Anti-prostate-specific membrane antigen liposomes loaded with 225Ac for potential targeted antivascular α-particle therapy of cancer

This study evaluates targeted liposomes loaded with the α-particle generator (225)Ac to selectively kill prostate-specific membrane antigen (PSMA)-expressing cells with the aim to assess their potential for targeted antivascular radiotherapy.

METHODS

In this study, PEGylated liposomes were loaded with (225)Ac and labeled with the mouse antihuman PSMA J591 antibody or with the A10 PSMA aptamer. The targeting selectivity, extent of internalization, and killing efficacy of liposomes were evaluated on monolayers of prostate cancer cells intrinsically expressing PSMA (human LNCaP and rat Mat-Lu cells) and on monolayers of HUVEC induced to express PSMA (induced HUVEC).

RESULTS

The loading efficiency of (225)Ac into preformed liposomes ranged from 58.0% ± 4.6% to 85.6% ± 11.7% of introduced radioactivity. The conjugation reactions resulted in approximately 17 ± 2 J591 antibodies and 9 ± 2 A10 aptamers per liposome. The average size of liposomes, 107 ± 2 nm in diameter, was not affected by conjugation or loading. LNCaP cells exhibit 2:1:0.5 relative PSMA expression, compared with MatLu and induced HUVEC, respectively, based on flow cytometry detecting association of the J591 antibody. J591-labeled liposomes display higher levels of total specific binding to all cell lines than A10 aptamer-labeled liposomes. Specific cell association of targeted liposomes increases with incubation time. Cytotoxicity studies demonstrate that radiolabeled J591-labeled liposomes are most cytotoxic, with median lethal dose values, after 24 h of incubation, equal to 1.96 (5.3 × 10(-5)), 2.92 × 10(2) (7.9 × 10(-3)), and 2.33 × 10(1) Bq/mL (6.3 × 10(-4) μCi/mL) for LNCaP, Mat-Lu, and induced HUVEC, respectively, which are comparable to the values for the radiolabeled J591 antibody. For A10 aptamer-labeled liposomes, the corresponding values are 3.70 × 10(1) (1.0 × 10(-3)), 1.85 × 10(3) (5.0 × 10(-2)), and 4.07 × 10(3) Bq/mL (1.1 × 10(-1) μCi/mL), respectively.

CONCLUSION

Our studies demonstrate that anti-PSMA-targeted liposomes loaded with (225)Ac selectively bind, become internalized, and kill PSMA-expressing cells including endothelial cells induced to express PSMA. These findings-combined with the unique ability of liposomes to be easily tuned, in terms of size and surface modification, for optimizing biodistributions-suggest the potential of PSMA-targeting liposomes encapsulating α-particle emitters for selective antivascular α radiotherapy.

Radionuclide carriers for targeting of cancer

This review describes strategies for the delivery of therapeutic radionuclides to tumor sites. Therapeutic approaches are summarized in terms of tumor location in the body, and tumor morphology. These determine the radionuclides of choice for suggested targeting ligands, and the type of delivery carriers. This review is not exhaustive in examples of radionuclide carriers for targeted cancer therapy. Our purpose is two-fold: to give an integrated picture of the general strategies and molecular constructs currently explored for the delivery of therapeutic radionuclides, and to identify challenges that need to be addressed. Internal radiotherapies for targeting of cancer are at a very exciting and creative stage. It is expected that the current emphasis on multidisciplinary approaches for exploring such therapeutic directions should enable internal radiotherapy to reach its full potential.

