Quantitative Biodistribution Studies Research Articles

In vitro and ex vivo storage phosphor imaging of short-living radioisotopes

Storage phosphor imaging may be of value for biodistribution studies of short-living radiotracers in small animals. Efficiency, sensitivity and resolution of imaging plates for short-living radioisotopes vary considerably but linear response to many radioisotopes was shown previously. However, these properties have not been compared directly for larger series of short-living radioisotopes, and only few studies have directly compared data obtained from phosphor images of tissue slices with results from dissection biodistribution studies. Therefore, we evaluated the properties of imaging plates for 11 short-living radioisotopes ( 18F, 32P, 67Ga, 89Sr, 99mTc, 90Y, 111In, 123I, 125I, 131I and 201Tl). We also evaluated the biodistribution of [ 123I]FP-CIT in rat brain using both the phosphor technique and conventional dissection methods. The imaging system showed a linear response for all tested radioisotopes over a wide range of radioactive concentrations and the efficiency, sensitivity and resolution varied greatly for the tested radioisotopes. Shielding experiments revealed the contribution of the various emission products of radioisotopes to these properties. However, quantitative biodistribution studies with radiotracers that are labeled with all tested radioisotopes, even 123I, are feasible. The results from the ex vivo biodistribution study, using [ 123I]FP-CIT as a radiotracer were similar for the phosphor imaging technique as compared to the dissection technique. Advantages of phosphor imaging in radiotracer distribution studies in rat brain as compared to dissection experiments may be more precise measurements, possibility to reanalyze imaging data and 3D-reconstruction. In conclusion, phosphor imaging is an attractive alternative for biodistribution studies of short-living radiotracers in small animals.

A Novel, Non-immunogenic Fyn SH3-derived Binding Protein with Tumor Vascular Targeting Properties

The generation of novel binding molecules based on protein frameworks ("scaffolds") represents an emerging field in protein engineering, with the potential to replace antibodies for many research and clinical applications. Here, we describe the design, construction, characterization, and use of a novel human Fyn SH3 phage library, containing 1.2 x 10(9) individual clone members. We also present the isolation and in vitro characterization of Fyn SH3-derived proteins binding to the extra-domain B of fibronectin, a marker of angiogenesis. One specific binding clone, named D3, was further evaluated and showed a remarkable ability to stain vascular structures in tumor sections. Furthermore, quantitative biodistribution studies in tumor-bearing mice revealed the ability of D3 to selectively accumulate in the tumor. In contrast to human scFv antibody fragments administered to mice, neither Fyn SH3 WT nor the D3 mutant was immunogenic in mice after four intravenous injections. The extra-domain B binding D3 protein opens new biomedical opportunities for the in vivo imaging of solid tumors and for the delivery of toxic agents to the tumoral vasculature.

Human monoclonal antibodies to domain C of tenascin-C selectively target solid tumors in vivo

We had previously reported that splice isoforms of tenascin-C containing the extra-domain C are virtually absent in normal adult tissues but are highly abundant in high-grade astrocytomas, with a prominent peri-vascular pattern of expression. We now report that the extra-domain C of tenascin-C is strongly expressed in the majority of lung cancers, with a vascular and stromal pattern of expression. Using antibody phage technology, we have generated a human monoclonal antibody (G11), with a dissociation constant K(D) = 4.2 nM for the human domain C. The G11 antibody, expressed in scFv and in mini-antibody (SIP) format, as well as a scFv-interleukin-2 fusion protein, was then characterized in quantitative biodistribution studies using mice grafted subcutaneously with U87 gliomas, revealing a selective tumor uptake, with tumor/blood ratios up to 11.8:1 at 24 h. A radioiodinated preparation of SIP(G11) was also investigated in a double tracer study using an orthotopic rat glioma model, confirming the antibody's ability to preferentially localize at the tumor site, with tumor/brain ratios superior to the ones observed with (18)F-fluorodeoxyglucose. These tumor-targeting properties, together with the strong immunohistochemical staining of human tumor sections, indicate that the G11 antibody may be used as a portable targeting moiety for the selective delivery of imaging and therapeutic agents to gliomas and lung tumors.

