VIP and breast cancer

VIP1 receptors are present in breast cancer cells. VIP elevates the cAMP and stimulates nuclear oncogene expression in MCF-7 cells. VIPhybrid is a VIP receptor antagonist that inhibits breast cancer proliferation. A VIP analog has been developed for imaging breast tumors. Therefore VIP1 receptors may be utilized for the early detection and treatment of breast cancer.

The gallium-deferoxamine complex: stability with different deferoxamine concentrations and incubation conditions

Previous studies report that deferoxamine (DFO) binds metallic ions such as Fe3+, In3+ and Ga3+ with very high affinity. This property of DFO has been utilized to label DFO-coupled compounds with radiometals such as 67Ga and 111In. We have studied the effect of low DFO concentrations and of different incubation conditions on the stability of the 67Ga-DFO complex. In our experience high (> 5 microM) DFO concentration appears to be critical in obtaining high radiochemical purity of such complexes.

Synthesis and screening of conformationally restricted and conformationally free N-(tertiary aminoalkyl)dithiocarbamic acids and esters as inhibitors of neuronal nitric oxide synthase

N-(Tertiary aminoalkyl)dithiocarbamic acids and esters were synthesized and evaluated for their ability to inhibit neuronal nitric oxide synthase. Preliminary results show these compounds are able to act at the binding site for L-arginine and the conformationally restricted esters may have a second site of activity involving the cofactor (6R)-5,6,7,8-tetrahydro-L-biopterin.

Muscarinic cholinergic receptor measurements with [18F]FP-TZTP: control and competition studies

[18F]Fluoropropyl-TZTP (FP-TZTP) is a subtype-selective muscarinic cholinergic ligand with potential suitability for studying Alzheimer's disease. Positron emission tomography studies in isofluorane-anesthetized rhesus monkeys were performed to assess the in vivo behavior of this radiotracer. First, control studies (n = 11) were performed to characterize the tracer kinetics and to choose an appropriate model using a metabolite-corrected arterial input function. Second, preblocking studies (n = 4) with unlabeled FP-TZTP were used to measure nonspecific binding. Third, the sensitivity of [18F]FP-TZTP binding to changes in brain acetylcholine (ACh) was assessed by administering physostigmine, an acetylcholinesterase (AChE) inhibitor, by intravenous infusion (100 to 200 microg x kg(-1) x h(-1)) beginning 30 minutes before tracer injection (n = 7). Tracer uptake in the brain was rapid with K1 values of 0.4 to 0.6 mL x min(-1) x mL(-1) in gray matter. A model with one tissue compartment was chosen because reliable parameter estimates could not be obtained with a more complex model. Volume of distribution (V) values, determined from functional images created by pixel-by-pixel fitting, were very similar in cortical regions, basal ganglia, and thalamus, but significantly lower (P < 0.01) in the cerebellum, consistent with the distribution of M2 cholinergic receptors. Preblocking studies with unlabeled FP-TZTP reduced V by 60% to 70% in cortical and subcortical regions. Physostigmine produced a 35% reduction in cortical specific binding (P < 0.05), consistent with increased ACh competition. The reduction in basal ganglia (12%) was significantly smaller (P < 0.05), consistent with its markedly higher AChE activity. These studies indicate that [18F]FP-TZTP should be useful for the in vivo measurement of muscarinic receptors with positron emission tomography.

Aminosyn II effectively blocks renal uptake of 18F-labeled anti-tac disulfide-stabilized Fv

