Carla J. Mathias

Indium‐III: a New Radionuclide Label for Studying Human Platelet Kinetics

Summary. Indium‐III, when complexed with 8‐hydroxyquinoline (oxine), has been employed as a radioactive platelet label for thrombus imaging in animals and man. The short half‐life (2.8 d) and high yield of gamma photons of 111In make it ideal for in vitro counting and external imaging. To evaluate its suitability for studies of platelet turnover in man, platelet kinetic studies were carried out on 10 healthy volunteers using 111In‐and 51Cr‐platelets concurrently. For 111In labelling, platelets were harvested by differential centrifugation from 43 ml of whole blood drawn into acid‐citrate dextrose (ACD) solution. The platelets were washed and suspended in a mixture of ACD and isotonic saline and then incubated with 111In‐oxine, rewashed, and suspended in plasma for reinfusion. 51Cr labelling was performed using standard methods. Mean labelling efficiency was 73% with 111In and 6.5% with 51Cr. In vitro studies demonstrated minimal release, elution, and reutilization of the 111In label. There was no significant difference in the aggregation response of 111In‐ and 51Cr‐platelets to ADP and collagen. The in vivo recovery of 111In‐platelets was approximately 50% greater than that of 51Cr‐platelets whereas the platelet life spans were similar. These results indicate that 111In labelled platelets may be useful for thrombokinetic studies in man. The new method offers the advantages of reduced blood requirements, higher labelling efficiency, and the ability to perform external imaging of platelet distribution in vivo.

Preparation of 66Ga- and 68Ga-labeled Ga(III)-deferoxamine-folate as potential folate-receptor-targeted PET radiopharmaceuticals

A folate-receptor-targeting radiopharmaceutical, Ga(III)-deferoxamine-folate (Ga-DF-Folate), was radiolabeled with two positron-emitting isotopes of gallium, cyclotron-produced (66)Ga (9.5 hour half-life) and generator-produced (68)Ga (68 minute half-life). The [(66)Ga]Ga-DF-Folate was administered to athymic mice with folate-receptor-positive human KB cell tumor xenografts to demonstrate that microPET mouse tumor imaging is feasible with (66)Ga, despite the relatively high positron energy of this radionuclide. Using the athymic mouse KB tumor xenograft model, dual-isotope autoradiography was also performed following i.v. co-administration of [(18)F]-FDG, a marker of regional metabolic activity, and folate-receptor-targeted [(111)In]In-DTPA-Folate. The autoradiographic images of 1 mm tumor sections demonstrate the gross heterogeneity of the KB cell tumor xenograft, as well as subtle disparity in the regional accumulation of the two radiotracers.

Receptor-mediated targeting of 67Ga-Deferoxamine-Folate to folate-receptor-positive human kb tumor xenografts

The radiochemical synthesis and stability of 67Ga-deferoxamine-folate ([67Ga]Ga-DF-Folate) were examined as a function of DF-Folate concentration. Optimal labeling occurred at DF-Folate concentrations > or =2.5 microg/mL. To define the possible biological significance of variations in product formulation, the biodistribution of [67Ga]Ga-DF-Folate was examined as a function of administered deferoxamine-folate dose in an athymic mouse KB tumor model. The folate-receptor-positive KB tumors were found to concentrate the 67Ga radiolabel in a dose-dependent fashion, consistent with saturable involvement of the folate receptor in mediating tumor accumulation of the radiopharmaceutical.

The folate receptor as a molecular target for tumor-selective radionuclide delivery

The cell-membrane folate receptor is a potential molecular target for tumor-selective drug delivery, including radiolabeled folate-chelate conjugates for diagnostic imaging. We review here some background on the folate receptor as tumor-associated molecular target for drug delivery, and briefly survey the literature on tumor-targeting with radiolabeled folate-chelate conjugates.

Assessment of regional myocardial and renal blood flow with copper-PTSM and positron emission tomography.

We recently demonstrated in isolated, perfused hearts that radiolabeled pyruvaldehyde bis(N4-methylthiosemicarbazonato)copper(II) (Cu-PTSM) is well extracted throughout a range of conditions including ischemia, hypoxia, and hyperemia. Once extracted, binding of radioactivity by the isolated heart was essentially irreversible, giving this tracer microspherelike qualities. Because Cu-PTSM can be readily prepared with the generator-produced positron-emitting copper 62 and other gamma- or positron-emitting copper radionuclides, we evaluated its usefulness for measuring regional myocardial and renal blood flow in vivo in intact dogs at rest, after ischemia, or after coronary hyperemia was induced by intravenous administration of dipyridamole. After intravenous administration of radiolabeled Cu-PTSM, the tracer cleared rapidly from the blood. Myocardial uptake of single photon-emitting 67Cu-labeled Cu-PTSM was measured directly in myocardial samples 15 minutes after tracer administration, and it increased proportionally with blood flow throughout the flow range (estimated concomitantly with radiolabeled microspheres) of 0.0-6.0 ml/g/min (n = 340 samples from 17 dogs, r = 0.99, Ycopper radioactivity = 85Xmicrosphere flow -7 chi 2 + 17). Renal uptake of radiolabeled Cu-PTSM was also proportional to blood flow. Positron emission tomography was performed in four intact dogs after intravenous administration of 64Cu-labeled Cu-PTSM (19% positron decay, t1/2 = 12.8 hours). High-quality images of heart and kidney were obtained. Accordingly, radiolabeled Cu-PTSM should be a useful, generator-produced tracer for estimating regional myocardial and renal blood flow with positron emission tomography.

