Paramagnetic thermosensitive liposomes for MR-thermometry

Design of multi-modal antenna arrays for microwave hyperthermia and 1H/1⁹F MRI monitoring of drug release

This simulation-based study presented a novel hybrid RF antenna array designed for neck cancer treatment within a 7T MRI system. The proposed design aimed to provide microwave hyperthermia to release 19F-labeled anticancer drugs from thermosensitive liposomes, facilitating drug concentration monitoring through 19F imaging and enabling 1H anatomical imaging and MR thermometry for temperature control. The design featured a bidirectional microstrip for generating the magnetic |B1|-fields required for 1H and 19F MR imaging, along with a patch antenna for localized RF heating. The bidirectional microstrip was operated at 300 MHz and 280 MHz through the placement of excitation ports at the ends of the antenna and an asymmetric structure along the antenna. Additionally, a patch antenna was positioned at the center. Based on this setup, an array of six antennas was designed. Simulation results using a tissue-mimicking simulation model confirmed the intensity and uniformity of |B1|-fields for both 19F and 1H nuclei, demonstrating the suitability of the design for clinical imaging. RF heating from the patch antennas was effectively localized at the center of the cancer model. In simulations with a human model, average |B1|-fields were 0.21 μT for 19F and 0.12 μT for 1H, with normalized-absolute-average-deviation values of 81.75% and 87.74%, respectively. Hyperthermia treatment was applied at 120 W for 600 s, achieving an average temperature of 40.22°C in the cancer model with a perfusion rate of 1 ml/min/kg. This study demonstrated the potential of a hybrid antenna array for integrating 1H MR, 19F drug monitoring, and hyperthermia.

A Dual-Gene Reporter-Amplifier Architecture for Enhancing the Sensitivity of Molecular MRI by Water Exchange

The development of genetic reporters for magnetic resonance imaging (MRI) is essential for investigating biological functions in vivo. However, current MRI reporters have low sensitivity, making it challenging to create significant contrast against the tissue background, especially when only a small fraction of cells express the reporter. To overcome this limitation, we developed an approach for amplifying the sensitivity of molecular MRI by combining a chemogenetic contrast mechanism with a biophysical approach to increase water diffusion through the co-expression of a dual-gene construct comprising an organic anion transporting polypeptide, Oatp1b3, and a water channel, Aqp1. We first show that the expression of Aqp1 amplifies MRI contrast in cultured cells engineered to express Oatp1b3. We demonstrate that the contrast amplification is caused by Aqp1-driven increase in water exchange, which provides the gadolinium ions internalized by Oatp1b3-expressing cells with access to a larger water pool compared with exchange-limited conditions. We further show that our methodology allows cells to be detected using approximately 10-fold lower concentrations of gadolinium than that in the Aqp1-free scenario. Finally, we show that our approach enables the imaging of mixed-cell cultures containing a low fraction of Oatp1b3-labeled cells that are undetectable on the basis of Oatp1b3 expression alone.

A dual-gene reporter-amplifier architecture for enhancing the sensitivity of molecular MRI by water exchange

The development of genetic reporters for magnetic resonance imaging (MRI) is essential for investigating biological functions in intact animals. However, current MRI reporters have low sensitivity, making it challenging to create significant contrast against the tissue background, especially when only a small percentage of cells express the reporter. To overcome this limitation, we developed an approach that amplifies signals by co-expressing an MRI reporter gene, Oatp1b3, with a water channel, aquaporin-1 (Aqp1). We first show that the expression of Aqp1 amplifies the paramagnetic relaxation effect of Oatp1b3 by facilitating transmembrane water exchange. This mechanism provides Oatp1b3-expressing cells with access to a larger water pool compared with typical exchange-limited conditions. We further demonstrated that our methodology allows dual-labeled cells to be detected using approximately 10-fold lower concentrations of contrast agent than that in the Aqp1-free scenario. Finally, we show that our approach enables the imaging of mixed-cell populations containing a low fraction of Oatp1b3-labeled cells that are otherwise undetectable based on Oatp1b3 expression alone.

MR Thermometry during Transcranial MR Imaging-Guided Focused Ultrasound Procedures: A Review

Interest in transcranial MR imaging-guided focused ultrasound procedures has recently grown. These incisionless procedures enable precise focal ablation of brain tissue using real-time monitoring by MR thermometry. This article will provide an updated review on clinically applicable technical underpinnings and considerations of proton resonance frequency MR thermometry, the most common clinically used MR thermometry sequence.

