Pharmacological and physical vessel modulation strategies to improve EPR-mediated drug targeting to tumors

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.

Normalizing tumor microenvironment with nanomedicine and metronomic therapy to improve immunotherapy

Nanomedicine offered hope for improving the treatment of cancer but the survival benefits of the clinically approved nanomedicines are modest in many cases when compared to conventional chemotherapy. Metronomic therapy, defined as the frequent, low dose administration of chemotherapeutics – is being tested in clinical trials as an alternative to the conventional maximum tolerated dose (MTD) chemotherapy schedule. Although metronomic chemotherapy has not been clinically approved yet, it has shown better survival than MTD in many preclinical studies. When beneficial, metronomic therapy seems to be associated with normalization of the tumor microenvironment including improvements in tumor perfusion, tissue oxygenation and drug delivery as well as activation of the immune system. Recent preclinical studies suggest that nanomedicines can cause similar changes in the tumor microenvironment. Here, by employing a mathematical framework, we show that both approaches can serve as normalization strategies to enhance treatment. Furthermore, employing murine breast and fibrosarcoma tumor models as well as ultrasound shear wave elastography and contrast-enhanced ultrasound, we provide evidence that the approved nanomedicine Doxil can induce normalization in a dose-dependent manner by improving tumor perfusion as a result of tissue softening. Finally, we show that pretreatment with a normalizing dose of Doxil can improve the efficacy of immune checkpoint inhibition.

Actively Targeted Nanomedicines in Breast Cancer: From Pre-Clinal Investigation to Clinic

Simple Summary

Despite all the efforts and advances made in the treatment of breast cancer, this pathology continues to be one of the main causes of cancer death in women, particularly triple-negative breast cancer (TNBC), and, although to a lesser degree, HER-2 receptor-positive tumors. Chemotherapy is one of the main treatments available. However, it shows numerous limitations due to its lack of selectivity. In this sense, the selective delivery of antineoplastics to cancer cells can reduce their adverse effects and increase their efficacy. The use of active targeted nanomedicine is a good strategy to achieve this selective chemotherapy. In fact, in recent decades, several active targeted nanoformulations have been approved or reached clinical investigation with excellent results. Among all nanomedicines, antibody-drug conjugates are the most promising.

Abstract

Breast cancer is one of the most frequently diagnosed tumors and the second leading cause of cancer death in women worldwide. The use of nanosystems specifically targeted to tumor cells (active targeting) can be an excellent therapeutic tool to improve and optimize current chemotherapy for this type of neoplasm, since they make it possible to reduce the toxicity and, in some cases, increase the efficacy of antineoplastic drugs. Currently, there are 14 nanomedicines that have reached the clinic for the treatment of breast cancer, 4 of which are already approved (Kadcyla^®, Enhertu^®, Trodelvy^®, and Abraxane^®). Most of these nanomedicines are antibody–drug conjugates. In the case of HER-2-positive breast cancer, these conjugates (Kadcyla^®, Enhertu^®, Trastuzumab-duocarmycin, RC48, and HT19-MMAF) target HER-2 receptors, and incorporate maytansinoid, deruxtecan, duocarmicyn, or auristatins as antineoplastics. In TNBC these conjugates (Trodelvy^®, Glembatumumab-Vedotin, Ladiratuzumab-vedotin, Cofetuzumab-pelidotin, and PF-06647263) are directed against various targets, in particular Trop-2 glycoprotein, NMB glycoprotein, Zinc transporter LIV-1, and Ephrin receptor-4, to achieve this selective accumulation, and include campthotecins, calicheamins, or auristatins as drugs. Apart from the antibody–drug conjugates, there are other active targeted nanosystems that have reached the clinic for the treatment of these tumors such as Abraxane^® and Nab-rapamicyn (albumin nanoparticles entrapping placlitaxel and rapamycin respectively) and various liposomes (MM-302, C225-ILS-Dox, and MM-310) loaded with doxorubicin or docetaxel and coated with ligands targeted to Ephrin A2, EPGF, or HER-2 receptors. In this work, all these active targeted nanomedicines are discussed, analyzing their advantages and disadvantages over conventional chemotherapy as well as the challenges involved in their lab to clinical translation. In addition, examples of formulations developed and evaluated at the preclinical level are also discussed.

Progress and Hurdles of Therapeutic Nanosystems against Cancer

Nanomedicine against cancer, including diagnosis, prevention and treatment, has increased expectations for the solution of many biomedical challenges in the fight against this disease. In recent decades, an exhaustive design of nanosystems with high specificity, sensitivity and selectivity has been achieved due to a rigorous control over their physicochemical properties and an understanding of the nano–bio interface. However, despite the considerable progress that has been reached in this field, there are still different hurdles that limit the clinical application of these nanosystems, which, along with their possible solutions, have been reviewed in this work. Specifically, physiological processes as biological barriers and protein corona formation related to the administration routes, designing strategies to overcome these obstacles, promising new multifunctional nanotherapeutics, and recent clinical trials are presented in this review.

Noninvasive Visualization of Sub-5 mm Orthotopic Hepatic Tumors by a Nanoprobe-Mediated Positive and Reverse Contrast-Balanced Imaging Strategy

Delineation of small malignant lesions and their vasculature enables early and accurate diagnosis of hepatocellular carcinoma (HCC). However, it remains challenging to identify these features simultaneously by noninvasive imaging technology. Reverse contrast imaging emerges as a powerful means to detect early-stage HCC by taking inspiration from the intrinsic liver phagocytosis toward exogenous agents to generate negative tumor-to-normal tissue signals. However, this mechanism conflicts with the signal-enhancing requirements for vasculature visualization. Here, we solve this conundrum by designing a positive and reverse contrast-balanced imaging strategy based on a multifunctional PEG-Ta₂O₅@CuS nanoprobe that combines advanced gemstone spectral computer tomography (GSCT) with photoacoustic (PA) imaging. The nanoprobe exhibits preferential accumulation in Kupffer cells and hepatocytes over tumor cells, and its spectral properties are well matched with GSCT, leading to the enhancement of reverse contrast signals that enable clear delineation of 2-4 mm orthotopic HCC lesions. Meanwhile, its strong PA imaging capability at the second near-infrared (NIR-II) window makes vascular evaluation accessible by monitoring the positive signal enhancement derived from the limited tumor accumulation of the nanoprobe. In addition, the nanoprobe enables NIR-II photohyperthermia for timely tumor ablation. Overall, this proposed strategy shows potential in early detection and theranostics of HCC for improved clinical outcomes.

Augmented EPR effect post IRFA to enhance the therapeutic efficacy of arsenic loaded ZIF-8 nanoparticles on residual HCC progression

BACKGROUND

Insufficient radiofrequency ablation (IRFA) can promote the local recurrence and distal metastasis of residual hepatocellular carcinoma (HCC), which makes clinical treatment extremely challenging. In this study, the malignant transition of residual tumors after IRFA was explored. Then, arsenic-loaded zeolitic imidazolate framework-8 nanoparticles (As@ZIF-8 NPs) were constructed, and their therapeutic effect on residual tumors was studied.

RESULTS

Our data showed that IRFA can dramatically promote the proliferation, induce the metastasis, activate the epithelial-mesenchymal transition (EMT) and accelerate the angiogenesis of residual tumors. Interestingly, we found, for the first time, that extensive angiogenesis after IRFA can augment the enhanced permeability and retention (EPR) effect and enhance the enrichment of ZIF-8 nanocarriers in residual tumors. Encouraged by this unique finding, we successfully prepared As@ZIF-8 NPs with good biocompatibility and confirmed that they were more effective than free arsenic trioxide (ATO) in sublethal heat-induced cell proliferation suppression, apoptosis induction, cell migration and invasion inhibition, and EMT reversal in vitro. Furthermore, compared with free ATO, As@ZIF-8 NPs exhibited remarkably increased therapeutic effects by repressing residual tumor growth and metastasis in vivo.

