ANALYSIS AND PHARMACOKINETICS OF N-L-LEUCYLDOXORUBICIN AND METABOLITES IN TISSUES OF TUMOR-BEARING BALB/C MICE

Leucyldoxorubicin (Leu-Dox) was developed as a prodrug of doxorubicin (Dox) with the aim of lowering the cardiotoxicity and improving the therapeutic index produced by Dox. To support the preclinical findings on its antitumor activity and cardiotoxicity, concentrations of Leu-Dox and its metabolites were determined in plasma, heart, and tumor after the administration of Leu-Dox to tumor-bearing mice. A liquid-liquid extraction procedure employing a chloroform/2-propanol/dimethylsulfoxide (DMSO) mixture was developed. By means of high-performance liquid chromatography (HPLC) with fluorescence detection, Leu-Dox and six of its metabolites could be assayed in the tissues with high sensitivity. Detection limits ranged from 0.01 nmol/g tissue for the aglycons to 0.06 nmol/g for Dox. Recoveries were in the range of 82%-110%, and calibration curves were linear over the concentration range tested (0.1-10 nmol/g tissue, r greater-than-or-equal-to 0.998). Concentration versus time curves were constructed for plasma, heart, and tumor over the first 72 h, and areas under the curves (AUCs) for the first 48 h were determined by the trapezoidal rule. Dox was rapidly formed from Leu-Dox, reaching peak levels in plasma within 5 min and in tissues within 1 h after i.v. administration of Leu-Dox (12 mg/kg). The elimination of Leu-Dox was also fast as illustrated by final half-lives of 1.1, 0.8, and 0.9 h in the plasma, heart, and tumor, respectively. For Dox, the final half-lives were 16.7 h in plasma, 15.3 h in heart tissues, and 27.4 h in tumor tissues. AUC values determined for Leu-Dox and Dox were 221 and 51 nmol ml-1 min, 443 and 4,262 nmol g-1 min, and 153 and 1,466 nmol g-1 min in the plasma, heart, and tumor, respectively. Comparison of these values with those obtained after an equimolar dose of Dox indicated 26%, 30%, and 16% of Leu-Dox appeared as Dox in the plasma, heart, and tumor, respectively. Thus, not only is the plasma compartment not representative for calculations of the conversion of Leu-Dox into Dox in tissue, but differences in its appearance also exist between the tissue compartments. The AUC values found for Dox in the heart may explain the reduced cardiotoxicity elicited by Leu-Dox as compared with Dox; however, the values obtained for Dox in the insensitive murine colon tumor cannot explain the enhanced antitumor activity exerted by Leu-Dox in the sensitive human tumor xenografts in nude mice.