner by a calcium channel blocker.

Measurement of aminotransferases: Part 1. Aspartate aminotransferase

Aminotransferases are ubiquitous enzymes of mammalian cells and several are of important diagnostic use. The application of aspartate aminotransferase activity measurements in serum from individuals suffering from myocardial infarction brought about a new dimension in clinical laboratory testing in the 1950s. This review focuses on measurement techniques for aspartate aminotransferase and their application (a subsequent article will review other aminotransferases). Assay techniques measuring enzyme activity are direct spectrophotometric measurements, manometric techniques, assays using dye substances, coupled enzyme techniques, and radiometric procedures. Of these procedures, the one employing malate dehydrogenase and NADH is the most important and is covered in particular detail. The estimation of the mitochondrial isoenzyme of aspartate aminotransferase is also of clinical interest, in particular for estimating severity of disease or in specific applications (e.g., chronic alcoholism). Methods reviewed for estimation of this enzyme are electrophoresis, chromatography, differential kinetic behavior, and immunochemical separation. Determination of the enzyme protein by techniques independent of its catalytic activity are also reviewed.

Effect of cefazolin on aminotransferase activity in the rat

Cefazolin given sc to male rats in daily doses of 0.5-2 g per kilogram of body weight significantly decreased alanine aminotranferase activity in serum, liver, kidney, heart, and brain 2-4 wk from the beginning of the treatment. Serum aspartate aminotransferase was also reduced, but serum alkaline phosphatase and tissue pyruvate decarboxylase activities remained unaltered. In female rats, daily sc administration of cefazolin at 0.1-1 g/kg also brought about a dose-related reduction of alanine and aspartate aminotransferase activities, which reached statistical significance at high dose levels. The effect of cefazolin at low concentrations was partly reversed by administration of pyridoxal in vivo. Paradoxically, at higher dose levels pyridoxal potentiated the action of cefazolin on serum aminotranferases. The low enzyme activities were elevated by subsequent addition of pyridoxal 5'-phosphate in vitro. Similar results were obtained when rats were treated with isoniazid at daily oral doses of 200 mg/kg; administration of pyridoxal completely restored alanine aminotransferase activity to the normal level within 2 wk. Cefazolin was metabolized in vivo, resulting in some metabolites that probably possessed a hydrazine group, since positive reactions were obtained with p-dimethylaminobenzaldehyde and Fast Blue B salt. The potentiation of decreased aminotransferase activity by pyridoxal indicated, however, some dissimilarity in the effect between isoniazid and cefazolin.