Synthesis and evaluation of [11C]FR194921 as a nonxanthine-type PET tracer for adenosine A1 receptors in the brain

This report describes the synthesis of [¹¹C]2-(1-methyl-4-piperidinyl)-6-(2-phenylpyrazolo[1,5-a]pyridin-3-yl)-3(2H)-pyridazinone ([¹¹C]FR194921), a highly selective, nonxanthine-type adenosine A₁ receptor antagonist, used in brain imaging in rats and conscious monkeys as a potential novel PET tracer. [¹¹C]FR194921 was successfully synthesized in 19 min after [¹¹C]CH₃I formation. The radiochemical yield was 38±3%; and radioactivity was 4.1±0.4 GBq, calculated from end of synthesis; radiochemical purity was higher than 99%; and the specific radioactivity was 25.0±8.1 GBq μmol⁻¹ (n=5). In a rat experiment, the distribution of [¹¹C]FR194921 was higher in the hippocampus, striatum and cerebellum regions. This accumulation was significantly decreased by approximately 50% by pretreatment with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A₁ receptor antagonist, which indicated specific binding of the radioligand to adenosine A₁ receptors. In conscious monkey PET experiments, [¹¹C]FR194921 accumulated in several regions of the brain, especially in the occipital cortex, thalamus and striatum. These results suggest that [¹¹C]FR194921 can be used as an agent for imaging adenosine A₁ receptors in vivo by positron emission tomography (PET).     

Fully automated synthesis of [18F]fluoromisonidazole using a conventional [18F]FDG module

We developed a new fully automated method for the synthesis of [¹⁸F]fluoromisonidazole ([¹⁸F]FMISO) by modifying a commercial FDG synthesizer and its disposable fluid pathway. A three-step procedure was used to prepare the tosylate precursor, 1-(2′-nitro-1′-imidazolyl)-2-O-tetrahydrofuranyl-3-O-toluenesulfonylpropanediol. Using glycerol as the starting material, the precursor was synthesized with a yield of 21%. The optimal labeling conditions for the automated synthesis of [¹⁸F]FMISO was 10 mg of precursor in acetonitrile (2 ml heated at 105°C for 360 s, followed by heating at 75°C for 280 s and hydrolysis with 1 N HCl at 105°C for 300 s. Using 3.7 GBq of [¹⁸F]F⁻ as a starting activity, [¹⁸F]FMISO was obtained with high end-of-synthesis (EOS) radiochemical yields of 58.5±3.5% for 60.0±5.2 min with high-performance liquid chromatography (HPLC) purification. When solid-phase purification steps were added, the EOS radiochemical yields were 54.5±2.8% (337±25 GBq/μmol) for 70.0±3.8 min (n=10 for each group, decay-corrected). With a high starting radioactivity of 37.0 GBq, we obtained radiochemical yields of 54.4±2.9% and 52.8±4.2%, respectively (n=3). The solid-phase purification removed unreacted [¹⁸F]fluoride and polar impurities before the HPLC procedure. Long-term tests showed a good stability of 98.2±1.5%. This new automated synthesis procedure combines high and reproducible yields with the advantage of using a disposable cassette system.     

Automated synthesis and purification of [F]bromofluoromethane at high specific radioactivity

[¹⁸F]Bromofluoromethane was synthesised from dibromomethane by substitution of bromine with [¹⁸F]fluoride. The synthesis and separation of the [¹⁸F]bromofluoromethane were automated. [¹⁸F]Bromofluoromethane was used to convert a phenolic and a thiophenolic precursor into a labelled ether and thioether, respectively. The specific radioactivity of these labelled products was determined with both high-performance liquid chromatography (with UV-absorbance detection) and liquid chromatography (with mass spectrometric detection). The median for the specific radioactivity, corrected at the end of radionuclide production, was 934 GBq/μmol (range 40–9900 GBq/μmol; n=83).     

