Unravelling the in vivo dynamics of liposomes: Insights into biodistribution and cellular membrane interactions

Liposomes, as a colloidal drug delivery system dating back to the 1960s, remain a focal point of extensive research and stand as a highly efficient drug delivery method. The amalgamation of technological and biological advancements has propelled their evolution, elevating them to their current status. The key attributes of biodegradability and biocompatibility have been instrumental in driving substantial progress in liposome development. Demonstrating a remarkable ability to surmount barriers in drug absorption, enhance stability, and achieve targeted distribution within the body, liposomes have become pivotal in pharmaceutical research. In this comprehensive review, we delve into the intricate details of liposomal drug delivery systems, focusing specifically on their pharmacokinetics and cell membrane interactions via fusion, lipid exchange, endocytosis etc. Emphasizing the nuanced impact of various liposomal characteristics, we explore factors such as lipid composition, particle size, surface modifications, charge, dosage, and administration routes. By dissecting the multifaceted interactions between liposomes and biological barriers, including the reticuloendothelial system (RES), opsonization, enhanced permeability and retention (EPR) effect, ATP-binding cassette (ABC) phenomenon, and Complement Activation-Related Pseudoallergy (CARPA) effect, we provide a deeper understanding of liposomal behaviour in vivo. Furthermore, this review addresses the intricate challenges associated with translating liposomal technology into practical applications, offering insights into overcoming these hurdles. Additionally, we provide a comprehensive analysis of the clinical adoption and patent landscape of liposomes across diverse biomedical domains, shedding light on their potential implications for future research and therapeutic developments.

IRGD-modified erythrocyte membrane biomimetic temozolomide nanodots for the treatment of glioblastoma

The crossing of the blood-brain barrier (BBB) for conventional anticancer drugs is still a big challenge in treating glioma. The biomimetic nanoparticle delivery system has attracted increasing attention and has a promising future for crossing the BBB. Herein, we construct a multifunctional biomimetic nanoplatform using the erythrocyte membrane (EM) with the tumor-penetrating peptide iRGD (CRGDK/RGPD/EC) as a delivery, and the inner core loaded with the chemotherapeutic drug temozolomide (TMZ). The resulting biomimetic nanoparticle has perfect biocompatibility and stealth ability, which will provide more chances to escape the reticuloendothelial system (RES) entrapment, and increase the opportunity to enter the tumor site. Moreover, the decorated iRGD has been extensively used to actively targeting and deliver therapeutic agents across the BBB into glioma tissue. We show that this biomimetic delivery of TMZ with a diameter of 22 nm efficiently slowed the growth of glioblastoma multiforme (GBM) and increased the survival rate of the 30 d from 0% to 100%.

A lipo-polymeric hybrid nanosystem with metal enhanced fluorescence for targeted imaging of metastatic breast cancer

Cancer metastasis plays a major role in failure of therapeutic avenues against cancer. Owing to metastasis, nearly 70-80% of stage IV breast cancer patients lose their lives. Nanodrug delivery systems are playing a critical role in the therapy of metastatic cancer in the recent times. This paper reports the enhanced permeation and retention (EPR) based targeting of metastatic breast cancer using a novel nano lipo-polymeric system (PIR-Au NPs). The PIR-Au NPs demonstrated an increase in fluorescence by virtue of surface coating with gold, owing to the metal enhanced fluorescence phenomenon as reported in our earlier reports. Enhanced fluorescence of PIR-Au NPs was observed in murine mammary carcinoma cell line (4T1), as compared to free IR780 or IR780 loaded nanosystems (P-IR NPs), when incubated for same time at same concentrations, indicating its potential application for imaging and an enhanced bioavailability of IR780. Significant cell death was noted with photothermal mediated cytotoxicity in-vitro against breast cancer cells (MCF-7 and 4T1). An enhanced fluorescence was observed in the zebra fish embryos incubated with PIR-Au NPs. The enhanced permeation and retention (EPR) effect was seen with PIR-Au NPs in-vivo. A strong fluorescent signal was recorded in mice injected with PIR-Au NPs. The tumor tissue collected after 72 h, clearly showed a greater fluorescence as compared to other groups, indicating the plasmon enhanced fluorescence. We also demonstrated the EPR-based targeting of the PIR-Au NPs in-vivo by means of photothermal heat. This lipo-polymeric hybrid nanosystem could therefore be successfully applied for image-guided, passive-targeting to achieve maximum therapeutic benefits.

Development, Characterization and Pharmacological Evaluation of Cannabidiol-Loaded Long Circulating Niosomes

Cannabidiol (CBD) is a promising drug candidate with pleiotropic pharmacological activity, whose low aqueous solubility and unfavorable pharmacokinetics have presented obstacles to its full clinical implementation. The rational design of nanocarriers, including niosomes for CBD encapsulation, can provide a plausible approach to overcoming these limitations. The present study is focused on exploring the feasibility of copolymer-modified niosomes as platforms for systemic delivery of CBD. To confer steric stabilization, the niosomal membranes were grafted with newly synthesized amphiphilic linear or star-shaped 3- and 4-arm star-shaped copolymers based on polyglycidol (PG) and poly(ε-caprolactone) (PCL) blocks. The niosomes were prepared by film hydration method and were characterized by DLS, cryo-TEM, encapsulation efficacy, and in vitro release. Free and formulated cannabidiol were further investigated for cytotoxicity and pro-apoptotic and anti-inflammatory activities in vitro in three human tumor cell lines. The optimal formulation, based on Tween 60:Span60:Chol (3.5:3.5:3 molar ration) modified with 2.5 mol% star-shaped 3-arm copolymer, is characterized by a size of 235 nm, high encapsulation of CBD (94%), and controlled release properties. Niosomal cannabidiol retained the antineoplastic activity of the free agent, but noteworthy superior apoptogenic and inflammatory biomarker-modulating effects were established at equieffective exposure vs. the free drug. Specific alterations in key signaling molecules, implicated in programmed cell death, cancer cell biology, and inflammation, were recorded with the niosomal formulations.

Targeting endothelial permeability in the EPR effect

The characteristics of the primary tumor blood vessels and the tumor microenvironment drive the enhanced permeability and retention (EPR) effect, which confers an advantage towards enhanced delivery of anti-cancer nanomedicine and has shown beneficial effects in preclinical models. Increased vascular permeability is a landmark feature of the tumor vessels and an important driver of the EPR. The main focus of this review is the endothelial regulation of vascular permeability. We discuss current challenges of targeting vascular permeability towards clinical translation and summarize the structural components and mechanisms of endothelial permeability, the principal mediators and signaling players, the targeted approaches that have been used and their outcomes to date. We also critically discuss the effects of the tumor-infiltrating immune cells, their interplay with the tumor vessels and the impact of immune responses on nanomedicine delivery, the impact of anti-angiogenic and tumor-stroma targeting approaches, and desirable nanoparticle design approaches for greater translational benefit.

Innovative Design of Targeted Nanoparticles: Polymer-Drug Conjugates for Enhanced Cancer Therapy

Polymer-drug conjugates (PDCs) have shown great promise in enhancing the efficacy and safety of cancer therapy. These conjugates combine the advantageous properties of both polymers and drugs, leading to improved pharmacokinetics, controlled drug release, and targeted delivery to tumor tissues. This review provides a comprehensive overview of recent developments in PDCs for cancer therapy. First, various types of polymers used in these conjugates are discussed, including synthetic polymers, such as poly(↋-caprolactone) (PCL), D-α-tocopheryl polyethylene glycol (TPGS), and polyethylene glycol (PEG), as well as natural polymers such as hyaluronic acid (HA). The choice of polymer is crucial to achieving desired properties, such as stability, biocompatibility, and controlled drug release. Subsequently, the strategies for conjugating drugs to polymers are explored, including covalent bonding, which enables a stable linkage between the polymer and the drug, ensuring controlled release and minimizing premature drug release. The use of polymers can extend the circulation time of the drug, facilitating enhanced accumulation within tumor tissues through the enhanced permeability and retention (EPR) effect. This, in turn, results in improved drug efficacy and reduced systemic toxicity. Moreover, the importance of tumor-targeting ligands in PDCs is highlighted. Various ligands, such as antibodies, peptides, aptamers, folic acid, herceptin, and HA, can be incorporated into conjugates to selectively deliver the drug to tumor cells, reducing off-target effects and improving therapeutic outcomes. In conclusion, PDCs have emerged as a versatile and effective approach to cancer therapy. Their ability to combine the advantages of polymers and drugs offers enhanced drug delivery, controlled release, and targeted treatment, thereby improving the overall efficacy and safety of cancer therapies. Further research and development in this field has great potential to advance personalized cancer treatment options.

