11 Metal Radionuclides for PET Imaging

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11 Metal Radionuclides for PET Imaging

Paul McQuade, Deborah W McCarthy and Michael J Welch

Introduction

Although the majority of PET radiopharmaceuticals in clinical and research use are labeled with the four common PET radionuclides, ¹⁵O, ¹³N, ¹¹C and ¹⁸F, a number of metal radionuclides have been studied.

68Ga, produced from a ⁶⁸Ge/⁶⁸Ga generator, was initially used for brain imaging in 1964 [1, 2]. The ⁸²Sr/⁸²Rb generator was originally developed by researchers at the Squibb Institute for Medical Research [3]. This generator, now marketed by Bracco, is the only industry- approved PET radiopharmaceutical with a Food and Drug Administration New Drug Application (NDA) in the United States. Over the past several years, there has been increasing interest in other metal-based radionuclides, particularly nuclides of copper,⁶⁶Ga and ⁸⁶Y.

Several of these nuclides can be distributed from a central production site and thereby have the potential for use in PET centers without a cyclotron.

In this chapter the available metal PET radionuclides will be summarized and those with the greatest potential for widespread use will be discussed in detail.

Cyclotron PET Radionuclide Production

In general, the majority of radionuclides used in PET imaging are produced by cyclotrons, either on-site or at a site near the scanner. Table 11.1 lists many of the

metal PET isotopes produced by a cyclotron. Isotopes that have a suitable half-life (⁶⁴Cu,⁶⁶Ga,⁸⁶Y) can poten- tially be transported over a long distance from the cyclotron facility.

The following section contains a brief description of the more commonly used cyclotron-produced metal PET isotopes.

Copper Radionuclides

A number of copper PET radionuclides can be produced on a biomedical cyclotron including:⁶⁰Cu,⁶¹Cu,

64Cu with half-lives of 23.4 min, 3.32 h, and 12.8 h respectively. ⁶⁴Cu, ⁶¹Cu and ⁶⁰Cu are produced at Washington University using a specially designed solid target holder [6, 7]. The isotopically enriched nickel targets are electroplated for irradiation, separated using ion exchange chromatography, and can then be recycled [7–9]. Large quantities of these Cu isotopes have been produced (yields of up to 33 GBq (~900 mCi) of⁶⁰Cu, 17 GBq (~150 mCi) of⁶¹Cu, and 40 GBq (~1 Ci) of⁶⁴Cu) [6, 7]. At present,⁶⁴Cu is produced routinely at Washington University. As a result of its 12.8-hour half- life, it can be distributed to investigators across the country.⁶⁴Cu is also produced as a by-product of the

68Zn(p,2n)⁶⁷Ga reaction, where it is separated from the ⁶⁷Ga [10]. However, the speciﬁc activity of⁶⁴Cu produced in this manner is much lower (30 TBq/mol (~860 Ci/mmol)) than that achieved using the

64Ni(p,n)⁶⁴Cu reaction (>370 TBq/mol (10,000 Ci/mmol)) [7].⁶⁴Cu complexed to pharmaceuticals are used for PET imaging, biodistribution studies, and therapy studies. The ⁶⁴Cu metal ion is used for studies 237

* Chapter reproduced from Valk PE, Bailey DL, Townsend DW, Maisey MN. Positron Emission Tomography: Basic Science and Clinical Practice. Springer-Verlag London Ltd 2003, 251–264.

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involving copper metabolism (Menkes’ syndrome and Wilson’s disease), nutrition, and copper transport [11].

In addition,⁶⁰Cu and ⁶⁴Cu have clinical applications in PET [12].

Gallium-66

Gallium-66 can be produced on a biomedical cyclotron via the ⁶⁶Zn(p,n)⁶⁶Ga nuclear reaction [13].⁶⁶Ga is produced at Washington University using isotopically enriched ⁶⁶Zn. The Zn can then be separated from the

66Ga either by cation exchange [14] or by solvent extraction techniques [15].⁶⁶Ga is of interest because it is a medium half-life isotope (t–¹₂= 9.45 h) with potential for both imaging and therapy as a result of its high- energy positron (4.1 MeV) and other high-energy gamma rays [16].

Technetium-94m

94mTc can be produced via the ⁹⁴Mo(p,n)^94mTc nuclear reaction [17]. Using enriched ⁹⁴Mo provides a good yield with relatively low levels of impurities [18].

Thermo-chromatographic separation of Mo from ^94mTc is achieved using a steam distillation method [19].^94mTc is an attractive PET isotope (t¹–₂= 52 min) as it can po-

tentially be used as a substitute in the typical ^99mTc- labeled single-photon-emitting radiopharmaceuticals.

