Tumor targeting via EPR: Strategies to enhance patient responses

Abstract

The tumor accumulation of nanomedicines relies on the enhanced permeability and retention (EPR) effect. In the last 5-10 years, it has been increasingly recognized that there is a large inter- and intra-individual heterogeneity in EPR-mediated tumor targeting, explaining the heterogeneous outcomes of clinical trials in which nanomedicine formulations have been evaluated. To address this heterogeneity, as in other areas of oncology drug development, we have to move away from a one-size-fits-all tumor targeting approach, towards methods that can be employed to individualize and improve nanomedicine treatments. To this end, efforts have to be invested in better understanding the nature, the complexity and the heterogeneity of the EPR effect, and in establishing systems and strategies to enhance, combine, bypass and image EPR-based tumor targeting. In the present manuscript, we summarize key studies in which these strategies are explored, and we discuss how these approaches can be employed to enhance patient responses.

Keywords: Cancer; Drug delivery; EPR; Nanomedicine; Tumor targeting.

Figure 1. Conventional low-molecular-weight chemotherapy versus EPR-based nanomedicine therapy.

A: Conventional small molecule chemotherapeutic drugs show high levels of off-target accumulation in healthy tissues during the distribution and elimination phase (upper parts of the panels on the left) and low levels of tumor accumulation (lower parts of the panels on the left). Conversely, nanodrugs prevent chemotherapy accumulation in healthy tissues (upper parts of the panels on the right), and promote accumulate at pathological sites (lower parts of the panels on the right). B: Typical pharmacokinetic profiles of small molecule drugs (left) and nanodrugs (right) in blood and tumors, exemplifying prolonged circulation properties and enhanced tumor accumulation over time.

Figure 2. Biological barriers contributing to heterogeneity in EPR-mediated tumor targeting.

Multiple different vascular and microenvironmental parameters contribute to heterogeneity in EPR-based nanomedicine accumulation. At the vessel level, these include vascular permeability, endothelial cell receptor expression and vascular maturation. Stromal parameters which contribute to heterogeneity in EPR-based nano-tumor targeting are the extracellular matrix, tumor cell density, hypoxia and the interstitial fluid pressure. All of these pathophysiological parameters have to be considered when aiming to developed individualized and improved nanomedicine treatments.

Figure 3. Strategies to overcome heterogeneity in EPR-based tumor targeting.

Several strategies can be employed to improve nanomedicine-based anticancer therapy. From left: Enhancing: Pharmacological and physical means, such as radiotherapy (RT), hyperthermia (HT) (adapted from [101]) and sonoporation (adapted from [133]) can be used to enhance the EPR effect in tumors. Combining: Synergism between nanomedicine-based chemotherapy and clinically relevant fractionated radiotherapy leads to increased nanomedicine accumulation and enhanced efficacy (adapted from [99]). Active targeting with pharmacologically active ligands (e.g. anti-EGFR nanobodies) synergizes with the drug molecules entrapped within a given nanomedicine formulation (adapted from [173]). Bypassing: In case of tumors with low or no EPR, vascular targeting (e.g. via RGD-targeted nanocarriers; adapted from [188]) or the use of triggerable nanocarriers that release their payload intravascularly (e.g. from drug-loaded microbubbles; adapted from [125]) can be used to improve drug delivery in spite of low/no EPR effect. Imaging: The heterogeneity in EPR-based tumor targeting can be addressed via direct or indirect imaging approaches, employing either nanotheranostics and companion nanodiagnostics to monitor the biodistribution and target site accumulation of nanomedicines, or employing the use of established images probes and protocols to visualize tumor blood vessels and the microenvironment. Imaging tumor blood vessels and EPR-based tumor targeting can help to pre-select patients for more personalized nanomedicine treatments (adapted from [210] and [205]).

Figure 4. Pharmacological and physical means to enhance tumor accumulation.