Noninvasive measurement of local temperature using ultrasound-switchable fluorescence

Measuring the local background temperature in diseased and inflamed tissues is highly desirable, especially in a non-invasive way. In this work, ultrasound-switchable fluorescence (USF) technique was utilized to estimate the local background temperature for the first time by analyzing the temperature dependence of fluorescence emission from USF contrast agents induced by a focused ultrasound (FU) beam. First, temperature-sensitive USF agents with distinct temperature switching-on thresholds were synthesized, and their thermal switching characteristics were quantified using an independent spectrometer system. Second, the USF contrast agent suspension was injected into a microtube that was embedded into a phantom and the dynamic USF signal was acquired using a camera-based USF system. The differential profile of the measured dynamic USF signal was computed and compared with the thermal switching characteristics. This allowed for the calculation of the local background temperature of the sample in the FU focal volume based on the estimation of heating speed. An infrared (IR) camera was used to acquire the surface temperature of the sample and further compare it with the USF system. The results showed that the difference between the temperatures acquired from the USF thermometry and the IR thermography was 0.64 ± 0.43 °C when operating at the physiological temperature range from 35.27 to 39.31 °C. These results indicated the potential use of the USF system for measuring the local temperature in diseased tissues non-invasively. The designed USF-based thermometry shows a broad application prospect in high spatial resolution temperature imaging with a tunable measurement range in deep tissue.

On the optimization of imaging parameters for magnetic resonance imaging thermometry using magnetic microparticles

Magnetic Resonance Imaging thermometry is an extremely useful technique which allows one to determine, noninvasively, the temperature deep in the tissue in two or three dimensions. Many methods of MR thermometry have been developed, including those that rely on the intrinsic MR properties of tissue and those which depend on the addition of contrast agents injected into the tissue to create temperature dependent MR images. One such method is to introduce magnetic particles whose magnetization's temperature dependence influences the MR properties of the surrounding tissue and obtain temperature from calibrated intensity changes of T₂* weighted MR images. One limitation of this method is the temperature resolution which is determined by the rate of change of the magnetization with temperature. One can change the MR response either through varying the particles properties or finding the MR scan parameters which maximize the image contrast due to T₂* weighting of images. In this work we calculate the MR signal strength, using known values of T₁ and T₂* relaxation times for agarose gel phantoms with embedded magnetic particles, and compared this with the temperature dependent intensity of experimental MR images. We seek to optimize the change in signal intensity with temperature by varying the selectable MR scanner parameters: echo time, repetition time, and flip angle. Based on comparison with experimental data we find that the change in signal with temperature can be significantly increased (by as much as 100%) through the appropriate choice of MR scan parameters.

ADAM 8 as a novel target for doxorubicin delivery to TNBC cells using magnetic thermosensitive liposomes

Metastatic breast cancer is one of the most common causes of cancer-related death in women worldwide. The transmembrane metalloprotease-disintegrin (ADAM8) protein is highly overexpressed in triple-negative breast cancer (TNBC) cells and potentiates tumor cell invasion and extracellular matrix remodeling. Exploiting the high expression levels of ADAM8 in TNBC cells by delivering anti-ADAM8 antibodies efficiently to the targeted site can be a promising strategy for therapy of TNBC. For instance, a targeted approach with the aid of ultra-high field magnetic resonance imaging (UHF-MRI) activatable thermosensitive liposomes (Lip_TS-GD) could specifically increase the intracellular accumulation of cytotoxic drugs. The surface of doxorubicin-loaded Lip_TS-GD was modified by covalent coupling of MAB1031 antibody (Lip_TS-GD-MAB) in order to target the overexpressed ADAM8 in ADAM8 positive MDA-MB-231 cells. Physicochemical characterization of these liposomes was performed using size, surface morphology and UHF-MRI imaging analysis. In vitro cell targeting was investigated by the washing and circulation method. Intracellular trafficking and lysosomal colocalization were assessed by fluorescence microscopy. Cell viability, biocompatibility and in-ovo CAM assays were performed to determine the effectiveness and safety profiles of liposome formulations. Our results show specific binding and induction of doxorubicin release after Lip_TS-GD-MAB treatment caused a higher cytotoxic effect at the cellular target site.

