DNA damage induced by a new 2-chloroethyl nitrosourea on malignant melanoma cells.

Different biological aspects of a novel 2-chloroethyl nitrosourea derived from cysteamine, N'-(2-chloroethyl)-N-[2-(methylsulfinyl)ethyl]-N'- nitrosourea (CMSOEN2), were studied. Drug-induced cytotoxic effects, uptake kinetics, DNA damage, and O6-alkylguanine-DNA alkyltransferase activity were determined in 3 melanoma cell lines: the murine B16 and 2 human metastatic-derived cell lines (M4 Beu and M3 Dau). We found that radioactivity uptake and incorporation in acido-precipitable material was inversely proportional to cell drug viability. The highly CMSOEN2-sensitive B16 line showed the lowest total radioactivity uptake. In fact, among the melanoma cell parameters studied, 3 of them were well correlated: (a) cytotoxicity as reflected by the colony-forming assay; (b) DNA cross-link frequency estimated by the alkaline elution technique; and (c) O6-alkylguanine-DNA alkyltransferase activity (Mer phenotype), defined as the ability of cell extracts to remove O6-methylguanine from N-methyl-N-nitrosourea-alkylated DNA. The 2 human cell lines (M4 Beu and M3 Dau), the most resistant to the cytostatic drug effects, showed little or no ability to form DNA lethal cross-links. These results correspond to the higher O6-alkylguanine-DNA alkyltransferase activity found in human-derived cell lines compared with that present in murine B16 cell lines. This study confirms that the cell content in this repair DNA protein is certainly one of the important factors implicated in the variability of response to 2-chloroethyl nitrosourea treatment observed in a number of established malignant cell lines. It has been shown that pretreatment of derived cell lines with methylating agents (N-methyl-N-nitrosourea, N-methyl-N'-nitro-N-nitrosoguanidine) or O6-methylguanine used as a free base, increased cytotoxic effects of this class of anticancer agents, likely by saturating receptor sites (sulfhydryl groups) of this specific DNA repair enzyme. Nevertheless, in preliminary Phase I and II clinical trials, 2 patients who had been treated with multiple chemotherapies including alkylating agents [1-(2-chloroethyl)-3- cyclohexyl-1-nitrosourea, 5-(3,3-dimethyl-1-triazeno)-imidazole-4-carboxamide, platinum derivatives], presented complete or partial remission after CMSOEN2 treatment. Our results raise the question of the exact relation between the Mer phenotype determined in derived murine or human cultured cells and that directly observed on surgically excised tumors in cancer patients. The original Mer phenotype could be modified by cell culture conditions since it has been shown that O6-alkylguanine-DNA alkyltransferase activity is widely distributed between normal and tumoral tissues without any real difference.

A potential melanoma tracer: synthesis, radiolabeling, and biodistribution in mice of a new nitridotechnetium bis(aminothiol) derivative pharmacomodulated by a N-(diethylaminoethyl)benzamide.

Radioiodobenzamides are the best-known agents under study for the diagnosis of cutaneous melanoma and its metastases. We report the synthesis of a new BAT derivative radiopharmaceutical in which radioiodine is replaced by 99m-technetium. The cyclic intermediary methyl 4-[3-(4,4,7,7-tetramethyl-5,6-dithia-2, 9-diazacyclodecyl)-2-oxapropyl]benzoate (5) occurred in two different conformations identified by spectroscopic analysis. The final BAT ligand was radiolabeled using the nitridotechnetium core by a ligand-exchange reaction. Two different complexes were purified. After macroscopic 99-technetium synthesis, syn and anti isomers were identified. The global radiochemical yield was over 80%. The biodistribution of these two complexes was evaluated in mice bearing murine B16 melanoma. Extensive liver and kidney uptake was observed, but the benzamide tropism for the tumor was partially preserved.

O6-(alkyl/aralkyl)guanosine and 2'-deoxyguanosine derivatives: synthesis and ability to enhance chloroethylnitrosourea antitumor action.

