Finding the optimal balance: Challenges of improving conventional cancer chemotherapy using suitable combinations with nano-sized drug delivery systems

Abstract

Anticancer drugs as well as nano-sized drug delivery systems face many barriers that hinder penetration deeply and evenly into solid tumors: a chaotic, tortuous vascular compartment resulting in tumor tissue distant from microvessels, a heterogeneous blood flow distribution with a concomitant defective microcirculatory exchange process, and a high interstitial fluid pressure. Furthermore, a resulting hostile tumor microenvironment characterized by hypoxia and/or extracellular acidosis can reduce the efficacy of anticancer drugs and confer drug resistance. Conversely, the enhanced permeation and retention effect has become the gold standard for developing macromolecular prodrugs and nano-sized drug delivery systems. Preclinically, there are meanwhile numerous in vivo proof-of-concepts that demonstrate not only a better tolerability of nano-sized drug delivery systems but also of enhanced antitumor efficacy compared to the conventional clinical standard. When faced with such a complex and heterogeneous disease as cancer in humans, it is more likely that a tailor-made combination of different therapeutic strategies will achieve the best results. In this respect, combining low-molecular weight cytostatic drugs with nano-sized drug delivery systems appears to be a natural choice for combination therapy that aims at distributing anticancer drugs at higher concentrations in the tumor in a more even manner. To date, such drug delivery approaches have been inadequately explored. In this review, we summarize the state-of-the-art of combination approaches with liposomal doxorubicin (Doxil™), the paclitaxel-albumin nanoparticle (Abraxane™) and the albumin-binding doxorubicin prodrug DOXO-EMCH (INNO-206), and discuss the insights obtained and perspectives for further research in this intriguing and promising field of drug delivery research.

Introduction

In 2008, Rakesh K. Jain depicted an intriguing illustration of a tumor and its surrounding vessels in Scientific American entitled: The problem: Abnormal vessels make trouble [1].

For us, when reading this title for the first time, it opened two avenues for interpretation: 1.) the abnormal vessels are making trouble for the drug developer and limiting his chances of a successful output; 2.) the abnormal vessels are causing an intrinsic problem for the tumor, and the chaotic, fenestrated vasculature that is formed during the tumor growth presents a biochemical and physiological property for targeting an anticancer drug to solid tumors as well as to metastases and thus improving the therapeutic index of conventional cancer chemotherapy.

The latter phenomenon of passive targeting discovered and pioneered by Hiroshi Maeda in 1986 is the enhanced permeation and retention effect (EPR effect) [2] – overlooked for many years by the scientific community – but meanwhile forms the basis and rationale for the era of polymer therapeutics and nanomedicine in oncology. It is described in detail in the contribution by Hiroshi Maeda in this special issue. Moreover, Hiroshi Maeda has shown preclinically as well as clinically that several agents, so-called EPR enhancers such as nitric oxide releasing agents or bradykinin, actually promote the abnormality of tumor vessels enhancing the leakiness of the tumor vasculature and thus improve the therapeutic outcome of drug delivery using drug–polymer conjugates [3].

Rakesh Jain's therapeutic approach of taming vessels to revert the EPR effect of abnormal tumor vessels would undermine the rationale of any nano-sized based drug delivery system designed to improve the tumor targeting properties of an encapsulated or chemically bound anticancer agent. Starting as a chemical engineer, his formidable insights into the characteristics of dysfunctional tumor blood vessels and the concomitant lack of drug penetration and uneven and variable intratumoral drug distribution have undoubtedly laid solid foundations why cancer chemotherapy with the approximately fifty low-molecular weight anticancer drugs used in the clinic often fails: the abnormal tumor vasculature fosters an environment characterized by irregular blood flow, hypoxia, high acidity, necrotic areas and an interstitial fluid pressure that prevents anticancer drugs from reaching tumor cells, impairs their mode of action as well as suppressing immune responses directed towards killing tumor cells. His proposal of restoring normal function to tumor vessels is therefore a plausible approach of creating a tumor vasculature through which cancer therapy can be optimized.

Are the different views of Rakesh Jain and Hiroshi Maeda regarding tumor biology and physiology and the proposed strategies of maximizing therapeutic options reconcilable? In our opinion, a closer inspection of the literature indicates that an antiangiogenic approach, whether induced by appropriate antibodies or metronomic therapy, combined with either low-molecular weight drugs or nano-sized drug delivery systems holds promise for improving drug delivery to solid tumors. Two fundamental properties of primary tumors and metastases that influence drug penetration and intratumoral drug distribution make us aware of the challenges we face when attempting to reduce the volume of a solid tumor: the size of the tumor on the one hand, and the heterogeneity not only of a single solid tumor but the metastases that have formed at the respective stage of the disease, on the other. Viewed from these perspectives, abnormal vessels can either create a considerable barrier for ensuring a deep and even drug distribution as well as an unfavorable environment in the tumor, or the enhanced permeation of abnormal vessels for macromolecules combined with an absent or defective lymphatic drainage system can be the decisive factor for achieving tumorstasis or remissions in a preclinical tumor model with a suitable nano-sized drug delivery system.

It is well known that due to the chaotic and hyperpermeable microvasculature of solid tumors, an interstitial fluid pressure builds up in the tumor which, however, can vary considerably in human malignant tumors as shown in Table 1.

