Doxorubicin binds in a cooperative manner to myocardial cells. Two binding sites

Experimental evidence indicates that the anthracycline antibiotic doxorubicin (adriamycin) localizes mainly in cell nuclei of cardiac cells and has a high affinity to several cellular constituents in addition to DNA. In the present study the cellular kinetics of doxorubicin in cultured rat myocardial cells were determined by measuring its uptake, its binding pattern over a concentration range of 0.1 mM to 80 microM, and the cellular release by means of [14-14C]doxorubicin. The binding kinetics of doxorubicin were compared with the doxorubicin-induced inhibition of [methyl-3H]thymidine incorporation into DNA. It is demonstrated that at micromolar concentrations doxorubicin is readily taken up by myocardial cells and that myocardial cells have the ability to bind doxorubicin at two specific binding sites and that a noncooperative high-affinity/low-capacity type and a positive cooperative type of binding are involved, as indicated by the positive slope in the initial region of the binding isotherm (Scatchard plot). A dose-dependent inhibition of [methyl-3H]thymidine incorporation into DNA is demonstrated. It is suggested that this is associated with the positive cooperative binding of doxorubicin. The cellular release of doxorubicin appeared to be biphasic, with estimated half-lives of about 5-6 h for the initial phase and 50-60 h for the terminal phase. The results of this study indicate that doxorubicin preferably binds to sites within myocardial cells and that the positive cooperative binding pattern is due to DNA as one of the binding sites. A relationship between the noncooperative high-affinity/low capacity binding and the pharmacological activity has yet to be determined.