Improving drug penetration to curb tumor drug resistance

Breaching the Blood-Brain Tumor Barrier for Tumor Therapy

Tumors affecting the central nervous system (CNS), either primary or secondary, are highly prevalent and represent an unmet medical need. Prognosis of these tumors remains poor, mostly due to the low intrinsic chemo/radio-sensitivity of tumor cells, a meagerly known role of the microenvironment and the poor CNS bioavailability of most used anti-cancer agents. The BBTB is the main obstacle for anticancer drugs to achieve therapeutic concentrations in the tumor tissues. During the last decades, many efforts have been devoted to the identification of modalities allowing to increase drug delivery into brain tumors. Until recently, success has been modest, as few of these approaches reached clinical testing and even less gained regulatory approval. In recent years, the scenario has changed, as various conjugates and drug delivery technologies have advanced into clinical testing, with encouraging results and without being burdened by a heavy adverse event profile. In this article, we review the different approaches aimed at increasing drug delivery to brain tumors, with particular attention to new, promising approaches that increase the permeability of the BBTB or exploit physiological transport mechanisms.

Depleting Tumor Cells Expressing Immune Checkpoint Ligands-A New Approach to Combat Cancer

Antibodies against inhibitory immune checkpoint molecules (ICPMs), referred to as immune checkpoint inhibitors (ICIs), have gained a prominent place in cancer therapy. Several ICIs in clinical use have been engineered to be devoid of effector functions because of the fear that ICIs with preserved effector functions could deplete immune cells, thereby curtailing antitumor immune responses. ICPM ligands (ICPMLs), however, are often overexpressed on a sizeable fraction of tumor cells of many tumor types and these tumor cells display an aggressive phenotype with changes typical of tumor cells undergoing an epithelial-mesenchymal transition. Moreover, immune cells expressing ICPMLs are often endowed with immunosuppressive or immune-deviated functionalities. Taken together, these observations suggest that compounds with the potential of depleting cells expressing ICPMLs may become useful tools for tumor therapy. In this article, we summarize the current state of the art of these compounds, including avelumab, which is the only ICI targeting an ICPML with preserved effector functions that has gained approval so far. We also discuss approaches allowing to obtain compounds with enhanced tumor cell-depleting potential compared to native antibodies. Eventually, we propose treatment protocols that may be applied in order to optimize the therapeutic efficacy of compounds that deplete cells expressing ICPMLs.

The Vicious Cross-Talk between Tumor Cells with an EMT Phenotype and Cells of the Immune System

Carcinoma cells that undergo an epithelial-mesenchymal transition (EMT) and display a predominantly mesenchymal phenotype (hereafter EMT tumor cells) are associated with immune exclusion and immune deviation in the tumor microenvironment (TME). A large body of evidence has shown that EMT tumor cells and immune cells can reciprocally influence each other, with EMT cells promoting immune exclusion and deviation and immune cells promoting, under certain circumstances, the induction of EMT in tumor cells. This cross-talk between EMT tumor cells and immune cells can occur both between EMT tumor cells and cells of either the native or adaptive immune system. In this article, we review this evidence and the functional consequences of it. We also discuss some recent evidence showing that tumor cells and cells of the immune system respond to similar stimuli, activate the expression of partially overlapping gene sets, and acquire, at least in part, identical functionalities such as migration and invasion. The possible significance of these symmetrical changes in the cross-talk between EMT tumor cells and immune cells is addressed. Eventually, we also discuss possible therapeutic opportunities that may derive from disrupting this cross-talk.

Antibody-Drug Conjugates (ADC) Against Cancer Stem-Like Cells (CSC)-Is There Still Room for Optimism?

Cancer stem-like cells (CSC) represent a subpopulation of tumor cells with peculiar functionalities that distinguish them from the bulk of tumor cells, most notably their tumor-initiating potential and drug resistance. Given these properties, it appears logical that CSCs have become an important target for many pharma companies. Antibody-drug conjugates (ADC) have emerged over the last decade as one of the most promising new tools for the selective ablation of tumor cells. Three ADCs have already received regulatory approval and many others are in different phases of clinical development. Not surprisingly, also a considerable number of anti-CSC ADCs have been described in the literature and some of these have entered clinical development. Several of these ADCs, however, have yielded disappointing results in clinical studies. This is similar to the results obtained with other anti-CSC drug candidates, including native antibodies, that have been investigated in the clinic. In this article we review the anti-CSC ADCs that have been described in the literature and, in the following, we discuss reasons that may underlie the failures in clinical trials that have been observed. Possible reasons relate to the biology of CSCs themselves, including their heterogeneity, the lack of strictly CSC-specific markers, and the capacity to interconvert between CSCs and non-CSCs; second, inherent limitations of some classes of cytotoxins that have been used for the construction of ADCs; third, the inadequacy of animal models in predicting efficacy in humans. We conclude suggesting some possibilities to address these limitations.