Development of an automated system for the quick production of 13N-labeled compounds with high specific activity using anhydrous [13N]NH3

An automated system was developed to synthesize ¹³N-labeled compounds with high specific activity using anhydrous [¹³N]NH₃ as a synthetic precursor. This system enabled (1) production of an aqueous solution of [¹³N]NH₃, (2) concentration and desiccation of the [¹³N]NH₃ solution, (3) reaction of anhydrous [¹³N]NH₃ with substrate, (4) purification and formulation.By the use of this system, [¹³N]p-nitrophenyl carbamate ([¹³N]NPC) ready for i.v. injection could be obtained in 5.1±0.1 min at the yield of 3.5±0.4 GBq, specific activity 460±55 GBq/μmol, and radiochemical purity >99% (n=3) by irradiating a 10 mM ethanol solution with 18 MeV protons at 17 μA for 25 min. With special precautions, [¹³N]NPC could be obtained at the extremely high specific activity of 1800±200 GBq/μmol.     

Radiosynthesis and ex vivo evaluation of [11C]-SIB-1553A as a PET radiotracer for beta4 selective subtype nicotinic acetylcholine receptor

[¹¹C]-SIB-1553A ((±)-4-[2-((N-[¹¹C]-methyl)-2-pyrrolidinyl)ethyl]thiophenol) was labelled with carbon-11 (t_1/2=20.4 min) and evaluated in vivo as potential radiotracer for noninvasive assessment of the β4 subunit nicotinic acetylcholine neurotransmission system with positron emission tomography (PET). The labelling precursor was obtained within five steps from N-Boc-prolinal in 45–56% overall yields. The radiosynthesis of [¹¹C]-SIB-1553A was achieved by a selective N-[¹¹C]-methylation in 32 min with a radiochemical purity greater than 97%, 7.5–30 GBq/μmol of specific radioactivity and 55–65% radiochemical yield (decay corrected, based on [¹¹C]methyl iodide). The ex vivo pharmacological profile of [¹¹C]-SIB-1553A was evaluated in rats with biodistribution studies in organs and in brain structures by autoradiography. The radiotracer uptake in the brain reached 0.49 %ID/g at 10 min and no brain radiometabolite was detected 40 min after intravenous injection. The quantification of radioactivity in various cerebral structures indicated a significantly higher radioactivity level at 15 min than at 30 min. Among the β4 nAChR subunit-rich structures studied in the rat brain, only the thalamus at 15 and 30 min and the hippocampus at 30 min showed significantly higher uptake. Moreover, competition studies performed with SIB-1553A (15 min before the radiotracer injection) revealed only a low specific binding estimated to 7% of the total binding at 15 min and 13% at 30 min.     

Synthesis and biological evaluation of 1-(4-[18F]fluorobenzyl)-4-[(5,6-dimethoxy-1-oxoindan-2-yl)methyl]piperidine for in vivo studies of acetylcholinesterase

We synthesized and evaluated 1-(4-fluorobenzyl)-4-[(5,6-dimethoxy-1-oxoindan-2-yl)methyl]piperidine (4-FDP), which is an analog of donepezil. The 4-[¹⁸F]FDP was prepared by reductive alkylation of debenzylated donepezil with 4-[¹⁸F]fluorobenzaldehyde in high radiochemical yield (decay-corrected, 40–52%) and with high effective specific activity (30–38 GBq/μmol). Tissue distribution studies in mice demonstrated nonspecific distribution of the 4-[¹⁸F]FDP in brain regions, suggesting that this radioligand may not be a suitable agent for in vivo studies of acetylcholinesterase (AChE), despite its potent in vitro biological activity.     