Enhanced retention of the alpha-particle-emitting daughters of Actinium-225 by liposome carriers

Targeted alpha-particle emitters hold great promise as therapeutics for micrometastatic disease. Because of their high energy deposition and short range, tumor targeted alpha-particles can result in high cancer-cell killing with minimal normal-tissue irradiation. Actinium-225 is a potential generator for alpha-particle therapy: it decays with a 10-day half-life and generates three alpha-particle-emitting daughters. Retention of (225)Ac daughters at the target increases efficacy; escape and distribution throughout the body increases toxicity. During circulation, molecular carriers conjugated to (225)Ac cannot retain any of the daughters. We previously proposed liposomal encapsulation of (225)Ac to retain the daughters, whose retention was shown to be liposome-size dependent. However, daughter retention was lower than expected: 22% of theoretical maximum decreasing to 14%, partially due to the binding of (225)Ac to the phospholipid membrane. In this study, Multivesicular liposomes (MUVELs) composed of different phospholipids were developed to increase daughter retention. MUVELs are large liposomes with entrapped smaller lipid-vesicles containing (225)Ac. PEGylated MUVELs stably retained over time 98% of encapsulated (225)Ac. Retention of (213)Bi, the last daughter, was 31% of the theoretical maximum retention of (213)Bi for the liposome sizes studied. MUVELs were conjugated to an anti-HER2/neu antibody (immunolabeled MUVELs) and were evaluated in vitro with SKOV3-NMP2 ovarian cancer cells, exhibiting significant cellular internalization (83%). This work demonstrates that immunolabeled MUVELs might be able to deliver higher fractions of generated alpha-particles per targeted (225)Ac compared to the relative fractions of alpha-particles delivered by (225)Ac-labeled molecular carriers.

Enhanced loading efficiency and retention of 225Ac in rigid liposomes for potential targeted therapy of micrometastases

Targeted alpha-particle emitters are promising therapeutics for micrometastatic disease. Actinium-225 has a 10-day half-life and generates a total of four alpha-particles per parent decay rendering (225)Ac an attractive candidate for alpha-therapy. For cancer cells with low surface expression levels of molecular targets, targeting strategies of (225)Ac using radiolabeled carriers of low specific radioactivities (such as antibodies) may not deliver enough alpha-particle emitters at the targeted cancer cells to result in killing. We previously proposed and showed using passive (225)Ac entrapment that liposomes can stably retain encapsulated (225)Ac for long time periods, and that antibody-conjugated liposomes (immunoliposomes) with encapsulated (225)Ac can specifically target and become internalized by cancer cells. However, to enable therapeutic use of (225)Ac-containing liposomes, high activities of (225)Ac need to be stably encapsulated into liposomes. In this study, various conditions for active loading of (225)Ac in preformed liposomes (ionophore-type, encapsulated buffer solution, and loading time) were evaluated, and liposomes with up to 73 +/- 9% of the initial activity of (225)Ac (0.2-200 microCi) were developed. Retention of radioactive contents by liposomes was evaluated at 37 degrees C in phosphate buffer and in serum-supplemented media. The main fraction of released (225)Ac from liposomes occurs within the first two hours of incubation. Beyond this two hour point, the encapsulated radioactivity is released from liposomes slowly with an approximate half-life of the order of several days. In some cases, after 30 days, (225)Ac retention as high as 81 +/- 7% of the initially encapsulated radioactivity was achieved. The (225)Ac loading protocol was also applied to immunoliposome loading without significant loss of targeting efficacy. Liposomes with surface-conjugated antibodies that are loaded with (225)Ac overcome the limitations of low specific activity for molecular carriers and are expected to be therapeutically useful against tumor cells having a low antigen density.

Surface-active liposomes for targeted cancer therapy

An overview of liposome-based drug-delivery carriers to cancer cells is presented. Properties related to interfacial interactions between liposomes and the biological milieu that determine the fate of liposomes in vivo are discussed. Original approaches to improve specificity for the target and to control the structural responsiveness of liposomes, depending on their immediate environment, with the aim of enhancing the delivered therapeutic doses, are summarized. This review is not exhaustive on research examples of liposomes as carriers for cancer therapy but, rather, aims to describe major directions of designs and strategies over recent years. The current therapeutic trends that exhibit increasingly higher complexity in structures and responses are also discussed.