Yttrium-86

86Y can be produced on a biomedical cyclotron via the

86Sr(p,n)⁸⁶Y nuclear reaction using isotopically enriched ⁸⁶Sr foil or carbonate pellet [20]. The separation of⁸⁶Sr from the ⁸⁶Y is carried out by dissolving the carbonate in an acidic solution and then co-precipitating the Sr with lanthanum. The precipitate is then dis- solved and separated using anion exchange [20].⁸⁶Y is appealing as an isotope (t¹_–

2= 14.74 h) because of its potential use for evaluating dosimetry prior to ⁹⁰Y radiotherapy.

With the current interest in small-animal PET imaging [21], a word should be said about the application of these metal radionuclides in such devices. One of the biggest constraints for these scanners is the positron range of the nuclide used. Many of the nuclides described in this chapter haveβ⁺maxenergies in excess of 1 MeV. Above this threshold, signiﬁcant re- duction in resolution is seen. One isotope,⁶⁴Cu, has demonstrated particular utility and promise in small- animal imaging. Its β⁺maxenergy of 646 keV puts it in the range of ¹⁸F, and the resolution of the images is comparable [22–24].

Table 111.1. Cyclotron-produced metal PET isotopes

Isotope Half-life Decay modes (%) Max β⁺energy (MeV) Reaction Natural abundance of target isotope (%)

60Cu 23.4 m β⁺(93), EC(7) 3.92 ⁶⁰Ni(p,n)⁶⁰Cu ⁴ 26.16

45Ti 3.09 h β⁺(86), EC(14) 1.04 ⁴⁵Sc(p,n)⁴⁵Ti ⁴ 100

61Cu 3.32 h β⁺(62), EC(38) 1.22 ⁶¹Ni(p,n)⁶¹Cu ⁴ 1.25

60Ni(d,n)⁶¹Cu ⁴ 26.16

66Ga 9.45 h β⁺(57), EC(43) 4.153 ⁶⁶Zn(p,n)⁶⁶Ga ⁴ 27.81

64Cu 12.8 h β⁺(19), EC(41), 0.656 ⁶⁴Ni(p,n)⁶⁴Cu ⁴ 1.16

β^–(40)

86Y 14.74 h β⁺(34), EC(66) 3.15 ⁸⁶Sr(p,n)⁸⁶Y [4] 9.86

94mTc 53 m β⁺(72), EC(28) 2.47 ⁹⁴Mo(p,n)^94mTc [4] 9.12

55Co 17.5 h β⁺(77), EC(23) 1.50 ⁵⁴Fe(d,n)⁵⁵Co [4] 5.84

89Zr 78.4 h β⁺(22), EC(78) 0.90 ⁸⁹Y(p,n)⁸⁹Zr [4] 100

89Y(d,2n)⁸⁹Zr [4] 100

83Sr 32.4 h β⁺(24), EC(76) 1.15 ⁸⁵Rb(p,3n)⁸³Sr [4] 72.15

38K 7.71 m β⁺(100) 2.68 ³⁸Ar(p,n)³⁸K [5] 0.063

40Ca(d,α)³⁸K [4] 96.97

52Mn 5.6 d β⁺(28), EC(72) 0.575 ⁵²Cr(p,n)⁵²Mn [4] 83.76

52Cr(d,2n)⁵²Mn [4] 83.76

72As 26 h β⁺(77), EC(23) 3.34 ⁷²Ge(p,n)⁷²As [4] 27.37

82mRb 6.3 h β⁺(26), EC(74) 0.78 ⁸²Kr(d,2,)^82mRb [4] 11.56

52Fe 8.2 h β⁺(57), EC(43) 0.80 ⁵⁰Cr(⁴He,2n)⁵²Fe [4] 4.31

51Mn 46.2 m β⁺(97), EC(3) 2.17 ⁵⁰Cr(d,n)⁵¹Mn [4] 4.31

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Generator PET Radionuclide Production

There are many short-lived radionuclides available from radionuclide generators. Generator systems consist of a long-lived parent radionuclide, which decay to a short-lived daughter radionuclide. These generator systems offer a continuous supply of relatively short- lived daughter radionuclides. It has been suggested that the use of generators for PET is very important because they make PET studies possible at centers that are remote from a cyclotron facility. Some of the generator metal PET isotopes are listed in Table 11.2.

Following is a brief description of the more commonly used generator-produced metal PET isotopes.

62Cu/⁶²Zn Generator

62Cu is the daughter isotope of⁶²Zn and is obtained by eluting the ⁶²Cu/⁶²Zn generator [25, 26]. Due to the short half-life of⁶²Cu (t¹–₂= 9.76 m), its production is well suited to a generator. The relatively short half-life of the parent (⁶²Zn, t¹_–

2= 9.13 h) means that these generators must be replaced every few days. An automated

62Cu/⁶²Zn generator that produces the perfusion agent [⁶²Cu]pyruvaldehyde-bis(n⁴-methylthiosemicarbazone) (PTSM) has been used in clinical trials [27].

