Lima J.J.Pedroso, de (ed.). Nuclear Medicine Physics

Подождите немного. Документ загружается.

98 Nuclear Medicine Physics

TABLE 4.1

Examples of Some Radiopharmaceuticals Used in Molecular Imaging Studies and

Their Application

Radiopharmaceutical Physiological Process/Molecular Target Reference

O Blood ﬂow (neurology) [3]

O Blood volume [4]

[

F]ﬂuoromisonidazole (FMISO) Cellular hypoxia [5]

[

F]annexin V Apoptosis [6]

2-[

F]ﬂuoro-2-deoxy-D-glucose (FDG) Oxidative metabolism [7]



-deoxy-3



F]ﬂuorothymidine (FLT) Proliferation (cell replication) [8]

[

C]choline Proliferation (membrane synthesis) [9]



-deoxy-2



F]ﬂuoro-5



-iodo-1β-

D-arabinofuranosyluracil (FIAU) Gene expression [10]

[

N]NH

Blood ﬂow (cardiology) [11]

[

C]acetate Oxidative metabolism (cardiology) [12]

[

C]palmitate Fatty acid metabolism (cardiology) [13]

14-[

F]ﬂuoro-

6-thia-heptadecanoic acid (FTHA) Fatty acid metabolism (cardiology) [14]

6-[

F]ﬂuoro-L-DOPA (FDOPA) Dopamine biosynthesis [15]

[

C]methyl-methionine (MET) Proliferation (protein synthesis) [16]

O-(2-[

F]ﬂuoroethyl)tyrosine (FET) Proliferation (protein synthesis) [17]

L-[3-

F]-α-methyl-tyrosine (FMT) Proliferation (protein synthesis) [18]

[

C]WAY1000635 Serotonin 5-HT

receptor [19]

[

C-methyl]SCH 23390 Dopamine D

receptor [20]

[

C]raclopride (RAC) Dopamine D

receptor [21]

4.3 Tracer Molecule

A fundamental characteristic of a radiopharmaceutical is that it can access

the physiological compartment where the target is located. To achieve this,

two fundamental characteristics are needed: the availability of the tracer in

blood plasma and its distribution across the relevant biological barriers. The

majority of drugs are transported in blood bound to plasma fractions, either

proteic or lipidic. The extent of binding as well as the kinetics of the binding

process (fast or slow) determine the availability of the radiopharmaceutical

to enter the tissue. In addition, the passage of the radiopharmaceuticals into

the tissue depends on the existence of a transport mechanism or the pres-

ence of favorable physicochemical characteristics of the molecules for their

passive diffusion across the barriers. For example, in the central nervous sys-

tem, the main physical barrier involves the tight junctions of the capillary

endothelium (blood–brain barrier). The extent by which a certain compound

Radiopharmaceuticals 99

can cross this barrier can be estimated by multiplying the permeability of the

compound by the capillary surface (the so-called PS product). In this context,

the permeability of the barrier to a certain compound is a characteristic value

easily related by structure–function studies with physicochemical properties

such as lipophilicity, charge, and molecular size [22].

The maximization of the signal measured in vivo and, consequently, the via-

bility of a radiopharmaceutical for molecular imaging (Figure 4.2), ultimately

depend on the balance between the molecular interactions established with

the active site (speciﬁc signal) and all other interactions (nonspeciﬁc signal).

We can therefore write, assuming ﬁrst-order kinetics that the tissue concen-

tration of a radiopharmaceutical C

is a function of its plasma concentration

over time:

(t) =



i=1



(τ)e

−β

(t−τ)

dτ. (4.2)

In this expression, components with a high value of α

correspond to

signiﬁcant contributions to the observed signal, whereas β

values for the

speciﬁc signal that are signiﬁcantly different from the β

values of the other

components allow us to separate the speciﬁc signal. A common situation is

when the β value of the speciﬁc process is much lower than those of all the

others (β →0). In this case, the protocol of the exam can allow sufﬁcient time

for the activity due to nonspeciﬁc processes to decay before image acquisition

takes place. This is, for example, the case of the studies with 2-ﬂuoro-deoxy-

D-glucose (see Section 4.7.2.1), in which an essentially irreversible speciﬁc

Selectivity

Blood–tissue

partition

Nonspecific

binding

Affinity

Metabolism

FIGURE 4.2

Overview of the main physiological processes involved in a molecular imaging study with radio-

pharmaceuticals. (Adapted from Physiological Imaging of the Brain with PET, Academic Press, San

Diego, pp. 51–56, 2001.)

