The clinical significance of unusual sites of thallium-201 uptake

To chelate thallium(I) - synthesis and evaluation of Kryptofix-based chelators for 201Tl

While best known for its toxic properties, thallium has also been explored for applications in nuclear diagnostics and medicine. Indeed, [201Tl]TlCl has been used extensively for nuclear imaging in the past before it was superceded by other radionuclides such as 99mTc. One reason for this loss of interest is the severe lack of suitable organic chelators able to effectively coordinate ionic forms of Tl and deliver it to specific diseased tissue by means of attached biological vectors. Herein, we describe the synthesis and characterisation of a series of Kryptofix 222-based chelators that can be radiolabelled with 201Tl(I) in high radiochemical yields at ambient temperature. We demonstrate that from these simple chelators, targeted derivatives are readily accessible and describe the synthesis and preliminary biological evaluation of a PSMA-targeted 201Tl-labelled Kryptofix 222-peptide conjugate. While the Kryptofix system is demonstrably capable of binding the thallium cation, no PSMA-mediated cell-uptake could be detected with the PSMA conjugate, suggesting that this targeting moiety may not be ideal for use in conjunction with 201Tl.

In vitro proof of concept studies of radiotoxicity from Auger electron-emitter thallium-201

BACKGROUND

Auger electron-emitting radionuclides have potential in targeted treatment of small tumors. Thallium-201 (201Tl), a gamma-emitting radionuclide used in myocardial perfusion scintigraphy, decays by electron capture, releasing around 37 Auger and Coster-Kronig electrons per decay. However, its therapeutic and toxic effects in cancer cells remain largely unexplored. Here, we assess 201Tl in vitro kinetics, radiotoxicity and potential for targeted molecular radionuclide therapy, and aim to test the hypothesis that 201Tl is radiotoxic only when internalized.

METHODS

Breast cancer MDA-MB-231 and prostate cancer DU145 cells were incubated with 200-8000 kBq/mL [201Tl]TlCl. Potassium concentration varied between 0 and 25 mM to modulate cellular uptake of 201Tl. Cell uptake and efflux rates of 201Tl were measured by gamma counting. Clonogenic assays were used to assess cell survival after 90 min incubation with 201Tl. Nuclear DNA damage was measured with γH2AX fluorescence imaging. Controls included untreated cells and cells treated with decayed [201Tl]TlCl.

RESULTS

201Tl uptake in both cell lines reached equilibrium within 90 min and washed out exponentially (t1/2 15 min) after the radioactive medium was exchanged for fresh medium. Cellular uptake of 201Tl in DU145 cells ranged between 1.6 (25 mM potassium) and 25.9% (0 mM potassium). Colony formation by both cell lines decreased significantly as 201Tl activity in cells increased, whereas 201Tl excluded from cells by use of high potassium buffer caused no significant toxicity. Non-radioactive TlCl at comparable concentrations caused no toxicity. An estimated average 201Tl intracellular activity of 0.29 Bq/cell (DU145 cells) and 0.18 Bq/cell (MDA-MB-231 cells) during 90 min exposure time caused 90% reduction in clonogenicity. 201Tl at these levels caused on average 3.5-4.6 times more DNA damage per nucleus than control treatments.

CONCLUSIONS

201Tl reduces clonogenic survival and increases nuclear DNA damage only when internalized. These findings justify further development and evaluation of 201Tl therapeutic radiopharmaceuticals.

Identification of AIDS-related tuberculosis with concordant gallium-67 and three-hour delayed thallium-201 scintigraphy

Concordant gallium-67 and thallium-201 uptake has been described in malignant lesions. More recently, 201Tl accumulation has been described in some benign conditions. The authors report three HIV-positive patients who underwent 67Ga and 201Tl scintigraphy. These studies revealed concordant 67Ga and 201Tl uptake and tumour was erroneously diagnosed. All three patients were finally diagnosed as having tuberculosis.

