Theodore S. T. Wang

Stability of [123I]IBZM SPECT measurement of amphetamine-induced striatal dopamine release in humans.

Binding competition between endogenous dopamine (DA) and the D2 receptor radiotracer [123I]IBZM allows measurement of the change in synaptic DA following amphetamine challenge with SPECT in the living human brain. Previous investigations using this technique in healthy subjects have shown that the magnitude of amphetamine effect on [123I]IBZM binding potential (BP) is small (range between 5 to 15% decrease), and that a large between‐subject variability in this effect is observed. Therefore, it was unclear how much of the apparent between‐subject variability was due to a low signal‐to‐noise ratio in the measurement, vs. true between‐subject differences in the magnitude of the response. The goals of this investigation were to test the within‐subject reproducibility and reliability of amphetamine‐induced decrease in [123I]IBZM BP with a test/retest paradigm, and to establish the presence or absence of tolerance or sensitization to single administration of i.v. amphetamine. Six healthy male subjects, never previously exposed to psychostimulants, twice underwent measurement of striatal amphetamine‐induced DA release (between‐measurement interval 16 ± 10 days) using SPECT and the [123I]IBZM constant infusion technique. Results demonstrated an excellent within‐subject reproducibility of amphetamine‐induced DA release: amphetamine‐induced decreases in [123I]IBZM BP were significant on each day, and had an intraclass correlation coefficient (ICC) of 0.89. Moreover, values from the second experiment were not significantly different from first experiment, suggesting the absence of either sensitization or tolerance to the effect of amphetamine on DA release in these experimental conditions. The subjective activation, as rated by the subjects on analog scales, was also highly reproducible. In conclusion, this scanning technique provides a reliable measurement of amphetamine‐induced reduction of [123I]IBZM BP and enables detection of between‐subject differences that appear stable over time. Synapse 31:302–308, 1999.

Use of indium-111-labeled cells in measurement of cellular dynamics of experimental cardiac allograft rejection

This study evaluates the kinetics and utility of infused indium-111-labeled cells in detecting rejection in ACI to Lewis rat heart allografts. Syngeneic leukocytes, lymph node lymphocytes, and platelets were isolated and labeled with indium-111 (111In) oxine, respectively, and were infused i.v. into Lewis rats carrying beating ACI or syngeneic hearts from post-transplant days 0 to 6. Recipients were imaged serially at 24 hr after infusion of labeled cells followed by excision of both native and transplanted hearts for direct isotope count. Labeled leukocytes accumulate progressively in the allograft with the scan becoming positive by post-transplant day 4. The ratio of allograft to native heart isotope counts rose from 1.25 on day 1 to 10.07 (P < 0.0001) on day 7. The Lewis recipients infused with labeled lymphocytes showed a positive scan on days 6 and 7 whereas the allograft to native heart isotope count ratio rose from 0.97 on day 1 to 5.33 (P < 0.001) on day 7. Recipients infused with 111In-labeled platelets showed a positive scan on days 5 to 7 and the allograft to native heart isotope count ratio rose sharply from 2.56 on day 4 to 16.98 (P < 0.005) on day 7. Syngeneic heart grafts failed to demonstrate significant accumulation of any of the labeled cell population. These studies confirm the importance of nonlymphocytic cells in cellular rejection, evaluate the kinetics of graft invasion by the various cell types, and suggest that the techniques used afford a method for a safe and an early detection of allograft rejection.

Migration patterns of dendritic cells in the rat: Comparison of the effects of γ and UV-B irradiation on the migration of dendritic cells and lymphocytes☆

To further define the underlying mechanisms of immune suppression induced by UV-B irradiation, we have examined the kinetics of homing patterns of in vitro UV-B-irradiated and gamma-irradiated-thoracic duct lymphocytes (TDL) compared to dendritic cells (DC). Our findings show that 111In-oxine-labeled TDL specifically home to the spleen, liver, lymph nodes, and bone marrow with subsequent recirculation of a large number of cells from the spleen to lymph nodes. In contrast, DC preferentially migrate to the spleen and liver with a relatively insignificant distribution to lymph nodes and an absence of subsequent recirculation. Splenectomy prior to cell injection significantly diverts the spleen-seeking DC to the liver but not to the lymph nodes, while the homing of TDL to lymph nodes is significantly increased. In vitro exposure of 111In-oxine labeled TDL to gamma irradiation does not significantly impair immediate homing to lymphoid tissues but inhibits cell recirculation between 3 and 24 hr. In contrast, gamma irradiation does not affect the tissue distribution of labeled DC, suggesting that DC are more radioresistant to gamma irradiation than TDL. Unlike the findings in animals injected with gamma-irradiated cells, UV-B irradiation virtually abolished the homing of TDL to lymph nodes and significantly reduced the homing of the spleen-seeking DC to the splenic compartment while a large number of cells were sequestered in the liver. The results of in vitro cell binding assay show that TDL, unlike DC, have the capacity to bind to high endothelial venules (HEV) within lymph node frozen sections while gamma and UV-B irradiation significantly inhibit the binding of TDL to lymph node HEV. These findings suggest that: (i) DC, unlike TDL, are unable to recirculate from blood to lymph nodes through HEV; (ii) although gamma irradiation impairs TDL recirculation, it does not affect DC tissue distribution; and (iii) UV-B irradiation impairs both TDL and DC migration patterns. We conclude that the lack of capacity of irradiated TDL to home to lymph nodes is due to damage to cell surface homing receptors and that the failure of DC to home to the lymph node microenvironment is related to the absence of HEV homing receptors on their cell surface.

