PHARMACOKINETICS AND DRUG DELIVERY

Our laboratory conducts preclinical and clinical pharmacological research. Our preclinical studies are centered on chemotherapy of brain tumors, in which the focus is on the design and evaluation of targeted drug delivery systems, and on the characterization of drug interactions. We are also active participants in several clinical investigations in which we quantitate drug and metabolite concentrations in biological media and develop pharmacokinetic/pharmacodynamic models.

In the past year we have completed the evaluation of different types of colloidal drug delivery systems in animal models for brain-tumors. Particle size, charge, magnetic properties and composition were the primary variables that were assessed. From these investigations, we have found that cationic liposomes have favorable properties for targeting brain-tumors, and will be the focus of future investigations. A number of in vitro studies of drug-loaded liposomes have been completed, and we have now designed a series of studies to fully characterize the role of colloidal systems as drug delivery devices in brain-tumors.

The angiogenic phenotype is associated with a state of vascular hyperpermeability due either to structural defects in capillaries, and/or the overexpression of growth factors, such as vascular endothelial growth factor (VEGF). Treatment of solid tumors with angiogenesis inhibitors may enable a reversion from a hyperpermeable state to a normapermeability state. Since cytotoxic drugs are likely to be used in combination with angiogenesis inhibitors, their uptake into tumors may be reduced in the presence of antiangiogenic therapy. We have shown previously that in a rat subcutaneous C6 glioma model, that the angiogenesis inhibitor, TNP-470 does reduce tumor concentrations of temozolomide.

In the past year, we have developed xenograft glioma models that have different expression levels of VEGF so we can understand how various biological parameters (i.e., VEGF and its endothelial cell receptors) relate to drug transport, and drug action. We are in the process of conducting studies in these animal models. Through an understanding of how VEGF and related parameters interrelate, the determinants of tumor drug delivery will be characterized, and optimal combination regimens can be designed.

Apoptosis was investigated as a mechanism to trigger delivery of drugs from carrier cells to tumor cells. Two possible mechanisms were considered; either drug release through a more permeable apoptotic cell membrane, or phagocytosis of drug-loaded apoptotic cells by tumor cells. Using S49 mouse lymphoma cells as model carrier cells that apoptose upon exposure to dexamethasone and RG2 rat glioma cells as target cells, it was found that both mechanism were operative.

Second-generation apoptotic-induced drug delivery (AIDD) systems were designed based on endothelial cells that expressed a flk1:fas fusion protein (EC/FF). These cells will apoptose in response to VEGF, an important growth factor secreted by solid tumors. Such genetically-engineered carrier cells would also serve as decoy receptors for VEGF, and could possibly interfere with neovascularization of the tumor. Drug-loaded EC/FF have been prepared and are being used in animal efficacy studies. There are numerous design variables that can be manipulated in AIDD systems that support the further development of this novel drug delivery strategy.

Peripheral benzodiazepine receptors (PBR) are located on the outer membrane of mitochondria, and their density is increased in brain-tumors. Thus, they may serve as a unique intracellular and selective target for antineoplastic agents. We have synthesized numerous PBR ligand-cytotoxic drug conjugates targeted to both rat and human PBRs. These conjugates are of low molecular weight, and screened based on a receptor-binding displacement and cytotoxicity assays. From these screens, a number of lead compounds have been identified that will be the basis of future synthetic efforts and pharmacokinetic studies.

Several phase I and II investigations of anticancer drugs and chemopreventive agents require pharmacokinetic and pharmaco-dynamic determinations. Most investigations start with the development of assays, such as high performance liquid chromatography or gas chromatograph-mass spectrometry, to quantitate drug and metabolite concentrations in patient samples. These data provide the foundations to conduct pharmacokinetic and pharmacodynamic analyses that characterize the biological disposition of the drug.

We are particularly interested in the development and application of population pharmacokinetic/pharmacodynamic models. We have developed a time-dependent pharmacodynamic model for glutathione depletion due to buthionine sulfoximine, and are applying indirect response models to characterize myelosuppresion. We have also developed a population pharmacokinetic model for topotecan that has identified a number of covariates or patient factors that impact on the drug's pharmacokinetic properties. Application of such models prospectively will lead to optimized and individualized drug dosage regimens.

CHEN, S.-H., GALLO, J.M. Use of capillary electrophoresis methods to characterize the pharmacokinetics of antisense drugs. Electrophoresis 19:2861-2869, 1998.

GALLO, J.M. Pharmacokinetics: Model structure and transport systems. In Drug Diffusion and Transport Processes in Pharmaceutical Systems, edited by K.J. Himmelstein, G.L. Amidon, P.I. Lee. Marcel Dekker, Inc., New York (in press).

PULFER, S.K., CICCOTTO, S.L., GALLO, J.M. Distribution of small magnetic particles in brain tumor-bearing rats. J Neuro-Oncology 41:99-105, 1999.

MA, J., GALLO, J.M. Delivery of cytotoxic drugs from carrier cells to tumor cells by apoptosis. Apoptosis 3:195-202, 1998.

MA, J., ZHOU-LI, F., KLEIN-SZANTO, A., GALLO, J.M. Modulation of angiogenesis by human glioma xenograft models that differentially express vascular endothelial growth factor. Clin. Exp. Metas. 16:559-568, 1998.

PULFER, S.K., GALLO, J.M. In vivo distribution of magnetic polysaccharide microspheres in normal and brain-tumor bearing rats. J. Drug Targeting 6:215-227, 1998.

^§   Fox Chase researcher

^a   P.B. Laub: Present address--Incyte Pharmaceuticals, 3174 Porter Drive, Palo Alto, CA 94304

Illustrations or unpublished data in these reports should not be used without permission of the author.

Fox Chase Cancer Center

Scientific Report 1998