The Structures of Gd(III) Chelates Conjugated at the Periphery of 3-(1'-Hexyloxy)ethyl-3-devinylpyropheophorbide-a (HPPH) Have a Significant Impact on the Imaging and Therapy of Cancer

3-(1'-Hexyloxyethyl)-3-devinyl-pyropheophorbide-a (HPPH or Photochlor), a tumor-avid chlorophyll-a derivative currently undergoing human clinical trials, was conjugated at various peripheral positions (position-17 or 20) of HPPH with either Gd(III)-aminobenzyl-DTPA (Gd(III) DTPA) or Gd(III)-aminoethylamido-DOTA (Gd(III) DOTA). The corresponding conjugates were evaluated for in vitro PDT efficacy, T₁ , T₂ relaxivities, in vivo fluorescence, and MR imaging under similar treatment parameters. Among these analogs, the water-soluble Gd(III)-aminoethylamido-DOTA linked at position-17 of HPPH, i. e., HPPH-17-Gd(III) DOTA, demonstrated strong potential for tumor imaging by both MR and fluorescence, while maintaining the PDT efficacy in BALB/c mice bearing Colon-26 tumors (7/10 mice were tumor free on day 60). In contrast to Gd(III) DTPA (Magnevist) and Gd(III) DOTA (Dotarem), the HPPH-Gd(III) DOTA retains in the tumor for a long period of time (24 to 48 h) and provides an option of fluorescence-guided cancer therapy. Thus, a single agent can be used for cancer-imaging and therapy. However, further detailed pharmacokinetic, pharmacodynamic, and toxicological studies of the conjugate are required before initiating Phase I human clinical trials.

Magnetic resonance activatable thermosensitive liposomes for controlled doxorubicin delivery

To limit the massive cytotoxicity of chemotherapeutic agents, it is desirable to establish an appropriate subtle blend of formulation design based on a dual-responsive strategy. In this study, a combined therapeutic platform based on magnetic thermosensitive liposomes (Lip_TS-GD) was developed. The incorporation of chelated-gadolinium imparted magnetic properties to thermosensitive liposomes (Lip_TS). The application of an ultra high field magnetic resonance imaging (UHF-MRI) induced hyperthermia, thus provided an improved chemotherapeutic effect of Doxorubicin (DOX). The paramagnetic platform demonstrated thermal sensitivity over a narrow temperature range starting at 37.8 °C, hence the release of DOX from Lip_TS-GD can be well triggered by inducing hyperthermia using UHF-MRI application. The prepared Lip_TS-GD were below 200 nm in diameter and an adequate release of DOX reaching 68% was obtained after 1 h UHF-MRI exposure. Profoundly, triple-negative breast cancer (TNBC) cells that were treated with Lip_TS-GD and subjected thereafter to UHF-MRI exposure for 60 min showed 36% viability. Hemocompatibility studies of Lip_TS-GD showed a physiological coagulation time and minimal hemolytic potential. Conclusively, Lip_TS-GD guided local delivery of DOX to solid tumors will potentially raise the therapeutic index, thus reducing the required dose and frequency of DOX administered systemically without influencing the adjacent tissues.

MR Thermometry in Cerebrovascular Disease: Physiologic Basis, Hemodynamic Dependence, and a New Frontier in Stroke Imaging

The remarkable temperature sensitivity of the brain is widely recognized and has been studied for its role in the potentiation of ischemic and other neurologic injuries. Pyrexia frequently complicates large-vessel acute ischemic stroke and develops commonly in critically ill neurologic patients; the profound sensitivity of the brain even to minor intraischemic temperature changes, together with the discovery of brain-to-systemic as well as intracerebral temperature gradients, has thus compelled the exploration of cerebral thermoregulation and uncovered its immutable dependence on cerebral blood flow. A lack of pragmatic and noninvasive tools for spatially and temporally resolved brain thermometry has historically restricted empiric study of cerebral temperature homeostasis; however, MR thermometry (MRT) leveraging temperature-sensitive nuclear magnetic resonance phenomena is well-suited to bridging this long-standing gap. This review aims to introduce the reader to the following: 1) fundamental aspects of cerebral thermoregulation, 2) the physical basis of noninvasive MRT, and 3) the physiologic interdependence of cerebral temperature, perfusion, metabolism, and viability.

Ferromagnetic particles as magnetic resonance imaging temperature sensors

Magnetic resonance imaging is an important technique for identifying different types of tissues in a body or spatial information about composite materials. Because temperature is a fundamental parameter reflecting the biological status of the body and individual tissues, it would be helpful to have temperature maps superimposed on spatial maps. Here we show that small ferromagnetic particles with a strong temperature-dependent magnetization, can be used to produce temperature-dependent images in magnetic resonance imaging with an accuracy of about 1 °C. This technique, when further developed, could be used to identify inflammation or tumours, or to obtain spatial maps of temperature in various medical interventional procedures such as hyperthermia and thermal ablation. This method could also be used to determine temperature profiles inside nonmetallic composite materials.