Evaluation of palpable breast masses with 201Tl scintigraphy

39 female patients (age range: 31-84 years) with palpable breast masses detected by physical examination, underwent 201Tl scintigraphy in order to assess its value in the detection of breast carcinomas and to differentiate them from benign breast masses. Planar images were carried out at 20-30 min and 2-3 h after intravenous administration of 111-185 MBq (3-5 mCi) of 201Tl chloride. In 12 patients single photon emission tomography (SPET) studies were also performed. In 18 patients the scintigraphic studies were positive and in 17 of these cases, breast carcinomas were confirmed. Tumour sizes ranged from 1.3 to 6 cm in diameter. In the remaining patient a false positive result was obtained where there was benign breast change. In three of seven cases, malignant axillary nodes were also detected. All 21 patients with negative scintigraphy had benign breast lesions. There were no differences between images obtained at 20-30 min and 2-3 h or between planar images and SPET studies. In 10 patients there was disagreement between mammography and 201Tl scans. 201Tl scans confirmed the presence of carcinoma in three cases and discarded malignancy in the other six cases. In the remaining case, 201Tl scan was false positive. 201Tl scintigraphy is useful in distinguishing malignant from benign breast masses, even when compared with mammography.

Sensitivity and specificity of thallium-201 single-photon emission tomography in the functional detection and differential diagnosis of brain tumours

The aim of this retrospective study was to assess the contribution of thallium-201 single-photon emission tomography (SPET) in the detection and differential diagnosis of brain tumours. In 90 patients 201Tl SPET was performed because of clinical or radiological suspicion of tumoral invasion, completed by technetium-99m hexamethylpropylene amine oxime and 99mTc-sestamibi SPET in some patients. For all tumours, diagnosis was based on biopsy or autopsy. Other diagnoses were made only after clinical and radiological follow-up for at least 6 months. Histologically tumours consisted of astrocytoma stage I or II (number of patients, n = 6), astrocytoma stage III (n = 8), glioblastoma multiforme (n = 14) and oligodendroglioma (n = 3), brain metastasis (n = 14), lymphoma (n = 3), meningioma (n = 3), pituitary adenoma (n = 2), pineal tumour (n = 1), colloid cyst (n = 1) and craniopharyngioma (n = 1). False-negative studies included pineal tumour (n = 1), colloid cyst (n = 1), craniopharyngioma (n = 1), astrocytomas stage I or II (n = 6) and stage III (n = 3), oligodendroglioma (n = 2) and metastasis in the brain stem (n = 1). Additional metastases approximately < 1.5 cm were not detected in two patients and 201Tl SPET underestimated tumoral extent in one patient suffering from glioblastoma multiforme (n = 1). A false-positive study was obtained in a patient with skull metastasis (n = 1). All 15 patients who were finally shown to suffer from ischaemic infarction had a normal SPET study 9-28 days after the onset of symptomatology. Of five patients with haemorrhagic infarction, studied within 2 weeks, four were false-positive. Of six patients with intracranial haemorrhage, studied 9-39 days later, one showed focal 201Tl accumulation. Two further false-positive studies consisted of angioma and epidural haematoma. Finally, SPET studies were normal in six patients with definite diagnosis of (reactive) gliosis (n = 3), Binswanger's encephalopathy (n = 1), postinfectious encephalopathy (n = 1) and multiple sclerosis (n = 1). In the patient population presented, sensitivity of 201Tl SPET for supratentorial brain tumours was 71.7% and specificity was 80.9%. Clinical information and control SPET studies in combination with early, 30-min and 3- to 4-h delayed imaging may be expected to improve on these figures. On the other hand it seems that, in addition to tumoral histology, the presence of tumours in the fossa posterior and small volumes contribute to the occurrence of false-negative 201Tl SPET studies.

The role of functional imaging in neoplasms of the thyroid

Iodine-123 and Iodine-131 have suitable physical properties that enable them to be used for functional imaging of the thyroid. Iodine-123 is used for routine testing for hyperthyroidism and thyroid nodules, whereas I-131, which has a longer half-life, is used for whole body imaging for detecting metastatic thyroid cancer in patients who have undergone thyroidectomy. The radionuclides of iodine are trapped and organified like nonradioactive iodine. In contrast, technetium as pertechnetate is trapped by the thyroid and can be used for imaging immediately after intravenous injection. There can be differences in scintiscans made in the same patient using radioiodine vs. technetium. Thallium is a useful adjuvant imaging agent for thyroid cancer. It should be stressed that correlation with the clinical findings and biochemical thyroid function tests are very important when evaluating thyroid scintiscans.