Enhancing solid tumor therapy with sequential delivery of dexamethasone and docetaxel engineered in a single carrier to overcome stromal resistance to drug delivery

Nanomedicines are often designed to target and treat solid tumors. Unfortunately, tumor and stroma composed of dense extracellular matrix, abnormal vascular barriers, elevated interstitial fluid pressure, et al., which impede the access and accumulation of nanomedicines into tumors. Strategies to disrupt these deterministic obstacles require a unique combination of promoter drugs and cytotoxic agents to target stroma and tumor simultaneously. Here, we engineered a novel strategy by co-delivery dexamethasone (DEX) and docetaxel (DTX) in the 2-in-1 liposome, namely (DEX + DTX)-Lip, with sequential release property. We proved that the engineered liposomal therapy approach could potentially achieve two objectives in tumor drug delivery: modulate tumor stroma and promote drug penetration and accumulation in tumor. Thus more DTX tenured in intratumoral site to kill tumor cells in a strong way with minimize systemic toxicity. The sequentially released liposomes won excellent antitumor efficacy in multifarious models, including KB, multidrug resistant KBv and metastatic 4 T1 tumor models and low toxicities compared with the combination of free drugs in vivo. Moreover, they demonstrated the potential of prevention tumor cells colonization and anti-metastasis in vivo models. These findings give insights in overcoming the deterministic stroma obstacles and provide a rational strategy to increase antitumor efficacy of combination nanomedicines with practical value.

How Tumor Cells Choose Between Epithelial-Mesenchymal Transition and Autophagy to Resist Stress-Therapeutic Implications

Tumor cells undergo epithelial-mesenchymal transition (EMT) or macroautophagy (hereafter autophagy) in response to stressors from the microenvironment. EMT ensues when stressors act on tumor cells in the presence of nutrient sufficiency, and mechanistic target of rapamycin (mTOR) appears to be the crucial signaling node for EMT induction. Autophagy, on the other hand, is induced in the presence of nutrient deprivation and/or stressors from the microenvironment with 5' adenosine monophosphate-activated protein kinase (AMPK) playing an important, but not exclusive role, in autophagy induction. Importantly, mTOR and EMT on one hand, and AMPK and autophagy on the other hand, negatively regulate each other. Such regulation occurs at different levels and suggests that, in many instances, these two stress responses are mutually exclusive. Nevertheless, EMT and autophagy are able to interconvert and we suggest that this may depend on spatiotemporal changes in the tumor microenvironment and/or on duration/intensity of the stressor signal(s). Eventually, we propose a three-pronged therapeutic approach aimed at targeting these three major tumor cell populations. First, cytotoxic drugs that act on differentiated and proliferating tumor cells and which, per se, may promote induction of EMT or autophagy in surviving tumor cells. Second, inhibitors of mTOR in order to prevent EMT induction. Third inducers of autophagic cell death (autosis) in order to deplete tumor cells that are constitutively in an autophagic state or are induced to enter an autophagic state in response to antitumor therapy.

High throughput scaffold-based 3D micro-tumor array for efficient drug screening and chemosensitivity testing

Oncology drug development is greatly hampered by inefficient drug screening using 2D culture. Herein, we present ready-to-use micro-scaffolds in 384-well format to generate uniform 3D micro-tumor array (3D-MTA, CV < 0.15) that predicts in vivo drug responses more accurately than 2D monolayer. 3D-MTA generated from both cell lines and primary cells achieved high screen quality (Z' > 0.5), and were compatible with standard high throughput and high content instruments. Doxorubicin identified by 3D-MTA and 2D successfully inhibited tumor growth in mice bearing lung cancer cell line (H226) xenografts, but not gemcitabine and vinorelbine, which were selected solely by 2D. Resistance towards targeted therapy was modeled on 3D-MTA, which elicited SK-BR-3 to express higher proliferation-related genes in response to gefitinb, as compared to 2D. Screening of 56 MAPK inhibitors identified pisamertib to synergistically improve cytotoxicity effect in combination with gefitinib. Primary tumor cells derived from patient-derived xenografts further attested concordance of drug response in 3D-MTA with in vivo response. 3D-MTA was further extended to realize chemosensitivity testing using patient-derived cells. Overall, 3D-MTA demonstrated strong potential to accelerate drug discovery and improve cancer treatment by providing efficient drug screening.