The interstitial fluid pressure impedes the transport of macromolecules that is mediated by convection to a much larger extent than for low-molecular weight drugs that primarily are distributed within the tumor by diffusion. However, the oncostatic tumor pressure is more uniform in the center of the tumor dropping steeply in the periphery and thus interstitial fluid oozes out of the tumor from the high- to the low-pressure regions carrying away nano-sized drug delivery systems as well as low-molecular weight anticancer drugs.

Likewise, we are well aware that macromolecules which have a size above the renal threshold and generally a diameter of < 500 nm enter through the fenestrated tumor vessels into the tumor interstitium and are subsequently not removed owing to a defective or absent lymphatic drainage system. Nano-sized drug delivery systems thus accumulate passively due to the enhanced permeation and retention effect in solid tumors, and there are ample examples in preclinical tumor models that demonstrate their increased uptake compared to the respective low-molecular weight anticancer drug. Finally, we need to consider that the EPR effect is not a property that is evenly distributed within a solid tumor: some microvessels will be very leaky, while others will not be any different to normal microvessels.

In 2000, Jain and colleagues published a first paper that an antibody directed against the vascular endothelial growth factor (VEGF) lowers the interstitial fluid pressure in subcutaneously growing tumors in nude mice [5] and subsequently proposed an original and seemingly paradox concept that antibodies designed to destroy blood vessels indeed in part repair the latter and improve the delivery of anticancer agents [6]. Clinical proof of concept was obtained in a phase III study in metastatic colorectal cancer that demonstrated an approximately 5 months benefit in overall survival when adding bevacizumab (Avastin™) to a standard regimen of irinotecan and 5-fluorouracil [7].

Meanwhile, we have a better understanding why an antibody that is designed to destroy blood vessels can at the same time improve the drug delivery of low-molecular weight anticancer agents such as 5-fluorouracil and irinotecan although a number of open questions remain when assessing the different response rates in the different stages of cancer disease (see below) [8], [9], [10]. The mode of action of anti-VEGF directed antibodies seems to take place in two steps: in a first step, blood vessels are destroyed, blood flow is diminished, the density of microvessels is decreased, and the interstitial fluid pressure in the tumor is concomitantly reduced. This phenomenon has been coined “pruning tumor microvessels” [1]. However, the remaining tumor vessels supplying the tumor with nutrients become les abnormal and more efficient thus creating a more favorable tumor microenvironment that leads to enhanced proliferation of tumor cells and makes them more susceptible to the antiproliferative activity of the co-administered anticancer agents. These insights serve as a working model for explaining the 5 months overall survival benefit in the combination trial of bevacizumab with irinotecan and 5-fluorouracil over therapy compared to the control arms, i.e. bevacizumab alone or the combination of irinotecan and 5-fluorouracil. It needs to be emphasized that this phase III study was performed in stage IV colorectal cancer, i.e. in those patients where metastases had formed in either liver, lungs or both [7].

It was anticipated that clinical trials comprising a combination of bevacizumab and clinically established agents such as 5-fluorouracil, capecitabine, leucovorin, and oxaliplatin would also improve the outcome of patients with high-risk stage II and stage III of colorectal cancer, i.e. at a stage where distant metastases were not present. Surprisingly, the large trials performed at stage II and stage III of this cancer disease (NSABP C-08 trial and AVANT trial) showed no benefit in overall survival compared to chemotherapy alone, but only in progression-free survival [10], [11]. These results demonstrate that the blockade of VEGF expression and its exploitation for improving chemotherapy is a very complex issue, and its success will differ depending on the patient's tumor burden and extent of metastases. To add to the complexity of this paradigm, cytostatic agents such as taxanes, cyclophosphamide or anthracyclines when used in a metronomic schedule are known to cause apoptosis in endothelial cells thereby inducing an antiangiogenic effect [12], [13], but they also induce VEGF expression after cytotoxic treatment as noted with paclitaxel [14], [15], [16], vinblastine [17], [18], carboplatin [19], [20], cisplatin [20], [21], 5-fluorouracil [20], dacarbazine [22] and anthracyclines [23].

Would it therefore be reasonable to combine a metronomic, antiangiogenic therapy with a cytotoxic one? And to take this one step further, would it be feasible to combine antiangiogenic agents with anticancer nano-sized drug delivery systems? Prima faciae, one would expect this to be contradictory, but as we will describe below there are several recent investigations that demonstrate that such an approach actually enhances antitumor efficacy and tumor targeting. Independently, it would intuitively make sense to sound out the potential of combining the antitumor effect of combinations of nano-sized drug delivery systems with conventional clinically established anticancer agents for two other straight-forward reasons: not only will the intratumoral distribution of both drug systems differ but also their cellular uptake mechanism, low-molecular weight drugs entering tumor cells by diffusion and nano-sized drug delivery systems by endocytosis.

In this discourse, we describe the state-of-the-art regarding in vivo experiments of combinations of anticancer agents with liposomal doxorubicin (PLD, Doxil™), the paclitaxel-albumin nanoparticle (nab-paclitaxel, Abraxane™) and the albumin-binding doxorubicin prodrug DOXO-EMCH (INNO-206), the insights obtained and the challenges and perspectives that lie ahead in this field of drug delivery research. Despite the fact that the preclinical data are limited when we reviewed the literature on the above outlined combination therapy approaches, the in vivo proof-of-concepts point towards several promising therapeutic options which are worthy to be explored in sufficient detail.