Antitumor activity of poly(ethylene glycol)–camptothecin conjugate: The inhibition of tumor growth in vivo

Abstract

Antitumor effect of poly(ethylene glycol)–camptothecin conjugate (PEG–CPT) was studied in the nude mouse model of human colon cancer xenografts. The animals were treated intravenously with 15 mg/kg of camptothecin (CPT) or PEG–CPT conjugate at equivalent CPT dose. Antitumor activity, apoptosis induction and caspase-dependent signaling pathways were studied 12, 24, 48 and 96 h after single injection. In addition, pharmacokinetics, tumor distribution and accumulation of PEG polymer labeled with green fluorescence protein (GFP) were studied. The data obtained showed that the conjugation of low molecular weight anticancer drug CPT with low solubility to high molecular weight water-soluble PEG polymer provides several advantages over the native drug. First, the conjugation improves drug pharmacokinetics in the blood and tumor. Second, such conjugation provides passive tumor targeting by the Enhanced Permeability and Retention (EPR) effect, increasing drug concentration in the tumor. Third, the coupling increases the bioavailability of CPT, induces apoptosis in tumor and, therefore, enhances anticancer activity of PEG–CPT. Thus, the use of macromolecular conjugate provided passive tumor targeting of the drug, improved pharmacokinetics and increased the stability of the drug during circulation. It offered better uptake by the targeted tumor cells and substantially enhanced apoptosis and antitumor activity of the conjugated drug in the tumor and decreased apoptosis in liver and kidney as compared with the native drug. All these characteristics make PEG–CPT conjugate an attractive anticancer drug for the effective chemotherapy of solid tumors.

Introduction

Conjugation of low molecular weight anticancer drugs to high molecular weight water-soluble polymers leads to several advantages [1], [2], [3], [4], [5], [6], [7], [8], [9]. First, in many cases this conjugation increases the aqueous solubility of often lipophilic anticancer drugs and substantially enhances the bioavailability of the drug. Second, coupling of an anticancer drug to a water-soluble polymer often improves its pharmacodynamic and pharmacokinetic properties. This improvement is achieved by protecting the drug from deactivation during circulation and by increasing its circulation time due to lower renal excretion and thereby prolonging its specific activity. Consequently, high molecular weight drug-conjugates are characterized by their prolonged systemic and/or local tissue half-lives. Third, polymer–drug complexes generally reduce the antigenic activity of the drug leading to a less pronounced immunological response. This property is particularly important for immunogenic biological drugs. Finally, coupling of low molecular weight drugs with a high molecular weight polymer provides for the so-called “passive tumor targeting” [10], [11] increasing the drug accumulation in solid tumors through the Enhanced Permeability and Retention (EPR) effect [12], [13], [14], [15]. The underlying mechanisms of the EPR effect include enhanced vascular permeability and impaired reticuloendothelial/lymphatic clearance of macromolecules from the tumor. These unique characteristics of tumor tissue result in the intratumor retention of macromolecular drugs. Such passive targeting to solid tumor tissues leads to enhancing the drug uptake by tumor cells and potentially preventing severe systemic adverse side effects.

We have previously shown the antitumor activity of poly(ethylene glycol) conjugated camptothecin (PEG–CPT) against a broad spectrum of tumors [16]. However, the tumor-inhibiting effect was evaluated merely based on the tumor size or mass, and the mechanism of antitumor activity remained unclear. The aims of the present study are to investigate the passive tumor targeting and the enhancement of antitumor activity as well as the apoptosis-inducing potential of PEG–CPT in tumor in vivo. In this research we show that high molecular weight PEG–CPT conjugate provides passive tumor targeting, augments apoptosis induction in tumor cells and potentially prevents adverse side effects by limiting cell death induction in healthy tissues.