Nitroxide conjugate of a thermally responsive elastin-like polypeptide for noninvasive thermometry

Rapid Scan Electron Paramagnetic Resonance Opens New Avenues for Imaging Physiologically Important Parameters In Vivo

We demonstrate a superior method of 2D spectral-spatial imaging of stable radical reporter molecules at 250 MHz using rapid-scan electron-paramagnetic-resonance (RS-EPR), which can provide quantitative information under in vivo conditions on oxygen concentration, pH, redox status and concentration of signaling molecules (i.e., OH^•, NO^•). The RS-EPR technique has a higher sensitivity, improved spatial resolution (1 mm), and shorter acquisition time in comparison to the standard continuous wave (CW) technique. A variety of phantom configurations have been tested, with spatial resolution varying from 1 to 6 mm, and spectral width of the reporter molecules ranging from 16 µT (160 mG) to 5 mT (50 G). A cross-loop bimodal resonator decouples excitation and detection, reducing the noise, while the rapid scan effect allows more power to be input to the spin system before saturation, increasing the EPR signal. This leads to a substantially higher signal-to-noise ratio than in conventional CW EPR experiments.

Imaging of nitroxides at 250MHz using rapid-scan electron paramagnetic resonance

Projections for 2D spectral-spatial images were obtained by continuous wave and rapid-scan electron paramagnetic resonance using a bimodal cross-loop resonator at 251MHz. The phantom consisted of three 4mm tubes containing different (15)N,(2)H-substituted nitroxides. Rapid-scan and continuous wave images were obtained with 5min total acquisition times. For comparison, images also were obtained with 29s acquisition time for rapid scan and 15min for continuous wave. Relative to continuous wave projections obtained for the same data acquisition time, rapid-scan projections had significantly less low-frequency noise and substantially higher signal-to-noise at higher gradients. Because of the improved image quality for the same data acquisition time, linewidths could be determined more accurately from the rapid-scan images than from the continuous wave images.

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.

NANO AND MICRO-STRUCTURES OF ELASTIN-LIKE POLYPEPTIDE-BASED MATERIALS AND THEIR APPLICATIONS: RECENT DEVELOPMENTS

Elastin-like polypeptide (ELP) containing materials have spurred significant research interest for biomedical applications exploiting their biocompatible, biodegradable and nonimmunogenic nature while maintaining precise control over their chemical structure and functionality through genetic engineering. Physical, mechanical and biological properties of ELPs could be further manipulated using genetic engineering or through conjugation with a variety of chemical moieties. These chemical and physical modifications also achieve interesting micro- and nanostructured ELP-based materials. Here, we review the recent developments during the past decade in the methods to engineer elastin-like materials, available genetic and chemical modification methods and applications of ELP micro and nanostructures in tissue engineering and drug delivery.

Hydration layer coupling and cooperativity in phase behavior of stimulus responsive peptide polymers

It is shown that hydrophilic (backbone) and hydrophobic (side chain) hydration layers of elastin-like polypeptides (ELPs), a class of stimulus responsive peptide polymers that exhibit lower critical solution temperature (LCST) phase transition behavior, can exist in a coupled and decoupled state. The decoupled hydration state consists of hydrophobic and hydrophilic hydration layers that respond independently to temperature, while the coupled hydration state is characterized by a common, cooperative dehydration of both hydration layers. It is further shown that the primary sequence of an ELP can be tuned to exhibit either of the hydration layer coupling modes. Charged side chains lead to decoupling, while strongly hydrophobic side chains trigger stronger interaction between hydrophilic and hydrophobic hydration, leading to coupling of both layers. Further, for aprotic residues this coupling is fostered by decreasing bulkiness of hydrophobic side chains due to larger hydration numbers and water molecules mediating coupling between side chain and backbone hydration shells. For coupled hydration shells, the LCST phase transition characterized by spin probing continuous wave electron paramagnetic resonance spectroscopy is reminiscent of a first-order process even on nanoscopic length scales. In contrast, analogous synthetic polymers exhibit nanoscale phase transitions over a broad temperature range, indicating that their nanoscale phase behavior is not of first order. Hence, our results indicate that ELPs are the first identified class of polymers that exhibit a first-order inverse phase transition on nanoscopic length scales. These results may also provide insights into the role of hydration layers in governing the structure-function relationship of intrinsically disordered proteins.