100 Nuclear Medicine Physics

process (phosphorylation by hexokinase) and the deﬁnition of an appropri-

ate waiting time after the injection allow the quantiﬁcation of the speciﬁc

process (oxidative metabolism).

Metabolism of the radiopharmaceuticals can be another obstacle for the

quantiﬁcation of a speciﬁc component of the signal. In fact, the formation,

during the study, of metabolites that maintain the labeling are able to cross

relevant biological barriers and present afﬁnity for molecular targets in the

region of interest can preclude the quantiﬁcation of the speciﬁc binding of

the radiopharmaceutical to its speciﬁc site. The development of metabolism-

resistant radiopharmaceuticals has been one of the main concerns, especially

in the case of molecules that are present in several metabolic pathways (e.g.,

carbohydrates or amino acids).

4.4 Radionuclide Selection

The success of a radiopharmaceutical for in vivo imaging requires that the

labeling process has a minimal interference in the physicochemical char-

acteristics of the molecule. This requires, ideally, an isotopic substitution

or, in case this is not possible, the substitution by an atom or chemical

group that does not signiﬁcantly change the in vivo behavior of the labeled

molecule.

In the case of PET, the existence of radioactive isotopes of carbon (

1/2

= 20.4 min), nitrogen (

N, t

1/2

= 9.97 min), and oxygen (

O, t

1/2

122 s) that are positron emitters allows the labeling of virtually any organic

molecule. However, the relatively short half-lives of these nuclides signif-

icantly limit the complexity of the molecules to be labeled as well as the

range of processes that can be studied in vivo. In contrast, halogen positron

emitters presenthalf-lives that aremore appropriate for complex labeling pro-

cedures and more prolonged studies (

F, t

1/2

= 109.8 min,

Br, t

1/2

= 16 h,

124

I, t

1/2

= 4.2 days); but their presence is not very common in organic

molecules, and the introduction of a halogen atom frequently causes signiﬁ-

cant changes in the physicochemical properties of the compounds. A notable

exception is ﬂuorine-18, which has been used with great success as a sub-

stitute for the hydrogen atom that has a close Van der Waals radius and,

sometimes, even for hydroxyl groups with which it shares a similar electronic

conﬁguration.

The substitution of hydrogen by Fluorine-18 is, as a consequence, the most

common form of nonisotopic labeling in PET, as the changes introduced in

a molecule by this substitution are minimal. The physicochemical changes

introduced in a molecule by a change of a certain chemical group are usu-

ally evaluated in a diagram similar to that presented in Figure 4.3 (Craig

plot) in which a series of chemical groups are plotted in terms of two main

physicochemical characteristics: lipophilicity (σ) and polarity (π). This dia-

Radiopharmaceuticals 101

–1.00

–0.75

–0.50

CONH

)

123

OCF

CONH

[

C]t-but.

–0.25

2.01.61.20.80.4–0.4–0.8–1.2–1.6–2.0

0.25

0.50

0.75

1.00

FIGURE 4.3

Craig plot for the two main physicochemical properties, charge (σ) and lipophilicity (π). Points

that are close in the diagram represent chemical groups with similar physicochemical proﬁle. H,

by deﬁnition, is the origin. (Adapted from P. N. Craig. J. Med. Chem., 14(8): 682, 1971.)

gram allows us to identify which chemical group is the best to substitute at a

certain position of a molecule to minimize the effect of the substitution over

the biological activity of the compound.

As can easily be seen in Figure 4.3, ﬂuorine is the element closest to the ori-

gin, which makes it an excellent substitute for hydrogen [by deﬁnition, σ(H) =

π(H) = 0], a fact that is the basis for the development of many PET radiophar-

maceuticals such as [

F]ﬂuoro-2-deoxy-D-glucose ([

F]FDG). This is also

another considerable advantage of molecular imaging with PET compared

with that by single photon emission tomography (SPECT) as the most appro-

priate γ emitter,

123

I, for use in this would undoubtedly lead to considerable

modiﬁcations in the physicochemical characteristics of a molecule.