Highly efficient automated synthesis of [C]choline for multi dose utilization

[¹¹C]Choline has been under investigation as a PET ligand for imaging tumor tissue, especially prostate cancer. An improved, automated synthesis of the tracer now was established. [¹¹C] Choline was produced by labeling 2-(dimethylamino)-ethanol (DMAE) with [¹¹C]CH₃I in a Tefzel^® tube at room temperature without solvent. The product was purified using a cation exchange cartridge. Reaction conditions were optimized with respect to synthesis time and amount of DMAE, resulting in radiochemical yields higher than 80% using 60 μl of DMAE in 20 min, radiochemical purity was >99% and residual DMAE was below 10 ppm. After ¹¹C-production of 1 h at 50 μA [¹¹C]choline activities of 30.0±5.6 GBq (n=29) were obtained in sterile solution ready for intravenous administration.     

Preparation of high specific activity (86)Y using a small biomedical cyclotron

⁸⁶Y is an attractive PET radionuclide due to its intermediate half-life. ⁸⁶Y was produced via the ⁸⁶Sr(p,n)⁸⁶Y nuclear reaction. Enriched SrCO₃ or SrO was irradiated with 2-6 μA of beam current for <4 h on a CS-15 cyclotron. It was shown that the SrO target could withstand at least 6 μA of beam current, a significant improvement over a maximum of 2 μA on the SrCO₃ target. Average yields of 4.5 mCi/μA·h were achieved with SrO, which represent 71% of the theoretical yield, compared to 2.3 mCi/μA·h with SrCO₃. The radioisotopic contaminants were ^86mY (220%), ⁸⁷Y (0.27%), ^87mY (0.43%) and ⁸⁸Y (0.024%). ⁸⁶Y was isolated in an electrochemical cell consisting of three Pt electrodes. The solution was electrolyzed at 2000 mA (40 min) using two Pt plate electrodes. A second electrolysis (230 mA for 20 min) was performed using one Pt plate and a Pt wire. On average, 97.1% of the ⁸⁶Y was recollected on the Pt wire after a second electrolysis. The ⁸⁶Y was collected from the Pt wire using 2.8 M HNO₃/EtOH (3:1). After evaporation, ⁸⁶Y was reconstituted in 100 μl of 0.1 M HCl. Target materials were recovered as SrCO₃ and then converted to SrO by thermal decomposition at 1150°C. Specific activity of ⁸⁶Y was determined to be 29±19 mCi/μg via titration of ⁸⁶Y(OAc)₃ with DOTA or DTPA. We have established techniques for the routine, economical production of high purity, high specific activity ⁸⁶Y on a small biomedical cyclotron that are translatable to other institutions.     

[C]NNC 687 and [C]NNC 756, dopamine D-1 receptor ligands. Preparation, autoradiography and PET investigation in monkey

NNC 687 and NNC 756 [(+)-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl-8-nitro-2,3,4,5-tetrahydro-1H-3-benzazepine and (+)-8-chloro-5-(2,3-dihydrobenzofuran-7-yl)-7-hydroxy-3-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine] are two new potent dopamine D-1 receptor antagonists. [¹¹C]NNC 687 and [¹¹C]NNC 756 were both prepared by N-methylation of the corresponding desmethyl compounds with [¹¹C]methyl iodide. The reactions were performed in acetone with subsequent normal-phase semi-preparative HPLC and resulting in 50–60% radiochemical yield (from EOB and decay-corrected) with a total synthesis time of 30–35 min and a radiochemical purity higher than 99%. The specific radioactivity obtained at time of injection was about 1500 Ci/mmol (55 GBq/μmol). Autoradiographic examination of [¹¹C]NNC 687 and [¹¹C]NNC 756 binding in post-mortem human brain sections showed specific binding in the striatum, a region with high density of dopamine D-1 receptors. PET examination of the radioligands in a Cynomolgus monkey demonstrated accumulation of radioactivity predominantly in the striatum. The ratio between radioactivities in the striatum and the cerebellum was about 2 and 8 for [¹¹C]NNC 687 and [¹¹C]NNC 756 after 60 min. [¹¹C]NNC 756 should have potential as PET ligand for examination of central dopamine D-1 receptors in man.     

