Novel delivery approaches for cancer therapeutics

Currently, a majority of cancer treatment strategies are based on the removal of tumor mass mainly by surgery. Chemical and physical treatments such as chemo- and radiotherapies have also made a major contribution in inhibiting rapid growth of malignant cells. Furthermore, these approaches are often combined to enhance therapeutic indices. It is widely known that surgery, chemo- and radiotherapy also inhibit normal cells growth. In addition, these treatment modalities are associated with severe side effects and high toxicity which in turn lead to low quality of life. This review encompasses novel strategies for more effective chemotherapeutic delivery aiming to generate better prognosis. Currently, cancer treatment is a highly dynamic field and significant advances are being made in the development of novel cancer treatment strategies. In contrast to conventional cancer therapeutics, novel approaches such as ligand or receptor based targeting, triggered release, intracellular drug targeting, gene delivery, cancer stem cell therapy, magnetic drug targeting and ultrasound-mediated drug delivery, have added new modalities for cancer treatment. These approaches have led to selective detection of malignant cells leading to their eradication with minimal side effects. Lowering multi-drug resistance and involving influx transportation in targeted drug delivery to cancer cells can also contribute significantly in the therapeutic interventions in cancer.

Construction and in vitro/in vivo targeting of PSMA-targeted nanoscale microbubbles in prostate cancer

BACKGROUND

Prostate-specific membrane antigen (PSMA) is a highly specific biological marker and treatment target for prostate cancer. So ultrasound molecular imaging using PSMA antibody-loaded targeted nanoscale microbubbles (MBs) may contribute to the early diagnosis of prostate cancer.

METHODS

PSMA monoclonal antibody-loaded targeted nanoscale MBs were prepared using biotin-avidin technology. Antibody binding was evaluated with immunofluorescence. Using MKN45 gastric cancer cells as controls, the targeting capability of the targeted MBs was observed in prostate cancer cells (LNCaP and C4-2) under optical microscope. Contrast enhancement was monitored by an ultrasound system in C4-2, LNCaP, and MKN45 transplanted tumors in nude mice. The arrival time, time to peak, peak intensity, and duration of contrast enhancement of targeted and blank nanoscale MBs were compared and analyzed.

RESULTS

Targeted PSMA monoclonal antibody-loaded nanoscale MBs were successfully synthesized. These MBs were stable and could specifically bind to LNCaP and C4-2 cells in vitro but did not bind to MKN45 cells. There were significant differences in peak intensity and duration of contrast enhancement between targeted and blank nanoscale MBs in both transplanted prostate tumors (P < 0.05). Among the three types of transplanted tumors with targeted nanoscale MBs, the peak intensity was significantly higher in prostate tumors (LNCaP and C4-2) than in gastric tumors (MKN45) (P < 0.05).

CONCLUSIONS

PSMA monoclonal antibody-loaded targeted nanoscale MBs can target and bind to prostate cancer cells specifically and allow for obvious contrast enhancement in vivo. Therefore, this study lays a foundation for early diagnosis and targeted therapy for prostate cancer.

An experimental study on the stiffness of size-isolated microbubbles using atomic force microscopy

To fully assess contrast-enhanced acoustic bioeffects in diagnostic and therapeutic procedures, the mechanical properties of microbubbles need to be considered. In the present study, direct measurements of the microbubble stiffness were performed using atomic force microscopy by applying nanoscale compressions (up to 25 nN/s) on size-isolated, lipidcoated microbubbles (diameter ranges of 4 to 6 μm and 6 to 8 μm). The stiffness was found to lie between 4 and 22 mN/m and to decrease exponentially with the microbubble size within the diameter range investigated. No cantilever spring constant effect was found on the measured stiffness. The Young's modulus of the size-isolated microbubbles used in our study ranged between 0.4 and 2 MPa. Microstructures on the surface of the microbubbles were found to influence the overall microbubble elasticity. Our results indicated that more detailed theoretical models are needed to account for the size-dependent microbubble mechanical properties to accurately predict their acoustic behavior. The findings provided useful insights into guidance of cavitation-induced drug and gene delivery and could be used as part of the framework in studies on the shear stresses induced on the blood vessel walls by oscillating microbubbles.

