Localized fetomaternal hyperglycemia: spatial and kinetic definition by positron emission tomography

BACKGROUND

Complex but common maternal diseases such as diabetes and obesity contribute to adverse fetal outcomes. Understanding of the mechanisms involved is hampered by difficulty in isolating individual elements of complex maternal states in vivo. We approached this problem in the context of maternal diabetes and sought an approach to expose the developing fetus in vivo to isolated hyperglycemia in the pregnant rat.

METHODOLOGY AND PRINCIPAL FINDINGS

We hypothesized that glucose infused into the arterial supply of one uterine horn would more highly expose fetuses in the ipsilateral versus contralateral uterine horn. To test this, the glucose tracer [18F]fluorodeoxyglucose (FDG) was infused via the left uterine artery. Regional glucose uptake into maternal tissues and fetuses was quantified using positron emission tomography (PET). Upon infusion, FDG accumulation began in the left-sided placentae, subsequently spreading to the fetuses. Over two hours after completion of the infusion, FDG accumulation was significantly greater in left compared to right uterine horn fetuses, favoring the left by 1.9+/-0.1 and 2.8+/-0.3 fold under fasted and hyperinsulinemic conditions (p<10(-11) n=32-35 and p<10(-12) n=27-45) respectively. By contrast, centrally administered [3H]-2-deoxyglucose accumulated equally between the fetuses of the two uterine horns. Induction of significant hyperglycemia (10(3) mg/dL) localized to the left uterine artery was sustained for at least 48 hours while maternal euglycemia was maintained.

CONCLUSIONS AND SIGNIFICANCE

This approach exposes selected fetuses to localized hyperglycemia in vivo, minimizing exposure of the mother and thus secondary effects. Additionally, a set of less exposed internal control fetuses are maintained for comparison, allowing direct study of the in vivo fetal effects of isolated hyperglycemia. Broadly, this approach can be extended to study a variety of maternal-sided perturbations suspected to directly affect fetal health.

Kinetic analysis of 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) in head and neck cancer patients before and early after initiation of chemoradiation therapy

The purpose of this study was to investigate the kinetic behavior of 3'-deoxy-3'-(18)F-fluorothymidine ((18)F-FLT) before and early after initiation of chemoradiation therapy in patients with squamous cell head and neck cancer.

METHODS

A total of 8 patients with head and neck cancer underwent (18)F-FLT PET scans (7 patients at baseline and after 5 d [10 Gy] of radiation therapy given with concomitant chemotherapy and 1 patient only at baseline). Dynamic PET images were obtained with concurrent arterial or venous blood sampling. Kinetic parameters including the flux constant of (18)F-FLT based on compartmental analysis (K-FLT), the Patlak influx constant (K-Patlak), and standardized uptake value (SUV) were calculated for the primary tumor and (18)F-FLT-avid cervical lymph nodes for all scans.

RESULTS

Mean pretreatment values of uptake for the primary tumor and cervical nodes were 0.075 +/- 0.006 min(-1), 0.042 +/- 0.004 min(-1), and 3.4 +/- 0.5 (mean +/- SD) for K-FLT, K-Patlak, and SUV, respectively. After 10 Gy of radiation therapy, these values were 0.040 +/- 0.01 min(-1), 0.018 +/- 0.016 min(-1), and 1.8 +/- 1.1 for K-FLT, K-Patlak, and SUV, respectively. For all lesions seen on pretherapy and midtherapy scans, the correlation was 0.90 between K-FLT and K-Patlak, 0.91 between K-FLT and SUV, and 0.99 between K-Patlak and SUV.

CONCLUSION

The initial (18)F-FLT uptake and change early after treatment in squamous head and neck tumors can be adequately characterized with SUV obtained at 45-60 min, which demonstrates excellent correlation with influx parameters obtained from compartmental and Patlak analyses.