CONCLUSIONS

This work provides a new paradigm for the treatment of residual HCC after IRFA.

GSH-sensitive polymeric prodrug: Synthesis and loading with photosensitizers as nanoscale chemo-photodynamic anti-cancer nanomedicine

Precisely delivering combinational therapeutic agents has become a crucial challenge for anti-tumor treatment. In this study, a novel redox-responsive polymeric prodrug (molecular weight, MW: 93.5 kDa) was produced by reversible addition-fragmentation chain transfer (RAFT) polymerization. The amphiphilic block polymer-doxorubicin (DOX) prodrug was employed to deliver a hydrophobic photosensitizer (PS), chlorin e6 (Ce6), and the as-prepared nanoscale system [NPs(Ce6)] was investigated as a chemo-photodynamic anti-cancer agent. The glutathione (GSH)-cleavable disulfide bond was inserted into the backbone of the polymer for biodegradation inside tumor cells, and DOX conjugated onto the polymer with a disulfide bond was successfully released intracellularly. NPs(Ce6) released DOX and Ce6 with their original molecular structures and degraded into segments with low MWs of 41.2 kDa in the presence of GSH. NPs(Ce6) showed a chemo-photodynamic therapeutic effect to kill 4T1 murine breast cancer cells, which was confirmed from a collapsed cell morphology, a lifted level in the intracellular reactive oxygen species, a reduced viability and induced apoptosis. Moreover, ex vivo fluorescence images indicated that NPs(Ce6) retained in the tumor, and exhibited a remarkable in vivo anticancer efficacy. The combinational therapy showed a significantly increased tumor growth inhibition (TGI, 58.53%). Therefore, the redox-responsive, amphiphilic block polymeric prodrug could have a great potential as a chemo-photodynamic anti-cancer agent.

SonoVue® vs. Sonazoid™ vs. Optison™: Which Bubble Is Best for Low-Intensity Sonoporation of Pancreatic Ductal Adenocarcinoma?

The use of ultrasound and microbubbles to enhance therapeutic efficacy (sonoporation) has shown great promise in cancer therapy from in vitro to ongoing clinical studies. The fastest bench-to-bedside translation involves the use of ultrasound contrast agents (microbubbles) and clinical diagnostic scanners. Despite substantial research in this field, it is currently not known which of these microbubbles result in the greatest enhancement of therapy within the applied conditions. Three microbubble formulations-SonoVue^®, Sonazoid™, and Optison™-were physiochemically and acoustically characterized. The microbubble response to the ultrasound pulses used in vivo was simulated via a Rayleigh-Plesset type equation. The three formulations were compared in vitro for permeabilization efficacy in three different pancreatic cancer cell lines, and in vivo, using an orthotopic pancreatic cancer (PDAC) murine model. The mice were treated using one of the three formulations exposed to ultrasound from a GE Logiq E9 and C1-5 ultrasound transducer. Characterisation of the microbubbles showed a rapid degradation in concentration, shape, and/or size for both SonoVue^® and Optison™ within 30 min of reconstitution/opening. Sonazoid™ showed no degradation after 1 h. Attenuation measurements indicated that SonoVue^® was the softest bubble followed by Sonazoid™ then Optison™. Sonazoid™ emitted nonlinear ultrasound at the lowest MIs followed by Optison™, then SonoVue^®. Simulations indicated that SonoVue^® would be the most effective bubble using the evaluated ultrasound conditions. This was verified in the pre-clinical PDAC model demonstrated by improved survival and largest tumor growth inhibition. In vitro results indicated that the best microbubble formulation depends on the ultrasound parameters and concentration used, with SonoVue^® being best at lower intensities and Sonazoid™ at higher intensities.

Tumor-Microenvironment-Responsive Nanomedicine for Enhanced Cancer Immunotherapy

The past decades have witnessed great progress in cancer immunotherapy, which has profoundly revolutionized oncology, whereas low patient response rates and potential immune-related adverse events remain major clinical challenges. With the advantages of controlled delivery and modular flexibility, cancer nanomedicine has offered opportunities to strengthen antitumor immune responses and to sensitize tumor to immunotherapy. Furthermore, tumor-microenvironment (TME)-responsive nanomedicine has been demonstrated to achieve specific and localized amplification of the immune response in tumor tissue in a safe and effective manner, increasing patient response rates to immunotherapy and reducing the immune-related side effects simultaneously. Here, the recent progress of TME-responsive nanomedicine for cancer immunotherapy is summarized, which responds to the signals in the TME, such as weak acidity, reductive environment, high-level reactive oxygen species, hypoxia, overexpressed enzymes, and high-level adenosine triphosphate. Moreover, the potential to combine nanomedicine-based therapy and immunotherapeutic strategies to overcome each step of the cancer-immunity cycle and to enhance antitumor effects is discussed. Finally, existing challenges and further perspectives in this rising field with the hope for improved development of clinical applications are discussed.

Co-delivery of doxorubicin and SIS3 by folate-targeted polymeric micelles for overcoming tumor multidrug resistance

Multidrug resistance (MDR) is considered as a critical limiting factor for the successful chemotherapy, which is mainly characterized by the overexpression of ATP-binding cassette (ABC) transporter ABCB1 or ABCG2. In this study, folate-targeted polymeric micellar carrier was successfully constructed to co-delivery of doxorubicin (DOX) and SIS3 (FA/DOX/SIS3 micelles), a specific Smad3 inhibitor which sensitizes ABCB1- and ABCG2-overexpressing cancer cells to chemotherapeutic agents. The ratio of DOX to SIS3 in polymeric micelles was determined based on the anti-tumor activity against resistant breast cells. In addition, FA/DOX/SIS3 micelles exhibited a much longer circulation time in blood and were preferentially accumulated in resistant tumor tissue. Pharmacodynamic studies showed that FA/DOX/SIS3 micelles possessed superior anti-tumor activity than other DOX-based treatments. Overall, FA/DOX/SIS3 micelles are a promising formulation for the synergistic treatment of drug-resistant tumor.

Enzyme-Loaded Catalytic Macrophage Vesicles with Cascade Amplification of Tumor-Targeting for Oxygenated Photodynamic Therapy

Background

Realizing that the potential of photodynamic therapy (PDT) is hindered by hypoxic microenvironment of tumor section, it is desirable to provide a cascade oxygenation strategy to enhance PDT.

Methods

The hydrophilic catalase protein was covalently linked to the hydrophobic photosensitizer Ce6 to form the nanocomplex Catalase-Ce6 with self-assembly. And the Catalase-Ce6 was loaded in the M1 macrophage vesicles (EVs) with GOX-modified to construct the nanosystem Catalase-Ce6@MEVs. The synergistic effects of PDT induced by Catalase-Ce6@MEVs were evaluated on the subcutaneous MFC tumor model.

Results

The construction of Catalase-Ce6 not only solved the insoluble problem of Ce6, but also induced a cascade effects for hydrolyzing glucose and increasing the hydrogen peroxide content, achieving the purpose of oxygenated PDT. Cascade tumor targeting was also realized through the binding between vascular cell adhesion molecule 1 (VCAM-1) of tumor tissue and α4 integrin of EVs and enhanced vascular permeability, triggering by PDT. Besides, in vivo experiments found that the Catalase-Ce6@MEVs presented M2 macrophage polarization effect.

Conclusion

Catalase-Ce6@MEVs exhibit the cascade targeting ability after laser irradiation and prominent tumor treatment effect in vivo, which may provide new ideas and methods for targeted PDT in clinical practice.

