First in vivo evaluation of a potential SPECT brain radiotracer for the gonadotropin releasing hormone receptor

OBJECTIVES

In vivo evaluations of a gonadotropin releasing hormone-receptor single photon emission computed tomography radiotracer for non-invasive detection of gonadotropin releasing homone-receptors in brain.

RESULTS

We have used a simple, robust and high-yielding procedure to radiolabel an alpha-halogenated bioactive compound with high radiochemical yield. Literature findings showed similar alpha-halogenated compounds suitable for in vivo evaluations. The compound was found to possess nano molar affinity for the gonadotropin releasing hormone-receptor in a competition dependent inhibition study. Furthermore, liquid chromatography-mass spectrometry analysis in saline, human and rat serum resulted in 46%, 52% and 44% stability after incubation for 1 h respectively. In addition, rat brain single photon emission computed tomography and biodistribution studies gave further insight into the nature of the compound as a radiotracer.

One size does not fit all: the merit of absorbed doses to the blood in 131I therapy for differentiated thyroid carcinoma

The amount of 131I necessary for successful ablation in patients with differentiated thyroid cancer (DTC) is still subject to debate. This study investigates the relationship of the absorbed dose of radiation to the blood while administering 131I activity with several other parameters in DTC patients. This prospective study included 90 DTC patients who were classified into three groups according to their level of dosage: 3.7 GBq (38.9%), 5.55 GBq (55.6%), and 7.4 GBq (5.5%). Blood dosimetry of treated patients was performed using external whole-body counting with a Geiger Muller dosimeter located 2 m away from the patients. Dose rate was measured at 2, 4, 5, 24, and 48 h after the administration of radioiodine. Based on the results of whole-body dose rate measurements, 48 h after administration of 3.7, 5.55, and 7.4 GBq of radioiodine, absorbed doses to patients' blood were estimated at 0.49 ± 0.12, 0.71 ± 0.21, and 0.76 ± 0.11 Gy, respectively. Increasing radioiodine dosage from 3.7 GBq to 5.55 GBq significantly increased blood dose, while there was no significant difference in blood dose between radioiodine dosages of 5.55 GBq and 7.4 GBq. The absorbed dose to the blood was significantly correlated to the patients' gender and the presence of lymph node metastases, but it was not significantly correlated to the type of pathology and regional or distant metastases. Ablation activities exceeding 5.55 GBq produce no further increase in the accumulated activity per volume of blood. The literature regarding this issue is scarce, and further studies are required to verify these preliminary results.

Regional brain imaging of vesicular acetylcholine transporter using o-[125 I]iodo-trans-decalinvesamicol as a new potential imaging probe

In this study, the regional rat brain distribution of radioiodinated o-iodo-trans-decalinvesamicol ([(125) I]OIDV) was determined in vivo to evaluate its potential as a single-photon emission computed tomography (SPECT) imaging probe for vesicular acetylcholine transporter (VAChT). Following intravenous injection, [(125) I]OIDV passed freely across the blood-brain barrier and accumulated in rat brain. The accumulation of [(125) I]OIDV in rat brain was significantly reduced by coadministration of (+/-)-vesamicol (0.125 µmol). In contrast, the coadministration of σ-receptor ligands, such as (+)-pentazocine (0.125 µmol) as a σ-1 receptor ligand and (+)-3-(3-hydroxyphenyl)-N-propylpiperidine (0.125 µmol) as a σ-1 and σ-2 receptor ligands, barely affected the accumulation of [(125) I]OIDV in rat brain. These findings in vivo were corroborated by autoradiographic analysis ex vivo. The authors found that the tracer binds with pharmacological selectivity to VAChT in rat brain and predicted that it may likewise serve in translational SPECT imaging studies of this marker in the integrity of cholinergic innervations.

Comparison of different dosimetric methods for red marrow absorbed dose calculation in thyroid cancer therapy

Several dosimetric methods have been proposed for estimating red marrow absorbed dose (RMAD) when radionuclide therapy is planned for differentiated thyroid cancer, although to date, there is no consensus as to whether dose calculation should be based on blood-activity concentration or not. Our purpose was to compare RMADs derived from methods that require collecting patients' blood samples versus those involving OLINDA/EXM software, thereby precluding this invasive procedure. This is a retrospective study that included 34 patients under treatment for metastatic thyroid disease. A deviation of <10 % between RMADs was found, when comparing the doses from the most usual invasive dosimetric methods and those from OLINDA/EXM. No statistical difference between the methods was discovered, whereby the need for invasive procedures when calculating the dose is questioned. The use of OLINDA/EXM in clinical routine could possibly diminish data collection, thus giving rise to a simultaneous reduction in time and clinical costs, besides avoiding any kind of discomfort on the part of the patients involved.

Inter-relationship between different platelet measures of 5-HT and their relationship to aggression in human subjects

The objective of this study was to explore the inter-relationship of three platelet measures of serotonergic function (5-HT): 5-HT Transporter Binding, 5-HT-2 Receptor Binding and 5-HT Content and to explore their inter-relationship with measures of aggression and impulsivity. 58 male subjects with personality disorder were studied. Numbers of platelet 5-HT Transporter and 5-HT-2 Receptor sites were assessed by examining the Bmax of ³H-Paroxetine Binding and the Bmax of ¹²⁵I-LSD Binding to the blood platelet; 5-HT Content was assessed by measuring the amount of 5-HT in the platelet material. Life history of aggression was assessed by Life History of Aggression. Impulsivity was assessed by the Impulsivity Scale of the Eysenck Personality Questionnaire-II. Platelet 5-HT Transporter Binding correlated with both 5-HT-2 Receptor Binding and 5-HT Content; the latter two variables did not correlate with each other. Only Platelet 5-HT Transporter binding correlated significantly with LHA Aggression. These data suggest that while Platelet 5-HT Transporter binding correlates with both 5-HT-2 Receptor Binding and with 5-HT Content, that only 5-HT Transporter Binding represents a correlate of aggression in male personality disordered subjects.

Biodistribution of 131I-labeled anti-CK8 monoclonal antibody in HNSCC in xenotransplanted SCID mice

BACKGROUND

A new promising approach to improve the outcome of head and neck squamous cell carcinoma (HNSCC) is the application of radio-labeled antibodies directed against tumor-associated antigens. Cytokeratin 8 (CK8), an intermediate filament forming protein, is shown to be de novo expressed in dysplastic lesions as well as in HNSCC. Therefore like the epithelial cell adhesion molecule CK8 seems to be a suitable anchor molecule for targeted radioimmunotherapy (RIT). The aim of this study was to investigate the biodistribution of a radio-labeled Cytokeratin 8-specific monoclonal antibody (mAb) in a SCID (severe combined immunodeficiency disease) mouse model.

MATERIALS AND METHODS

The mAb against CK8 was labeled with (131)I and biodistribution was tested in established HNSCC xenografts in SCID mice. The biodistribution of the mAb in the tumor and different organs was determined with a gamma counter and was calculated as % injected dose/gram tissue.

RESULTS

Initially, after systemic administration of (131)I-anti CK8 monoclonal antibody high activity was seen in all the organs. Over time the general activity decreased, whereas activity accumulated in the tumor. This activity decayed compared to the other tissues with a two- to threefold prolonged radioactive half-life.

CONCLUSION

Specific antibody-antigen-binding is probably responsible for the prolonged radioactive half-life in the tumor and the resulting cumulative activity due to enrichment of the (131)I-anti CK8 mAb, so that Cytokeratin 8 seems to be a suitable anchor molecule for radioimmunotherapy in HNSCC.

Radioactivity of blood samples taken from thyroidectomized thyroid carcinoma patients after therapy with (131)I

BACKGROUND

Occasionally, blood samples may be required from thyroid cancer patients after they have been given the therapy dose of (131)I, as part of necessary medical management of comorbidities. Thus, in the days after (131)I administration, medical health professionals may be involved in the withdrawal, handling, and manipulation of radioactive blood samples. The purpose of this study was to quantify the amount of radioactivity in blood samples taken from thyroidectomized thyroid carcinoma patients after the administration of therapeutic activities of (131)I.

METHODS

For dosimetry purposes, serial blood sampling is performed on thyroidectomized thyroid carcinoma patients prior to therapy with (131)I. The quantities of radioactive material present in these blood samples were expressed as a percentage of the administered activity and then extrapolated to the high levels of (131)I used in therapy for 377 patients in this study. The corresponding radiation exposure rate from the blood samples was then calculated to determine what radiation protection methods were required for staff handling these samples.

