Advances in the production, processing and microPET image quality of technetium-94m.

This work involves the production, processing and imaging of the short-lived, rarely used positron emission tomography (PET) radionuclide technetium-94m (94mTc). Our procedures are an extension of methods reported in the literature and are detailed within. A key modification was the development of a single step that combines purification and concentration of an aqueous 94mTc-pertechnetate solution, which both reduces processing time and increases the final concentration of the solution. Additionally, a convenient method for the direct recovery of 94mTc into an organic solvent was developed, eliminating the solvent transfer step needed for organic syntheses using 94mTc. Each of these advances potentially extends the scope of syntheses possible with this short-lived radionuclide. To explore the imaging potential of 94mTc, we carried out phantom imaging studies on small-scale high-resolution PET scanners to estimate the limitations of detection associated with 94mTc and PET. Preliminary studies demonstrate that useful images can be obtained with modern image reconstruction algorithms when using a correction for the cascade gamma ray contamination.

Monitoring the effect of mild hyperthermia on tumour hypoxia by Cu-ATSM PET scanning.

PURPOSE: Mild hyperthermia can improve tumour oxygenation and enhance radiosensitivity. Imaging the hypoxic fraction of a tumour can guide hyperthermia treatment planning and facilitate treatment optimization. 64Cu-ATSM (Copper-diacetyl-bis(N4-methylthiosemicarbazone)) is a positron emitting compound that has been demonstrated to have rapid uptake and selective retention in hypoxic cells and has been used for imaging human and animal tumours. The purpose of the present report is to establish methodology that will allow one to use Cu-ATSM PET scanning to detect the impact of hyperthermia on tumour physiology in as little time as possible. MATERIALS AND METHODS: EMT6 tumours (mouse mammary carcinoma) were implanted into the subcutaneous tissue of both thighs of 10 BALB/c mice (one heated, one control tumour per animal). The target thermal dose was 41.5 degrees C x 45 min. Without interrupting heating, 64Cu-ATSM (mean activity 1.8 mCi) was then injected and serial PET scans were obtained. In a sub-group of four animals, a low administered activity (approximately 0.3 mCi) 64Cu-ATSM scan was also conducted before heating to permit a direct comparison of the effects of hyperthermia on the same tumours. In another sub-group of five animals, a low activity (approximately 0.3 mCi) 64Cu-PTSM (pyruvaldehyde-bis(N*-methylthiosemicarbazone)) scan was conducted before heating, to confirm a posited correlation between perfusion and early 64Cu-ATSM uptake. RESULTS: This study corrected for perfusion differences by dividing tumour uptake by the average early (first minute) uptake ('self-normalized uptake'). The 10 heated tumours showed a significantly (p = 0.007) lower self-normalized uptake than control tumours by 2 min. For the four mice with low activity Cu-ATSM scans performed before hyperthermia, the tumours to be heated demonstrated self-normalized uptake consistent with the unheated control tumours and which departed significantly (p < or = 0.02) from their post-hyperthermia scans by 5 min. Comparisons between scans and needle electrode surveys were performed in an additional four animals with eight tumours. For technical reasons electrode surveys were done after the end of hyperthermia-and, therefore, these animals also had comparison scans taken after hyperthermia. Reduced self-normalized uptake on scans was associated with increased pO2 on electrode surveys. These data also suggested a substantial degradation of the effect on tumour hypoxia by approximately 15-45 min after the end of mild hyperthermia. CONCLUSION: Short imaging times of approximately 5 min with modest (approximately 4-10) numbers of mice can discriminate the effects of mild hyperthermia on tumour physiology. The long-term objective is to use this tool to identify as short and mild a hyperthermia session as possible.

Design and synthesis of functionalized cyclopentadienyl tricarbonylmetal complexes for technetium-94m PET imaging of estrogen receptors.

Cyclopentadienyl tricarbonylmetal (CpTM, M = Re, (94m)Tc) complexes, some based on a typical nonsteroidal estrogen, were prepared with the aim of developing technetium- and rhenium-labeled imaging agents for estrogen receptor (ER) positive breast tumors. CpT[(94m)Tc] compounds with simple cyclopentadienyl substituents were first synthesized using a modified double ligand transfer reaction. The in vivo biodistribution of one of these CpT[(94m)Tc] complexes was determined by tissue dissection and microPET imaging. Novel C-ring substituted analogues of cyclofenil, a nonsteroidal compound known to bind the ER, were also prepared, and their ER binding was measured. Because of their low ER affinity, however, labeling and imaging studies of these compounds were not pursued. It is notable that the highest ER binding analogue, a CpTRe cyclofenil derivative, could be synthesized from the corresponding ferrocenyl cyclofenil analogue by the double ligand transfer reaction. This further demonstrates the versatility of the double ligand transfer reaction and indicates that the synthesis of technetium and rhenium radiolabeled agents for breast tumor imaging and therapy is also likely to be successful.