A series of O6-(alkyl/aralkyl)guanosines and 2'-deoxyguanosine analogs extended to peracetyl and N2-acetyl derivatives, potentially water soluble, was synthesized. Each was associated with N'-(2-chloroethyl)-N-[2-(methylsulfonyl)ethyl]-N'-nitrosourea for in vitro evaluation on M4Beu melanoma cells of their ability to enhance the cytotoxic effect of this chloroethylnitrosourea, which is frequently reduced by repairs performed by O6-alkylguanine-DNA-alkyltransferase. Structure-activity analysis revealed that (i) benzyl and 4-halobenzyl are the O6-substituents required to afford a significant activity, (ii) 2'-deoxyguanosine derivatives demonstrate greater potency than guanosine analogs, (iii) acetylation, especially at the N2 position, generally results in compounds with moderate ability but may prevent incorporation of such nucleosides into DNA. Accordingly, O6-(4-iodobenzyl)-N2-acetylguanosine (3b) and O6-benzylperacetyl-2'-deoxyguanosine (2a), as well as O6-benzyl-N2-acetylguanosine (1b) and O6-benzyl-N2-acetyl-2'-deoxyguanosine (2b), by far the most water soluble, exhibit a good profile for further in vivo trials by the intravenous route.

Reorientation of the left ventricular long-axis on myocardial transaxial tomograms by a linear fitting method.

A method is described for reorientating the left ventricular (LV) long-axis from myocardial transaxial tomographic data. On a midventricular transverse slice and on a midventricular sagittal slice, the apical and basal limits are selected successively by the operator. The linear activity profiles between these two limits are plotted line by line. In each profile, the two points with the maximum counts in the septal and lateral walls on the transverse slice, or in the anterior and inferior walls on the sagittal slice, are detected. The intermediate point with the minimum counts is then determined. The set of points with minimum counts are fitted by a straight line using the least squares method. This line is taken as the LV long-axis. In a series of 15 cases with stress-delayed 201Tl SPECT, the reproducibility of the reorientation with this semi-automatic method was compared with manual selection of the LV long-axis. In all patients, a successful reorientation was obtained with the present method. The reproducibility was significantly better with the semi-automatic method than with the manual selection of the LV long-axis.

Dissolution rate and transit times of technetium-99m DTPA-labeled tablets.

In this study we demonstrate that the dissolution rate and gastroduodeno-cecal transit time of radiolabeled tablets of theophylline can be determined in vivo using technetium-99m (99mTc). Six healthy male volunteers ingested a tablet containing 300 mg of theophylline mixed with 3.7 MBq of [99mTc]DTPA. Anterior and posterior scintigraphic views of the abdomen were collected serially over 8 hr, after which a 200-ml solution containing 37 MBq of [99mTc] pertechnetate was ingested in order to visualize the contours of the stomach. The in vivo activity contained in the tablet was calculated from the scintigraphic views after correction of background activity, radioactive decay, and depth attenuation. The dissolution rate of [99mTc] DTPA was also measured in vitro and compared with the dissolution rate of theophylline. The results showed close dissolution rates between [99mTc]DTPA and theophylline in vivo (T1/2 184 min and 176 min, respectively), and a faster early dissolution rate of [99mTc]DTPA in vitro (T1/2 92 min versus 156 min for theophylline). The mean gastroduodenal and duodenocecal times were 72 +/- 25 min (m +/- s.d.) and 245 +/- 15 min, respectively. Scintigraphic imaging of labeled formulations with 99mTc present useful applications in pharmaceutics and pharmacology.

In vitro uptake of technetium-99m-teboroxime in carcinoma cell lines and normal cells: comparison with technetium-99m-sestamibi and thallium-201.

Since 201Tl, 99mTc-sestamibi and 99mTc-teboroxime concentrate in cardiac cells through different mechanisms, we compared their uptake in cultured normal cells and carcinoma cell lines in order to define their possible use for tumor evaluation in vivo. Four lines of normal cells from animals, including myocytes from newborn rats, and four lines of human carcinoma cell lines were incubated for 1 hr with 37 kBq of either tracer. Results, expressed in percent of the total activity taken up by 1 million cells, showed a 9% difference between the uptake of teboroxime by normal and carcinoma lines (24.6% +/- 2.8% versus 22.5% +/- 2.1%, respectively, p < 0.05). Mean uptake was 80% higher in tumor than in normal cells for 201Tl (5.39% +/- 1.33% versus 3.00% +/- 1.08%, respectively, p < 0.001) and nearly 4 times higher for sestamibi (5.37% +/- 2.34% versus 1.44% +/- 1.88%, p < 0.001). For both agents, uptake by the myocytes and carcinoma cells was comparable (5.14% +/- 0.11% for 201Tl and 5.28% +/- 1.03% for sestamibi). When the myocytes are excluded from the group of normal cells, the uptake is 112% higher in tumor than in normal cells for 201Tl (5.39% +/- 1.33% versus 2.54% +/- 0.44%, p < 0.001) but it becomes nearly nine times higher for sestamibi (5.37% +/- 2.34% versus 0.60% +/- 0.23%, p < 0.001). It is concluded that these experiments show that the uptake of sestamibi was the most discriminant to separate between normal and malignant cells, while teboroxime was the less discriminant. Potential clinical applications for tumor visualization based on differences in sestamibi and teboroxime uptake could be envisioned.

