Iodinated Neuroprobe for Mapping Monoamine Reuptake Sites

This invention relates to neuroprobes for mapping monoamine reuptake sites in the brain, and particularly to a neuroprobe that can also serve as a radiotracer for use in single-photon emission computed tomography (SPECT) and positron emission tomography (PET) for imaging of such reuptake sites.

A brain consists of a plurality of neurons that interact by exchanging chemical messengers. Each neuron generates neurochemicals, referred to as neurotransmitters; neurotransmitters act at sites on the cellular membrane of a neuron, the sites being referred to as receptors. Receptors are associated with either ion channels through the cellular membrane or secondary neurochemical messenger systems. By contrast, reuptake sites are molecular complexes which transport chemicals across the cellular membrane of a neuron. When a neurotransmitter has served its function, it is removed from the vicinity of the receptor by being bound to a reuptake site which transports the neurotransmitter to the interior of the neuron.

Just as there are many specialized neurons in the brain, there are also a variety of neurotransmitters, associated receptors, and reuptake sites. The distribution of specialized neurons depends upon the particular organism under study, and the state of health of that organism.

A neuron can be classified according to the type of neurotransmitter that it uses to communicate with other neurons. Certain types of neurons can be found predominantly in particular regions of the brain. For example, the striatal region of a mammalian brain is innervated by neurons using dopamine as a neurotransmitter. The striatum also contains a large number of non-dopaminergic neurons that have dopamine receptors. Certain compounds, such as cocaine, have a preferential affinity for dopamine reuptake sites, and therefore tend to bind to such reuptake sites. The effect of a molecule such as cocaine upon a dopamine reuptake site is to inhibit reuptake of the neurotransmitter dopamine, leaving more dopamine available in the vicinity of the dopamine receptors.

In certain neurological diseases, such as Parkinson's disease, distinct groups of neurons lose their normal physiological functioning. Consequently, the abnormal neurons may behave differently in the presence of some neurotransmitters, and may also produce neurotransmitters in a manner that differs from a healthy neuron.

The major neurotransmitters, dopamine, norepinephrine, and serotonin, are referred to collectively as the monoamine neurotransmitters. Many neurons have receptors adapted to receive at least one of these neurotransmitters. Parkinson's disease is caused by the degeneration of some of the dopaminergic neurons in the brain. The neurons lost in Parkinson's disease have a large number of dopamine reuptake sites; cocaine and chemical analogs of cocaine have an affinity for such reuptake sites.

A radioisotope is commonly incorporated in molecules that have a demonstrated binding affinity for a particular type of neuroreceptor, and such molecules are commonly used as neuroprobes. The localization of neuroprobes can be used to find specialized neurons within particular regions of the brain. It is also known that a neurological disease can be detected by observing abnormal binding distributions of a neuroprobe. Such abnormal binding distributions can be observed by incorporating a radionuclide within each molecule of the neuroprobe with a high binding affinity for the particular reuptake sites of interest. Then, an imaging technique can be used to obtain a representation of the in vivo spatial distribution of the reuptake sites of interest.

In single photon emission computed tomography (SPECT) imaging, the most commonly used radionuclides are heavy metals, such as .sup.99m Tc. Heavy metals are very difficult to incorporate into the molecular structure of neuroprobes because such probes are relatively small molecules (molecular weight less than 400).

In positron emission tomography (PET), the radiohalide .sup.18 F (fluorine) is commonly used as a substitute for H (hydrogen) in radiopharmaceuticals because it is similar in size. Not all halogens will work, however. For example, I (iodine) is much larger than both H and F, being approximately half the size of a benzene ring. However, due to the small size of typical radiopharmaceuticals for use as neuroprobes, the presence of iodine markedly changes the size of the compound, thereby altering or destroying its biological activity.

In addition, the presence of iodine in a neuroprobe tends to increase its lipophilicity, and therefore increases the tendency of the neuroprobe to engage in non-specific binding. For example, paroxetine is a drug with high affinity and selectivity for serotonin reuptake sites, and [3H]paroxetine has been shown in rodents to be a useful in vivo label (Scheffel, U. and Hartig, PR. J. Neurochem., 52: 1605-1612, 1989). However, several iodinated analogs of this compound with iodine attached at several different positions had unacceptably low affinity, in fact being one tenth of the affinity of the parent compound. Furthermore, when the iodinated compound was used as an in vivo radiolabled neuroprobe, non-specific binding activity was found to be so high that no measurable portion of the brain uptake appeared to be specifically bound to the serotonin reuptake site. Thus, the iodinated form of paroxetine is not useful as an in vivo probe.

The addition of iodine to a neuroprobe can unfavorably alter its biological properties. For example, tomoxetine has high affinity and selectivity for norepinephrine reuptake sites. However, when tomoxetine is iodinated, e.g. to form R-4-iodotomoxetine, the resulting labeled compound has low affinity for such reuptake sites, and relatively high affinity for serotonin reuptake sites. In vivo labeling studies have shown that it is an unacceptably poor probe even for the serotonin reuptake sites because it exhibits low total brain uptake and immeasurably low specific uptake.