Targeted and Nontargeted α-Particle Therapies

α-Particle irradiation of cancerous tissue is increasingly recognized as a potent therapeutic option. We briefly review the physics, radiobiology, and dosimetry of α-particle emitters, as well as the distinguishing features that make them unique for radiopharmaceutical therapy. We also review the emerging clinical role of α-particle therapy in managing cancer and recent studies on in vitro and preclinical α-particle therapy delivered by antibodies, other small molecules, and nanometer-sized particles. In addition to their unique radiopharmaceutical characteristics, the increased availability and improved radiochemistry of α-particle radionuclides have contributed to the growing recent interest in α-particle radiotherapy. Targeted therapy strategies have presented novel possibilities for the use of α-particles in the treatment of cancer. Clinical experience has already demonstrated the safe and effective use of α-particle emitters as potent tumor-selective drugs for the treatment of leukemia and metastatic disease.

Time-restricted feeding and the realignment of biological rhythms: translational opportunities and challenges

It has been argued that circadian dysregulation is not only a critical inducer and promoter of adverse health effects, exacerbating symptom burden, but also hampers recovery. Therefore understanding the health-promoting roles of regulating (i.e., restoring) circadian rhythms, thus suppressing harmful effects of circadian dysregulation, would likely improve treatment. At a critical care setting it has been argued that studies are warranted to determine whether there is any use in restoring circadian rhythms in critically ill patients, what therapeutic goals should be targeted, and how these could be achieved. Particularly interesting are interventional approaches aiming at optimizing the time of feeding in relation to individualized day-night cycles for patients receiving enteral nutrition, in an attempt to re-establish circadian patterns of molecular expression. In this short review we wish to explore the idea of transiently imposing (appropriate, but yet to be determined) circadian rhythmicity via regulation of food intake as a means of exploring rhythm-setting properties of metabolic cues in the context of improving immune response. We highlight some of the key elements associated with his complex question particularly as they relate to: a) stress and rhythmic variability; and b) metabolic entrainment of peripheral tissues as a possible intervention strategy through time-restricted feeding. Finally, we discuss the challenges and opportunities for translating these ideas to the bedside.

Kinetic Analysis of Nanostructures Formed by Enzyme-Instructed Intracellular Assemblies against Cancer Cells

Recent studies have demonstrated that enzyme-instructed self-assembly (EISA) in extra- or intracellular environments can serve as a multistep process for controlling cell fate. There is little knowledge, however, about the kinetics of EISA in the complex environments in or around cells. Here, we design and synthesize three dipeptidic precursors (ld-1-SO₃, dl-1-SO₃, dd-1-SO₃), consisting of diphenylalanine (l-Phe-d-Phe, d-Phe-l-Phe, d-Phe-d-Phe, respectively) as the backbone, which are capped by 2-(naphthalen-2-yl)acetic acid at the N-terminal and by 2-(4-(2-aminoethoxy)-4-oxobutanamido)ethane-1-sulfonic acid at the C-terminal. On hydrolysis by carboxylesterases (CES), these precursors result in hydrogelators, which self-assemble in water at different rates. Whereas all three precursors selectively kill cancer cells, especially high-grade serous ovarian carcinoma cells, by undergoing intracellular EISA, dl-1-SO₃ and dd-1-SO₃ exhibit the lowest and the highest activities, respectively, against the cancer cells. This trend inversely correlates with the rates of converting the precursors to the hydrogelators in phosphate-buffered saline. Because CES exists both extra- and intracellularly, we use kinetic modeling to analyze the kinetics of EISA inside cells and to calculate the cytotoxicity of each precursor for killing cancer cells. Our results indicate that (i) the stereochemistry of the precursors affects the morphology of the nanostructures formed by the hydrogelators, as well as the rate of enzymatic conversion; (ii) decreased extracellular hydrolysis of precursors favors intracellular EISA inside the cells; (iii) the inherent features ( e.g., self-assembling ability and morphology) of the EISA molecules largely dictate the cytotoxicity of intracellular EISA. As the kinetic analysis of intracellular EISA, this work elucidates how the stereochemistry modulates EISA in the complex extra- and/or intracellular environment for developing anticancer molecular processes. Moreover, it provides insights for understanding the kinetics and cytotoxicity of aggregates of aberrant proteins or peptides formed inside and outside cells.

