Interaction of neomycin, tobramycin and amikacin with melanin in vitro in relation to aminoglycosides-induced ototoxicity

The aim of this study was to examine in vitro the interaction between aminoglycoside antibiotics displaying adverse ototoxic effects and melanin which is a constituent of the inner ear. The binding of neomycin, tobramycin and amikacin to model synthetic melanin was studied. It has been demonstrated that all the investigated aminoglycosides form stable complexes with melanin biopolymer. The obtained results show that the amount of drug bound to melanin increases with the increase of initial drug concentration and the incubation time. An analysis of drugs binding to melanin by the use of Scatchard plots has shown that at least two classes of independent binding sites must be implicated in the studied aminoglycoside antibiotic-melanin complexes formation: strong binding sites (n1) with the association constant K1 approximately 0.2-2.0 x 10(5) M(-1) and weak binding sites (n2) with K2 approximately 1.0-4.9 x 10(3) M(-1). Based on the values of association constants the following order of drugs affinity to DOPA-melanin was found: tobramycin > amikacin >> neomycin. The ability of the analyzed aminoglycoside antibiotics to form complexes with melanin in vitro may be one of the reasons for their ototoxicity in vivo, as a result of their accumulation in melanin in the inner ear.

Chronic intracerebroventricular infusion of MCH causes obesity in mice. Melanin-concentrating hormone

Melanin-concentrating hormone (MCH) is a cyclic amino acid neuropeptide localized in the lateral hypothalamus. Although MCH is thought to be an important regulator of feeding behavior, the involvement of this peptide in body weight control has been unclear. To examine the role of MCH in the development of obesity, we assessed the effect of chronic intracerebroventricular infusion of MCH in C57BL/6J mice that were fed with regular or moderately high-fat (MHF) diets. Intracerebroventricular infusion of MCH (10 microg/day for 14 days) caused a slight but significant increase in body weight in mice maintained on the regular diet. In the MHF diet-fed mice, MCH more clearly increased the body weight accompanied by a sustained hyperphagia and significant increase in fat and liver weights. Plasma glucose, insulin, and leptin levels were also increased in the MCH-treated mice fed the MHF diet. These results suggest that chronic stimulation of the brain MCH system causes obesity in mice and imply that MCH may have a major role in energy homeostasis.

Binding of drugs to eye melanin is not predictive of ocular toxicity

Ocular melanin is found in the uveal tract and in the pigmented epithelial layer of the retina. Many structurally and pharmacologically unrelated drugs from different therapeutic classes bind to melanin. Examples include numerous drugs acting on the central nervous system, beta-blockers, beta-agonists, antimalarial drugs, sympathomimetic amines, and antibiotics. The critical factors are the acid/base status and the lipophilicity of the molecule. In all cases, there are no direct consequences of drug-melanin binding. Drug-related toxic effects on the retina described in humans and animals are unrelated to melanin binding: melanin binding and retinal toxicity are two separate entities, the latter being related to the intrinsic toxicity of the compound rather than its ability to bind. Chloroquine and phenothiazines are often used as examples of drugs with retinal toxicity linked to melanin binding. In both cases however, experimental data show that the toxic mechanism is unrelated to binding. Melanin binding has also been found to be protective against the ocular toxicity of some drugs. In conclusion, we believe that potential ocular toxicity of future drugs can be assessed adequately by conducting well-designed toxicology studies, and using nonpigmented rodents in addition to pigmented nonrodent species remains fully justified. Binding of drugs to eye melanin is not predictive of ocular toxicity.

Induction of experimental autoimmune anterior uveitis by a self-antigen: melanin complex without adjuvant

PURPOSE

Experimental autoimmune anterior uveitis (EAAU) is an organ-specific autoimmune disease induced by immunization with bovine melanin-associated antigen (MAA) and two adjuvants (complete Freund's adjuvant and purified pertussis toxin). This study was undertaken to explore whether an adjuvant is required in the induction of EAAU.

METHODS

Insoluble MAA was extracted from the bovine iris and ciliary body. Soluble bovine MAA was derived by treatment of insoluble MAA with the proteolytic enzyme, V8 protease. Lewis rats were immunized with the insoluble or soluble antigen, with or without adjuvant (complete Freund's adjuvant and purified pertussis toxin). Adoptive transfer of CD4+ and CD8+ T cells was performed to investigate the pathogenesis of EAAU.

RESULTS

Experimental autoimmune anterior uveitis can be induced in Lewis rats by immunization with 100 g insoluble bovine MAA alone without the use of adjuvants. The disease can be adoptively transferred to naive syngenic rats by primed CD4+ T cells. In contrast, soluble bovine MAA was not uveitogenic unless adjuvants were employed.

CONCLUSIONS

The data suggest that EAAU can be induced in the Lewis rat without addition of an adjuvant. Future studies concerning the pathogenesis of EAAU can now be performed without the possible confounding effect of an adjuvant.

A paramagnetic agent causing ochronotic arthropathy

Magnetic resonance imaging (MRI) identified a paramagnetic substance in the hyaline cartilage of the hips and knees in a patient with ochronosis. Chemical studies characterized the paramagnetic agent as melanin. The free radicals contained in melanin were shown to initiate cytotoxicity. The loss of cartilage in ochronotic arthropathy now can be explained at the electron level using the superoxide theory of oxygen toxicity. Inappropriate metabolism of oxygen also may explain early cartilage degeneration in hemochromatosis, hemosiderosis, and Wilson's disease.

Mutagenicity of melanin from human red hair

The Salmonella typhimurium histidine reversion test of Ames et al. was used to demonstrate the pheomelanin, the red-brown polymeric pigment produced in human skin and hair, becomes mutagenic after exposure to long wave-length UV-light; a finding consistent with the UV-induced somatic mutation hypothesis for the origin of freckles and the high susceptibility of redheads and blonds to sunlight-induced skin cancers.