In vivo electron paramagnetic resonance imaging of differential tumor targeting using cis-3,4-di(acetoxymethoxycarbonyl)-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl

PURPOSE

Electron paramagnetic resonance spectroscopy promises quantitative images of important physiologic markers of animal tumors and normal tissues, such as pO(2), pH, and thiol redox status. These parameters of tissue function are conveniently reported by tailored nitroxides. For defining tumor physiology, it is vital that nitroxides are selectively localized in tumors relative to normal tissue. Furthermore, these paramagnetic species should be specifically taken up by cells of the tumor, thereby reporting on both the site of tumor formation and the physiological status of the tissue. This study investigates the tumor localization of the novel nitroxide, cis-3,4-di(acetoxymethoxycarbonyl)-2,2,5,5-tetramethyl-1-pyrrolidin-yloxyl 3 relative to the corresponding di-acid 4.

METHODS

We obtained images of nitroxide 3 infused intravenously into C3H mice bearing 0.5-cm(3) FSa fibrosarcoma on the leg, and compared these with images of similar tumors infused with nitroxide 4.

RESULTS

The ratio of spectral intensity from within the tumor-bearing region to that of normal tissue was higher in the mice injected with 3 relative to 4.

CONCLUSION

This establishes the possibility of tumor imaging with a nitroxide with intracellular distribution and provides the basis for EPR images of animal models to investigate the relationship between crucial aspects of tumor microenvironment and malignancy and its response to therapy.

Anti-HER2 immunoliposomes for selective delivery of electron paramagnetic resonance imaging probes to HER2-overexpressing breast tumor cells

Electron paramagnetic resonance (EPR) imaging is an emerging modality that can detect and localize paramagnetic molecular probes (so-called spin probes) in vivo. We previously demonstrated that nitroxide spin probes can be encapsulated in liposomes at concentrations exceeding 100 mM, at which nitroxides exhibit a concentration-dependent quenching of their EPR signal that is analogous to the self-quenching of fluorescent molecules. Therefore, intact liposomes encapsulating high concentrations of nitroxides exhibit greatly attenuated EPR spectral signals, and endocytosis of such liposomes represents a cell-activated contrast-generating mechanism. After endocytosis, the encapsulated nitroxide is liberated and becomes greatly diluted in the intracellular milieu. This dequenches the nitroxides to generate a robust intracellular EPR signal. It is therefore possible to deliver a high concentration of nitroxides to cells while minimizing background signal from unendocytosed liposomes. We report here that intracellular EPR signal can be selectively generated in a specific cell type by exploiting its expression of Human Epidermal Growth Factor Receptor 2 (HER2). When targeted by anti-HER2 immunoliposomes encapsulating quenched nitroxides, Hc7 cells, which are novel HER2-overexpressing cells derived from the MCF7 breast tumor cell line, endocytose the liposomes copiously, in contrast to the parent MCF7 cells or control CV1 cells, which do not express HER2. HER2-dependent liposomal delivery enables Hc7 cells to accumulate 750 μM nitroxide intracellularly. Through the use of phantom models, we verify that this concentration of nitroxides is more than sufficient for EPR imaging, thus laying the foundation for using EPR imaging to visualize HER2-overexpressing Hc7 tumors in animals.

Scaling of EPR spectral-spatial images with size of sample: images of a sample greater than 5 cm in linear dimension

The authors have obtained spectral-spatial EPR images of a phantom significantly larger than those previously obtained. Images of a homogeneous phantom 4.2 cm in diameter and 6.5 cm in length with B1 equivalent to that used for smaller samples give a similar linewidth resolution both with linewidth population distributions of width 0.1 microT. Spatial resolution appeared to have modest degradation. Images of the large homogeneous phantom provide maps of the magnetic field of a partially shimmed magnet.