RESULTS

The average amount of radioactivity in a 1 mL blood sample at 1 hour postadministration of 5.5 GBq (150 mCi) of (131)I was 0.2 ± 0.15 MBq (5.4 ± 4.0 μCi). This corresponds to an exposure rate of 1.23 μSv/h (0.123 mrem/h) at 10 cm from the sample. For samples obtained beyond 24 hours after a therapeutic administration of 5.55 GBq (150 mCi), the exposure levels are approximately equal to background radiation.

CONCLUSION

The data in this study indicate that the radiation exposure from blood samples withdrawn from thyroidectomized thyroid cancer patients is low. However, to ensure that staff members are exposed to minimal levels of radiation, it is imperative that staff members who are involved in withdrawing, handling, or manipulating radioactive blood samples adhere to the recommended radiation safety practices.

Data on repeated (131)I-WB scans and the incidence of positive Tg and negative (131)I-WBS in DTC patients from a 24 months study

We present data on repeated iodine-131 whole body scans ((131)I-WBS) in differentiated thyroid cancer patients (DTC) after surgery and (131)I remnant ablation and on increased thyroglobulin (Tg) with negative (131)I-WBS, in a retrospective study at our hospital. A total of 106 patients (91 female and 15 male) treated with (131)I for DTC met the inclusion criteria. The mean age of the patients was 45 years, age range 16-81 years. A total of 101 patients had complete 24 months follow-up following (131)I remnant ablation treatment. The mean (131)I dose administered after the first 6 months of follow- up was 3GBq while mean total dose was 4.9GBq, range 1.1-7.4GBq. Our results showed that at the end of the first 6 months post treatment, 58/101 patients had a negative (131)I-WBS. By the end of the 4th (131)I treatment at 24th months, the remaining 43 patients became negative for (131)I-WBS. We found increased Tg and negative (131)I-WBS in 2 of the 101 patients at the 24th months examination the so called Tg elevated negative (131)I-WBS (TENIS syndrome). The possible explanation of this syndrome is discussed. In conclusion, our study in DTC operated patients does not support the use of repeated diagnostic (131)I-WBS after an undetectable Tg because we found no Tg rebound in patients with negative (131)I-WBS, after 24 months of follow-up with serial measurements of Tg on and of suppression with L thyroxine.

The use of dosimetry in the treatment of differentiated thyroid cancer

The standard treatment for the ablation of thyroid remnant tissue following surgery as well as for the treatment of iodine avid metastases in patients suffering from differentiated thyroid cancer (DTC) is therapy with radioactive iodine. Mostly fixed standard activities are used with the inherent risk of under- or overdosing the patient. Therefore, the rationale for using a dosimetry-based approach is to replace the conventional fixed activity regimen by a patient-tailored approach which allows the administered therapeutic activity to be increased while avoiding unwanted side effects. The purpose of this review was to describe the presently used dosimetric concepts: 1) the blood dosimetry approach (optimizing the "safety" aspect of the treatment); and 2) lesion-based dosimetry (optimizing the "efficacy" of the treatment) and their respective clinical findings. In addition, a simplified method for performing blood dosimetry and its application towards further enhancement of radioiodine therapies are introduced. Finally, a new concept for potentially determining patient-specific radiation sensitivity using blood dosimetry is introduced.

Regional cerebral blood flow changes in early-onset anorexia nervosa before and after weight gain

To investigate the changes of regional cerebral blood flow (rCBF) in early-onset anorexia nervosa (AN) before and after weight gain, we examined resting rCBF using single photon emission computed tomography (SPECT) with [(123)I]iodoamphetamine ((123)I-IMP). Ten female children with AN (mean age 13.2 years old) participated in this study. SPECT examinations were performed in all patients twice at the beginning of treatment and after weight gain. The mean body mass index (BMI) was changed from 13.1 to 16.6 during 4 months treatment period. Automatic voxel-based analysis of the images was carried out using statistical parametric mapping (SPM) software. Relatively increased rCBF in the bilateral parietal lobe and limbic lobe including the posterior cingulate cortex (PCC) were observed after weight gain in early-onset AN. There was no significant decrease in the rCBF after weight gain. A significant positive correlation was observed between BMI and rCBF in the right thalamus, right parietal lobe, and right cerebellum. These results suggested that weight gain during the process of recovery from early-onset AN might activate specific brain regions which are possibly relevant to the pathophysiological aspects of the disorder.

Red cell volume can be accurately determined in sheep using a nonradioactive biotin label

The sheep has served as an informative animal model for investigation of human fetal and newborn erythropoiesis and red blood cell (RBC) kinetics. We previously validated the permanent label (14C)cyanate for measuring red cell volume (RCV) in sheep. Here, we validate biotin labeling of RBCs as a nonradioactive method for measuring RCV in sheep with the anticipation that it can be applied in studies of human infants. The RCV was determined simultaneously using two techniques for quantitation of the biotin label. The first one quantified total blood concentration of biotin label on biotin-labeled RBCs using (125I)streptavidin. The second one enumerated biotin-labeled RBCs by flow cytometry after incubation with fluorescein-conjugated avidin. RCV measurements made using the two biotin quantitation techniques were validated against both (14C)cyanate and 51Cr as reference methods. Both biotin techniques produced RCV values that agreed well with the reference methods and with each other, producing correlation coefficients averaging >or =0.93. Sequential repetitive measurements in the same animal also agreed with the (14C)cyanate method and each other (average difference <10%). These results establish biotin-labeled RBCs as an accurate method for performing RCV measurements in sheep. This biotin method can be applied in studies that model neonatal erythropoiesis.

Quantitative serial imaging of an 124I anti-CEA monoclonal antibody in tumor-bearing mice

OBJECTIVE

The 4.2-day half-life of (124)I favors its use for positron emission tomography (PET) of monoclonal antibodies (mAbs). However, high positron energy and beta(+) -associated cascade gamma rays pose image resolution and background noise problems for (124)I. This study evaluated quantitative PET of an (124)I mAb in tumor-bearing mice.

METHODS

An R4 microPETtrade mark (Siemens/CTIMI, Knoxville, TN) was used with standard energy and coincidence timing windows (350-750 keV and 6 ns, respectively), delayed random coincidence subtraction, iterative image reconstruction, and no attenuation or scatter correction. Image resolution, contrast, and response linearity were compared for (124)I and (18)F, using phantoms. Nude mice bearing human colon tumors (LS-174T) were injected intravenously with a chimeric (124)I anti-CEA mAb (cT84.66) and imaged serially 1 hour to 7 days postinjection. Venous blood was sampled to validate image-derived blood curves. Mice were sacrificed after the final scan, and the biodistribution of (124)I was measured by direct tissue assay. Images were converted to units of kBq/g for each tissue of interest by comparing the final scans with the direct assays.

RESULTS

Measured resolution (FWHM) 0-16 mm from the scanner axis was 2.3-2.7 mm for (124)I versus 1.9-2.0 mm for (18)F. Due to true coincidence events between annihilation photons and cascade gamma rays, background was greater for (124)I than (18)F, but the signal-to-background ratio was still more than 20, and (124)I image intensities varied linearly with activity concentration. Tissue-based calibration worked well (i.e., PET blood curves agreed with direct measurements within 12% at all time points), while calibration, based on a cylindrical phantom approximating the mouse body, yielded tumor quantitation that was 46%-66% low, compared with direct assay.

CONCLUSIONS

Images of quantitative accuracy sufficient for biodistribution measurements can be obtained from tumor-bearing mice by using (124)I anti-CEA mAbs with standard microPET acquisition and processing techniques, provided the calibration is based on the direct assay of excised tissue samples.

Radiosynthesis and evaluation of 5-[125I]iodoindol-3-yl-beta-D-galactopyranoside as a beta-galactosidase imaging radioligand

The synthesis and investigation of 5-[125I]iodoindol-3-yl-beta-d-galactopyranoside ([125I]IBDG) as a radioligand for single-photon emission computed tomography (SPECT) imaging of b-galactosidase expression are described. No-carrier-added [125I]IBDG was synthesized by a radioiododestannylation approach in > 75% overall radiochemical yield and > 99% radiochemical purity. [125I]IBDG was evaluated as a substrate using beta-galactosidase-expressing (D54L) and nonexpressing (D54) human glioma cell lines. A 24-hour incubation of this substrate with cultured cells revealed a 6.5-fold greater intracellular trapping of radioactivity in D54L cells compared with D54 cells. Systemic delivery of [125I]IBDG in nude mice bearing D54L tumors failed to show significant trapping of radioactivity within these tumors by SPECT imaging. In contrast, intratumoral injection of the substrate resulted in efficient trapping of radioactivity in D54L tumors but not D54 tumors, resulting in clear SPECT visualization of the former tumor. Based on dynamic SPECT imaging and blood metabolite analysis, we conclude that although [125I]IBDG is an efficient in vivo substrate for beta-galactosidase, its rapid renal clearance hampers its intratumoral availability on systemic administration.