Uptake of technetium-99m-teboroxime in cultured myocardial cells: comparison with thallium-201 and technetium-99m-sestamibi.

The effects of metabolic inhibition on the uptake of 99mTc-teboroxime were assessed in cultured myocardial cells and compared with 201Tl and 99mTc-sestamibi. Metabolic impairment was induced by cyanide (CN), a blocker of the mitochondrial respiratory chain, iodoacetate (IAA), an inhibitor of the glycolytic pathway, and ouabain, an inhibitor of Na(+)-K+ sarcolemmal ATPase. Cellular viability was appreciated by the trypan blue exclusion method. The effects of low temperature and of cellular death resulting from osmotic lysis were also assessed. Net cellular uptake of the radiotracers and the amount of proteins in the culture dishes were measured. All experiments were performed in parallel with control conditions and the results were expressed relatively to the control values. Teboroxime uptake was clearly decreased at low temperature (29.6% +/- 2.2% at 0 degree C, p < 0.001), while metabolic inhibition or osmotic lysis had no definite effect. The uptake of 201Tl and sestamibi was severely diminished in the presence of a mixture of 5 mM CN and 0.1 mM IAA, but 201Tl was less resistant than sestamibi (13.7% +/- 0.3% and 73.5% +/- 3.3%, respectively, after 1 hr of preincubation, p < 0.001 for both). Uptake of both tracers was very low in the presence of dead cells (12.1% +/- 1.3% for 201Tl and 4.1% +/- 0.2% for sestamibi, p < 0.001 for both). Ouabain had a detrimental effect only on 201Tl uptake at doses higher than 100 microM. Of these three currently available coronary blood flow imaging agents, teboroxime shows the lowest sensitivity to metabolic impairment.

A comparison between conventional scintigraphy and emission tomography with thallium-201 in the detection of myocardial infarction: concise communication.

We have compared emission tomography and conventional scintigraphy with thallium-201 in a series including 15 normal subjects and 64 patients showing transmural myocardial necrosis in various locations, fully documented by clinical, electrocardiographic, and enzymatic evidence. The reconstruction was derived from 32 projection images collected around the left side of the patient's chest by a rotating scintillation camera. The conventional views and the transverse, frontal, and sagittal sections were interpreted independently by two observers. The final calculated sensitivity was 89% with conventional scintigraphy and 98% with emission tomography, and the specificity was 93% in the two cases. Thus, emission tomography provides a better sensitivity and also a better interobserver agreement than conventional scintigraphy in the detection of transmural myocardial necrosis with thallium-201.

Comparison between 180 degrees and 360 degrees data collection in technetium-99m MIBI SPECT of the myocardium.

In a series of 12 patients presenting with a single-vessel coronary artery disease and who were injected with 370 MBq of 99mTc-2-methoxyisobutylisonitrile at peak exercise, two consecutive single photon emission computed tomography (SPECT) data collections, i.e., 32 views of 30 sec during a 180 degrees rotation and 64 views of 15 sec during a 360 degrees rotation, were performed 1 hr later. In both cases, transverse sections were reconstructed using (a) a backprojection method with a ramp filter, (b) a correction for downscatter, (c) a correction for depth attenuation by the Chang method, or (d) both corrections. Each reconstructed myocardium was then divided into four short-axis sections which were radially divided into nine sectors. Sectors with an activity below 80% of the maximum were considered as abnormal. Sensitivity and specificity were calculated relative to a sector-by-sector theoretical anatomic distribution of the perfusion abnormalities. Results demonstrate that, of all situations, the best balance between sensitivity and specificity was achieved with the 180 degrees data collection and no correction at reconstruction. Using the 360 degrees data sampling technique mainly lowered the sensitivity in the patients with a circumflex or right coronary artery disease. It is concluded that there does not seem to be any definite advantage in performing a 360 degrees rather than a 180 degrees data collection in 99mTc myocardial SPECT.