The pH-dependent association with cancer cells of tunable functionalized lipid vesicles with encapsulated doxorubicin for high cell-kill selectivity

To enable selective cell-kill, we designed functionalized lipid vesicles with pH-triggered heterogeneous membranes and encapsulated doxorubicin that exhibit tunable surface topography. These vesicles "hide" (mask) the targeting ligands from their surface during circulation in the blood, and only progressively "expose" these ligands as they gradually penetrate deeper into the tumor interstitium, where after endocytosis they burst release their contents. The stimulus to activate the binding reactivity is the pH gradient between the blood stream (pH 7.4-7.0) and the increasingly acidic pH inside the tumor interstitium (pH 6.7-6.5). Doxorubicin release is activated at the endosomal pH 5.5-5.0. We show that tunable functionalized vesicles exhibit environmentally-dependent (pH-dependent) association with cancer cells resulting in high cell-kill selectivity. When lowering the extracellular pH from 7.4 to 6.5, tunable functionalized vesicles deliver doxorubicin to cancer cells that increases from 41% to 93% of maximum resulting in cancer cell killing that increases from 23 to 71% of maximum, respectively. This proof-of-concept shows the potential of tunable targeted liposomal chemotherapy to selectively kill cancer cells in an environmentally-dependent way.

Antitumor efficacy following the intracellular and interstitial release of liposomal doxorubicin

pH-triggered lipid-membranes designed from biophysical principles are evaluated in the form of targeted liposomal doxorubicin with the aim to ultimately better control the growth of vascularized tumors. We compare the antitumor efficacy of anti-HER2/neu pH-triggered lipid vesicles encapsulating doxorubicin to the anti-HER2/neu form of an FDA approved liposomal doxorubicin of DSPC/cholesterol-based vesicles. The HER2/neu receptor is chosen due to its abundance in human breast cancers and its connection to low prognosis. On a subcutaneous murine BT474 xenograft model, superior control of tumor growth is demonstrated by targeted pH-triggered vesicles relative to targeted DSPC/cholesterol-based vesicles (35% vs. 19% decrease in tumor volume after 32 days upon initiation of treatment). Superior tumor control is also confirmed on SKBR3 subcutaneous xenografts of lower HER2/neu expression. The non-targeted form of pH-triggered vesicles encapsulating doxorubicin results also in better tumor control relative to the non-targeted DSPC/cholesterol-based vesicles (34% vs. 41% increase in tumor volume). Studies in BT474 multicellular spheroids suggest that the observed efficacy could be attributed to release of doxorubicin directly into the acidic tumor interstitium from pH-triggered vesicles extravasated into the tumor but not internalized by cancer cells. pH-triggered liposome carriers engineered from gel-phase bilayers that reversibly phase-separate with lowering pH, form transiently defective interfacial boundaries resulting in fast release of encapsulated doxorubicin. Our studies show that pH-triggered liposomes release encapsulated doxorubicin intracellularly and intratumorally, and may improve tumor control at the same or even lower administered doses relative to FDA approved liposomal chemotherapy.

