The formation of new blood vessels from pre-existing ones—denoted as angiogenesis—is required for tumour maintenance and development as well as for metastasis formation ^[1][2][1,2]. Angiogenesis is regulated by a delicate balance between a host of pro-angiogenic and antiangiogenic factors [3]. Hence, intensive attention has been placed on the assessment of cancer-associated neovascularization ^[4][5][4,5]. Producing various angiogenic factors, proteases, heparanase, digestive enzymes, and chemotactic and stimulatory factors, tumour cells exert a direct effect on capillary endothelial cells, induce the assembly of activated immune cells, and trigger the activity of stromal cells; thus, they create a pro-angiogenic niche in the tumour microenvironment ^[2][6][2,6].

Given that angiogenesis/neo-angiogenesis has crucial role in tumour development, propagation, and metastatic spread, angiogenic biomarkers are recorded as promising candidates for tumour imaging [7]. Vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), fibroblast growth factor-2 (FGF-2), ephrins, α_vβ₃ integrins, aminopeptidase N (APN/CD13), fibronectin, nitroimidazole, matrix metalloproteinases, gastrin-releasing peptide receptor (GRPR), and E-selectin represent the most important angiogenic molecules [8]. The detection of angiogenesis-related markers makes timely tumour identification possible, and these biomolecules also serve as potential targets for anti-tumour treatment and patient follow-up ^[1][9][1,9]. Anti-angiogenic drugs can inhibit tumour expansion and decelerate, delay, or even inhibit tumour augmentation and metastasis formation ^[10][11][10,11]. Therefore, angiogenesis-directed molecular probes may lay the basis of personalised cancer diagnostics and treatment.

Non-invasive positron emission tomography (PET) or positron emission tomography/computed tomography (PET/CT) are regarded as the mainstay diagnostic tools in the detection of primary tumours and pertinent metastases. Furthermore, PET can be a useful means of angiogenesis-targeted drug development, authorization, and dose estimation ^[8][12][8,12]. Several PET radionuclides have been used in angiogenesis-directed diagnostical settings, including fluoride-18 (¹⁸F), gallium-68 (⁶⁸Ga), copper-64 (⁶⁴Cu), and even the gamma emitter indium-111 (¹¹¹In). Although scandium (Sc) radioisotopes were already recognized as valuable radiometals for isotope diagnostic use in the late 1990s, they have been set aside for almost 20 years [13].

The following three radionuclides of Sc are well suited for either diagnostic or therapeutic applications: scandium-43 (⁴³Sc), scandium-44 (⁴⁴Sc), and scandium-47 (⁴⁷Sc) [14]. Positron emitters ⁴³Sc and ⁴⁴Sc are relevant diagnostic radiometals, whereas β⁻-emitting ⁴⁷Sc is the therapeutic sister with accompanying γ rays for imaging purposes ^[15][16][15,16].

There are two major ways to produce ⁴⁴Sc. Via the proton irradiation of natural calcium or enriched ⁴⁴Ca targets (through a ⁴⁴Ca(p,n)⁴⁴Sc nuclear reaction), the production of ⁴⁴Sc can be easily obtained by applying low-energy cyclotrons ^[17][18][19][17,18,19]. Cyclotron-based production provides outstanding radiochemical yield (RCY) and radiochemical purity (RCP) for clinical applications ^[18][20][18,20]. Of note, ⁴⁴Sc production via applying a cyclotron has been confirmed to be economically viable [17]. Although ⁴⁴Sc can also be eluated from titanum-44/scandium-44 (⁴⁴Ti/⁴⁴Sc) generator systems, the radioactive-waste handling of long-lived ⁴⁴Ti (62 ± 2 years) makes this approach challenging ^[21][22][23][21,22,23]. In addition, the demanding production of ⁴⁴Ti implies another obstacle regarding the routine usage of ⁴⁴Ti/⁴⁴Sc generators ^[17][24][17,24]. Difficulties around the production of ⁴⁴Sc may underpin why this radiometal came to the focus of exhaustive investigation as a potential labelling isotope so late.

⁴⁴Sc, with physical properties of T_1/2: 3.97 h (lately reported as 4.04 h), E_β⁺_average: 632 KeV, E_β⁺_mean: 0.63 MeV, and I:94.3%, has begun to emerge as a radiometal of particular interest in imaging, dosimetry, and treatment follow-up ^[20][25][26][20,25,26]. Positron emitter ⁴⁴Sc also has a gamma co-emission of 1157 keV (99.9%), which may limit the radioactive dose that can be injected into patients [26]. However, this high-energy gamma radiation made the radiometal a precious candidate for the novel β + γ coincidence PET imaging [27]. Since the positron energy of the applied PET radionuclide is inversely proportional to the resolution of the reconstructed image, better spatial-image resolution and better quality could be obtained with ⁴⁴Sc relative to other radiometals—for example, ⁶⁸Ga (⁶⁸Ga: E_β⁺_max: 1.9 MeV and E_β⁺_mean: 0.89 MeV vs. ⁴⁴Sc: E_β⁺_max: 1474 KeV and E_β⁺_mean: 0.63 MeV) ^[28][29][30][28,29,30]. These advantageous physical characteristics together with its decay to nontoxic Ca make ⁴⁴Sc widely feasible in PET and PET/CT imaging ^[13][31][13,31]. Given its small ionic radius, the chemistry of ⁴⁴Sc is comparable to that of ⁶⁸Ga; thus, ⁴⁴Sc could be applied in several fields, including dosimetry investigations in theranostic settings and in trafficking ligands with longer pharmacokinetics and subsequent requirements for longer imaging times (even 24 h post administration) ^[32][33][32,33].

