The anticancer efficacy of pixantrone-loaded liposomes decorated with sialic acid–octadecylamine conjugate

Abstract

Based on the knowledge that sialic acid is a critical element for tumor development and its receptors are highly expressed on the tumor-associated macrophages (TAMs) which play important roles in the growth and metastasis of tumors, we synthesized a sialic acid–octadecylamine conjugate (SA–ODA) and anchored it on the surface of pixantrone (Pix)-loaded liposomes, to achieve an improved anticancer effect. Four Pix formulations (Pix-S, Pix-CL, Pix-PL and Pix-SAL represent solution, conventional liposome, stealth liposome, and SA–ODA modified liposome, respectively) were developed, and various parameters, including drug loading, stability, in vitro release, cytotoxicity and pharmacokinetics, were evaluated. The tumor growth inhibition and toxicity studies were performed in S180-bearing Kunming mice. Pix-S exhibited a strong toxicity to the immune system, accelerated the growth of tumors and reduced the lifespan of mice. In contrast, Pix-SAL displayed the strongest anticancer and life-prolonging effects among all of the formulations in this study. More importantly, injection of Pix-SAL induced a phenomenon whereby the cancerous tissues were “shed” from mice, after which the wound healed. We speculate that this special efficacy may be partly due to the killing of TAMs by Pix-SAL. This study suggests that SA–ODA modified liposomes may serve as an effective intravenous delivery vehicle for Pix.

Introduction

Sialic acid (SA) is a generic term for the N- or O-substituted derivatives of neuraminic acid, which is a monosaccharide with a nine-carbon backbone. SA is also the name for the most common member of this group, N-acetylneuraminic acid. Located at the terminus of numerous cell-surface glycoproteins and glycolipids, SA that is exposed to the cellular environment functions in intrinsic and extrinsic communication [1]. SA thereby plays many important roles in a variety of physiological and pathologic processes, including neuronal growth, microbe binding that leads to infections, regulation of the immune response, and the progression and spread of tumors [2], [3].

The positive relationship between SA and tumors was reported by Kimura et al. [4] in 1958; he proposed that tumor cells might excrete and contain excess sialyl glycans, glycoproteins, or glycolipids. These characteristics were later verified in several human cancers [5] and found to correlate with highly metastatic tumor types [3], [6]. Recent studies have demonstrated that surface SA can help tumor cells escape recognition and elimination by the immune system, thus contributing to tumor metastasis [7]. Therefore, SA could serves as a necessary nutrient during the growth and metastasis of carcinomas.

Solid tumors comprise malignant cells, extracellular matrix (ECM) and many other non-malignant cell types, including fibroblasts, blood/lymphatic vessels' endothelial cells and inflammatory cells (such as macrophages, neutrophils, mast cells and lymphocytes) [8]. Macrophages tend to be the crucial inflammatory cells within the tumor stroma [9]. These macrophages are referred to as tumor-associated macrophages (TAMs), and most of them are derived from peripheral blood monocytes recruited into the tumor mass. TAMs promote cancer proliferation, invasion and metastasis [10], [11] through several mechanisms. These mechanisms include tumor angiogenesis, matrix remodeling, secretion of effector molecules, suppression of adaptive immunity and additional unknown mechanisms [8], [12]. The presence of extensive TAMs infiltration has been shown to correlate with poor cancer patient prognosis [13]. In mammalians, sialic acid binding receptors [Sialoadhesin (Sn), is the prototypic member of the sialic acid binding family of lectins called Siglecs] are highly expressed on TAMs [14]. These findings suggest that targeting delivery of drug into TAMs (mediated by SA) is a promising strategy for cancer therapy.

Because of the important roles of SA and its receptors in the progression of tumors, SA has been utilized as a ligand to improve the efficacy of anticancer drugs. Recently, Zheng [15] prepared selenium nanoparticle (Se-NP) and SA modified Se-NP, and the cellular uptake of the latter in HeLa cells was approximately three times-higher than that of the Se-NP. Jayant [16] constructed a prodrug constituted of PEG, SA and doxorubicin (DOX), and it exhibited rapid uptake by A2870 cells and enhanced cytotoxicity compared to PEG-DOX and free DOX. These studies demonstrated that SA moieties can enhance both cell uptake and cytotoxicity of the molecules/carriers that were modified. However, there are only a few reports on the in vivo anticancer effects – the golden standard of clinical efficacy evaluation – of SA-based nanocarriers [3].

Pixantrone (Pix) is a aza-anthracenedione, it acts as a topoisomerase II poison and has dose–response effects against hematological and solid tumor models [17]. Recently, Pix received marketing authorization from the EMEA for the treatment of non-Hodgkin's B-cell lymphomas, and the Phase 2 study in patients with advanced solid tumors was also completed [18]. The commodity form of Pix is a dark-blue lyophilized powder for infusion solution. However, extensive use of this form is limited in oncology because of undesired side effects, including neutropenia, thrombocytopenia, lymphopenia, alopecia, nausea, and vomiting [19], [20]. Thus, there is a need for developing a Pix formulation with reduced toxicity and enhanced efficacy.

Liposomes are small vectors composed of phospholipid bilayers. Numerous studies have shown that encapsulation of anticancer agents by liposomes can reduce the blood clearance rate [21], decrease systematic toxicity, and enhance antitumor activity [22]. Pix is rapidly cleared (1.15 l/h/kg) after intravenous injection into humans [23]. Therefore, liposome encapsulation is a strategy that has been used to prolong Pix's blood circulation time and subsequently enhance its tumor accumulation via the enhanced permeation and retention (EPR) effect.

In the present study, with the aim to enhance the therapeutic effect of Pix and reduce its systemic side effects, conventional and pegylated Pix-loaded liposomes were developed using remote loading technology. To improve the anticancer effects of the Pix liposomes further, a sialic acid–octadecylamine conjugate (SA–ODA) was synthesized and used to decorate the surface of the Pix liposomes. Parameters including drug loading, stability, in vitro release, cytotoxicity and pharmacokinetics were evaluated for the three types of liposomes carrying Pix.