Magnetic resonance imaging can distinguish between different tissue types by measuring the proton distribution in a living sample. Here, the authors demonstrate how this technique can be extended to provide temperature information by using ferromagnetic particles with temperature-dependent magnetization.

Six-coordinate Iron(II) and Cobalt(II) paraSHIFT Agents for Measuring Temperature by Magnetic Resonance Spectroscopy

Paramagnetic Fe(II) and Co(II) complexes are utilized as the first transition metal examples of (1)H NMR shift agents (paraSHIFT) for thermometry applications using Magnetic Resonance Spectroscopy (MRS). The coordinating ligands consist of TACN (1,4,7-triazacyclononane) and CYCLEN (1,4,7,10-tetraazacyclododecane) azamacrocycles appended with 6-methyl-2-picolyl groups, denoted as MPT and TMPC, respectively. (1)H NMR spectra of the MPT- and TMPC-based Fe(II) and Co(II) complexes demonstrate narrow and highly shifted resonances that are dispersed as broadly as 440 ppm. The six-coordinate complex cations, [M(MPT)](2+) and [M(TMPC)](2+), vary from distorted octahedral to distorted trigonal prismatic geometries, respectively, and also demonstrate that 6-methyl-2-picolyl pendents control the rigidity of these complexes. Analyses of the (1)H NMR chemical shifts, integrated intensities, line widths, the distances obtained from X-ray diffraction measurements, and longitudinal relaxation time (T1) values allow for the partial assignment of proton resonances of the [M(MPT)](2+) complexes. Nine and six equivalent methyl protons of [M(MPT)](2+) and [M(TMPC)](2+), respectively, produce 3-fold higher (1)H NMR intensities compared to other paramagnetically shifted proton resonances. Among all four complexes, the methyl proton resonances of [Fe(TMPC)](2+) and [Co(TMPC)](2+) at -49.3 ppm and -113.7 ppm (37 °C) demonstrate the greatest temperature dependent coefficients (CT) of 0.23 ppm/°C and 0.52 ppm/°C, respectively. The methyl groups of these two complexes both produce normalized values of |CT|/fwhm = 0.30 °C(-1), where fwhm is full width at half-maximum (Hz) of proton resonances. The T1 values of the highly shifted methyl protons are in the range of 0.37-2.4 ms, allowing rapid acquisition of spectroscopic data. These complexes are kinetically inert over a wide range of pH values (5.6-8.6), as well as in the presence of serum albumin and biologically relevant cations and anions. The combination of large hyperfine shifts, large temperature sensitivity, increased signal-to-noise ratio, and short T1 values suggests that these complexes, in particular the TMPC-based complexes, show promise as paraSHIFT agents for thermometry.

Multimodal targeted high relaxivity thermosensitive liposome for in vivo imaging

Liposomes are spherical, self-closed structures formed by lipid bilayers that can encapsulate drugs and/or imaging agents in their hydrophilic core or within their membrane moiety, making them suitable delivery vehicles. We have synthesized a new liposome containing gadolinium-DOTA lipid bilayer, as a targeting multimodal molecular imaging agent for magnetic resonance and optical imaging. We showed that this liposome has a much higher molar relaxivities r1 and r2 compared to a more conventional liposome containing gadolinium-DTPA-BSA lipid. By incorporating both gadolinium and rhodamine in the lipid bilayer as well as biotin on its surface, we used this agent for multimodal imaging and targeting of tumors through the strong biotin-streptavidin interaction. Since this new liposome is thermosensitive, it can be used for ultrasound-mediated drug delivery at specific sites, such as tumors, and can be guided by magnetic resonance imaging.

Absolute MR thermometry using nanocarriers

Accurate time-resolved temperature mapping is crucial for the safe use of hyperthermia-mediated drug delivery. We here propose a magnetic resonance imaging temperature mapping method in which drug delivery systems serve not only to improve tumor targeting, but also as an accurate and absolute nano-thermometer. This method is based on the temperature-dependent chemical shift difference between water protons and the protons in different groups of drug delivery systems. We show that the chemical shift of the protons in the ethylene oxide group in polyethylene glycol (PEG) is temperature-independent, whereas the proton resonance of water decreases with increasing temperature. The frequency difference between both resonances is linear and does not depend on pH and physiological salt conditions. In addition, we show that the proton resonance of the methyl group in N-(2-hydroxypropyl)-methacrylamide (HPMA) is temperature-independent. Therefore, PEGylated liposomes, polymeric mPEG-b-pHPMAm-Lac2 micelles and HPMA copolymers can provide a temperature-independent reference frequency for absolute magnetic resonance (MR) thermometry. Subsequently, we show that multigradient echo MR imaging with PEGylated liposomes in situ allows accurate, time-resolved temperature mapping. In conclusion, nanocarrier materials may serve as highly versatile tools for tumor-targeted drug delivery, acting not only as hyperthermia-responsive drug delivery systems, but also as accurate and precise nano-thermometers.