Joint scintigraphy in rabbits with 99mtc-N-[3-(triethylammonio)propyl]-15ane-N5, a new radiodiagnostic agent for articular cartilage imaging.

UNLABELLED: The aim of this study was to investigate joint scintigraphy in rabbits with 99mTc-N-[3-(triethylammonio)propyl]-15ane-N5 (NTP 15-5), a new radiopharmaceutical that specifically localizes in cartilaginous tissues. METHODS: Scans obtained after intravenous injection of the 99mTc-labeled compound in normal and arthropathy-induced rabbits were compared with those of the bone-imaging agent 99mTc-methylene diphosphonate (99mTc-MDP). RESULTS: The radioactive uptake of 99mTc-NTP 15-5 was detected in cartilaginous tissues 5 min after injection and was stable for 2 h. The uptake intensity was related to age and joint disease severity, and cartilage alterations not revealed by radiography induced a significant decrease of radiotracer uptake. On the other hand, imaging performed with 99mTc-MDP did not reveal the early changes in arthrosis but was more specific for bone remodeling in advanced stages of diseases or in inflammatory processes. CONCLUSION: Our results indicate that 99mTc-NTP 15-5 could be a good tracer for human arthrosic and arthritic cartilage detection, especially for the early diagnosis of joint diseases.

Technetium-99m-sestamibi uptake in breast tumor and associated lymph nodes.

UNLABELLED: The aim of this study was to measure the accumulation of 99mTc-sestamibi in breast tumors and their axillary lymph nodes in patients undergoing scintimammography. METHODS: Eighteen patients who were scheduled for breast surgery underwent scintimammography with 740 MBq of 99mTc-sestamibi on the day before the operation. The next morning, reinjection with 370 MBq was performed. Immediately after the surgical procedure, the 99mTc activity of the tumor samples and, when available, the related lymph nodes was measured in a gamma counter. The samples were weighed and prepared for histological analysis. The activity of each sample was normalized to the mean activity of normal tissue samples obtained from the same patient. RESULTS: Among the 198 samples analyzed, the relative uptake of sestamibi was increased in 111 containing normal lymph nodes (1.80+/-0.79 vs 1.00+/-0.22, p<0.05), as well as in the seven containing invaded lymph nodes (2.01+/-0.83, p<0.01) and more dramatically, in the 22 with a carcinoma (5.64+/-3.06, p<0.001). In two patients with a benign lesion, both scintigraphy and counting demonstrated increased activity in the tumor. Four patients had negative scan results despite the presence of malignant tumor and a more than fourfold increase of sestamibi concentration in two of them. CONCLUSION: Technetium-99m-sestamibi concentrates strongly in breast carcinoma, sometimes even when the scan results appear normal, and mildly in lymph nodes, especially when invaded; it also concentrates in some benign tumors, possibly in relation to the presence of epithelial hyperplasia.

Synthesis and evaluation of new iodine-125 radiopharmaceuticals as potential tracers for malignant melanoma.

The synthesis, labeling, and biodistribution of four 125I radiopharmaceuticals designed to localize in melanoma were tested. Uptake in tumors was demonstrated by autoradiography of whole-body sections and quantitated by measurement of radioactivity of selected tissues and tumors using melanoma-bearing mice. N-(2-diethylaminoethyl)-4-iodobenzamide was selected for its highest melanoma uptake: 60 min after IV injection of 6.5% and 4% ID/g, respectively for murine B16 and human melanotic melanoma. Tumor uptake showed the highest values of all analyzed tissues from 6 to 24 hr after injection. High uptake in melanotic tumor tissue with relatively low uptake in blood, muscle, brain, lung, and liver tissue resulted in high tumor/nontumor ratios (at 24 hr for B16, tumor/blood = 37, tumor/brain = 147, tumor/muscle = 95). This agent was compared with iodoamphetamine. Scintigraphic images of the tumor confirmed that external detection of melanoma is possible with this new radiopharmaceutical.

Phase II scintigraphic clinical trial of malignant melanoma and metastases with iodine-123-N-(2-diethylaminoethyl 4-iodobenzamide).