Similarly, we can conclude that the possible substitution of the OH group

by ﬂuorine-18 would not be very favorable. As we can see, these two groups

are present in opposite quadrants of the diagram in Figure 4.3 (+σ/+π for

F−σ/−π for OH). Thehydroxylgroupis, therefore, muchbetter substituted by

the O-methyl groupand in fact, the labeling by the O-[

C]methyl groupis one

102 Nuclear Medicine Physics

TABLE 4.2

Positron Emitting Nuclides with Indication of Half-lives and an Example of a

Production Reaction

Nuclide t

1/2

%β

Reaction Nuclide t

1/2

%β

Reaction

C 20.4 m 99.8

N(p, α)

Ga 1.13 h 90 (Generator)

N 9.97 m 100

C(d,n)

Ge 270.8 d

Ga(p,2n)

O 122.24 s 99.9

N(d,n)

Se 7.1 h 65

As(p,3n)

F 109.8 m 97

Ne(d,α)

Br 1.62 h 75.5

Se(p,2n)

K 7.6 m 100

Cl(α,n)

Br 16 h 57

As(

He,2n)

Fe 8.28 h 57

Kr(

He,3n)

Rb 1.26 m 96 (Generator)

Co 17.5 h 77

Fe(p,2n)

Sr 25.6 d 100 Mo(p,spall)

Cu 9.74 m 98 (Generator)

Y 14.74 m 34

Sr(p,3n)

Zn 9.22 h 93

Cu(p,2n)

Zr 3.27 d 25

Y(p,n)

Cu 12.7 h 18

Ni(p,n)

94m

Tc 53 m 72

Mo(p,n)

94m

Ga 9.5 h 57

Zn(p,2n)

124

I 4.18 d 25

124

Te(p,n)

124

Note: d: days; h: hours; m: minutes; s: seconds.

ofthe most widely used systems for the developmentof radiopharmaceuticals

labeled with carbon-11.

Table 4.2 presents a list of some of the main positron emitting nuclides

used in PET. Although the majority of the PET tracers have been developed

around the ﬁrst four of these, there are positron emitters from elements across

the entire periodic table including, for example, isotopes of technetium and

iodine, elements that are usually associated with SPECT studies, as well as

nuclides available from reactors such as gallium-68 and rubidium-82.

Other important considerations involved in the selection of a positron emit-

ting nuclide include the percentage of β emission, the energy of the positron

(that inﬂuences its range in tissue), and its half-life, which should be long

enough to permit chemical synthesis and the exam to take place but not too

long to avoid exposing the patient to unnecessary doses of radiation after the

exam is ﬁnished.

4.5 Labeling Position

The selection of the labeling position in a radiopharmaceutical is critical, as it

can determine the fate of the emitting nuclide as the molecule is metabolized

inthe body.As anexample, Figure4.4indicates two possible labeling positions

for the molecule dihydroxyphenylalanine (DOPA), a marker of dopaminer-

gic function in Parkinson’s disease: [1-

C]DOPA [23] and [2-

C]DOPA [24].

In the ﬁrst case (labeling on carbon 1), the action of the enzyme

L-amino

acid decarboxylase present in the target tissue causes the loss of the labeling

Radiopharmaceuticals 103

[1-

C]DOPA

L-amino acid

decarboxylase

Dopamine

[2-

C]DOPA

[

C]Dopamine

FIGURE 4.4

Fate of the emitting nuclide for two carbon-11 labeling positions of DOPA. Labeling in carbon 1

causes the loss of the labeling through

whereas labeling in carbon 2 leads to the production

of [

C]dopamine.

with the production of

. In the second case (labeling on carbon 2),

the same metabolic mechanism leads to the production of the dopaminergic

neurotransmitter [

C]dopamine.

4.6 Radiosynthesis

The speciﬁc nature of the radiopharmaceuticals imposes some conditions on

the synthesis process as well as on the necessary quality control procedures.

Additionally, considering that it will be applied in clinical studies, the ﬁnal

productshould also be sterile, apyrogenic,and suitably formulated for human

use. Also for clinical reasons, the compounds have to be produced with very

high speciﬁc activity, which means that the entire synthesis process has to be

made within a very short time. In the case of nuclides with very short half-

lives, such as oxygen-15 and nitrogen-13 (t

1/2

< 10 min), their chemistry is

limited to very simple molecules (CO, H

O, and NH

) that do not have sig-

niﬁcant requirements in terms of quality control. In the case of nuclides with

longer half-lives, such as carbon-11 (t

1/2

= 20.4 min), it is possible to syn-

thesize small organic molecules such as amino acids, carbohydrates, or fatty

acids and perform typical quality control procedures. For radionuclides with

longer half-lives, such as ﬂuorine-18 (t

1/2

= 109.8 min), it is possible to have a

more complex chemistry and to include detailed quality control procedures.