An iodinated compound can be useful as an in vitro probe, but may be useless as an in vivo probe, because an in vivo probe must meet the requirements associated with intravenous administration of the probe to a living subject. Reasons for the loss of in vivo utility include the fact that the compound may be metabolized too quickly, that it may not cross the blood-brain-barrier, and that it may have high non-specific uptake into the lipid stores of the brain. In vitro homogenate binding studies remove these obstacles by isolating the brain tissue from hepatic metabolic enzymes, by homogenizing the brain tissue so as to destroy the blood-brain-barrier, and by diluting the brain tissue so as to decrease the concentration of lipids in the assay tube. Accordingly, it cannot be assumed that a probe will be useful in both in vivo and in vitro modalities.

An in vivo SPECT probe was developed by iodinating cocaine. However, this probe shows a binding affinity and specificity no better than cocaine itself, which is inadequate for purposes of SPECT imaging.

An iodinated neuroprobe is provided for mapping monoamine reuptake sites. The iodinated neuroprobe is of the formula: ##STR2## wherein R can be a C.sub.n H.sub.2+1 group, where n=0-6, an alkenyl group, a monofluoroalkyl group including .sup.n F where n=18 or 19, or a .sup.m C.sub.n H.sub.2+1 group where n=1-6 and where m=11 or 14 for at least one .sup.m C. Also R' can be a C.sub.n H.sub.2n+1 group where n=0-6, a p-iodophenylmethyl group, a p-iodophenylethyl group, a phenylmethyl group, or a phenylethyl group. X can be an isotope of F, an isotope of Cl, an isotope of Br, an isotope of I, CH.sub.3, or Sn(R'.sub.1 R".sub.2 R".sub.3). R".sub.1 can be a C.sub.n H.sub.2n+1 group where n=1-6, or an aryl group. R".sub.2 can be a C.sub.n H.sub.2n+1 group where n=1-6, or an aryl group. R".sub.3 can be a C.sub.n H.sub.2n+1 group where n=1-6, or an aryl group. Y can be H only if X is an isotope of I, or R' is a p-iodophenylmethyl group, or R' is a p-iodophenylethyl group. Otherwise Y must be an isotope of I. Also provided is a diastereomer of this embodiment wherein the carboxyl-R' group is in the alpha position.

In a further embodiment, the iodinated neuroprobe for mapping monoamine reuptake sites of the invention is of the formula: ##STR3## wherein R can be a C.sub.n H.sub.2n+1 group where n=0-6, an alkenyl group, a monofluoroalkyl group including .sup.n F where n=18 or 19, or a .sup.m C.sub.n H.sub.2n+1 group where n=1-6 and where m=11 or 14 for at least one .sup.m C. R' can be a C.sub.n H.sub.2n+1 group where n=0-6, a p-iodophenylmethyl group, a p-iodophenylethyl group, a phenylmethyl group, or a phenylethyl group. X can be an isotope of F, an isotope of Cl, an isotope of Br, an isotope of I, CH.sub.3, or Sn(R".sub.1 R".sub.2 R".sub.3). R".sub.1 can be a C.sub.n H.sub.2n+1 group where n=1-6, or an aryl group. R".sub.2 can be a C.sub.n H.sub.2n+1 group where n=1-6, or an aryl group. R".sub.3 can be a C.sub.n H.sub.2n+1 group where n= 1-6, or an aryl group. Y can be H only if X is an isotope of I, or R' is a p-iodophenylmethyl group, or R' is a p-iodophenylethyl group. Otherwise, Y must be an isotope of I. Further, W can be 0, S, (CH.sub.2).sub.n, O(CH.sub.2).sub.n where n=1-6, wherein X resides on a benzene ring of the formula at an ortho, meta, or para position with respect to W, and Y resides at any remaining position on the benzene ring. Also provided is a further embodiment which is a diastereomer of this embodiment wherein the carboxyl-R' group is in the alpha position.

For each of the foregoing embodiments there is provided a precursor of the radiolabled neuroprobe that lacks a radiotracer atom, and a kit for preparing an associated iodinated neuroprobe.

Both the radiostable and radioactive variants of the iodinated neuroprobe of the invention are useful for human and non-human research. For example, in vivo and in vitro experiments can be performed using the compounds of the invention to study dopamine reuptake sites generally, and cocaine binding sites in particular.

The invention will be more fully understood from the following detailed description, in conjunction with the accompanying figures in which:

FIG. 1 shows prior art compounds compared to compounds of the invention;

FIG. 2 shows regional activity in a baboon brain following injection of a compound of the invention;

FIG. 3 shows a synthesis route for a compound of the invention;

FIG. 4 shows regional areas of brain uptake of a compound of the invention;

FIG. 5A shows regional activity in a baboon brain following injection of compound of the invention; and

FIG. 5B shows regional activity in a baboon brain following injection of a compound of the invention.