Preparation of the hypoxia imaging PET tracer [F]FAZA: reaction parameters and automation

¹⁸F-labeling of the nitroimidazole nucleoside analogue 1-(5-fluoro-5-deoxy--D-arabinofuranosyl)-2-nitroimidazole (FAZA) was developed to use this tracer in PET for detection of hypoxia. Parameters for labeling and hydrolysis were optimized with regard to amount of precursor, temperature and time. Labeling yields reached a maximum of 62±4% at 100 °C within 5 min using 5 mg of precursor. Hydrolysis was best performed with 1 mL of 0.1 N NaOH at 20 °C for 2 min. Transfer of these conditions to an automated synthesizer resulted in an overall radiochemical yield of 20.7±3.5%. Absolute yields at EOS were 9.8±2.3 GBq of [¹⁸F]FAZA ready for injection (n=21; 50 min after EOB; irradiation parameters: 35 μA, 60 min). Thus, a convenient approach suitable for large-scale production of [¹⁸F]FAZA was developed by an automated process.     

Comparison of non-isotopic exchange methods for n.c.a. labelling of 2-iodophenyl-metyrapone

The Cu(I)-assisted aromatic halogen exchange proved useful for preparation of macroscopic amounts of 2-iodophenyl-metyrapone as well as for the n.c.a. radioiodinated analogue. Semi-preparative HPLC-isolation provided the compound with high purity for use as chromatographic standard and for the determination of the inhibition constant of the 2-iodo-analogue. The non-isotopic exchange led to a high specific activity (>5000 GBq/μmol) of 2-[¹²³I]iodophenyl-metyrapone. A reaction in acetic acid at elevated temperatures proved superior to an exchange in aqueous solution with in-situ reduction of Cu²⁺. Even without Cu⁺ high radiochemical yields of > 80% were obtained, while addition of Cu⁺ provided the radiotracer with 95% radiochemical yield within 5 minutes in acetic acid.     

Preliminary results on the production of short-lived radioisotopes with a Plasma Focus device

An experimental campaign was conducted to assess the feasibility of short-lived radioisotope (SLR) production within the pulsed discharges of a Plasma Focus (PF) device. This so-called “endogenous production” technique rests on the exploitation of nuclear reactions for the creation of SLR directly within the plasma, rather than on irradiating an external target. Until now only one research group has published data relevant to PF endogenous production of SLR, and the data seem to confirm that the PF has the capability to breed SLR. The campaign demonstrated production of ¹⁵O, ¹⁷F and ¹³N from the ¹⁴N(d,n)¹⁵O, ¹²C(d,n)¹³N and ¹⁶O(d,n)¹⁷F reactions. A 7 kJ, 17 kV Mather-type PF was operated with natural nitrogen, oxygen, CO₂ and deuterium in the vacuum chamber. Results to date confirm that, with a PF of this type, up to 1 μCi of SLRs per discharge can be obtained.     

Efficient synthesis of 2-bromo-1-[F]fluoroethane and its application in the automated preparation of F-fluoroethylated compounds

An efficient synthesis of 2-bromo-1-[¹⁸F]fluoroethane from commercially available 1,2-dibromoethane and its integration into an automated preparation device was developed for the routine synthesis of ¹⁸F-fluoroethylated compounds. The 1,2-dibromoethane was reacted with the [¹⁸F]fluoride/Kryptofix^®2.2.2./carbonate complex in acetonitrile at 70°C for 3 min resulting in 60–70% radiochemical yields. The crude reaction mixture was diluted with water, loaded on a LiChrolute^®EN-cartridge, eluted with acetonitrile and passed through an AluminaB^®-cartridge. This method provides 2-bromo-1-[¹⁸F]fluoroethane with 98% radiochemical purity and <0.1 μmol of 1,2-dibromoethane within 10 min, thus avoiding a purifying distillation step. This method was easily integrated into an automated system for the routine synthesis of ¹⁸F-fluoroethylated compounds.     