Comparison in animal models of 18F-spiroperidol and 18F-haloperidol: Potential agents for imaging the dopamine receptor

Fluorine-18-labeled haloperidol and spiroperidol have been prepared by an exchange reaction using the corresponding non-labeled compound or the nitro analog. Studies in rats have shown that the distribution of labeled spiroperidol has a high striatum to cerebellum ratio which is not observed with haloperidol. A ratio of 10.66 +/- 1.6 is obtained two hours after administration of the 18F-spiroperidol. When 18F-spiroperidol was administered to a baboon and tomographic images obtained, the dopamine receptor rich areas were clearly visualized two hours after administration.

Synthesis and evaluation of 99mTc(CO)3-DTPA-folate as a folate-receptor-targeted radiopharmaceutical

A folate-receptor-targeted 99mTc-radiopharmaceutical, [99mTc]Tc(CO)(3)DTPA-folate, was prepared by heating [99mTc]Tc(CO)(3)(H(2)O)(3)(+) in an aqueous solution of the previously reported DTPA-folate conjugate. The radiotracer was HPLC purified (> 98% radiochemical purity) and evaluated in vitro and in vivo as an agent for targeting folate-receptor-positive cells. [99mTc]Tc(CO)(3)DTPA-folate experienced high, folate-receptor-specific uptake in human KB tumor cells. Intravenous administration of [99mTc]Tc(CO)(3)DTPA-folate to athymic mice bearing KB cell tumor xenografts resulted in 99mTc tumor uptake of 1.8 +/- 0.5 and 3.3 +/- 0.2%ID/g (n = 3) at 30 minutes and 4 hours post-injection, respectively. Tumor uptake was reduced when folic acid was co-administered with the intravenous [99mTc]Tc(CO)(3)DTPA-folate, consistent with radiopharmaceutical localization being mediated by the folate receptor.

Characterization of the uptake of 16α-([18F]fluoro)-17β-estradiol in DMBA-induced mammary tumors☆

Abstract In order to investigate possible correlations between the uptake of 16α-([18F]fluoro)-17β-estradiol (18F-ES) by 7,12-dimethylbenz(a) anthracene (DMBA)-induced tumors in rats and the estrogen receptor (ER) content of these tumors, a comprehensive study was performed in which the tissue distribution of 18F-ES was measured in tumor-bearing rats, together with simultaneous measurements of blood volume (by technetium-labeled red blood cells) and blood flow (by iodoantipyrine infusion). In addition, the time course of 18F-ES metabolism and the tissue distribution of the metabolites was studied. Metabolism of 18F-ES is very rapid, and after 2 h, most of the activity in blood and nontarget tissues is due to metabolites; target tissue activity, however, is due mainly to unmetabolized compound. Most of the circulating activity, both 18F-ES and its metabolites, is strongly associated with macromolecules or cells, and while the metabolites are not taken up selectively by target tissues, they do enter nontarget tissues. Tumor blood volume and blood flow vary widely, but not in a way that appears related to tumor necrosis. The uptake of 18F-ES by the uterus and DMBA-induced mammary tumors of adult rats reaches maximum levels (ca 0.35 and 0.10% I.D./g·kg, respectively) at early times (0–1 h), and drops slowly thereafter. The uterus to nontarget or tumour to nontarget tissue ratios, however, start low and continue to increase, reaching maximum levels (ca 20 and 15, respectively) at 2–3 h. There does not, however, appear to be a simple relationship between tumor uptake (either as % I.D./g·kg or tumor to nontarget ratio) measured at a single 3 h time point and tumor ER content, even considering differences in tumor blood flow. This suggests that an estimation of tumor ER content will require the application of more complex pharmacodynamic models that involve the measurement of the complete profile of receptor lignad uptake, retention, and washout from target to nontarget areas. The application of such models will be assisted by the development of estrogen receptor binding ligands that are not converted to circulating metabolites.

Indium-111 platelet scintigraphy in cerebrovascular disease.

We obtained scintigraphic images of the neck from 100 patients with suspected cerebrovascular disease after injecting indium-111-labeled autologous platelets. One or more focuses of increased activity, implying local platelet accumulation, were seen along the course of the cervical carotid arteries in 52 patients. In 64 patients, there was a highly significant correlation between the results of scintigraphy and carotid arteriography (p = 10 6). There was no significant correlation between the scintigraphic findings and the previous or subsequent occurrence of transient ischemic attack or cerebral infarction in the carotid circulation. These data suggest that factors other than the simple formation of platelet thrombi in the cervical carotid arteries are of primary importance in the pathogenesis of stroke.

A kit formulation for preparation of [111In]In-DTPA-folate, a folate- receptor-targeted radiopharmaceutical

A kit formulation has been developed for convenient, routine compounding of (111)In-labeled In(III)-DTPA-Folate, an investigational radiopharmaceutical for targeting tumor-associated folate receptors. The kit consists of the DTPA-Folate conjugate in sodium citrate solution, from which [(111)In]In-DTPA-Folate can be rapidly and reliably compounded by the addition of aqueous [(111)In]In(III)-chloride.