Christopher Scarfone

Integrin-mediated targeting of drug delivery to irradiated tumor blood vessels

The objective of this study was to target drug delivery to radiation-induced neoantigens, which include activated receptors within the tumor vasculature. These responses include posttranslational changes in pre-existing proteins, which can be discovered by phage-displayed peptide libraries administered to mice bearing irradiated tumors. Phage-displayed peptides recovered from irradiated tumors included the amino acid sequence RGDGSSV. This peptide binds to integrins within the tumor microvasculature. Immunohistochemical staining of irradiated tumors showed accumulation of fibrinogen receptor alpha(2b)beta(3) integrin. We studied tumor targeting efficiency of ligands to radiation-induced alpha(2b)beta(3). Radiopharmaceuticals were localized to irradiated tumors by use of alpha(2b)beta(3) ligands conjugated to nanoparticles and liposomes. Fibrinogen-conjugated nanoparticles bind to the radiation-activated receptor, obliterate tumor blood flow, and significantly increase regression and growth delay in irradiated tumors. Radiation-guided drug delivery to tumor blood vessels is a novel paradigm for targeted drug delivery.

Phantom validation of coregistration of PET and CT for image-guided radiotherapy

Radiotherapy treatment planning integrating positron emission tomography (PET) and computerized tomography (CT) is rapidly gaining acceptance in the clinical setting. Although hybrid systems are available, often the planning CT is acquired on a dedicated system separate from the PET scanner. A limiting factor to using PET data becomes the accuracy of the CT/PET registration. In this work, we use phantom and patient validation to demonstrate a general method for assessing the accuracy of CT/PET image registration and apply it to two multi-modality image registration programs. An IAEA (International Atomic Energy Association) brain phantom and an anthropomorphic head phantom were used. Internal volumes and externally mounted fiducial markers were filled with CT contrast and 18F-fluorodeoxyglucose (FDG). CT, PET emission, and PET transmission images were acquired and registered using two different image registration algorithms. CT/PET Fusion (GE Medical Systems, Milwaukee, WI) is commercially available and uses a semi-automated initial step followed by manual adjustment. Automatic Mutual Information-based Registration (AMIR), developed at our institution, is fully automated and exhibits no variation between repeated registrations. Registration was performed using distinct phantom structures; assessment of accuracy was determined from registration of the calculated centroids of a set of fiducial markers. By comparing structure-based registration with fiducial-based registration, target registration error (TRE) was computed at each point in a three-dimensional (3D) grid that spans the image volume. Identical methods were also applied to patient data to assess CT/PET registration accuracy. Accuracy was calculated as the mean with standard deviation of the TRE for every point in the 3D grid. Overall TRE values for the IAEA brain phantom are: CT/PET Fusion = 1.71 +/- 0.62 mm, AMIR = 1.13 +/- 0.53 mm; overall TRE values for the anthropomorphic head phantom are: CT/PET Fusion = 1.66 +/- 0.53 mm, AMIR = 1.15 +/- 0.48 mm. Precision (repeatability by a single user) measured for CT/PET Fusion: IAEA phantom = 1.59 +/- 0.67 mm and anthropomorphic head phantom = 1.63 +/- 0.52 mm. (AMIR has exact precision and so no measurements are necessary.) One sample patient demonstrated the following accuracy results: CT/PET Fusion = 3.89 +/- 1.61 mm, AMIR = 2.86 +/- 0.60 mm. Semi-automatic and automatic image registration methods may be used to facilitate incorporation of PET data into radiotherapy treatment planning in relatively rigid anatomic sites, such as head and neck. The overall accuracies in phantom and patient images are < 2 mm and < 4 mm, respectively, using either registration algorithm. Registration accuracy may decrease, however, as distance from the initial registration points (CT/PET fusion) or center of the image (AMIR) increases. Additional information provided by PET may improve dose coverage to active tumor subregions and hence tumor control. This study shows that the accuracy obtained by image registration with these two methods is well suited for image-guided radiotherapy.

Targeting drug delivery to radiation-induced neoantigens in tumor microvasculature.