Synthesis and biodistribution of R- and S-isomers of [18F]-fluoropropranolol, a lipophilic ligand for the beta-adrenergic receptor

The S and R isomers of [¹⁸F]-fluoropropranolol (1-[1-fluoro-2-isopropylamino]-3-naphthalen-1-yloxy-propan-2-ol) have been prepared by reductive alkylation of the appropriate aminoalcohols. The radiosynthesis provides a reasonable yield (25%) to give products of 99% enantiomeric excess and specific activities of 1–3 Ci/μmol. The dissociation constants for the β₂ adrenergic receptor are 0.5 and 2.5 nM for the S and the R isomers, respectively. The biodistribution data in rats show that uptake and egress of the tracer is rapid but that the result of blocking studies and the difference between the R and the S isomers suggest receptor-mediated uptake in receptor-rich tissue.     

A liquid-scintillation-based primary standardization of Pb

A new radioactivity solution standard of ²¹⁰Pb has been developed and will be disseminated by the National Institute of Standards and Technology (NIST) as standard reference material (SRM) 4337. This new ²¹⁰Pb solution standard is contained in a 5 mL flame-sealed borosilicate glass ampoule, consists of (5.133±0.002) g of a nominal 1 mol L⁻¹ nitric acid solution, has a density of (1.028±0.002) g mL⁻¹ at 20 °C, has carrier ion concentrations of about 11 μg Pb²⁺ and 21 μg Bi³⁺ per gram of solution, and is certified to contain a massic activity (9.037±0.22) kBq g⁻¹ as of the reference time 1200 EST, 15 June 2006. All of the uncertainties cited above correspond to standard uncertainties multiplied by a coverage factor k=2. The standardization for the ²¹⁰Pb content of the solution was based on 4πβ liquid scintillation (LS) measurements using CIEMAT/NIST ³H-standard efficiency tracing (CNET). Confirmatory determinations were also performed by high-resolution HPGe γ-ray spectrometry, by 2π spectrometry with a Si surface barrier detector of separated ²¹⁰Po, and by 4πβ(LS)–γ(NaI) anticoincidence counting.     

Improved target system for production of high purity [F]fluorine via the O(p,n)F reaction

An improved aluminium target system for production of elemental fluorine via the ¹⁸O(p,n)¹⁸F reaction using a two-step irradiation protocol is described. In the first step highly enriched gaseous oxygen-18 is irradiated with protons to form fluorine-18 which gets deposited on the inner target surface. In the second step, after cryogenic recovery of oxygen-18 target gas, a mixture of elemental ‘cold’ fluorine and krypton is introduced and a short proton irradiation is done, whereby an isotopic exchange between the gaseous fluorine and the deposited radiofluorine occurs. The second step leads to the recovery of the radiofluorine as [¹⁸F]fluorine. Optimisation studies were performed regarding the yield and specific radioactivity of [¹⁸F]fluorine. Furthermore, some irradiation parameters relevant to the recovery step were investigated. It was found that a 15 to 20 min irradiation with a beam current of 20 μA is sufficient for the isotopic exchange between the fluorine-carrier and the ¹⁸F-radioactivity deposited on the inner wall of the target. The distribution of the ¹⁸F-radioactivity deposited on the inner target surface is inhomogeneous, probably due to convection effects. Extensive radioanalytical techniques were applied to characterise the reactivity of [¹⁸F]fluorine and to identify undesired nonreactive ¹⁸F-compounds, mainly [¹⁸F]tetrafluoromethane and [¹⁸F]nitrogentrifluoride. The [¹⁸F]fluorine produced in the system used has the distinction of having a negligible contamination from those inert ¹⁸F-compounds. This is a combined effect of the use of highest purity gases and a welded target construction, which avoids any contact of the gases with organic material during irradiations. The target has proved to be very reliable for production of [¹⁸F]fluorine in high yields of up to 34 GBq and specific activities of 350–600 GBq/mmol, both at 30 min after end of activation bombardment.     