Thermosensitive liposomal drug delivery systems: state of the art review

Thermosensitive liposomes are a promising tool for external targeting of drugs to solid tumors when used in combination with local hyperthermia or high intensity focused ultrasound. In vivo results have demonstrated strong evidence that external targeting is superior over passive targeting achieved by highly stable long-circulating drug formulations like PEGylated liposomal doxorubicin. Up to March 2014, the Web of Science listed 371 original papers in this field, with 45 in 2013 alone. Several formulations have been developed since 1978, with lysolipid-containing, low temperature-sensitive liposomes currently under clinical investigation. This review summarizes the historical development and effects of particular phospholipids and surfactants on the biophysical properties and in vivo efficacy of thermosensitive liposome formulations. Further, treatment strategies for solid tumors are discussed. Here we focus on temperature-triggered intravascular and interstitial drug release. Drug delivery guided by magnetic resonance imaging further adds the possibility of performing online monitoring of a heating focus to calculate locally released drug concentrations and to externally control drug release by steering the heating volume and power. The combination of external targeting with thermosensitive liposomes and magnetic resonance-guided drug delivery will be the unique characteristic of this nanotechnology approach in medicine.

A method to convert MRI images of temperature change into images of absolute temperature in solid tumours

PURPOSE

During hyperthermia (HT), the therapeutic response of tumours varies substantially within the target temperature range (39-43 °C). Current thermometry methods are either invasive or measure only temperature change, which limits the ability to study tissue responses to HT. This study combines manganese-containing low temperature sensitive liposomes (Mn-LTSL) with proton resonance frequency shift (PRFS) thermometry to measure absolute temperature in tumours with high spatial and temporal resolution using MRI.

METHODS

Liposomes were loaded with 300 mM MnSO(4). The phase transition temperature (T(m)) of Mn-LTSL samples was measured by differential scanning calorimetry (DSC). The release of manganese from Mn-LTSL in saline was characterised with inductively coupled plasma atomic emission spectroscopy. A 2T GE small animal scanner was used to acquire dynamic T1-weighted images and temperature change images of Mn-LTSL in saline phantoms and fibrosarcoma-bearing Fisher-344 rats receiving hyperthermia after Mn-LTSL injection.

RESULTS

The T(m) of Mn-LTSL in rat blood was 42.9 ± 0.2 °C (DSC). For Mn-LTSL samples (0.06 mM-0.5 mM Mn(2+) in saline) heated monotonically from 30 °C to 50 °C, a peak in the rate of MRI signal enhancement occurred at 43.1° ± 0.3 °C. The same peak in signal enhancement rate was observed during heating of fibrosarcoma tumours (N = 3) after injection of Mn-LTSL, and the peak was used to convert temperature change images into absolute temperature. Accuracies of calibrated temperature measurements were in the range 0.9-1.8 °C.

CONCLUSION

The release of Mn(2+) from Mn-LTSL affects the rate of MR signal enhancement which enables conversion of MRI-based temperature change images to absolute temperature.

Non-invasive temperature mapping using temperature-responsive water saturation shift referencing (T-WASSR) MRI

We present a non-invasive MRI approach for assessing the water proton resonance frequency (PRF) shifts associated with changes in temperature. This method is based on water saturation shift referencing (WASSR), a method first developed for assessing B0 field inhomogeneity. Temperature-induced water PRF shifts were determined by estimating the frequency of the minimum intensity of the water direct saturation spectrum at each temperature using Lorentzian line-shape fitting. The change in temperature was then calculated from the difference in water PRF shifts between temperatures. Optimal acquisition parameters were first estimated using simulations and later confirmed experimentally. Results in vitro and in vivo showed that the temperature changes measured using the temperature-responsive WASSR (T-WASSR) were in good agreement with those obtained with MR spectroscopy or phase-mapping-based water PRF measurement methods,. In addition, the feasibility of temperature mapping in fat-containing tissue is demonstrated in vitro. In conclusion, the T-WASSR approach provides an alternative for non-invasive temperature mapping by MRI, especially suitable for temperature measurements in fat-containing tissues.

