Drug Induced Retinopathy

Morphologic characteristics of N-methyl-N-nitrosourea-induced retinal degeneration in C57BL mice

Morphologic characteristics of retinal degeneration induced by a single systemic administration of N-methyl-N-nitrosourea (MNU) in mice was investigated. The aim was to characterize the MNU-induced retinal lesions in mice and compare them with human retinitis pigmentosa. A dose of 60 mg/kg body weight MNU, injected intraperitoneally into male and female C57BL mice, evoked progressive retinal degeneration in all treated mice, while control mice remained normal. An early change was photoreceptor apoptosis followed by infiltration of macrophages and swelling of the pigment epithelial cells with phagosomal inclusions for apoptotic photoreceptor cell removal. Loss of the majority of photoreceptor cells occurred within a week. Then, Feulgen-positive corpuscles, indicative of an aggregation of degenerative photoreceptor elements, vitread the outer limiting membrane were surrounded by Müller cell processes, and the duplication of the pigment epithelial cells sclerad the outer limiting membrane were seen 2 and 3 weeks after the treatment. Finally, the Feulgen-positive corpuscles disappeared and Müller cell processes were in direct contact with the continuous lining of the single layer of pigment epithelial cells. As in retinitis pigmentosa in humans, the primary event was loss of photoreceptor cells by apoptosis, but the migration of the pigment epithelial cells within the retina was not seen in the present model.

Chromatographic modelling of interactions between melanin and phenothiazine and dibenzazepine drugs

Differences in drug-melanin interactions were determined for 13 phenothiazine neuroleptics and 2 dibenzazepine thymoleptics by means of high-performance liquid chromatography. The chromatographic column was packed with a stationary phase obtained by chemical immobilization of synthetic L-dopa melanin on silica particles. For six phenothiazines the melanin-binding parameters were also determined by an ultrafiltration method. Correlation between measures of drug-melanin interaction determined chromatographically and by the standard slow-equilibrium method was significant, however moderate. The chromatographic method of assessing interactions between drugs and melanin permitted reliable and quantitatively comparable data for representative series of solutes to be readily obtained. Such data were subjected to the analysis of quantitative structure-retention relationships (QSRR). It was found that retention of the agents on the immobilized melanin column could be described by two-parameter regression equations comprising the energy of the lowest unoccupied molecular orbital and either the water-accessible surface area of a drug molecule or its hydrophobicity parameter, determined chromatographically on an immobilized artificial membrane column. The QSRR equation derived allows for the estimation of melanin binding based on the structure of a compound candidate, and thus rationalizes predictions of potential toxicity of drugs or drug candidates.

Retinal degeneration induced by N-methyl-N-nitrosourea and detection of 7-methyldeoxyguanosine in the rat retina

Retinal degeneration induced by a single parenteral dose (40 mg) of N-methyl-N-nitrosourea (MNU) was studied in rats and rabbits. Sequential observations showed that severe pathological changes, that is, complete or widespread destruction of the photoreceptor layer and the outer nuclear layer of the retina, occurred in both animals by 72 hr after the administration of MNU. At week 5, at the end stage of the retinal degeneration induced by MNU, thin remnants remained with the inner nuclear layer in direct contact with the pigment epithelium. An additional immunohistochemistry study using antibodies against 7-methyldeoxyguanosine (7-mdGua), revealed the presence of high levels of 7-mdGua DNA adducts in the nuclei of the outer nuclear layer cells. This finding suggests that the alkylation of the outer nuclear layer by MNU was the direct cause of the retinal degeneration seen in these animals.

Toxicity study in rats of a tellurium based immunomodulating drug, AS-101: a potential drug for AIDS and cancer patients

Male and female Sprague Dawley rats were injected intraperitoneally for 4 weeks with ammonium trichloro (dioxyethylene-0-0'-) tellurate, an immunomodulating drug at doses ranging from 3 to 24 mg/kg/week. Routine laboratory examinations included body weight, food consumption, clinical chemistry and hematological examinations. At termination of the experiment, all rats were sacrificed and subjected to a detailed necropsy. Few mortalities were recorded during the course of the study. Clinical signs included hind limb paresis and paraphimosis. A garlic odor pervaded the room. Body weight and food consumption were adversely affected in a dose-related manner. Effects were elicited on the hematological system; changes being noted in the platelet and leukocyte counts as well. Clinical chemistry evaluation revealed signs of hepatoxicity, especially in the female treated groups. The level of beta-globulin was increased. At necropsy organs were found to have a grayish-blue discoloration. Tellurium related histopathological changes were observed in the eyes, liver, thymus, bone marrow, heart and kidneys. An attempt has been made to compare the toxicity of this drug with other tellurium-containing compounds. A good correlation was found. Novel effects of the drug were retinopathy and replacement of bone marrow by bony or fibrous tissue. The possibility that some of the effects may have been elicited due to selenium-vitamin E deficiency has been considered.

Toxicological screening

Well-defined dose- and time-related toxic effects of chemicals can often be detected using simple tests with small numbers of animals. The strategies for the establishment of toxicological screening tests are discussed. The most important steps are the definition of the targets, the selection of the methods, and the setting of the test criteria. Screening tests must then be validated with standard reference chemicals, and the test criteria must be adjusted so that the standards can be detected regularly. Maximal flexibility is allowed in the design of the tests. For evaluation, the results obtained in treated animals can be compared with those of controls, using conventional concepts of biostatistics. It is also possible to base the evaluation on preset test criteria. Toxicological screening tests do not replace conventional safety studies, but they help in selecting the most promising candidates in a series of related chemicals and in establishing priorities for further testing. As an example, a screening for the hemolytic effects of chemicals is presented.

Studies of the mechanism of chloroquine binding to synthetic DOPA-melanin

In order to elucidate the mechanism of drugs binding to melanin, effects of pH, ionic strength and organic solvent on the interaction of chloroquine with synthetic dopa-melanin were studied. The results indicate that electrostatic, hydrophobic and van der Waals' forces participate in the formation of the chloroquine-melanin complex. Binding analysis by the Scatchard method showed that two classes of binding sites take part in the complex formation: strong binding sites with the association constant k1 approximately to 10(5) and weak binding sites with K2 approximately 10(4). Experiments with chemically modified melanin yielded some information about binding sites of this biopolymer. The obtained results suggest that strong binding involves both hydrophobic interaction and electrostatic attraction between the protonated ring system of chloroquine and the ortho-semiquinone groups of melanin. However, the weakly reacting sites can be identified as ionic bonds between protonated aliphatic nitrogen of chloroquine molecule and carboxyl groups of melanin. Van der Waals' forces occurring at the conjunctions of the aromatic rings of the drug and the aromatic indole-nuclei of the melanin probably take part in the weak binding too.