PET imaging in rats to discern temporal onset differences between 6-hydroxydopamine and tau gene vector neurodegeneration models

We attempted to monitor the nigrostriatal dopaminergic system in rats with positron emission tomography (PET) during the progression of two experimental disease states. One model was 6-hydroxydopamine (6-OHDA) lesioning and the other was direct gene transfer of the microtubule-associated protein tau to the substantia nigra using an adeno-associated virus vector (AAV9). The PET ligand was 6-[18F]fluoro-L-m-tyrosine (FMT), imaged prior to, and at two intervals after initiating dopaminergic neurodegeneration. The striatum was delineated with the aid of repeated PET imaging (FMT and sodium fluoride for bone), realignment to subsequent computed axial tomography scans, and registration to an atlas, which proved essential to tracking disease progression. The striata on the two sides of the brain were compared over time after unilateral lesioning treatments. 6-OHDA reduced uptake on the ipsilateral side relative to the untreated contralateral side at both 1 and 4 weeks after lesioning, while the AAV9 tau led to reduced uptake of the tracer in the striatum at 4 weeks, but not 1 week after treatment. The amplitude of the loss of FMT uptake in striatum at 4 weeks with either model was subtle relative to the postmortem histological analysis of the tissue, but the multi-modal imaging analysis yielded statistical effects that matched well with the histology in terms of the timing of the loss of dopaminergic markers. Live longitudinal imaging successfully tracked two distinct types of disease progression in individual rats, although the FMT is not a sensitive ligand to monitor the extent of the lesion.

Reduced presynaptic dopamine activity in fibromyalgia syndrome demonstrated with positron emission tomography: a pilot study

Although the pathophysiology underlying the pain of fibromyalgia syndrome (FMS) remains unknown, a variety of clinical and investigational findings suggests a dysregulation of dopaminergic neurotransmission. We therefore investigated presynaptic dopaminergic function in 6 female FMS patients in comparison to 8 age- and gender-matched controls as assessed by positron emission tomography with 6-[(18)F]fluoro-L-DOPA as a tracer. Semiquantitative analysis revealed reductions in 6-[(18)F]fluoro-L-DOPA uptake in several brain regions, indicating a disruption of presynaptic dopamine activity wherein dopamine plays a putative role in natural analgesia. Although the small sample size makes these findings preliminary, it appears that FMS might be characterized by a disruption of dopaminergic neurotransmission.

PERSPECTIVE

An association between FMS and reduced dopamine metabolism within the pain neuromatrix provides important insights into the pathophysiology of this mysterious disorder.

Synthesis and biological evaluation of [11C]MK-912 as an alpha2-adrenergic receptor radioligand for PET studies

In vitro studies showed that MK-912 ((2S, 12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12b-octahydro -2H-benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one) is a potent alpha2-adrenergic receptor antagonist with high affinity (Ki = 0.42, 0.26 and 0.03 nM to alpha2A, alpha2B and alpha2C, respectively) and high selectivity (alpha2A/alpha1A = 240; alpha2A/D-1 = 3600; alpha2A/D-2 = 3500; alpha2A/5-HT1 = 700; alpha2A/5-HT2 = 4100). The compound was labeled with 11C and evaluated in rodents and monkey as a specific radioligand for studying alpha2-adrenergic receptors using PET. [11C]MK-912 was synthesized by methylation of its desmethyl precursor, L-668,929, with [11C]CH3I in (Bu3O)P=O at 85 degrees C for 8 min followed by purification with HPLC in 18% yield in a synthesis time of 45 min from end of bombardment (EOB). The specific activity was 0.83-0.93 Ci/micromol and the radiochemical purity was 97%. The initial uptake of [11C]MK-912 in mouse brain, heart, lung, liver and kidney was high (5%, 4%, 5%, 17% and 8% per gram of organ, respectively, at 5 min postinjection) and the activities were then slowly cleared from these organs. The uptake of [11C]MK-912 in rat olfactory tubercle, a brain region with high density of alpha2-adrenergic receptors, was reduced by 30%, and the ratio of radioactivity in olfactory tubercle/cerebellum was reduced from 2:1 to 1:1 by coinjection of [11C]MK-912 with a potent alpha2-adrenergic receptor antagonist, atipamezole (3 mg/kg), indicating that compound 2 binds to alpha2-adrenergic receptors. However, a PET study in a rhesus monkey revealed that the initial influx of [11C]MK-912 into various brain regions (cerebellum, cortex, olfactory tubercle and striatum) was high (0.02%/cc), and the radioactivity was then washed out slowly and without significantly differential retention in these brain regions. This, coupled with the fact that none of the high-density alpha2-adrenergic receptor brain regions exceeds a few millimeters in diameter, suggests that [11C]MK-912 is probably not an ideal radioligand for studying alpha2-adrenergic receptors in humans using commercially available PET.