131I labeling of tamoxifen and biodistribution studies in rats

Tamoxifen [TAM ([Z]-2-[4-(1,2-diphenyl-1-di-butenyl)-phenoxy]-N,N-dimethylethanamine)] has been used as an antiestrogen drug for treatment and prevention of human breast cancer. Tamoxifen was labeled with 131I using iodogen as an oxidizing agent. Mass spectroscopy of the cold standard showed that the labeling occurs in ortho position to the phenyl ether position of TAM as expected. Quality control, radiochemical yield and stability were established using the radioelectrophoresis method. The radiolabeled compound maintained its stability throughout working period of 24 h. Scintigraphic imaging was performed and tissue distribution was determined in Albino Wistar rats. According to biodistribution and imaging experiments the radiolabeled compound presented estrogen receptor (ER) specificity and it was uptaken by endometrium as well as breast tissue.

125I used for labelling of proteins in an absorption model changes the absorption rate of insulin aspart

The aim of this study is to validate the ability of the disappearance model to predict absorption rates of insulin aspart in pigs. The disappearance model is used as a screening tool to estimate absorption rates after subcutaneous injections in humans or pigs especially of insulin and insulin analogues. The disappearance model measures remaining radioactivity at the injection site and therefore radioactive labelling of the insulin analogue is necessary. The labelling is done with 125I. One of the assumptions for the disappearance model to be reliable is that absorption rates of the labelled and non-labelled molecules are comparable. In this study, we compared disappearance data with absorption calculated from plasma samples of insulin aspart. The calculated absorption is based on non-labelled insulin aspart. The absorption rate from the disappearance data was statistical significant (p = 0.0028) different from the absorption rate based on plasma samples. A control study was carried out where 125I labelled insulin aspart was compared to 127I (the natural non-radioactive isotope) insulin aspart. In this study, absorption rate from the disappearance data and absorption rate based on plasma samples were similar (p = 0.63).

CONCLUSION

Iodination of insulin aspart changes the subcutaneous absorption rate.

Biodistribution of Free 211At and 125I– in Nude Mice Bearing Tumors Derived from Anaplastic Thyroid Carcinoma Cell Lines

Free 211At has been proposed for therapy of anaplastic thyroid carcinoma (ATC). However, no extensive biodistribution study comparing tumor-bearing and nontumor-bearing mice has previously been performed. The aim of this study was to perform a complete evaluation of the biodistribution of 211At, both for normal and ATC-bearing mice. For comparison, the biodistribution of 125I- was simultaneously studied. Dosimetric evaluations were performed to investigate if (211)At can be used for therapy of ATC.

METHODS

Athymic nude mice were subcutaneously injected with either of two human ATC cell lines, HTh83 and KAT-4. Tumor-bearing and nontumor-bearing mice were injected intravenously with 0.3 MBq 211At and 0.3 MBq 125I- simultaneously. The mice were sacrificed 4-24 hours after injection, and the activity concentrations in tissues were determined.

RESULTS

Except for the thyroid, the concentration of 211At was higher than that of 125I- in the tissues. The uptake of 211At was primarily high in NIS-expressing organs. Furthermore, the absorbed doses to these organs were higher than both tumor types.

CONCLUSIONS

The biodistribution of 211At and 125I- differed in this animal model. The higher mean absorbed dose from 211At in several organs than in tumor tissue restricts the possibility of using free 211At for therapy of ATC.

Monoclonal antibody A7 coupled to magnetic particles as a contrast enhancing agent for magnetic resonance imaging of human colorectal carcinoma

BACKGROUND

Local recurrence, the most frequent pattern of recurrence of rectal carcinoma, is almost always fatal. The difficulty of diagnosing local recurrence contributes importantly to the poor prognosis.

METHODS

We coupled monoclonal antibody (Mab) A7, which reacts specifically with human colorectal carcinoma, to ferromagnetic lignosite (FML) particles to distinguish rectal carcinoma from other tissues by magnetic resonance (MR) imaging. We examined retention of immunoreactivity by the A7-FML complexes in vitro, and also their distribution in vivo according to radiolabeling and MR imaging when injected into nude mice bearing human colorectal carcinoma xenografts.

RESULTS

A7-FML retained binding activity nearly identical to that of Mab A7. Significantly more (125)I-labeled A7-FML accumulated in engrafted tumors than did (125)I-labeled normal mouse IgG-FML complexes (P < 0.05). A7-FML disappeared rapidly from the blood. Normal tissues accumulated less (125)I-labeled A7-FML than tumors; this accumulation decreased linearly with time. In MR imaging, signal intensity was reduced in the tumor by the injection of A7-FML.

CONCLUSIONS

A7-FML is potentially useful as a MR contrast enhancing agent for human colorectal carcinoma xenografts implanted subcutaneously.

[I131 in blood samples: management in the laboratory]

OBJECTIVES

Patients treated by (131)I may require blood sampling in the days following its administration. We investigated the safety of such samples in terms of radioactivity and the possible disturbance of the analyses by these "131I-spiked" samples.

METHOD

1) The radioactivity of blood samples from 131I-treated patients was measured (dose rate, surface activity, total activity) ; 2) The risk for the personnel was subsequently evaluated and ; 3) The interference of this 131I-generated radioactivity on the results of routine automated and IRMA assays was investigated.

RESULTS

1) All RA measures but two were found below the European limits ; 2) Irradiation of personnel was negligible ; 3) The faint radioactivity did not disturb any analyses.

CONCLUSION

These data demonstrate the safety that results from the negligible radioactivity in these blood samples.

131I in blood samples: a danger for professionals? A problem for immunoassays?

OBJECTIVE

Our objective was to investigate the safety of radioactive blood samples from patients receiving 131I and whether the radioactivity affects the validity of assays.

METHODS

First, the activity of samples from patients given 131I was measured by 3 methods and compared with the upper threshold. Then, pilot sera were spiked with 131I, and possible interference was investigated using 2 immunoradiometric assays.

RESULTS

The activity of 13 of the 15 samples was below the European limit; the other 2 samples were from patients with reduced renal clearance rates. No differences in thyroglobulin level or thyroid-stimulating hormone level were found between sera that were spiked with 131I and sera that were not.

CONCLUSION

These blood samples are safe because they contain negligible activity, and the use of radioimmunoassays or immunoradiometric assays on them produces reliable results.

[Results of a risk adapted and functional radioiodine therapy in Graves' disease]

Aim of this study was to find out, if results of a functional orientated radioiodine therapy in Graves' disease could be optimized using a risk adopted dose concept.

PATIENTS, METHOD

351 patients with Graves' disease were treated for the first time between 11/97 and 8/01. The basic dose was 125 Gy, which was increased up to 250 Gy in a cumulative manner depending on clinical parameters (initial thyroid metabolism, thyroid volume, immunoreactivity). Two different methods of dosimetry were used. Occasional thyreostasis was withdrawn two days before the radioiodine test was started. Follow up was done on average 8 +/- 2.4 (4-17.2) months. TSH > or = 0.27 microIU/mL confirmed as a measure of the success.

RESULTS

With improved pretherapeutic dosimetry and a mean target dose of 178 +/- 31 Gy (n=72) therapeutic success occurred in 66.7%, in 51.4% euthyreosis was restalled and in 15.3% of patients hypothyroidism was seen (TSH > 4.20 microIU/mL). With simplified pretherapeutic dosimetry and a mean target dose of 172 +/- 29 Gy (n=279) results were moderately impaired (63.8%, 40.1% and 23.7%). With increasing target dose therapeutic failure increased, as insufficiently adopted risk factors for therapeutic failure turned out the initial thyroid metabolism, the TcTU(s) as the (h)TRAb titer.

CONCLUSION

Functional orientated RIT can be optimized by including illness specific characteristics, principal limitations are a high initial thyroid metabolism, a large thyroid volume and a high (h)TRAb-titer.

