Schedule of administration of doxorubicin may influence risk of cardiomyopathy. One study looked at the effect of continuous (48 hour) versus bolus (1 hour) infusions of doxorubicin in 121 children who received a cumulative dose of 360 mg/m2 for treatment of acute lymphoblastic leukemia (ALL) and found no difference in the degree or spectrum of cardiotoxicity in the two groups. Because the follow-up time in this study was relatively short, it is not yet clear whether the frequency of progressive cardiomyopathy will differ between the two groups over time. Another study compared cardiac dysfunction in 113 children who received doxorubicin either by single-dose infusion or by a consecutive divided daily-dose schedule. The divided-dose patients received one-third of the total cycle dose over 20 minutes for 3 consecutive days. Patients treated according to a single-dose schedule received the cycle dose as a 20-minute infusion. There was no significant difference in the incidence of cardiac dysfunction between the divided-dose and single-dose infusion groups. Earlier studies in adults have shown decreased cardiotoxicity with prolonged infusion; thus, further evaluation of this question is warranted.
Prevention or amelioration of doxorubicin-induced cardiomyopathy is clearly important because the continued use of doxorubicin is required in cancer therapy. Dexrazoxane is a bisdioxopiperazine compound that readily enters cells and is subsequently hydrolyzed to form a chelating agent. Evidence supports its capacity to mitigate cardiac toxicity in patients treated with doxorubicin.[27,28,29,30,31] Studies suggest that dexrazoxane is safe and does not interfere with chemotherapeutic efficacy. There is a single-study experience suggesting that there could be an increase in malignancies when multiple topoisomerase inhibitors are administered in close proximity; other studies, however, do not show increased risk of malignancies.[31,32,33,34] However, at this time, this should not preclude treatment with dexrazoxane.[35,36]
Two closed Pediatric Oncology Group therapeutic phase III studies for Hodgkin lymphoma (HL) [36,37] measured myocardial toxicity clinically and sequentially over time by echocardiography and electrocardiography, and by determination of levels of cardiac troponin T (cTnT), a protein that is elevated after myocardial damage.[30,38,39,40,41,42] Long-term outcomes for these patients are not yet available.
The angiotensin-converting enzyme inhibitor enalapril has been used in the attempt to ameliorate doxorubicin-induced left ventricular dysfunction. Although a transient improvement in left ventricular function and structure was noted in 18 children, left ventricular wall thinning continued to deteriorate; thus, the intervention with enalapril was not considered successful. For this reason, studies to date in doxorubicin-treated cancer survivors have not demonstrated a benefit of enalapril in preventing progressive cardiac toxicity.[28,29]
A number of studies have examined cardiac function after radiation therapy and doxorubicin exposure using cardiopulmonary exercise stress tests and have found abnormalities in exercise endurance, cardiac output, aerobic capacity, echocardiography during exercise testing, and ectopic rhythms.[43,44,45,46,47] In addition to subclinical abnormalities of systolic function observed by conventional echocardiography, diastolic dysfunction (impaired ventricular relaxation) has also been observed, which may precede impairment of systolic function. Specific abnormalities of cardiac function may progress over time after therapy, as suggested by a report targeting parameters of left ventricular contractility. An increased prevalence of diastolic dysfunction has also been reported in childhood cancer survivors, consistent with the hypothesis of increased vascular and ventricular stiffness associated with precocious cardiovascular aging. It remains unclear whether these abnormalities will have clinical impact. Asymptomatic cardiac toxic effects can be demonstrated in patients who have normal clinical assessments, and abnormalities can be linked to lower self-reported health and New York Heart Association cardiac function scores.[51,52] Clearly, additional studies with long-term follow-up will be necessary to determine optimal screening modalities and frequencies.