Prior studies have already dealt with the investigation of the feasibility of ⁴⁴Sc in pre-therapeutic dosimetry ^[34][35][34,35]. Khawar et al. published a paper indicating that the pharmacokinetics of [⁴⁴Sc]Sc-PSMA-617-based (PSMA (prostate-specific membrane antigen)) PET/CT imaging serves as a useful tool for the calculation of normal organ-absorbed doses and the maximum allowable activity in prostate-cancer patients prior to lutetium-177 (¹⁷⁷Lu)-labelled PSMA-617 ([¹⁷⁷Lu]Lu-PSMA-617) radiotherapy [36]. Pioneering clinical studies further strengthened the efficacy of ⁴⁴Sc-labelled somatostatin analogue [⁴⁴Sc]Sc-DOTATOC and [⁴⁴Sc]Sc-PSMA-617 as valuable radiotracers in dosimetry settings [35]. Delayed acquisition granted by the longer half-life of ⁴⁴Sc enables better pre-therapeutic dosimetry for [¹⁷⁷Lu]Lu-PSMA-617 radiotherapy ^[34][37][34,37]. Clinical successes in the treatment of leukaemia, non-Hodgkin lymphoma, malignant melanoma, urinary bladder cancer, glioma, neuroendocrine tumours, and prostate cancer with alpha emitter bismuth-213 (²¹³Bi)- and actinium-225 (²²⁵Ac)-linked radiopharmaceuticals project that these isotopes could also benefit from dosimetry estimations based on ⁴⁴Sc [38]. Moreover, in the long run, other radionuclides, such as yttrium-90 (⁹⁰Y), ioide-131 (¹³¹I), strontium-90 (⁹⁰Sr), and phosphorus-32 (³²P) may be potential candidates for theranostic applications. Although preclinical research findings are available on radiation-absorbed doses from ⁴⁷Sc, the limited availability of the radiometal hampers its usage as a potential radiotherapeutic agent [39].

Similar to ⁶⁸Ga, ⁴⁴Sc is also able to form thermodynamically stable complexes with chelator DOTA (1,4,7,10-teraazacyclododecane-N,N′,N″,N″′-teraacetic acid) ^[40][41][40,41]. In addition to this, ⁴⁴Sc seems superior to ⁶⁸Ga in several facets. Given its longer half-life (T_1/2 ⁴⁴Sc: 3.97 h vs. T_1/2 ⁶⁸Ga: 68 min) and cyclotron-dependent extensive synthesis, ⁴⁴Sc can be easily shipped to remote nuclear medical facilities. This contributes not only to the wider distribution of the radioisotope but also to the accomplishment of protein- or antibody-based PET studies with lengthened examination times [42]. Furthermore, radiopharmaceuticals with longer pharmacokinetic characteristics can also be proposed due to the longer half-life of the radiometal [17]. Moreover, the pharmacokinetic properties of ⁴⁴Sc-appended imaging probes are largely comparable to those of the ⁶⁸Ga-labelled counterparts [43]. The nearly four-hour half-life of ⁴⁴Sc could be easily fitted to the pharmacokinetics of several targeting molecules, including peptides, antibodies, or their fragments, as well as oligonuclides, which makes facile radiopharmaceutical synthesis possible ^[13][17][13,17]. Additionally, long-lived ⁴⁴Sc favours delayed imaging and the achievement of appropriate tumour-to-background (T/M) ratios. The time of imaging is of critical importance from the point of view of patient management and the scheduling of examinations. Therefore, the use of ⁴⁴Sc-labelled radiopharmaceuticals would contribute to the establishment of a fluent workflow.