New SDS-Based Polyelectrolyte Multicore Nanocarriers for Paclitaxel Delivery-Synthesis, Characterization, and Activity against Breast Cancer Cells

The low distribution of hydrophobic anticancer drugs in patients is one of the biggest limitations during conventional chemotherapy. SDS-based polyelectrolyte multicore nanocarriers (NCs) prepared according to the layer by layer (LbL) procedure can release paclitaxel (PTX), and selectively kill cancer cells. Our main objective was to verify the antitumor properties of PTX-loaded NCs and to examine whether the drug encapsulated in these NCs retained its cytotoxic properties. The cytotoxicity of the prepared nanosystems was tested on MCF-7 and MDA-MB-231 tumour cells and the non-cancerous HMEC-1 cell line in vitro. Confocal microscopy, spectrophotometry, spectrofluorimetry, flow cytometry, and RT PCR techniques were used to define the typical hallmarks of apoptosis. It was demonstrated that PTX encapsulated in the tested NCs exhibited similar cytotoxicity to the free drug, especially in the triple negative breast cancer model. Moreover, SDS/PLL/PTX and SDS/PLL/PGA/PTX significantly reduced DNA synthesis. In addition, PTX-loaded NCs triggered apoptosis and upregulated the transcription of Bax, AIF, cytochrome-c, and caspase-3 mRNA. Our data demonstrate that these novel polyelectrolyte multicore NCs coated with PLL or PLL/PGA are good candidates for delivering PTX. Our discoveries have prominent implications for the possible choice of newly synthesized, SDS-based polyelectrolyte multicore NCs in different anticancer therapeutic applications.

Nanoparticle-based drug delivery systems to enhance cancer immunotherapy in solid tumors

Immunotherapy has developed rapidly in solid tumors, especially in the areas of blocking inhibitory immune checkpoints and adoptive T-cell transfer for immune regulation. Many patients benefit from immunotherapy. However, the response rate of immunotherapy in the overall population are relatively low, which depends on the characteristics of the tumor and individualized patient differences. Moreover, the occurrence of drug resistance and adverse reactions largely limit the development of immunotherapy. Recently, the emergence of nanodrug delivery systems (NDDS) seems to improve the efficacy of immunotherapy by encapsulating drug carriers in nanoparticles to precisely reach the tumor site with high stability and biocompatibility, prolonging the drug cycle of action and greatly reducing the occurrence of toxic side effects. In this paper, we mainly review the advantages of NDDS and the mechanisms that enhance conventional immunotherapy in solid tumors, and summarize the recent advances in NDDS-based therapeutic strategies, which will provide valuable ideas for the development of novel tumor immunotherapy regimen.

Perspectives for Improving the Tumor Targeting of Nanomedicine via the EPR Effect in Clinical Tumors

Over the past few decades, the enhanced permeability and retention (EPR) effect of nanomedicine has been a crucial phenomenon in targeted cancer therapy. Specifically, understanding the EPR effect has been a significant aspect of delivering anticancer agents efficiently to targeted tumors. Although the therapeutic effect has been demonstrated in experimental models using mouse xenografts, the clinical translation of the EPR effect of nanomedicine faces several challenges due to dense extracellular matrix (ECM), high interstitial fluid pressure (IFP) levels, and other factors that arise from tumor heterogeneity and complexity. Therefore, understanding the mechanism of the EPR effect of nanomedicine in clinics is essential to overcome the hurdles of the clinical translation of nanomedicine. This paper introduces the basic mechanism of the EPR effect of nanomedicine, the recently discussed challenges of the EPR effect of nanomedicine, and various strategies of recent nanomedicine to overcome the limitations expected from the patients' tumor microenvironments.

Human solid tumors and clinical relevance of the enhanced permeation and retention effect: a 'golden gate' for nanomedicine in preclinical studies?

Nanocarriers passively accumulate in solid tumors through irregular wide fenestrations in neovasculature and increased retention due to poor lymphatic drainage, a phenomenon termed the enhanced permeation and retention (EPR) effect. Although several preclinical reports have described the role of EPR in nanomedicine, its role in human solid tumor is obscure. There are several distinct factors for tumors in mice versus humans, including size, heterogeneity and nanomedicine pharmacokinetics. This review focuses on preclinical and clinical studies demonstrating the role of the EPR effect and passive targeting. The article illustrates the gaps that limit clinical effectiveness of the EPR effect and elaborates strategies to boost its efficiency, relaying future clinical outcomes for designing clinically applicable EPR-based nanomedicine.

pH-Responsive Poly(ethylene glycol)-b-poly(2-vinylpyridine) Micelles for the Triggered Release of Therapeutics

The use of pH-responsive polymeric micelles is a promising approach to afford the targeted, pH-mediated delivery of hydrophobic drugs within the low-pH tumour milieu and intracellular organelles of cancer cells. However, even for a common pH-responsive polymeric micelle system-e.g., those utilising poly(ethylene glycol)-b-poly(2-vinylpyridine) (PEG-b-PVP) diblock copolymers-there is a lack of available data describing the compatibility of hydrophobic drugs, as well as the relationships between copolymer microstructure and drug compatibility. Furthermore, synthesis of the constituent pH-responsive copolymers generally requires complex temperature control or degassing procedures that limit their accessibility. Herein we report the facile synthesis of a series of diblock copolymers via visible-light-mediated photocontrolled reversible addition-fragmentation chain-transfer polymerisation, with a constant PEG block length (90 repeat units (RUs)) and varying PVP block lengths (46-235 RUs). All copolymers exhibited narrow dispersity values (Đ ≤ 1.23) and formed polymeric micelles with low polydispersity index (PDI) values (typically <0.20) at physiological pH (7.4), within a suitable size range for passive tumour targeting (<130 nm). The encapsulation and release of three hydrophobic drugs (cyclin-dependent kinase inhibitor (CDKI)-73, gossypol, and doxorubicin) were investigated in vitro at pH 7.4-4.5 to simulate drug release within the tumour milieu and cancer cell endosome. Marked differences in drug encapsulation and release were observed when the PVP block length was increased from 86 to 235 RUs. With a PVP block length of 235 RUs, the micelles exhibited differing encapsulation and release properties for each drug. Minimal release was observed for doxorubicin (10%, pH 4.5) and CDKI-73 exhibited moderate release (77%, pH 4.5), whereas gossypol exhibited the best combination of encapsulation efficiency (83%) and release (91% pH 4.5) overall. These data demonstrate the drug selectivity of the PVP core, where both the block molecular weight and hydrophobicity of the core (and accordingly the hydrophobicity of the drug) have a significant effect on drug encapsulation and release. These systems remain a promising means of achieving targeted, pH-responsive drug delivery-albeit for select, compatible hydrophobic drugs-which warrants their further investigation to develop and evaluate clinically relevant micelle systems.

Influence of lung cancer model characteristics on tumor targeting behavior of nanodrugs

Evidence is mounting that there is a significant gap between the antitumor efficacy of nanodrugs in preclinical mouse tumor models and in clinical human tumors, and that differences in tumor models are likely to be responsible for this gap. Herein, we investigated the enhanced permeability and retention (EPR) effect in mouse lung cancer models with different tumor growth rates, volumes and locations, and analyzed the nanodrug tumor targeting behaviors limited by tumor vascular pathophysiological characteristics in various tumor models. The results showed that the fast-growing tumors were characterized by lower vascular tight junctions, leading to higher vascular paracellular transport activity and nanodrug tumor accumulation. The paracellular transport activity increased with the growth of tumor, but the vascular density and transcellular transport activity decreased, and as a result, the average tumor accumulation of passive targeting nanodrugs decreased. Orthotopic tumors were rich in blood vessels, but had low vascular transcellular and paracellular transport activities, making it difficult for nanodrug accumulation in orthotopic tumors via passive targeting strategies. The antitumor efficacy of passive targeting nanodrugs in various lung cancer-bearing mice validated the aforementioned nanodrug accumulation behavior, and nanodrugs based on the angiogenesis-tumor sequential targeting strategy achieved obviously improved efficacy in orthotopic lung cancer-bearing mice. These results suggest that the EPR effect varies in different tumor models and should not be used as a universal targeting strategy for antitumor nanodrugs. Besides, attention should be paid to the animal tumor models in the evaluation of nanodrugs so as to avoid exaggerating the antitumor efficacy.