PBPK model for radioactive iodide and perchlorate kinetics and perchlorate-induced inhibition of iodide uptake in humans

Detection of perchlorate (ClO4-) in several drinking water sources across the U.S. has lead to public concern over health effects from chronic low-level exposures. Perchlorate inhibits thyroid iodide (I-) uptake at the sodium (Na+)-iodide (I-) symporter (NIS), thereby disrupting the initial stage of thyroid hormone synthesis. A physiologically based pharmacokinetic (PBPK) model was developed to describe the kinetics and distribution of both radioactive I- and cold ClO4- in healthy adult humans and simulates the subsequent inhibition of thyroid uptake of radioactive I- by ClO4-. The model successfully predicts the measured levels of serum and urinary ClO4- from drinking water exposures, ranging from 0.007 to 12 mg ClO4-/kg/day, as well as the subsequent inhibition of thyroid 131I- uptake. Thyroid iodine, as well as total, free, and protein-bound radioactive I- in serum from various tracer studies, are also successfully simulated. This model's parameters, in conjunction with corresponding model parameters established for the male, gestational, and lactating rat, can be used to estimate parameters in a pregnant or lactating human, that have not been or cannot be easily measured to extrapolate dose metrics and correlate observed effects in perchlorate toxicity studies to other human life stages. For example, by applying the adult male rat:adult human ratios of model parameters to those parameters established for the gestational and lactating rat, we can derive a reasonable estimate of corresponding parameters for a gestating or lactating human female. Although thyroid hormones and their regulatory feedback are not incorporated in the model structure, the model's successful prediction of free and bound radioactive I- and perchlorate's interaction with free radioactive I- provide a basis for extending the structure to address the complex hypothalamic-pituitary-thyroid feedback system. In this paper, bound radioactive I- refers to I- incorporated into thyroid hormones or iodinated proteins, which may or may not be bound to plasma proteins.

Bone marrow dosimetry using blood-based models for radiolabeled antibody therapy: a multiinstitutional comparison

Standardization of marrow dosimetry is of considerable importance when estimating dose-response for a multicentered clinical trial involving radionuclide therapy. However, it is only within the past five years that the intercomparison of marrow dosimetry results among separate clinical trials that use the same agent has become scientifically feasible. In this work, we have analyzed reported marrow dosimetry results from radioimmunotherapy trials and recalculated marrow absorbed doses at a central facility using a standard blood model with patient-specific source data. The basic approach used in the American Association of Physicists in Medicine (AAPM)/Sgouros marrow dosimetry methodology was common to calculation performed at all participating institutions, including the central facility. Differences in dose estimates associated with starting assumptions and the exact implementation of the AAPM/Sgouros calculation methodology used by the source institutions and the central facility were quantified and compared.

METHODS

Data from 22 patients enrolled in radiolabeled antibody clinical trials were randomly selected from 7 participating institutions for the assessment of marrow dose. The analysis was restricted to those patients who were treated with 131I- or 186Re-labeled antibody and had no marrow involvement. Calculation of bone marrow dose at each participating institution was unique to the trial or institution, but all used some form of the AAPM/Sgouros blood model approach. The central facility adopted a marrow dosimetry model based on the AAPM/Sgouros model for radiolabeled antibodies using the standard MIRD approach to the remainder-of-body contribution. A standardized approach to account for variations in patient mass was used for the remainder-of-body component. To simplify clinical implementation, regional marrow uptake and time-dependent changes in the marrow-to-blood concentration ratio were not included. Methods of formatting the collection of standard datasets useful in defining dose-response parameters are also presented.

RESULTS

Bone marrow doses were calculated according to the method described for each of the 22 patients based on the patient-specific data supplied by the participating institutions. These values were then individually compared with the marrow doses originally reported by each institution. Comparison of the two calculation methods was expressed as a ratio of the marrow doses for each patient. The mean ratio for the dose estimates at the participating institution calculation compared with the central laboratory value was 0.920 +/- 0.259 (mean +/- SD), with a range from 0.708 to 1.202.

CONCLUSION

The independent use of the AAPM/Sgouros method blood model approach to marrow dosimetry has brought these dose estimates to within 30% of the results obtained centrally compared with substantially higher uncertainties reported previously. Variations in calculation methodology or initial assumptions adopted by individual institutions may still contribute significant uncertainty to dose estimates, even when the same data are used as a starting point for the calculation comparison shown here. A clinically relevant, standard method for marrow dosimetry for radiolabeled antibodies is proposed as a benchmark for intercomparison purposes. A parameter sensitivity analysis and a summary discussion of the use of this model for potentially improving dose-response data correlation are also presented.

Failure of radioiodine treatment in Graves’ disease intentionally caused by a patient: Suspected Munchausen syndrome

We experienced a case with Graves' disease in which radioiodine treatment failed probably because of intentional spitting out of capsules of radioactive iodide. Chemical analysis of the substances collected from the trash in the treatment room demonstrated that its profile was the same as that of the capsules for radioiodine administration. Measurement of the iodine concentrations in a blood sample obtained at 24 h after the administration of radioiodine indicated that the patient showed iodine excess. These findings suggest that this may be a case of Munchausen syndrome.

Uncertainties in dose coefficients from ingestion of 131I, 137Cs, and 90Sr

Quantification of uncertainties in doses from intakes of radionuclides is important in risk assessments and epidemiologic studies of individuals exposed to radiation. In this study, the uncertainties in the doses per unit intake (i.e., dose coefficients) for ingestion of 131I, 137Cs, and 90Sr by healthy individuals have been determined. Age-dependent thyroid dose coefficients were derived for 131I. The analysis for 131I uses recent measurements of thyroid volume obtained by ultrasonography, which indicate a thyroid mass lower than that previously obtained using autopsy measurements. The coefficients for 137Cs are determined using the relationship between the biological half-lives and the amount of potassium in the human body. The most recent International Commission on Radiological Protection biokinetic model was employed to determine the uncertainties for 90Sr. For 137Cs and 90Sr, the dose coefficients represent exposure in adulthood and they were determined for all organs of radiological importance. The uncertainty in the estimated dose coefficients represent state of knowledge estimates for a reference individual, and they are described by lognormal distributions with a specified geometric mean (GM) and geometric standard deviation (GSD). The estimated geometric means vary only slightly from the dose coefficients reported by ICRP publications. The largest uncertainty is observed in the dose coefficients for bone surface (GSD = 2.6), and red bone marrow (GSD = 2.4) in the case of ingestion of 90Sr. For most other organs, the uncertainty in the 90Sr dose coefficients is characterized by a GSD of 1.8 (or less for some organs). For 131I, the uncertainty in the thyroid dose coefficients is well represented by a GSD of 1.7 for both sexes and all ages other than infants for whom a GSD of 1.8 is more appropriate. The lowest uncertainties are obtained for the dose coefficients from ingestion of 137Cs (GSD = 1.24 for males; 1.4 for females). A dominant source of uncertainty in the ingestion dose coefficients is the variation of the biokinetic parameters. For 131I, the largest contribution to the uncertainty comes from the variation in the thyroid mass, but the contribution of the biokinetic parameters is comparable. The biokinetic parameters with the largest contribution to the uncertainty are (a) the fractional uptake from blood to thyroid in the case of ingestion of 131I, (b) the absorbed fraction from the gastrointestinal tract (f1) in the case of 90Sr, and (c) the amount of potassium in the body for 137Cs. The contribution to the uncertainty of the absorbed fraction (which accounts for the fraction of energy deposited in the target organ) is the smallest contributor to the uncertainty in the dose coefficients for most organs. To reduce the uncertainty in the dose estimated for a real individual, one should determine the above-mentioned parameters for the specified individual rather than to rely on assumptions for a reference individual.

Biologic dosimetry of 188Re-HDD/lipiodol versus 131I-lipiodol therapy in patients with hepatocellular carcinoma

One approach to treatment of primary hepatocellular carcinoma (HCC) is intraarterial injection of (131)I-lipiodol. Although clinical results have been positive, the therapy can be improved by using (188)Re instead of (131)I as the radionuclide. (188)Re is a high-energy beta-emitter, has a shorter half-life than (131)I, and has only low-intensity gamma-rays in its decay. The present study compared the cytotoxic effect of the radionuclide therapy in HCC patients treated with (131)I-lipiodol and (188)Re-4-hexadecyl 2,2,9,9-tetramethyl-4,7-diaza-1,10-decanethiol (HDD)/lipiodol. To this end, dicentric chromosomes (DCs) were scored in metaphase spreads of peripheral blood cultures. The equivalent total-body dose was deduced from the DC yields using an in vitro dose-response curve.