A potential lung perfusion imaging agent of synthetic origin

Abstract99mTc‐labelled macroaggregated albumin (MAA) is the radiopharmaceutical routinely used for perfusion lung scans. However MAA formulations contain excipients of biological origin, that may potentially cause allergic hypersensitivity in patients. The aim of this study was to prepare a non‐biological lung imaging agent, with physiological uptake based on a mechanism of colloid localisation in the pulmonary vasculature. To a frozen stannous fluoride cold kit (RAH Radiopharmacy) was added 99mTc‐pertechnetate (⩽2 GBq) in saline (1–4 ml), and the radioactive contents were mixed by rotation (40 rpm) in a syringe at room temperature for 30–180 min. The preparative conditions were varied to control particle growth by: the addition of metal ions, halide ions, or oxidants; different mixing times; and temperatures. The 99mTc products were analysed for % radiolabelling efficiency (RE), radioactive particle size distribution (RPSD), qualitative and quantitative rat biodistribution studies. Results indicated that all radioactive particles were formed with >99% RE, and 1–47% were >8 µm. The optimum radiotracer formulation containing the highest proportion of the largest particles, was prepared by mixing SnF2 and 99mTc‐pertechnetate with a low [Na+] at room temperature for 50 min. Results from the quantitative organ assays gave 88±1% tracer in the lungs, and less than 10% in the liver and spleen. The images showed excellent, uniform lung uptake with minimal interference from liver and spleen to the lower regions of right and left lobes. In conclusion, the synthetic radiopharmaceutical 99mTc–tin fluoride colloid can be prepared with a large particle size, from a commercially available cold kit in a simple and practical manner, and it has good potential for use as a perfusion imaging agent in lung scans. Copyright © 2006 John Wiley & Sons, Ltd.

Enhanced delivery of iodine for synchrotron stereotactic radiotherapy by means of intracarotid injection and blood–brain barrier disruption: Quantitative iodine biodistribution studies and associated dosimetry

Synchrotron stereotactic radiotherapy (SSR) is a binary cancer treatment modality that involves the selective accumulation of a high Z element, such as iodine, in tumors, followed by stereotactic irradiation with kilovoltage X-rays from a synchrotron source. The success of SSR is directly related to the absolute amount of iodine achievable in the tumor. The purposes of this preclinical study were to determine whether the delivery of iodine to brain tumor models in rats could be enhanced by the means of its intracarotid injection with or without a hyperosmotic solution and to evaluate corresponding absorbed X-ray doses. Experiments were performed on four groups of F98 glioma-bearing rats, which received either intracarotid (IC) or intravenous (IV) infusions of a mixture (6 mL in 12 min) of an iodinated contrast agent associated or not with a transient blood-brain barrier opener (mannitol). The mixture volumetric proportions were 8/13 of Iomeron (C = 350 mg/mL) for 5/13 of mannitol or saline, respectively. Absolute iodine concentration kinetic was measured in vivo in the tumor, blood, contralateral and ipsilateral brain, and muscle by monochromatic computed tomography. Associated dosimetry was performed by computing the iodine dose enhancement factor (DEF) in each region and building dose distribution maps by analytical simulations. Infusion of mannitol significantly enhanced iodine tumor uptake compared with the control values (p < 0.0001 and p = 0.0138, for IC and IV protocols, respectively). The mean iodine concentrations (C) reached 20.5 +/- 0.98 mg/mL (DEF = 4.1) after administration of iodine and mannitol vs. 4.1 +/- 1.2 mg/mL i.c. with serum (DEF = 1.6). The tumor iodine uptakes after jugular injection with mannitol (C = 4.4 +/- 2.1 mg/mL, DEF = 1.7) were not significantly different from IC injection of iodine without mannitol (p = 0.8142). The IV injection of iodine with saline led to an iodine concentration in the tumor of 1.2 +/- 0.98 mg/mL and a DEF of 1.2. This study established that optimizing the delivery of iodine by means of IC injection combined with a blood-brain barrier opener (mannitol) significantly increases the iodine uptake of F98 rat gliomas. This infusion protocol could potentially enhance the efficacy of SSR treatment, because the radiation dose is proportional to the iodine amount present in the irradiation bed.