82Rb/⁸²Sr Generator

82Rb (t¹–₂= 76 s) is the daughter isotope of⁸²Sr. The generator is a microprocessor-controlled, self-contained infusion system [3].⁸²Rb is mainly used in myocardial perfusion studies in which the ⁸²Rb⁺acts as an analog of potassium [28]. In addition, it has been utilized to evaluate blood–brain barrier changes in patients with Alzheimer’s-type dementia [29].

68Ga/⁶⁸Ge Generator

The ⁶⁸Ga/⁶⁸Ge generator produces ⁶⁸Ga as either [⁶⁸Ga]ethylenediaminetetraacetic-acid ([⁶⁸Ga]EDTA) or [⁶⁸Ga]Cl₃in 1M HCl [30].⁶⁸Ga has been used to label blood constituents, proteins, peptides, and antibodies (see section below on Gallium). There have been a limited number of patient studies using the ⁶⁸Ga/⁶⁸Ge generator. The most common use of⁶⁸Ga is in the form of [⁶⁸Ga]citrate, which upon administration produces [⁶⁸Ga]transferrin. This approach has been used to measure pulmonary transcapillary escape rate in various disease states [31–35].

Metal-based Radiopharmaceuticals

The next section of this chapter will deal with the labeling studies that have been carried out with these PET radionuclides.

Gallium

Introduction

In 1871 Dmitri Mendeleev predicted the existence of gallium (which he named eka aluminum) on the basis of the periodic table. It was not until 1875 that this pre- diction was conﬁrmed, when gallium was discovered spectroscopically and obtained as the free metal by Lecoq de Boisbaudian. Naturally occurring gallium has two isotopes,⁶⁹Ga (60.1% natural abundance) and ⁷¹Ga (39.9% natural abundance). Three radioisotopes can be produced, all of which are useful for incorporation into radiopharmaceuticals. Two of these,⁶⁶Ga (t–¹₂= 9.45 h) and ⁶⁸Ga (t¹_–

2= 68 min), decay by β⁺-emission and are Table 111.2. Generator-produced metal PET isotopes [4].

Daughter Isotope Daughter half-life Daughter decay Max β⁺energy Parent Isotope Parent half-life Parent decay

mode (%) (MeV) mode (%)

128Cs 3.8 m β⁺(61), EC(39) 2.90 ¹²⁸Ba 2.43 d EC(100)

44Sc 3.92 h β⁺(95), EC(5) 1.47 ⁴⁴Ti 48 y EC(100)

62Cu 9.76 m β⁺(98), EC(2) 2.91 ⁶²Zn 9.13 h β⁺(7), EC(93)

82Rb 1.25 m β⁺(96), EC(4) 3.15 ⁸²Sr 25 d EC(100)

68Ga 68.3 m β⁺(90), EC(10) 1.90 ⁶⁸Ge 275 d EC(100)

52mMn 21.1 m β⁺(98), EC(2) 1.63 ⁵²Fe 8.2 h β⁺(57), EC(43)

110In 66 m β⁺(71), EC(29) 2.25 ¹¹⁰Sn 4.0 h EC(100)

118Sb 3.5 m β⁺(77.5), EC(22.5) 2.67 ¹¹⁸Te 6.00 d EC(100)

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used for PET imaging, and ⁶⁷Ga (t¹–₂= 78 h) decays by γ-emission and is used for single photon imaging [4].

Chemistry of Gallium

Gallium is a group 13 element and exists most commonly in the +3 oxidation state. While lower oxidation states have been observed, all relevant radiopharmaceuticals occur in this oxidation state. Similar to other group 13 elements (B, Al, and In), Ga³⁺is classiﬁed as a

“hard acid”, bonding strongly to highly ionic, non-po- larizable Lewis bases. As a result, gallium chemistry is dominated by ligands containing oxygen and nitrogen donor atoms [36]. There are two requirements for using gallium complexes as radiopharmaceuticals. The ﬁrst is that gallium complexes must resist hydrolysis at physiological pH. Ga(OH)₃, the primary product formed at physiological pH, is insoluble. It is not until the pH is increased above 9.6 that the soluble species [Ga(OH)₄]^–is formed. The second concern is that Ga³⁺

has an electronic configuration of 3d¹⁰, and this is similar to that of high spin Fe³⁺, which has a half-filled 3d shell. As a result, their ionic radii, ionization potential, and coordination environment are similar. Thus, gallium radiopharmaceuticals must be stable enough to avoid trans-chelation of Ga³⁺ to various iron- binding proteins, particularly transferrin. Transferrin has two binding sites, for which the gallium binding constants are 20.3 and 19.3 [37]. In practice, these requirements necessitate the coordination of Ga³⁺ by a polydentate ligand, typically forming gallium species that are six-coordinate. However, several Ga(III) complexes with coordination numbers four or five are stable in vivo.