Heterogeneity in EPR-based tumor targeting can be overcome by using different pharmacological and physical means. A-B: Accumulation of radiolabeled liposomes in tumors was increased after TNF-α application, which enhances vascular permeability and tumor penetration. The concentration of liposomes was substantially higher in TNF-α-treated tumors than in control tumors (adapted from [70]). C-D: Losartan, an angiotensin II receptor blocker, decompresses tumor blood vessels and leads to improved vessel perfusion. This results in enhanced accumulation of 5-fluorouracil (5-FU; adapted from [77]). E: Extravasation of liposomes from tumor blood vessels upon applying hyperthermia at different temperatures (adapted from [120]). F: CT-FMT images showing enhanced accumulation of fluorophore-labeled liposomes in tumors after sonoporation (adapted from [123]). G: Sonoporation in combination with gemcitabine has a positive impact on the survival of patients suffering from inoperable pancreatic cancer (adapted from [131]). H: Site-specific sonoporation in combination with liposomal doxorubicin inhibits the growth of rat glioma (FUS+DOX; indicated by yellow circles) more efficiently compared to treatment with liposomal doxorubicin alone (DOX only; adapted from [136]).

Figure 5. Imaging EPR to predict nanomedicine response.

A: Mouse study with a Zirconium-89-labeled liposomal PET nanoreporter showing highly heterogeneous tumor accumulation in individual animals. B: Relative tumor increase in different 4T1 tumor-bearing mice showing that the extent of tumor accumulation correlates with antitumor efficacy (A-B: adapted from [211]). C: PEGylated liposomes were labeled with a fluorophore and with a ⁶⁴Cu PET-tracer to follow their tumor accumulation. Left image shows HER2-targeted doxorubicin liposomes in fluorescence microscopy, right image shows liposomes labeled with the PET-tracer. D: The accumulation of the companion diagnostic liposomes correlates with antitumor reponse, showing the smallest tumor volume changes for tumor with the highest levels of liposome accumulation (C-D: adapted from [212]). E: Color-coded MR images of patients before and after administration of the companion diagnostic ferumoxytol (FMX), allowing for quantification of nanoparticle tumor (encircled) accumulation. F: Clinical outcomes show that a high degree of FMX accumulation in tumors (i.e. above median; high EPR) corresponds to better therapeutic outcome, as exemplified by an overall decrease in average tumor size (E-F: adapted from [214]). G: PET-CT images exemplifying the accumulation of ⁶⁴Cu-labeled HER2-targeted PEGylated liposomes loaded with doxorubicin in breast (left) and brain (right) tumor lesions. H: Correlation between liposome accumulation at the pathological site(s) and progression-free survival, showing that patients with higher uptake tend to present with better outcomes (adapted from [215]).

Figure 6. Limited clinical translation: the role of (s.c.) cancer xenograft models.

A-B: EPR-based tumor accumulation of different nanomedicine formulations in immunocompetent versus immunocompromised mice. As compared to immunocompromised animals, immunocompetent mice tend to show increased accumulation. The location of xenograft tumors also impacts nanomedicine accumulation (adapted from [219]). C: Schematic overview of discrepancies between typically used preclinical tumor xenograft models and the real-life clinical situation. D: Comparing histology for a human primary tumor, its PDX model and the traditionally used cell line-based xenograft tumor model illustrates the fairly high similarity between the primary tumor and the PDX model, and the fairly low similarity between the primary tumor and the cell line-based xenograft tumor model (adapted from [232]).

Figure 7. Use of organoids and PDX models to promote translational (nanomedicine) research.

Tumor cells harvested via biopsies can be used for the development of organoids as well as for PDX models. Organoids enable drug screening and cytotoxicity studies, while PDX models allow for in vivo drug accumulation and treatment response studies. When performed together, these setups may help to perform more efficient and more predictive preclinical research, and they may assist in identifying the the right (nano-) drug treatment for the right patient.