Metabolically stable cocaine analogs such as 2.beta.-carbomethoxy-3.beta.-(4-iodophenyl)-tropane), an iodine-containing analog of .beta.-CIT (also designated RTI-55), as shown in FIG. 1, compound 3, have high affinities for dopamine and serotonin reuptake sites in brain. As will be discussed below, [.sup.123 I]-.beta.-CIT is shown to be a SPECT (single photon emission computed tomography) radiotracer for dopamine and serotonin reuptake sites.

[.sup.123 I]-.beta.-CIT was prepared by reaction of the corresponding tributyltin precursor with no-carrier added Na[.sup.123 I] in the presence of peracetic acid, followed by preparative HPLC on a C-18 column with methanol/water/triethylamine (75/25/0.2) at a flow rate of 1.0 ml/min. The final product was formulated in 6 ml sterile saline containing 5-10% ethanol.

Six SPECT experiments were performed in four female baboons (10 kg Papio anubis) under isoflurane anesthesia. The animals were injected with 10.6.+-.1.4 mCi[.sup.123 I]-.beta.-CIT and scanned for 333.+-.25 min in either the 810X Brain Imager (Strichman Medical Equipment; five experiments) or the ASPECT device (Digital Sintigraphics, Cambridge, Mass.; one experiment), with these and subsequent data expressed as means .+-.S.E.M. Serial 2-6 min images were reconstructed assuming uniform attenuation equal to that of water in an ellipse drawn around the brain. Data were decay-corrected to the time of injection.

FIG. 2 illustrates regional activity in baboon brain following injection of 9.6 mCi [.sup.123 I]CIT. Activity is expressed in arbitrary units known from phantom studies to be linear with radioactive concentrations. The activities in three brain regions are graphed wherein the trace of open circles is the striatum, the trace of open squares is the midbrain, and the trace of open diamonds is the cerebellum.

The highest activities were found in the striatal region and reached peak levels at 179.+-.9 min (n=6) post injection (p.i.)(FIG. 2). Striatal activity was monitored in two animals for an additional 190 and 260 min post peak values. In one animal, striatal activity was virtually unchanged for the remaining 190 min of the experiment. With reference to FIG. 2, in the second animal, washout of striatal activity was fit to an exponential function and had T.sub.1/2 =27 h (r=0.92).

The brain region which approximately overlay the mesencephalon or midbrain area had the second highest levels of activity. Midbrain values peaked earlier (45.+-.16 min p.i.; n=6) and washed out more rapidly (T.sub.1/2 =294.+-.59 min; r=0.98.+-.0.01; n=3) than that in the striatum.

At the time of peak striatal uptake, the ratios of regional brain activities were: striatum (100%); hypothalamus (38.1.+-.5.2%); occipital lobe (13.5.+-.0.8%); temporo-parietal lobes (14.3.+-.2.0%); frontal lobe (10.3.+-.1.0%); and cerebellum (10.0.+-.1.5%), all measured with n=6.

(-)Cocaine (FIG. 1, compound 1) and CFT (FIG. 1, compound 2), both potent dopamine and serotonin reuptake inhibitors, induced rapid and dose-dependant displacement of both striatal and midbrain activity. (-)Cocaine (2.9 .mu.mol/kg) administered at 200 min p.i. caused displacement of 17% of striatal and 49% of midbrain levels within 30-65 min. At 14.7 .mu.mol/kg administered at 230 min p.i., the corresponding cumulative displacements were 62% and 77%, respectively, within the same period of time.

CFT (0.4 .mu.mol/kg) administered i.v. at 180 min p.i. caused displacement of 57% of striatal and 72% of midbrain levels within 60-120 min. At 2.0 .mu.mol/kg administered at 298 min p.i., the corresponding cumulative displacements were 83% and 91%, respectively, within the same period of time.

In contrast, citalopram (a selective serotonin reuptake inhibitor) caused greater displacement of midbrain than striatal activity. At a dose of 8.3 .mu.mol/kg i.v. at 190 min p.i., midbrain levels decreased by 57% during the following 110 min, compared to only 5% decrease in striatal activity during the same period.

[.sup.123 I]-.beta.-CIT appears to be a useful SPECT tracer of the dopamine and serotonin reuptake sites. Brain uptake and washout are relatively slow in comparison to cocaine itself and are consistent with the metabolically resistant chemical structure of .beta.-CIT and the location of the radioiodine in a chemically stable position. Striatal uptake appears to largely represent labeling of the dopamine reuptake site, whereas that in the midbrain is largely associated with the serotonin reuptake site. The high ratios of striatal to cerebellar activity of [.sup.123 I]-.beta.-CIT are consistent with low non-specific uptake of the tracer, and suggest that [.sup.123 I]-.beta.-CIT may be a useful clinical marker of dopaminergic deficiencies in Parkinson's disease.