Efficacy of paclitaxel/dexamethasone intra-tumoral delivery in treating orthotopic mouse breast cancer

The effect of topical co-administration of promoter drugs with paclitaxel to increase anti-tumor effects of paclitaxel was investigated. Mice with orthotopic 4T1-Luc breast cancer received single intra-tumoral injection of a polymeric formulation with paclitaxel and a specific promoter drug. Several promoter drugs were evaluated, including: dexamethasone, losartan, nicotinamide, Azone, and oleic acid. Dexamethasone exhibited the highest effect on paclitaxel anti-tumor activity, in a dose-dependent fashion. However, this effect was accompanied by systemic effects of dexamethasone, and inability to prevent tumor metastasis to the lungs. Topical co-administration of promoter drugs with anti-cancer agents can enhance their anti-tumor effects. Further investigations are needed to identify the most efficient combinations of promoter and anti-cancer drugs, and their suitability for the clinical management of the breast cancer disease.

Roles of the first-generation claudin binder, Clostridium perfringens enterotoxin, in the diagnosis and claudin-targeted treatment of epithelium-derived cancers

Given that most malignant tumors are derived from epithelium, developing a strategy for treatment of epithelium-derived cancers (i.e., carcinomas) is a pivotal issue in cancer therapy. Carcinomas, including ovarian, breast, prostate, and pancreatic cancers, are known to overexpress various claudins (CLDNs); in particular, CLDN-3 and -4 are frequently overexpressed in malignant case. The generation of CLDN binders is a key for expanding CLDN-targeted cancer therapy but has been delayed due to the small size of CLDN extracellular domains (approximately 50 amino acids for the first domain and 15 amino acids for the second) and their high homology among species. Interestingly, however, the receptors for Clostridium perfringens enterotoxin (CPE), a foodborne toxin in humans, happen to be identical to CLDN-3 and -4. Thus, the first CLDN binder, CPE, has provided us CLDN-targeted cancer therapy from a concept into a potential reality. In this review, we describe roles of CPE technology in cancer therapy and discuss future directions in the CLDN-targeting concept-to-therapy process.

Core-shell nanocarriers with high paclitaxel loading for passive and active targeting

Rapid blood clearance and premature burst release are inherent drawbacks of conventional nanoparticles, resulting in poor tumor selectivity. iRGD peptide is widely recognized as an efficient cell membrane penetration peptide homing to αVβ3 integrins. Herein, core-shell nanocapsules (NCs) and iRGD-modified NCs (iRGD-NCs) with high drug payload for paclitaxel (PTX) were prepared to enhance the antitumor activities of chemotherapy agents with poor water solubility. Improved in vitro and in vivo tumor targeting and penetration were observed with NCs and iRGD-NCs; the latter exhibited better antitumor activity because iRGD enhanced the accumulation and penetration of NCs in tumors. The NCs were cytocompatible, histocompatible, and non-toxic to other healthy tissues. The endocytosis of NCs was mediated by lipid rafts in an energy-dependent manner, leading to better cytotoxicity of PTX against cancer cells. In contrast with commercial product, PTX-loaded NCs (PTX-NCs) increased area under concentration-time curve (AUC) by about 4-fold, prolonged mean resident time (MRT) by more than 8-fold and reduced the elimination rate constant by greater than 68-fold. In conclusion, the present nanocarriers with high drug-loading capacity represent an efficient tumor-targeting drug delivery system with promising potential for cancer therapy.

Anti-Cancer Stem-like Cell Compounds in Clinical Development - An Overview and Critical Appraisal

Cancer stem-like cells (CSC) represent a subpopulation of tumor cells with elevated tumor-initiating potential. Upon differentiation, they replenish the bulk of the tumor cell population. Enhanced tumor-forming capacity, resistance to antitumor drugs, and metastasis-forming potential are the hallmark traits of CSCs. Given these properties, it is not surprising that CSCs have become a therapeutic target of prime interest in drug discovery. In fact, over the last few years, an enormous number of articles describing compounds endowed with anti-CSC activities have been published. In the meanwhile, several of these compounds and also approaches that are not based on the use of pharmacologically active compounds (e.g., vaccination, radiotherapy) have progressed into clinical studies. This article gives an overview of these compounds, proposes a tentative classification, and describes their biological properties and their developmental stage. Eventually, we discuss the optimal clinical setting for these compounds, the need for biomarkers allowing patient selection, the redundancy of CSC signaling pathways and the utility of employing combinations of anti-CSC compounds and the therapeutic limitations posed by the plasticity of CSCs.