Radiation can be used to guide drugs to specific sites such as neoplasms or aberrant blood vessels. When blood vessels are treated with ionizing radiation, they respond by expressing a number of cell adhesion molecules and receptors that participate in homeostasis. Examples of radiation-induced molecules in blood vessels include ICAM-1, E-selectin, P-selectin and the beta(3) integrin. We have observed that the endothelium and blood components respond to oxidative stress in a similar, if not identical manner in all tumor models. Although we have identified several other radiation-induced molecules within tumor blood vessels, the beta(3) target for drug delivery achieves the greatest site-specific peptide binding within irradiated tumor blood vessels. We have focused on peptides and antibodies that bind to integrin beta(3). beta(3)-binding proteins have been conjugated to fluorochromes and radionuclides to study the site specificity and microscopic distribution. We have found immunofluorescent and immunohistochemical staining of beta(3) within the lumen of blood vessels immediately following irradiation. To determine whether it is feasible to guide drug delivery to irradiated tumors, we studied ligands to alpha(2b)beta(3) (fibrinogen). Peptides within fibrinogen that bind to alpha(2b)beta(3) includes the dodecapeptide, HHLGGAKQAGDV and the RGD peptide. We utilized 131I conjugation to these ligands to study the biodistribution in tumor bearing mice. Our clinical trial consists of the RGD peptidomimetic, biapcitide, labeled with 99mTc. This study shows that it is feasible to guide drugs to human neoplasms by use of radiation-guided peptides. These studies have shown that peptides that bind to these integrins bind to tumors following exposure to ionizing radiation.

Radiation-Mediated Control of Drug Delivery

Clinical trials of radiotherapy to control drug delivery were initiated in 1999 at Vanderbilt University. The initial studies exploited the findings that platelets are activated in tumor blood vessels after high-dose irradiation as used in radiosurgery and high-dose-rate brachytherapy. Platelets labeled with 111In showed binding in tumor blood vessels. However, the platelet labeling process caused platelets to also accumulate in the spleen. That clinical trial was closed, and subsequent clinical trials targeted protein activation in irradiated tumor blood vessels. Preclinical studies showed that peptide libraries that bind within irradiated tumor blood vessels contained the peptide sequence Arg-Gln-Asp (RGD). RGD binds to integrin receptors (e.g., receptors for fibrinogen, fibronectin, and vitronectin). We found that the fibrinogen receptor (GPIIb/IIIa, &agr;2b&bgr;3) is activated within irradiated tumor blood vessels. RGD peptidemimetics currently in clinical trials include GPIIb/IIIa antagonists and the platelet-imaging agent biapcitide. Biapcitide is an RGD mimetic that is labeled with 99Tc to allow gamma camera imaging of the biodistribution of the GPIIb/IIIa receptor in neoplasms of patients treated with radiosurgery. This study has shown that the schedule of administration of the RGD mimetic is crucial. The peptide mimetic must be administered immediately before irradiation, whereas the natural ligands to the receptor compete for biapcitide binding if biapcitide is administered after irradiation. The authors currently are conducting a dose deescalation study to determine the threshold dosage required for RGD mimetic binding to radiation activated receptor. Radiation-guided clinical trials have been initiated by use of high-dose-rate brachytherapy. In a separate trial, the pharmacokinetics of radiation-inducible gene therapy are being investigated. In this trial, the radiation-activated promoter Egr-1 regulates expression of the tumor necrosis factor &agr; gene, which is administered by use of the attenuated adenovirus vector. The Ad.Egr-TNF (ADGV) gene is administered by intratumoral injection of vector followed by irradiation in patients with soft-tissue sarcomas. This review highlights recent findings in these phase I pharmacokinetic studies of radiation-controlled drug delivery systems.