Stable adhesion of phospholipid vesicles to modified gold surfaces

Phospholipid vesicles are well-studied biomembrane mimics that are of increasing interest in drug delivery, immunoassays, and sensor chips. In a number of biosensor applications it is desirable to be able to adhere vesicles to a surface in a manner which does not result in their rupture or fusion. Such behavior should, in principle, be achievable by controlling the vesicle-surface and vesicle-vesicle interactions. We have varied vesicle composition and charge (phosphatidylcholine, phosphatidylcholine-phosphatidic acid 18 mol%) and solution ionic strength, to study the adhesion of fluorescent vesicles to glass, gold, and gold modified with chemisorbed acetyl-cysteine. The extent of chemisorption was characterized with angle-resolved X-ray photoelectron spectroscopy (ARXPS), and vesicle integrity and behavior was studied using entrapped and lipophilic fluorescent markers, together and in separate measurements. Vesicle fusion (by energy transfer), adhesion of intact vesicles (with entrapped calcein) and diffusion coefficients (by photobleaching recovery) were monitored using confocal fluorescence microscopy. Acetyl-cysteine modified gold surfaces were shown to be appropriate substrates for adhesion of intact vesicles. Finally, as a 'proof of principle' for fluorescence amplification, release of a self-quenching entrapped reporter dye (calcein) by the detergent Triton X-100 was followed in real time.

Nanoconjugation of PSMA-Targeting Ligands Enhances Perinuclear Localization and Improves Efficacy of Delivered Alpha-Particle Emitters against Tumor Endothelial Analogues

This study aims to evaluate the effect on killing efficacy of the intracellular trafficking patterns of α-particle emitters by using different radionuclide carriers in the setting of targeted antivascular α-radiotherapy. Nanocarriers (lipid vesicles) targeted to the prostate-specific membrane antigen (PSMA), which is unique to human neovasculature for a variety of solid tumors, were loaded with the α-particle generator actinium-225 and were compared with a PSMA-targeted radiolabeled antibody. Actinium-225 emits a total of four α-particles per decay, providing highly lethal and localized irradiation of targeted cells with minimal exposure to surrounding healthy tissues. Lipid vesicles were derivatized with two types of PSMA-targeting ligands: a fully human PSMA antibody (mAb) and a urea-based, low-molecular-weight agent. Target selectivity and extent of internalization were evaluated on monolayers of human endothelial cells (HUVEC) induced to express PSMA in static incubation conditions and in a flow field. Both types of radiolabeled PSMA-targeted vesicles exhibit similar killing efficacy, which is greater than the efficacy of the radiolabeled control mAb when compared on the basis of delivered radioactivity per cell. Fluorescence confocal microscopy demonstrates that targeted vesicles localize closer to the nucleus, unlike antibodies which localize near the plasma membrane. In addition, targeted vesicles cause larger numbers of dsDNAs per nucleus of treated cells compared with the radiolabeled mAb. These findings demonstrate that radionuclide carriers, such as PSMA-targeted lipid-nanocarriers, which localize close to the nucleus, increase the probability of α-particle trajectories crossing the nuclei, and, therefore, enhance the killing efficacy of α-particle emitters.

Heterogeneous liposome membranes with pH-triggered permeability enhance the in vitro antitumor activity of folate-receptor targeted liposomal doxorubicin

The killing efficacy of doxorubicin from liposome-based delivery carriers has been shown to correlate strongly with its intracellular trafficking and, in particular, its fast and extensive release from the delivery carrier. However, previously explored pH-triggered mechanisms that were designed to become activated during liposome endocytosis have also been shown to interfere with the liposome stability in vivo. We have designed pH-triggered gel-phase liposomes with heterogeneous membranes for the delivery of doxorubicin. These liposomes are triggered to form "leaky" interfacial boundaries between gel-gel phase separated domains on the membrane bilayer with lowering pH. The pH-triggered mechanism does not compromise liposome stability in vivo and results in superior in vitro killing efficacy of delivered doxorubicin when liposomes are endocytosed by a clathrin-mediated pathway. In the present work, we evaluate the general applicability of these liposomes when targeted to the folate receptor (FR) of KB cancer cells in vitro and become endocytosed by a less acidic pathway: the caveolae pathway. FR-targeting liposomes exhibit almost 50% decrease in cell association for increase in liposome size from 120 to 280 nm in diameter after relatively short incubation times (up to 4 h). The fraction of internalized vesicles, however, is approximately 60% of the cell associated vesicles independent of their size. Our findings demonstrate that, for the same doxorubicin uptake per cancer cell, the killing effect of doxorubicin delivered by pH-triggered lipid vesicles is greater (IC(50) = 0.032 mM for a 6 h incubation) than when delivered by a conventional non-pH-responsive composition (IC(50) = 0.194 mM). These findings suggest higher bioexposure of cells to the therapeutic agent possibly via faster and more extensive release from the carrier. Animal studies of FR-targeting non-pH-responsive liposomal doxorubicin report stronger therapeutic potential for the targeted approach relative to nontargeted liposomes and to free doxorubicin. The findings of the present study suggest that the targeted pH-triggered liposomes could potentially further enhance the therapeutic outcomes of doxorubicin in vivo.