In Vivo Biodistribution, Clearance, and Biocompatibility of Multiple Carbon Dots Containing Nanoparticles for Biomedical Application

Current research on the use of carbon dots for various biological systems mainly focuses on the single carbon dots, while particles that contain multiple carbon dots have scarcely been investigated. Here, we assessed multiple carbon dots-crosslinked polyethyleneimine nanoparticles (CDs@PEI) for their in vivo biodistribution, clearance, biocompatibility, and cellular uptake. The in vivo studies demonstrate three unique features of the CDs@PEI nanoparticles: (1) the nanoparticles possess tumor-targeting ability with steady and prolonged retention time in the tumor region. (2) The nanoparticles show hepatobiliary excretion and are clear from the intestine in feces. (3) The nanoparticles have much better biocompatibility than the polyethyleneimine passivated single carbon dots (PEI-CD). We also found that pegylated CDs@PEI nanoparticles can be effectively taken up by the cells, which the confocal laser scanning microscope can image under different excitation wavelengths (at 405, 488, and 800 nm). These prior studies provide invaluable information and new opportunities for this new type of intrinsic photoluminescence nanoparticles in carbon dot-based biomedical applications.

Emerging strategies in developing multifunctional nanomaterials for cancer nanotheranostics

Cancer involves a collection of diseases with a common trait - dysregulation in cell proliferation. At present, traditional therapeutic strategies against cancer have limitations in tackling various tumors in clinical settings. These include chemotherapeutic resistance and the inability to overcome intrinsic physiological barriers to drug delivery. Nanomaterials have presented promising strategies for tumor treatment in recent years. Nanotheranostics combine therapeutic and bioimaging functionalities at the single nanoparticle level and have experienced tremendous growth over the past few years. This review highlights recent developments of advanced nanomaterials and nanotheranostics in three main directions: stimulus-responsive nanomaterials, nanocarriers targeting the tumor microenvironment, and emerging nanomaterials that integrate with phototherapies and immunotherapies. We also discuss the cytotoxicity and outlook of next-generation nanomaterials towards clinical implementation.

Do Lipid-based Nanoparticles Hold Promise for Advancing the Clinical Translation of Anticancer Alkaloids?

Since the commercialization of morphine in 1826, numerous alkaloids have been isolated and exploited effectively for the betterment of mankind, including cancer treatment. However, the commercialization of alkaloids as anticancer agents has generally been limited by serious side effects due to their lack of specificity to cancer cells, indiscriminate tissue distribution and toxic formulation excipients. Lipid-based nanoparticles represent the most effective drug delivery system concerning clinical translation owing to their unique, appealing characteristics for drug delivery. To the extent of our knowledge, this is the first review to compile in vitro and in vivo evidence of encapsulating anticancer alkaloids in lipid-based nanoparticles. Alkaloids encapsulated in lipid-based nanoparticles have generally displayed enhanced in vitro cytotoxicity and an improved in vivo efficacy and toxicity profile than free alkaloids in various cancers. Encapsulated alkaloids also demonstrated the ability to overcome multidrug resistance in vitro and in vivo. These findings support the broad application of lipid-based nanoparticles to encapsulate anticancer alkaloids and facilitate their clinical translation. The review then discusses several limitations of the studies analyzed, particularly the discrepancies in reporting the pharmacokinetics, biodistribution and toxicity data. Finally, we conclude with examples of clinically successful encapsulated alkaloids that have received regulatory approval and are undergoing clinical evaluation.

Multifunctional Size-Expandable Nanomedicines Enhance Tumor Accumulation and Penetration for Synergistic Chemo-Photothermal Therapy

Size expansion can effectively improve tumor accumulation of nanocarriers where precise control is required. A dual-responsive nanocarrier stimulated by both endogenous pH and exogenous heat stimuli can change its size. Herein, a nanoparticle composed of poly(N,N-diethyl acrylamide) (PDEAA) and poly(2-(diisopropylamino) ethyl methacrylate) (PDPA) is developed. The antitumor drug celastrol (CLT) and the photosensitizer indocyanine green (ICG) are then loaded in it to form CIPP. ICG generates heat under near-infrared (NIR) stimulation to kill tumor cells and enhance CIPP penetration. Meanwhile, CIPP expands in response to hyperthermia and acid tumor microenvironments, preventing itself from returning to the blood flow, thus accumulating in tumor sites. Ultimately, the acidic lysosomal environment in tumor cells disintegrates CIPP to release CLT, directly inducing immunogenic cell death and sensitizing tumor cells for hyperthermia by disrupting the interaction of heat shock protein 90 and P50cdc37. Most of the tumors in B16F10-bearing mice are eradicated after single laser irradiation. The dual-responsive CIPP with multiple functions and simple design displays a synergistic antitumor effect. This study provides a basis for developing size-expandable stimulus-responsive drug delivery systems against tumors.

A novel protein-drug conjugate, SSH20, demonstrates significant efficacy in caveolin-1-expressing tumors

In recent years, human serum albumin (HSA) has been characterized as an ideal drug carrier in the cancer arena. Caveolin-1 (Cav-1) has been established as the principal structural protein of caveolae and, thus, critical for caveolae-mediated endocytosis. Cav-1 has been shown to be overexpressed in cancers of the lung and pancreas, among others. We found that Cav-1 expression plays a critical role in both HSA uptake and response to albumin-based chemotherapies. As such, developing a novel albumin-based chemotherapy that is more selective for tumors with high Cav-1 expression or high levels of caveolar-endocytosis could have significant implications in biomarker-directed therapy. Herein, we present the development of a novel and effective HSA-SN-38 conjugate (SSH20). We find that SSH20 uptake decreases significantly by immunofluorescence assays and western blotting after silencing of Cav-1 expression through RNA interference. Decreased drug sensitivity occurs in Cav-1-depleted cells using cytotoxicity assays. Importantly, we find significantly reduced sensitivity to SSH20 in Cav-1-silenced tumors compared to Cav-1-expressing tumors in vivo. Notably, we show that SSH20 is significantly more potent than irinotecan in vitro and in vivo. Together, we have developed a novel HSA-conjugated chemotherapy that is potent, effective, safe, and demonstrates improved efficacy in high Cav-1-expressing tumors.

Applications of Aptamer-Bound Nanomaterials in Cancer Therapy

Cancer is still a major disease that threatens human life. Although traditional cancer treatment methods are widely used, they still have many disadvantages. Aptamers, owing to their small size, low toxicity, good specificity, and excellent biocompatibility, have been widely applied in biomedical areas. Therefore, the combination of nanomaterials with aptamers offers a new method for cancer treatment. First, we briefly introduce the situation of cancer treatment and aptamers. Then, we discuss the application of aptamers in breast cancer treatment, lung cancer treatment, and other cancer treatment methods. Finally, perspectives on challenges and future applications of aptamers in cancer therapy are discussed.

Recent Advances in Exosome-Based Drug Delivery for Cancer Therapy

Simple Summary

Exosomes derived from various sources can deliver therapeutic agents such as small molecule drugs, nucleic acids, and proteins to cancer cells by passive or active targeting. These exosomes can encapsulate drugs inside the exosomes, extending drug half-life and increasing drug release stability. In addition, exosomes are highly biocompatible due to their endogenous origin and can be used as nanocarriers for tissue-specific targeted delivery. This review discusses recent advances in exosome-based drug delivery for cancer therapy.

Abstract

Exosomes are a class of extracellular vesicles, with a size of about 100 nm, secreted by most cells and carrying various bioactive molecules such as nucleic acids, proteins, and lipids, and reflect the biological status of parent cells. Exosomes have natural advantages such as high biocompatibility and low immunogenicity for efficient delivery of therapeutic agents such as chemotherapeutic drugs, nucleic acids, and proteins. In this review, we introduce the latest explorations of exosome-based drug delivery systems for cancer therapy, with particular focus on the targeted delivery of various types of cargoes.

