Targeted delivery of platinum-taxane combination therapy in ovarian cancer

Abstract

Biodegradable polypeptide-based nanogels have been developed from amphiphilic block copolymers, poly(ethylene glycol)-b-poly(l-glutamic acid)-b-poly(l-phenylalanine), which effectively co-incorporate cisplatin and paclitaxel, the clinically used drug combination for the treatment of advanced ovarian cancer. In order to target both drugs selectively to the tumor cells, we explored the benefits of ligand-mediated drug delivery by targeting folate receptors, which are overexpressed in most ovarian cancers. Drug-loaded nanogels were surface-functionalized with folic acid (FA) with the help of a PEG spacer without affecting the ligand binding affinity and maintaining the stability of the carrier system. FA-decorated nanogels significantly suppressed the growth of intraperitoneal ovarian tumor xenografts outperforming their nontargeted counterparts without extending their cytotoxicity to the normal tissues. We also confirmed that synchronized co-delivery of the platinum-taxane drug combination via single carrier to the same targeted cells is more advantageous than a combination of targeted single drug formulations administered at the same drug ratio. Lastly, we demonstrated that the same platform can also be used for localized chemotherapy. Our data indicate that intraperitoneal administration can be more effective in the context of targeted combination therapy. Our findings suggest that multifunctional nanogels are promising drug delivery carriers for improvement of current treatment for ovarian cancer.

Introduction

Combination chemotherapy is preferred over treatment with single agents to combat most cancers as it targets multiple cell-survival pathways at the same time and delays the onset of resistance. This helps in achieving long-term tumor remission and increases median survival [1]. Cisplatin (CDDP)-based therapy has been the standard treatment for ovarian cancer since its discovery. After paclitaxel (PTX) was shown to be effective in ovarian cancer, multiple clinical trials studied the overall efficacy of CDDP and PTX and found significant benefit over the pre-existing treatments. Since then, this combination has been the treatment of choice for both early stage as well as advanced cases of ovarian cancer [2], [3]. However, administration of two different agents comes with the inconvenience of repeated or extended duration of drug infusion in patients. Moreover, the most extensively used conventional formulation of paclitaxel, Taxol®, utilizes Cremophor EL (polyethoxylated castor oil) that has been linked to significant toxicities including allergic, hypersensitivity and anaphylactic reactions during infusion that require premedication and prolonged peripheral neuropathy. Combining such drugs in one delivery carrier is therefore a well-suited and convenient strategy for controlling the pharmacokinetics and co-delivery of the desired drug ratio in vivo, to maximize the therapeutic potency and minimize drug-associated toxicities.

Cross-linked nanogels have been found to be promising drug carriers to achieve this goal. Being mostly hydrophilic in nature, nanogels are highly biocompatible with a high loading capacity for guest molecules [4]. The nanogel structure can be readily adjusted to integrate features of different materials and, thus, offer advantages for combinatorial encapsulation of drugs with varying physicochemical properties. One of the widespread synthetic techniques for the synthesis of nanogels is the crosslinking of preformed core-shell self-assemblies such as polymer micelles that allows introducing a high degree of spatial organization into the nanogels [4], [5]. Cross-linking is known to impart control over the swelling behavior of the nanogels and thus helps in achieving controlled release of the incorporated cargo, which is an added advantage over the structural integrity imparted to the carrier system upon in vivo administration [4], [6]. The enhanced stability also makes the prolonged circulation of the nanogels possible, which in turn allows for increased drug accumulation at the target site [5], [6], [7], [8], [9].

We have previously described a biodegradable hybrid nanogel carrier system (NG) for the combination of CDDP and PTX for ovarian cancer therapy, which not only mitigated the toxicities associated with the use of free drugs but also improved treatment outcome [10]. However, our system relied solely on the enhanced permeation and retention (EPR) effect to facilitate the delivery of the drug combination to the tumor site [11]. Regardless of the importance and popularity of EPR effect-based drug delivery, this strategy has some limitations related to the inter- and intra-tumor heterogeneity, variations in the density as well as permeability of the tumor vasculature that can affect the accumulation of nanocarriers. One of the popular approaches to circumvent these problems is by surface-functionalization of the drug carrier with ligands that can target receptors with differential expression on the cancer cell surface, which helps in increasing the mean residence time of the delivery system at the tumor site and improving target cell uptake [12], [13], [14], [15]. One such receptor of interest is the folate receptor (FR). Malignant cells, due to their high rate of cell division, have an increased requirement of folic acid (FA), since it is an essential component of cell metabolism and DNA synthesis and repair. To fulfill this higher need of FA, FR is known to be over-expressed in a large number of malignant tissues, including ovarian cancer, compared to normal tissues with the exception of the kidney and choroid plexus [16], [17]. Furthermore, this receptor becomes accessible via the plasma compartment only after the cells lose their polarity owing to malignant transformation which makes it a differential target for cancer tissue that is easily accessible for intravenously administered FA-conjugated systems. Its natural ligand, FA, comes with the advantages of high binding affinity, stability and a simple chemical structure together with ease of availability, making it a suitable targeting ligand for ovarian cancer therapy. FA can thus be successfully conjugated to macromolecular systems without loss of binding affinity to its receptor [18]. Many different agents targeting the folate pathway are currently in clinical development [19]. To date, FA-targeted agents showed significant promise in phase II clinical trials but it has not been confirmed in phase III studies [20], [21], [22]. Accordingly, there is a need for further identification of new therapeutic combinations and refinement of patient selection. To this end, FA-conjugated imaging agents could be used for pre-selection of patients based on the expression of FR [23] and several methods have been already developed for this purpose [24], [25].

Our group has previously demonstrated a tumor-specific delivery and improved anti-cancer effect in vivo of CDDP-loaded NGs decorated with FA targeting groups [26]. In the current study, we designed FA-linked NGs incorporating platinum-taxane combination, and examined whether FR-targeted concurrent delivery of synergistic combination of CDDP and PTX can lead to enhanced therapeutic efficacy compared to nontargeted NG system.