Imaging multidrug resistance P-glycoprotein transport function using microPET with technetium-94m-sestamibi.

The best characterized mechanism of multidrug resistance (MDR) in cancer involves the MDR1 efflux transporter P-glycoprotein (Pgp). The positron-emitting radiotracer hexakis(2-methoxyisobutylisonitrile)-(94m)Tc ((94m)Tc-MIBI) was synthesized and validated in cell transport studies as a substrate for MDR1 Pgp. In vivo small-scale PET imaging and biodistribution studies of mdr1a/1b (-/-) gene deleted and wild-type mice demonstrated the use of (94m)Tc-MIBI to detect Pgp function. The reversal effect of a Pgp modulator was shown in tissue distribution studies of KB 3-1 (Pgp-) and KB 8-5 (Pgp+) tumor-bearing nude mice. The current (94m)Tc-MIBI experiments parallel previous studies employing (99m)Tc-MIBI, showing essentially identical performance of the two technetium radiotracers and providing biological validation of (94m)Tc-MIBI for PET imaging of multidrug resistance.

7alpha- and 17alpha-substituted estrogens containing tridentate tricarbonyl rhenium/technetium complexes: synthesis of estrogen receptor imaging agents and evaluation using microPET with technetium-94m.

To develop technetium and rhenium-labeled imaging agents for estrogen receptor (ER) positive breast tumors, we have prepared tridentate metal tricarbonyl chelates substituted at the 7alpha- and 17alpha-positions of estradiol. Some of the Re(CO)(3) conjugates have high binding for the ER in vitro. The in vivo biodistribution of the highest affinity of these novel metal tricarbonyl conjugates, prepared as the (94m)Tc labeled analogue, was evaluated by tissue dissection and microPET imaging. Although target tissue-selective uptake was not apparent, it is notable that microPET imaging identified the stomach as a major site of activity deposition, a site that might have been missed by standard tissue distribution studies.

MicroPET imaging of a gastrin-releasing peptide receptor-positive tumor in a mouse model of human prostate cancer using a 64Cu-labeled bombesin analogue.

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a variety of carcinomas and has been the target for detection and treatment of these neoplasms in animals. In particular, analogues of the tetradecapeptide bombesin (BN) have been radiolabeled with (99m)Tc and (111)In for detection of GRPR-positive tumors by gamma ray scintigraphy. The goal of this study was to evaluate the potential of the bombesin analogue, DOTA-Aoc-BN(7-14), for positron-emission tomographic (PET) imaging after radiolabeling with the positron-emitter (64)Cu. A saturation binding assay on PC-3 human prostate cancer cells showed that (64)Cu-DOTA-Aoc-BN(7-14) had an equilibrium binding constant (K(d)) of 6.1 +/- 2.5 nM and a receptor concentration (B(max)) of 2.7 +/- 0.6 x 10(5) receptors/cell. The radiolabeled analogue also showed rapid internalization with 18.2% internalized into 10(5) PC-3 cells by 2 h. The tumor localization of (64)Cu-DOTA-Aoc-BN(7-14) was 5.5% injected dose per gram in athymic nude mice bearing PC-3 xenografts at 2 h postinjection. The tumor retention with respect to the 2 h value was 76% and 45% at 4 and 24 h, respectively, and was GRPR-mediated as shown by inhibition with a coinjection of excess peptide. MicroPET imaging of (64)Cu-DOTA-Aoc-BN(7-14) in athymic nude mice bearing subcutaneous PC-3 tumors showed good tumor localization. Further studies with (64)Cu-pyruvaldehyde-bis(N(4)-methylthiosemicarbazone) ((64)Cu-PTSM) suggested that low blood flow to the PC-3 tumors may have limited the localization of (64)Cu-DOTA-Aoc-BN(7-14). This study demonstrates that (64)Cu-DOTA-Aoc-BN(7-14) can be used to detect GRPR-positive tumors by PET imaging.