Mediastinal lymph node staging of non-small-cell lung cancer: a prospective comparison of computed tomography and positron emission tomography

We compared the abilities of positron emission tomography and computed tomography to detect N2 or N3 lymph node metastases (N2 or N3) in patients with lung cancer. Positron emission tomography detects increased rates of glucose uptake, characteristic of malignant cells. Patients with peripheral tumors smaller than 2 cm and a normal mediastinum were ineligible. All patients underwent computed tomography, positron emission tomography, and surgical staging. The American Thoracic Society lymph node map was used. Computed and positron emission tomographic scans were read by separate radiologists blinded to surgical staging results. Lymph nodes were "positive" by computed tomography if larger than 1.0 cm in short-axis diameter. Standardized uptake values were recorded from areas on positron emission tomography corresponding to those from which biopsy specimens were taken; if greater than 4.2, they were called "positive." Seventy-five lymph node stations (2.8 per patient) were analyzed in 27 patients. Computed tomography incorrectly staged the mediastinum as positive for metastases in three patients and as negative for metastases in three patients. Sensitivity and specificity of computed tomographic scans were 67% and 83%, respectively. Positron emission tomography correctly staged the mediastinum in all 27 patients. When analyzed by individual node station, there were four false positive and four false negative results by computed tomography (sensitivity = 60%, specificity = 93%, positive predictive value = 60%). Positron emission tomography mislabeled one node station as positive (100% sensitive, 98% specific, positive predictive value 91%). The differences were significant when the data were analyzed both for individual lymph node stations (p = 0.039) and for patients (p = 0.031) (McNemar test). Positron emission tomography and computed tomography are more accurate than computed tomography alone in detecting mediastinal lymph node metastases from non-small-cell lung cancer.

Detection of scalene lymph node metastases from lung cancer. Positron emission tomography

Preliminary data indicate that positron emission tomography (PET) following injection of fluorodeoxyglucose F18 (FDG) is sensitive and specific for detecting malignant cells in chest tumors and mediastinal lymph nodes. We report a case of non-small cell lung cancer metastatic to clinically normal scalene lymph nodes that was correctly staged by FDG-PET.

Fluorine-18 and carbon-11 labeled amphetamine analogs--synthesis, distribution, binding characteristics in mice and rats and a PET study in monkey

No-carrier-added (NCA) (+-)-p-[18F]fluoroamphetamine (2a) and (+-)-6-[18F]fluoro-3,4-methylene-dioxy-amphetamine (2b) were synthesized through a multistep synthesis by nucleophilic substitution of the appropriate precursors (p-nitrobenzaldehyde, 1a and 6-nitropiperonal 1b, respectively) with [18F]fluoride followed by condensation with nitroethane and reduction with LAH in 20-30% yield (EOB) in a synthesis time of 90-109 min from EOB. NCA (-)-[11C]methamphetamine (4a) and (+-)-3,4-methylene-dioxy-N-[11C]methamphetamine (4b) were synthesized by methylation of the appropriate desmethyl precursors 3a and 3b with [11C]H3I in 40-60% yield (EOB) in a synthesis time of 30 min from EOB. Animal studies in mouse and rat revealed that the relative tissue uptake of these radiotracers was kidneys > lungs > liver > spleen > brain > heart > blood. The uptakes of these radiotracers in mouse brain were high and similar at 5 min post-injection (approx. 5%/g) but radioactivity then declined rapidly (approx. 1%/g at 60 min post-injection). For compounds 2a and 2b, the activity in the femur did not increase with time indicating in vivo defluorination may not be the major route of metabolism. Monoamine uptake inhibitors (nomifensine, fluoxetine and nisoxetine) did not inhibit but enhance the uptake of (-)-[11C]methamphetamine (4a) in the rat brain by greater than 50%. A PET study in a Rhesus monkey revealed that the uptakes of (-)-[11C]methamphetamine in different brain regions were similar and the retention of the radioactivity in these regions remained constant throughout the study. Analysis of arterial plasma by HPLC showed that 50% of radioactivity remained as 4a at 60 min post-injection.