Phase-shift, stimuli-responsive drug carriers for targeted delivery

The intersection of particles and directed energy is a rich source of novel and useful technology that is only recently being realized for medicine. One of the most promising applications is directed drug delivery. This review focuses on phase-shift nanoparticles (that is, particles of submicron size) as well as micron-scale particles whose action depends on an external-energy triggered, first-order phase shift from a liquid to gas state of either the particle itself or of the surrounding medium. These particles have tremendous potential for actively disrupting their environment for altering transport properties and unloading drugs. This review covers in detail ultrasound and laser-activated phase-shift nano- and micro-particles and their use in drug delivery. Phase-shift based drug-delivery mechanisms and competing technologies are discussed.

Phase-shift, stimuli-responsive perfluorocarbon nanodroplets for drug delivery to cancer

This review focuses on phase-shift perfluorocarbon nanoemulsions whose action depends on an ultrasound-triggered phase shift from a liquid to gas state. For drug-loaded perfluorocarbon nanoemulsions, microbubbles are formed under the action of tumor-directed ultrasound and drug is released locally into tumor volume in this process. This review covers in detail mechanisms involved in the droplet-to-bubble transition as well as mechanisms of ultrasound-mediated drug delivery.

Effect of surface architecture on in vivo ultrasound contrast persistence of targeted size-selected microbubbles

Ultrasound molecular imaging is a powerful diagnostic modality using microbubbles coated with targeting ligands specific for endothelial biomarkers. The circulation persistence of ligand-bearing contrast agents is a key determinant in their contrast enhancement and targeting capability. Prior studies have shown that targeted microbubbles with ligands attached to the shell using the conventional exposed-ligand architecture (ELA) could trigger undesired ligand-induced complement activation and decreased circulation time. Microbubbles with the buried-ligand architecture (BLA), however, were found to inhibit complement activation and prolong circulation time. In the present study, we extended the stealth BLA microbubble design to size-selected (4 to 5-μm diameter) microbubbles targeted with cyclic RGD peptide using the postlabeling technique. Microbubble circulation persistence was measured in the healthy mouse kidney using a Visualsonics Vevo 770 scanner operating at 40 MHz in fundamental mode. The circulation persistence for targeted BLA microbubbles was significantly longer compared with their ELA counterparts and similar to no-ligand controls. Use of the BLA instead of the ELA increased the circulation half-life approximately two-fold. Analysis of the time-intensity and time-fluctuation curves with a two-compartment pharmacokinetic model showed a minimal degree of nonspecific vascular adhesion for any group. These results demonstrate the importance of surface architecture in the design of targeted microbubbles for ultrasound molecular imaging.

Imaging prostate cancer lymph node metastases with a multimodality contrast agent

BACKGROUND

Methods to detect lymph node (LN) metastases in prostate cancer (PCa) are limited. Pelvic LN dissection is commonly performed during prostatectomy, but often followed by morbid complications. More refined methods for detecting LN invasion are needed.

METHODS

We developed a dual-labeled targeting agent having a near-infrared (NIR) fluorophore for intraoperative guidance, and a conventional radiotracer for detection of LN metastasis. Nu/Nu mice were orthotopically implanted with DsRed-expressing human PCa (PC3) cells. Antibody (Ab) specific for epithelial cell adhesion molecule was conjugated to DOTA, IRDye 800CW, and radiolabeled with (64) Cu. Dual-labeled Ab was administered intravenously at 10-12 weeks post-implantation, and positron emission tomography/computed tomography (PET/CT) and fluorescence imaging were performed within 18-24 hr.

RESULTS

Metastasis to lumbar LNs was detected by DsRed fluorescence imaging, as well as pathology, in 75% of mice having pathology-confirmed primary prostate tumors. These metastases were also detected by NIR fluorescence imaging. In some cases, metastases to sciatic, medial, renal, and axillary nodes were also detected. For all LNs examined, no significant differences were found between the percentages of metastases detected by NIR imaging (63%) and µPET/CT (64%) (P = 0.93), or between those detected by DsRed imaging (25%) and pathological examination (19%) (P = 0.12).