METHODS

Twenty (131)I-lipiodol treatments and 11 (188)Re-HDD/lipiodol treatments were performed on, respectively, 16 and 7 patients with inoperable HCC. Patients received a mean activity of 1.89 GBq of (131)I-lipiodol or 3.56 GBq of (188)Re-HDD/lipiodol into the liver artery by catheterization. For each patient, a blood sample was taken during the week before therapy. A blood sample was also taken 7 and 14 d after administration for the patients treated with (131)I-lipiodol and 1 or 2 d after administration for the patients treated with (188)Re-HDD/lipiodol.

RESULTS

The mean DC yield of (188)Re-HDD/lipiodol therapy (0.087 DCs per cell) was significantly lower than that of (131)I-lipiodol therapy (0.144 DCs per cell) for the administered activities. Corresponding equivalent total-body doses were 1.04 Gy for (188)Re-HDD/lipiodol and 1.46 Gy for (131)I-lipiodol. Data analysis showed that, in comparison with (131)I-lipidol, (188)Re-HDD/lipiodol yielded a smaller cytotoxic effect and a lower radiation exposure for an expected higher tumor-killing effect.

CONCLUSION

(188)Re is a valuable alternative for (131)I in the treatment of HCC with radiolabeled lipiodol, and a dose escalation study for (188)Re-HDD/lipiodol therapy is warranted.

A bolus/infusion paradigm for the novel NMDA receptor SPET tracer [123i]CNS 1261

We have previously performed quantitative kinetic modeling of [(123)I]CNS 1261, a new SPET ligand for the MK801 intrachannel site of the NMDA receptor. We now report a bolus-infusion protocol, which eliminates the need for arterial blood sampling. Dynamic SPET scanning and venous blood sampling were performed in 7 healthy volunteers. Good agreement was obtained between kinetic and equilibrium analysis. SPET scanning with a bolus-infusion protocol is a valid method to estimate the total volume of distribution for [(123)I]CNS 1261 in clinical populations.

A minimally invasive method to evaluate 131I kinetics in blood

The dose limiting factor for 131I therapy in patients with thyroid cancer is myelotoxicity, thus an accurate determination of radioiodine activity in the red marrow is of paramount importance. The reference method for red marrow dosimetry in radiometabolic therapy is based on the measurement of radioiodine kinetics, particularly the activity/time curve in blood. Such a measurement requires withdrawal of blood samples at various times after 131I administration. This procedure involves some potential risk from the radiation protection point of view, such as possible contamination of personnel with blood and disposal of the radioactive blood samples (and syringes). We present here a minimally invasive method to evaluate radioiodine kinetics in the blood, which only requires one blood sample and a set of measurements on the patient's thigh made with a collimated NaI(Tl) probe. The method has been validated in four patients treated with 131I for thyroid cancer.

Radioiodinated SB 207710 as a radioligand in vivo: imaging of brain 5-HT 4 receptors with SPET

Single-photon emission tomography (SPET) and positron emission tomography (PET), when coupled to suitable radioligands, are uniquely powerful for investigating the status of neurotransmitter receptors in vivo. The serotonin subtype-4 (5-HT(4)) receptor has discrete and very similar distributions in rodent and primate brain. This receptor population may play a role in normal cognition and memory and is perhaps perturbed in some neuropsychiatric disorders. SB 207710 [(1-butyl-4-piperidinylmethyl)-8-amino-7-iodo-1,4-benzodioxan-5-carboxylate] is a selective high-affinity antagonist at 5-HT(4) receptors. We explored radioiodinated SB 207710 as a possible radioligand for imaging 5-HT(4) receptors in vivo. Rats were injected intravenously with iodine-125 labelled SB 207710, euthanised at known times and dissected to establish radioactivity content in brain tissues. Radioactivity entered brain but cleared rapidly and to a high extent from blood and plasma. Between 45 and 75 min after injection, the ratios of radioactivity concentration in each of 12 selected brain tissues to that in receptor-poor cerebellum correlated with previous measures of 5-HT(4) receptor density distribution in vitro. The highest ratio was about 3.4 in striatum. SB 207710 was labelled with iodine-123 by an iododestannylation procedure. A cynomolgus monkey was injected intravenously with [(123)I]SB 207710 and examined by SPET. Maximal whole brain uptake of radioactivity was 2.3% of the injected dose at 18 min after radioligand injection. Brain images acquired between 9 and 90 min showed high radioactivity uptake in 5-HT(4) receptor-rich regions, such as striatum, and low uptake in receptor-poor cerebellum. At 169 min the ratio of radioactivity concentration in striatum to that in cerebellum was 4.0. In a second SPET experiment, the cynomolgus monkey was pretreated with a selective 5-HT(4) receptor antagonist, SB 204070, at 20 min before [(123)I]SB 207710 injection. Radioactivity in all brain regions was reduced almost to the level in cerebellum by 176 min after radioligand injection. These findings show that [(123)I]SB 207710 is an effective radioligand for imaging brain 5-HT(4) receptors in vivo.

Comparison of radioiodine biokinetics following the administration of recombinant human thyroid stimulating hormone and after thyroid hormone withdrawal in thyroid carcinoma

Iodine kinetics were studied in patients with differentiated thyroid cancer while euthyroid under exogenous thyroid stimulating hormone (TSH) and while hypothyroid to detect differences in radioiodine uptake, distribution and elimination. Nine patients with total or near-total thyroidectomy on thyroid hormone suppressive therapy received two or three daily doses of 0.9 mg recombinant human TSH (rhTSH) followed by administration of a diagnostic activity of 2 mCi (74 MBq) iodine-131. After the biokinetics assessments had been performed, patients stopped taking thyroid hormones to become hypothyroid. A second 2 mCi (74 MBq) diagnostic activity of 131I was administered, followed by a second set of biokinetics assessments. One week later the patients underwent remnant ablation with a therapeutic activity of 131I. A comparison of the 131I kinetics in the patients while euthyroid and while hypothyroid showed major differences in the doses to the remnant as well as in residence times and radiation exposure to the blood. In the first diagnostic assessment the remnant dose was higher in eight of the nine patients and clearance of the activity from the blood was faster in all of them. The data from this study suggest that radioiodine administration is potent and safe when administered to euthyroid patients following rhTSH administration. Enhanced residence time in the remnant and decreased radiation exposure to the blood were noted when patients were euthyroid compared to when they were rendered hypothyroid. However, all patients received diagnostic activities in the same order: first while euthyroid, followed by hypothyroidism. It is quite possible that "stunning" from the radioiodine administered in the initial uptake study inhibited the subsequent uptake of radioiodine by the remnant lesions in the second uptake study.

Biodistribution of 131I-, 186Re-, 177Lu-, and 88Y-Labeled hLL2 (Epratuzumab) in Nude Mice with CD22-Positive Lymphoma

Radioimmunotherapy (RIT) is a new and effective treatment modality in patients with non-Hodgkin's lymphoma. The monoclonal antibody (mAb) hLL2 (epratuzumab), a humanized mAb directed against the CD22 antigen, and which internalizes, can be labeled with various radionuclides. The biodistribution of hLL2 labeled with (131)I, (186)Re, (177)Lu, and (88)Y was studied in nude mice with subcutaneous human lymphoma xenografts in order to determine the most suitable of these four radionuclides for RIT with hLL2.

METHODS

Human Ramos lymphoma xenografts were transplanted in cyclophosphamide-pretreated athymic BALB/c mice. Four groups of mice were injected intravenously with (131)I-, (186)Re-, (88)Y-, or (177)Lu-labeled hLL2, respectively. To determine the nonspecific tumor uptake, two groups of mice received (88)Y-labeled or (131)I-labeled control antibody, cG250. The biodistribution of the radiolabel was determined 1, 3, and 7 days postinjection (p.i.).

RESULTS

Radiolabeled hLL2 had a higher tumor uptake than the nonspecific mAb at all time-points, irrespective of the radiolabel used. Tumor accretion of (88)Y- and (177)Lu-hLL2 was higher than tumor uptake of (131)I- and (186)Re-hLL2. Activity in the bone, represented by the femur without bone marrow, was higher for (177)Lu- and (88)Y-hLL2 than for (131)I- and (186)Re-hLL2 on day 7 p.i.

CONCLUSION

The use of the residualizing radiolabels (88)Y and (177)Lu in combination with a mAb directed against an internalizing antigen resulted in higher uptake and better retention of the radiolabel in the tumor.

