Abstract 992: A switch in mechanism of action attenuates doxorubicin-mediated cardiotoxicity

Chemotherapy remains as the mainstay systemic therapy for triple negative breast cancer patients since targeted therapies are not suitable for this breast cancer subtype. Among the chemotherapies for triple negative breast cancer, doxorubicin is known to be effective but its use is hampered by a risk of developing doxorubicin-mediated cardiomyopathy. Doxorubicin causes cardiotoxicity primarily through topoisomerase-IIβ poisoning, resulting in cardiomyocyte apoptosis. Notably, cardioprotection has been reported after combination treatment of doxorubicin with an acyloxyalkyl ester, AN-7 in an in vivo model. As an acyloxyalkyl ester, AN-7 also releases formaldehyde upon esterase hydrolysis. Therefore, the aim of this study was to elucidate the mechanism of cardioprotection conferred by the co-treatment of doxorubicin with formaldehyde-releasing prodrugs in BALB/c mice bearing mammary 4T1.2 tumours. Both experimental and clinically available formaldehyde-releasing prodrugs were investigated in this triple negative breast cancer model. Cardiac damage was induced with a single dose of doxorubicin at 16 mg/kg in mammary tumour-bearing mice. Combined treatment of doxorubicin and formaldehyde-releasing prodrugs significantly reduced γH2AX foci (a marker for topoisomerase-IIβ poisoning) and protected the heart from doxorubicin-induced myocardial apoptosis. To investigate if the combination treatments were cardioprotective due to alteration in doxorubicin biodistribution, the initial response to the first dose of doxorubicin was investigated by treating tumour-bearing mice with 4 mg/kg 14C-doxorubicin. Addition of formaldehyde-releasing prodrugs enhanced formation of formaldehyde-mediated doxorubicin-DNA adducts in the hearts, independent of doxorubicin biodistribution in the heart, blood and tumours. This doxorubicin-DNA adduct forming treatment was associated with a novel cardiac transcriptomic profile as compared to doxorubicin single agent treatment, as assessed by RNA-sequencing. These combination treatments also maintained doxorubicin antitumour efficacy with respect to inducing dsDNA breaks, promoting apoptosis and inhibiting mammary tumour proliferation. The switch in doxorubicin action from cardiac topoisomerase-IIβ poisoning to doxorubicin-DNA adduct formation protects the heart from doxorubicin-mediated cardiac damage. Hence, the combination treatment of doxorubicin and formaldehyde-releasing prodrugs may be a promising cardioprotective therapy that does n