Neurotoxicity of amphotericin B methyl ester in dogs

Amphotericin B potentiates the activation of inducible nitric oxide synthase and causes nitric oxide-dependent mitochondrial dysfunction in cytokine-treated rodent astrocytes

Because the neurotoxic effects of the antifungal drug amphotericin B (AMB) closely resemble those ascribed to the highly reactive gaseous free radical nitric oxide (NO), we investigated the effect of AMB on NO production in rodent astrocytes. AMB caused a dose-dependent increase of NO generation in interferon-gamma (IFN-gamma)-stimulated rat and mouse astrocytes, as well as in IFN-gamma + tumor necrosis factor-alpha (TNF-alpha)-activated rat astrocytoma cell line C6. Treatment of rat astrocytes with AMB markedly potentiated IFN-gamma-triggered expression of mRNA for iNOS, but not for its transcription factor IRF-1. The activation of transcription factor NF-kappaB was apparently required for AMB-induced iNOS mRNA expression, as the latter was abolished by NF-kappaB inhibitors: pyrrolidine dithiocarbamate and MG132. AMB-mediated enhancement of astrocyte NO production was partly dependent on endogenous IL-1, as shown by partial inhibition of AMB effect with IL-1 receptor antagonist. IFN-gamma + AMB treatment led to reduction of astrocyte mitochondrial respiration (measured by MTT assay) that has been completely reverted by selective iNOS inhibitor aminoguanidine. AMB toxicity toward IFN-gamma-stimulated astrocytes was dependent on both AMB and NO action, since AMB and NO-releasing substance SNP synergized in inducing astrocyte mitochondrial dysfunction. These results suggest that the enhancement of cytokine-induced iNOS activation in astrocytes and the subsequent release of high amounts of NO might be at least partly responsible for AMB neurotoxicity.

Comparative neurotoxicities of amphotericin B and its mono-methyl ester derivative in rats

The intracisternal administration of amphotericin B (AmB) and its mono-methyl ester derivative (AME), via direct intraventricular injection (0.01 to 5 mg/ml, 6 microliters) in adult female Wistar rats, revealed that AmB was significantly more toxic than AME, as measured by weight loss, lethargy, death, and central nervous system histopathology. Light and electron microscopy confirmed a greater neurotoxicity for AmB, manifested as edema and modest gliosis extending along and beyond the injection tract. Neuronal degeneration and myelin damage were present in AmB-treated (1 mg/ml) animals but were present only modestly in animals treated with AME at a fivefold greater concentration. Intravenous administration of AmB to adult female Wistar rats as five daily doses of 5 mg/kg of body weight resulted in significant weight loss and some deaths. Histopathologic examination of the brains, spinal cords, and sural nerves of surviving animals revealed neurotoxicity manifested by neuronal degeneration, gliosis, and myelin edema. In sharp contrast, similar treatment with AME at a 10-fold greater dose resulted in neither death nor significant neurotoxicity. The administration of five daily doses of a mixture of AME-AmB (9:1; wt/wt) at 50 mg/kg of body weight resulted in neurotoxicity. These results indicate that AmB exhibits significantly greater in vivo neurotoxicity than AME.

Comparative toxicities of amphotericin B and its monomethyl ester derivative on glial cells in culture

Amphotericin B (AmB) is a potent antifungal polyene macrolide antibiotic and is the drug of choice for the treatment of deep-seated mycotic infections. Its use is limited, owing to its nephrotoxicity, and it must be dispersed in deoxycholate for parenteral administration. In contrast, AME (the monomethyl derivative of AmB) is water dispersible, is appreciably less cytotoxic than AmB toward a variety of cell types, and is reportedly active against the acquired immunodeficiency syndrome virus (human immunodeficiency virus type 1). The latter activity has generated interest in AME as an antiviral drug. However, AME is perceived to be neurotoxic, based on the outcome of a human clinical trial of AME as an antifungal drug. AmB is not regarded as neurotoxic, presumably because any neurotoxicity in vivo is precluded by its nephrotoxicity. It was important, therefore, to determine the potential for neurotoxicity of the two agents in comparative tests, assessing the effects of their direct action against neural cells in culture. Rat cortical cells, comprising astrocytes and oligodendrocytes, were used. AME was at least 10 times less toxic than AmB and equally less toxic against several other nonneural cell types also included in these tests. Equally important, AmB disrupted the myelin sheath in these cultures, and it inhibited its generation. AME did not, even at a concentration 10 times greater than the toxic concentration of AmB. AmB is, therefore, potentially more neurotoxic than AME, contrary to current perception. AME is effective as an antifungal and antiviral drug at a concentration far below its toxic concentration for neural cells. Also, AME does not cross the blood-brain barrier appreciably, so that a therapeutic level in blood can be expected without encountering neurotoxicity.

In vitro and in vivo evaluation of antifungal agents

The evaluation of any antifungal agent involves the determination of its in vitro and in vivo activity against pathogenic and/or opportunistic fungi. The in vitro evaluation is followed by an in vivo evaluation in animal models, and clinical trials in humans. From the first report of the efficacy of the iodides for the treatment of sporotrichosis (1903) until the introduction of the imidazoles (azoles, 1960s), the number of antifungal agents available was very limited, including griseofulvin (1939), nystatin (1950), amphotericin B (1956), and flucytosine (1964). This paper briefly reviews the status of the antifungal agents currently used, and gives a more in depth evaluation of progress during recent years in the search for new antifungal drugs. Efforts to improve the efficacy of the current antifungal agents are also reviewed.