Spin crossover iron(ii) complexes as PARACEST MRI thermometers

We demonstrate the potential utility of spin crossover iron(ii) complexes as temperature-responsive paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents in magnetic resonance imaging (MRI) thermometry.

MRI-guided monitoring of thermal dose and targeted drug delivery for cancer therapy

Application of localized hyperthermia treatment for solid tumor therapy is under active clinical investigation. The success of this treatment methodology, whether for tumor ablation or drug delivery, requires accurate target localization and real-time temperature mapping of the targeted region. Magnetic Resonance Imaging (MRI) can monitor temperature elevations in tissues in real-time during tumor therapy. MRI can also be applied in concert with methods such as High Intensity Focused Ultrasound (HIFU) to enable image-guided drug delivery (IGDD) from temperature sensitive nanocarriers, by exploiting not only its anatomic resolution, but its ability to detect and measure drug release using markers co-loaded with drugs within the nanocarriers. We review this rapidly emerging technology, providing an overview of MRI-guided tissue thermal dose monitoring for HIFU and Laser therapy, its role in targeted drug delivery and its future potential for clinical translation.

Hyperthermia-induced drug targeting

INTRODUCTION

Specific delivery of a drug to a target site is a major goal of drug delivery research. Using temperature-sensitive liposomes (TSLs) is one way to achieve this; the liposome acts as a protective carrier, allowing increased drug to flow through the bloodstream by minimizing clearance and non-specific uptake. On reaching microvessels within a heated tumor, the drug is released and quickly penetrates. A major advance in the field is ThermoDox® (Celsion), demonstrating significant improvements to the drug release rates and drug uptake in heated tumors (∼ 41°C). Most recently, magnetic resonance-guided focused ultrasound (MRgFUS) has been combined with TSL drug delivery to provide localized chemotherapy with simultaneous quantification of drug release within the tumor.

AREAS COVERED

In this article the field of hyperthermia-induced drug delivery is discussed, with an emphasis on the development of TSLs and their combination with hyperthermia (both mild and ablative) in cancer therapy. State-of-the-art image-guided heating technologies used with this combination strategy will also be presented, with examples of real-time monitoring of drug delivery and prediction of efficacy.

EXPERT OPINION

The specific delivery of drugs by combining hyperthermia with TSLs is showing great promise in the clinic and its potential will be even greater as the use of image-guided focused ultrasound becomes more widespread - a technique capable of penetrating deep within the body to heat a specific area with improved control. In conjunction with this, it is anticipated that multifunctional TSLs will be a major topic of study in this field.

Thermally-triggered 'off-on-off' response of gadolinium-hydrogel-lipid hybrid nanoparticles defines a customizable temperature window for non-invasive magnetic resonance imaging thermometry

For effective and safe thermotherapy, real-time, accurate, three-dimensional tissue thermometry is required. Magnetic resonance imaging (MRI)-based thermometry in combination with current temperature responsive contrast agents only provides an 'off-on' signal at a certain temperature, not indicating temperature increases beyond the desired therapeutic levels. To overcome this limitation, a novel Gd-chelated hydrogel-lipid hybrid nanoparticle (HLN) formulation was developed that provides an 'off-on-off' signal defining a thermometric window for MR thermometry. Novel thermally responsive poly(N-isopropylacrylamide-co-acrylamide) (NIPAM-co-AM) hydrogel nanoparticles (<15 nm) with bisallylamidodiethylenetriaminetriacetic acid, a novel crosslinker with Gd(3+) chelation functionality, were synthesized. The Gd-hydrogel nanoparticles were encapsulated in a solid lipid nanoparticle matrix that prevented T(1)-weighted contrast signal enhancement. Melting of the matrix lipid freed the Gd-hydrogel nanoparticles into the bulk water and an 'off-on' contrast signal enhancement occurred. As the temperature was further increased to temperatures greater than, the volume phase transition temperature of the hydrogel nanoparticles, they collapsed and provided an 'on-off' signal diminution. Both the 'off-on' and the 'on-off' transition temperature could be tailored by changing the lipid matrix and altering the NIPAM/AM ratio in the hydrogel, respectively. This allowed MRI thermometry of different temperature windows using the Gd-HLN system.