Gallium-citrate/transferrin

[⁶⁷Ga]citrate has been used as a tumor imaging agent for over 30 years [38]. It was subsequently discovered that trans-chelation of gallium to the iron-binding protein transferrin was the actual tumor imaging agent [39]. Further work demonstrated that gallium is com- pletely bound to transferrin as soon as 15 minutes after administration of [⁶⁷Ga]citrate [40]. The effectiveness of this radiopharmaceutical is such that it remains in use today in the clinical diagnosis of certain types of neoplasia, lung cancer, non-Hodgkin’s disease, lym- phoma, malignant melanoma, and leukemia. To date, the mechanism by which gallium–transferrin enters tumors is unknown.

[⁶⁸Ga]citrate also has been used in diagnostic imaging, but due to its shorter half-life, the disease states studied are different. Upon injection, Ga-

citrate/transferrin is immediately taken up by the lungs and has been used to study pulmonary vascular per- meability (PVP) using PET imaging techniques that are not possible with ⁶⁷Ga and single photon imaging, due to improved quantiﬁcation [32, 41–43].

Only a limited number of studies have been carried out with [⁶⁶Ga]transferrin. These show that with the longer half-life of ⁶⁶Ga compared to ⁶⁸Ga and with the higher sensitivity of PET versus single photon imaging, [⁶⁶Ga]transferrin may provide an alternative to [⁶⁷Ga]transferrin [44].

Somatostatin Analogs

Somatostatin is a cyclic 14-amino acid peptide that was initially found in the hypothalamus, which has an in- hibiting effect on growth hormone secretion [45].

Receptors for somatostatin can be found in the brain, pituitary gland, gastrointestinal tract, endocrine and exocrine pancreas, and the thyroid [46, 47].

Somatostatin receptors have also been found in a large number of human tumors [48]. Unfortunately, somatostatin has a short plasma half-life, therefore analogs such as octreotide and DOTATOC have been developed. The structural formulas of somatostatin, octreotide, and DOTATOC are shown in Fig. 11.1.

Octreotide is an 8-amino acid cyclic peptide that can be labeled with ⁶⁸Ga using the bifunctional chelator (BFC) DTPA. However, in vitro studies have shown that trans-chelation of gallium takes place [49]. An in vitro and in vivo stable conjugate of octreotide was synthe- sized using the bifunctional chelator desferrioxoamine B (DFO) [49, 50], and it was rapidly cleared from the circulation via the kidneys and showed high tumor uptake [50].

Similar to octreotide, the radiopharmaceutical DOTATOC (DOTA-(D)Phe¹-Tyr³-octreotide) was designed. This can be labeled with a variety of metals in the +3 oxidation state such as Ga, In, B, and Y, and has shown high stability in human serum and has a high afﬁnity to somatostatin receptors. Studies have shown that DOTATOC labeled with gallium is one of the best somatostatin analogs developed to date due to its high tumor-to-blood ratio [51]. DOTADOC has been labeled with both ⁶⁶Ga and ⁶⁸Ga [51–53].⁶⁶Ga with its longer half-life may be preferred over ⁶⁸Ga for PET diagnosis, and, due to other aspects of its decay properties, may have potential for tumor radiotherapy.

Serum Albumin Microspheres

The classical radionuclide technique for measurement of organ perfusion involves vascular injection of

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labeled microspheres of such a size that can be trapped by the ﬁrst capillary bed they encounter [36]. For this purpose, commercial kits of biodegradable albumin colloids are available for ^99mTc labeling. As mentioned earlier, however, the labeling with PET radionuclides offers a distinct advantage over single-photon radionuclides.⁶⁶Ga- and ⁶⁸Ga-labeled albumin microspheres have been studied. The microspheres can be labeled directly with gallium, or through a bifunctional chelator that is covalently bound to the microspheres.⁶⁸Ga- labeled microspheres have been used as regional pulmonary and cerebral blood ﬂow agents [54–57].

However, for studying slow dynamic processes such as lymphatic transport, ⁶⁶Ga-labeled albumin colloids have been proposed [58].

Gallium Labeling of Antibodies and Proteins

The monoclonal antibody antimyosin has been labeled with both ⁶⁶Ga and ⁶⁸Ga, via the bifunctional chelator DTPA, for the imaging of acute myocardial infarction [59]. The slow antigen–antibody reaction during the diffusion of antimyosin in necrotic myocardium re- quires several hours to equilibrate, therefore the longer-lived ⁶⁶Ga would be more appropriate [59].

Twenty-nine hours after administrating [⁶⁶Ga]DTPA- antimyosin, the normal-to-infarcted myocardial ratio was 2.7, and this agent can be viewed as a valid tracer for myocardial necrosis via PET imaging.

DTPA has also been used to label ⁶⁸Ga to the monoclonal antibody fragment BB5-G [60], which is speciﬁc for a human parathyroid surface antigen. This study showed that ⁶⁸Ga-labeled antibodies are potential can- didates for imaging with PET. However, the slow antibody clearance along with the slow antigen–antibody uptake may require the use of a longer-lived radionu-

clide, such as ⁶⁶Ga, to improve target-to-non-target ratios.