Because intact IgG has limitations as a tumor-imaging agent, radiolabeled Fv fragments are being evaluated. Due to the high renal accumulation of Fv fragments, methods to block renal uptake are being sought. This study evaluated how well Aminosyn II, a Food and Drug Administration-approved 15% amino acid solution, would block the renal accumulation of 18F anti-Tac disulfide-stabilized Fv (dsFv) fragments (small fragments with high renal uptake). The anti-Tac dsFv is directed against the alpha subunit of the interleukin 2 receptor. It was labeled at specific activities of 1.1-2.7 mCi/mg using N-succinimidyl 4-[18F]fluoromethyl benzoate. Four adult baboons were injected i.v. with 0.7-1.9 mCi and 150 microg of dsFv. Each baboon was preinjected with Aminosyn II i.v. and, on a separate occasion, with a control solution. Thirty min before injection of 18F-labeled anti-Tac dsFv, a bolus of either solution was given, followed by a constant infusion of 13.3 ml/kg/h. Quantitative positron emission tomography imaging was performed. The amino acid levels in serum were measured serially. The baseline levels of lysine (and other amino acids) in plasma were not significantly different in either the Aminosyn II or control infusion group and did not change during the control infusion. In the Aminosyn II group, lysine levels in plasma 5 min before anti-Tac dsFv infusion were 5-15 times higher than the baseline value and continued to rise during the infusion. The areas under the curve in blood of the 18F-labeled anti-Tac dsFv, from time of injection to end of imaging, expressed as percentage injected dose (%ID), were 28.94 +/- 4.05%ID x h/liter (mean +/- SD) for the control group and 32.09 +/- 11.15%ID x h/liter for the Aminosyn II group (P = 0.54). The peak concentration of 18F-labeled anti-Tac dsFv in the kidney of the controls was 24.53 +/- 4.34%ID; the value in the Aminosyn II group was 5.39 +/- 1.89%ID, representing a mean decrease of 78.5%. The times to reach 90% of the peak levels of 18F in the kidney were 5.6 +/- 3.0 min for the Aminosyn II group and 33.8 +/- 4.8 min for the control group. The amounts excreted in urine by 90 min were 47.7 +/- 8.55%ID and 78.5 +/- 12.8%ID (P = 0.01) for the controls and Aminosyn II group, respectively. In conclusion, Aminosyn II effectively blocks the renal accumulation of 18F-labeled anti-Tac dsFv. Use of Aminosyn II should allow much higher tracer administration for the same radiation exposure to the target organ (kidney).

Preparation of 211At-labeled humanized anti-Tac using 211At produced in disposable internal and external bismuth targets

These studies describe the production and purification of 211At as well as the procedure for labeling humanized anti-Tac, the antibody to the alpha-chain of the IL-2 receptor (IL-2R alpha), which has been shown to be a useful target for immunotherapy. The optimized protocol combines the advantages of the two-stage dry distillation procedure with the astatination of trialkylstannyl substances as labeling compounds for proteins. The 211At was produced by bombarding either an external or a recently developed disposable internal bismuth target with alpha-particles from a Cyclotron Corporation CS-30 cyclotron. The 211At was found to contain less than 0.01% 210At. The production rate for the external target was 0.15 mCi +/- 0.056 microA(-1) h(-1) (n = 9) (5.55 MBq mcroA[-1] h[-1]). The production rate for the internal target was 0.44 +/- 0.14 mCi microA(-1) h(-1) (n = 16) (16.28 MBq mcroA[-1] h[-1]).

(Arg15, Arg21) VIP: evaluation of biological activity and localization to breast cancer tumors

VIP analogs, which contain a single lysine amino acid, were synthesized and evaluated using breast cancer cells. (Arg15, Arg20) VIP, (Argl5, Arg21) VIP, and (Arg20, Arg21) VIP inhibited 125I-VIP binding to T47D cells with high affinity (IC50 values of 1.2, 1.0, and 0.8 nM, respectively). The VIP analogs elevated cAMP in T47D cells with ED50 values ranging from 0.1-1 nM. Because (Arg15, Arg21) VIP was the most potent at elevating cAMP, it was characterized further. (Arg15, Arg21) VIP transiently increased c-fos gene expression in breast cancer cells. N-Succinimidyl-4-18F (fluoromethly) benzoate was prepared in one chemical step from N-succinimidyl-4-(4-nitrobenzenesulfonyl)oxomethyl)benzoate by adding 18F in acetone at room temperature. This prosthetic group was then reacted with (Arg15, Arg21) VIP ((RR) VIP). (18F-RR) VIP bound with high affinity to T47D cells and was rapidly internalized. (18F-RR) VIP was injected intravenously into nude mice bearing breast cancer xenografts and after 4 h, the density of (18F-RR) VIP was elevated in the tumors relative to normal organs. These data suggest that VIP receptors may be used to localize breast cancer tumors.