Promoting tumor penetration of nanoparticles for cancer stem cell therapy by TGF-β signaling pathway inhibition

Cancer stem cells (CSCs), which hold a high capacity for self-renewal, play a central role in the development, metastasis, and recurrence of various malignancies. CSCs must be eradicated to cure instances of cancer; however, because they can reside far from tumor vessels, they are not easily targeted by drug agents carried by nanoparticle-based drug delivery systems. We herein demonstrate that promoting tumor penetration of nanoparticles by transforming growth factor β (TGF-β) signaling pathway inhibition facilitates CSC therapy. In our study, we observed that although nanoparticles carrying siRNA targeting the oncogene polo-like kinase 1 (Plk1) efficiently killed breast CSCs derived from MDA-MB-231 cells in vitro, this intervention enriched CSCs in the residual tumor tissue following systemic treatment. However, inhibition of the TGF-β signaling pathway with LY364947, an inhibitor of TGF-β type I receptor, promoted the penetration of nanoparticles in tumor tissue, significantly ameliorating the intratumoral distribution of nanoparticles in MDA-MB-231 xenografts and further leading to enhanced internalization of nanoparticles by CSCs. As a result, synergistic treatment with a nanoparticle drug delivery system and LY364947 inhibited tumor growth and reduced the proportion of CSCs in vivo. This study suggests that enhanced tumor penetration of drug-carrying nanoparticles can enhance CSCs clearance in vivo and consequently provide superior anti-tumor effects.

Physicochemical properties of novel protein kinase inhibitors in relation to their substrate specificity for drug transporters

INTRODUCTION

Small molecule tyrosine and serine-threonine kinase inhibitors (TKIs and STKIs) are emerging drugs that interfere with downstream signaling pathways involved in cancer proliferation, invasion, metastasis and angiogenesis. The understanding of their pharmacokinetics, the identification of their transporters and the modulating activity exerted on transporters is pivotal to predict therapy efficacy and to avoid unwarranted drug treatment combinations.

AREAS COVERED

Experimental or in silico data were collected and summarized on TKIs and STKIs physico-chemical properties, which influence their transport, metabolism and efficacy, and TKIs and STKIs as influx transporter substrates and inhibitors. In addition, the uptake by tumor cell influx transporters and some factors in the tumor microenvironment affecting the uptake of TKIs and STKIs by cancer cells are briefly covered.

EXPERT OPINION

Membrane transporters play an important role in the pharmacokinetics and hence the efficacy of anticancer drugs, including TKIs and STKIs. These drugs are substrates and inhibitors of various transporters. Drug resistance may be bypassed not only by identifying the proper transporter but also by selective combinations, which may either downregulate or increase transporter activity. However, care has to be taken because this profile might be disease, drug and patient specific.

Nanoscale drug delivery for taxanes based on the mechanism of multidrug resistance of cancer

Taxanes are one type of the most extensively used chemotherapeutic agents to treat cancers. However, their clinical use is severely limited by intrinsic and acquired resistance. A diverse variety of mechanisms has been implicated about taxane resistance, such as alterations of drug targets, overexpression of efflux transporters, defective apoptotic machineries, and barriers in drug transport. The deepening understanding of molecular mechanisms of taxane resistance has spawned a number of targets for reversing resistance. However, circumvention of taxane resistance would not only possess therapeutic potential, but also face with clinical challenge, which accelerates the development of optimal nanoscale delivery systems. This review highlights the current understanding on the mechanisms of taxane resistance, and provides a comprehensive analysis of various nanoscale delivery systems to reverse taxane resistance.

Pushing tumor cells towards a malignant phenotype: stimuli from the microenvironment, intercellular communications and alternative roads

The tumor microenvironment produces different types of stimuli capable of endowing tumor cells with an aggressive behavior that is characterized by increased motility, invasiveness and propensity to metastasize, gain of a tumor-initiating phenotype, and drug resistance. The following classes of stimuli have been reported to promote such a malignant phenotype: (i) solid- or fluid-induced stress; (ii) altered composition of the extracellular matrix; (iii) hypoxia and low pH; (iv) innate and adaptive immune responses; (v) antitumor drugs. The simultaneous presence of more than one of these stimuli, as likely occurs in vivo, may lead to synergistic interactions in the induction of malignant traits. In many cases, the gain of a malignant phenotype is not the result of a direct effect of the stimuli on tumor cells but, rather, a stimulus-promoted cross-talk between tumor cells and other cell types within the tumor microenvironment. This cross-talk is mainly mediated by two classes of molecules: paracrine factors and adhesion receptors. Stimuli that promote a malignant phenotype can promote additional outcomes in tumor cells, including autophagy and cell death. We summarize here the available evidence about the variables that induce tumor cells to take one or the other of these roads in response to the same stimuli. At the end of this review, we address some unanswered questions in this domain and indicate future directions of research.