Elevated plasma concentrations of low-density lipoprotein (LDL) have been linked to atherogenesis and coronary heart disease [61]. To study this, LDL was labeled with ⁶⁸Ga via the bifunctional chelator DTPA.

Studies involving [⁶⁸Ga]DTPA-LDL showed signiﬁcant uptake in LDL receptor-rich tissue, and this gallium radiopharmaceutical has potential to evaluate lipoprotein metabolism non-invasively [61]. A similar study in- volved the labeling of platelets with ⁶⁸Ga [62], since platelet deposition occurs at the site of endothelial injury and this can be caused by LDL. ⁶⁸Ga-labeled platelets have potential for the imaging of platelet accumulation at the site of vascular lesions.

The examples given thus far have all utilized DTPA as the bifunctional chelator agent, but other ligands can be used for this purpose as well. Other examples include desferrioxamine-B (DFO), which through three hydroxamate groups can co-ordinate the gallium and can be attached to the C- or N-terminus of the biomolecule, and also 1,4,7-triazacyclononane-1-succinic acid 4,7-diacetic acid (NODASA) [63], shown in Fig. 11.2.

Studies have also been undertaken to synthesize new bifunctional chelators containing a more stable and lipophilic metal center to increase clearance of the labeled biomolecule from the liver [64].

Ga-labeled Myocardial Imaging Agents

Due to the convenient half-life of ⁶⁸Ga, considerable work has been done in the development of⁶⁸Ga-labeled myocardial agents. Gallium radiopharmaceuticals that localize in the heart are lipophilic in nature and can be either neutral or cationic.

Ala Gly Cys Lys

Cys Ser Asn

Thr Phe

Phe Phe

Thr Trp

Lys Somatostatin

(D)Phe Cys Phe

Cys Thr

(D)Trp

Lys Thr(OL)

Octreotide

N N

C

COOH HOOC

HOOC

O N H

(D)Phe Cys Tyr

Cys Thr

(D)Trp

Lys Thr(OL)

DOTATOC Figure 111.1. Structural formulas of

somatostatin, octreotide, and DOTATOC.

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A series of uncharged lipophilic tripodal tris(salicylaldimine) ligands (Fig. 11.3) have been investigated with limited success [65]. This work was continued, and analogous species were prepared in which alkoxy sub- stituents were placed on the ethane backbone, resulting in more lipophilic ligands [66]. These ligands provided increased uptake in the heart and higher heart-to-blood ratios, but their increased lipophilicity resulted in higher accumulation in the liver. Along a similar vein, the cationic species [⁶⁸Ga(4,6-MeO₂sal)₂BAPEN]⁺ showed signiﬁcant myocardial uptake and retention, with a heart-to-blood ratio of 45.6 ± 4.0 achieved two hours post injection [67]. This represents an improvement over the analogous neutral salicylaldimine ligands.

Other ligands examined include the neutral complexes of the type 1-aryl-3-hydroxy-2-methyl-4- pyridones, which were found to have heart uptake in animal models [68]. Unfortunately, these species had a short plasma half-life and were only stable long enough for a ﬁrst-pass extraction by the heart.

The ligands described so far have the gallium in a hexadentate environment, but four co-ordinate ligands have also been examined. These include ligands of the type N₂S₂(BAT-TECH) [69] and tris(2-mercaptoben- zyl)amine (S₃N), which showed both brain and myocardial uptake [70] (Fig. 11.4).

Ga-labeled Brain Imaging Agents

The work on gallium radiopharmaceuticals that can cross the blood–brain barrier (BBB) has been con- ducted for some time with only limited success. Species such as [Ga]THM₂BED show low brain uptake immediately after injection, but have a very fast washout [71].

[Ga]EDTA has also been used to show BBB defects at the site of brain tumors and multiple sclerosis plaques [72–75]. The most promising gallium brain imaging radiopharmaceutical developed to date is that complexed with the small lipophilic S₃N species [70]. This agent has a gradual increase in brain uptake, followed by a slow washout. This was demonstrated by a blood-to- brain ratio of 3.5 at 15 minutes after administration that increased to 5.2 after one hour.

Copper

Introduction

Copper has two natural occurring isotopes, ⁶³Cu (69.2%) and ⁶⁵Cu (30.8%). Several radionuclides can be produced for use in PET imaging,⁶⁰Cu,⁶¹Cu,⁶²Cu and

64Cu, with ⁶⁷Cu and ⁶⁴Cu used for therapeutic purposes.

N N

HO2C HO2C

N CO2H CO2H

CO2H

DTPA

N C OH

CH3

O N C

OH

(CH2)2CONH(CH2)5

O H2N(CH2)5 N C

OH O

(CH2)2CONH(CH2)5

DFO

N N

N

CO₂H HO₂C

CO₂H HO2C

NODASA

Figure 111.2. Bifunctional chelators for labeling with ^66/68Ga.

