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Advanced targeted drug delivery systems for cancer therapy

T Minko,Department of Pharmaceutics, Rutgers,
The State University of New Jersey

The long-term objective of our research is to develop a novel four component targeted proapoptotic anticancer drug delivery system which includes (I) a carrier; (2) a cell surface targeting moiety; (3) an anticancer drug as an inducer of cell death and (4) a suppressor of antiapoptotic cellular defense. We hypothesise that the use of this DDS will significantly increase the efficacy of cancer treatment by: (I) targeting anticancer drug specifically to cancer cells; (2) simultaneous induction of programmed cell death and (3) the suppression of the main antiapoptotic cellular defense mechanisms. The present study examines the role of the individual components and the molecular targets of proposed drug delivery system (DDS). Based on our prior experience we selected polyethylene glycol (PEG) polymer as a carrier for the DDS. Leutinizing homlone-releasing hormone (LHRH) has been selected as a targeting moiety to deliver the DDS specifically to ovarian cancer cells and facilitate its cellular uptake. Camptothecin (CPT) -a potent anticancer drug with a novel mode of action involving the nuclear enzyme topoisomerase la -was selected as an apoptosis inducing agent and synthetic BCL2 homology 3 (BH3) domain peptide was used to suppress cellular defense.
The results showed that: (I) the targeted receptor for LHRH peptide is overexpressed in ovarian cancer cells and does not express in normal cells; (2) LHRH as a targeting moiety significantly enhanced cellular uptake of BH3 peptide; (3) BH3 peptide alone does not induce significant mitochondrial dysfunction; (4) BH3 peptide shifts the ratio of pro-apoptotic to anti-apoptotic members of BCL2 protein family towards the predominant expression of pro-apoptotic member; (5) conjugation of CPT to PEG polymer significantly enhances its cytotoxicity; (6) BH3 and LHRH peptides sigtuficantly et1hance anticancer activity of CPT -PEG conjugate; (7) BH3 peptide inhibits cellular antiapoptotic defense and therefore increases the ability of CPT -PEG conjugate to activate caspase-dependent pathways of apoptosis; (8) LHRH peptide enhances the ability of CPT -PEG conjugate to activate caspase-dependent signaling pathway of apoptosis; (9) BH3 and LHRH peptides significantly increase apoptosis induction by CPT-PEG conjugate.
Taken together results indicate much higher cytotoxicity and apoptosis inducing activity of PEG- CPT conjugates when compared to free CPT. Moreover, the effects of targeted CPT-PEG-BH3 and CPT -PEG-LHRH conjugates were more pronounced than non-targeted PEG-CPT conjugate. The data allow us to suggest that the two tier targeted CPT -PEG-BH3-LHRH conjugate, which we are currently developing, will demonstrate higher cytotoxicity and ability to induce apoptosis than one tier targeted CPT-PEG-BH3 and CPT-PEG-LHRH conjugates. Moreover, the combination of anticancer drug, BH3 and LHRH peptides in one drug delivery system (CPT- PEG-BH3-LHRH) will make this system specific to ovarian tumor and non-toxic to healthy organs. The study also supports our selection of molecular targets -LHRH receptors and antiapoptotic members of BCL2 protein family -for enhancing the anticancer efficacy of drug delivery systems.

Utilisation of Membrane Transporters for Improvement of Drug Absorption

lkumi Tamai, Faculty of Pharmaceutical Sciences,
Tokyo University of Science

Since intestinal absorptive/influx transporters for nutrients efficiently absorb physiological compounds, they should be promising as the tools for the improvement of drug absorption. However, when compared with other tissues such as liver and kidney, the information and the numbers of transporters work for drug absorption are limited. Furthermore, secretory/efflux transporters are also expressed at the apical membranes and may reduce the apparent membrane permeability. So, it is essential to evaluate both of influx and efflux transporters to improve intestinal membrane permeability.
To utilise influx transporters for improvement of intestinal absorption, the chemical structure of drugs should be changed to be substrates for the target transporters. Peptide transporter PEPT I is most studied drug transporter at the apical membranes and modification of amino acid-like drugs to peptides will be effective to improve membrane permeability via PEPTl. PEPTl utilises proton gradient as the driving force and luminal pH affects apparent activity of PEPTl. It has been reported that flavonoids are likely to activate sodium-proton exchanger 3 (NHE3) and acidify luminal pH, resulting in the increased membrane permeability of beta-Iactam antibiotics via PEPT 1. We often evaluate substrates of transporters by the in vitro experimental systems such as transfected cells of transporters, other cultured cells or isolated membranes. However, a good substrate of PEPT I examined by such in vitro methods does not necessarily exhibit high membrane permeability in vivo. This apparent discrepancy of PEPT I-mediated membrane permeability between in vivo and in vitro evaluations may be explained by the difference of pH in the two experiments, namely higher physiological pH in vivo than in vitro. In such case, acidification of lumen by polymers that can release protons and maintain luminal pH low is effective to increase permeability via PEPTl. We demonstrated that apparent in vivo activity of PEPTl can be manipulated by controlling the intestinal luminal pH using acidic polymers, resulting in the improved intestinal absorption of peptide-mimetic drugs that do not have high enough bioavailability when administered conventional method. This strategy is useful for substrates of PEPTl with low absorption in vivo, since membrane permeability can be improved via PEPTl by formulation without changes of chemical structures of active compounds. Efflux transporters like P-pg may decrease the membrane permeability by pumping out the substrates to lumen. However, many clinically used drugs exhibit good absorption even though they are substrates of P-gp. We need to consider affinity, luminal concentration, and influx intestinal permeability to evaluate significance of P-gp on drug absorption.
In conclusion, precise information that affects influx/efflux transporter activity will be essential for the utilisation of membrane transporters to improve intestinal absorption of drugs.