Interaction of a doxorubicin-transferrin conjugate with isolated transferrin receptors

Drug targeting systems for cancer chemotherapy

Tumour specific drug targeting has been a very actively investigated area for over 2 decades. Various approaches have involved the use of drug delivery systems that can localise the anticancer agent at the tumour site without damaging the normal cells. For this purpose, various delivery systems that have been utilised are liposomes, microspheres and recently, nanoparticles. Two liposome formulations containing anticancer drugs for example, adriamycin and daunomycin are already on the market in the USA and Europe. Microspheres are also being investigated for delivering various anticancer drugs and protein/peptides for anticancer treatment, and several formulations are in Phase I/II clinical trials. Antitumour drugs have also been linked to tumour specific monoclonal antibodies via various chemical linkages. Doxorubicin was linked to a chimeric monoclonal antibody that was targeted to the Lewis Y antigen. Though this conjugate initially showed potential, it was recently dropped from Phase II clinical trials. Another approach with monoclonal antibodies has been the use of immunotoxins. Immunotoxins initially showed promise as potential anticancer agents at picomolar concentrations but several clinical and preclinical studies have not shown much promise in this regard. Drug containing liposomes and microspheres have been further linked to tumour specific monoclonal antibodies to enhance their tumour specificity. Most of the studies with immunoliposomes or targeted microspheres have not gone beyond the preclinical studies. New therapeutic approaches are presently emerging based on natural products like cytokines, peptide growth factor antagonists, antisense oligonucleotides and specific genes. These approaches need the help of delivery systems to deliver these complex molecules to tumour cells. One of the current pursued approaches is the use of cationic liposomes. Several clinical studies are undergoing with various cationic liposomes and the next few years will demonstrate the usefulness of this approach. In recent years, the problems in cancer treatment have been complicated with the emergence of resistance strains leading to resistant and cross-resistant tumour cells. Several agents have been used to overcome or reverse drug-resistance in solid tumours and it remains a highly pursued area in cancer treatment.

Enhanced Antiproliferative Activity of Transferrin-Conjugated Paclitaxel-Loaded Nanoparticles Is Mediated via Sustained Intracellular Drug Retention

We studied the molecular mechanism of greater efficacy of paclitaxel-loaded nanoparticles (Tx-NPs) following conjugation to transferrin (Tf) ligand in breast cancer cell line. NPs were formulated using biodegradable polymer, poly(lactic-co-glycolide) (PLGA), with encapsulated Tx and conjugated to Tf ligand via an epoxy linker. Tf-conjugated NPs demonstrated greater and sustained antiproliferative activity of the drug in dose- and time-dependent studies compared to that with drug in solution or unconjugated NPs in MCF-7 and MCF-7/Adr cells. The mechanism of greater antiproliferative activity of the drug with conjugated NPs was determined to be due to their greater cellular uptake and reduced exocytosis compared to that of unconjugated NPs, thus leading to higher and sustained intracellular drug levels. The increase in antiproliferative activity of the drug with incubation time in MCF-7/Adr cells with Tf-conjugated NPs suggests that the drug resistance can be overcome by sustaining intracellular drug retention. The intracellular disposition characteristics of Tf-conjugated NPs following their cellular uptake via Tf receptors could have been different from that of unconjugated NPs via nonspecific endocytic pathway, thus influencing the NP uptake, their intracellular retention, and hence the therapeutic efficacy of the encapsulated drug.

The plasma membrane NADH-diaphorase is active during selective phases of the cell cycle in mouse neuroblastoma cell line NB41A3. Its relation to cell growth and differentiation

Plasma membrane oxidoreductases have been described in all cells and use extracellular impermeant electron acceptors (DCIP, Ferricyanide) that are reduced by NADH. They appear to regulate the overall cell activity in response to oxidative stress from the cellular environment. An NADH-DCIP reductase has been described at the plasma membrane of NB41A3, a neuroblastoma cell line (Zurbriggen and Dryer (1993) Biochim. Biophys. Acta 1183, 513-520) whose activation with extracellular impermeant substrates promotes cell growth. Elutriation was performed to separate cells and the various fractions were analysed for enzyme activity on intact cells combined with flow cytometry. These studies showed that the enzyme is mostly induced and activated during the G1 and during the G2/M-phases. These observations were further corroborated with specific inhibitors of the cell cycle. A three-fold increase in enzyme activity was observed in the presence of alpha-amanitin, a specific cell cycle inhibitor of the G1-phase. Taxol, a specific inhibitor of the M-phase, also induces a significant increase in enzyme activity. FACS analysis of taxol -treated and alpha-amanitin-treated cells corroborated these data. The cells have been synchronized and the enzyme activity was measured at different time intervals. An activity increase was observed after ca. 2-3 h, that corresponds to a raise in the M-phase, according to FACS data. Furthermore, NTera-2 cells - a human neuroblastoma cell line that differentiates into fully mature neurones in the presence of retinoic acid - exhibit a 50% decrease in the enzyme activity during the G0-phase upon differentiation, compared to undifferentiated cells. Together the data presented in this paper show that this plasma membrane NADH-diaphorase affects cell growth and differentiation and is strongly modulated at various phases of the cell cycle.

Cytotoxic effects of a doxorubicin-transferrin conjugate in multidrug-resistant KB cells

Cancer chemotherapy is often limited by the emergence of multidrug-resistant tumor cells. Multidrug resistance (MDR) can be caused by amplification of the MDR genes and overexpression of the P-glycoprotein, which is capable of lowering intracellular drug concentrations. A doxorubicin-transferrin conjugate has been synthesized and exerts its cytotoxic effects through a transmembrane mechanism. We have examined the cytotoxicity of this conjugate and compared it with doxorubicin in sensitive (KB-3-1) and in multidrug-resistant KB cell lines (KB-8-5, KB-C1, and KB-V1). In the clonogenic assay, doxorubicin exhibited IC50 concentrations of 0.03 and 0.12 microM in the sensitive (KB-3-1) and resistant (KB-8-5) cell lines, respectively, whereas, doxorubicin-transferrin conjugate was more effective with IC50 concentrations of 0.006 and 0.028 microM, respectively. In highly multidrug-resistant KB-C1 and KB-V1 cells, doxorubicin up to 1 microM did not cause any cytotoxic effects, while the doxorubicin-transferrin conjugate inhibited colony formation of these cells with IC50 levels of 0.2 and 0.025 microM, respectively. These results demonstrate that doxorubicin-transferrin is effective against multidrug-resistant tumor cells.