HPMA copolymer conjugated 5-aminolevulinic acid exhibits superior efficacy for photodynamic therapy with tumor-responsive and targeting properties

In this study, we developed a nanoformulation of 5-aminolevulinic acid (5-ALA) for tumor-targeted photodynamic therapy, in which 5-ALA was conjugated with a biocompatible polymer N-(2-hydroxypropyl)methacrylamide (HPMA) through the hydrazone bond, i.e., P-ALA. P-ALA behaves as the nano-sized molecule with an average size of 5.5 nm in aqueous solution. P-ALA shows a largely increased release rate in acidic pH than physiological pH, suggesting the rapid release profile in acidic tumor environment. P-ALA did not show apparent cytotoxicity up to 0.1 mg/ml, however, under light irradiation, remarkable cell death was induced with the IC₅₀ of 20-30 μg/ml. More importantly, we found significantly higher tumor accumulation of P-ALA than 5-ALA which benefit from its nano-size by taking advantage of the enhanced permeability and retention (EPR) effect. Consequently, P-ALA exhibited much improved in vivo antitumor efficacy without any apparent side effects. We thus anticipate the application of P-ALA as a nano-designed photosensitizer for anticancer photodynamic therapy.

Effect of Cellular and Microenvironmental Multidrug Resistance on Tumor-Targeted Drug Delivery in Triple-Negative Breast cancer

Multidrug resistance (MDR) reduces the efficacy of chemotherapy. Besides inducing the expression of drug efflux pumps, chemotherapy treatment alters the composition of the tumor microenvironment (TME), thereby potentially limiting tumor-directed drug delivery. To study the impact of MDR signaling in cancer cells on TME remodeling and nanomedicine delivery, we generated multidrug-resistant 4T1 triple-negative breast cancer (TNBC) cells by exposing sensitive 4T1 cells to gradually increasing doxorubicin concentrations. In 2D and 3D cell cultures, resistant 4T1 cells are presented with a more mesenchymal phenotype and produced increased amounts of collagen. While sensitive and resistant 4T1 cells showed similar tumor growth kinetics in vivo, the TME of resistant tumors was enriched in collagen and fibronectin. Vascular perfusion was also significantly increased. Fluorophore-labeled polymeric (∼10 nm) and liposomal (∼100 nm) drug carriers were administered to mice with resistant and sensitive tumors. Their tumor accumulation and penetration were studied using multimodal and multiscale optical imaging. At the whole tumor level, polymers accumulate more efficiently in resistant than in sensitive tumors. For liposomes, the trend was similar, but the differences in tumor accumulation were insignificant. At the individual blood vessel level, both polymers and liposomes were less able to extravasate out of the vasculature and penetrate the interstitium in resistant tumors. In a final in vivo efficacy study, we observed a stronger inhibitory effect of cellular and microenvironmental MDR on liposomal doxorubicin performance than free doxorubicin. These results exemplify that besides classical cellular MDR, microenvironmental drug resistance features should be considered when aiming to target and treat multidrug-resistant tumors more efficiently.

Dual-targeting of brain tumors with nanovesicles

The delivery of chemotherapies to brain tumors faces the difficult task of crossing the blood-brain barrier (BBB).^1-4 The brain capillary endothelial cells (BCECs) along with other cell lines, such as astrocytes and pericytes, form the BBB. This highly selective semipermeable barrier separates the blood from the brain parenchyma. The BBB controls the movement of drug molecules in a selective manner⁵ and maintains central nervous system (CNS) homeostasis. Depending on the properties of drugs such as their hydrophilic-lipophilic balance (HLB), some can cross the BBB through passive diffusion.⁶ However, this approach alone has not led to successful drug developments due to low net diffusion rates and systemic toxicity. Although the use of nanomedicine has been proposed to overcome these drawbacks, many recent studies still rely on the so-called 'enhanced permeability and retention (EPR)' effect though there is a realization in the field of drug delivery that EPR effect may not be sufficient for successful drug delivery to brain tumors. Since, compared to many other solid tumors, brain tumors pose additional challenges such as more restrictive blood-tumor barrier as well as the well-developed lymphatic drainage, the selection of functional moieties on the nanocarriers under consideration must be carried out with care to propose better solutions to this challenge.

Effect of Hypertension on EPR effect Induced by Polymer Nanomicelles in Renal Cell Carcinoma in vitro

Objective

To investigate the effect of hypertension on the (enhanced permeability and retention, EPR) effect induced by polymer nanomicelles in renal cell carcinoma in vitro.

Methods

A total of 80 patients with renal cell carcinoma treated at the Department of Urology Surgery in the Dept. of Urology of the Affiliated Hospital of Hebei University from Oct. 2019 to Oct. 2020, were analyzed retrospectively. The hypertension group (experimental group) included 40 patients, and the normal blood pressure group (control group) included 40 patients. The diagnosis of renal clear cell carcinoma was confirmed by preoperative auxiliary examinations, such as ultrasonography and CT combined with postoperative pathological analysis. All patients underwent laparoscopic radical nephrectomy for renal cell carcinoma. Polymer nanomicelles (loaded with prolonium iodide) were perfused into the resected kidney specimens within the specified time. The iodine enrichment of polymer nanomicelles in renal tumors was assessed by CT scanning. The peak EPR effect and the time to the peak were statistically compared between the two groups.

Results

No significant differences were found in age, sex, location of kidney disease, tumor location or tumor size between the two groups (p> 0.05). The peak (χ̄±S) of the EPR effect in experimental group was 3.60±0.95 ug/cm3 and 3.01±0.96 ug/cm3 in control group, respectively. There was significant difference between the two groups (p< 0.05). The time to the peak of the EPR effect was 3.76±0.75 h in experimental group and 3.82±0.93 hour in control group, respectively. No statistically significant difference was found in the time to the peak of the EPR effect between the two groups (p> 0.05).

Conclusion

Hypertension has a certain effect on the EPR effect induced by polymer nanomicelles in renal cell carcinoma in vitro.

Supermagnetic Human Serum Albumin (HSA) Nanoparticles and PLGA-Based Doxorubicin Nanoformulation: A Duet for Selective Nanotherapy

Predicting the ability of nanoparticles (NP) to access the tumor is key to the success of chemotherapy using nanotherapeutics. In the present study, the ability of the dual NP-based theranostic system to accumulate in the tumor was evaluated in vivo using intravital microscopy (IVM) and MRI. The system consisted of model therapeutic doxorubicin-loaded poly(lactide-co-glycolide) NP (Dox-PLGA NP) and novel hybrid Ce^3/4+-doped maghemite NP encapsulated within the HSA matrix (hMNP) as a supermagnetic MRI contrasting agent. Both NP types had similar sizes of ~100 nm and negative surface potentials. The level of the hMNP and PLGA NP co-distribution in the same regions of interest (ROI, ~2500 µm²) was assessed by IVM in mice bearing the 4T1-mScarlet murine mammary carcinoma at different intervals between the NP injections. In all cases, both NP types penetrated into the same tumoral/peritumoral regions by neutrophil-assisted extravasation through vascular micro- and macroleakages. The maximum tumor contrasting in MRI scans was obtained 5 h after hMNP injection/1 h after PLGA NP injection; the co-distribution level at this time reached 78%. Together with high contrasting properties of the hMNP, these data indicate that the hMNP and PLGA NPs are suitable theranostic companions. Thus, analysis of the co-distribution level appears to be a useful tool for evaluation of the dual nanoparticle theranostics, whereas assessment of the leakage areas helps to reveal the tumors potentially responsive to nanotherapeutics.

The tumor EPR effect for cancer drug delivery: Current status, limitations, and alternatives

Over the past three decades, the enhanced permeability and retention (EPR) effect has been considered the basis of tumor-targeted drug delivery. Various cancer nanomedicines, including macromolecular drugs, have been designed to utilize this mechanism for preferential extravasation and accumulation in solid tumors. However, such nanomedicines have not yet achieved convincing therapeutic benefits in clinics. Increasing evidence suggests that the EPR effect is over-represented in human tumors, especially in metastatic tumors. This review covers the evolution of the concept, the heterogeneity and limitation of the EPR effect in clinical realities, and prospects for alternative strategies independent of the EPR effect.