Preclinical studies established [125I]-N-(2-diethylaminoethyl) 4-iodobenzamide (BZA) as a potential radiopharmaceutical in the management of patients with malignant melanoma. External detection of both murine and human melanotic melanomas was possible after intravenous injection of 125I-BZA in tumor-bearing mice. This article reports a Phase II clinical trial evaluating 123I-BZA as an imaging agent of primary melanomas and metastases. A total of 110 patients with a history of melanoma were investigated in two nuclear medicine departments. Subjects were imaged from 20 to 24 hr after the intravenous injection of 3.5 mCi (130 MBq) of 123I-BZA. After injection, no short-term or long-term side effects were noted. Calculated on a lesion-site basis, diagnostic sensitivity was 81%, accuracy was 87% and specificity was 100%. The melanoma nature of previously occult lesions was confirmed by clinical criteria and/or additional investigations in follow-up studies. The scintigraphies were normal in 44 patients in clinical remission after treatment of malignant melanoma and in seven patients with nonmelanoma disease. No false positive results were observed. Iodine-123-BZA scintigraphy appears to be a safe and useful agent for the detection and follow-up of patients with malignant melanoma. BZA also allowed the detection of unsuspected lesions and the evaluation of the results of various therapeutic procedures such as surgery, chemotherapy, immunobiology, biological therapy or radiotherapy.

Comparison between thallium-201 and technetium-99m methoxyisobutyl isonitrile defect size in single-photon emission computed tomography at rest, exercise and redistribution in coronary artery disease.

Defect size on myocardial tomograms was measured in 30 patients who underwent 2 separate studies, 1 with thallium-201 (TI-201), the other with technetium-99m-methoxyisobutyl isonitrile (MIBI). A group of 15 patients with myocardial infarction was studied at rest and received both tracers on the same day. The other 15 patients had documented coronary artery disease. They were were given injections of TI-201 at peak exercise and underwent imaging immediately after exercise and again 4 hours later. They then received a dose of MIBI for imaging at rest. A week later they underwent a second exercise test with the same work load and received a second dose of MIBI. Defect size on single-photon emission computed tomographic images was measured and repeated twice. Results were expressed in percentage of the volume of the whole myocardium. Reproducibility of the defect size measurement was high for TI-201 (r = 0.978; SEE = 1.59) as well as for MIBI (r = 0.981; SEE = 0.80). In patients with coronary artery disease the mean size of the defects was significantly larger with TI-201 than with MIBI at exercise (6.7 +/- 5.2 vs 4.6 +/- 5.2%, respectively, p less than 0.05) and at redistribution (5.1 +/- 4.4 vs 2.8 +/- 3.2%, respectively, p less than 0.05), where no difference was seen in patients with myocardial infarction studied only at rest (11.2 +/- 10.4 vs 12.0 +/- 11.5%, respectively, p = not significant). Smaller MIBI defect sizes, when compared with TI-201, in the exercise and redistribution studies were not due to technical artefacts since there was no difference when they were compared at rest.

Comparable uptake of thallium-201 and technetium-99m MIBI in hibernating and "maimed" myocardium.

Thallium-201 and technetium-99m-MIBI uptake are comparable in "maimed" (i.e., partially viable) and hibernating myocardium. The appreciation of myocardial viability should be based not only on the presence of a regional contractility improvement, but also on the evaluation of the initial level of contractility and of tracer uptake in the concerned area.

In vitro binding of oxime acetylcholinesterase reactivators to proteoglycans synthesized by cultured chondrocytes and fibroblasts.

The incorporation of the 14C-labelled acetylcholinesterase reactivators 1-(methyl-imidazolium)-3 (4-carbaldoxime-pyridinium) propane dibromide (pyrimidoxime) and N,N'-trimethylene bis(pyridinium-4-aldoxime) dibromide (TMB4) into cultured chondrocytes and fibroblasts was measured and their binding to macromolecules synthesized by these cells studied. The results showed that these drugs concentrated slowly and poorly into these cells, but bound firmly to high molecular mass materials in the culture supernatants. The chromatographic properties of these macromolecules on Sepharose CL-2B in non-dissociative or dissociative conditions were similar to those of the proteoglycans synthesized by these cells. Dialysis of the macromolecule-bound drugs against increasing pH buffers showed half-dissociation pH > 8, identical to those for chondroitin sulphate. These results suggest strongly that pyrimidoxime and TMB4 are bound to proteoglycans by ionic interactions, and this together with their poor lipophilicity can explain their high selectivity for the cartilaginous tissues as opposed to other proteoglycan-containing structures such as skin.