99mTc-labeled murine ior C5 monoclonal antibody in colorectal carcinoma patients: Pharmacokinetics, biodistribution, absorbed radiation doses to normal organs and tissues and tumor localization

Monoclonal Antibody (mAb) ior C5 is a murine IgG(1) that recognizes the tumor associated antigen (TAA) ior C2, a cell surface O-linked glycoprotein carbohydrate chain not present in most normal tissues and homogeneously expressed in the cytoplasm of normal colon epithelium and heterogeneously expressed in more than 83% of primary colorectal carcinomas. This study was designed to investigate the pharmacokinetics, biodistribution and the absorbed radiation doses of (99m)Tc-labeled mAb ior C5 antibody in colorectal tumor patients. Ten patients were administered 3 mg of anti-O-linked glycoprotein carbohydrate chain TAA ior C2 murine monoclonal antibody ior C5 radiolabeled with (99m)Tc activity of 1435.0 +/- 123 MBq by intravenous (i.v.) bolus infusion. Blood and urine samples were collected from 4 out of 10 patients at timed intervals from 10 min and up to 24 h after injection of the (99m)Tc-labeled mAb ior C5 for pharmacokinetic studies. Whole body images were taken in 5 out of 10 patients for quantitative normal organ biodistribution and dosimetry studies and planar anterior and posterior and SPECT images were taken in 5 out of 10 patients for tumor localization. Mean absorbed doses were estimated using the methods developed by the Medical Internal Radiation Dose (MIRD) committee. The effective dose equivalent (EDE) and effective dose (ED) were calculated as prescribed in International Commission on Radiological Protection (ICRP) publications 30 and 60. Plasma disappearance curves of (99m)Tc-labeled murine antibody ior C5 were best fit by a two-compartment model in all patients with (t(1/2alpha)) of 4.32 +/- 2.18 h and (t(1/2beta) of 32.6 +/- 3.82 h. Among the main target organs, accumulation of the radiolabeled antibody was found in liver (9.38 +/- 0.80%), heart (8.92 +/- 0.94%) and spleen (1.37 +/- 0.30%) at 5 min post-administration. These values were reduced at 24 h to (5.91 +/- 0.73%) and (0.62 +/- 0.22%), respectively, for the heart and spleen and increased to (9.78 +/- 1.99%) for liver. Estimates of radiation absorbed dose to normal organs in rad/mCi administered were: whole body, 0.0181 +/- 0.0017; heart wall, 0.0768 +/- 0.0090; kidneys, 0.0530 +/- 0.0260; liver, 0.0565 +/- 0.0109 and spleen, 0.0540 +/- 0.0128. The effective dose equivalent and effective dose estimates for adults were 0.0314 +/- 0.0031 and 0.0249 +/- 0.0027 rem/mCi administered. This feasibility study indicates that the O-linked glycoprotein carbohydrate chain TAA ior C2 is expressed in primary and metastatic colorectal carcinomas and shows very limited expression in normal adult tissues. The very good pattern of biodistribution of (99m)Tc-labeled mAb ior C5 in patients will allow imaging of colorectal carcinoma lesions.

99mTc-Alafosfalin: an antibiotic peptide infection imaging agent

The radiolabeled antibiotic peptide 99mTc-alafosfalin was assessed as an infection imaging agent in a rat model by comparison with 99mTc-DTPA and 99mTc-leukocytes. 99mTc-alafosfalin was prepared via an instant cold kit and 99mTc-leukocytes were prepared using 99mTc-stannous fluoride colloid in an ex vivo labeling procedure of whole blood. In separate experiments, the three radiotracers were administered to rats infected with staphylococcus aureus. Quantitative biodistribution studies were performed as well as scintigraphic images and histopathology. 99mTc-alafosfalin is a stable product, obtained in high radiochemical purity (>95%). This agent was mainly renally excreted, with low liver, spleen and bone uptake, and resulted in a mean ratio of infected/non-infected thighs of 4.3/1.0 at 4 hr post radiotracer injection. 99mTc-DTPA gave a corresponding ratio of 1.9/1.0 and 99mTc-leukocytes gave 20.0/1.0 at the same time point. An in vitro assay found the level of 99mTc-alafosfalin binding to staphylococcus aureas higher than 99mTc-DTPA (10% versus 1% respectively). 99mTc-alafosfalin accumulates at sites of infection in a rat model better than the perfusion molecule 99mTc-DTPA, yet less than 99mTc-leukocytes. The distribution characteristics of this 99mTc-antibiotic peptide would be an advantage in imaging abdominal and soft tissue infection.

Synthesis and biological evaluation of EC20: a new folate-derived, (99m)Tc-based radiopharmaceutical.