Heterogeneous domains and membrane permeability in phosphatidylcholine-phosphatidic acid rigid vesicles as a function of pH and lipid chain mismatch

Heterogeneous lipid membranes tuned by pH were evaluated at 37 degrees C in the form of PEGylated vesicles composed of lipid pairs with dipalmitoyl ( n = 16) and distearoyl ( n = 18) chain lengths. One lipid type was chosen to have the titratable moiety phosphatidic acid on its headgroup, and the other lipid type was chosen to have a phosphatidylcholine headgroup. The effect of pH on the formation of lipid heterogeneities and on membrane permeability was studied on vesicles composed of lipid pairs with matching and nonmatching chain lengths. The formation of lipid heterogeneities increases with decreasing pH in membranes composed of lipid pairs with either matching or nonmatching chain lengths. Increased permeability with decreasing pH was exhibited only by membranes composed of lipid pairs with nonmatching chain lengths. Permeability rates correlate strongly with the predicted extent of interfacial boundaries of heterogeneities, suggesting defective packing among nonmatching acyl chains of lipids. In heterogeneous mixtures with one lipid type in the fluid state ( n = 12), the dependence of membrane permeability on pH is weaker. In the presence of serum proteins, PEGylated gel-phase vesicles containing lipid pairs with nonmatching chain lengths exhibit faster release rates with decreasing pH compared to measured release rates in phosphate buffer, suggesting a second mechanism of formation of separated phases. PEGylated vesicles composed of lipid pairs with nonmatching chain lengths labeled with internalizing anti-HER2/neu antibodies that target overexpressed antigens on the surface of SKOV3-NMP2 ovarian cancer cells exhibit specific cancer cell targeting, followed by extensive internalization (more than 84% of bound vesicles) and fast release of contents intracellularly. These PEGylated vesicles composed of rigid membranes for long blood circulation times that exhibit pH-dependent release of contents intracellularly could become potent drug delivery carriers for the targeted therapy of solid tumors.

Floret-shaped solid domains on giant fluid lipid vesicles induced by pH

Lateral lipid phase separation of titratable PS or PA lipids and their assembly in domains induced by changes in pH are significant in liposome-based drug delivery: environmentally responsive lipid heterogeneities can be tuned to alter collective membrane properties such as permeability (altering drug release) and surface topography (altering drug carrier reactivity) impacting, therefore, the therapeutic outcomes. At the micrometer scale fluorescence microscopy on giant unilamellar fluid vesicles (GUVs) shows that lowering pH (from 7.0 to 5.0) promotes condensation of titratable PS or PA lipids into beautiful floret-shaped domains in which lipids are tightly packed via hydrogen-bonding and van der Waals interactions. The order of lipid packing within domains increases radially toward the domain center. Lowering pH enhances the lipid packing order, and at pH 5.0 domains appear to be entirely in the solid (gel) phase. Domains phenomenologically comprise a circular "core" cap beyond which interfacial instabilities emerge resembling leaf-like stripes. At pH 5.0 stripes are of almost vanishing Gaussian curvature independent of GUVs' preparation path and in agreement with a general condensation mechanism. Increasing incompressibility of domains is strongly correlated with a larger number of thinner stripes per domain and increasing relative rigidity of domains with smaller core cap areas. Line tension drives domain ripening; however, the final domain shape is a result of enhanced incompressibility and rigidity maximized by domain coupling across the bilayer. Introduction of a transmembrane osmotic gradient (hyperosmotic on the outer lipid leaflet) allows the domain condensation process to reach its maximum extent which, however, is limited by the minimal expansivity of the continuous fluid membrane.

