A docetaxel-carboxymethylcellulose nanoparticle outperforms the approved taxane nanoformulation, Abraxane, in mouse tumor models with significant control of metastases

Abstract

Cellax is a PEGylated carboxymethylcellulose conjugate of docetaxel (DTX) which condenses into a 120-nm nanoparticle, and was compared against the approved clinical taxane nanoformulation (Abraxane®) in mouse models. Cellax increased the systemic exposure of taxanes by 37 × compared to Abraxane, and improved the delivery specificity: Cellax uptake was selective to the tumor, liver and spleen, with a 203 × increase in tumor accumulation compared to Abraxane. The concentration of released DTX in Cellax treated tumors was well above the IC50 for at least 10 d, while paclitaxel released from Abraxane was undetectable after 24 h. In s.c. PC3 (prostate) and B16F10 (melanoma) models, Cellax exhibited enhanced efficacy and was better tolerated compared to Abraxane. In an orthotopic 4T1 breast tumor model, Cellax reduced the incidence of lung metastasis to 40% with no metastasic incidence in other tissues. Mice treated with Abraxane displayed increased lung metastasic incidence (> 85%) with metastases detected in the bone, liver, spleen and kidney. These results confirm that Cellax is a more effective drug delivery strategy compared to the approved taxane nanomedicine.

Introduction

A key feature of tumor pathophysiology is vascular abnormality [1], [2]. After visualizing high density, dilated, immature, and chaotically branched blood vessels in tumors [3], investigators went on to demonstrate that tumor vasculature tended to be leaky, signified by unusual transfers of large molecules such as proteins [4], [5], [6]. Comprehensive studies of tumor vascular pathobiology and hyperpermeability published in 1986 [6], [7] were coincident with a report on the exploitation of the phenomenon for therapeutic benefit: nanoparticles and macromolecules loaded with chemotherapeutic were observed to passively accumulate in tumor tissue, with measurable improvement to efficacy [8]. The enhanced permeability and retention (EPR) effect is the foundation science upon which nanomedicine is founded: nanoparticles circulating in the bloodstream will not penetrate most tissues (excepting the reticuloendothelial system (RES)), but will extravasate into tumor tissue [1]. Tumor tissue is further abnormal in that lymphatic drainage is impaired, and accordingly, the tumor represents a biological cul-de-sac for nanoparticles, and passive accumulation occurs [1], [2]. In principal, the numerous off-target chemotherapeutic effects experienced in standard small molecule therapy can be alleviated due to improved specificity of distribution, and anti-tumor efficacy can be magnified by nanoparticle delivery systems. Nanomedicine approaches to drug formulation for the treatment of solid tumors are reaching clinical application, largely securing approval due to improved safety profiles. For example, Doxil® is an approved alternative to conventional doxorubicin for treatment of Kaposi's sarcoma, and Abraxane® is an alternative to Taxol® for treatment of metastatic breast cancer [2]. Safety enhancements arise due to elimination of irritating excipients such as Polysorbate 80 or Cremophor (Taxotere and Taxol, respectively) and reduced non-specific distribution to sensitive tissues (eg: reduced cardiotoxicity with Doxil®).

In the field of polymer therapeutics, wherein drugs are conjugated to polymeric carriers and may condense into nanoparticle structures, promising advancements have been reported: compounds clustered around the PEG, polyglutamate, hydroxypropylmethacrylate (HPMA) and polysaccharide family have demonstrated sufficient safety to reach this level of evaluation, but none except Opaxio has exemplified sufficient improvement to efficacy to reach Phase III evaluation [9]. Although Phase I and II data for Opaxio was positive, Phase III evaluation failed to demonstrate enhancement relative to the standard of care [10], [11]. The failure of polymeric products has served to inform the field regarding design principles of an improved system: an increased drug carrying capacity can minimize premature drug release during blood transport, exert sustained drug release and boost the effect of each particle that accumulates in the tumor [12], [13], [14]; compositions should remain stable during blood transport [15], [16]; and polymer composition should not activate the immune system or have any undesirable biological activity [17], [18], [19].

Based on these important designing factors, we have designed Cellax, which is a conjugate of docetaxel (DTX) and an acetylated and PEGylated carboxymethylcellulose polymer [20], [21], [22]. The Cellax polymer contains ~ 5 wt.% PEG and ~ 37 wt.% DTX, and condenses into a defined nanoparticle of 120 nm with a low polydispersity index (PDI) of 0.1 [21], and in preclinical evaluation in mice exhibited prolonged blood circulation and increased tumor delivery compared to Taxotere [22]. DTX was selected as it is broadly indicated for cancer therapy, including NSCLS, breast, prostate, stomach and head and neck cancer [24], and although this opinion is open to debate [23], is clinically preferred due to a perceived benefit over paclitaxel [24]. DTX is a potent antineoplastic compound known to promote tubulin assembly, kill cells passaging through the cell cycle at low nM levels, and is active against a wide range of murine and human tumor cells [25], [26]. In efficacy testing employing a Taxotere benchmark, Cellax displayed improved activity in a panel of chemo-insensitive tumor models [20], [22]. Moreover, Cellax was better tolerated in mice compared to Taxotere and could be administered at 170 mg DTX/kg without inducing significant stress or any abnormality in the mice [22]. Taxotere, on the other hand, caused significant stress and neutropenia to the mice at 40 mg DTX/kg [22]. The Taxotere benchmark was a critical test given that Cellax delivers DTX [20], [21], [22], but the benchmark only addresses one aspect of the current field, as Abraxane has been gaining traction over Taxol and Taxotere. Abraxane is a nanoparticle formulation of paclitaxel (NAb: nanoparticle albumen-bound paclitaxel), and a significant part of its benefit arises from the elimination of the toxic excipients used to formulate Taxol and Taxotere. Abraxane increases the safety and maximum tolerated dose (MTD) of paclitaxel (PTX) in human patients, reduces neutropenia, and increases overall survival rates for metastatic breast cancer patients compared to Taxol [27], [28], [29]. As Abraxane is a clinically approved nanoparticle formulation demonstrating significant improvements over standard taxanes, it is of importance to extend the benchmark comparison to Abraxane in animal models to evaluate the potential of a nanoformulation in enhancing taxane therapy. Here, we report the comparisons of Cellax and Abraxane in pharmacokinetics, biodistribution, toxicity and efficacy in preclinical models including metastatic breast cancer, advanced melanoma and pancreatic cancer, which are either approved indications or are in clinical trials for Abraxane therapy.