CONCLUSION

This study demonstrates that a multimodality contrast agent is useful for early detection of metastatic disease, and has applications for intraoperative PCa treatment. Further agent optimization is necessary to enhance specificity, and provide validation for prostate and other LN metastasizing epithelial cancers.

Ultrasound molecular imaging of tumor angiogenesis with an integrin targeted microbubble contrast agent

RATIONALE AND OBJECTIVES

Ultrasound molecular imaging is an emerging technique for sensitive detection of intravascular targets. Molecular imaging of angiogenesis has strong potential for both clinical use and as a research tool in tumor biology and the development of antiangiogenic therapies. Our objectives are to develop a robust ultrasound contrast agent platform using microbubbles (MB) to which targeting ligands can be conjugated by biocompatible, covalent conjugation chemistry, and to develop a pure low mechanical index (MI) imaging processing method and corresponding quantification method. The MB and the imaging methods were evaluated in a mouse model of breast cancer in vivo.

MATERIALS AND METHODS

We used a cyclic arginine-glycine-aspartic acid (cRGD) pentapeptide containing a terminal cysteine group conjugated to the surface of MB bearing pyridyldithio-propionate (PDP) for targeting αvβ3 integrins. As negative controls, MB without a ligand or MB bearing a scrambled sequence (cRAD) were prepared. To enable characterization of peptides bound to MB surfaces, the cRGD peptide was labeled with FITC and detected by plate fluorometry, flow cytometry, and fluorescence microscopy. Targeted adhesion of cRGD-MB was demonstrated in an in vitro flow adhesion assay against recombinant murine αvβ3 integrin protein and αvβ3 integrin-expressing endothelial cells (bEnd.3). The specificity of cRGD-MB for αvβ3 integrin was demonstrated by treating bEnd.3 EC with a blocking antibody. A murine model of mammary carcinoma was used to assess targeted adhesion and ultrasound molecular imaging in vivo. The targeted MB were visualized using a low MI contrast imaging pulse sequence, and quantified by intensity normalization and 2-dimensional Fourier transform analysis.

RESULTS

The cRGD ligand concentration on the MB surface was ∼8.2 × 10(6) molecules per MB. At a wall shear stress of 1.0 dynes/cm, cRGD-MB exhibited 5-fold higher adhesion to immobilized recombinant αvβ3 integrin relative to nontargeted MB and cRAD-MB controls. Similarly, cRGD-MB showed significantly greater adhesion to bEnd.3 EC compared with nontargeted MB and cRAD-MB. In addition, cRGD-MB, but not nontargeted MB or cRAD-MB, showed significantly enhanced contrast signals with a high tumor-to-background ratio. The adhesion of cRGD-MB to bEnd.3 was reduced by 80% after using anti-αv monoclonal antibody to treat bEnd.3. The normalized image intensity amplitude was ∼0.8, 7 minutes after the administration of cRGD-MB relative to the intensity amplitude at the time of injection, while the spatial variance in image intensity improved the detection of bound agents. The accumulation of cRGD-MB was blocked by preadministration with an anti-αv blocking antibody.

CONCLUSIONS

The results demonstrate the functionality of a novel MB contrast agent covalently coupled to an RGD peptide for ultrasound molecular imaging of αvβ3 integrin and the feasibility of quantitative molecular ultrasound imaging with a low MI.

Vascular endothelial growth factor receptor 2-specific microbubbles for molecular ultrasound detection of prostate cancer in a rat model

OBJECTIVES

To investigate whether rat prostate cancer can be detected by means of molecular ultrasound (US) using target-specific microbubbles versus a nonspecific contrast agent.

MATERIALS AND METHODS

A total of 20 Copenhagen rats were randomly examined 75 to 104 days after orthotopic implantation of G-Dunning rat prostatic tumor cells was using a high-end US system and either 1.2 mL/kg of the nonspecific agent A or 0.1 mL/kg of the target-specific agent B, containing vascular endothelial growth factor receptor 2 binding peptide. Contrast inflow (areas under the curve analysis) was determined during the first 30s, and all microbubbles were destroyed in the scan plane by means of the flash technique 20 minutes after intravenous administration to investigate specific accumulation of individual bubbles in tumors. Differences between normal prostate tissue and tumor were analyzed using luminance images. Sonographically determined tumor localization and extent were compared with magnetic resonance imaging and histology.