Specific endothelial binding and tumor uptake of radiolabeled angiostatin

Angiostatin (AS) is a potent antiangiogenic agent which inhibits tumor growth through specific action on proliferating endothelial cells. Imaging of radiolabeled AS would enhance our knowledge on the pharmacokinetics of AS and might provide useful information relating to tumor neovasculature. We therefore investigated the potential of radiolabeled AS as a novel tumor imaging agent. Human angiostatin was radioiodine labeled using the lactoperoxidase method. Competition binding studies showed a dose-dependent inhibition of (125)I-AS binding to endothelial cells by excess unlabeled AS, and a displacement curve demonstrated that specific binding was dose dependent and saturable, with a K(d) value of 169 n M. Gel analysis showed that (125)I-AS remained stable in serum for up to 24 h without significant degradation. Intravenously injected (125)I-AS in rats was cleared from the blood in an exponential fashion. Biodistribution data from human colon cancer-bearing Balb/C nude mice showed high uptake in the kidneys, stomach, liver, and lungs. Tumor uptake was 3.2+/-0.7, 2.6+/-0.2, and 1.7+/-0.2%ID/g at 2, 4, and 9 h after injection, respectively. Tumor to muscle count ratio increased from 3.1+/-0.5 at 2 h to 4.4+/-0.5 at 9 h. Serial scintigraphy from 1 to 5 h after (123)I-AS injection demonstrated high uptake in the kidneys and bladder, consistent with renal excretion. There was clear demarcation of tumor by 1 h, with gradual increase in contrast over time (4-h tumor to contralateral thigh ratio =4.7+/-1.1). Thus, radioiodine-labeled angiostatin binds specifically to endothelial cells and has potential as a novel tumor imaging agent.

Influence of acetylcholine levels on the binding of a SPECT nicotinic acetylcholine receptor ligand [123I]5-I-A-85380

Although in vitro theory indicates that ligand binding is sensitive to competition with neurotransmitters, only some imaging ligands have shown such competition in vivo. The purpose of this study was to determine whether increases in acetylcholine (ACh) levels induced by an acetylcholinesterase inhibitor, physostigmine, inhibit in vivo binding of [(123)I]5-iodo-3-(2(S)-2-azetidinyl-methoxy) pyridine (5-I-A-85380), a single photon emission computed tomography ligand for the high-affinity type nicotinic ACh receptor (nAChR). Baboons were used for seven studies with a bolus plus constant infusion equilibrium paradigm. After achieving equilibrium at 5 h, physostigmine (0.02 (n = 1), 0.067 (n = 3), and 0.2 (n = 3) mg/kg) was administered intravenously and data were acquired for up to 8 h. To confirm equilibrium conditions, [(123)I]5-I-A-85380 plasma levels were measured in four studies, including all studies with 0.2 mg/kg physostigmine. Prior to physostigmine administration, thalamic activities were stable, with changes of 1.1%/h or less, except in one study with a gradual increase of 4.2%/h. Thalamic activities were decreased by 15% in one study with 0.067 mg/kg and 14-17% in all studies with 0.2 mg/kg physostigmine administration (P = 0.009). In these studies with 0.2 mg/kg physostigmine administration, [(123)I]5-I-A-85380 plasma levels showed a transient or a sustained increase after physostigmine administration that would have increased thalamic activities. These results suggest that elevated ACh levels induced by physostigmine can effectively compete in vivo with [(123)I]5-I-A-85380 binding at nAChRs. However, decreased thalamic activities could have been caused by other mechanisms, including internalization of the receptor with an associated decreased affinity for radioligand.

Low-molecular-weight polyethylenimine as a non-viral vector for DNA delivery: comparison of physicochemical properties, transfection efficiency and in vivo distribution with high-molecular-weight polyethylenimine

Low-molecular-weight polyethylenimine (LMW-PEI) was synthesized by the acid-catalyzed, ring-opening polymerization of aziridine and compared with commercially available high-molecular-weight PEI (HMW-PEI) of 25 kDa. Molecular weights were determined by size-exclusion chromatography in combination with multi-angle laser light scattering. The weight average molecular weight (M(w)) of synthesized LMW-PEI was determined as 5.4+/-0.5 kDa, whereas commercial HMW-PEI showed a M(w) of 48+/-2 kDa. DNA polyplexes of LMW-PEI and HMW-PEI were characterized with regard to DNA condensation (ethidium bromide fluorescence quenching), size (photon correlation spectroscopy) and surface charge (laser Doppler anemometry). Compared with HMW-PEI, DNA condensation of LMW-PEI was slightly impaired at lower N/P ratios. Complexes with plasmid DNA at a N/P ratio of 6.7 showed significantly increased hydrodynamic diameters (590+/-140 vs. 160+/-10 nm), while the zeta-potential measurements were similar (23+/-2 vs. 30+/-3 mV). The cytotoxicity of LMW-PEI in L929 fibroblasts was reduced by more than one order of magnitude compared with HMW-PEI, as shown by MTT assay. LMW-PEI exhibited increased transfection efficiency in six different cell lines. Reporter gene expression was found to be increased by a factor of 2.1-110. The pharmacokinetics and biodistribution of 125I-PEI in mice were similar for both molecular weights with an AUC of ca. 330+/-100% ID/ml min. Approximately half of the injected dose accumulated in the liver. LMW-PEI proved to be an efficient gene delivery system in a broad range of cell lines. Due to differences in polyplex structure, as well as its relatively low cytotoxicity, which makes the application of high N/P ratios possible, LMW-PEI appears to possess advantageous qualities with regard to transfection efficiency over PEI of higher molecular weight.

Radiolabeled monoclonal antibodies specific to the extracellular domain of prostate-specific membrane antigen: preclinical studies in nude mice bearing LNCaP human prostate tumor

Prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein, is highly expressed by virtually all prostate cancers. PSMA is also expressed on the tumor vascular endothelium of virtually all solid carcinomas and sarcomas but not on normal vascular endothelium. PSMA is currently the focus of several diagnostic and therapeutic strategies. We have previously reported on the radiolabeling and in vitro binding properties of monoclonal antibodies (mAbs) (J415, J533, and J591) that recognize and bind with high affinity to the extracellular domain of PSMA (PSMA(ext)). This article reports on the in vivo behavior and tumor uptake of (131)I- and (111)In-labeled antiPSMA(ext) mAbs (J415, J533, and J591) and their potential utility for radioimmunotherapy.

METHODS

In nude mice bearing PSMA-positive human LNCaP tumors, the pharmacokinetics, biodistribution, and tumor uptake of these antibodies was compared with (111)In-7E11 mAb, specific to the intracellular domain of PSMA (PSMA(int)). Autoradiographic studies were done to identify intratumoral distribution of radiolabeled mAbs.

RESULTS

With (131)I-labeled antibodies, the net tumor retention of radioactivity by day 6 was significantly higher with J415 (15.4% +/- 1.1%) and 7E11 (14.5% +/- 1.7%) than with J591 (9.58% +/- 1.1%). By contrast, the tumor uptake of (111)In-1,4,7,10-tetraazacyclododecane-N,N',N", N"'-tetraacetic acid-labeled J415 and J591 gradually increased with time and was quite similar to that of 7E11. In addition, the blood clearance of (111)In-labeled J415 and J591 antibodies was relatively faster than that of radiolabeled 7E11. As a consequence, the tumor-to-blood ratios with J415 and J591 were higher than that of 7E11. The localization of radiolabeled anti-PSMA(ext) antibodies in PSMA-positive LNCaP tumors was highly specific because the tumor uptake of (131)I-labeled J415 and J591 was more than twice that of a nonspecific antibody. Furthermore, the tumor uptake of (131)I-J591 was almost 20 times higher in PSMA-positive LNCaP tumors than in PSMA-negative PC3 and DU145 tumor xenografts. Autoradiographic studies suggested that 7E11 (anti-PSMA(int)) distinctly favors localization to areas of necrosis whereas J415 and J591 (anti-PSMA(ext)) demonstrated a distinct preferential accumulation in areas of viable tumor.

CONCLUSION

These results clearly demonstrate that PSMA-specific internalizing antibodies such as J415 and J591 may be the ideal mAbs for the development of novel therapeutic methods to target the delivery of beta-emitting radionuclides ((131)I, (90)Y, and (177)Lu) for the treatment of PSMA-positive tumors. In addition, because J591 and J415 mAbs are specific to PSMA(ext), thus targeting viable tumor, these immunoconjugates are better candidates for targeted radioimmunotherapy than are antibodies targeting PSMA(int).