Frozen cyclohexane-in-water emulsion as a sacrificial template for the synthesis of multilayered polyelectrolyte microcapsules

This paper reports the application of frozen cyclohexane-in-water emulsions as sacrificial templates for the fabrication of hollow microcapsules through layer-by-layer assembly of polyelectrolytes, poly(styrenesulfonate sodium salt), and poly(allylamine hydrochloride). Extraction of the cyclohexane phase from frozen emulsions stabilized with 11 polyelectrolyte layers by compatibilization with 30% v/v ethanol leads to the formation of water-filled microcapsules while preserving the spherical geometry. The majority of microcapsules (>90%) are prepared with intact polyelectrolyte membranes as measured by their deformation induced by osmotic pressure. This work provides a new route for the synthesis of hollow multilayered microcapsules under mild operating conditions.

Interstitial Release of Cisplatin from Triggerable Liposomes Enhances Efficacy against Triple Negative Breast Cancer Solid Tumor Analogues

Liposomal cisplatin, a promising triple negative breast cancer treatment modality, has been shown to decrease toxicities associated with cisplatin's free agent form. However, the heterogeneous intratumoral distributions of the liposomes themselves, combined with limited release of cisplatin from them contribute to limited penetration of cisplatin within tumors reducing efficacy. This study uses pH-responsive liposomes designed to release cisplatin within the acidic tumor interstitium (7.0 > pH ≥ 6.0) with a dual aim (1) to improve the penetration of the free drug within tumors on the assumption of greater diffusivities based on the free drug's much smaller size than its carrier's size and (2) to increase the availability of the free agent near cancer cells deep into the tumor. On cell monolayers treated with pH-releasing liposomal cisplatin, acidification of the extracellular solution resulted in decreased LD50 values, which were significantly lower than the LD50 values for non-pH-releasing liposomal cisplatin. In multicellular spheroids with acidic interstitia, pH-releasing liposomal cisplatin significantly decreased spheroid volumes relative to non-pH-releasing liposomal cisplatin. Improved efficacy was correlated with increased spheroid penetration of a fluorescent cisplatin surrogate. These findings demonstrate that interstitial release of cisplatin by pH-responsive liposomes may improve the intratumoral distributions of the free drug enhancing efficacy.

Sticky Patches on Lipid Nanoparticles Enable the Selective Targeting and Killing of Untargetable Cancer Cells

Effective targeting by uniformly functionalized nanoparticles is limited to cancer cells expressing at least two copies of targeted receptors per nanoparticle footprint (approximately ≥2 × 10(5) receptor copies per cell); such a receptor density supports the required multivalent interaction between the neighboring receptors and the ligands from a single nanoparticle. To enable selective targeting below this receptor density, ligands on the surface of lipid vesicles were displayed in clusters that were designed to form at the acidic pH of the tumor interstitium. Vesicles with clustered HER2-targeting peptides within such sticky patches (sticky vesicles) were compared to uniformly functionalized vesicles. On HER2-negative breast cancer cells MDA-MB-231 and MCF7 {expressing (8.3 ± 0.8) × 10(4) and (5.4 ± 0.9) × 10(4) HER2 copies per cell, respectively}, only the sticky vesicles exhibited detectable specific targeting (KD ≈ 49-69 nM); dissociation (0.005-0.009 min(-1)) and endocytosis rates (0.024-0.026 min(-1)) were independent of HER2 expression for these cells. MDA-MB-231 and MCF7 were killed only by sticky vesicles encapsulating doxorubicin (32-40% viability) or α-particle emitter (225)Ac (39-58% viability) and were not affected by uniformly functionalized vesicles (>80% viability). Toxicities on cardiomyocytes and normal breast cells (expressing HER2 at considerably lower but not insignificant levels) were not observed, suggesting the potential of tunable clustered ligand display for the selective killing of cancer cells with low receptor densities.