Kinetic modeling of [11C]raclopride: combined PET-microdialysis studies

The in vivo binding of D2 receptor ligands can be affected by agents that alter the concentration of endogenous dopamine. To define a more explicit relation between dopamine and D2 receptor binding, the conventional compartment model for reversible ligands has been extended to account for a time-varying dopamine pulse. This model was tested with [11C]raclopride positron emission tomography and dopamine microdialysis data that were acquired simultaneously in rhesus monkeys. The microdialysis data were incorporated into the model assuming a proportional relation to synaptic dopamine. Positron emission tomography studies used a bolus-plus-infusion tracer delivery with amphetamine given at 40 minutes to induce dopamine release. The extended model described the entire striatal time-activity curve, including the decrease in radioactivity concentration after an amphetamine-induced dopamine pulse. Based on these results, simulation studies were performed using the extended model. The simulation studies showed that the percent decrease in specific binding after amphetamine measured with the bolus-plus-infusion protocol correlates well with the integral of the postamphetamine dopamine pulse. This suggests that changes in specific binding observed in studies in humans can be interpreted as being linearly proportional to the integral of the amphetamine-induced dopamine pulse.

In vivo muscarinic binding selectivity of (R,S)- and (R,R)-[18F]-fluoromethyl QNB

We have developed a multistep radiochemical synthesis of two diastereomers of quinuclidinyl-4-[18F]-fluoromethyl-benzilate ([18F]-FMeQNB), a high-affinity ligand for muscarinic acetylcholine receptors. Previously, we have shown that the nonradioactive (R,R)-diastereomer displays an eightfold selectivity for M1 over M2 while the nonradioactive (R,S)-diastereomer displays a sevenfold selectivity for M2 over M1 in vitro. This paper reports the results of in vivo comparison studies. In the rat, uptake of (R,S)-[18F]-FMeQNB was nearly uniform in all brain regions following the concentration of M2 subtype. The uptake was reduced by 36-54% in all brain regions on coinjection with 50 nmol of unlabeled ligand. An injection of (R,S)-[18F]-FMeQNB followed at 60 min by injection of unlabeled ligand and subsequent sacrifice at 120 min displaced 30-50% of radioactivity in the pons, medulla, and cerebellum, which contain a high proportion of M2 subtype. The most dramatic displacement and inhibition of uptake on coinjection of (R,S)-[18F]-FMeQNB was observed in the heart. In rhesus monkey, the compound showed prolonged uptake and retention in the brain. In the blood, the parent compound degraded rapidly to a single radiolabeled polar metabolite believed to be fluoride. Within 30 min the parent compound represented less than 5% of the plasma activity. Displacement with (R)-QNB was generally slow, but was more rapid from those tissues which contain a higher proportion of M2 subtype. The results are consistent with the hypothesis that (R,S)-[18F]-FMeQNB is M2 selective in vivo. On the other hand, (R,R)-[18F]-FMeQNB showed higher uptake in those brain regions containing a higher concentration of M1 subtype. Uptake in the heart at 60 min was much lower than that observed with the (R,S)-diastereomer. Inhibition of uptake on coinjection with unlabeled (R,S)-FMeQNB is only significant in the heart, thalamus, and pons. Inhibition of uptake on coinjection with unlabeled (R,R)-FMeQNB is quite uniform in all brain regions. Displacement with (R)-QNB shows a more varying amount displaced. These results are consistent with (R,R)-[18F]-FMeQNB being M1 selective in vivo.

Factors influencing the in vivo pharmacokinetics of peptides and antibody fragments: the pharmacokinetics of two PET-labeled low molecular weight proteins

Monoclonal antibodies (MoAbs) were proposed as candidates for selective tumor targeting based on their high binding affinity for tumors and the absence of binding by normal tissue. However, the exclusive and complete transport of the drug have been found lacking in the use of intact MoAbs, especially in the case of solid tumors. Smaller fragments that maintained the desiderable tumor targeting characteristics appear to have an advantage because of the increase in whole body clearance and the shorter time to maximum target to non-target ratio. But the binding to normal tissue increases as the molecular weight or size decreases, especially in the kidney, because the glomerular sieving coefficient increases. We have used two approaches to overcome the normal tissue binding: a) the use of lysine to block the uptake of the Low Molecular Weight Proteins (LMWP) in the proximal tubules and b) the labeling of different amino acids in the LMWP to alter the residence time in the kidney. The combination of these two methods, blocking the uptake with lysine and shortening the residence time of the radioactive compound produced in the kidney, can lead to substantially decreased normal kidney targeting. The lysine paradigm was effectively demonstrated using 18F-labeled anti-Tac dsFv. Shortening the residence time was illustred by comparing the kinetics of the lysine catabolite versus the methionine metabolite. Furthermore, the rapid pharmacokinetics of the LMWP are consistent with the use of the shorter half-lives of PET radionuclides and the concomitant increase in quantitation and sensitivity afforded by PET.