Nanoscale Drug Delivery and Hyperthermia: The Materials Design and Preclinical and Clinical Testing of Low Temperature-Sensitive Liposomes Used in Combination with Mild Hyperthermia in the Treatment of Local Cancer

The overall objective of liposomal drug delivery is to selectively target drug delivery to diseased tissue, while minimizing drug delivery to critical normal tissues. The purpose of this review is to provide an overview of temperature-sensitive liposomes in general and the Low Temperature-Sensitive Liposome (LTSL) in particular. We give a brief description of the material design of LTSL and highlight the likely mechanism behind temperature-triggered drug release. A complete review of the progress and results of the latest preclinical and clinical studies that demonstrate enhanced drug delivery with the combined treatment of hyperthermia and liposomes is provided as well as a clinical perspective on cancers that would benefit from hyperthermia as an adjuvant treatment for temperature-triggered chemotherapeutics. This review discusses the ideas, goals, and processes behind temperature-sensitive liposome development in the laboratory to the current use in preclinical and clinical settings.

Imaging in interventional oncology

Medical imaging in interventional oncology is used differently than in diagnostic radiology and prioritizes different imaging features. Whereas diagnostic imaging prioritizes the highest-quality imaging, interventional imaging prioritizes real-time imaging with lower radiation dose in addition to high-quality imaging. In general, medical imaging plays five key roles in image-guided therapy, and interventional oncology, in particular. These roles are (a) preprocedure planning, (b) intraprocedural targeting, (c) intraprocedural monitoring, (d) intraprocedural control, and (e) postprocedure assessment. Although many of these roles are still relatively basic in interventional oncology, as research and development in medical imaging focuses on interventional needs, it is likely that the role of medical imaging in intervention will become even more integral and more widely applied. In this review, the current status of medical imaging for intervention in oncology will be described and directions for future development will be examined.

Focused ultrasound as a local therapy for liver cancer

Conventional surgical treatments of liver cancer are invasive (including minimally invasive) with a high incidence of new metastasis and poor success, even after multiple resections or ablations. These limitations motivated research into new, less invasive solutions for liver cancer treatment.Focused ultrasound surgery (FUS), or high-intensity focused ultrasound, has been recognized as a noninvasive technology for benign and malignant tumor treatment. Previously, FUS was guided with ultrasound that has limited target definition and monitoring capability of the ablation process. Combining magnetic resonance imaging (MRI) with multiple-element phased-array transducers to create MRI-guided focused ultrasound thermal therapy provides more accurate targeting and real-time temperature monitoring. This treatment is hindered by the ribcage that limits the acoustic windows to the liver and the respiratory motion of the liver. New advances in MRI and transducer design will likely resolve these limitations and make MRI-guided FUS a powerful tool in local liver cancer therapy. This article reviews this technology and advances that can expand its use for cancer treatment in general and liver cancer in particular.

Pushing the sensitivity envelope of lanthanide-based magnetic resonance imaging (MRI) contrast agents for molecular imaging applications

Contrast in magnetic resonance imaging (MRI) arises from changes in the intensity of the proton signal of water between voxels (essentially, the 3D counterpart of pixels). Differences in intervoxel intensity can be significantly enhanced with chemicals that alter the nuclear magnetic resonance (NMR) intensity of the imaged spins; this alteration can occur by various mechanisms. Paramagnetic lanthanide(III) complexes are used in two major classes of MRI contrast agent: the well-established class of Gd-based agents and the emerging class of chemical exchange saturation transfer (CEST) agents. A Gd-based complex increases water signal by enhancing the longitudinal relaxation rate of water protons, whereas CEST agents decrease water signal as a consequence of the transfer of saturated magnetization from the exchangeable protons of the agent. In this Account, we survey recent progress in both areas, focusing on how MRI is becoming a more competitive choice among the various molecular imaging methods. Compared with other imaging modalities, MRI is set apart by its superb anatomical resolution; however, its success in molecular imaging suffers because of its intrinsic insensitivity. A relatively high concentration of molecular agents (0.01-0.1 mM) is necessary to produce a local alteration in the water signal intensity. Unfortunately, the most desirable molecules for visualization in molecular imaging are present at much lower concentrations, in the nano- or picomolar range. Therefore, augmenting the sensitivity of MRI agents is key to the development of MR-based molecular imaging applications. In principle, this task can be tackled either by increasing the sensitivity of the reporting units, through the optimization of their structural and dynamic properties, or by setting up proper amplification strategies that allow the accumulation of a huge number of imaging reporters at the site of interest. For Gd-based agents, high sensitivities can be attained by exploiting a range of nanosized carriers (micelles, liposomes, microemulsions, and the like, as well as biological structures such as apoferritin and lipoproteins) properly loaded with Gd-based chelates. Furthermore, the sensitivity of Gd-based agents can be markedly affected either by their interactions with biological structures or by their cellular localization. For CEST agents, a huge sensitivity enhancement has been obtained by using the water molecules contained in the inner cavity of liposomes as the exchangeable source of protons for magnetization transfer. Several "tricks" (for example, the use of multimeric lanthanide(III) shift reagents, changes in the shape of the liposome container, and so forth) have been devised to improve the chemical shift separation between the intraliposomal water and the "bulk" water resonances. Overall, excellent sensitivity enhancements have been obtained for both classes of agents, enabling their use in MR molecular imaging applications.