Referring again to FIG. 1, in a second study (Neumeyer, J. L. et al., J. Med. Chem., 34: 3144-3146, 1991), the potent cocaine analog 2.beta.-carbomethoxy-3.beta.-(4-fluorophenyl)tropane (compound 2) (also referred to as CFT or WIN 35,428 (Clarke, R. L., et al., 1973; Madras, B. K. et al., 1989)) when tritiated or labeled with .sup.11 CH.sub.3 was found to be superior to [.sup.3 H]cocaine or [.sup.11 C]cocaine (Fowler, J. S. et al., Synapse 4: 371-377, 1989) as a radioligand probe for cocaine receptors in terms of higher affinity and larger residence time on the dopamine reuptake site. For further development of analogues suitable for PET and SPECT imaging, 2.beta.-carbomethoxy-3.beta.-(4-iodophenyl)tropane were synthesized and characterized (compound 3a; designated as .beta.-CIT in analogy to CFT, its corresponding, N-demethylated derivative (compound 4; designated as nor-CIT), and the C.sub.2.alpha. isomer (compound 3b), as shown in FIG. 1.

Referring to FIG. 3, a synthesis protocol for [.sup.123 I]-.beta.-CIT is described. Ecgonidine methyl ester (compound 5) was prepared from cocaine by the procedure of Clarke et al. (1973.) Treatment of compound 5 with phenylmagnesium bromide and subsequent workup with trifluoroacetic acid at low temperature gave a mixture of C.sub.2 epimers (compound 6) (45%) and (compound 7) (31%), which were separated by flash chromatography (silica; CH.sub.2 Cl.sub.2 /CH.sub.3 OH, 25:1). Direct iodination of compound 6 with I.sub.2 /HNO.sub.3 /H.sub.2 SO.sub.4 gave the para-substituted compound 3a (.beta.-CIT) as an oil; 62%; [.alpha.].sub.25 D-2.0.degree. (c=0.85, CHCl.sub.3). D-Tartrate salt; mp 72.degree.-74.degree. C.; [.alpha.].sup.25 D-87.7.degree. (c=1.5, CH.sub.3 OH). Iodination of compound 7 by the same procedure gave compound 3b (.alpha.-CIT) as an oil; 39% [.alpha.].sup. 25 D+44.degree. (c=2.5, CHCl.sub.3). 1,5-naphthalenedisulfonate salt; mp 139.degree.-140.degree. C. N-Demethylation of compound 6 was accomplished by conversion to its 2,2,2,-trichloroethyl carbamate followed by reduction (Zn/acetic acid) to yield compound 8 by the procedure previously described by Milius, R. A., et al., J. Med. Chem. Vol. 34, No. 5, 1728-1731, 1991, herein incorporated by reference, followed by iodination to yield nor-CIT (compound 4), which was isolated as a yellow crystalline solid (free base 48% from compound 6): mp 149.degree.-151.degree. C; [.alpha.].sup.25 D-67.4.degree. (c=1, CHCl.sub.3).

[.sup.123 I]-.beta.-CIT (compound .sup.123 I-3a) was synthesized from nonradioactive .beta.-CIT (compound 3a) by conversion to the corresponding tributyltin derivative (compound 9). Treatment of compound 3a with bis(tributyltin), tetrakis(triphenylphosphate)palladium(0), and palladium(II) acetate in refluxing tetrahydrofuran gave compound 9 as a colorless waxy solid after flash chromatography (silica, stepwise gradient, hexane to hexane/ether, 75:25) in 26% yield from 3a. The 300-MH.sub.3 NMR (CDCl.sub.3) of compound 9 was consistent with the assigned structure. Reaction of compound 9 with no-carrier-added Na.sup.123 I in the presence of peracetic acid gave compound [.sup.123 I]-3a. The radioiodinated product compound [.sup.123 I]-3a was purified by preparative HPLC (Novapak C.sub.18, MeOH/H.sub.2 O/Et.sub.3 N, 75:25:0.2, 1.0 mL/min; t.sub.R 6.7 min) and formulated in normal saline containing 5% ethanol an 1% ascorbic acid. Compound [.sup.123 I]-3a was obtained in average overall yield of 60.0.+-.13.4% and with radiochemical purity of 97.6.+-.1.6%. The tributyltin precursor used in radiolabeling contained about 7 mol % CIT carrier, resulting in an .sup.123 I product having a specific activity of about 2000 ci/mmol.

The affinities of cocaine (compound 1), .alpha.-CIT (compound 3b), .beta.-CIT (compound 3a), and .beta.-CFT (compound 2) for the dopamine and serotonin reuptake sites were determined from radioligand displacement studies using tissue homogenates prepared from baboon and rat brain, shown in Table 1 below.