Solitary pulmonary nodules: detection of malignancy with PET with 2-[F-18]-fluoro-2-deoxy-D-glucose

It is estimated that nearly one-third of solitary pulmonary nodules (SPNs) may represent bronchogenic carcinoma. The noninvasive imaging methods used currently (ie, plain radiography, computed tomography) are not reliable for accurate detection of malignancy in most SPNs. The authors prospectively evaluated use of positron emission tomography (PET) with 2-[fluorine-18]-fluoro-2-deoxy-D-glucose (FDG) for identification of malignancy in 20 patients with noncalcific, radiographically indeterminate SPNs. PET-FDG imaging demonstrated focal hypermetabolism in 13 biopsy-proved malignant nodules, whereas no increased FDG uptake was seen in the seven benign SPNs. Semiquantitative analysis with computation of differential uptake ratios also helped clearly differentiate benign nodules (mean +/- standard deviation, 0.56 +/- 0.27) from malignant nodules (mean +/- standard deviation, 5.63 +/- 2.38) (P less than .001). Thus, PET-FDG imaging may be a potentially useful noninvasive technique for accurate differentiation of benign and malignant SPNs that are radiographically indeterminate.

[18F]fluoro-beta-fluoromethylene-m-tyrosine analogs, potential PET agents for presynaptic dopamine terminals: synthesis and spectroscopic characterization

18F-labeled (E)-beta-fluoromethylene-DL-m-tyrosine (FMMT) was prepared by the direct reaction of FMMT with [18F]acetylhypofluorite (AcOF) resulting into three product isomers. Extensive 1H, 13C and 19F-NMR spectroscopic analysis identify these products to be 2-fluoro, 6-fluoro-FMMT and 2,6-difluoro-FMMT. The HPLC isolated radiochemical EOB yields of these products were 22, 25 and 14%, respectively, based on starting [18F]AcOF. The specific activity at the end of a synthesis time of an hour was ca 200 mCi/mmol. With the possible advantage of "metabolic trapping" in dopamine nerve terminals via covalent binding to MAO and reduced metabolite formation, [18F]F-FMMT may potentially be the optimal PET tracer for CNS dopamine nerve terminals.

Considerations in setting up a positron emission tomography center

Clinically oriented imaging with position emission tomography (PET) has come of age. Given an adequate referral base and physician interest, a compelling argument can be made at all levels of the review process for setting up a PET program in a clinical setting. PET is expensive. It is obvious that the cost of running a PET service depends heavily on an institution's ability to obtain reasonable financing. Educational institutions have the opportunity to acquire special funding through a variety of sources. On the other hand, money can be expensive for private entrepreneurs. It appears that in the near future PET centers will probably remain at educational institutions or large well-financed community hospitals able to raise money at reasonable rates until reimbursement issues are better resolved. Finally, the future of clinical PET may hinge significantly on the ability of commercial radiopharmaceutical suppliers to provide regional fluorodeoxyglucose distribution. As an institutional program development, PET offers opportunities by providing unique clinical data aiding the referral pattern. PET may serve as a magnet for recruitment in many areas and may promote interdisciplinary cooperation. A clinical PET center serves both as a model for future and more widespread use of PET and as a training ground for medical personnel. Finally, the unique capabilities of PET may facilitate grant opportunities.