Biodistribution and catabolism of Ga-67-labeled anti-Tac dsFv fragment

The disulfide-linked fragment (dsFv) of the antibody to the alpha subunit of the IL2 receptor has been radiolabeled with a [Ga-67] Ga-2-(p-SCN-Bz)-NOTA derivative linked through an isothiocyanato group to either the epsilon-amino group of lysine or the alpha-amino group of the N-terminal amino acids. This low molecular weight protein (LMWP) has been proposed as a tumor diagnostic agent. However, > 60% of the injected dose localized in the mouse kidney. The major catabolites (> 95%) in the kidney were identified as the Ga-2-(p-SCN-Bz)-NOTA conjugate with either lysine or methionine, with no evidence of transchelation of Ga-67. Since different amino acids in the dsFv were radiolabeled according to this procedure, it was possible to study the relative residence times of the various catabolites. The methionine conjugate had a significantly shorter residence time than the lysine conjugate in the same kidney. Labeling the appropriate amino acid in a LMWP may lead to reduced residence times and increased diagnostic or therapeutic ratios.

Brain incorporation of [1-11C]arachidonate in normocapnic and hypercapnic monkeys, measured with positron emission tomography

Positron emission tomography (PET) was used to determine brain incorporation coefficients k* of [1-11C]arachidonate in isoflurane-anesthetized rhesus monkeys, as well as cerebral blood flow (CBF) using [15O]water. Intravenously injected [1-11C]arachidonate disappeared from plasma with a half-life of 1.1 min, whereas brain radioactivity reached a steady-state by 10 min. Mean values of k* were the same whether calculated by a single-time point method at 20 min after injection began, or by least-squares fitting of an equation for total brain radioactivity to data at all time points. k* equalled 1.1-1.2 x 10(-4) ml x s(-1) x g(-1) in gray matter and was unaffected by a 2.6-fold increase in CBF caused by hypercapnia. These results indicate that brain incorporation of [1-11C]arachidonate can be quantified in the primate using PET, and that incorporation is flow-independent.

Quantification of amphetamine-induced changes in [11C]raclopride binding with continuous infusion

Positron emission tomography and single-photon emission computer tomography receptor-binding ligands can be used to measure changes in neurotransmitter levels. In particular, amphetamine-induced dopamine release has been assessed with [11C]raclopride by paired bolus injections and with [123I]iodobenzamide by using a single bolus plus infusion (B/I) study. Here, we measured the change in [11C]raclopride-specific binding in rhesus monkeys after i.v. administration of 0.4 mg/kg amphetamine by using both the bolus and B/I paradigms. Paired bolus studies (control and postamphetamine) were analyzed using compartment modeling and graphical analysis with a new plasma metabolite model to measure the total distribution volume (VT). Specific binding, calculated with three measures linearly proportional to the binding potential, demonstrated a 22-42% reduction in the postamphetamine study. VT values from B/I studies were determined by the tissue-to-plasma ratio at equilibrium, in addition to the bolus methods. There was good agreement between the control VT values between bolus and B/I studies. The amphetamine-induced change in specific binding in B/I studies was 19 +/- 16%, measured directly from tissue radioactivity levels. This study demonstrates that stimulus-induced changes in specific binding can be measured with a single [11C]raclopride study using the B/I method.