Extracellular matrix degrading enzyme with stroma-targeting peptides enhance the penetration of liposomes into tumors

Various anti-tumor nanomedicines have been developed based on the enhanced permeability and retention effect. However, the dense extracellular matrix (ECM) in tumors remains a major barrier for the delivery and accumulation of nanoparticles into tumors. While ECM-degrading enzymes, such as collagenase, hyaluronidase, and bromelain, have been used to facilitate the accumulation of nanoparticles, serious side effects arising from the current non-tumor-specific delivery methods limit their clinical applications. Here, we report targeted delivery of bromelain into tumor tissues through its covalent attachment to a hyaluronic acid (HA)-peptide conjugate with tumor ECM targeting ability. The ECM targeting peptide, collagen type IV-binding peptide (C4BP), was chosen from six candidate-peptides based on their ability to bind to frozen sections of triple-negative breast cancer, 4T1 tumor ex vivo. The HA- C4BP conjugate showed a significant increase in tumor accumulation in 4T1-bearing mice after intravenous administration compared to unmodified HA. We further demonstrated that the systemic administration of bromelain conjugated C4BP-HA (C4BP-HA-Bro) potentiates the anti-tumor efficacy of liposomal doxorubicin. C4BP-HA-Bro decreased the number and length of collagen fibers and improved the distribution of doxorubicin within the tumor. No infusion reaction was noted after delivery of C4BP-HA-Bro. C4BP-HA thus offers a potential for effective and safe delivery of bromelain for improved intratumoral delivery of therapeutics.

Poly(Styrene-Co-Maleic Acid)-Conjugated 6-Aminofluorescein and Rhodamine Micelle as Macromolecular Fluorescent Probes for Micro-Tumors Detection and Imaging

Styrene-co-maleic acid (SMA) copolymer was evaluated as a polymer platform to conjugate with two fluorescent dyes, i.e., 6-aminofluorescein (AF) and Rhodamine (Rho); which spontaneously self-assembles in an aqueous medium and forms a micelle through a non-covalent interaction. These SMA-dye conjugates showed the nanosized micelle formation through dynamic light scattering (DLS) with discrete distributions having mean particle sizes of 135.3 nm, and 190.9 nm for SMA-AF, and SMA-Rho, respectively. The apparent molecular weight of the micelle was evaluated using Sephadex G-100 gel chromatography and it was found that the 49.3 kDa, and 28.7 kDa for SMA-AF, and SMA-Rho, respectively. Moreover, the biodistribution study showed the selective accumulation of the SMA-dye conjugates in the tumor of mice. Taken together, the SMA-dye conjugated micelles appear in high concentrations in the tumor by utilizing the enhanced permeability and retention (EPR) effect of the tumor-targeted delivery. These results indicate that SMA-dye conjugates have the advanced potential as macromolecular fluorescent probes for microtumor imaging by means of a photodynamic diagnosis.

Prediction the clinical EPR effect of nanoparticles in patient-derived xenograft models

Many preclinically tested nanoparticles in existing animal models fail to be directly translated into clinical applications because of their poor resemblance to human cancer. Herein, the enhanced permeation and retention (EPR) effect of glycol chitosan nanoparticles (CNPs) in different tumor microenvironments (TMEs) was compared using different pancreatic tumor models, including pancreatic cancer cell line (BxPC3), patient-derived cancer cell (PDC), and patient-derived xenograft (PDX) models. CNPs were intravenously injected into different tumor models, and their accumulation efficiency was evaluated using non-invasive near-infrared fluorescence (NIRF) imaging. In particular, differences in angiogenic vessel density, collagen matrix, and hyaluronic acid content in tumor tissues of the BxPC3, PDC, and PDX models greatly affected the tumor-targeting efficiency of CNPs. In addition, different PDX models were established using different tumor tissues of patients to predict the clinical EPR effect of CNPs in inter-patient TMEs, wherein the gene expression levels of PECAM1, COL4A1, and HAS1 in human tumor tissues were observed to be closely related to the EPR effect of CNPs in PDX models. The results suggested that the PDX models could mimic inter-patient TMEs with different blood vessel structures and extracellular matrix (ECM) content that critically affect the tumor-targeting ability of CNPs in different pancreatic PDX models. This study provides a better understanding of the heterogeneity and complexity of inter-patient TMEs that can predict the response of various nanoparticles in individual tumors for personalized cancer therapy.

Strategies for persistent retention of macromolecules and nanoparticles in the blood circulation

The enhanced permeability and retention (EPR) effect has been the gold standard in developing drug delivery systems for passive tumor targeting. Although the importance of this concept remains unchanged, some controversies have arisen. In this review, various strategies for tumor targeting using macromolecules and nanoparticles based on the EPR effect are discussed from the viewpoint of pharmacokinetics. Overall, such strategies seek to retain therapeutic material in the blood circulation, which is a key factor for successful targeting. Strategies using macromolecules, including antibody-drug conjugates, serum albumin-based delivery systems, PEGylated recombinant proteins, and stealth liposomes as well as nanoparticle-based strategies such as those based on lipid nanoparticles, and polymeric micelles, have been discussed. The feasibility of small extracellular vesicles, a new class of nanosized delivery carriers, is also discussed.

The role of size in PEGylated liposomal doxorubicin biodistribution and anti-tumour activity

The size of nanoliposome‐encapsulated drugs significantly affects their therapeutic efficacy, biodistribution, targeting ability, and toxicity profile for the cancer treatment. In the present study, the biodistribution and anti‐tumoral activity of PEGylated liposomal Doxorubicin (PLD) formulations with different sizes were investigated. First, 100, 200, and 400 nm PLDs were prepared by remote loading procedure and characterised for their size, zeta potential, encapsulation efficacy, and release properties. Then, in vitro cellular uptake and cytotoxicity were studied by flow cytometry and MTT assay, and compared with commercially available PLD Caelyx^®. In vivo studies were applied on BALB/c mice bearing C26 colon carcinoma. The cytotoxicity and cellular uptake tests did not demonstrate any statistically significant differences between PLDs. The biodistribution results showed that Caelyx^® and 100 nm liposomal formulations had the most doxorubicin (Dox) accumulation in the tumour tissue and, as a result, considerably suppressed tumour growth compared with 200 and 400 nm PLDs. In contrast, larger nanoparticles (200 and 400 nm formulations) had more accumulation in the liver and spleen. This study revealed that 90 nm Caelyx^® biodistribution profile led to the stronger anti‐tumour activity of the drug and hence significant survival extension, and showed the importance of vesicle size in the targeting of nanoparticles to the tumour microenvironment for the treatment of cancer.

Ultrasonic Microbubble Cavitation Enhanced Tissue Permeability and Drug Diffusion in Solid Tumor Therapy

Chemotherapy has an essential role not only in advanced solid tumor therapy intervention but also in society’s health at large. Chemoresistance, however, seriously restricts the efficiency and sensitivity of chemotherapeutic agents, representing a significant threat to patients’ quality of life and life expectancy. How to reverse chemoresistance, improve efficacy sensitization response, and reduce adverse side effects need to be tackled urgently. Recently, studies on the effect of ultrasonic microbubble cavitation on enhanced tissue permeability and retention (EPR) have attracted the attention of researchers. Compared with the traditional targeted drug delivery regimen, the microbubble cavitation effect, which can be used to enhance the EPR effect, has the advantages of less trauma, low cost, and good sensitization effect, and has significant application prospects. This article reviews the research progress of ultrasound-mediated microbubble cavitation in the treatment of solid tumors and discusses its mechanism of action to provide new ideas for better treatment strategies.

Navigations of the targeting pathway of nanomedicines towards tumor

INTRODUCTION

Nanomedicines (NMs) have emerged as a promising approach for revolutionizing cancer treatment outcomes, mainly due to their benefits in the tumor targeted delivery of therapeutics. The preferential accumulation of NMs in tumor has been widely verified by macroscopical technologies. Accordingly, several classic and emerging targeting mechanisms have been proposed to support the tumor-specific delivery of NMs. The targeting mechanism has been a topic of intensive interest and controversy in the field of NMs development. Especially, the mechanisms by which NMs target tumor remain elusive.

AREA COVERED

This topical review mainly discussed the evolution of the targeting mechanisms, crucial issues associated with each mechanism, and confused debates among the mechanisms. The targeting mechanisms of tumor-specific NMs discussed here include the enhanced permeability and retention (EPR) effect, protein corona-mediated targeting delivery, circulating cell mediated transportation, and transcytosis.

EXPERT OPINION

It is of great significance for ultimate clinical translation to have more comprehensive considerations on the mechanism driving the pathway of NMs toward tumors. Our thoughts in this review are expected to provide comprehensive understanding on the mechanisms and elicit thorough explorations on new mechanism to renovate the knowledge framework of NMs delivery.