Synthesis and biodistribution of the alpha 2-adrenergic receptor antagonist (11C)WY26703. Use as a radioligand for positron emission tomography

The purpose of these experiments was to label an alpha 2-adrenergic receptor ligand with a positron emitting isotope and then test this radioligand in vivo. No-carrier-added [11C]WY26703 was synthesized by methylation of its desmethyl precursor, WY27050 with [11C]H3I followed by purification with HPLC in 14% yield in a synthesis time of 35 min from EOB. Ki values for unlabeled WY26703, ranged from 0.52-1.55 nM in tissues that express a single alpha 2-adrenergic receptor subtype. Tail vein injections of [11C]WY26703 in mice revealed that the compound was distributed in the brain, heart, lungs, spleen, and kidneys. In the brains of rats treated with atipamezole, an alpha 2-adrenergic receptor antagonist, there was no decrease in [11C] accumulation indicating a lack of observable specific binding of the radioligand. When brain tissue was homogenized and filtered, however, atipamezole decreased [11C] activity by 53%. Therefore, [11C]WY26703 crosses the blood-brain barrier and specifically binds to alpha 2-adrenergic receptors with high affinity. Atipamezole treatment decreased only the area of the locus coeruleus [11C] value of the various regions of the brain. The affinity, however, of [11C]WY26703 does not appear to distinguish alpha 2-receptors from nonspecific binding sites. PET study of [11C]WY26703 in a Rhesus monkey showed that influx of [11C]WY26703 into the brain was high for the first few minutes but radioactivity then declined rapidly and did not retain in a specific brain region. This suggests that [11C]WY26703 may not be a useful ligand for imaging human alpha 2-adrenergic receptors by positron emission tomography.

Synthesis of radiofluorinated analogs of m-tyrosine as potential L-dopa tracers via direct reaction with acetylhypofluorite

The direct electrophilic radiofluorination of m-tyrosine using [18F]acetylhypofluorite was investigated. Results showed that this reaction was both rapid and efficient with recovered decay corrected yield of 71% radiofluorinated m-tyrosines based on starting [18F]acetylhypofluorite. Specific activity of the product obtained in this study was 100-200 mCi/mmol although 1-5 Ci/mmol are easily achievable with our improved production of [18F]AcOF. Three positional isomers were found and identified by 19F-NMR to be 2-, 4-, 6-fluoro-m-tyrosine with a distribution of 36:11:52, respectively. This measured distribution allowed the assignment of the radio-HPLC peaks. Biological studies are currently underway in our laboratory using these fluoro-m-tyrosines to determine which isomer would be most suited for the evaluation of the dopamine system by positron tomography.

Noninvasive testing of cerebral perfusion reserve prior to coronary artery bypass graft surgery

Cerebral perfusion reserve testing using fluorine-18-fluoromethane and positron emission tomographic brain scanning to define cerebral blood flow abnormalities was performed in 5 patients being considered for combined coronary and carotid reconstructive surgery. Blood flow testing during normocapnia and following hypercapnia was utilized in these patients to determine the hemodynamic significance of known extracranial carotid artery occlusive lesions. Reserve diminution in 2 of these patients prompted combined surgery, whereas normal reserve values in the other 3 prompted coronary surgery alone. Results obtained in this preliminary series show how preoperative noninvasive testing of cerebral perfusion reserve adds to the diagnostic evaluation of patients with widespread vascular disease.

Cerebral perfusion reserve indexes determined by fluoromethane positron emission scanning

An index of cerebral perfusion reserve (RES%), defined as the percent change of regional cerebral blood flow over baseline per mm Hg of end-tidal CO2 tension, was determined for each middle cerebral artery (MCA) territory in patients with unilateral carotid distribution transient ischemic attacks or minor cerebrovascular accidents and was compared with that of age-matched, neurologically normal volunteers. Vasodilator responses to induced hypercapnia were tested during inhalation of 5% CO2 in 95% O2 while regional cerebral blood flow was measured by fluoromethane inhalation positron emission tomography. Mean RES% for 24 normal MCA territories was 5.2 +/- 0.8%. Mean RES% for 15 patient nonischemic MCA territories was 3.8 +/- 1.3% and for 15 ischemic MCA territories was 2.8 +/- 1.9% (both p less than 0.001). Individual RES% values and symmetry ratios between ischemic and nonischemic regions were also determined and compared with angiographic data. Areas of diminished, asymmetric, or paradoxical (two patients) CO2 reactivity appear to correspond to areas of compensatory vasodilation. We found this technique to be a safe and reproducible method for defining and recording localized areas of cerebral tissue at apparent risk for hemodynamically related damage.