OH

R' C N

RCH₂

3

Ligand R R'

H3[(5-MeOsal)3tame] H OCH3

H₃[(sal)₃tame-O-n-Bu] O(CH₂)₃CH₃ H H3[(sal)3tame-O-iso-B OCH2CH(CH3)2 H H3[5-MeOsal)3tame-O-sec-Bu] OCH(CH3)CH2CH3 H H3[(5-MeOsal)3tame-O-n-Pr] O(CH2)2CH3 OCH3

Figure 111.3. Neutral salicyladimine ligands.

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For a brief description of the radioactive properties and production see the section on copper radionuclides (above).

Chemistry of Copper

Copper is situated in the ﬁrst row of the transition elements and has an electron conﬁguration of [Ar]4s¹3d¹⁰. The chemistry of copper is dominated by two oxidation states, I and II, although Cu(III) complexes have been reported.

Copper (I) has the electron conﬁguration [Ar]3d¹⁰, so its complexes tend to be colorless and diamagnetic.

It prefers “soft” Lewis bases such as thioethers, phos- phines, and nitriles, and usually forms tetradentate complexes that adopt a tetrahedral environment. If Cu(I) is bound by weakly coordinating ligands, it dis- proportionates in solution to give Cu(0) and Cu(II).

Cu(II) has an electron conﬁguration of [Ar]3d⁹, so all mononuclear Cu(II) complexes are paramagnetic.

Cu(II) can be termed an “intermediate” Lewis acid and as such is bound most strongly by nitrogen- and sulfur-containing ligands. The co-ordination number can vary from four to six, with tetradentate complexes preferring a square–pyramidal arrangement and hexadentate complexes adopting a distorted octahedral environment. The distorted octahedral environment is caused by the partially ﬁlled d-orbital that causes tetragonal elongation along the z-axis.

Cu(III) complexes are rare and require strong π-donating ligands for stability. The copper in these species have a [Ar]3d⁸electronic conﬁguration similar to that for Ni(II) and form predominately square planar complexes. Cu(III) complexes are powerful oxi- dants and in solution require a high pH to remain stable; because of this they cannot be utilized in bio- logical systems.

Biochemistry of Copper

The human body contains about 100 mg of copper, the third most abundant trace metal after iron and zinc, and it is distributed mainly in the muscle, bone, liver, and blood. Although the amount of copper is small, it is an essential component in several enzymatic systems [76].

Copper bis (thiosemicarbazones)

One of the most commonly used Cu(II)-bis(thiosemicarbazone) is Cu(II)-pyruvaldehyde-bis(N⁴- methylthiosemicarbazone) ([Cu]PTSM), (Fig. 11.5).

[^62/64Cu]PTSM is a highly lipophilic complex, and is used as a blood ﬂow tracer. Following administration of [^62/64Cu]PTSM, tissue uptake is rapid, and the tracer is trapped in most major tissues such as the brain [77–80], heart [77, 78, 81], and tumors [82–85]. Tissue retention of [^62/64Cu]PTSM and other bis(thiosemicarbazone) derivatives is believed to result when the lipophilic complex diffuses across the cell membrane.

The Cu(II) is then reduced to Cu(I), at which time the copper complex decomposes. Cu(I) is trapped by binding to intracellular macromolecules [86]. Unlike [^62/64Cu]PTSM, studies involving [^60/62/64Cu(II)]diacetyl- bis([N⁴]methylthiosemicarbazone) ([^60/62/64Cu]ATSM) (Fig. 11.5) have shown that [^60/62/64Cu]ATSM has selec- tive uptake in hypoxic tissue [6, 87, 88]. Hypoxia is a condition in which the oxygen demands exceed the oxygen available. Hypoxic tissue can be found in myocardial infarction, brain injury, and tumors. The selec- tivity of hypoxic imaging agents is a balance between its redox potential and cell uptake. The cell uptake is important because hypoxic tissue has poor blood supply, and therefore the ability to deliver the tracer to the tissue is reduced. The difference in uptake of [^62/64Cu]PTSM and [^60/62/64Cu]ATSM is believed to result from the additional methyl group that results in a lower redox potential for [^60/62/64Cu]ATSM. Research is on-going to develop new mixed bis(thiosemicarbazone) ligands in an attempt to develop superior hypoxic imaging agents [89–92].

Design of Copper Celates

The design of copper complexes that are inert poses a signiﬁcant challenge due to the lability of Cu(II).

The choice of ligand can greatly inﬂuence the bioki- netics, biodistribution, and metabolism of the radiopharmaceutical, thus affecting its usefulness. In an attempt to reduce Cu(II) lability, sterically encum- bered ligands and macrocyclic ligands based on Cyclan have been utilized [94–97]. Macrocyclic Figure 111.4. ⁶⁸Ga-labelled myocardial and brain imaging agents.