Engineering Nanorobots for Tumor-Targeting Drug Delivery: From Dynamic Control to Stimuli-Responsive Strategy

Nanotechnology has been widely applied to the fabrication of drug delivery systems in the past decades. Recently, with the progress made in microfabrication approaches, nanorobots are steadily becoming a promising means for tumor-targeting drug delivery. In general, nanorobots can be divided into two categories: nanomotors and stimuli-responsive nanorobots. Nanomotors are nanoscale systems with the ability to convert surrounding energies into mechanical motion, whereas stimuli-responsive nanorobots are featured with activatable capacity in response to various endogenous and exogenous stimulations. In this minireview, the dynamic control of nanomotors and the rational design of stimuli-responsive nanorobots are overviewed, with particular emphasis on their contribution to tumor-targeting therapy. Moreover, challenges and perspectives associated with the future development of nanorobots are presented.

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.

Poly(methacrylic Acid)-Cellulose Brushes as Anticancer Porphyrazine Carrier

The prospective strategy for treatment of cancer is based on the application of nano-sized macromolecular carriers, which are able penetrate inside and can be accumulated within tumor tissue. In this work graft copolymers of cellulose and poly(methacrylic acid) has been prepared and tested as a nanocontainers for the delivery of drug to tumor. For this purpose, two derivatives of porphyrazine suitable for photodynamic cancer therapy were loaded into prepared polymer brush. Fluorescence imaging was applied for monitoring of accumulation of porphyrazine in the CT26 murine colon carcinoma. The selective accumulation of cellulose brush loaded with porphyrazine in tumor was demonstrated by fluorescence intensity contrast between the tumor area and normal tissues. The tumor growth rate after photodynamic therapy were assessed and inhibition of its growth was revealed.

Nanocarriers Used in Drug Delivery to Enhance Immune System in Cancer Therapy

Cancer, a group of diseases responsible for the second largest cause of global death, is considered one of the main public health problems today. Despite the advances, there are still difficulties in the development of more efficient cancer therapies and fewer adverse effects for the patients. In this context, nanobiotechnology, a materials science on a nanometric scale specified for biology, has been developing and acquiring prominence for the synthesis of nanocarriers that provide a wide surface area in relation to volume, better drug delivery, and a maximization of therapeutic efficiency. Among these carriers, the ones that stand out are those focused on the activation of the immune system. The literature demonstrates the importance of this system for anticancer therapy, given that the best treatment for this disease also activates the immune system to recognize, track, and destroy all remaining tumor cells.

Evaluation of release and pharmacokinetics of hexadecylphosphocholine (miltefosine) in phosphatidyldiglycerol-based thermosensitive liposomes

Hexadecylphosphocholine (HePC, Miltefosine) is a drug from the class of alkylphosphocholines with an antineoplastic and antiprotozoal activity. We previously reported that HePC uptake from thermosensitive liposomes (TSL) containing 1,2-dipalmitoyl-sn-glycero-3-phosphodiglycerol (DPPG₂) into cancer cells is accelerated at mild hyperthermia (HT) resulting in increased cytotoxicity. In this study, we compared HePC release of different TSL formulations in serum. HePC showed rapid but incomplete release below the transition temperature (T_m) of investigated TSL formulations in serum. Short heating (5 min) to 42 °C increased HePC release from DPPG₂-TSL (T_m = 41 °C) by a factor of two in comparison to body temperature (37 °C). Bovine serum albumin (BSA) induced HePC release from DPPG₂-TSL comparable to serum. Furthermore, multilamellar vesicles (MLV) were capable to extract HePC from DPPG₂-TSL in a concentration- and temperature-dependent manner. Repetitive exposure of DPPG₂-TSL to MLV at 37 °C led to a fast initial release of HePC which slowed down after subsequent extraction cycles finally reaching approx. 50% HePC release. A pharmacokinetic study in rats revealed a biphasic pattern with an immediate clearance of approx. 50% HePC whereas the remaining 50% HePC showed a prolonged circulation time. We speculate that HePC located in the external leaflet of DPPG₂-TSL is rapidly released upon contact with suitable biological acceptors. As demonstrated by MLV transfer experiments, asymmetric incorporation of HePC into the internal leaflet of DPPG₂-TSL might improve HePC retention in presence of complex biological media and still give rise to HT-induced HePC release.

Triple negative breast cancer and non-small cell lung cancer: Clinical challenges and nano-formulation approaches

Triple negative breast cancer (TNBC) and non-small cell lung cancer (NSCLC) are amongst the most aggressive forms of solid tumors. TNBC is highlighted by absence of genetic components of progesterone receptor, HER2/neu and estrogen receptor in breast cancer. NSCLC is characterized by integration of malignant carcinoma into respiratory system. Both cancers are associated with poor median and overall survival rates with low progression free survival with high incidences of relapse. These cancers are characterized by tumor heterogeneity, genetic mutations, generation of cancer-stem cells, immune-resistance and chemoresistance. Further, these neoplasms have been reported for tumor cross-talk into second primary cancers for each other. Current chemotherapeutic regimens include usage of multiple agents in tandem to affect tumor cells through multiple mechanisms with various such combinations being clinically tested. However, lack of controlled delivery and effective temporospatial presence of chemotherapeutics has resulted in suboptimal therapeutic response. Consequently, passive targeted albumin bound paclitaxel and PEGylated liposomal doxorubicin have been clinically used and tested with newer drugs for improved therapeutic efficacy in these cancers. Active targeting of nanocarriers against surface overexpressed proteins in both neoplasms have been explored. However, use of single agent nanoparticulate formulations against both cancers have failed to elicit desired outcomes. This review aims to identify clinical unmet need in these cancers while establishing a correlation with tested nano-formulation approaches and issues with preclinical to clinical translation. Lipid and polymer-based drug-drug and drug-gene combinatorial nanocarriers delivering multiple chemotherapeutics simultaneously to desired site of action have been detailed. Finally, emerging opportunities such as pharmacological targets (immune check point and epigentic modulators) as well as gene-based modulation (siRNA/CRISPR/Cas9) and the nano-formulation challenges for effective treatment of both cancers have been explored.

Critical quality attributes in the development of therapeutic nanomedicines toward clinical translation

Nanomedicine is a rapidly emerging field with several breakthroughs in the therapeutic drug delivery application. The unique properties of the nanoscale delivery systems offer huge advantages to their payload such as solubilization, increased bioavailability, and improved pharmacokinetics with an overall goal of enhanced therapeutic index. Nanomedicine has the potential for integrating and enabling new therapeutic modalities. Several nanoparticle-based drug delivery systems have been granted approval for clinical use based on their outstanding clinical outcomes. Nanomedicine faces several challenges that hinder the realization of its full potential. In this review, we discuss the critical formulation- and biological-related quality features that significantly influence the performance of nanoparticulate systems in vivo. We also discuss the quality-by-design approach in the pharmaceutical manufacturing and its implementation in the nanomedicine. A deep understanding of these nanomedicine quality checkpoints and a systematic design that takes them into consideration will hopefully expedite the clinical translation process. Graphical abstract.

[Construction and Anti-tumor Effect Evaluation of a Dual-Responsive Hyaluronic Acid Carbon Quantum Dot-Gelatin Nano-Drug Delivery System]

OBJECTIVE

To construct a pH and matrix metalloproteinase (MMP) dual-responsive nano drug delivery system with adjustable particle size so as to synergistically enhance the retention and penetration of chemotherapeutic drugs in tumor tissues and improve tumor treatment effect.

METHODS

Hyaluronic acid (HA) carbon quantum dots (CD) coupled with gelatin nanoparticle (GNP) were constructed, and were connected with doxorubicin (DOX), a chemotherapeutic drug, through pH-sensitive imine to produce GNP@HA-CD-DOX nanoparticles. The changes of particle size, drug release behavior, hemocompatibility, cell uptake and deep penetration of tumor spheroids, in vivo tumor targeting and therapeutic effect were analyzed.