A new peptide derivative of folic acid was designed to efficiently coordinate (99m)Tc. This new chelate, referred to as EC20, was found to bind cultured folate receptor (FR)-positive tumor cells in both a time- and concentration-dependent manner with very high affinity (K(D) approximately 3 nM). Using an in vitro relative affinity assay, EC20 was also found to effectively compete with (3)H-folic acid for cell binding when presented either alone or as a formulated metal chelate. Following intravenous injection into Balb/c mice, (99m)Tc-EC20 was rapidly removed from circulation (plasma t(1/2) approximately 4 min) and excreted into the urine in a nonmetabolized form. Data from gamma scintigraphic and quantitative biodistribution studies performed in M109 tumor-bearing Balb/c mice confirmed that (99m)Tc-EC20 predominantly accumulates in FR-positive tumor and kidney tissues. These results suggest that (99m)Tc-EC20 may be clinically useful as a noninvasive radiodiagnostic imaging agent for the detection of FR-positive human cancers.

Tumor-targeting properties of antibody-vascular endothelial growth factor fusion proteins.

A major problem of antibody-based targeting of solid tumors is the poor penetration of antibodies into tumor tissue. Vasoactive immunoconjugates have been proposed as a means of increasing antibody uptake in tumors. In principle, VEGF (also known as vascular permeability factor) could selectively alter vascular permeability, leading to improved tumor targeting. A possible role for VEGF in the targeting of tumor neovasculature has been postulated, based on the overexpression of VEGF receptors in tumor endothelial cells. However, quantitative biodistribution studies on this topic are not available. In this report, we describe the cloning, expression, characterization and biodistribution in tumor-bearing mice of antibodies fused to either VEGF(120) or VEGF(164) The MAb fragments chosen for analysis were scFv(L19), specific for the ED-B domain of fibronectin, a marker of angiogenesis, and scFv(HyHEL-10), a negative control antibody of irrelevant specificity in mice. Neither unconjugated VEGF nor scFv(HyHEL-10)-VEGF fusion proteins showed accumulation in the tumor (tumor:blood ratios approx. 1 at 4 hr and 24 hr postinjection). By contrast, scFv(L19)-VEGF(120) but not scFv(L19)-VEGF(164) showed significant accumulation in tumors (tumor:blood ratio = 9.3 at 24 hr) but was not superior to unconjugated scFv(L19). Preinjection of unlabeled scFv(L19)-VEGF(120) prior to administration of radiolabeled fusion protein led to increased accumulation of radiolabeled scFv(L19)-VEGF(120) in the tumor but only at very high concentrations (20 microg/mouse).

Detecting inflammation with 131I-labeled ornidazole

The aim of this study was to demonstrate the accumulation of 131I-labeled ornidazole ( 131I-ORN) in experimental abscesses. 131I-ORN was prepared by electrophilic radioiodination of ORN, using radioiodide in the presence of Iodogen®. An in vivo inflammation model was prepared by intramuscular injection of turpentine into the thigh of rabbits. Four days later 131I-ORN was intravenously administered to rabbits. Serial scintigrams were performed at different periods, using a Sophy DX Gamma Camera. 131I-ORN was visualized at 10 min after injection. 131I-ORN was also administered intraperitoneally to rats with turpentine-induced inflammation, for quantitative biodistribution studies. Counts of selected tissues were taken by a NaI(Tl) scintillation detector (gamma counter) after rats were decapitated. The target-to-non-target muscle ratios were 2.5, 2.6, 2.9 and 1.9 at 1, 3, 5 and 24 h, respectively.

Production of high purity 94mTc for positron emission tomography studies

The radioisotope 94mTc (T 1/2 = 52.5 min; I β+ = 72%; E β+ = 2.47 MeV) is of considerable interest for quantitative biodistribution studies of Tc radiopharmaceuticals using positron emission tomography. The nuclear processes 94Mo(p,n), 93Nb( 3He,2n), 92Mo(α,pn), and 92Mo(α,2n) 94Ru → 94mTc are of potential interest for the production of 94mTc. Detailed cross section and yield measurements showed that the 94Mo(p,n)-reaction over the energy range E p = 13 → 7 MeV is most suitable: The yield of 94mTc is high (2 GBq/μAh), the impurity level is low (provided highly-enriched 94Mo is used as target material), and a small-sized cyclotron is adequate. Using conventional targetry, sufficient quantities of the radioisotope can be produced. The chemical processing methods employed include both solvent extraction and thermochromatography. The quality of the final product is discussed.