N SH HS NH SH

HN HS SH

Et Et

BAT-TECH tris(2-mercatobenzyl)amine (S₃N)

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ligands are also used as bifunctional chelators to couple copper to monoclonal antibodies and other biomolecules. Studies of these macrocyclic ligands have shown that the charge carried and the number of functional groups attached to the nitrogens affect the stability and clearance properties of these radiopharmaceuticals [94, 96]. Other ligands examined (Fig. 11.7) include the cationic copper(I) bis(diphos- phines), which act as substrates for P-glycoprotein (Pgp) [98, 99], and Schiff base complexes of Cu(II) as possible perfusion tracers for the brain, heart, and kidneys [100–102].

Copper-labeled Antibodies, Proteins and Peptides

64Cu has labeled monoclonal antibodies (MAbs), proteins, and peptides for PET imaging. Copper is most commonly attached via macrocyclic ligands such as TETA (1,4,8,11-tetraazacyclotetradecane-N,N′,N′′,N′′′- tetraacetic acid), BAD (bromoacetamidobenzyl-1,4,8,11- tetraazacyclotetradecane- N,N′,N′′,N′′′-tetraacetic acid), CPTA (1,4,8,11-tetraazazcyclotetradecane-1-(α-1,4-toluic

acid)) and DOTA (1,4,7,10-tetraazacyclododecane- N,N′,N′′,N′′′-tetraacetic acid). Schematic representations of these ligands are shown in Fig. 11.7. Direct labeling of Cu(I) to antibodies has also been attempted. Stannous tartrate was used to reduce disulﬁde groups, which then bound Cu(I) [103].

Examples of biomolecules labeled with copper include: the anti-colorectal monoclonal antibody (Mab) 1A3 for PET imaging of primary and metastatic colorectal tumors [104–106]; human serum albumin for perfusion and blood pool imaging [107–109]; and somatostatin analogs, such as octreotide and Y3-TATE, for imaging of somatostatin receptors (SSR) [12, 110–112].

Technetium

Introduction

Technetium was predicted on the basis of the periodic table and was incorrectly reported to be discovered in Cu

S S

N N

CH₃ H

N N

H H₃C

H

CH₃ Cu

S S

N N

CH₃ H₃C

N N

H H₃C

H CH₃

Cu-PTSM Cu-ATSM Figure 111.5. Cu(II) bis(thiosemicarbazone)

complexes, [Cu]PTSM and [Cu]ATSM.

N N

COOH

COOH HOOC

HOOC

N O N Br

HOOC N

N

COOH

DOTA

TETA BAD

N N

COOH

COOH HOOC

HOOC NH

NH N HN

CO₂H

CTPA Figure 111.6. Cu(I) and Cu(II) chelates.

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1925, at which time it was named masurium [113].

Technetium-99 was first produced by Perrier and Segre in 1937 and was the first artificially produced element [114]. Naturally occurring technetium-99 was isolated from African pitchblende (a uranium-rich ore) in 1961 [115]. Twenty-two isotopes of technetium with masses ranging from 90 to 111 have been reported; all of them are radioactive. Technetium is one of only two elements with mass number less than 83 that have no stable isotopes, the other is promethium with a mass number of 61. There are three long-lived radioactive isotopes of technetium:⁹⁷Tc (t¹–₂=2.6 x 10⁶years),⁹⁸Tc (t¹–₂= 4.2 x 10⁶years) and ⁹⁹Tc (t–¹₂= 2.1 x 10⁵years). The most-used isotope of technetium is ^99mTc (t¹_–

2 = 6.01 hours), which accounts for about 80% of the radiopharmaceuticals in use. It is used frequently as a result of its short half-life, its low-energy gamma ray, and its ability to be chemically bound to many biologically active molecules as well as the availability and long shelf life of ^99mTc generators. More recently, ^94mTc (t¹–₂ = 52.5 min) has been examined.^94mTc can be produced on a biomedical cyclotron and decays by β⁺-emission and therefore can be used in PET imaging.

Chemistry of Technetium

Technetium is a second-row group-VII transition metal, with an electron conﬁguration of [Kr]5s²4d⁵. The chemistry exhibited by Tc is vast as a result of the fact that it can occupy several different oxidation states (–1 to +7, although only +1 through +7 can exist in solution) and the large number of coordination geome- tries it can adopt [116]. These two factors mean that Tc can bind to a large number of donor ligands. However, this ﬂexibility can cause problems in the design of Tc

radiopharmaceuticals, as well as the in vivo biodistribution of Tc coordination compounds. Technetium is a group-VII congener of rhenium, and as a result their chemistries are similar. The only major difference is that the redox potential of analogous complexes can differ signiﬁcantly, with Tc complexes being more easily reduced. Due to their chemical similarities, rhenium has often been used as an alternative to technetium in preliminary radiopharmaceutical investigations.

