Girard H. Hottendorf

Differences in the Sensitivity of Fischer and Sprague-Dawley Rats to Aminoglycoside Nephrotoxicity

Because of the anecdotal but unconfirmed inferences of the greater sensitivity of Fischer rats to aminoglycoside nephrotoxicity, an intrastudy comparison of the sensitivity of Fischer and Sprague-Dawley rats to aminoglycoside nephrotoxicity was undertaken. Tobramycin was administered at 3 dose levels to each strain of rat for 10 days. Nephrotoxicity was evaluated utilizing a spectrum of both functional and morphologic assessments of renal proximal tubular integrity. The results confirm that the aged matched Fischer rat is more sensitive to the nephrotoxic effect of tobramycin than the Sprague-Dawley. These results also suggest an opportunity to study quantitative and perhaps qualitative differences in pathogenic mechanisms of aminoglycoside nephrotoxicity in a single laboratory animal species.

Carcinogenicity testing of antitumor agents.

Carcinogenicity testing of antitumor agents in animal bioassays has been proposed because of the potential for carcinogenicity of this class of agents and the expectation that such testing may indicate prospectively the target organs of any related human oncogenesis. The literature reveals the anticipated confirmations in animals of the carcinogenicity of many antitumor agents. Furthermore, these agents have been associated with human tumors in numerous case reports. Review of the literature also indicates the inability of animal studies to predict the sites of carcinogen-induced tumors in man. The carcinogenic risk assessment of antitumor agents should begin with the determination of the ability of the agent to interact with DNA. Those agents which are capable of alkylating or binding DNA should be tested for mutagenic and teratogenic potential. The presumption of carcinogenicity should be made for DNA-reactive, mutagenic/teratogenic antitumor agents without requiring confirmation in long-term carcinogenicity bioassays in large numbers of animals. The inability of carcinogenicity studies in animals to accurately predict potential human tumor sites must also be recognized.

Review and Evaluation of the NCI/NTP Carcinogenesis Bioassays

A comparison of the carcinogenesis bioassay results obtained by the National Cancer Institute (NCI) and the National Toxicology Program (NTP) indicates that approximately one-half of the bioassays directed by both institutions were positive for carcinogenicity. The more recent 85 bioassays completed by NTP reveal a higher proportion of studies interpreted as demonstrating no evidence of carcinogenicity than represented in the initial 198 bioassays conducted by NCI. Of the 100 NCI bioassays that were not positive for carcinogenicity 3 (3%) were classified in the category of “no evidence for carcinogenicity in two animal species.” Of the 43 NTP bioassays that were not positive for carcinogenicity 36 (84%) were placed in the category of “no carcinogenic effects.” The reason for this shift from a 33:1 positive to negative ratio in the NCI bioassays to an approximately 1:1 ratio in the NTP bioassays appears to be a difference in interpretation of the adequacy of the testing. For example, 6 of the 36 NTP negative bioassays involved testing in only one species. Uniform criteria for concluding that a bioassay is negative must be developed and the results of all existing and future carcinogenesis bioassays must be interpreted with these exclusive criteria. Other bioassay problems are explored, including the incomplete validation of the carcinogenesis bioassay protocol by confirmatory results with positive and negative reference agents, the apparent lack of bioavailability data for some orally administered negative compounds, the continued use of mouse hepatic neoplasia as a single discriminating parameter, the variability in the inter- and intrastudy incidence of spontaneous tumors, and the continued reliance on the maximum tolerated dose.

Functional-Morphologic Correlations in Assessing Renal Toxicity*

Evaluation of the correlation of functional and morphologic assessments of renal toxicity are obviously important to the toxicologic pathologist. All clinical assessments of the nephrotoxicity of drugs are based on renal function tests while preclinical assessments involve functional and/or morphologic assessments in laboratory animals. However, there are considerable concerns about the sensitivity of the tests available to monitor renal function. In an attempt to assess the relative sensitivity of functional and morphologic indicators of nephrotoxicity, three data bases were examined for their ability to rank the nephrotoxicity of the three most widely used aminoglycosides. The functional data base was comprised of reported comparative clinical trial data and the morphologic data base was composed of preclinical studies available in the literature which used morphology as the basis of comparison. A third data base involved biochemical or mechanistic comparisons of aminoglycosides and represented data that was not based on clinical function test results or morphologic analysis. When these three data bases were compared, it was obvious that clinical function tests could not discriminate between the nephrotoxic liability of the three aminoglycosides. The preclinical renal morphology data base and the biochemical mechanistic data base both clearly discriminated the drugs one from another and ranked the three aminoglycosides in the same order. Several conclusions were suggested by these results. Comparisons of the nephrotoxic potentials of drugs in patients utilizing available renal function tests may be inconclusive primarily because clinical renal function tests are insensitive. Comparisons of the nephrotoxic liabilities of drugs in laboratory animals utilizing morphologic assessments of renal tissue damage may be the best comparison available. Such comparisons indicate that amikacin is less nephrotoxic than tobramycin which is less nephrotoxic than gentamicin.

Correlation Between the in vitro and in vivo Nephrotoxicity of Parenteral Antibiotics in the Rabbit

The ability of in vitro models to predict nephrotoxicity was studied by comparing in vivo and in vitro toxicities of parenteral antibiotics. Rabbits were treated with a single dose of carbapenem antibiotics. After 24 h, the toxicity ranking was imipenem > BMY-25174 > BMY-26225 based on blood urea nitrogen (BUN) and histopathology; previous studies showed the cephalosporin ranking to be cephaloridine > cefazolin > cephalothin. In vitro models used were from rabbit renal cortex and included primary proximal tubule cultures (RPTC), the cell line LLC-RK1, and cortical slices. Cell viability was determined after 24-h exposure of RPTC and LLC-RK1 monolayers to the drugs; in some cases the S9 fraction of cortical homogenate was added concomitantly. Based on the 50% effective concentration (EC50s), the toxicity ranking for cephalosporins in both culture models was cephalothin > cephaloridine > cefazolin > cephalothin + S9. The ranking of carbapenems in RPTC was imipenem > BMY-25174 > BMY-26225, but for LLC-RK1 wa...

Lack of in vivo evidence of a cytochrome P450 metabolite participating in aminoglycoside nephrotoxicity

Recent in vitro evidence has suggested that the cytotoxicity of aminoglycosides may be mediated by a metabolite generated by the hepatic cytochrome P450 drug-metabolizing system. This postulate has been tested by pretreating rats with cobalt protoporphyrin IX (CoP) to suppress hepatic P450 levels prior to administration of gentamicin. CoP pretreatment was observed to suppress antipyrine clearance markedly but not to alter gentamicin nephrotoxicity.