The data in Table 1 represent radioligand binding of [.sup.3 H]CFT (0.5 nM) to dopamine reuptake sites in tissue homogenates prepared from primate striatum and binding of [.sup.3 H]paroxetine to serotonin reuptake sites in homogenates prepared from rat cortical membranes. The IC.sub.50 value is the concentration of displacing analogue required to decrease specific radioligand binding by 50%. Values represent means .+-.SEM (of n experiments).

With reference to FIG. 4, five SPECT (single photon emission computer tomography) experiments were performed with four female baboons (Papio anubis, 10-12 kg) under isoflurane anesthesia. Animals were injected i.v. with 8.1.+-.1.4 mCi [.sup.123 I]-.beta.-CIT (with these and subsequent data expressed as mean .+-.SEM) and scanned for 300.+-.41 min with the 810X Brain Imager (Strichman Medical Equipment, Medfield, Mass.). Serial 1-2 min images were reconstructed assuming uniform attenuation equal to that of water in an ellipse drawn around the brain. Data were decay corrected to time of injection.

FIGS. 5A and 5B illustrate regional activity in baboon brain following iv injection of 12.1 mCi (FIGS. 5A) and 4.2 mCi (FIG. 5B) [.sup.123 I]CIT. Activity is expressed in arbitrary units known from phantom studies to be linear with radioactive concentrations. Displacing agents (FIG. 5A: 13 .mu.mol Lu-19-005 per kg; FIG. 5B: 7.4 .mu.mol Citalopram per kg) were injected iv at the times marked with arrows. Activities in three brain regions are graphed wherein the trace of filled squares is the striatum, the trace of open circles is the midbrain, and the trace of Xs is the cerebellum.

Highest brain uptake overlay the striatal region and peaked at 154.+-.19 min postinjection (pi) of the radioligand and showed striatal to cerebellar ratios at that time of 9.8.+-.1.6. Washout of striatal activity was followed for an additional 200 and 260 min in two of three control animals and showed 0% and 12% decreases, respectively, from time of striatal peak to end of the experiment.

With reference to FIGS. 5A and 5B, the brain area with second highest activities approximately overlays the midbrain and showed peak levels at 43.+-.5 min pi (n=5) and had a faster washout than striatal activity.

The pharmacological specificity of the in vivo labeling of [.sup.123 I]-.beta.-CIT was examined with displacement of brain activity by indatraline (also designated Lu 19-005), a potent agent for the dopamine and serotonin reuptake sites, and citalopram, an agent selective for the serotonin reuptake site. Indatraline (3 .mu.mol/kg iv) injected at 200 min pi radioligand caused significant decrease of both striatal and midbrain activity, as shown in FIG. 5A. During the 100 min period after injection of Lu 19-005, striatal activity decreased by 65% compared to a mean decrease of 2% during the same period in the two control animals followed for that length of time. In contrast, citalopram (7.4 .mu.mol/kg iv) injected 60 min pi radioligand showed a selective decrease of midbrain activity, as shown in FIG. 5B. Citalopram caused a 48% decrease of midbrain activity during the 60-min period after injection, in comparison to 16.+-.3% decrease (n=3) of midbrain activity in control animals followed during this same period.

These results showed that [.sup.123 I]-.beta.-CIT was a useful SPECT probe of monoamine reuptake sites in primates. The majority of striatal activity was associated with dopamine reuptake sites, and the majority of midbrain activity was associated with serotonin reuptake sites, which is consistent with the densities of these monoamine transporters measured in postmortem primate brains. Brain washout of activity was relatively slow, in part because of the high affinities of .beta.-CIT for the monoamine transporters. In addition, the iodine atom appears to be in a relatively metabolically resistant position, since whole body scanning showed low thyroid uptake, which is indicative of a slow in vivo rate of deiodination. [.sup.123 I]-.beta.-CIT and [.sup.11 C]-.beta.-CIT may be useful clinical markers of dopaminergic and serotonergic innervation in human disorders such as Parkinson's disease and depression, which are thought to have abnormalities in these neuro-transmitter systems.

2-beta-Carbomethoxy-3-beta-(4-iodophenyl)tropane

A mixture of 2-beta-carbomethoxy-3-beta-phenyltropane (See Example 1A below and Milius et al. J. Med. Chem., 1991, 34, 1728) (2.9 g, 11.5 mmol) and I.sub.2 (3 g. 11.8 mmol) in 25 ml of glacial acetic acid was stirred and treated dropwise with a mixture of 4.7 mL of concentrated nitric acid and 4.7 mL of concentrated sulfuric acid. The reaction mixture was heated to 55.degree. C. and stirred for 2 hours, then cooled to room temperature and poured onto ice (100 g) and filtered. The pH of the filtrate was adjusted to 9.5 by the addition of concentrated ammonium hydroxide at 0.degree.-5.degree. C. The resulting precipitate was removed by filtration and dissolved in methylene chloride (250 ml). The filtrate was extracted with two 50 mL portions of methylene chloride. The extracts and solution of precipitate were combined, washed with brine (50 ml) and dried over magnesium sulfate. After the removal of the solvent, 3.9 g (90.4%) of 2-beta-carbomethoxy-3-beta-4-iodophenyltropane free base was obtained as an oil.