Strategies to enhance drug delivery to solid tumors by harnessing the EPR effects and alternative targeting mechanisms

The Enhanced Permeability and Retention (EPR) effect has been recognized as the central paradigm in tumor-targeted delivery in the last decades. In the wake of this concept, nanotechnologies have reached phenomenal levels in research. However, clinical tumors display a poor manifestation of EPR effect. Factors including tumor heterogeneity, complicating tumor microenvironment, and discrepancies between laboratory models and human tumors largely contribute to poor efficiency in tumor-targeted delivery and therapeutic failure in clinical translation. In this article, approaches for evaluation of EPR effect in human tumor were overviewed as guidance to employ EPR effect for cancer treatment. Strategies to augment EPR-mediated tumoral delivery are discussed in different dimensions including enhancement of vascular permeability, depletion of tumor extracellular matrix and optimization of nanoparticle design. Besides, the recent development in alternative tumor-targeted delivery mechanisms are highlighted including transendothelial pathway, endogenous cell carriers and non-immunogenic bacteria-mediated delivery. In addition, the emerging preclinical models better reflect human tumors are introduced. Finally, more rational applications of EPR effect in other disease and field are proposed. This article elaborates on fundamental reasons for the gaps between theoretical expectation and clinical outcomes, attempting to provide some perspective directions for future development of cancer nanomedicines in this still evolving landscape.

35 years of discussions with Prof. Maeda on the EPR effect and future directions

In May 2021, 35 years after first announcing the enhanced permeability and retention (EPR) effect, Dr. Maeda passed away. As a theoretical pillar of high molecular weight drug delivery systems (DDS) with high biocompatibility, the EPR effect has been proven worldwide in experimental mouse models. However, in clinical solid tumors, awareness of the EPR effect is insufficient, and more importantly, DDS has not become the mainstream cancer treatment. Both Dr. Maeda and I were acutely aware of this, and for 35 years, we discussed what to do about it and strived to make up for the inadequacies of the EPR effect by employing different strategies. Dr. Maeda came up with ways to use tumor vascular permeability more effectively and to apply oxidative stress to tumor cells. I proposed cancer stromal targeting (CAST) therapy using the anti-insoluble fibrin antibody conjugated with an anticancer agent in order to overcome the insufficiency of the EPR effect in clinical solid cancers, which possess abundant stromal tissue. Clinical cancers are surrounded by an abundant stroma and survive even under hypoxia and malnutrition due to this stromal barrier. Cancer cells become resistant to any external attack, including with anticancer drugs and radiation. While it goes without saying that EPR effects are important in clinical solid cancer strategies, DDSs that offer both accumulation and even distribution in solid cancers are also required.

Combined legumain- and integrin-targeted nanobubbles for molecular ultrasound imaging of breast cancer

Molecular ultrasound imaging is a promising strategy for non-invasive and precise cancer diagnosis. Previously reported ultrasound contrast agents (UCAs) are mostly microbubbles or nanobubbles (NBs) larger than 200 nm, leading to less efficient tumor delivery. Here we synthesized NBs with a small size (~49 nm) and modified the NB surface with alanine-alanine-asparagine (NB-A) or arginine-glycine-aspartic acid peptide (NB-R) for concurrent active targeting towards legumain in tumor cells and integrin in tumor neovasculature. In vitro, the NB-A and NB-R presented echogenicity comparable with SonoVue MBs and showed specific binding with tumors cells and endothelial cells, respectively. In vivo, the combined NB-A/NB-R accumulated in tumor tissues selectively and provided ultrasound signals with prolonged duration and that were significantly stronger than non-targeted NBs, single-targeted NBs and SonoVue MBs. Overall, the dual targeted NBs served as efficient UCAs for specific imaging of breast cancer, and hold great potential for general cancer diagnosis/monitoring in the future.

Changeable net charge on nanoparticles facilitates intratumor accumulation and penetration

The Enhanced Permeability and Retention (EPR) effect is a golden strategy for the nanoparticle (NP)-based targeting of solid tumors, and the surface property of NPs might be a determinant on their targeting efficiency. Poly(ethylene glycol) (PEG) is commonly used as a shell material; however, it has been pointed out that PEG-coated NPs may exhibit accumulation near tumor vasculature rather than having homogenous intratumor distribution. The PEG shell plays a pivotal role on prolonged blood circulation of NPs but potentially impairs the intratumor retention of NPs. In this study, we report on a shell material to enhance tumor-targeted delivery of NPs by maximizing the EPR effect: polyzwitterion based on ethylenediamine-based carboxybetaine [PGlu(DET-Car)], which shows the changeable net charge responding to surrounding pH. The net charge of PGlu(DET-Car), is neutral at physiological pH 7.4, allowing it to exhibit a stealth property during the blood circulation; however, it becomes cationic for tissue-interactive performance under tumorous acidic conditions owing to the stepwise protonation behavior of ethylenediamine. Indeed, the PGlu(DET-Car)-coated NPs (i.e., gold NPs in the present study) exhibited prolonged blood circulation and remarkably enhanced tumor accumulation and retention than PEG-coated NPs, achieving 32.1% of injected dose/g of tissue, which was 4.2 times larger relative to PEG-coated NPs. Interestingly, a considerable portion of PGlu(DET-Car)-coated NPs clearly penetrated into deeper tumor sites and realized the effective accumulation in hypoxic regions, probably because the cationic net charge of PGlu(DET-Car) is augmented in more acidic hypoxic regions. This study suggests that the changeable net charge on the NP surface in response to tumorous acidic conditions is a promising strategy for tumor-targeted delivery based on the EPR effect.

Monitoring EPR Effect Dynamics during Nanotaxane Treatment with Theranostic Polymeric Micelles

Cancer nanomedicines rely on the enhanced permeability and retention (EPR) effect for efficient target site accumulation. The EPR effect, however, is highly heterogeneous among different tumor types and cancer patients and its extent is expected to dynamically change during the course of nanochemotherapy. Here the authors set out to longitudinally study the dynamics of the EPR effect upon single- and double-dose nanotherapy with fluorophore-labeled and paclitaxel-loaded polymeric micelles. Using computed tomography-fluorescence molecular tomography imaging, it is shown that the extent of nanomedicine tumor accumulation is predictive for therapy outcome. It is also shown that the interindividual heterogeneity in EPR-based tumor accumulation significantly increases during treatment, especially for more efficient double-dose nanotaxane therapy. Furthermore, for double-dose micelle therapy, tumor accumulation significantly increased over time, from 7% injected dose per gram (ID g^-1 ) upon the first administration to 15% ID g^-1 upon the fifth administration, contributing to more efficient inhibition of tumor growth. These findings shed light on the dynamics of the EPR effect during nanomedicine treatment and they exemplify the importance of using imaging in nanomedicine treatment prediction and clinical translation.

Strategies to improve the EPR effect: A mechanistic perspective and clinical translation

Many efforts have been made to achieve targeted delivery of anticancer drugs to enhance their efficacy and to reduce their adverse effects. These efforts include the development of nanomedicines as they can selectively penetrate through tumor blood vessels through the enhanced permeability and retention (EPR) effect. The EPR effect was first proposed by Maeda and co-workers in 1986, and since then various types of nanoparticles have been developed to take advantage of the phenomenon with regards to drug delivery. However, the EPR effect has been found to be highly variable and thus unreliable due to the complex tumor microenvironment. Various physical and pharmacological strategies have been explored to overcome this challenge. Here, we review key advances and emerging concepts of such EPR-enhancing strategies. Furthermore, we analyze 723 clinical trials of nanoparticles with EPR enhancers and discuss their clinical translation.

Feasibility of using a 99mTc-hydroxamamide complex containing an albumin binder moiety for in vivo albumin labeling-based tumor imaging

Human serum albumin (HSA), which is distributed throughout the blood, is used as a carrier for transporting drugs to tumors based on the enhanced permeability and retention (EPR) effect. To develop an agent for the in vivo radiolabeling of endogenous albumin, we designed and synthesized novel hydroxamamide (Ham)-based technetium-99m (^99mTc) complexes, which contained a monovalent or bivalent 4-(4-iodophenyl)butyric acid (IA) derivative as an albumin binder (ALB) moiety ([^99mTc]AB2 and [^99mTc]ALB2, respectively), and evaluated their utility for in vivo tumor imaging. In an in vitro HSA-binding assay, [^99mTc]AB2 and [^99mTc]ALB2 showed greater binding to HSA than [^99mTc]BHam, a ^99mTc-Ham complex without an ALB moiety. In an in vivo biodistribution assay, [^99mTc]ALB2 showed marked blood and tumor retention (25.13 and 4.61% injected dose (ID)/g, respectively, at 1 h postinjection), suggesting that the EPR effect had been induced. However, [^99mTc]AB2 showed no marked blood or tumor retention (4.16 and 0.75% ID/g, respectively, at 1 h postinjection), probably because the affinity of the monovalent IA derivative for albumin was insufficient to induce the EPR effect. These findings indicated that the multivalent interactions of [^99mTc]ALB2 had enhanced its affinity for albumin. ^99mTc-complexes containing multivalent ALB moieties may be useful for tumor imaging.