Highly potent indanamine serotonin uptake blockers as radiotracers for imaging serotonin uptake sites

Two highly potent indanamine serotonin (5-HT) uptake blockers, trans-3′-(4′-bromophenyl)-1-indanamine (trans-[¹¹C]DBPI or [¹¹C]Lu 19-056) and its iodo analog, trans-3′(4′-[¹²⁵I]iodophenyl)-1-indanamine (trans-[¹²⁵I]DIPI) were evaluated as radiotracers for imaging 5-HT uptake sites in vivo. Trans-[¹¹C]DBPI was synthesized by N-methylation of the normethyl precursor with [¹¹C]iodomethane. Trans-[¹²⁵I]DIPI was synthesized by iododestannylation of the tributyltin precursor with [¹²⁵I]NaI. Radiochemical yields for the [¹¹C] and [¹²⁵I] radiotracers were 34 and 40%, with specific activities of 4000 and 1800 mCi/μmol, respectively. In vitro, the iodo analog, trans-DIPI, showed an IC₅₀ value of 0.26 nM in inhibition of [³H]paroxetine binding to 5-HT uptake sites in rat cortex. The potency was found to be equivalent to that of paroxetine or McN5652. In vivo, after i.v. injection into mice, both radiotracers showed high uptake in brain (3–4% dose/whole brain at 15 min) and high accumulation into target tissues such as hypothalamus and olfactory tubercles (7–8% dose/g at 60 min). The binding was blocked by pre-injection of 5 mg/kg of paroxetine. While the in vivo distribution agreed with previously reported 5-HT uptake site distribution, the radiotracers showed high uptake in non-target tissues such as cerebellum, resulting in low target-to-non-target ratios (1.5–1.6 at 60 min). Since washout from non-target regions was slower than from target regions, longer-time observation with ¹²⁵I up to 6 h did not improve the ratios. HPLC analyses of mouse brain homogenates and blocking studies indicated that the high uptake in non-target regions is not the result of metabolism or any interaction of the radiotracers with those tissues via specific binding sites. In spite of low target-to-non-target ratios, target regions with high density of 5-HT uptake sites, such as the raphe nuclei, superior colliculi and substantia nigra, were visualized with trans-[¹²⁵I]DIPI by ex vivo autoradiography, since the radiotracer showed high specific binding (total minus nonspecific binding).     

Improved quality control of [18F]FDG by HPLC with UV detection

A conventional high-performance liquid chromatographic (HPLC) method for the analysis of 2-fluoro-2-deoxy-d-glucose (FDG) and 2-deoxy-2-chloro-d-glucose (ClDG) in [¹⁸F]FDG preparations is described. This method was based on a postcolumn derivatization with 2-cyanoacetamide (2-CA) and UV detection. FDG and ClDG were separated on a normal-phase column using acetonitrile/water as the mobile phase. The eluate was mixed with 2-CA in sodium borate buffer solution at the outlet of a PTFE coil (10 m×0.5 mm id) from the column, and the reaction was carried out at 100°C during the passage through the coil. The UV absorbance of the resultant product was monitored at 276 nm. Under optimum conditions, the detection limits [signal-to-noise (S/N) ratio=3] for FDG and ClDG were 0.31 and 0.17 μg/ml for a 20-μl injection volume, respectively, and the linearity ranges were 0.5–100 μg/ml for both compounds. The intra- and interday reproducibilities were better than 2.2% [relative standard deviation (R.S.D.)]. This HPLC separation procedure is also useful for determining the radiochemical purity of [¹⁸F]FDG preparations since it allows the analysis of 2-[¹⁸F]fluoro-1,3,4,6-tetra-O-acetyl-d-glucose ([¹⁸F]TAG), partially hydrolyzed [¹⁸F]TAG and [¹⁸F]F⁻. This method can be used at many positron emission tomography (PET) facilities since it does not require an expensive, sophisticated electrochemical detector.     

