A compound of the formula: ##STR1## wherein R.sup.1 represents a lower alkyl; R.sup.2 represents H, an optionally substituted alkyl or an optionally substituted alkenyl; R.sup.3 and R.sup.4 each represents an optionally substituted lower alkyl or a lower alkoxy, or R.sup.3 and R.sup.4 form, taken together, an optionally substituted butadienylene; and X.sup.1 and X.sup.2 each represents an optionally esterified or etherified hydroxy, or a salt thereof is useful for protecting cells from the cytotoxicity of .beta.-amyloid protein.
Methods of treating or suppressing mitochondrial diseases, such as Friedreich's ataxia (FRDA), Leber's Hereditary Optic Neuropathy (LHON), mitochondrial myopathy, encephalopathy, lactacidosis, stroke (MELAS), or Kearns-Sayre Syndrome (KSS) are disclosed, as well as compounds useful in the methods of the invention. Energy biomarkers useful in assessing the metabolic state of a subject and the efficacy of treatment are also disclosed.