RESULTS

GNP@HA-CD-DOX nanoparticles had a particle size of (162.93±2.55) nm, which could be degraded to release HA-CD-DOX with a particle size of about 40 nm under the treatment of MMP. The drug loading of DOX was (4.94±0.22)%. DOX was released in the tumor microenvironment and lysosomes in response to the low pH. No obvious hemolysis was observed in GNP@HA-CD-DOX. GNP@HA-CD-DOX showed a reduction in particle size after co-incubation with MMP-2. The MMP-sensitive GNP@HA-CD-DOX had significantly improved cell uptake and better deep penetration in tumor spheres. GNP@HA-CD-DOX displayed better distribution in tumor and anti-tumor ability in tumor-bearing mice compared with the small particle size HA-CD-DOX group. In addition, it has better safety.

CONCLUSION

The pH and MMP dual-sensitive nano-tech drug delivery system with adjustable particle sizes synergistically enhances the retention and deep penetration of drugs in tumors as well as the anti-tumor effect, suggesting new approaches to tumor treatment.

Effect of an Endothelin B Receptor Agonist on the Tumor Accumulation of Nanocarriers

Enhancing blood flow to tumors is a prominent strategy for improving the tumor accumulation of macromolecular drugs through the enhanced permeability and retention (EPR) effect. IRL-1620 is an agonist of the endothelin B receptor, and is a promising molecule to enhance tumor blood flow by activating endothelial nitric oxide synthase. However, contradictory effects on tumor blood flow modulation have been reported because the effects of IRL-1620 may differ in different animal models. Here, we examined for the first time the effect of IRL-1620 on the EPR effect for PEGylated liposomes in a CT-26 murine colon cancer model. Co-injection of IRL-1620 at an optimum dose (3 nmol/kg) nearly doubled the tumor accumulation of liposomes compared with controls, indicating that IRL-1620 enhanced the EPR effect in the present colon cancer model. Co-injection of IRL-1620 is a promising strategy to improve the therapeutic effects of macromolecular drugs while reducing their side effects.

Hierarchical nano-to-molecular disassembly of boron dipyrromethene nanoparticles for enhanced tumor penetration and activatable photodynamic therapy

The development of activatable photosensitizers (PSs) is of particular interest for achieving tumor photodynamic therapy (PDT) with minimal side effects. However, the in vivo applications of PSs are limited by complex physiological and biological delivery barriers. Herein, boron dipyrromethene (BDP)-based nanoparticles are developed through the self-assembly of a multifunctional "one-for-all" building block for enhanced tumor penetration and activatable PDT. The nanoparticles show excellent colloidal stability and long circulation lifetime in blood. Once they reach the tumor site, the first-stage size reduction occurs due to the hydrolysis of the Schiff base bond between polyethylene glycol and the cyclic Arg-Gly-Asp peptide in the acidic tumor microenvironment (pH~6.5), facilitating tumor penetration and specific recognition by cancer cells overexpressing integrin α_νβ₃ receptors. Upon the endocytosis by cancer cells, the second-stage size reduction is triggered by more acidic pH in lysosomes (pH~4.5). Importantly, the protonated diethylamino groups can block photoinduced electron transfer from the amine donor to the excited PSs and accelerate complete disassembly of the nanoparticles into single PS molecule, with the recovery of the fluorescence and photoactivity for efficient PDT. This study presents a smart PS delivery strategy involving acidity-triggered hierarchical disassembly from the nano to molecular scale for precise tumor PDT.

Reappraisal of anticancer nanomedicine design criteria in three types of preclinical cancer models for better clinical translation

Anticancer nanomedicines are designed to improve anticancer efficacy by increasing drug accumulation in tumors through enhanced permeability retention (EPR) effect, and to reduce toxicity by decreasing drug accumulation in normal organs through long systemic circulation. However, the inconsistent efficacy/safety of nanomedicines in cancer patients versus preclinical cancer models have provoked debate for nanomedicine design criteria. In this study, we investigate nanomedicine design criteria in three types of preclinical cancer models using five clinically used nanomedicines, which identifies the factors for better clinical translations of their observed clinical efficacy/safety compared to free drug or clinical micelle formulation. When those nanomedicines were compared with drug solution or clinical micelle formulation in breast tumors, long and short-circulating nanomedicines did not enhance tumor accumulation by EPR effect in transgenic spontaneous breast cancer model regardless of their size or composition, although they improved tumor accumulations in subcutaneous and orthotopic breast cancer models. However, when tumors were compared to normal breast tissue, nanomedicines, drug solution and clinical micelle formulation showed enhanced tumor accumulation regardless of the breast cancer models. In addition, long-circulating nanomedicines did not further increase tumor accumulation in transgenic mouse spontaneous breast cancer nor universally decrease drug accumulations in normal organs; they decreased or increased accumulation in different organs, potentially changing the clinical efficacy/safety. In contrast, short-circulating nanomedicines decreased blood concentration and altered drug distribution in normal organs, which are correlated with their clinical efficacy/safety. A reappraisal of current nanomedicine design criteria is needed to ensure consistent clinical translation for improvement of their clinical efficacy/safety in cancer patients.

Liposomal Formulation of a Melphalan Lipophilic Prodrug: Studies of Acute Toxicity, Tolerability, and Antitumor Efficacy

BACKGROUND

Recently we developed a scalable scheme of synthesis of melphalan ester conjugate with 1,2-dioleoyl-sn-glycerol (MlphDG) and a protocol for the fabrication of its lyophilized liposomal formulation.

OBJECTIVE

Herein we compared this new convenient in use formulation of MlphDG with parent drug Alkeran® in rats concerning several toxicological parameters and evaluated its antitumor efficacy in the model of breast cancer in mice.

METHOD

Liposomes of approximately 100 nm in diameter, consisting of egg phosphatidylcholine, soybean phosphatidylinositol, and MlphDG, or placebo liposomes without the drug were produced by extrusion and lyophilized. Alkeran® or liposomes recovered by the addition of water were injected into the tail vein of animals. Clinical examination of rats consisted of detailed inspection of the behavior, general status, and hematological parameters. Mice with transplanted breast cancer WNT-1 were subjected to multiple treatments with the drugs; tumor growth inhibition was assessed, together with cellular immunity parameters.

RESULTS

Liposomes showed approximately two times lower acute toxicity and better tolerability than Alkeran® in terms of behavioral criteria. The toxic effects of liposomes on hemopoiesis were manifested at higher doses than in the case of Alkeran®, proportionally to the difference in LD50 values. The formulation inhibited tumor growth significantly more effectively than Alkeran®, delaying the start of the exponential growth phase and exhibiting no additional toxic effects toward bone marrow.

CONCLUSION

Lower toxicity of the liposomal formulation of MlphDG promises improved quality of life for cancer patients in need of treatment with melphalan. Presumably, the list of indications for melphalan therapy could be extended.

Hypoxia Engineered Bone Marrow Mesenchymal Stem Cells Targeting System with Tumor Microenvironment Regulation for Enhanced Chemotherapy of Breast Cancer

Improving the tumor targeting of docetaxel (DTX) would not only be favored for the chemotherapeutic efficacy, but also reduce its side effects. However, the regulation of the tumor microenvironment could further inhibit the growth of tumors. In this study, we introduced a system consisting of hypoxia-engineered bone marrow mesenchymal stem cells (H-bMSCs) and DTX micelles (DTX-M) for breast cancer treatment. First, the stem cell chemotherapy complex system (DTX@H-bMSCs) with tumor-targeting ability was constructed according to the uptake of DTX-M by hypoxia-induced bMSCs (H-bMSCs). DTX micellization improved the uptake efficiency of DTX by H-bMSCs, which equipped DTX@H-bMSCs with satisfactory drug loading and stability. Furthermore, the migration of DTX@H-bMSCs revealed that it could effectively target the tumor site and facilitate the drug transport between cells. Moreover, in vitro and in vivo pharmacodynamics of DTX@H-bMSCs exhibited a superior antitumor effect, which could promote the apoptosis of 4T1 cells and upregulate the expression of inflammatory factors at the tumor site. In brief, DTX@H-bMSCs enhanced the chemotherapeutic effect in breast cancer treatment.

