Synthesis of a lipophilic daunorubicin derivative and its incorporation into lipidic carriers developed for LDL receptor-mediated tumor therapy.

Abstract

Many tumors express elevated levels of LDL receptors (apoB, E receptors) on their membranes. Selective delivery of anti-neoplastic drugs to tumors by incorporation of these drugs into LDL or LDL-resembling particles should improve the efficacy of tumor therapy and minimize the severe side-effects. Since the apolipoproteins on the particles are essential for the LDL receptor recognition, drugs should preferably be incorporated into the lipid moiety. Most anti-tumor agents are too hydrophilic for incorporation into these carriers. We synthesized LAD, a lipophilic prodrug of daunorubicin, by coupling the drug via a lysosomally degradable peptide spacer to a cholesteryl oleate analog. The overall yield of the synthesis was 50% with a purity of > 90%. Radioactively ([3H]) labeled LAD was obtained via a slightly modified procedure (yield 40%). The octanol/water partition coefficient of LAD is 30-fold higher than that of daunorubicin. LAD could be incorporated into triglyceride-rich lipid emulsions and small liposomes, which, if provided with apoE, have been demonstrated earlier to be cleared in vivo via the LDL receptor. The liposomes contained approximately 10 molecules of LAD per liposomal particle. Analysis of differently sized LAD-containing emulsions suggests that LAD associates with the surface of lipidic particles. In the presence of human serum, LAD did not dissociate from the emulsion particles, indicating a firm association of LAD with the carrier. The coupling of a cholesterol ester analog to daunorubicin results in a lipophilic prodrug that can be firmly anchored into lipidic carries. LAD-loaded emulsions and liposomes provided with recombinant apoE will be tested in the near future for their ability to deliver LAD to tumor tissue in vivo via the LDL receptor.