The free base was dissolved in methanol (20 ml) and combined with 1.5 g of D-(-)tartaric acid in 20 ml of methanol. After the removal of methanol under reduced pressure, the residue was recrystallized from methanol ether (3:1) to give 2-beta-carbomethoxy-3-beta-(4-iodophenyl)tropane D-tartrate salt as white crystals, m.p. 72.degree.-74.degree. C. C.sub.16 H.sub.20 NO.sub.2 I.C.sub.4 H.sub.6 O.sub.6. Calculated: C: 44.88, H: 4.89, N: 2.62. Found: C: 44.70, H: 4.94, N: 2.57. [alpha].sub.D.sup.22 =-87.7.degree. (c=0.3, CH.sub.3 OH).

2-beta-Carbomethoxy-3-beta-phenyltropane

A 2M ethereal solution of phenylmagnesium bromide (83 mL, 166 mmol) in a 500-mL 3-neck round-bottom flask equipped with mechanical stirrer, addition funnel, and nitrogen inlet tube was diluted with 83 mL of anhydrous diethyl ether and cooled to -20.degree. C. under an atmosphere of dry nitrogen. A solution of anhydroecgonine methyl ester, prepared from cocaine (1) (15 g, 82.8 mmol) in anhydrous ether (75 mL) was added dropwise. The heterogeneous mixture was stirred for 1 h at -20.degree. C., then poured into an equal volume of ice and water, and acidified by the dropwise addition of 2M HCl. The aqueous layer was made basic by the addition of concentrated ammonium hydroxide, saturated with NaCl, and extracted with diethyl ether. The combined extracts were dried (Na.sub.2 SO.sub.4) and concentrated in vacuo to give a brown oil. Bulb to bulb distillation (70.degree. C., 0.9 Torr) of the crude product gave a pale yellow oil (16 g, 70 %). TLC analysis of the oil (silica, pentane/diethyl ether/2-propylamine, 15:5:0.8) showed it to be a mixture of the C-2 alpha and beta epimers. The beta isomer was isolated by silica gel chromatography (pentane: diethyl ether: isopropyl amine, 70:30:3). m.p. 63.degree.-66.degree. C. (lit: 62.degree.-64,5.degree. C.: Clarke et al. J. Med. Chem. 16: 1260 (1973)).

2-alpha-Carbomethoxy-3-beta-iodophenyltropane

The mixture of alpha and beta-2-carbomethoxy-3-beta-iodophenyltropanes prepared as described in Example 1 were separated by silica gel chromatography as described in Example 1. Fractions containing the alpha-2-carbomethoxy-3-beta-iodophenyltropane were pooled and concentrated in vacuo. The free base thus obtained was treated with naphthalene-1,5-disulfonic acid. The crude salt was recrystallized from acetonitrile to give the 2-alpha-carbomethoxy-3-beta-iodophenyltropane naphthalene-1,5-disulfonate salt, m.p. 166.degree.-168.degree. C. C.sub.16 H.sub.20 NO.sub.2 I.C.sub.10 H.sub.6 (SO.sub.3 H).sub.2.2H.sub.2 O. Calculated: C:40.01, H:4:55, N:1.97, I:17.90; Found: C:43.94, H:4.55, N:1.91, I:17.99.

2-beta-Carbomethoxy-3-beta-(4-iodophenyl)nortropane.

A solution of 2-beta-carbomethoxy-3-beta-(4-iodophenyl)tropane (410 mg, 1.5 mmol) in toluene (20 mL) was treated with of 2,2,2-trichloroethyl chloroformate (1 mL, 7.3 mmol). The mixture was heated at 120.degree. C. for 1 hour, cooled to room temperature, and evaporated to dryness in vacuo. The residue was partitioned between methylene chloride and water. The organic layer was separated, dried (Na.sub.2 SO.sub.4), and concentrated in vacuo to give the trichloroethyl chloroformate as a dry foam. The crude carbamate was dissolved in 50% aqueous acetic acid, treated with 200 mg (0.0067 g-atom) of zinc dust, and stirred at room temperature for 16 hours. The reaction mixture was filtered adjusted to pH 7 with concentrated ammonium hydroxide, saturated with NaCl, and extracted with diethyl ether. The extracts were combined, dried (Na.sub.2 SO.sub.4), and concentrated in vacuo. The residue was purified by flash chromatography (silica, pentane/diethyl ether/isopropylamine, 3:7:0.7) to afford 2-beta-carbomethoxy-3-beta-(4-iodophenyl)nortropane, which was isolated as a yellow crystalline solid, m.p 149.degree.-151.degree. C.; [alpha].sup.25.sub.D -67.4.degree. (C=1, CHCl.sub.3).