Tumor microenvironment remodeling-based penetration strategies to amplify nanodrug accessibility to tumor parenchyma

Remarkable advances in nano delivery systems have provided new hope for tumor prevention, diagnosis and treatment. However, only limited clinical therapeutic effects against solid tumors were achieved. One of the main reasons is the presence of abundant physiological and pathological barriers in vivo that impair tumoral penetration and distribution of the nanodrugs. These barriers are related to the components of tumor microenvironment (TME) including abnormal tumor vasculature, rich composition of the extracellular matrix (ECM), and abundant stroma cells. Herein, we review the advanced strategies of TME remodeling to overcome these biological obstacles against nanodrug delivery. This review aims to offer a perspective guideline for the implementation of promising approaches to facilitate intratumoral permeation of nanodrugs through alleviation of biological barriers. At the same time, we analyze the advantages and disadvantages of the corresponding methods and put forward possible directions for the future researches.

Transient and Efficient Vascular Permeability Window for Adjuvant Drug Delivery Triggered by Microbeam Radiation

BACKGROUND

Microbeam Radiation Therapy (MRT) induces a transient vascular permeability window, which offers a novel drug-delivery system for the preferential accumulation of therapeutic compounds in tumors. MRT is a preclinical cancer treatment modality that spatially fractionates synchrotron X-rays into micrometer-wide planar microbeams which can induce transient vascular permeability, especially in the immature tumor vessels, without compromising vascular perfusion. Here, we characterized this phenomenon using Chicken Chorioallantoic Membrane (CAM) and demonstrated its therapeutic potential in human glioblastoma xenografts in mice.

METHODS

the developing CAM was exposed to planar-microbeams of 75 Gy peak dose with Synchrotron X-rays. Similarly, mice harboring human glioblastoma xenografts were exposed to peak microbeam doses of 150 Gy, followed by treatment with Cisplatin. Tumor progression was documented by Magnetic Resonance Imaging (MRI) and caliper measurements.

RESULTS

CAM exposed to MRT exhibited vascular permeability, beginning 15 min post-irradiation, reaching its peak from 45 min to 2 h, and ending by 4 h. We have deemed this period the "permeability window". Morphological analysis showed partially fragmented endothelial walls as the cause of the increased transport of FITC-Dextran into the surrounding tissue and the extravasation of 100 nm microspheres (representing the upper range of nanoparticles). In the human glioblastoma xenografts, MRI measurements showed that the combined treatment dramatically reduced the tumor size by 2.75-fold and 5.25-fold, respectively, compared to MRT or Cisplatin alone.

CONCLUSIONS

MRT provides a novel mechanism for drug delivery by increasing vascular transpermeability while preserving vessel integrity. This permeability window increases the therapeutic index of currently available chemotherapeutics and could be combined with other therapeutic agents such as Nanoparticles/Antibodies/etc.

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.

The Use of Alternative Strategies for Enhanced Nanoparticle Delivery to Solid Tumors

Nanomaterial (NM) delivery to solid tumors has been the focus of intense research for over a decade. Classically, scientists have tried to improve NM delivery by employing passive or active targeting strategies, making use of the so-called enhanced permeability and retention (EPR) effect. This phenomenon is made possible due to the leaky tumor vasculature through which NMs can leave the bloodstream, traverse through the gaps in the endothelial lining of the vessels, and enter the tumor. Recent studies have shown that despite many efforts to employ the EPR effect, this process remains very poor. Furthermore, the role of the EPR effect has been called into question, where it has been suggested that NMs enter the tumor via active mechanisms and not through the endothelial gaps. In this review, we provide a short overview of the EPR and mechanisms to enhance it, after which we focus on alternative delivery strategies that do not solely rely on EPR in itself but can offer interesting pharmacological, physical, and biological solutions for enhanced delivery. We discuss the strengths and shortcomings of these different strategies and suggest combinatorial approaches as the ideal path forward.

Combined cancer therapeutics-Tackling the complexity of the tumor microenvironment

Cancer treatment has yet to find a "silver bullet" capable of selectively and effectively kill tumor cells without damaging healthy cells. Nanomedicine is a promising field that can combine several moieties in one system to produce a multifaceted nanoplatform. The tumor microenvironment (TME) is considered responsible for the ineffectiveness of cancer therapeutics and the difficulty in the translation from the bench to bed side of novel nanomedicines. A promising approach is the use of combinatorial therapies targeting the TME with the use of stimuli-responsive nanomaterials which would increase tumor targeting. Contemporary combined strategies for TME-targeting nanoformulations are based on the application of external stimuli therapies, such as photothermy, hyperthermia or ultrasounds, in combination with stimuli-responsive nanoparticles containing a core, usually composed by metal oxides or graphene, and a biocompatible stimuli-responsive coating layer that could also contain tumor targeting moieties and a chemotherapeutic agent to enhance the therapeutic efficacy. The obstacles that nanotherapeutics must overcome in the TME to accomplish an effective therapeutic cargo delivery and the proposed strategies for improved nanotherapeutics will be reviewed. This article is categorized under: Therapeutic Approaches and Drug Discovery > Nanomedicine for Oncologic Disease Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Factors affecting the dynamics and heterogeneity of the EPR effect: pathophysiological and pathoanatomic features, drug formulations and physicochemical factors

INTRODUCTION

The enhanced permeability and retention (EPR) effect serves as the foundation of anticancer nanomedicine design. EPR effect-based drug delivery is an effective strategy for most solid tumors. However, the degree of efficacy depends on the pathophysiological conditions of tumors, drug formulations, and other factors.

AREAS COVERED

Vascular mediators including nitric oxide, bradykinin , and prostaglandins are vital for facilitating and maintaining EPR effect dynamics. Progression to large, advanced cancers may induce activated blood coagulation cascades, which lead to thrombus formation in tumor vasculature. Rapidly growing tumors cause obstructed or suppressed blood flow in tumor vasculature related to embolism or occluded blood vessels. The resulting limited tumor blood flow leads to less drug delivered to tumors, i.e. no or poor EPR effect. High stromal content also suppresses vascular permeability and drug diffusion. Restoring obstructed tumor blood flow and improving tumor vascular permeability via vascular mediators will improve drug delivery and the EPR effect. Physicochemical features of nanomedicines also influence therapeutic outcomes and are vital for the EPR effect.

EXPERT OPINION

The tumor microenvironment, especially tumor blood flow, is critical for a potent EPR effect. A rational strategy for circumventing EPR effect barriers must include restoring tumor blood flow.

Galactose Modified Liposomes for Effective Co-Delivery of Doxorubicin and Combretastatin A4

Background

Tumor angiogenesis plays a crucial role in tumor development, and recent efforts have been focused on combining proapoptotic and antiangiogenic activities to enhance antitumor therapy.

Methods

In this study, galactose-modified liposomes (Gal-LPs) were prepared for co-delivery of doxorubicin (DOX) and combretastatin A4 phosphate (CA4P). The co-cultured system composed of BEL-7402 and human umbilical vein endothelial cells (HUVEC) cells was established to effectively evaluate in vitro anti-tumor activity through cell viability and cell migration assay. Furthermore, both in vivo bio-distribution and anti-hepatoma effect of DOX&CA4P/Gal-LPs were investigated on H22 tumor cell-bearing mice.