Styrene Maleic Acid Copolymer-Based Micellar Formation of Temoporfin (SMA@ mTHPC) Behaves as A Nanoprobe for Tumor-Targeted Photodynamic Therapy with A Superior Safety

Tumor-targeted photodynamic therapy (PDT) using polymeric photosensitizers is a promising anticancer therapeutic strategy. Previously, we developed several polymeric nanoprobes for PDT using different polymers and PDT agents. In the study, we synthesized a styrene maleic acid copolymer (SMA) micelle encapsulating temoporfin (mTHPC) that is a clinically used PDT drug, SMA@mTHPC, with a hydrodynamic size of 98 nm, which showed high water solubility. SMA@mTHPC maintained stable micelle formation in physiological aqueous solutions including serum; however, the micelles could be disrupted in the presence of detergent (e.g., Tween 20) as well as lecithin, the major component of cell membrane, suggesting micelles will be destroyed and free mTHPC will be released during intracellular uptake. SMA@mTHPC showed a pH-dependent release profile, for which a constant release of ≈20% per day was found at pH 7.4, and much more release occurred at acidic pH (e.g., 6.5, 5.5), suggesting extensive release of free mTHPC could occur in the weak acidic environment of a tumor and further during internalization into tumor cells. In vitro cytotoxicity assay showed a lower cytotoxicity of SMA@mTHPC than free mTHPC; however, similar in vivo antitumor effects were observed by both SMA@mTHPC and free THPC. More importantly, severe side effects (e.g., body weight loss, death of the mice) were found during free mTHPC treatment, whereas no apparent side effects were observed for SMA@mTHPC. The superior safety profile of SMA@mTHPC was mostly due to its micelle formation and the enhanced permeability and retention (EPR) effect-based tumor accumulation, as well as the tumor environment-responsive release properties. These findings suggested SMA@mTHPC may become a good candidate drug for targeted PDT with high safety.

Acid-responsive HPMA copolymer-bradykinin conjugate enhances tumor-targeted delivery of nanomedicine

Obstructed blood flow and erratic blood supply in the tumor region attenuate the distribution and accumulation of nanomedicines in the tumor. Therefore, improvement of these conditions is crucial for efficient drug delivery. In this study, we designed and synthesized a novel N-(2-hydroxypropyl)methacrylamide (HPMA)-based copolymer conjugate of BK, which possessed adequate systemic stability and tumor-selective action required to improve the accumulation of nanomedicines in the tumor. Levulinoyl-BK (Lev-BK) was conjugated to an HPMA-based polymer via an acid-cleavable hydrazone bond (P-BK). An acid-responsive release of Lev-BK from P-BK was observed, and P-BK alone after intradermal application showed below 10% of the BK activity, thus proving a reduction in the vascular permeability activity of BK when attached to the polymer carrier. P-BK pre-treatment improved blood flow in the tumor tissue by 1.4-1.7-fold, which was maintained for more than 4 h. In addition, P-BK pre-treatment increased the tumor accumulation of pegylated liposomal doxorubicin (PLD) by approximately 3-fold. Furthermore, P-BK pre-treatment led to superior antitumor activity of PLD and significantly improved the survival of tumor-bearing mice. The release of BK from P-BK in the acidic milieu of the tumor was a prerequisite for P-BK to exert its effect, as the vascular permeability enhancing activity of P-BK was negligible. Collectively, P-BK pre-treatment improved intratumoral blood flow and augmented tumor accumulation of nanomedicine, thereby resulting in a significant suppression of tumor growth. Therefore, these findings demonstrate that P-BK is a potential concomitant drug for improving the tumor delivery of nanomedicines.

Electrochemotherapy of Deep-Seated Tumors: State of Art and Perspectives as Possible "EPR Effect Enhancer" to Improve Cancer Nanomedicine Efficacy

Simple Summary

Electroporation-based therapies (reversible electroporation, irreversible electroporation, electrochemotherapy) are used for the selective treatment of deep-seated tumors. The combination of the structural modifications of the lipid bilayer of cell membranes, due to the application of electrical pulses in the targeted tissue, with the concomitant systemic (intravenous) administration of drugs can be considered as a sort of bridge between local-regional and systemic treatments. A possible further application of these techniques can be envisaged in their use as enhancers of the so-called “enhanced permeability and retention” effect. The intratumoral uptake of drug-loaded nanocarriers concomitant with the application of electric pulses in the target tumor is a new scenario worthy of attention and can represent a potential new frontier for drug delivery in oncology.

Abstract

Surgical resection is the gold standard for the treatment of many kinds of tumor, but its success depends on the early diagnosis and the absence of metastases. However, many deep-seated tumors (liver, pancreas, for example) are often unresectable at the time of diagnosis. Chemotherapies and radiotherapies are a second line for cancer treatment. The “enhanced permeability and retention” (EPR) effect is believed to play a fundamental role in the passive uptake of drug-loaded nanocarriers, for example polymeric nanoparticles, in deep-seated tumors. However, criticisms of the EPR effect were recently raised, particularly in advanced human cancers: obstructed blood vessels and suppressed blood flow determine a heterogeneity of the EPR effect, with negative consequences on nanocarrier accumulation, retention, and intratumoral distribution. Therefore, to improve the nanomedicine uptake, there is a strong need for “EPR enhancers”. Electrochemotherapy represents an important tool for the treatment of deep-seated tumors, usually combined with the systemic (intravenous) administration of anticancer drugs, such as bleomycin or cisplatin. A possible new strategy, worthy of investigation, could be the use of this technique as an “EPR enhancer” of a target tumor, combined with the intratumoral administration of drug-loaded nanoparticles. This is a general overview of the rational basis for which EP could be envisaged as an “EPR enhancer” in nanomedicine.

Sequential administration of PEG-Span 80 niosome enhances anti-tumor effect of doxorubicin-containing PEG liposome

To improve the anti-tumor effect of polyethylene glycol-modified liposome containing doxorubicin (DOX-PEG liposome), the effect of sequential administration of PEG-Span 80 niosome was investigated for Colon-26 cancer cells (C26)-bearing mice. The concept of the current study is as follows: Since both particulates would be accumulated in the tumor tissue due to the enhanced permeability and retention (EPR) effect, PEG-Span 80 niosome, mainly composed of synthetic surfactant (Span 80), would interact with DOX-PEG liposome and be a trigger to induce the release of DOX from the liposome within the tumor tissue, leading to the improvement of anti-tumor effect of DOX-PEG liposome. To find out an adequate liposome for this strategy, several PEG liposomes with different compositions were examined in terms of drug release enhancement and it was found that PEG-Span80 niosome could significantly enhance the release of calcein and DOX from a PEG liposome composed of 90% hydrogenated soybean phosphatidylcholine (HSPC) and 10% cholesterol. The sequential administration of PEG-Span 80 niosome at 24 or 48 h after dosing of DOX-PEG liposome provided a higher anti-tumor effect than the single dose of DOX-PEG liposome in the C26-bearing mice. Particularly, the 24 h-later dosing of PEG-Span 80 niosome has been found to be more effective than the 48 h-later dosing. It was also confirmed that the coexistence of PEG-Span 80 niosome with DOX-PEG liposome in 50% serum or in 50% supernatant of tumor tissue homogenate significantly increased DOX release from PEG liposome, suggesting that DOX release from DOX-PEG liposome within tumor tissue would be enhanced via the interaction with PEG-Span 80 niosome. This strategy would lead to the safer and more inexpensive chemotherapy, since it could make it possible to provide the better anti-tumor effect by utilizing the lower dose of DOX.

From Passive Targeting to Personalized Nanomedicine: Multidimensional Insights on Nanoparticles' Interaction with the Tumor Microenvironment

Nanomedicine is revolutionizing the treatment of cancer and has achieved unprecedented outcomes over the past decades. The accumulation of Nanoparticles (NPs) in different tumors relies mainly on the Enhanced Permeability and Retention (EPR) effect benefiting from the wide fenestrae of the tumor vasculature and the lack of lymphatic drainage. However, the EPR effect is recognized as a heterogeneous phenomenon resulting in heterogeneous outcomes of clinical trials. Extensive efforts are exerted to enhance the outcomes of nanomedicine in a larger cohort of patients by employing active targeting strategies. However, actively targeted NPs accumulate in tumors by the EPR effect and hence fail to achieve convincing therapeutic outcomes. These obstacles are gradually being removed by improving the understanding of the Tumor Microenvironment (TME) and the mechanistic interaction of the NPs with its different components. In this review, we provide detailed insights into the past concerns of drug targeting, the current trends of TME reengineering, and the future implications for overcoming past hurdles. Strategies explored in this regard included the use of companion diagnostics and the modulation of the protein corona associated with the systemic administration of NPs and their interaction with biological macromolecules.