In this latter case, it is also possible to distribute the radiopharmaceuticals

to centers that do not have a cyclotron available, a feature that facilitates the

clinical use of these compounds.

In order to minimize synthesis time, the process is optimized to include

the radionuclide in the last possible step. The process relies, therefore, on the

use of (cold) prevalidated precursor molecules that are added to the labeling

104 Nuclear Medicine Physics

agent (radionuclide). It is also common to use a large stoichiometric excess

of the cold reagents to compensate for the relative lack of the labeling agent

and to improve the general yield of the reaction.

In compounds labeled with positron emitters, the short half-lives also

require working with very high initial activities. The principles of radio-

protection, particularly considering the high-energy γ rays that result from

annihilation of the positrons (511 keV), require the use of automated synthesis

and dispensing modules housed in heavily shielded hot cells with a minimum

thickness of 75 mm of lead in their walls. This increasing automation of the

processes also helps in establishing reproducibility, allowing the optimiza-

tion of the synthesis conditions and facilitating production according to good

manufacturing practice (GMP) regulations.

4.7 PET Nuclides: Reactions and Radiopharmaceuticals

4.7.1 Carbon-11

Carbon-11 is usually produced by proton irradiation of a nitrogen target

through the reaction

N(p,α)

C. The presence of small quantities of oxy-

gen leads to the formation of

, whereas the presence of hydrogen

leads to the formation of

. After a puriﬁcation stage, each of these

gases is propelled by a helium overpressure and is cryogenically trapped for

the radiolabeling of a variety of important chemical precursors (Table 4.3).

Of these, the most important is methyl iodide (

I), which is used for

the radiolabeling of numerous radiopharmaceuticals that are based on this

nuclide.

TABLE 4.3

Examples of Chemical Species Labeled with Carbon-11 as Radiopharmaceuticals

Per Se or as Precursors for Other More Complex Molecules

Reaction Via Labeled Species

N(p,α)

C →

→

OH →

I →

OSO

→ H

CHO

→ R

COOMgX → R

COOH

→ R

COCl

→ CH

COOLi →CH

COOH

→

CHCl

→

→ H

CN →

→

CO(NH

)

→

CCl

→

COCl

Radiopharmaceuticals 105

4.7.2 Fluorine-18

Two distinct nuclear reactions are used to produce the two chemical main

forms of ﬂuorine-18 used in PET (Table 4.4):

−

(nucleophilic ﬂuorine) and

(electrophilic ﬂuorine).

Fluoride ion is normally produced by the reaction

O(p,n)

F through

the cyclotron irradiation of

O-enriched water. The resulting

−

is then

trappedin ananionic exchange resinand extracted fromthe aqueous medium.

Since the ﬂuoride ion is not very reactive in aqueous solution, it is taken to

dryness and resuspended in an aprotic solvent such as DMSO or acetonitrile.

A key factor for the production of reactive nucleophilic ﬂuoride is the

choice of the counter ion. Potassium is normally used, although it is usu-

ally chelated in a poli-ether macrocyclic complex (Cryptand 222), which

increases the distance between both ions and therefore, helps the release of

the ﬂuoride for nucleophilic substitution reactions. This is, in fact, the most

common radiolabeling mechanism with

F used, for example, for the syn-

thesis of [

F]FDG through the nucleophilic attack of F

−

on the precursor

tetra-O-acetyl-2-O-triﬂuoro-methanesulfonyl-β-

D-mannopyranose [25].

In contrast, the production of molecular ﬂuorine is traditionally done by

the reaction

Ne(d,n)

F by deuteron bombardment of neon in a nickel tar-

get. Molecular ﬂuorine is a very reactive species, and the activity produced

is trapped by adsorption onto the target walls. To remove this, we need to

ﬂush the target with cold ﬂuorine, which, by competition, will remove the

adsorbed onto the nickel walls of the target. The ﬂuorine-18 obtained

is consequently heavily diluted with nonradiolabeled ﬂuorine, which leads

to a very low speciﬁc activity of the radiopharmaceuticals produced [26].

This constraint limits the use of this synthesis pathway for the labeling of

most radiopharmaceuticals, although it is still used as the major path for the

TABLE 4.4

Examples of Chemical Species Labeled with [

F]ﬂuoride and [

F]ﬂuorine, in Most

Cases with the Objective of being Used as Hot Precursors for the Labeling of More

Complex Molecules

Reaction Via Labeled species

Ne(d,n)

F →

→ CH

COO

→ RSO

O(p,n)

F →

−

→ Br

→ I(CH

)

→ CH

FCOOCH

→

FArCHO

106 Nuclear Medicine Physics

production of a few radiotracers (e.g., [

F]DOPA) for which the speciﬁc

activity factor is not so critical.