Results

The results showed that DOX&CA4P/Gal-LPs were spherical with a mean particle size of 143 nm, and could readily be taken up by BEL-7402 cells. Compared with a mixture of free DOX and CA4P, the DOX&CA4P/Gal-LPs were more effective in inhibiting cell migration and exhibited stronger cytotoxicity against BEL-7402 cells alone or a co-cultured system. The in vitro studies showed that the co-cultured system was a more effective model to evaluate the anti-tumor activity of combination therapy. Moreover, DOX&CA4P/Gal-LPs exhibited a greater anti-hepatoma effect than other drug formulations, indicating that Gal-LPs could promote drug accumulation in the tumor region and improve the anti-tumor activity.

Conclusion

Gal-LPs co-loaded with chemotherapeutic and antiangiogenic drugs are a promising strategy for anti-hepatoma therapy.

Chemo-physical Strategies to Advance the in Vivo Functionality of Targeted Nanomedicine: The Next Generation

The past few decades have witnessed an evolution of nanomedicine from biologically inert entities to more smart systems, aimed at advancing in vivo functionality. However, we should recognize that most systems still rely on reasonable explanation-including some over-explanation-rather than definitive evidence, which is a watershed radically determining the speed and extent of advancing nanomedicine. Probing nano-bio interactions and desirable functionality at the tissue, cellular, and molecular levels is most frequently overlooked. Progress toward answering these questions will provide instructive insight guiding more effective chemo-physical strategies. Thus, in the next generation, we argue that much effort should be made to provide definitive evidence for proof-of-mechanism, in lieu of creating many new and complicated systems for similar proof-of-concept.

Stimuli-responsive phospholipid-drug conjugates (PDCs)-based nanovesicles for drug delivery and theranostics

Liposomes represent one of the most successful nano-drug delivery systems among enormous nano-carriers. Although great progress has been made in conventional liposomes, the emerging shortcomings still impair the therapeutic index. The proposal of stimuli-responsive phospholipid-drug conjugates (PDCs)-based nanovesicles solves the challenges that conventional liposomes are faced with, showing great potential for cancer diagnosis and therapy. Herein, we intend to overview the current progress and unique advantages of stimuli-responsive PDCs-based nanovesicles. First, the challenges of conventional liposomes and the development of PDCs-based nanovesicles are summarized. Next, the stimuli-responsive elements used in current stimuli-responsive PDCs-based nanovesicles are outlined. Then, the unique superiorities of stimuli-responsive PDCs-based nanovesicles for drug delivery and theranostics are highlighted in detail. Finally, the future opportunities and challenges of stimuli-responsive PDCs-based nanovesicles for clinical translation are put forward.

Advances in Exosome-Based Drug Delivery and Tumor Targeting: From Tissue Distribution to Intracellular Fate

Exosomes or small extracellular vesicles are considered a new generation of bioinspired-nanoscale drug delivery system (DDS). Endogenous exosomes function as signalosomes since they convey signals via ligands or adhesion molecules located on the exosomal membrane, or packaged inside the exosome. Recently, exosome membrane modification, therapeutic payloads encapsulation, and modulation of in vivo disposition of exosomes have been extensively investigated, among which significant advances have been made to optimize exosome-mediated delivery to solid tumors. Exosomes, specifically tumor cell-derived exosomes, are presumed to have tumor-preferential delivery due to the homotypic features. However, quality attributes that dictate the tissue distribution, cell type-selective uptake, and intracellular payload release of the administered exosomes, as well as the spatiotemporal information regarding exosome fate in vivo, remain to be further investigated. This review summarizes recent advances in developing exosomes as drug delivery platforms with a focus on tumor targeting. The pharmacokinetic features of naive exosomes and factors influencing their intracellular fate are summarized. Recent strategies to improve tumor targeting of exosomes are also reviewed in the context of the biological features of tumor and tumor microenvironment (TME). Selected approaches to augment tumor tissue deposition of exosomes, as well as methods to enhance intracellular payload delivery, are summarized with emphasis on the underlying mechanisms (eg, passive or active targeting, endosomal escape, etc.). In conclusion, this review highlights recently reported tumor-targeting strategies of exosome-based drug delivery, and it's in the hope that multiple approaches might be employed in a synergistic combination in the development of exosome-based cancer therapy.

Biomineralization-inspired dasatinib nanodrug with sequential infiltration for effective solid tumor treatment

The complex blood environment, heterogenic enhanced permeability and retention (EPR) effect, and dense matrix comprise the primary "leakage obstacles" impeding specific accumulation and penetration of nanodrugs against solid tumors, thus forming a key bottleneck for their clinical application. Herein, we present a biomineralization-inspired dasatinib (DAS) nanodrug (CIPHD/DAS) that sequentially permeates all of the abovementioned hindrances for efficient treatment of solid tumors. CIPHD/DAS exhibited a robust hybrid structure constructed from an iRGD-modified hyaluronic acid-deoxycholic acid organic core and a calcium phosphate mineral shell. In vitro and in vivo data demonstrated the mechanism of sequential tumoral infiltration was based on mineral-stiffened blood circulation with decreased premature drug leakage, iRGD-endowed tumor-specific transendothelial transport for "first-order promotion of accumulation" and DAS-mediated restoration of fibrotic stromal homeostasis for "second-order promotion of penetration". Resultantly, CIPHD/DAS showed remarkable distal drug availability in desmoplastic 4T1/CAFs orthotropic mouse models and significantly suppressed tumor growth and metastasis. This optimized strategy with sequential permeabilization of the capital "leakage obstacles" validates a promising paradigm to conquer the "impaired delivery and penetration" associated bottleneck of nanodrugs in the clinical treatment of solid tumors.

Liposome-Based Drug Delivery Systems in Cancer Immunotherapy

Cancer immunotherapy has shown remarkable progress in recent years. Nanocarriers, such as liposomes, have favorable advantages with the potential to further improve cancer immunotherapy and even stronger immune responses by improving cell type-specific delivery and enhancing drug efficacy. Liposomes can offer solutions to common problems faced by several cancer immunotherapies, including the following: (1) Vaccination: Liposomes can improve the delivery of antigens and other stimulatory molecules to antigen-presenting cells or T cells; (2) Tumor normalization: Liposomes can deliver drugs selectively to the tumor microenvironment to overcome the immune-suppressive state; (3) Rewiring of tumor signaling: Liposomes can be used for the delivery of specific drugs to specific cell types to correct or modulate pathways to facilitate better anti-tumor immune responses; (4) Combinational therapy: Liposomes are ideal vehicles for the simultaneous delivery of drugs to be combined with other therapies, including chemotherapy, radiotherapy, and phototherapy. In this review, different liposomal systems specifically developed for immunomodulation in cancer are summarized and discussed.