Tumor-penetrating peptide functionalization enhances the anti-glioblastoma effect of doxorubicin liposomes

The targeted therapeutic effect of nano drug delivery system for glioblastoma has been hampered by the weak enhanced permeability and retention (EPR) effect of glioblastoma and the low delivering efficiency of NDDS in glioblastoma tissue. In this study, a tumor-penetrating peptide (RGERPPR), the specific ligand of neuropilin-1 overexpressed on glioblastoma and endothelial cells, was used as a targeting moiety to enhance the anti-glioblastoma effect of doxorubicin liposomes. Firstly, RGERPPR-PEG-DSPE was synthesized and used to prepare the RGERPPR peptide-functionalized liposomes (RGE-LS), which showed vesicle sizes of around 90 nm and narrow size distributions. The cellular uptake and in vivo near-infrared fluorescence imaging test displayed that RGE-LS exhibited increased uptake by glioblastoma cells and intracranial glioblastoma tissues. The cytotoxicity assay and anti-glioblastoma study proved that RGERPPR functionalization significantly enhanced the in vitro inhibitory effect of doxorubicin liposomes on glioblastoma cells and prolonged the median survival time of nude mice bearing intracranial glioblastoma. Finally, the immunofluorescence analysis evidenced that RGE-LS were able to penetrate through tumor vessels and stroma and deep into the whole tumor tissue. The results indicated that tumor-penetrating peptide functionalization is an effective strategy for enhancing the anti-glioblastoma effect of doxorubicin liposomes.

Influence of the duration of intravenous drug administration on tumor uptake

Enhancing tumor uptake of anticancer drugs is an important therapeutic goal, because insufficient drug accumulation is now considered to be an important reason for unresponsiveness or resistance to antitumor therapy. Based on a mechanistic tumor uptake model describing tumor exposure to molecules of different molecular size after bolus administration, we have investigated the influence of the duration of intravenous administration on tumor uptake. The model integrates empirical relationships between molecular size and drug disposition (capillary permeability, interstitial diffusivity, available volume fraction, and plasma clearance), together with a compartmental pharmacokinetics model and a drug/target binding model. Numerical simulations were performed using this model for protracted intravenous drug infusion, a common mode of administration of anticancer drugs. The impact of mode of administration on tumor uptake is described for a large range of molecules of different molecular size. Evaluation was performed not only for the maximal drug concentration achieved in the tumor, but also for the dynamic profile of drug concentration. It is shown that despite a lower maximal uptake for a given dose, infusion allows for a prolonged exposure of tumor tissues to both small- and large-sized molecules. Moreover, infusion may allow higher doses to be administered by reducing Cmax-linked toxicity, thereby achieving a similar maximal uptake compared to bolus administration.

Microdevices for nanomedicine

This review surveys selected methods of manufacture and applications of microdevices-miniaturized functional devices capable of handling cell and tissue cultures or producing particles-and discusses their potential relevance to nanomedicine. Many characteristics of microdevices such as miniaturization, increased throughput, and the ability to mimic organ-specific microenvironments are promising for the rapid, low-cost evaluation of the efficacy and toxicity of nanomaterials. Their potential to accurately reproduce the physiological environments that occur in vivo could reduce dependence on animal models in pharmacological testing. Technologies in microfabrications and microfluidics are widely applicable for nanomaterial synthesis and for the development of diagnostic devices. Although the use of microdevices in nanomedicine is still in its infancy, these technologies show promise for enhancing fundamental and applied research in nanomedicine.

Approaches to improve tumor accumulation and interactions between monoclonal antibodies and immune cells

Monoclonal antibodies (mAb) have become a mainstay in tumor therapy. Clinical responses to mAb therapy, however, are far from optimal, with many patients presenting native or acquired resistance or suboptimal responses to a mAb therapy. MAbs exert antitumor activity through different mechanisms of action and we propose here a classification of these mechanisms. In many cases mAbs need to interact with immune cells to exert antitumor activity. We summarize evidence showing that interactions between mAbs and immune cells may be inadequate for optimal antitumor activity. This may be due to insufficient tumor accumulation of mAbs or immune cells, or to low-affinity interactions between these components. The possibilities to improve tumor accumulation of mAbs and immune cells, and to improve the affinity of the interactions between these components are reviewed. We also discuss future directions of research that might further improve the therapeutic efficacy of antitumor mAbs.