Measurement of transferrin receptor kinetics in the baboon liver using dynamic positron emission tomography imaging and [18F]holo-transferrin

We have evaluated the use of [18F]holo-transferrin ([18F]Tf) and positron emission tomography (PET) to measure in vivo Tf receptor expression and recycling using the baboon liver as a model. [18F]Tf was intravenously injected in three baboons and dynamic PET was performed over the region containing liver and spleen. In two of the three baboons, [18F]albumin ([18F]Alb), labeled with the same technique, was administered 3 hours later. Time activity curves (TACs) were obtained from liver and spleen for both tracers. TACs for [18F]Tf over the liver were fit to a pharmacokinetic model including vascular radioactivity and an extravascular tissue compartment corresponding to transferrin uptake and release. [18F]Alb data provided an independent estimate of plasma volume. Kinetic analysis showed the presence of a tissue compartment for [18F]Tf that rapidly reaches equilibrium (half time 7-10 minutes). In this organ, the measured rates for Tf turnover obtained with quantitative PET are similar to previously published data using cell culture systems. A model for [18F]Tf in the spleen was not statistically improved by adding a tissue compartment. These data and the pharmacokinetic modeling provide in vivo evidence of a high flux equilibrium binding compartment in the liver, consistent with Tf internalization and recycling.

Schizophrenia is associated with elevated amphetamine-induced synaptic dopamine concentrations: evidence from a novel positron emission tomography method

A major line of evidence that supports the hypothesis of dopamine overactivity in schizophrenia is the psychomimetic potential of agents such as amphetamine that stimulate dopamine outflow. A novel brain imaging method provides an indirect measure of in vivo synaptic dopamine concentration by quantifying the change in dopamine receptor radiotracer binding produced by agents that alter dopamine release but do not themselves bind to dopamine receptors. The purpose of this investigation is (i) to determine the sensitivity (i.e., amount of dopamine reflected in radiotracer binding changes) of this method by examining the relationship between amphetamine-induced changes in simultaneously derived striatal extracellular dopamine levels with in vivo microdialysis and striatal binding levels with the dopamine D2/D3 positron-emission tomography radioligand [11C]raclopride in nonhuman primates, and (ii) to test the hypothesis of elevated amphetamine-induced synaptic dopamine levels in schizophrenia. In the nonhuman primate study (n = 4), doubling the amphetamine dose produced a doubling in [11C]raclopride specific binding reductions. In addition, the ratio of percent mean dopamine increase to percent mean striatal binding reduction for amphetamine (0.2 mg/kg) was 44:1, demonstrating that relatively small binding changes reflect large changes in dopamine outflow. In the clinical study, patients with schizophrenia (n = 11) compared with healthy volunteers (n = 12) had significantly greater amphetamine-related reductions in [11C]raclopride specific binding (mean +/- SEM): -22.3% (+/-2.7) vs. -15.5% (+/-1.8),P = 0.04, respectively. Inferences from the preclinical study suggest that the patients' elevation in synaptic dopamine concentrations was substantially greater than controls. These data provide direct evidence for the hypothesis of elevated amphetamine-induced synaptic dopamine concentrations in schizophrenia.

An automated radiopharmaceutical dispenser

An automated radiopharmaceutical dispenser, that offers several advantages over manual procedures, has been developed. It employs a personal computer interfaced to a precision syringe drive module, a dose calibrator, and a printer. The operating program provides menu-selection operation and documentation of procedures and individual doses delivered. All materials in contact with the radiopharmaceutical are sterile and disposable. A novel transport safe is employed to further reduce radiation exposure.

Technetium-99m labeling and biodistribution of anti-TAC disulfide-stabilized Fv fragment

We used a preformed 99mTc chelate approach to label a genetically engineered disulfide-bonded Fv fragment of anti-Tac monoclonal antibody (dsFv). The biodistribution of this 99mTc-labeled dsFv was evaluated in athymic mice with IL-2 alpha-receptor-positive ATAC4 tumor xenografts.

METHODS

Benzoylmercaptoacetyl-triglycine (BzMAG3) was first labeled with 99mTc, and the carboxy group of 99mTc-MAG3 was then activated to the corresponding tetrafluorophenyl ester. This activated ester was purified with a Sep-Pak C18 column and conjugated to dsFv. The resulting 99mTc-MAG3-dsFv was purified with PD-10 size-exclusion chromatography. The immunoreactivity of 99mTc-MAG3-dsFv was 76% +/- 9%. When incubated in serum at 37 degrees C for 24 hr, there was no appreciable dissociation of 99mTc. The mice were co-injected with 125I-dsFv labeled by the Iodo-Gen method as a control. The mice were killed at 15 to 720 min for analysis of biodistribution and radiocatabolites.