The Enhanced Permeability and Retention (EPR) Effect: The Significance of the Concept and Methods to Enhance Its Application

Chemotherapy for human solid tumors in clinical practice is far from satisfactory. Despite the discovery and synthesis of hundreds of thousands of anticancer compounds targeting various crucial units in cancer cell proliferation and metabolism, the fundamental problem is the lack of targeting delivery of these compounds selectively into solid tumor tissue to maintain an effective concentration level for a certain length of time for drug-tumor interaction to execute anticancer activities. The enhanced permeability and retention effect (EPR effect) describes a universal pathophysiological phenomenon and mechanism in which macromolecular compounds such as albumin and other polymer-conjugated drugs beyond certain sizes (above 40 kDa) can progressively accumulate in the tumor vascularized area and thus achieve targeting delivery and retention of anticancer compounds into solid tumor tissue. Targeting therapy via the EPR effect in clinical practice is not always successful since the strength of the EPR effect varies depending on the type and location of tumors, status of blood perfusion in tumors, and the physical-chemical properties of macromolecular anticancer agents. This review highlights the significance of the concept and mechanism of the EPR effect and discusses methods for better utilizing the EPR effect in developing smarter macromolecular nanomedicine to achieve a satisfactory outcome in clinical applications.

Barriers to antibody therapy in solid tumors, and their solutions

Antibody drugs have become the mainstream of cancer treatment due to advances in cancer biology and Ab engineering. However, several barriers to Ab therapy have also been identified. These include various mechanisms for Ab drug resistance, such as heterogeneity of antigen expression in tumor cells and reduction in antitumor immunity due to expression diversity, polymorphism of Fc receptors (FcR) in effector cells, and reduced function of effector cells. Countermeasures to each resistance mechanism are being investigated. This review focuses on barriers that impede the delivery of Ab drugs due to features of the solid tumor microenvironment. Unlike hematological malignancies, in which the target tumor cells are in blood vessels, clinical solid tumors contain cancer stroma, which interferes with the delivery of Ab drugs. In addition, the cancer mass itself interferes with the penetration of Ab drugs. In this article, I will consider the etiology of cancer stroma and propose a new Ab drug development strategy for solid cancer treatment centering on cancer stromal targeting (CAST) therapy using anti-insoluble fibrin Ab-drug conjugate (ADC), which can overcome the cancer stroma barrier. The recent success of ADCs, chimeric antigen receptor T cells (CAR-Ts), and Bi-specific Abs is changing the category of Ab drugs from molecular-targeted drugs based on growth signal inhibition to cancer-specific targeted therapies. Therefore, at the end of this review, I argue that it is time to reorient the concept of Ab drug development.

Development of a Selective Tumor-Targeted Drug Delivery System: Hydroxypropyl-Acrylamide Polymer-Conjugated Pirarubicin (P-THP) for Pediatric Solid Tumors

Simple Summary

Hydroxypropyl acrylamide polymer-conjugated pirarubicin (P-THP), an innovative polymer-conjugated anticancer agent, theoretically has highly tumor-specific distribution via the enhanced permeability and retention (EPR) effect. While anthracyclines are extremely important in the treatment of most pediatric solid tumors, P-THP may serve as a less toxic and more effective substitute for conventional anthracyclines in both newly diagnosed and refractory/recurrent pediatric cancers.

Abstract

Most pediatric cancers are highly chemo-sensitive, and cytotoxic chemotherapy has always been the mainstay of treatment. Anthracyclines are highly effective against most types of childhood cancer, such as neuroblastoma, hepatoblastoma, nephroblastoma, rhabdomyosarcoma, Ewing sarcoma, and so forth. However, acute and chronic cardiotoxicity, one of the major disadvantages of anthracycline use, limits their utility and effectiveness. Hydroxypropyl acrylamide polymer-conjugated pirarubicin (P-THP), which targets tumor tissue highly selectively via the enhanced permeability and retention (EPR) effect, and secondarily releases active pirarubicin molecules quickly into the acidic environment surrounding the tumor. Although, the latter rarely occurs in the non-acidic environment surrounding normal tissue. This mechanism has the potential to minimize acute and chronic toxicities, including cardiotoxicity, as well as maximize the efficacy of chemotherapy through synergy with tumor-targeting accumulation of the active molecules and possible dose-escalation. Simply replacing doxorubicin with P-THP in a given regimen can improve outcomes in anthracycline-sensitive pediatric cancers with little risk of adverse effects, such as cardiotoxicity. As cancer is a dynamic disease showing intra-tumoral heterogeneity during its course, continued parallel development of cytotoxic agents and molecular targeting agents is necessary to find potentially more effective treatments.

Selective Targeting of Breast Cancer by Tafuramycin A Using SMA-Nanoassemblies

Triple-negative breast cancer (TNBC) is a heterogeneous subtype of tumors that tests negative for estrogen receptors, progesterone receptors, and excess HER2 protein. The mainstay of treatment remains chemotherapy, but the therapeutic outcome remains inadequate. This paper investigates the potential of a duocarmycin derivative, tafuramycin A (TFA), as a new and more effective chemotherapy agent in TNBC treatment. To this extent, we optimized the chemical synthesis of TFA, and we encapsulated TFA in a micellar system to reduce side effects and increase tumor accumulation. In vitro and in vivo studies suggest that both TFA and SMA–TFA possess high anticancer effects in TNBC models. Finally, the encapsulation of TFA offered a preferential avenue to tumor accumulation by increasing its concentration at the tumor tissues by around four times in comparison with the free drug. Overall, the results provide a new potential strategy useful for TNBC treatment.

Thiolated-Polymer-Based Nanoparticles as an Avant-Garde Approach for Anticancer Therapies-Reviewing Thiomers from Chitosan and Hyaluronic Acid

Thiomers (or thiolated polymers) have broken through as avant-garde approaches in anticancer therapy. Their distinguished reactivity and properties, closely linked to their final applications, justify the extensive research conducted on their preparation and use as smart drug-delivery systems (DDSs). Multiple studies have demonstrated that thiomer-rich nanoformulations can overcome major drawbacks found when administering diverse active pharmaceutical ingredients (APIs), especially in cancer therapy. This work focuses on providing a complete and concise review of the synthetic tools available to thiolate cationic and anionic polymers, in particular chitosan (CTS) and hyaluronic acid (HA), respectively, drawing attention to the most successful procedures. Their chemical reactivity and most relevant properties regarding their use in anticancer formulations are also discussed. In addition, a variety of NP formation procedures are outlined, as well as their use in cancer therapy, particularly for taxanes and siRNA. It is expected that the current work could clarify the main synthetic strategies available, with their scope and drawbacks, as well as provide some insight into thiomer chemistry. Therefore, this review can inspire new research strategies in the development of efficient formulations for the treatment of cancer.

EPR-Effect Enhancers Strongly Potentiate Tumor-Targeted Delivery of Nanomedicines to Advanced Cancers: Further Extension to Enhancement of the Therapeutic Effect

For more than three decades, enhanced permeability and retention (EPR)-effect-based nanomedicines have received considerable attention for tumor-selective treatment of solid tumors. However, treatment of advanced cancers remains a huge challenge in clinical situations because of occluded or embolized tumor blood vessels, which lead to so-called heterogeneity of the EPR effect. We previously developed a method to restore impaired blood flow in blood vessels by using nitric oxide donors and other agents called EPR-effect enhancers. Here, we show that two novel EPR-effect enhancers—isosorbide dinitrate (ISDN, Nitrol^®) and sildenafil citrate—strongly potentiated delivery of three macromolecular drugs to tumors: a complex of poly(styrene-co-maleic acid) (SMA) and cisplatin, named Smaplatin^® (chemotherapy); poly(N-(2-hydroxypropyl)methacrylamide) polymer-conjugated zinc protoporphyrin (photodynamic therapy and imaging); and SMA glucosamine-conjugated boric acid complex (boron neutron capture therapy). We tested these nanodrugs in mice with advanced C26 tumors. When these nanomedicines were administered together with ISDN or sildenafil, tumor delivery and thus positive therapeutic results increased two- to four-fold in tumors with diameters of 15 mm or more. These results confirmed the rationale for using EPR-effect enhancers to restore tumor blood flow. In conclusion, all EPR-effect enhancers tested showed great potential for application in cancer therapy.