The use of radionuclides for tumor therapy

The successful use of radionuclides for tumor therapy depends to a major extent on the ability to achieve a high concentration of radioactivity in the tumor relative to other radiosensitive organs not involved by tumor, such as bone marrow, intestinal mucosa, liver, and kidneys. Techniques designed to achieve such differential localization of the radionuclides include the use of radiopharmaceuticals that enter specific metabolic pathways unique to certain tumor types; radiolabeled antibodies that attach to tumor-associated antigens present on tumor cell surfaces; heterologous antibodies that attach to tumor-associated antigens present on tumor cell surfaces and which are then identified by radiolabeled antibodies directed against the species in which the original, unlabeled antibody was made, and radiolabeled compounds injected regionally at the tumor site. Although both clinical and experimental evidence on the use of radionuclides for tumor therapy is encouraging in preliminary studies, extensive further research needs to be done in this area to insure the clinical efficacy of radionuclides for tumor therapy.

In vivo biodistribution of a radiotracer for imaging serotonin-1a receptor sites with pet: [11C]Ly274601

LY274601 [R-(+)8-thiomethyl-2-(di-n-propyl-amino)tetralin], a full agonist of the 5-HT1A receptor with high affinity and selectivity, was labeled with 11C and 3H, and its in vivo behavior was studied to evaluate [11C]LY274601 as a PET radiotracer for imaging 5-HT1A receptor sites in living brain. Following intravenous tail injection into mice, [11C]LY274601 showed high blood-brain barrier permeability and accumulated in regions known to have high densities of 5-HT1A receptor sites such as the brain stem including the raphe nuclei. The binding of the radiotracer in target tissues is blocked by pre-injection of the 5-HT1A receptor selective ligand 8-OH-DPAT (1 mg/kg, s.c.), suggesting that the binding is specific to 5-HT1A receptor sites. Using ex vivo autoradiography, the target tissues such as hippocampus CA1-4 fields, piriform cortex, dorsal raphe nucleus and lateral septum were visualized as hot spots. These tissues were observed to have binding 2-2.7 times greater than the cerebellum. The distribution of the radiotracer agrees well with the distribution of 5-HT1A receptors revealed by in vitro autoradiography with [3H]8-OH-DPAT. However, the radiotracer was metabolized quickly and cleared from target tissues with a half life of approximately 15 min. [11C]LY274601 showed high non-specific binding in regions with low number of 5-HT1A receptor sites such as cerebellum.

Syntheses of [11C] and [3H] LY274601, a serotonin1A receptor agonist

LY274601, R-(+)-8-thiomethyl-2-(di-n-propylamino)tetralin, a serotonin (5-HT) 1A receptor full agonist with high affinity (Ki: 0.6nM) and selectivity, was labeled with 11 C for imaging 5-HT 1A receptor sites in vivo by positron emission tomography (PET). [ 11 C]LY274601 was synthesized by S-methylation of the normethyl precursor unmasked via hydrolysis of the butyrate thioester of LY274601. The methylation reaction with [ 11 C]iodomethane proceeded quickly and efficiently in DMF at 40°C, yielding the radiotracer in an average overall radiochemical yield of 35.7±9.8%, The synthesis time including HPLC purification and formulation for injection was approximately 30 min. The specific activity was 630±78mCi/μmol at the end of synthesis (E.O.S.). This labeling procedure was also employed in the preparation of [ 3 H]LY274601, R-(+)-8-[ 3 H]methylthio-2-(di-n-propyl-amino)tetralin, from [ 3 H]iodomethane.

In nonhuman primates intracarotid adenosine, but not sodium nitroprusside, increases cerebral blood flow

