The use of explant lens culture to assess cataractogenic potential.

Explanted cultures of crystalline lenses have been used to investigate mechanisms of xenobiotic-induced cataract formation. However, very few studies have utilized mechanistic information to predict the cataractogenic potential of structurally diverse xenobiotics. The present investigation outlines how visual assessment of lens clarity, biochemical endpoints of toxicity, and mechanisms of lenticular opacity formation can be used to select compounds with a lower probability of causing cataract formation in vivo. The rat lens explant culture system has been used to screen thiazolidinediones against ciglitazone for their direct cataractogenic potential in vitro. The two compounds that were selected as development candidates (englitazone and darglitazone) did not produce cataracts in rats exposed daily for 3 months. The culture system has also been used to illustrate that the lens is capable of metabolizing compounds to reactive intermediates. In this example, the toxicity of S-(1,2-dichlorovinyl)-L-cysteine (DCVC), a model cataractogen, was attenuated by inhibiting lenticular cysteine conjugate beta-lyase metabolism using aminooxyacetic acid. Finally, this model was used retrospectively to investigate the cataractogenic potential of CJ-12,918 and CJ-13,454 in rats. These compounds showed differences in the incidence of cataract formation in vivo based on differences in hepatic metabolism and penetration of parent drug and metabolites into the lens. The rank order of cataractogenic potential in vitro correlated better with in vivo results when an induced S9 microsomal fraction was added to the culture media. However, the model did not correctly predict the cataractogenic potential of ZD2138, a structurally similar compound. These studies illustrate the use of explant culture to assess mechanisms of cataract formation and outline its use and limitations for predicting cataractogenic potential in vivo.

Metabolism and excretion of trovafloxacin, a new quinolone antibiotic, in Sprague-Dawley rats and beagle dogs. Effect of bile duct cannulation on excretion pathways.

The excretion and metabolism of trovafloxacin was investigated after administration of a single oral dose of [14C]trovafloxacin to Sprague-Dawley rats and beagle dogs. The bile was the major route of excretion in rats (59% of the dose). Trovafloxacin was extensively metabolized in this species, and only 3% of the dose was excreted unchanged. Glucuronidation and acetylation were the major metabolic pathways involved in the elimination, and no oxidative metabolites were detected in rats. In dogs, 97.6 and 2.7% of the dose was recovered in feces and urine, respectively, in 72 hr. However, excretion studies in bile duct-cannulated dogs revealed that 28.2% of the radioactivity was recovered in bile, whereas 45.6% was in urine. This suggested that bile duct cannulation had affected the disposition of trovafloxacin. Analysis of bile and urine of bile duct-cannulated dogs by LC/MS/MS indicated that glucuronidation was the major metabolic pathway in dogs as well. Two novel metabolites were identified in the bile of this species. One was confirmed as a pyrroline analog of trovafloxacin (M7), and the second was tentatively identified as the hydroxycarboxylic acid analog (M6). The differences in metabolism of trovafloxacin in the bile duct-cannulated and noncannulated dogs were investigated by comparison of the metabolite profiles in urine and feces of these animals. Although the metabolites in urine were similar, the extracts of fecal samples obtained from noncannulated animals revealed the presence of N-acetyltrovafloxacin (M3). Incubation of trovafloxacin with cecal contents of dogs under anaerobic conditions suggested the involvement of intestinal microflora in the formation of this metabolite. Metabolite M3 was absent from fecal extracts of bile duct-cannulated dogs, suggesting that surgery had affected the metabolism of trovafloxacin by gut microflora.