RESULTS

The median tumor size in the 20 rats determined on US (2.3 mm) and magnetic resonance imaging (2.4 mm) showed a very high correlation (r = 0.92, P = 0.01). Both agent A and agent B demonstrated higher vascularization of tumor periphery compared with normal prostate (P < 0.05) based on contrast inflow and areas under the curve analysis. Twenty minutes after administration, luminance for agent B in the tumor was significantly higher (P = 0.003) compared with nonspecific agent A (11.8-0.1). In consensus reading, the increase in signal intensity of the tumor compared with normal prostate tissue was significantly higher for agent B (P = 0.005), whereas no significant difference was found for agent A (P = 0.180).

CONCLUSIONS

The target-specific contrast agent was superior to the unspecific US contrast agent both with regard to early inflow analysis and specific accumulation in prostate cancer after 20 minutes.

scVEGF microbubble ultrasound contrast agents: a novel probe for ultrasound molecular imaging of tumor angiogenesis

OBJECTIVE

To develop a novel microbubble (MB) ultrasound contrast agent covalently coupled to a recombinant single-chain vascular endothelial growth factor construct (scVEGF) through uniform site-specific conjugation for ultrasound imaging of tumor angiogenesis.

METHODS

Ligand conjugation to maleimide-bearing MB by thioether bonding was first validated with a fluorophore (BODIPY-cystine), and covalently bound dye was detected by fluorometry and flow cytometry. MBs were subsequently site-specifically conjugated to cysteine-containing Cys-tag in scVEGF, and bound scVEGF was quantified by enzyme-linked immunosorbent assay. Targeted adhesion of scVEGF-MB was investigated with in vitro parallel plate flow chamber assays with recombinant murine VEGFR-2 substrates and human VEGFR-2-expressing porcine endothelial cells (PAE/KDR). A wall-less ultrasound flow phantom, with flow channels coated with immobilized VEGFR-2, was used to detect adhesion of scVEGF-MB with contrast ultrasound imaging. A murine model of colon adenocarcinoma was used to assess retention of scVEGF-MB with contrast ultrasound imaging during tumor angiogenesis in vivo.

RESULTS

Proof-of-principle of ligand conjugation to maleimide-bearing MB was demonstrated with a BODIPY-cysteine fluorophore. Conjugation of BODIPY to MB saturated at 10-fold molar excess BODIPY relative to maleimide groups on MB surfaces. MB reacted with scVEGF and led to the conjugation of 1.2 × 10(5) molecules scVEGF per MB. Functional adhesion of sc-VEGF-MB was shown in parallel plate flow chamber assays. At a shear stress of 1.0 dynes/cm2, scVEGF-MB exhibited 5-fold higher adhesion to both recombinant VEGFR-2 substrates and VEGFR-2-expressing endothelial cells compared with nontargeted control MB. Additionally, scVEGF-MB targeted to immobilized VEGFR-2 in an ultrasound flow phantom showed an 8-fold increase in mean acoustic signal relative to casein-coated control channels. In an in vivo model of tumor angiogenesis, scVEGF MB showed significantly higher ultrasound contrast signal enhancement in tumors (8.46 ± 1.61 dB) compared with nontargeted control MB (1.58 ± 0.83 dB).

CONCLUSIONS

These results demonstrate the functionality of a novel scVEGF-bearing MB contrast agent, which could be useful for molecular imaging of VEGFR-2 in basic science and drug discovery research.

Systems biology through mouse imaging centers: experience and new directions

The completed sequencing of genomes has forced upon us the challenge of understanding how the detailed information in the genome gives rise to the specific characteristics--phenotype--of the individual. This is crucial for understanding not only normal development but also, from a medical perspective, the genetic basis of disease. Much of the mammalian genome-to-phenotype relationship will be worked out in the mouse, for which powerful genetic-manipulation tools are available. Mouse imaging combined with powerful statistical methods has a unique and growing role to play in phenotyping genetically modified mice. This review outlines the challenges for image-based phenotyping, summarizes the current state of three-dimensional imaging technologies for the mouse, and highlights new opportunities in systems biology that are opened by imaging mice.