Réjean Lebel

Novel solubility-switchable MRI agent allows the noninvasive detection of matrix metalloproteinase-2 activity in vivo in a mouse model

A novel MRI proteinase‐modulated contrast agent (PCA) was developed to detect the activity of the proinvasive enzyme matrix metalloproteinase‐2 (MMP‐2) in vivo. The PCA2‐switch agent incorporates a solubility switch, where cleavage of a peptide substrate by MMP‐2 decreases the water solubility of the agent. Evidence suggests that this leads to an accumulation of cleaved PCA2‐switch in an MMP‐2‐positive, wild‐type, MC7‐L1 mammary carcinoma tumor in a Balb/c mouse model compared to a MC7‐L1 MMP‐2‐knockdown tumor. When a scrambled peptide sequence is inserted into the agent (PCA2‐scrambled), the in vitro cleavage efficiency of MMP‐2 is markedly reduced. In vivo, PCA2‐scrambled does not accumulate in the wild‐type tumor and the pharmacokinetics is similar in both tumors. In conclusion, in vivo cleavage of PCA2‐switch by MMP‐2 results in a significant accumulation of the cleaved PCA2‐switch in an MMP‐2‐positive tumor. Magn Reson Med 60:1056–1065, 2008.

New enzyme-activated solubility-switchable contrast agent for magnetic resonance imaging: from synthesis to in vivo imaging.

We designed and synthesized a novel contrast agent (CA) to image the activity of matrix metalloproteinase-2 (MMP-2) in a tumor, noninvasively using magnetic resonance imaging (MRI). We exploited the concept of solubility-switchable CAs in the design of a protease-modulated CA (PCA), referred to as PCA2-switch. This PCA contains a paramagnetic gadolinium chelate (Gd-DOTA), which was attached to the N-terminus of a MMP-2 cleavable peptide sequence via a hydrophobic chain. The aqueous solubility of the CA depends on the presence of a polyethylene glycol chain (PEG) on the C-terminus of the peptide. Upon proteolytic cleavage of the peptide by MMP-2, the PEG chain is detached from the CA, which becomes less water soluble. This compound and control compounds were successfully tested in an animal model bearing two tumors with different levels of MMP-2 activity.

Conversion of arterial input functions for dual pharmacokinetic modeling using Gd-DTPA/MRI and 18F-FDG/PET.

Reaching the full potential of magnetic resonance imaging (MRI)‐positron emission tomography (PET) dual modality systems requires new methodologies in quantitative image analyses. In this study, methods are proposed to convert an arterial input function (AIF) derived from gadolinium‐diethylenetriaminepentaacetic acid (Gd‐DTPA) in MRI, into a 18F‐fluorodeoxyglucose (18F‐FDG) AIF in PET, and vice versa. The AIFs from both modalities were obtained from manual blood sampling in a F98‐Fisher glioblastoma rat model. They were well fitted by a convolution of a rectangular function with a biexponential clearance function. The parameters of the biexponential AIF model were found statistically different between MRI and PET. Pharmacokinetic MRI parameters such as the volume transfer constant (Ktrans), the extravascular–extracellular volume fraction (νe), and the blood volume fraction (νp) calculated with the Gd‐DTPA AIF and the Gd‐DTPA AIF converted from 18F‐FDG AIF normalized with or without blood sample were not statistically different. Similarly, the tumor metabolic rates of glucose (TMRGlc) calculated with 18F‐FDG AIF and with 18F‐FDG AIF obtained from Gd‐DTPA AIF were also found not statistically different. In conclusion, only one accurate AIF would be needed for dual MRI‐PET pharmacokinetic modeling in small animal models. Magn Reson Med, 2013.

Effect of thrombin and bradykinin on endothelial cell mechanical properties monitored through membrane deformation.

The endothelium is closely implicated in the control and maintenance of the vascular homeostasis. The functions of endothelial cells are highly regulated by several agonists of G protein‐coupled receptors (GPCR), which can mediate signals involved in morphological remodeling. Here, we evaluated the mechanical properties of human umbilical vein endothelial cells (HUVEC) in responses to two physiological agonists namely thrombin and bradykinin. We used the atomic force microscopy (AFM) technique to study changes in cell membrane stiffness and interaction between the actin cytoskeleton and the cell membrane. HUVEC stimulated with thrombin (10 nM) and bradykinin (1 µM) showed a temporal increase in their membrane stiffness from 5.0 ± 0.1 kPa (control) to 8.2 ± 0.4 kPa (thrombin) and 7.3 ± 0.5 kPa (bradykinin) and in membrane tethers elongation forces from 43.9 ± 0.9 pN (control) to 49.5 ± 0.8 pN (thrombin) and 53.1 ± 0.8 pN (bradykinin). These results were consistent with the reorganization of the actin cytoskeleton observed in fluorescence microscopy. This study demonstrates that these agonists induce important modifications of the cell membrane properties that can be directly linked to the reorganization and the interaction of the actin cytoskeleton near the apical side of the membrane. These changes in the mechanical properties of endothelial cells provide relevant informations in the biological and pathophysiological behaviors of endothelial cells. Copyright

A comprehensive review on controls in molecular imaging: lessons from MMP‐2 imaging

Metalloproteinases (MMPs), including MMP-2, play critical roles in tissue remodeling and are involved in a large array of pathologies, including cancer, arthritis and atherosclerosis. Their prognostic value warranted a large investment or resources in the development of noninvasive detection methods, based on probes for many current clinical and pre-clinical imaging modalities. However, the potential of imaging techniques is only matched by the complexity of the data they generate. This complexity must be properly assessed and accounted for in the early steps of probe design and testing in order to accurately determine the efficacy and efficiency of an imaging strategy. This review proposes basic rules for the evaluation of novel probes by addressing the specific case of MMP targeted probes.

