La educación en valores en los procesos educativos actuales

Radiolabelled biomolecules include protein based radiopharmaceuticals ranging from small peptides to intact antibodies. These radiopharmaceuticals are mostly applied in oncology. Well known examples are somatostatin receptors, CCK, gastrin releasing peptide receptors for peptides and HER2, VEGF and PDL1 for antibodies.

Summarizing the pharmacokinetics of these protein based radiopharmaceuticals, it can be concluded that size of the protein molecule matters regarding its pharmacokinetics After the intravenous administration, factors like extravasation, diffusion, accumulation, and clearance determine tissue uptake and image contrast. For full antibodies these processes take quite long resulting in optimal scanning protocols, taking 3 to 7 days after injection. Therefore, a longer lived radionuclide needs to be used for PET imaging such as ¹²⁴I or ⁸⁹Zr. Because of the favourable characteristics of ¹⁸F, fragments of antibodies (Fab) retaining the antigen binding properties have been generated.

The advantage of the use of radiolabelled Fab and (Fab’)2 fragment of monoclonal antibodies (mAbs) are:

(a) Shorter time interval between injection and imaging;

(b) Often better contrast in comparison with full antibodies;

(c) Lower radiation dose to patients.

However, there are still some challenges such as:

(a) Decrease of apparent binding affinity compared to full antibodies. This is typical for Fab fragments due to the loss of the avidity effect of bivalent binding;

(b) Absolute tumour uptake is often lower and kidney uptake is often increased;

(c) Both Fab and (Fab)2 are still too large to have efficient extravasation;

(d) Both Fab and (Fab)2 are still above the border of enhanced permeability and retention (45 kDa for globular proteins). Small peptides do not suffer from slow PK; they can rapidly penetrate into tissue and bind to the receptor of interest.

After binding of the protein based tracer with the receptor/antigen (=target), the combined tracer/target complex internalized into the cancer cell by endocytosis (Fig. 89) [171]. The target will undergo recycling and will be available for another interaction with a tracer. The internalized tracer can undergo 1. degradation by peptidases, 2. release the radiolabel. In most cases the radiolabel stays in the cell.

FIG. 89. Uptake mechanism of peptide/protein based radiopharmaceuticals in the cancer cell (reproduced from Ref. [171] with permission).

Another approach to address the slow PK of full antibodies is pretargeting (Fig. 90) [172], where tumour target antigen binding specificity is obtained by: (a) injecting unlabelled antibody derivatives followed; (b) by a radiolabelled low molecular weight compound that specifically binds to the antibody and is rapidly cleared from the circulation. The binding between the antibody and the radiolabelled compound should follow a bio-orthogonal fashion and can be achieved via click chemistry of ultra high affinity interactions such avidin-streptavidin.

FIG. 90. Principle of pretargeting strategy (reproduced from Ref. [172] with permission).

Advantages of this approach are:

(a) Taking advantage of the biodistribution pattern of high affinity full antibodies;

(b) Short time interval between injection of radiotracer and scanning;

(c) Possibility of using ¹⁸F with its favourable radiation characteristics.

Challenges still to be addressed

(a) Non-specifically bound antibody can affect the PET image;

(b) Two iv injections required (two visits to hospital);

(c) Time intervals between injections are critical;

(d) A clearing agent using non-radioactive IEDDA counterpart can be used but involves a 3^rd iv injection;

(e) 2–3 FDA or EMA approvals required for each administered agent.

6.10. HORMONE RECEPTORS

Breast cancer is the most common cancer in the women, with 2 million new cancer cases diagnosed every year. Breast cancer diagnosis are mainly based on estrogen receptor and progesterone receptor positive, ¹⁸F radiolabelled steroid hormones can be used to study and image tumour expression of estrogen receptor and progesterone receptors in patients with primary and metastatic breast cancer.

16α-[¹⁸F]fluoro-17β-estradiol ([¹⁸F]FES) and [¹⁸F]fluoro-furanyl-norprogesterone ([¹⁸F]FFNP) (Fig. 91) are two promising radiopharmaceuticals for estrogen/progesterone receptors content evaluation of tumours using PET, allowing whole body receptor scanning without invasive techniques such as biopsy. The advantages of non-invasive in vivo assessment include avoiding sampling error, assessing the entire tumour volume receptor status rather than part of the tumour (addressing the heterogeneity of ER expression), assessing the status of all the lesions expressing ER, and assessing the biological activity of the receptor at diagnosis and in response to treatment. In patients with estrogen receptor expressing tumours, [¹⁸F]FES-PET may prove useful for patient stratification, selection of patients eligible for hormonal therapy, assessment of estrogen receptor occupancy, response prediction and follow-up. Recently, FDA approved

18FF-fluoroestradiol, manufactured by Zionexa as Cerianna, as radiopharmaceutical for

‘estrogen receptor +’ in breast cancer, as an adjunct to biopsy in patients with recurrent or metastatic breast cancer.

FIG. 91. Structures of estrogen receptor PET-agents labelled with ¹⁸F (courtesy of E. Cazzola, Sacro Cuore Hospital).