Breast tumour imaging using incomplete circular orbit pinhole SPET: A phantom study

Improvements in 99Tcm-sestamibi breast lesion visualization using single photon emission tomography (SPET) may help define the clinical role of this technique alongside X-ray mammography in the diagnosis and management of breast cancer. Pinhole SPET offers the advantages of high resolution and sensitivity when compared to conventional parallel-beam collimation for sources located near the pinhole aperture. In this work, the potential of incomplete (180 degrees) circular orbit (ICO) SPET with pinhole collimation is investigated as a means to visualize small (6.4 and 9.6 mm diameter) spherical simulated tumours, at clinical count densities and tumour-to-background ratios, in a breast phantom. ICO pinhole SPET is compared to complete circular orbit (CCO) pinhole SPET for reference, and planar breast imaging (scintimammography) using parallel-beam and pinhole collimators. A prototype box-shaped pinhole collimator with a 4 mm diameter circular aperture was used to acquire projections of an 890 ml breast phantom both in isolation and mounted on a cylinder filled with a mixture of 99Tcm-pertechnetate and water. A heart phantom containing 99Tcm activity in the myocardium was placed in the cylinder. Simulated tumours containing 99Tcm were placed in the breast phantom and scanned at clinically relevant count densities and scan times with tumour-to-normal tissue concentration ratios of 5.0:1 (9.6 mm sphere) and 7.7:1 (6.4 mm sphere). Phantom data were reconstructed using pinhole filtered backprojection (FBP) and maximum likelihood-expectation maximization (ML-EM). The tumours were not visualized with scintimammography, in which lesion contrast and signal-to-noise were estimated from region of interest analysis to be < 2% and 0.01, respectively. Average (over lesion size and scan time) contrast and signal-to-noise in the ICO (CCO) SPET images were 33% and 1.72 (34% and 1.3), respectively. These values indicate that ICO pinhole SPET has the potential to improve visualization of small (< 10 mm) breast tumours when compared with scintimammography, which may be beneficial for the early classification of cancers of the breast.

The effect of truncation reduction in fan beam transmission for attenuation correction of cardiac SPECT

A limitation of fan beam transmission imaging using a 40 cm field-of-view scintillation camera is the data truncation that occurs when imaging medium to large-sized patients. With filtered backprojection, truncation may cause bright rings in the reconstructed image. The primary objective of this study is to evaluate a method to extrapolate the truncated transmission data under clinically relevant count density conditions. The method involves obtaining the patient contour by processing the scatter and photopeak emission data, filling the contour with the attenuation coefficient for soft tissue, reprojecting the contour image, extrapolating the truncated projection set with the projections. A long focal length (114 cm) fan collimator is used on one head of a triple camera SPECT system to acquire transmission data. The two remaining detectors are equipped with low energy, ultra high resolution parallel hole collimators. A large thorax phantom (38 cm/spl times/26 cm) and patient data are used to evaluate the method. For SPECT image reconstruction, non-uniform attenuation correction is performed with a truncated attenuation map, an extrapolated attenuation map and the untruncated attenuation map. The SPECT results indicate that image uniformity changes very little using any of the three different attenuation maps when a long focal length fan beam collimator is used for transmission data acquisition. Truncation artifacts that are apparent in the transmission image can be substantially reduced for objects up to 40 cm wide.

Gamma camera-based PET inverse treatment planning for head and neck cancer using hybrid imaging instrumentation and IMRT

Abstract Purpose : To demonstrate the feasibility of incorporating gamma camera-based positron emission tomography (GC-PET) nuclear medicine molecular imaging into inverse conformal radiotherapy treatment planning using commercially available hardware and software. Materials and methods : Anatomical X-ray computed tomography (X-ray CT) and GC-PET imaging of the base of the tongue region were performed on a hybrid nuclear medicine—X-ray CT scanner (General Electric Millennium VG Hawkeye, Milwaukee, WI). Patient positioning included a carbon composite flat-table insert and Aquaplast™ U-frame head immobilization mask. Both anatomical and molecular images were acquired and then transferred to the treatment planning and dose calculation workstations via a Local Area Network (LAN). GC-PET molecular information was registered with the anatomy using a four-point external registration technique. A five-field conformal inverse treatment plan, which targets radiation dose to the GC-PET-defined lesion, was then developed using the Varian SomaVision™, CadPlan™ and Helios™ treatment planning modules. Results : The radiation dose distribution was made to conform to the tumor region, as indicated by the area of increased flouro-2-deoxyglucose (FDG) uptake in the GC-PET image, using the inverse treatment planning technique. Conclusions : Information from molecular imaging techniques such as GC-PET may be incorporated into the inverse treatment planning process using the combined molecular and anatomical imaging methods, and commercially available hardware and software.