Determination of an optimal pharmacokinetic model of FET for quantitative applications in rat brain tumors

O-(2-18F-fluoroethyl)-l-tyrosine (18F-FET) is a radiolabeled artificial amino acid used in PET for tumor delineation and grading. The present study compares different kinetic models to determine which are more appropriate for 18F-FET in rats. Methods: Rats were implanted with F98 glioblastoma cells in the right hemisphere and scanned 9–15 d later. PET data were acquired during 50 min after a 1-min bolus of 18F-FET. Arterial blood samples were drawn for arterial input function determination. Two compartmental pharmacokinetic models were tested: the 2-tissue model and the 1-tissue model. Their performance at fitting concentration curves from regions of interest was evaluated using the Akaike information criterion, F test, and residual plots. Graphical models were assessed qualitatively. Results: Metrics indicated that the 2-tissue model was superior to the 1-tissue model for the current dataset. The 2-tissue model allowed adequate decoupling of 18F-FET perfusion and internalization by cells in the different regions of interest. Of the 2 graphical models tested, the Patlak plot provided adequate results for the tumor and brain, whereas the Logan plot was appropriate for muscles. Conclusion: The 2-tissue-compartment model is appropriate to quantify the perfusion and internalization of 18F-FET by cells in various tissues of the rat, whereas graphical models provide a global measure of uptake.

Optimization of the reference region method for dual pharmacokinetic modeling using Gd-DTPA/MRI and 18F-FDG/PET

The combination of MRI and positron emission tomography (PET) offers new possibilities for the development of novel methodologies. In pharmacokinetic image analysis, the blood concentration of the imaging compound as a function of time, [i.e., the arterial input function (AIF)] is required for MRI and PET. In this study, we tested whether an AIF extracted from a reference region (RR) in MRI can be used as a surrogate for the manually sampled 18F‐FDG AIF for pharmacokinetic modeling.

Impact of H‐aggregation on activatable MMP‐2‐specific probes for optical imaging

In order to target and image MMP-2 activity using optical imaging, we developed a panel of new MMP-2 probes based on Cy5 and QSY21 as fluorophore/quencher FRET partners, separated by various MMP-2 specific peptide substrates. We compared these probes for their specificity against other MMPs, their rate of activation by MMP-2 and their initial quenching.

Real-Time Microfluidic Blood-Counting System for PET and SPECT Preclinical Pharmacokinetic Studies

Small-animal nuclear imaging modalities have become essential tools in the development process of new drugs, diagnostic procedures, and therapies. Quantification of metabolic or physiologic parameters is based on pharmacokinetic modeling of radiotracer biodistribution, which requires the blood input function in addition to tissue images. Such measurements are challenging in small animals because of their small blood volume. In this work, we propose a microfluidic counting system to monitor rodent blood radioactivity in real time, with high efficiency and small detection volume (∼1 μL). Methods: A microfluidic channel is built directly above unpackaged p-i-n photodiodes to detect β-particles with maximum efficiency. The device is embedded in a compact system comprising dedicated electronics, shielding, and pumping unit controlled by custom firmware to enable measurements next to small-animal scanners. Data corrections required to use the input function in pharmacokinetic models were established using calibrated solutions of the most common PET and SPECT radiotracers. Sensitivity, dead time, propagation delay, dispersion, background sensitivity, and the effect of sample temperature were characterized. The system was tested for pharmacokinetic studies in mice by quantifying myocardial perfusion and oxygen consumption with 11C-acetate (PET) and by measuring the arterial input function using 99mTcO4− (SPECT). Results: Sensitivity for PET isotopes reached 20%–47%, a 2- to 10-fold improvement relative to conventional catheter-based geometries. Furthermore, the system detected 99mTc-based SPECT tracers with an efficiency of 4%, an outcome not possible through a catheter. Correction for dead time was found to be unnecessary for small-animal experiments, whereas propagation delay and dispersion within the microfluidic channel were accurately corrected. Background activity and sample temperature were shown to have no influence on measurements. Finally, the system was successfully used in animal studies. Conclusion: A fully operational microfluidic blood-counting system for preclinical pharmacokinetic studies was developed. Microfluidics enabled reliable and high-efficiency measurement of the blood concentration of most common PET and SPECT radiotracers with high temporal resolution in small blood volume.

MRI-Guided Derivation of the Input Function for PET Kinetic Modeling

Blood samples obtained by arterial cannulation are the gold standard to measure the input function for PET pharmacokinetic modeling. There is interest in less invasive methods, such as image-derived input functions (IDAIF). MRI can be used to segment and correct partial volume effects of the PET images, improving IDAIF extraction. Preclinical studies have shown that the input function of PET tracers, namely fluorodeoxyglucose and [(18)F]fluoroethyl-l-tyrosine, can be derived from the Gd-DTPA input function. Noninvasive, MRI-guided, PET input function derivation is a promising avenue to reduce or eliminate the need for arterial plasma samples in preclinical and clinical settings.