Uptake of radiolabelled herceptin by experimental mammary adenocarcinoma

BACKGROUND

The aim of this study was to examine the biodistribution of (131)I-herceptin in C3H/Bi mice with transplantable mammary adenocarcinoma with a high frequency of C-erbB2 receptor expression.

MATERIAL AND METHODS

Mice C3H/Bi with subcutaneously transplanted mammary adenocarcinoma were used as animal model to study the interaction between C-erbB2 receptor and hercepin, a humanized anti-C-erbB2 monoclonal antibody. The expression of the gene encoding C-erbB2 receptor in the tumours was studied by the RT-PCR technique.

RESULTS

Expression of this gene was found in 66% of the studied cases. Similarly, the presence of the C-erbB2 receptor in 77% of the tumours was detected by a Western blot analysis with the use of herceptin. Biodistribution experiments of iodine-labelled herceptin in mice C3H/Bi with adenocarcinoma revealed its maximal accumulation in the tumours at 48 hours since the i.v. injection (7% ID/g). The tumour/muscle radioactivity ratio reached its highest value (above 20) also at 48 hours after the injection.

CONCLUSIONS

C3H/Bi mice with this adenocarcinoma may be a good experimental model to study herceptin, or its fragments, labelled with different radionuclids for preliminary evaluation of their usefulness in the therapeutic and diagnostic aspects of breast cancer.

Evaluation of DNA aptamers directed to thrombin as potential thrombus imaging agents

Two DNA aptamers directed against two separate exosites on human alpha-thrombin were evaluated for thrombus-imaging potential. Aptamer ODN 1 is directed to the thrombin substrate binding site (exosite 1). Our finding that ODN 1 competes with fibrin for binding to exosite 1 on thrombin suggests that ODN 1 will not be useful for thrombus imaging. Aptamer ODN 2 is directed against the thrombin heparin binding site (exosite 2). ODN 2 bound to model thrombi that were formed either by clotting purified fibrinogen with thrombin, or by recalcifying citrated plasma. As the thrombin content of thrombi was increased the rate of ODN 2 uptake into preformed thrombi increased, whereas the rate of release of ODN 2 out of preformed thrombi decreased. This in vitro data suggested that ODN 2 might be useful for thrombus imaging because it can bind to exosite 2 on fibrin-bound thrombin. However, in a rabbit jugular vein model using thrombus supplemented with human thrombin, ODN 2 uptake was equal to the ovalbumin control, and did not reflect thrombin content. While the in vitro results with ODN 2 were consistent with thrombus imaging, the rapid clearance of ODN 2 from circulation, combined with slow mass transfer in the clot, seem to work against in vivo thrombin-dependent imaging or washout analysis.

Development of radioimmunoassay for the measurement of human leptin in serum

BACKGROUND

Leptin is a 16 kDa polypeptide hormone encoded by the obese gene (ob) and secreted by adipose tissue. This hormone plays a major role in energy homeostasis and regulation of food intake and body weight. It also affects the metabolic, neuroendocrine and reproductive systems.

MATERIAL AND METHODS

Labelling of recombinant human leptin with (125)I was best performed by the Chloramine-T method. New Zealand white rabbits were immunised with recombinant human leptin, cross-reaction of obtained antisera was analyzed with 10 different antigens. The separation of bound and free fractions was performed using the second antibody - PEG method.

RESULTS

The obtained tracer had specific activities of 2.8-3.3 kBq/ microg and had a stability of 5 weeks. A highly specific polyclonal antibody was obtained without measurable cross-reaction against the analysed antigens. Concentrations of human leptin were measured by a single overnight incubation assay with a sensitivity of 0.5 ng/ml and a measuring range of 0.5-100 ng/ml. The intra-assay and inter-assay coefficient of variation was under 6% and 8%, respectively. Recovery ranged from 88% to 106%.

CONCLUSIONS

Serum human leptin concentrations can be accurately and precisely measured by this new radioimmunoassay. Preliminary results obtained from the measurement of serum leptin in lean, overweight and obese patients are presented. Serum leptin concentrations correlated with body mass index and were significantly higher in women than in men, except for obese patients.

Diagnostic sensitivity of two radio receptor assays (TRAK Assay and TRAK Dyno Human) for detection of TSH receptor antibodies

Radio receptor assays for the detection of TSH receptor antibodies in serum are typically based on binding the competition of TSH-R antibodies and (125)I "labeled" TSH for membrane preparation of thyrocytes (TBII tests). The sensitivity of the available tests utilizing porcine cell membranes was found to be around 80%. A new test (TRAK Dyno human, BRAHMS) utilizes human recombinant TSH receptor and human standard material that is supposed to improve the performance of the test. We have compared the results of these two assays. The sensitivity of the TRAK Assay tested in 356 patients with untreated Graves' disease was found to be 85%, and 97.5% for TRAK Dyno human in 111 newly diagnosed patients. Both tests were performed from the same serum specimen for 60 of the investigated patients. The TRAK Assay was positive in 50 patients (83.2%) and TRAK Dyno human in 59 patients (98.3%). The specificity of the new radio receptor assay was also improved.

Biodistribution and kinetics of (131)I-labelled anti-CD20 MAB IDEC-C2B8 (rituximab) in relapsed non-Hodgkin's lymphoma

The native chimeric human-mouse anti-CD20 antibody IDEC-C2B8 (rituximab) is therapeutically applied in relapsed non-Hodgkin's lymphoma (NHL). The purpose of this study was to evaluate the distribution and pharmacokinetics of iodine-131 labelled rituximab in humans for radioimmunotherapy of relapsed CD20-positive NHL. Thirty-five patients with relapsed NHL were administered 20-40 mg rituximab labelled with 250 MBq (131)I. Biodistribution was determined by the gamma camera whole-body scans, whole-body probe measurements and the analysis of serial blood and urine samples. Dosimetry was performed using the MIRDOSE 3 program. Antibody administration was well tolerated. The whole-body activity showed a mono-exponential decrease with a wide range of effective half-lives, the mean value (88 h) being significantly longer than the half-life of its murine counterpart, tositumomab. This led to appropriately higher dose factors for the whole body and organs. Activity was excreted mainly through the kidneys. Normal organs showed decreasing ratios of organ to whole-body activity over time, whereas the tumour tissue presented different kinetics, with increasing ratios of tumour to whole-body activity as evidence for specific antibody binding. It is concluded that (131)I-labelled rituximab is suitable for pretherapeutic dosimetry. Due to the wide range of whole-body and organ dose factors, individual dosimetry is necessary for radioimmunotherapy with (131)I-labelled rituximab. The therapeutic activities of (131)I-labelled rituximab required to deliver similar doses should be lower than those of its murine counterpart.

Diagnostic 131-iodine whole-body scan may be avoided in thyroid cancer patients who have undetectable stimulated serum Tg levels after initial treatment

The follow-up of differentiated thyroid cancer after total thyroidectomy and thyroid ablation is commonly based on serum Tg determination and 131-iodine ((131)I) diagnostic whole-body scan (WBS) performed in the hypothyroid state, 6-12 months after thyroid ablation. Based on the greater sensitivity of Tg measurement, with respect to WBS, the diagnostic yield of diagnostic WBS has been questioned in patients who are off L-T(4) therapy and have undetectable Tg levels. The aim of the present retrospective study was to evaluate the diagnostic relevance of (131)I WBS performed after thyroid remnant ablation, in patients with undetectable serum Tg and off thyroid hormone therapy. The study included 315 of 662 consecutive patients (47.6%) treated in our department between 1980 and 1990, who, at the first control WBS after thyroid ablation, had undetectable serum Tg levels in the hypothyroid state. There were 54 men (17%) and 261 women (83%), with a mean age of 40.9 +/- 13.1 yr (range, 12-76), followed for a mean of 12 +/- 2.8 (range, 9-19) yr. The control WBS was negative in 225 (71.4%) patients and positive for persistent areas of thyroid bed uptake, frequently of very low significance, in 90 (28.6%). No local or distant metastases were discovered. At the last follow-up visit (1999-2000), 281 (89.2%) patients showed complete remission, with undetectable serum Tg off L-T(4) and negative WBS. Persistent thyroid bed uptake, with undetectable levels of Tg, was observed in 29 patients (9.2%) studied during L-T(4) withdrawal. Only 2 patients (0.6%) experienced local recurrence (lymph-node metastases) during their follow-up. In conclusion, our data suggest that the presence of undetectable levels of serum Tg off L-T(4) at the time of the first control WBS after initial treatment, is highly predictive of complete and persistent remission. With the exception of detecting persistent thyroid bed uptake in a minority of cases, the control WBS has never given information that could influence the following therapeutic strategy. On this basis, we propose that the diagnostic (131)I WBS may be avoided in patients with undetectable levels of Tg off L-T(4). These patients may be monitored with clinical examination, neck ultrasound, and serum Tg measurements on L-T(4).