6.11. [¹⁸F]FLUOROCHOLINE HO

OH

18F

O

O H O

H

O

18F O

[¹⁸F]FFNP [¹⁸F]FES

Choline is used in all cells as precursor for phospholipids biosynthesis, essential as membrane component. Therefore, choline is transported into the cells, metabolized and then trapped on both normal and cancer cells. Many tumours are characterized by increased cell proliferation that include increased metabolism and increased choline demand. Choline demand originated from hyperactivity of the enzyme (choline-kinases) that is essential to create a phophatidylcholine, which is fundamental for synthesis of the cell membrane. A ¹⁸F derivative of choline, that follows the same biological pathway of choline can be used for image of altered cell function.

Historically [¹¹C]Choline was used to evaluate many kinds of tumours like in the brain, lung, urinary bladder or prostate where was very effective on diagnosis. Due to the short half-life of

11C (20 min), [¹¹C]Choline is very difficult to use in routine clinical care, to cover the high radiopharmaceutical demand. For this reason, several [¹⁸F]fluorinated derivatives of choline were studied and [¹⁸F]fluorocholine (Fig. 92) was selected as a promising fluorine analogue.

The presence of monograph in EU and USA Pharmacopoeia combined with several approved marketing authorizations in many countries make F-Choline availability easy and well spread around countries.

FIG. 92. Structure of [¹⁸F]fluorocholine (courtesy of E. Cazzola, Sacro Cuore Hospital).

6.12. CLASSES OF PET RADIOPHARMACEUTICALS APPLIED IN HUMANS 6.12.1. PET radiopharmaceuticals for brain studies

Regarding PET radiopharmaceuticals for human brain studies several new applications to image new targets have emerged. Third generation tracers for TSPO (related to image activated microglia and neuroinflammation) [173], ¹⁸F-labelled tracers for the cholinergic system, tracers to image protein misfolding (beta-amyloid, tau and alpha-synuclein for Alzheimer’s disease, Parkinson’s disease and other neurodegenerative diseases). Very recently also PET radiopharmaceuticals were published for Synaptic vesicle glycoprotein 2A (SV2A) receptors representing synaptic density which is a factor that changes over time during neurodegeneration [174]. Table 17 gives an overview of neuroPET radiopharmaceuticals applied in humans and appeared in literature

TABLE 19. NEURO PET-TRACERS APPLIED IN HUMANS (courtesy of P. Elsinga, University of Groningen)

Target Tracer Physiological process

TSPO DAA and PBR derivatives, [¹⁸F]GE180 Antagonist

[

F]Fluorocholine N

18

F

OH

GABA [¹⁸F]Flumazenil Antagonist

Dopaminergic system [¹⁸F]FDOPA Vesicular storage

[¹⁸F]Fallypride D2 antagonist

[¹⁸F]FP-CIT, [¹⁸F]FE-PE2I Dopamine transporter [¹⁸F]Florbetaben/florbetapir Staining agent

NMDA [¹⁸F]GE179 Antagonist

Cholinergic system [¹⁸F]FEOBV VAChT ligand

[¹⁸F]FP-TZTP

[¹⁸F]ASEM, [¹⁸F]A-85360, [¹⁸F]flubatine

M2 antagonist α7-nAChR ligands α4β2-nAChR ligands

mGlu-5 [¹⁸F]PSS232, [¹⁸F]FPEB Antagonist

VMAT2 [¹⁸F]AV-133, [¹⁸F]FP-DTBZ Inhibitor

5-HT1A [¹⁸F]MPPF antagonist

P2X7 [¹⁸F]JNJ-64413739 Antagonist

GLUT-transporters and Hexokinase [¹⁸F]FDG Glucose consumption

TABLE 17. NEURO PET-TRACERS APPLIED IN HUMANS (‘cont’)

Target Tracer Physiological process

Tau protein [¹⁸F]THK523, [¹⁸F]AV1541 Protein misfolding in AD

Phosphodiesterase-PDE-4 [¹⁸F]MNI589 Breakdown of cAMP

SV2A [¹⁸F]UCB-J Synaptic density Ligands

Sigma [¹⁸F]fluspidine Antagonists

6.12.2. Oncology PET tracers

Table 18 represents published PET radiopharmaceuticals that are used in oncological patients.

The main development is the emerging of tracers for PSMA ([¹⁸F]PSMA1007) to diagnose and treat prostate cancer [175]. Furthermore, tracers for EGFR-tk are developed for treatment follow-up. In comparison to the neurology tracers there are a few new tracers labelled with ¹¹C or ¹⁸F published in the last 5 years. Physicians still use established tracers such as [¹⁸F]FDG, [¹⁸F]FET, [¹⁸F]FDOPA and [¹⁸F]FLT.

TABLE 20. ONCOLOGY PET TRACERS APPLIED IN HUMANS (courtesy of P. Elsinga, University of Groningen, the Netherlands)

Target Tracer Physiological process

GLUT-transporters and Hexokinase [¹⁸F]FDG Glucose consumption

Thymidine kinase type 1 [¹⁸F]FLT DNA synthesis

Choline synthase [¹⁸F]fluormethyl choline Membrane synthesis

Dopamine storage [¹⁸F]fluoro-DOPA Dopaminergic system

Amino acid transporter [¹⁸F]fluoroethyl tyrosine Amino acid transports

Hydroxyapatite matrix [¹⁸F]sodium fluoride Bone metastases