A tumor-targeted theranostic nanomedicine with strong absorption in the NIR-II biowindow for image-guided multi-gradient therapy

Developing new strategies to enhance drug accumulation in the tumor and therapeutic efficacy is of great importance in the field of tumor therapy. Herein, a peanut-like multifunctional nanomedicine (CuS-PGH NMs) made of CuS nanoparticles encapsulated in poly(l-lysine)(PLL)/glucose oxidase (GOx)-hyaluronic acid (HA) shells has been constructed via layer-by-layer (LbL) assembly, and shows good biocompatibility and effective multi-gradient therapy. Because of the enhanced permeability and retention (EPR) effect, the CuS-PGH NMs could significantly enhance the cellular uptake by tumors overexpressing CD44 receptors, which respond to hyaluronidase (HAase)-triggered surface charge conversion. Once internalized by the tumor, GOx was the first to be exposed and could effectively deplete endogenous glucose for starvation therapy, and the excess H2O2 was then converted into highly toxic hydroxyl radicals (˙OH) via a Cu+-mediated Fenton-like reaction for chemodynamic therapy (CDT). Meanwhile, the as-obtained Cu+ ions accompanied the regenerated less-active Cu2+ ions. Interestingly, the high content of H2O2 could, in turn, accelerate Cu2+/Cu+ conversion to promote the Cu+-H2O2 reaction for enhanced chemodynamic therapy (CDT), thereby achieving efficient tumor growth suppression via synergistic starvation/CDT therapy. Subsequently, owing to the strong NIR-II absorption capability of CuS-PGH NMs, effective photothermal tumor ablation of the weakened tumor cells could be realized with the precise guidance of NIR-II PAI. This multi-gradient therapeutic strategy has been demonstrated to have excellent antitumor activity with minimal nonspecific damages, and offers a new avenue to precise tumor therapy.

What Went Wrong with Anticancer Nanomedicine Design and How to Make It Right

The three design criteria of anticancer nanomedicines to improve anticancer efficacy and to reduce toxicity have been debated for decades: (1) Nanomedicines increase drug accumulation through enhanced permeability and retention (EPR) in tumors to improve anticancer efficacy. (2) Long systemic circulation of nanomedicines with high plasma concentration reduces reticuloendothelial system (RES) clearance and decreases drug accumulation in the normal organs to reduce toxicity, and to enhance the EPR effect. (3) A universal nanodelivery platform based on EPR and long systemic circulation can be developed to deliver different anticancer drugs. Although these criteria have repeatedly been confirmed in preclinical xenograft cancers, the majority of anticancer nanomedicines have failed to improve clinical efficacy, while the clinical efficacies/safety of successful nanomedicines are inconsistent with these design criteria. First, the debate over tumor EPR may have mixed two different questions and missed more clinically relevant comparisons for nanomedicines versus free drugs. When tumors are compared with normal tissues, tumor EPR has been confirmed in both mouse xenograft tumors and human cancers. However, nanomedicines may not enhance drug accumulation in human tumors compared with free drugs, despite outstanding improvement in preclinical cancers. Heterogeneity of enhanced permeability and retention in human cancers occurs for small/large molecules and nanomedicines, which cannot fully explain the poor translation of nanomedicines' efficacy from preclinical cancer models to cancer patients. Second, long-circulation nanomedicine should not be used as a universal design criterion because it does not further improve tumor accumulation by tumor EPR in human patients nor universally reduce distribution in normal organs. In contrast, nanomedicines change the drug tissue distribution to alter anticancer efficacy/safety. Third, a universal nanodelivery platform that uses the same design criteria for different drugs is not feasible. Rather, drug-specific nanodelivery systems are required to overcome the intrinsic shortcomings of delivered drugs, which are determined by the physicochemical, pharmacokinetic, and pharmacodynamic properties of the delivered drugs and nanocarriers to improve their efficacy/safety.

Barriers and Strategies of Cationic Liposomes for Cancer Gene Therapy

Cationic liposomes (CLs) have been regarded as the most promising gene delivery vectors for decades with the advantages of excellent biodegradability, biocompatibility, and high nucleic acid encapsulation efficiency. However, the clinical use of CLs in cancer gene therapy is limited because of many uncertain factors in vivo. Extracellular barriers such as opsonization, rapid clearance by the reticuloendothelial system and poor tumor penetration, and intracellular barriers, including endosomal/lysosomal entrapped network and restricted diffusion to the nucleus, make CLs not the ideal vector for transferring extrinsic genes in the body. However, the obstacles in achieving productive therapeutic effects of nucleic acids can be addressed by tailoring the properties of CLs, which are influenced by lipid compositions and surface modification. This review focuses on the physiological barriers of CLs against cancer gene therapy and the effects of lipid compositions on governing transfection efficiency, and it briefly discusses the impacts of particle size, membrane charge density, and surface modification on the fate of CLs in vivo, which may provide guidance for their preclinical studies.

Vascular and extracellular matrix remodeling by physical approaches to improve drug delivery at the tumor site

INTRODUCTION

Modern comprehensive studies of tumor microenvironment changes allowed scientists to develop new and more efficient strategies that will improve anticancer drug delivery on site. The tumor microenvironment, especially the dense extracellular matrix, has a recognized capability to hamper the penetration of conventional drugs. Development and co-applications of strategies aiming at remodeling the tumor microenvironment are highly demanded to improve drug delivery at the tumor site in a therapeutic prospect.

AREAS COVERED

Increasing indications suggest that classical physical approaches such as exposure to ionizing radiations, hyperthermia or light irradiation, and emerging ones as sonoporation, electric field or cold plasma technology can be applied as standalone or associated strategies to remodel the tumor microenvironment. The impacts on vasculature and extracellular matrix remodeling of these physical approaches will be discussed with the goal to improve nanotherapeutics delivery at the tumor site.

EXPERT OPINION

Physical approaches to modulate vascular properties and remodel the extracellular matrix are of particular interest to locally control and improve drug delivery and thus increase its therapeutic index. They are particularly powerful as adjuvant to nanomedicine delivery; the development of these technologies could have extremely widespread implications for cancer treatment.[Figure: see text].

Intratumoral Distribution and pH-Dependent Drug Release of High Molecular Weight HPMA Copolymer Drug Conjugates Strongly Depend on Specific Tumor Substructure and Microenvironment

Stimulus-sensitive polymer drug conjugates based on high molecular weight N-(2-hydroxypropyl) methacrylamide (HPMA) copolymers carrying doxorubicin via a pH-dependent cleavable bond (pHPMA-Dox) were previously shown to be able to overcome multi-drug resistance. Nevertheless, a tumor type dependent differential response was observed. Although an improved and more selective tumor accumulation of pHPMA-Dox is generally achieved due to the enhanced permeability and retention (EPR) effect, little is known about the fate of these conjugates upon entering the tumor tissue, which could explain the different responses. In this study, we compared in vitro and in vivo accumulation and Dox-activation of pHPMA-Dox in three cancer cell line models (1411HP, A2780cis, HT29) and derived xenograft tumors using a near-infrared fluorescence-labeled pHPMA-Dox conjugate. Firstly, cytotoxicity assays using different pH conditions proved a stepwise, pH-dependent increase in cytotoxic activity and revealed comparable sensitivity among the cell lines. Using multispectral fluorescence microscopy, we were able to track the distribution of drug and polymeric carrier simultaneously on cellular and histological levels. Microscopic analyses of cell monolayers confirmed the assumed mechanism of cell internalization of the whole conjugate followed by intracellular cleavage and nuclear accumulation of Dox in all three cell lines. In contrast, intratumoral distribution and drug release in xenograft tumors were completely different and were associated with different tissue substructures and microenvironments analyzed by Azan- and Hypoxisense^®-staining. In 1411HP tumors, large vessels and less hypoxic/acidic microenvironments were associated with a pattern resulting from consistent tissue distribution and cellular uptake as whole conjugate followed by intracellular drug release. In A2780cis tumors, an inconsistent pattern of distribution partly resulting from premature drug release was associated with a more hypoxic/acidic microenvironment, compacted tumor tissue with compressed vessels and specific pre-damaged tissue structures. A completely different distribution pattern was observed in HT29 tumors, resulting from high accumulation of polymer in abundant fibrotic structures, with small embedded vessels featuring this tumor type together with pronounced premature drug release due to the strongly hypoxic/acidic microenvironment. In conclusion, the pattern of intratumoral distribution and drug release strongly depends on the tumor substructure and microenvironment and may result in different degrees of therapeutic efficacy. This reflects the pronounced heterogeneity observed in the clinical application of nanomedicines and can be exploited for the future design of such conjugates.