Effects of low-dose neutrons applied at reduced dose rate on human melanoma cells.

Human melanoma cells that are resistant to gamma rays were irradiated with 14 MeV neutrons given at low doses ranging from 5 cGy to 1.12 Gy at a very low dose rate of 0.8 mGy min(-1) or a moderate dose rate of 40 mGy min(-1). The biological effects of neutrons were studied by two different methods: a cell survival assay after a 14-day incubation and an analysis of chromosomal aberrations in metaphases collected 20 h after irradiation. Unusual features of the survival curve at very low dose rate were a marked increase in cell killing at 5 cGy followed by a plateau for survival from 10 to 32.5 cGy. The levels of induced chromosomal aberrations showed a similar increase for both dose rates at 7.5 cGy and the existence of a plateau at the very low dose rate from 15 to 30 cGy. The existence of a plateau suggests that a repair process after low-dose neutrons might be induced after a threshold dose of 5-7.5 cGy which compensates for induced damage from doses as high as 32.5 cGy. These findings may be of interest for understanding the relative biological effectiveness of neutrons and the effects of environmental low-dose irradiation.

Synthesis, characterization and comparative biodistribution study of a new series of p-iodine-125 benzamides as potential melanoma imaging agents.

Iodobenzamides are reported to possess some affinity for melanoma. In order to identify the compound having the most appropriate pharmacokinetic properties as a potential melanoma imaging agent, thirteen new [125I]radioiodobenzamides with a butylene amide-amine spacer and various substituents on the terminal amino group were investigated. Their synthesis, radioiodination and biodistribution in B16 melanoma bearing C57BL6 mice are described and compared to [125I] labeled N-(2-diethylaminoethyl)-4-iodobenzamide ([125I]BZA), our reference compound. Changes in the terminal amino constituents induced modifications of lipophilicity, tumor uptake and organ distribution. The dimethylaminobutyl iodobenzamide appeared to be the most promising radiopharmaceutical imaging agent for the detection of melanoma and its metastases.

Melanin affinity of N-(2-diethylaminoethyl)-4-iodobenzamide, an effective melanoma imaging agent.

The cellular uptake and incorporation in macromolecules of iodine-125 labelled N-(2-diethylaminoethyl)-4-iodobenzamide ([125I]BZA), a melanoma imaging agent, was studied using human melanoma cells M3Dau (amelanotic) and M4Beu (melanotic). The interaction between [125I]BZA and synthetic melanin was examined in various conditions of incubation. The results showed that uptake was high only for M4Beu, whereas the incorporation in trichloroacetic acid-precipitable proteins was very low for both model cell lines, with no correlation with melanin content. Experiments with synthetic melanin showed that BZA binding to melanin was saturable and reversible, and involved several types of interaction. The influence of the ionic environment indicated that electrostatic forces play a role in the affinity, and the decrease in binding produced by the presence of an alcohol in the medium suggested that hydrophobic interactions may be involved in the binding mechanism. This was supported by the Scatchard analysis, which revealed two classes of binding sites, and the determination of two association constants (K1 = 3.9 +/- 1.9 x 106/M and K2 = 2.9 +/- 0.9 x 104/M). The affinity of BZA for melanin might explain the good results obtained in a phase II clinical trial for the diagnosis of malignant melanoma metastases, in which the specificity was 100%.

Comparison of the myocardial distributions of 201Tl and 99Tcm-MIBI on SPECT images.

The distributions of 201Tl and 99Tcm-MIBI on stress images were quantitatively measured in a series of 15 patients presenting with documented coronary artery disease. Following two sequential, one-week apart, peak-exercise injections of either 74 MBq of 201Tl or 370 MBq of 99Tcm-MIBI, tomographic views of the myocardium were reconstructed and two thick, central short-axis sections were divided into nine sectors. A sector was considered as showing a defect if its relative activity was lower than 30, 40, 50 or 60% of the highest level of activity in the 18 sectors. Results demonstrate that there were more defects with 201Tl than with 99Tcm-MIBI. However, the difference disappeared if the inferior wall was not included. These results suggest that depth attenuation could be, at least partly, responsible for the higher rate of positive results observed in single photon emission computed tomography with 201Tl than with 99Tcm-MIBI.