In Vivo evaluation of an In-labeled ST-peptide analog for specific-targeting of human colon cancers

In vitro competitive binding studies of In-DOTA-NCS-6-Ahx-Phe¹⁹-ST[1–19] vs. ¹²⁵I-Tyr⁵-6-Ahx-Phe¹⁹-ST[1–19] with guanylate cyclase -C (GC-C) receptors on human colon cancer LS-180 cells revealed an IC₅₀ value of 7.7 ± 0.1.6 nM. The in vitro cellular residualization studies of the ¹¹¹In-DOTA-NCS-ST peptide and GC-C receptor mediated stimulated cGMP production with LS-180 cells demonstrates that this peptide selectively binds to LS-180 cells in an agonistic fashion. In vivo biodistribution studies in LS-180 tumor bearing SCID mice demonstrates that the ¹¹¹In-DOTA-NCS-ST peptide targets the tumor with a specific uptake of 0.94 ± 0.31%ID/g at 1 hr p.i. and approximately 23% was retained by the tumor at 4 hrs p.i. The radioactivity cleared rapidly from the blood stream with 84.5 ± 3.4%ID at 1h p.i. found in the urine. High activity in urine and kidney, and minimal activity in liver and intestines, demonstrates preferential clearance of the radioactivity through the renal/urinary pathway. The specific in vitro and in vivo accumulation of the radioactivity by LS-180 human colonic cancer cells highlights the potential of radiometallated-DOTA-ST analogs as diagnostic/therapeutic radiopharmaceuticals.     

Tumor uptake of radioiodinated anti-human pulmonary surfactant-associated protein monoclonal antibody PE10 in nude mice bearing human pulmonary adenocarcinoma in combination with an unlabeled preload

This study assessed the potential use of radioimmunoscintigraphy of pulmonary alveolar Type II cells tumor with the radiolabeled anti-human surfactant-associated protein (SP) monoclonal antibody (MAb) PE 10 in combination with preloads of unlabeled MAb. The in vitro binding of iodine-125 (¹²⁵I)-labeled MAb PE 10 (1 μg), which had a specific radioactivity of 400 MBq/mg, on human pulmonary papillary adenocarcinoma NCI-H441 cells that produced SP was investigated. In NCI-H441 tumor-bearing nude mice, the tumor uptake of ¹²⁵I-MAb PE 10 (5 μg) was examined in combination with preloads of unlabeled MAb PE 10 (0, 5, 10, and 50 μg). An isotype-matched unassociated murine MAb was used as a control both in vitro and in vivo. ¹²⁵I-MAb PE 10 showed specific cell binding compared with ¹²⁵I-control MAb. Tumor uptake of ¹²⁵I-MAb PE 10 in vivo reached a peak of 4.97±0.33% injected dose per gram (%ID/g) at 48 h postinjection. Preloads of 5 and 10 μg unlabeled MAb PE 10 significantly enhanced tumor uptake at 48 h postinjection ( 5.94±0.29% ID/g and 5.72±0.29% ID/g, respectively), whereas preload of 50 μg unlabeled MAb PE 10 significantly decreased tumor uptake ( 2.75±0.32% ID/g) at 48 h. Preload of 5 μg unlabeled MAb PE 10 significantly increased the tumor-to-blood radioactivity ratio at 48 h ( 2.39±0.16). Preloads of unlabeled control MAb did not cause any significant change in tumor uptake. Immunohistochemistry showed the intracellular and pericellular patterns of SP expression in tumor cells. In conclusion, radioimmunoscintigraphy with MAb PE 10 labeled with a γ-emitting radioiodine such as ¹²³I might be a useful means of targeting pulmonary alveolar Type II tumor cells in combination with preloading with an optimal dose of the unlabeled MAb.