2-beta-Carbomethoxy-3-beta-(4-iodophenyl)-8-(3-fluoropropyl)-nortropane

A solution of 2-beta-carbomethoxy-3-beta-(4-iodophenyl)-nortropane (371 mg, 1.0 mmol), 1-bromo-3-fluoropropane (155 mg, 1.1 mmol), and triethylamine (0.5 mL) in dry toluene (20 mL) was stirred under an atmosphere of dry nitrogen and heated to reflux. After four hours, the reaction mixture was cooled to room temperature and filtered. The filtrate was concentrated under reduced pressure, and the residue chromatographed on a silica column (eluant: diethyl ether). Concentration of product-containing fractions gave 2-beta-carbomethoxy-3-beta-(4-iodophenyl)-8-(3-fluoropropyl)nortropan e as a white solid, m.p. 78.5.degree.-79.5.degree. C. C.sub.18 H.sub.23 NO.sub.2 FI, Calculated: C: 50.13, H:5.34, N: 3.25; Found: C: 50.27, H: 5.26, N:3.15.

2-beta-Carbomethoxy-3-beta-(3-fluoro-4-iodophenyl)tropane

A mixture of 2-beta-carbomethoxy-3-beta-(3-fluorophenyl) tropane (400 mg, 1.44 mmol), silver sulfate (400 mg, 1.3 mmol), iodine (600 mg, 2.36 mmol) and 80% sulfuric acid (9 Ml) was stirred for five days at room temperature. The reaction mixture was poured into 150 mL of ice and water, made basic by the addition of concentrated ammonium hydroxide, and extracted with three 60 mL portions of chloroform. The combined extracts were washed sequentially with solutions of 10% sodium bisulfite, 5% sodium carbonate and water, then dried over sodium sulfate, and filtered. The filtrate was concentrated in vacuo and the oily residue was redissolved in chloroform and treated with a solution of p-toluene sulfonyl chloride in chloroform. The resulting solid was repeatedly recrystallized from water and ethanol to give 2-beta-carbomethoxy-3-beta-(3-fluoro-4-iodophenyl)tropane tosylate salt as a white crystalline solid, m.p. 68.degree.-70.degree. C. (soften, 45.degree. C.), C.sub.16 H.sub.19 FINO.sub.2 C.sub.7 H.sub.8 SO.sub.3 H.sub.2 O: Calculated: C: 46.55,H: 4.93, N: 2.36; Found: C: 46.34, H: 4.86, N:1.99.

2-beta-Carboxy-3-beta-(4-iodophenyl)tropane

A suspension of 2-beta-carbomethoxy-3-beta-(4-iodophenyl)tropane (100 mg, 0.26 mmol) in 2 mL of H.sub.2 O was heated at reflux for 10 hours. The resulting solution was cooled to room temperature, and the resulting precipitate was collected by filtration and dried under vacuum overnight to give 70 mg (70%) of 2-beta-carboxy-3-beta-(4-iodophenyl)tropane m.p. 299.degree.-300.degree. C. C.sub.15 H.sub.18 NO.sub.2 I.0.5 H.sub.2 O: Calculated C: 47.51, H:5.05, N: 3.69: Found: C: 47.28, H: 4.84, N: 3.69.

2-beta-Carbomethoxy-3-beta-benzyloxytropane

A stirred suspension of benzyl bromide (3.0 g, 0.015 mol) and potassium iodide (3.0 g, 0.021 mol) in acetone (20 mL) was treated dropwise with a solution of ecgonine methyl ester (2.6 g, 0.014 mol) in acetone (10 mL) at room temperature. The mixture was stirred at room temperature for 70 hours, then heated to reflux and stirred for an additional 8 hours. The reaction mixture was cooled to room temperature and filtered. The filtrate was concentrated in vacuo, the residue dissolved in chloroform (200 mL) and extracted with four 50 mL portions of 2N hydrochloric acid. The combined extracts were made basic by the addition of concentrated ammonium hydroxide. The resulting mixture was extracted with four 20 mL portions of chloroform. The extracts were dried over sodium sulfate and concentrated in vacuo to give 1.7 g of 2-beta-carbomethoxy-3-beta-benzyloxytropane as an oil.

The product was dissolved in acetonitrile (20 mL) and treated with a solution of naphthalene-1,5-disulfonic acid (2.2 g) in acetonitrile (20 mL). The solution was concentrated in vacuo to a syrup, which was diluted with diethyl ether. The resulting precipitate was collected by filtration and dried to give 1.6 g of 2-beta-carbomethoxy-3-beta-benzyloxytropane naphthalene-1,5-disulfonate salt, m.p. 126.degree.-130.degree. C., C.sub.17 H.sub.23 NO.sub.3.C.sub.10 H.sub.6 (SO.sub.3 H).sub.2 2.5H.sub.2 O. Elemental analysis: Calculated, C: 52.08, H: 5.83, N: 2.25. Found, C: 52 02, H: 5 69, N: 2. 72[alpha].sub.D.sup.24 =25.4.degree. (c=1, CH.sub.3 OH).