Reduction-Responsive and Multidrug Deliverable Albumin Nanoparticles: An Antitumor Drug to Abraxane against Human Pancreatic Tumor-Bearing Mice

Many macromolecular antitumor drugs were developed based on the enhanced permeability and retention (EPR) effect, for example, albumin-bound paclitaxel nanoparticles (nab-PTX and Abraxane) and pegylated liposomal doxorubicin (Doxil). However, these EPR effect-based therapeutic systems are less effective in malignant tumors with low vascular permeability, such as pancreatic tumors. Because the EPR effect depends on nanoparticles' size, we first determined nanoparticles' size associated with a high tumor-targeting rate in a human pancreatic tumor xenograft model with low vascular permeability. Abraxane appears to behave as an albumin monomer (7 nm) in the blood circulation following intravenous injection. The in vitro and in vivo tumor-targeted delivery and antitumor activity of PTX-loaded albumin nanoparticles were significantly improved by optimizing the mean nanoparticle diameter to 30 nm. Furthermore, nitric oxide was added to 30 nm PTX-loaded albumin nanoparticles to examine the feasibility of albumin nanoparticles as a platform for multiple drug delivery. Their antitumor effect was evaluated in an orthotopic transplantation mouse model of a human pancreatic tumor. The nitric oxide PTX-loaded 30 nm albumin nanoparticle treatment on model mice achieved a significantly higher survival rate than Abraxane treatment. These findings suggest that 30 nm albumin nanoparticles have a high therapeutic effect as a useful platform for multiple drugs against human pancreatic tumors.

Unraveling the role of Intralipid in suppressing off-target delivery and augmenting the therapeutic effects of anticancer nanomedicines

Intralipid, a clinically used lipid emulsion, was reportedly utilized as one strategy to suppress off-target delivery of anticancer nanomedicines; Intralipid also effectively improved drug delivery to tumors and produced better therapeutic effects. However, the mechanisms involved-the why and how-in Intralipid's facilitation of delivery of nanomedicines to tumors have not yet been reported in detail. In this study, we investigated Intralipid and discovered the beneficial effects of Intralipid pretreatment when using three anticancer nanomedicines, including the clinically approved drug doxorubicin (Doxil). Intralipid pretreatment induced a 40% reduction in liver uptake of a polymeric nanoprobe used in photodynamic therapy as well as a 1.5-fold-increased nanomedicine accumulation in tumors. This increased accumulation consequently led to significantly better therapeutic effects, and this finding was validated by using Doxil. As an interesting result, Intralipid pretreatment significantly prolonged the plasma half-life of nanomedicines in normal healthy mice but not in tumor-bearing mice, which suggests that tumors become an alternative route of nanomedicine delivery when liver delivery is suppressed. Also, we found markedly increased tumor blood flow, as measured by fluorescence angiography, and significantly lower blood viscosity after Intralipid pretreatment. All our results together indicate that Intralipid treatment not only suppressed off-target nanomedicine delivery by the reticuloendothelial system, but more important, it enhanced nanomedicine delivery to tumors by improving tumor blood flow, which is key to satisfactory drug delivery via the enhanced permeability and retention effect. Significantly better therapeutic outcomes were thus achieved by the strategy of combining utilization of nanomedicines and Intralipid pretreatment. STATEMENT OF SIGNIFICANCE: Off-target delivery to organs such as the liver and obstructed tumor blood flow as is often seen in advanced cancers are major barriers to the therapeutic efficacy of anticancer nanomedicines. Intralipid has been shown effective for suppressing nanomedicine accumulation in the liver, resulting in improved anticancer effects. Unraveling the mechanisms involved in this process will be greatly helpful for the clinical application of anticancer nanomedicines. We reported here that Intralipid could also significantly increase tumor delivery of nanomedicine, which is beneficial for improving tumor blood flow and lowering blood viscosity. To our knowledge, this is the first study to investigate the role of Intralipid in this regard. This knowledge provides a solid rationale for the use of Intralipid in combination with anticancer nanomedicines.

The 35th Anniversary of the Discovery of EPR Effect: A New Wave of Nanomedicines for Tumor-Targeted Drug Delivery-Personal Remarks and Future Prospects

This Special Issue on the enhanced permeability and retention (EPR) effect commemorates the 35th anniversary of its discovery, the original 1986 Matsumura and Maeda finding being published in Cancer Research as a new concept in cancer chemotherapy. My review here describes the history and heterogeneity of the EPR effect, which involves defective tumor blood vessels and blood flow. We reported that restoring obstructed tumor blood flow overcomes impaired drug delivery, leading to improved EPR effects. I also discuss gaps between small animal cancers used in experimental models and large clinical cancers in humans, which usually involve heterogeneous EPR effects, vascular abnormalities in multiple necrotic foci, and tumor emboli. Here, I emphasize arterial infusion of oily formulations of nanodrugs into tumor-feeding arteries, which is the most tumor-selective drug delivery method, with tumor/blood ratios of 100-fold. This method is literally the most personalized medicine because arterial infusions differ for each patient, and drug doses infused depend on tumor size and anatomy in each patient. Future developments in EPR effect-based treatment will range from chemotherapy to photodynamic therapy, boron neutron capture therapy, and therapies for free radical diseases. This review focuses on our own work, which stimulated numerous scientists to perform research in nanotechnology and drug delivery systems, thereby spawning a new cancer treatment era.

Tumor Environment-Responsive Hyaluronan Conjugated Zinc Protoporphyrin for Targeted Anticancer Photodynamic Therapy

Targeted tumor accumulation, tumor environment responsive drug release, and effective internalization are critical issues being considered in developing anticancer nanomedicine. In this context, we synthesized a tumor environment-responsive nanoprobe for anticancer photodynamic therapy (PDT) that is a hyaluronan conjugated zinc protoporphyrin via an ester bond (HA-es-ZnPP), and we examined its anticancer PDT effect both in vitro and in vivo. HA-es-ZnPP exhibits high water-solubility and forms micelles of ~40 nm in aqueous solutions. HA-es-ZnPP shows fluorescence quenching without apparent 1O2 generation under light irradiation because of micelle formation. However, 1O2 was extensively generated when the micelle is disrupted, and ZnPP is released. Compared to native ZnPP, HA-es-ZnPP showed lower but comparable intracellular uptake and cytotoxicity in cultured mouse C26 colon cancer cells; more importantly, light irradiation resulted in 10-time increased cytotoxicity, which is the PDT effect. In a mouse sarcoma S180 solid tumor model, HA-es-ZnPP as polymeric micelles exhibited a prolonged systemic circulation time and the consequent tumor-selective accumulation based on the enhanced permeability and retention (EPR) effect was evidenced. Consequently, a remarkable anticancer PDT effect was achieved using HA-es-ZnPP and a xenon light source, without apparent side effects. These findings suggest the potential of HA-es-ZnPP as a candidate anticancer nanomedicine for PDT.

Nanodrug Delivery Systems Modulate Tumor Vessels to Increase the Enhanced Permeability and Retention Effect

The use of nanomedicine for antitumor therapy has been extensively investigated for a long time. Enhanced permeability and retention (EPR) effect-mediated drug delivery is currently regarded as an effective way to bring drugs to tumors, especially macromolecular drugs and drug-loaded pharmaceutical nanocarriers. However, a disordered vessel network, and occluded or embolized tumor blood vessels seriously limit the EPR effect. To augment the EPR effect and improve curative effects, in this review, we focused on the perspective of tumor blood vessels, and analyzed the relationship among abnormal angiogenesis, abnormal vascular structure, irregular blood flow, extensive permeability of tumor vessels, and the EPR effect. In this commentary, nanoparticles including liposomes, micelles, and polymers extravasate through the tumor vasculature, which are based on modulating tumor vessels, to increase the EPR effect, thereby increasing their therapeutic effect.

HPMA Copolymer-Based Nanomedicines in Controlled Drug Delivery

Recently, numerous polymer materials have been employed as drug carrier systems in medicinal research, and their detailed properties have been thoroughly evaluated. Water-soluble polymer carriers play a significant role between these studied polymer systems as they are advantageously applied as carriers of low-molecular-weight drugs and compounds, e.g., cytostatic agents, anti-inflammatory drugs, antimicrobial molecules, or multidrug resistance inhibitors. Covalent attachment of carried molecules using a biodegradable spacer is strongly preferred, as such design ensures the controlled release of the drug in the place of a desired pharmacological effect in a reasonable time-dependent manner. Importantly, the synthetic polymer biomaterials based on N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers are recognized drug carriers with unique properties that nominate them among the most serious nanomedicines candidates for human clinical trials. This review focuses on advances in the development of HPMA copolymer-based nanomedicines within the passive and active targeting into the place of desired pharmacological effect, tumors, inflammation or bacterial infection sites. Specifically, this review highlights the safety issues of HPMA polymer-based drug carriers concerning the structure of nanomedicines. The main impact consists of the improvement of targeting ability, especially concerning the enhanced and permeability retention (EPR) effect.