Intelligent biosynthetic nanobiomaterials for hyperthermic combination chemotherapy and thermal drug targeting of HSP90 inhibitor geldanamycin

An intelligent biosynthetic nanobiomaterial (IBN) platform was explored for drug delivery applications for hyperthermic combination chemotherapy and thermal drug targeting. Geldanamycin (GA), a heat shock protein 90 inhibitor, was conjugated to novel thermosensitive poly(K)(8)-poly(VPGXG)(60) block copolymers [K(8)-ELP(1-60)] with guest residues as valine, alanine and glycine in a 5:2:3 ratio at the 'X' position. The conjugates were completely soluble in PBS and showed a characteristic thermosensitive inverse phase transition. [K(8)-ELP(1-60)]-GA conjugate nanoparticles showed a size ranging from 50 to 200 nm depending upon temperature. Relevant to systemic drug delivery in vivo, these IBNs stably disperse in aqueous solution. Cytotoxicity assays have shown that the IBN from [K(8)-ELP(1-60)]-GA conjugates exhibits effective hyperthermic combination chemotherapy with facile heat modulation.

An infrared spectroscopic study of the conformational transition of elastin-like polypeptides

The infrared spectroscopy of elastin-like polypeptides and the relation to the inverse thermal transition are discussed. To correlate the spectroscopic observations with structure a density function theory model was created that captures the essential hydrogen bonding and packing of the beta-spiral structure proposed for elastin and elastin-like polypeptides. The infrared spectrum was calculated using periodic boundary conditions and a method for estimating the difference dipole moment permits both frequencies and intensities to be obtained for the modeling of spectra. The two observed amide I bands at 1615 cm(-1) and 1656 cm(-1) are shown to arise from the beta-spiral structure. The increase in intensity of these bands with increasing salt concentration and temperature is assigned to the closer association of strands of the beta-spiral. The sharp inverse temperature transition is observed within 1 degrees C and involves a change in secondary structure that involves formation of interstrand beta-sheets for approximately 25% of the amino acids. This conclusion is consistent with available data and simulations that have been reported to date.

Object dependent sweep width reduction with spectral-spatial EPR imaging

For spectral-spatial EPR imaging, prior knowledge about the spatial support of an imaged object can be exploited in two ways. We can shrink the spatial field of view (FOV) to closely wrap the object in a sphere or reduce the sweep width in a projection dependent fashion. Use of a smaller spatial FOV with the same number of samples enhances spatial resolution by reducing voxel volume at the expense of signal-to-noise and a consequent degraded line-width resolution. We have developed another approach to define sweep width that prunes away the portions of the projection sweep with no signal. This reduces data acquisition time for the continuous wave (CW) EPR image proportional to the sweep width reduction. This method also avoids voxel volume reduction. Using the reduced-sweep method, we decreased the data acquisition time by 20% maintaining spatial and linewidth resolution.

Spatially uniform sampling in 4-D EPR spectral-spatial imaging

Four-dimensional EPR imaging involves a computationally intensive inversion of the sampled Radon transform. Conventionally, N-dimensional reconstructions have been carried out with N-1 stages of 2-D backprojection to exploit a dimension-dependent reduction in execution time. The huge data size of 4-D EPR imaging demands the use of a 3-stage reconstruction each consisting of 2-D backprojections. This gives three orders of magnitude reduction in computation relative to a single stage 4-D filtered backprojection. The multi-stage reconstruction, however, requires a uniform angular sampling that yields an inefficient distribution of gradient directions. We introduce a solution that involves acquisition of projections uniformly distributed in solid angle and reconstructs in three 2-D stages with the spatial uniform solid angle data set converted to uniform linear angular projections using 2-D interpolation. Images were taken from the two sampling schemes to compare the spatial resolution and the line width resolution. The degradation in the image quality due to the additional interpolation was small, and we achieved approximately 30% reduction in data acquisition time.