2-beta-Carbomethoxy-3-beta-(4-tributylstannylphenyl)tropane

A mixture of 2-beta-carbomethoxy-3-beta-(4-iodophenyl)tropane (250 mg, 0.65 mmol), bis(tributyl)distannane (522 mg, 0.9 mmol), tetrakis(triphenylphosphine)palladium(0) (3 mg) and anhydrous toluene (10 mL) was heated to reflux under an atmosphere of dry nitrogen and stirred for 28 hours. The mixture was filtered, and the filtrate concentrated in vacuo. The residue was applied to a silica gel column and eluted with a mixture of hexane:diethyl ether:isopropyl amine (70:30:3). The fractions containing product were pooled, concentrated in vacuo and treated with pentane to precipitate 2-beta-carbomethoxy-3-beta-carbomethoxy-3-beta-(4-tributylstannylphenyl)tr opane as a solid. The 300 MHz NMR spectrum was consistent with the assigned structure [alpha].sub.D.sup.22 =-8.9.degree. (c=0.4, CHCl.sub.3).

[.sup.123 I]-2-beta-Carbomethoxy-3-beta-(4-iodophenyl)tropane

To a vial containing 50 .mu.g (0.094 .mu.mol) of 2-beta-carbomethoxy-3-beta-(4-tributylstannylphenyl)tropane was added 50 .mu.L ethanol, 150 .mu.L 0.5M H.sub.3 PO.sub.4, 125-500 .mu.L (20-30 mCi) [.sup.123 I]NaI solution, and 100 .mu.L (4.2 .mu.mol) 0.042M peracetic acid. After 20-30 minutes, 50 .mu.L of 100 mg/mL aqueous NaHSO.sub.3 solution was added. Saturated NaHCO.sub.3 solution was added, and the mixture extracted with ethyl acetate. The combined extracts were dried (Na.sub.2 SO.sub.4) and concentrated to dryness. The residue was redissolved in methanol and purified by HPLC (C-18 column, eluant: CH.sub.3 OH: H.sub.2 O: triethylamine; 75:25:0.2). The fraction eluting at the retention time of 2-beta-carbomethoxy-3-beta-(4-iodophenyl)tropane was collected evaporated to dryness and reconstituted in 5% ethanol and 0.1 nM ascorbic acid.

In SPECT applications, the radiostable iodinated neuroprobe of the invention is useful as a reference standard, and can also be used as a dilutant for the radioactive form of the neuroprobe. The radioiodinated compound is generally identified by its chromatographic mobility as compared with a fully characterized reference standard. Thus, preparation of the radioiodinated compound requires the non-radioactive iodinated compound.

To avoid the necessity of storing a radioactive neuroprobe, it is useful to provide a kit containing the non-radioactive iodinated compound and an appropriate oxidizing agent, such as perchloric acid, performic acid, peracetic acid, hydrogen peroxide, hydrogen peroxide with lactoperoxidase, 1,3,4,6-tetrachloro-3.alpha.,6.alpha.-diphenylglycouril, or a N-chloro-4-methylbenzenesulfonamide sodium salt. Then, the non-radioactive precursor compound can be oxidized in the presence of a suitable radioactive compound, such as the carrier free Na[.sup.123 I] shown in the synthesis route described herein, any other radioisotope source, such as any solution of a salt of a radioactive isotope of iodine a reagent containing .sup.m C.sub.n H.sub.2n+1 X, where n=0-6 and X is a leaving group, or a reagent containing .sup.18 F of the formula .sup.18 FC.sub.n H.sub.2n X, where n=0-6 and X is a leaving group, to prepare the iodinated neuroprobe at its time and place of use.

Radiolabled neuroprobes of the invention are also useful in other imaging procedures. For example, an .sup.125 I-labled neuroprobe can be used in autoradiography or therapy, and an .sup.131 I-labled neuroprobe is useful as a multiple photon emitter for use in animal studies. Also, .sup.11 C-, .sup.14 C-, and .sup.18 F-labeled neuroprobes can be used in PET imaging.

Both the radiostable and radioactive variants of the iodinated neuroprobe of the invention are useful for human and non-human research. For example, in vivo and in vitro experiments can be performed using the compounds of the invention to study the dopamine transporter generally, and cocaine binding sites in particular.

Additionally, the radiostable version of the neuroprobe of the invention can be used as a drug for influencing dopamine reuptake.

Other modifications and implementations will occur to those skilled in the art without departing from the spirit and the scope of the invention as claimed. Accordingly, the above-description is not intended to limit the invention except as indicated in the following claims.