RESULTS

The tumor uptake of 99mTc-MAG3-dsFv was similar to that of 125I-dsFv. The tumor uptake of 99mTc-MAG3-dsFv was rapid with tumor-to-blood or tumor-to-organ ratio higher than 1 for all organs except the kidneys. The peak tumor value of 5.1% injected dose per gram was obtained at 45 min, and the tumor-to-organ ratios increased steadily over time; a ratio of 15, 11, 7, 95 and 0.10 resulted at 6 hr for blood, liver, stomach, muscle and kidney. The radioactivity was primarily excreted through kidneys.

CONCLUSION

The rapid achievement of high tumor-to-blood and -tissue ratios makes 99mTc-MAG3-dsFv a promising agent for scintigraphic detection of various hematological malignancies that express IL-2 alpha receptors.

Labeling proteins at high specific activity using N-succinimidyl 4-[18F](fluoromethyl) benzoate

High effective specific activity N-succinimidyl 4-[18F](fluoromethyl)benzoate was prepared using a reversed phase HPLC procedure. Reversed phase HPLC removed several additional impurities not removed by normal phase HPLC, thereby increasing the effective specific activity. Small amount (< 100 micrograms) of sensitive proteins such as erythropoietin can be labeled with this reagent in high yield without aggregation.

Molecular modeling of the interaction of diagnostic radiopharmaceuticals with receptor proteins: m2 antagonist binding to the muscarinic m2 subtype receptor

Models of the m2 muscarinic receptor have been built and acetylcholine and an antagonist of the quinuclidinyl benzilate family docked to the putative active site. We have incorporated aspects of homology, site-directed mutagenesis studies and structure-activity studies of specific lead compounds in the construction of our receptor models with a primary focus on the structure of the binding sites. We have observed a deep pocket binding of 5-BrQNT, suggesting a plausible explanation for the observation that agonists and antagonists do not bind competitively. The results of these computational studies are interpreted within the context of the observed in vitro results. Our goal is to assist in the development of subtype receptor selective radiopharmaceuticals for use in PET and SPECT.

Autoradiographic evidence that (R)-3-quinuclidinyl (S)-4-fluoromethylbenzilate ((R,S)-FMeQNB) displays in vivo selectivity for the muscarinic m2 subtype

Alzheimer's disease (AD) involves selective loss of muscarinic m2, but not m1, subtype neuroreceptors in cortical and hippocampal regions of the human brain. Until recently, emission tomographic study of the loss of m2 receptors in AD has been limited by the absence of available m2-selective radioligands that can penetrate the blood-brain barrier. We now demonstrate the in vivo m2 selectivity of a fluorinated derivative of QNB, (R)-3-quinuclidinyl (S)-4-fluoromethylbenzilate ((R,S)-FMeQNB), by studying autoradiographically the in vivo inhibition of radioiodinated (R)-3-quinuclidinyl (S)-4-iodobenzilate ((R,S)-[125I]IQNB) binding by unlabelled (R,S)-FMeQNB. In the absence of (R,S)-FMeQNB, (R,S)-[125I]IQNB labels brain regions in proportion to the total muscarinic receptor concentration; in the presence of 75 nmol of (R,S)-FMeQNB, (R,S)-[125I]IQNB labelling in those brain regions containing predominantly m2 subtype is reduced to background levels. We conclude that (R,S)-FMeQNB is m2-selective in vivo, and that (R,S)-[18F]FMeQNB may be of potential use in positron emission tomographic (PET) study of the loss of m2 receptors in AD.

L-lysine effectively blocks renal uptake of 125I- or 99mTc-labeled anti-Tac disulfide-stabilized Fv fragment

