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Recent work suggests that adriamycin (ADM) cardiotoxicity results from the depletion of coenzyme 10 (CoQ10) activity in myocardial mitochondria. CoQ10 is indispensable in the bioenergetics of coupled respiration of oxidative phosphorylation. It exists naturally in mitochondria, especially in the myocardium. Bertazzoli et al have reported a decrease in ADM-induced cardiotoxicity by CoQ10 both in vivo and in the in vitro isolated rabbit heart. The systolic time interval (ATI) (pre-ejection period/left ventricular ejection time ratio) has been shown to increase (indicating cardiac dysfunction) with increasing doses of ADM. We have noted a gradual increase in the STI in eight of ten patients receiving 200-500 mg/m2 of ADM. Two of these eight patients had congestive heart failure (CHF) at doses of 200 and 350 mg/m2. The continued daily oral administration of 50 mg of CoQ10 beginning with the first dose of ADM resulted in a decreased incidence of cardiac dysfunction, and a gradual increase in STI occurred in only two of eight patients receiving 200-400 mg/m2 of ADM. CHF was observed inone patient at a dose of 350 mg/m2. It is suggested that CoQ10 was nontoxic and did not affect the antitumor activity or modify the ADM-induced bone marrow toxicity. A prospective randomized study comparing ADM with and without CoQ10 is in progress.