Nanocarriers to solid tumors: considerations on tumor penetration and exposure of tumor cells to therapeutic agents

Solid tumors constitute the majority of diagnosed cancers. For effective killing, therapeutic agents should ideally be delivered uniformly and at lethal doses to all cancer cells comprising the tumors, while keeping normal organ toxicities to a minimum. This requirement sets two of the major challenges in drug delivery to solid cancers: uniformity in delivery, and delivery of at least a minimum amount of therapeutics per cancer cell. Herein we review various approaches that aim to improve the penetration and content release of delivered therapeutic agents from nanocarriers of self-assembling nature. Biophysical characteristics of solid tumors are briefly discussed to motivate and rationalize the design of reported nanoparticle structures. This review does not aim to be exhaustive of the various designs and strategies, but to mostly give a flavor of the general current directions aiming to address these challenges.

Transport-driven engineering of liposomes for delivery of α-particle radiotherapy to solid tumors: effect on inhibition of tumor progression and onset delay of spontaneous metastases

PURPOSE

Highly cytotoxic α-particle radiotherapy delivered by tumor-selective nanocarriers is evaluated on metastatic Triple Negative Breast Cancer (TNBC). On vascularized tumors, the limited penetration of nanocarriers (<50-80 μm) combined with the short range of α-particles (40-100 μm) may, however, result in only partial tumor irradiation, compromising efficacy. Utilizing the α-particle emitter Actinium-225 (²²⁵Ac), we studied how the therapeutic potential of a general delivery strategy using nanometer-sized engineered liposomes was affected by two key transport-driven properties: (1) the release from liposomes, when in the tumor interstitium, of the highly diffusing ²²⁵Ac-DOTA that improves the uniformity of tumor irradiation by α-particles and (2) the adhesion of liposomes on the tumors' ECM that increases liposomes' time-integrated concentrations within tumors and, therefore, the tumor-delivered radioactivities.

METHODS

On an orthotopic MDA-MB-231 TNBC murine model forming spontaneous metastases, we evaluated the maximum tolerated dose (MTD), biodistributions, and control of tumor growth and/or spreading after administration of ²²⁵Ac-DOTA-encapsulating liposomes, with different combinations of the two transport-driven properties.

RESULTS

At 83% of MTD, ²²⁵Ac-DOTA-encapsulating liposomes with both properties (1) eliminated formation of spontaneous metastases and (2) best inhibited the progression of orthotopic xenografts, compared to liposomes lacking one or both properties. These findings were primarily affected by the extent of uniformity of the intratumoral microdistributions of ²²⁵Ac followed by the overall tumor uptake of radioactivity. At the MTD, long-term toxicities were not detected 9.5 months post administration.

CONCLUSION

Our findings demonstrate the potential of a general, transport-driven strategy enabling more uniform and prolonged solid tumor irradiation by α-particles without cell-specific targeting.

pH-dependent formation of lipid heterogeneities controls surface topography and binding reactivity in functionalized bilayers

During direct cell-to-cell communication, lipids on the extracellular side of plasma membranes reorganize, and membrane-associated communication-related molecules colocalize. At colocalization sites, sometimes identified as rafts, the local cell surface topography and reactivity are altered. The processes regulating these changes are largely unknown. On model lipid membranes, study of simplified processes that control surface topography and reactivity may potentially contribute to the understanding and control of related cell functions and associated diseases. Integration of these processes on nanometer-sized lipid vesicles used as drug delivery carriers would precisely control their interactions with diseased cells minimizing toxicities. Here we design such basic pH-dependent processes on model functionalized lipid bilayers, and we demonstrate reversible sharp changes in binding reactivity within a narrow pH window. Cholesterol enables tuning of the membrane reorganization to occur at pH values not necessarily close to the reported pK(a)'s of the constituent titratable lipids, and bilayer reorganization over repeated cycles of induced pH changes exhibits hysteresis.