MR thermometry

Minimally invasive thermal therapy as local treatment of benign and malignant diseases has received increasing interest in recent years. Safety and efficacy of the treatment require accurate temperature measurement throughout the thermal procedure. Noninvasive temperature monitoring is feasible with magnetic resonance (MR) imaging based on temperature-sensitive MR parameters such as the proton resonance frequency (PRF), the diffusion coefficient (D), T1 and T2 relaxation times, magnetization transfer, the proton density, as well as temperature-sensitive contrast agents. In this article the principles of temperature measurements with these methods are reviewed and their usefulness for monitoring in vivo procedures is discussed. Whereas most measurements give a temperature change relative to a baseline condition, temperature-sensitive contrast agents and spectroscopic imaging can provide absolute temperature measurements. The excellent linearity and temperature dependence of the PRF and its near independence of tissue type have made PRF-based phase mapping methods the preferred choice for many in vivo applications. Accelerated MRI imaging techniques for real-time monitoring with the PRF method are discussed. Special attention is paid to acquisition and reconstruction methods for reducing temperature measurement artifacts introduced by tissue motion, which is often unavoidable during in vivo applications.

Rationale for and measurement of liposomal drug delivery with hyperthermia using non-invasive imaging techniques

The purpose of this review is to present an overview of the state-of-the-art imaging modalities used to track drug delivery from liposomal formulations into tumors during or after hyperthermia treatment. Liposomes are a drug delivery system comprised of a phospholipid bilayer surrounding an aqueous core and have been shown to accumulate following hyperthermia therapy. Use of contrast-containing liposomes in conjunction with hyperthermia therapy holds great promise to be able to directly measure drug dose concentrations as well as to non-invasively describe patterns of drug distribution with MR and PET/SPECT imaging modalities. We will review the rationale for using this approach and the potential advantages of having such information available during and after treatment.

Role of membrane lipids in the mechanism of bacterial species selective toxicity by two α/β-antimicrobial peptides

We have previously shown that two synthetic antimicrobial peptides with alternating alpha- and beta-amino acid residues, designated simply as alpha/beta-peptide I and alpha/beta-peptide II, had toxicity toward bacteria and affected the morphology of bacterial membranes in a manner that correlated with their effects on liposomes with lipid composition similar to those of the bacteria. In the present study we account for the weak effects of alpha/beta-peptide I on liposomes or bacteria whose membranes are enriched in phosphatidylethanolamine (PE) and why such membranes are particularly susceptible to damage by alpha/beta-peptide II. The alpha/beta-peptide II has marked effects on unilamellar vesicles enriched in PE causing vesicle aggregation and loss of their internal aqueous contents. The molecular basis of these effects is the ability of alpha/beta-peptide II to induce phase segregation of anionic and zwitterionic lipids as shown by fluorescence and differential scanning calorimetry. This phase separation could result in the formation of defects through which polar materials could pass across the membrane as well as form a PE-rich membrane domain that would not be a stable bilayer. alpha/beta-Peptide II is more effective in this regard because, unlike alpha/beta-peptide I, it has a string of two or three adjacent cationic residues that can interact with anionic lipids. Although alpha/beta-peptide I can destroy membrane barriers by converting lamellar to non-lamellar structures, it does so only weakly with unilamellar vesicles or with bacteria because it is not as efficient in the aggregation of these membranes leading to the bilayer-bilayer contacts required for this phase conversion. This study provides further understanding of why alpha/beta-peptide II is more toxic to micro-organisms with a high PE content in their membrane as well as for the lack of toxicity of alpha/beta-peptide I with these cells, emphasizing the potential importance of the lipid composition of the cell surface in determining selective toxicity of anti-microbial agents.