Mitochondrial and cellular toxicity induced by fialuridine in human muscle in vitro

Fialuridine (FIAU), when used experimentally in the treatment of patients with chronic hepatitis B, caused irreversible acute hepatic failure, myopathy, myoglobinuria, severe lactic acidosis, neuropathy, and death. To investigate the primary cellular elements involved in the neuromuscular toxicity of FIAU, we examined its effects on human myotubes in cultures and searched for signs of recovery. From a total of 75 flasks of normal myotubes prepared from human muscle biopsies, 63 were exposed to various concentrations of FIAU (0.01 microM, 0.1 microM, 1 microM, 10 microM, 50 microM, and 100 microM) for 1 to 3 weeks, whereas 12 served as controls. After 3 weeks of FIAU treatment, 27 flasks were observed for 3 additional weeks to assess spontaneous recovery. All cultures were evaluated with: (a) light microscopy; (b) quantitative immunocytochemistry examining the number of myotubes immunostained for neural-cell adhesion molecule (N-CAM); (c) Oil-Red-O stain, to assess the lipid droplet accumulation; and d) electron microscopy with morphometric measurements of the volumetric density of each organelle per unit volume of tissue (magnification, x24,000). After 3 weeks of FIAU treatment, we found a severe reduction in the number of N-CAM-positive myotubes that varied according to the concentration of FIAU and the duration of treatment. Electron microscopy demonstrated a varying degree of destruction of the myotubes with significant increase in lipid droplets, lysosomes, and the rough endoplasmic reticulum. Major changes in mitochondria were noted even early in the treatment and consisted of concentric lamellar structures, paracrystalline inclusions, and vacuolization. These abnormalities remained unchanged without signs of recovery for up to 3 weeks after withdrawal of FIAU. We conclude that FIAU induces mitochondrial changes and intracellular lipid accumulations similar, but more severe, to those described with the other nucleoside analogues, such as zidovudine. In contrast to zidovudine, however, the FIAU-induced abnormalities do not improve or reverse after withdrawal of the drug. The observations are consistent with the irreversible mitochondrial changes noted in the FIAU-treated patients due to defective mitochondrial DNA replication.