Improved renal clearance and tumor targeting of 99mTc-labeled anti-Tac monoclonal antibody Fab by chemical modifications

This study was undertaken to improve the renal clearance and tumor targeting properties of 99mTc-labeled humanized anti-Tac (HuTac) monoclonal antibody Fab fragments using two chemical approaches: 1) labeling with a renal secretion agent 99mTc-mercaptoacetyltriglycine (MAG3) and 2) lowering its isoelectric point (pI) by acylation. HuTac Fab (3.3 mg/mL) was reacted with a trifluorophenyl ester (TFP) of 99mTc-MAG3 alone or was additionally reacted with TFP-glycolate to reduce the pI. In Balb/c mice, 99mTc-MAG3-Fab (pI > 9.3) rapidly accumulated in the kidneys (177% injected dose [ID]/g at 15 min) and then gradually cleared out of the kidneys. In contrast, the glycolation (pI 4.6 approximately 6.6) drastically reduced the renal uptake (31% ID/g) and also the whole-body retention (82% ID vs 101% for the nonglycolated) at 15 min, indicating that the glycolated 99mTc-MAG3-Fab (pI 4.6 approximately 6.6) was rapidly excreted. The glycolated remained in the blood longer than the nonglycolated (1.2% vs 0.3% ID/g at 360 min), but this effect was less drastic than the effect shown on the renal uptake. In nude mice bearing receptor-positive (ATAC4) tumors, the glycolated 99mTc-MAG3-Fab increased the peak tumor uptake to 14.8% ID/g from 8.3% ID/g for 99mTc-MAG3-Fab, whereas the glycolation resulted in a drastic reduction of the renal uptake at 15 min. We demonstrated that the renal clearance and the tumor targeting of Fab could be optimized by chemical modifications.

Gel electrophoresis-autoradiographic image analysis of radiolabeled protein drug concentration in serum for pharmacokinetic studies

INTRODUCTION

The purpose of this study was to evaluate the feasibility of using gel electrophoresis combined with autoradiographic image analysis for quantitating protein drug concentrations in biological fluid for pharmacokinetic studies.

METHODS

Protein drugs were iodinated using the Iodogen reagent and injected into Sprague-Dawley rats for pharmacokinetic evaluation. Serum samples were analyzed using trichloroacetic acid (TCA)-precipitable counts or sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Commercially available precasted Bis-Tris gradient gels were used for SDS-PAGE. Autoradiography of gel samples was performed using phosphoimager and quantitated using the ImageQuant software.

RESULTS

The maximum loading volume for protein drugs with molecular weight close to that of albumin ( approximately 70 kDa) was about 1 microl, whereas for protein drugs with larger or smaller molecular weight (i.e., >80 or <40 kDa), the maximum loading volume was up to 20 microl/lane. The optimal exposure time was about 18 h or overnight. Standard curves were constructed using serially diluted dosing solution, which was linear over a 10-fold concentration range with a correlation coefficient of.98. Comparing to the soluble human interleukin-13 receptor (shIL-13R) pharmacokinetic profiles from TCA-precipitable counts, the quantitative gel analysis revealed lower concentrations at later time points and a lower bioavailability from intraperitoneal injection.

DISCUSSION

This study provided the first systemic evaluation of gel electrophoresis technology for quantitative protein drug determination in serum and its application in pharmacokinetic studies. The combination of gel electrophoresis with autoradiographic image analysis provided accurate and specific quantitation results. The overnight turnover time allowed routine application in preclinical pharmacokinetic studies.

Biokinetics of a F(ab')3 iodine-131 labeled antigen binding construct (Mab 35) directed against CEA in patients with colorectal carcinoma

An 131I labeled trivalent antigen binding construct, formed from 3 Fab' fragments of murine anti-CEA monoclonal antibody (Mab) 35, has shown favorable biokinetics in animal studies.

OBJECTIVES

The aim of this study was to evaluate biodistribution and tumor uptake of 131I-F(ab')3 in patients and its potential utility for radioimmunotherapy of CEA expressing tumors.

PATIENTS AND METHODS

Six patients (5 M, 1 F; age 62 +/- 13 y) with liver metastases of colorectal cancer, scheduled for hepatic surgery were studied by 2-3 whole body scans immediately post infusion of 111-137 MBq of 131I labeled Mab 35 F(ab')3 and up to 72 h. Circulating CEA ranged from 1.2 to 1930 ng/ml. We evaluated plasma and whole body clearance, activity accumulation by post-surgical ex-vivo tissue measurement in primary tumor (T) and metastases (M), and calculated M to blood (M/B) and M to liver (M/L) ratios.

RESULTS

All known tumor sites were detected by immunoscintigraphy and confirmed at surgery. Whole body effective T1/2 calculated in two patients was 51.5 h and 55.6 h respectively. Effective serum T1/2 was mono-exponential in 3 patients (short observation interval) with 20.9 +/- 7 h and bi-exponential in three with alpha T1/2 of 6.3 +/- 1 h and beta T1/2 of 38.6 +/- 5 h. In a patient with concomitant colic and hepatic lesions uptake of primary tumor was 0.0071% injected dose per gram of tissue (%ID/g) and mean metastases activity was 0.0275 %ID/g at 48 h. In the 3 patients who had surgery at 48 h, mean uptake in metastases and normal liver was 0.0182 %ID/g and 0.0021 %ID/g, respectively (M/L 8.67). In the single subject followed until 7 days post infusion, residual activity in liver metastases was 10 times higher than in normal parenchyma.

CONCLUSIONS

Tumor uptake and tumor to blood ratio, as well as serum clearance of the triconstruct are similar to those observed with intact iodinated anti-CEA antibodies. In the patient studied for 7 days the tumor residence time was favorable. Further improvements, however, need to be obtained before considering this approach for radioimmunotherapy.

Simplified methods for estimation of 99mTc-pentetate and 131I-orthoiodohippurate plasma clearance in dogs and cats

The purpose of this study was to evaluate simplified methods for estimation of Technetium Tc 99m (99mTc)-pentetate and orthoiodohippurate I 131 (131I-OIH) plasma clearance in dogs and cats with 1 and 2 blood samples. Plasma clearances were calculated after a bolus injection of 1.85-11.1 MBq of 99mTc-pentetate and 131I-OIH with a 2-compartment model based on a 12-point curve as a reference method in 21 dogs and 18 cats. Three 2-sample and 3 single-sample methods were investigated. The method yielding the smallest standard deviation of the difference between the reference method and the simplified method was selected as the optimal one. Linear regression analysis was performed between the reference method and the simplified method and coefficient of determination (R2) was calculated. For 99mTc-pentetate plasma clearance, the optimal 2-sample method was the one with a mono-compartment model with samples taken at specific times. For 131I-OIH plasma clearance, the estimation was improved slightly by raising the clearance calculated with a mono-compartment model to the power of an empirically determined parameter. The optimal single-sample method was the one with a linear quadratic regression between the volume of distribution of the tracer at a specific time and the clearance calculated with 12 samples. Two-sample methods performed significantly better than did single-sample methods. The conclusion is made that 99mTc-pentetate and 131I-OIH plasma clearances can be estimated in dogs and cats with 1 or 2 blood samples with a reasonable margin of error compared to plasma clearances calculated with a 2-compartment model and 12 blood samples.

Value of protein-bound radioactive iodine measurements in the management of differentiated thyroid cancer treated with (131)I

Measurement of the protein-bound radioactive iodine level (PBI(131)) in the plasma of patients following (131)I-iodide administration for thyroid cancer has been re-examined in a retrospective study of 171 patient episodes. It is shown that whereas the previously used threshold value for the measurement at 6 days does not correlate well with the 3-day whole body scan, there is good agreement between the scan and the temporal changes in PBI(131) from 1-6 days: an increasing PBI(131) correlates with a positive scan, and a decreasing PBI(131) with a negative scan. The area under the curve (AUC) for the PBI(131)-time curve is related to the absorbed dose for the tumour. For a small group of 11 patients, dosimetry estimates were made from serial scans, quantified with phantoms; these absorbed doses correlated with the AUC and the 6-day PBI(131). Therefore, it is suggested that these parameters may be useful in predicting absorbed radiation dose in these patients.