4.7.2.1 Deoxyglucose Method

The method is based on the properties of 2-deoxy-

D-glucose (deoxyglu-

cose, DG), and it was originally developed to determine local glucose

utilization ex vivo in animals by autoradiography with carbon-14 (2-deoxy-

D-[1-

C]glucose) [27]. This method was later adapted for in vivo imaging

by labeling DG with positron emitters. Carbon-11 was tested (2-deoxy-

D-

[1-

C]glucose) [28], but it was the nonisotopic substitution of one of the

two hydrogens in position 2 by ﬂuorine-18 (2-[

F]ﬂuoro-2-deoxy-D-glucose

[

F]FDG) [6] that showed ideal characteristics to enable the determination

in vivo, in humans, the local energy metabolism (Figure 4.5).

Deoxyglucose shares with glucose the same passive transport mechanism

forentering cells (GLUTtransporters). Inthe cells, deoxyglucoseis alsometab-

olized by hexokinase through phosphorylation to produce deoxyglucose-

6-phosphate. This compound is effectively trapped within the cells due to

its high polarity and is considered, within a compartmental model, to be

in a different compartment with no direct access back to the blood pool

(Figure 4.6).

In normal glucose, phosphorylation by hexokinase is only the ﬁrst step

of glycolysis, the initial step of the process of oxidative metabolism. The

glycolytic pathway then proceeds with the action of an enzyme called

phosphoglucose isomerase, which catalyzes the reaction that transforms

glucose-6-phosphate into fructose-6-phosphate. This reaction, which can be

easily understood using Fischer projections (Figure 4.7), relies on the forma-

tion of an enediol intermediate, which is essential for the conversion of the

aldol group of glucose into the keto group of fructose.

In the case of deoxyglucose, the absence of the hydroxyl group in the

β position makes this reaction impossible; and as a consequence, there is

accumulation of the metabolic intermediate deoxyglucose-6-phosphate. It

2-deoxy-D-[1-

C]glucose 2-deoxy-D-[1-

C]glucose 2-[

F] fluro2-deoxy-D-glucose

OH CH

FIGURE 4.5

Structural formulas of the different forms of deoxyglucose labeled with β-emitters: 2-deoxy-

D-[1-

C]glucose ([

C]DG) and β+: 2-deoxy-D-[1-

C]glucose ([

C]DG) and 2-[

F]ﬂuoro-

2-deoxy-

D-glucose ([

F]FDG).

Radiopharmaceuticals 107

Glucose-6-phosphate

III III

Deoxyglucose-6-phosphate

Deoxyglucose

Glucose

+ H

Glucose

FIGURE 4.6

Three-compartment model for deoxyglucose labeled as a tracer for the determination of oxida-

tive metabolism. Compartment I: capillary lumen; compartment II: intact compounds in tissue;

compartment III: metabolic products. Cp, CE, and CM glucose concentrations in compartments

I, II, and III respectively. K1-k3, kinetic constants of passage between compartments. Variables

labeled with ‘∗’ are the same for deoxyglucose. EM, energetic metabolism; DM, deoxyglucose

metabolites.

should be noted that, although this chemical reaction is not possible, there

is evidence that deoxyglucose-6-phosphate binds effectively to phosphoglu-

cose isomerase [29], leading to a competitive inhibition of the binding of

glucose. This is, in fact, one of the mechanisms proposed for the toxic effect

of deoxyglucose when administered in higher doses, which is suggested to

cause an inhibition of glycolysis and the consequential development of symp-

toms similar to those of hypoglycemia. Although the simple reversion of

the phosphorylation reaction of deoxyglucose by hexoquinase is, as in the

case of glucose, highly unfavorable in terms of the energy balance, several

Glucose-6-phosphate Enediol intermediate Fructose-6-phosphate

HHO

OHH

–

HHO

OHH

HHO

OHH

OPO

2–

OPO

2–

OPO

2–

FIGURE 4.7

Schematic representation of the reaction catalyzed by the enzyme phosphoglucose isomerase

that converts glucose-6-phosphate in fructose-6-phosphate evidencing the enediol intermediate

fundamental to the formation of the ﬁnal product.