Radiolabeling Studies Using ^94mTc

The ﬁrst technetium complex to give consistently high uptake and slow washout in myocardial tissue was the technetium(I) cationic complex [Tc(tbi)₆]⁺ [117]. In this complex, the Tc(I) oxidation state is stabilized by the back-bonding isonitrile ligands, which adopt an octahedral geometry around the technetium. In an effort to improve the heart-to-non-target organ ratios, the isonitrile ligand was functionalized. This led to the development of the complex technetium-methoxy- isobutylisonitrile (Tc-sestamibi, Fig. 11.8) [117], which is now a commercially available instant kit for ^99mTc labeling. Studies have been undertaken in which ^94mTc was used to label sestamibi, in the hope that the increased resolution of PET imaging over SPECT would be useful in detecting myocardial perfusion [118, 119].

Another approach to imaging myocardial perfusion is via the neutral, lipophilic, seven-coordinate Tc(III) complex Tc-teboroxime. Tc-teboroxime can be produced in 90% yield from the reaction of TcO₄^–, dioxime, methyl boronic acid, and SnCl₂under acidic conditions [120]. [^94mTc]teboroxime was prepared by using ^94mTcO₄^– and a commercially available kit for teboroxime [121]. The results obtained were consistent Cu

P P

R R R

R

R R

+ R = CH₃ C₂H₅ C₆H₅

(CH₂)₂OC₂H₅

Cu N

N N

O H

+

Cu

N N

O O

Figure 111.7. Macrocyclic chelates for Cu(II).

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with those reported for [^99mTc]teboroxime [121], and demonstrate that ^94mTc provides an opportunity to study many of the new technetium compounds with PET. Another study involving instant labeling kits available for ^99mTc was the anti-carcinoembryonic antibody Fab’ fragment (CEA-Scan) [122]. This monoclonal Fab’ fragment when labeled with ^99mTc is used for imaging colorectal cancer [123]. The labeling with

94mTc was as straightforward as those observed for

99mTc, with labeling yields typically around 99%.

These studies have shown that ^94mTc labeling can be used as an alternative to ^99mTc and can directly replace

99mTc in instant labeling kits. By utilizing PET imaging,

94mTc should provide image resolution superior to that of the corresponding ^99mTc-labeled agent obtained with single photon imaging. Therefore,^94mTc labeled compounds would make superior radiopharmaceuticals relative to the analogous ^99mTc compounds.

Other PET Metaloradiopharmaceuticals

Rubidium-82

As mentioned previously,⁸²Rb is a short-lived (t¹–₂= 1.27 min) positron-emitting radionuclide that is commercially available as a generator system from the long- lived ⁸²Sr (t¹–₂= 25.6 days) [3]. This generator system is the most widely used in PET imaging, with its in vivo applications based on the ability of rubidium to mimic K⁺[28]. Hence,⁸²Rb has been used to study myocardial perfusion and blood ﬂow [124–127] as well as the in- tegrity of the BBB [128–131].

Potassium-38

38K decays 100% by β⁺-emission and has a half-life (t_–¹

2= 7.71 min) that is short enough to allow multiple studies in the same patient [4]. Similar to ⁸²Rb, PET imaging has focused on ³⁸K⁺as a tracer for myocardial perfusion [132–134] and as a cerebral tracer [134, 135].

Yttrium-86

86Y has a 14.7-hour half life and decays 33% by β⁺-emission [4]. The long half life means that PET images can be acquired several days after administrating the radiopharmaceutical. [⁸⁶Y]citrate and [⁸⁶Y]EDTMP ([⁸⁶Y]ethylenediamine-tetramethylene- phosphonate) have been used to study bone metastases [136–138].⁸⁶Y has also labeled somatostatin analogs for imaging of somatostatin receptors [139–141].

Manganese-51, -52 and -52m

Both ⁵¹Mn and ⁵²Mn are cyclotron-produced positron- emitting radionuclides with half-lives of 46.2 min and 5.6 days, respectively, while ^52mMn is generator produced with a half-life of 21.1 min [4].^51/52Mn have been suggested for use in the diagnosis and treatment of blood diseases [142] and as cationic perfusion tracers [143].^52mMn has been used to study the myocardial and cerebral perfusion [144, 145].

Cobalt-55

Finally,⁵⁵Co has a half life of 18.2 hours and decays 81% by β⁺-emission [4]. It has been used in PET imaging as [⁵⁵Co]Cl₂, where ⁵⁵Co²⁺has been used as a marker for calcium uptake in degenerating brain tissue [146–150] and complexed to molecules such as oxine and MPO (mercaptopyridine-N-oxide) for platelet labeling [151] and to EDTA for renal function assess- ments [152]. It has also been chelated to biomolecules including bleomycin for studies of lung cancer and brain metastases [153–155] and the MAb LS-174T [156].

Conclusions

There are many possible metal-based positron-emitting radionuclides that can be utilized in positron tomography. At the present time, the most promising nuclides are copper, gallium, and yttrium. It is highly Figure 111.8. [^94mTc]-sestamibi.

Tc C

C C

C C C

N N

O

O O N

N O

+