Intracarotid infusion of short-acting vasodilators, such as adenosine and nitroprusside, in doses that lack significant systemic side effects, may permit controlled manipulation of cerebrovascular resistance. In this experiment we assessed changes in cerebral blood flow (CBF) after intracarotid infusion of nitroprusside and adenosine. The study was conducted on six adult baboons under isoflurane anesthesia and controlled ventilation. Intracarotid drug infusion protocol avoided hypotension during nitroprusside infusion and tested for autoregulatory vasoconstriction. CBF (intraarterial 133Xe technique) was measured four times during infusions of 1) intracarotid saline, 2) IV phenylephrine (0.2 &mgr;g · kg−1 · min−1) aimed to increase mean arterial pressure by 10–15 mm Hg, 3) IV phenylephrine and intracarotid nitroprusside (0.5 &mgr;g · kg−1 · min−1), and 4) intracarotid adenosine (1 mg/min). IV phenylephrine increased mean arterial pressure (69 ± 8 to 91 ± 9 mm Hg, P < 0.0001, n = 6), and concurrent infusion of intracarotid nitroprusside reversed this effect. However, compared with baseline, CBF did not change with IV phenylephrine or with concurrent infusion of IV phenylephrine and intracarotid nitroprusside. Intracarotid adenosine profoundly increased CBF (from 29 ± 8 to 75 ± 32 mL · 100 g−1 · min−1;P < 0.0001). In nonhuman primates, intracarotid adenosine increases CBF in doses that lack significant systemic side effects, whereas intracarotid nitroprusside has no effect. Intracarotid adenosine may be useful for manipulating cerebrovascular resistance and augmenting CBF during cerebral ischemia.

Donor pretreatment: rat heart allograft survival and measurement of passenger leukocyte depletion with indium-111.

The present experiment was designed to study the relationship between rat heart allograft survival and passenger leukocyte depletion in donor-pretreated animals. Untreated Lewis rats served as recipients of cardiac allografts from treated Fischer rats. Passenger leukocyte depletion was assayed with indium-111 oxine-labeled leukocytes (predominantly lymphocytes) which were infused into donor rats 6 hr before treatment with cyclophosphamide, antilymphocyte globulin (ALG), sub-lethal total body irradiation, or in vitro perfusion-preservation of the isolated beating heart. In vivo pretreatment of the donor with cyclophosphamide resulted in a significant prolongation of heart allograft survival but effected no reduction in graft-labeled lymphocytes. In vitro perfusion-preservation of the donor heart, for 1 to 2 hr, led to a 50 to 60% reduction in graft-labeled lymphocytes but failed to significantly prolong the survival of the heart allografts. Both ALG and sublethal total body irradiation donor pretreatments resulted in significant prolongation of heart allograft survival and a 20 to 25% labeled passenger lymphocyte depletion. This study demonstrates that there is no direct correlation between allograft survival and the degree of mobile passenger lymphocyte depletion, suggesting that the efficacy of leukocytotoxic donor pretreatment methods may depend in part on alternative mechanisms.

Rapid monitoring of intraoperative cerebral blood flow using 133Xe.

This study examined the feasibility of rapid rCBF monitoring using 133Xe as a tracer during operative procedures. We compared the initial slope index derived from two bicompartmental and one monocompartmental physiological models. The single-compartment model requires only 3 min of monitoring, whereas the bicompartmental models, thought to be more reliable, require 11 min of clearance. Data were collected from 26 patients undergoing carotid endarterectomy. Approximately 20 mCi of 133Xe in saline was injected i.v. for up to five measurements per patient, for a total of 117 measurements. The robustness of the regression for the three parameters (r = 0.781–0.99, p < 0.0001) suggests that the three parameters are closely related. This is supported by similarity of the slopes of the regression lines (between 0.944 and 1.25) and the mean ± SD of the three rCBF models (24.9–27.5 ± 12.0–14.3 ml 100 g−1 min−1). Similar results were obtained for individual detectors, despite the expected higher variability. For intraoperative use in surgical procedures in which physiological conditions may change rapidly and i. v. injections of tracer must be used, a rCBF index that quickly and accurately reflects flow conditions is useful. Our data suggest that the single-compartmental Wyper index may be used to provide information about cerebral perfusion that is as accurate and robust as bicompartmental models, but requires only one-quarter of the data collection time.

Peritransplant streptavidin recipient treatment prolongs rat cardiac allograft survival

Background. Because streptavidin shows high localization in inflamed tissues, it might also interfere with the proliferation of cells involved in allograft rejection. Methods and Results. Treatment of naïve ACI recipients with 20 mg/kg streptavidin i.p. alone significantly prolonged Lewis cardiac allografts from a mean survival time of 9.8±0.7 days in controls to 19.8±6.5 days, with one recipient accepting the graft permanently (>250 days). Peritransplant streptavidin treatment combined with 0.5 ml of antilymphocyte serum (ALS) transient immunosuppression led to permanent graft survival (>250 days) in 6 of 10 recipients. Second-set skin grafts performed 60 days after the primary cardiac allograft were prolonged to 45 days, whereas the third party Wistar-Furth (WF) skin grafts were rejected in 15 days without the rejection of the primary Lewis cardiac allografts. Pathology of transplanted cardiac allografts at 100 days showed no mononuclear cell infiltration or chronic allograft vasculopathy. Streptavidin given for 5 days at 20 mg/kg caused a moderate initial weight loss but had no effect on hematologic, biochemical, and histologic parameters in the treated recipients. Conclusion. This study demonstrates that peritransplant recipient treatment with streptavidin combined with peritransplant ALS induces prolonged cardiac and second-set skin allograft survival. We conclude that recipient peritransplant streptavidin treatment may provide a new strategy for the induction of transplant tolerance.