In this study, we investigated the ability of L-lysine to block renal uptake of 125I- or 99mTc- labeled Fv fragments. Anti-Tac disulfide-stabilized Fv fragment (dsFv) was derived from a murine monoclonal antibody that recognizes the alpha subunit of the interleukin-2 receptor (IL-2R alpha). The 125I- or 99mTc-labeled dsFv was injected i.v. into non-tumor-bearing nude mice or into nude mice bearing SP2/Tac (IL-2R alpha positive) and SP2/0 (IL-2R alpha negative) tumor. We then evaluated the pharmacokinetics of L-[3H]lysine and the effect of L-lysine dose, timing of administration, and route of delivery on catabolism and biodistribution of i.v. dsFv. Peak renal uptake of i.v. or i.p. injected L-[3H]lysine occurred within 5 and 15 min, respectively. The kidney uptake of L-lysine exhibited a dose-response effect. When L-lysine was coinfused or injected shortly before dsFv, renal uptake of dsFv was blocked to < 5% of the control, but longer intervals were less effective. Aminosyn II and Travasol 10% (parenteral amino acid solutions) also blocked renal uptake of radiolabeled dsFv. Administration of L-lysine did not alter the blood kinetics and slightly increased the tumor uptake of dsFv, but it did prevent catabolism in the kidney and resulted in lower amounts of catabolites in the serum and urine. In conclusion, we have shown that a blocking dose of lysine, injected with or immediately before the injection of radiolabeled dsFv, is most effective in blocking the renal uptake of dsFv. This is consistent with the rapid uptake of L-[3H]lysine by the kidney and is further substantiated by the relative ineffectiveness of lysine injected immediately after the radiolabeled dsFv injection.

A review of new oncotropic tracers for PET imaging

We have developed three biochemical probes to determine if they are sensitive probes of early biochemical change in a tumor. All three probes appear to have the appropriate properties for in vivo imaging, but must now be evaluated as probes for the sensitive detection of changes in early malignant disease.

Evaluation of human transferrin radiolabeled with N-succinimidyl 4-[fluorine-18](fluoromethyl) benzoate

Iron metabolism plays a key role in cell proliferation and survival in rapidly growing cancer cells. Uptake is mediated by the carrier protein transferrin. The increased need for iron has been used as a method to target tumors and there is well-documented evidence that certain tumors can be imaged with tracers such as 67Ga, that mimic transferrin-mediated iron uptake. To obtain a tracer that would be better able to quantitate transferrin kinetics and indirectly evaluate iron metabolism, we have labeled human transferrin with the positron emitter, 18F, with a one-step high-specific activity method developed in our laboratory.

METHODS

We measured the binding affinities of [18F]diferric (holo-) and iron-free (apo-) transferrin on two human cell lines. We also compared cellular uptake of [18F]holo-transferrin and [67Ga]citrate in various conditions, and washout of label incorporated into cells.

RESULTS

The binding affinity of [18F]holo-transferrin was found to be the same as that reported for [125I]holo-transferrin. In our hands there was no significant difference in binding affinity between diferric holo-transferrin and iron-free apo-transferrin. [18F]holo-transferrin uptake rapidly reaches a steady-state equilibrium between the intracellular and extracellular environment, while gallium accumulation linearly increases with time. [18F]holo-transferrin is rapidly recycled out of the cell with similar kinetics to those reported for [125I]holo-transferrin.

CONCLUSION

[18F]holo-transferrin displays the properties of native transferrin and appears suitable for quantitative evaluation of transferrin kinetics in vivo.

Synthesis and evaluation of radioiodinated 6-iodo-L-DOPA as a cerebral L-amino acid transport marker

Regioselective radioiodination of N-trifluoroacetyl 3,4-dimethoxy-6-trifluoroacetoxymercurio-L-phenylalanine ethyl ester 1 under no-carrier-added condition gave 6-[125I]iodo protected L-DOPA with a labeling efficiency of more than 85%, and no-carrier-added 6-[125I]I-L-DOPA was obtained with a radio-chemical purity of over 95% after hydrolysis and chromatography. A nonradioactive standard of 6-iodo protected L-DOPA was synthesized by the iododemercuration of 6-mercuric trifluoroacetate protected L-DOPA 1 using I2 in chloroform. 6-[125I]I-L-DOPA showed high brain accumulation and rapid blood clearance in mice. The rat brain slice studies indicated high affinity of 6-[125I]I-L-DOPA for carrier-mediated and stereoselective active transport systems. The tissue homogenate analysis revealed that most of the accumulated radioactivity was intact 6-[125I]I-L-DOPA. Thus, 6-[123I]I-L-DOPA appears to be a suitable single photon emission computed tomography (SPECT) tracer for the selective measurement of cerebral L-amino acid transport, having no affinity for dopamine metabolism.