⁴⁴Sc-labelled PET radiopharmaceuticals could even be appropriate for intraoperative radio-guided surgery, including the detection of lymphatic metastases at later time points post injection [32]. Consequently, ⁴⁴Sc-based PET probes could stand out as valuable imaging agents. ⁴⁷Sc (T_1/2 = 3.35 days)—the therapeutic match of ⁴⁴Sc—emits a β⁻ radiation of a maximum energy of 0.600 MeV (31.6%) and 0.439 MeV (68.4%), which could be exploited in targeted radiotherapy ^[33][44][33,44]. Therefore, ⁴⁴Sc/⁴⁷Sc has exquisite potential as a radiotheranostic pair in PET diagnostics and in therapeutic settings ^[33][45][33,45]. Besides ⁴⁴Sc and ⁴⁷Sc, scandium-43 (⁴³Sc) is another significant member of the group of Sc isotopes. Investigating the quantitative capabilities of ⁴³Sc/⁴⁴Sc and comparing it to ¹⁸F and ⁶⁸Ga, Lima et al. published a paper indicating that the application of radiotracers labelled with the mixture of ⁴³Sc/⁴⁴Sc may be beneficial in clinical fields [46]. The findings of their phantom study proved that precise, quantitative PET/CT could be obtained by applying commercial PET systems [46]. Given that ⁴³Sc does not have high-energy gamma emission, the use of ⁴³Sc/⁴⁴Sc could be especially important in terms of overcoming the limitation of ⁴⁴Sc derived from gamma radiation. These favourable features propelled both preclinical and clinical investigations with ⁴⁴Sc down new lines. Some experiments have been spawned to focus on the evaluation of the clinical feasibility of ⁴⁴Sc-labelled molecules at a translational level as well as the comparison of ⁴⁴Sc with other widely used radionuclides, such as ⁶⁸Ga and ⁶⁴Cu [47].

The optimal short half-life (109.8 min) and the high positron abundance (β ≥ 97%) of ¹⁸F made it the most commonly used radioisotope for the labelling of PET biomolecules [48]. Positron emitter ¹⁸F possesses a relatively low positron energy (E_max =  0.635 MeV and E_mean =  0.250 MeV) and a short positron range within the tissue (maximum of 2.3 mm) ^[49][50][49,50]. These nuclear properties ensure ideal image quality and high spatial resolution obtained with ¹⁸F-labelled tracers [51]. The 109.8 min-long half-life is beneficial for synthesis procedures as well as for the performance of examinations of a few hours [51]. Owing to the short longevity, ¹⁸F-labelled radiopharmaceuticals can be safely administered without an increased risk of excess radiation. Moreover, another advantage is the facile cyclotron-based production, which provides immense quantities of the isotope at high specific activity [51]. Although a cyclotron is required to produce the radiometal, its decay characteristics make the distribution of ¹⁸F- and ¹⁸F-labelled radiotracers to distant nuclear medical laboratories without an on-site cyclotron possible. Even though ¹⁸F is well suited for the labelling of a wide range of small and medium-sized molecules, the radiolabelling of peptides with ¹⁸F is still cumbersome [51]. Hence, different isotopes have come into focus that address some of the difficulties associated with ¹⁸F-based radiotracers.

Up to now, the favourable physical characteristics of ⁶⁸Ga and ⁶⁴Cu made them attractive for the radiolabelling of a broad set of molecules. Currently, ⁶⁸Ga-labelled PET radiopharmaceuticals are applied most frequently for labelling purposes of radiometal-based PET tracers. The short-lived positron emitter ⁶⁸Ga (T_1/2: 67.71 min ≈ 68 min, Eβ⁺_average: 830 KeV, maximum β⁺ energy: 1.92 MeV, Iβ⁺: 89%, Eγ: 1077 KeV, Iγ: 3.2%) can be obtained from a germanium-68/gallium-68 (⁶⁸Ge/⁶⁸Ga)-generator system ^[52][53][52,53]. With such physical properties, decay characteristics, and easy accessibility from an on-site ⁶⁸Ge/⁶⁸Ga generator, ⁶⁸Ga has emerged as the mainstay PET-imaging radiometal ^[32][52][54][32,52,54]. Since the radiosynthesis of peptides—complexed with different macrocyclic ligands—with ⁶⁸Ga is easy to perform, the use of ⁶⁸Ga is effective in imaging settings [55]. ⁶⁸Ga-labelled DOTA-conjugated somatostatin analogues (SSTR), including DOTATOC, DOTATATE, and DOTANOC, meant a substantial step forward in the imaging of SSTR-positive neuroendocrine tumours [33].

The short half-life and related transport challenges, however, may limit the widespread implementation of ⁶⁸Ga into clinical and preclinical settings. Owing to the short longevity of the radiometal, radiolabelling procedures are only possible in laboratories equipped with in-house ⁶⁸Ge/⁶⁸Ga-generator systems. In addition, due to its short half-life, ⁶⁸Ga-labelling can be applied solely in case of small molecules and peptides featured with a rapid pharmacokinetic profile ^[17][56][17,56]. Furthermore, ⁶⁸Ga-labelling is only suitable for PET examinations of relatively short duration. Because of the breakthrough of ⁶⁸Ge and sorbent material, the purification and concentration of ⁶⁸Ga is warranted following elution, which could also restrict its applicability in routine diagnostic usage [57]. Image noise associated with the elevated positron energy of the radionuclide constitutes another shortcoming of ⁶⁸Ga imaging [32]. These facts render ⁶⁸Ga-labelled radiotracers of limited attractiveness for centralized distribution.

The application of ⁶⁴Cu may bridge the limitations derived from ⁶⁸Ga-associated imaging. Given its longer half-life (T_1/2: 12.7 h) and generator-independent production, the use of ⁶⁴Cu (β⁺ emission, E_average = 278 keV, abundance: 19%) seems to be economically more viable for PET imaging ^[58][59][58,59]. Due to its longer-lived nature, the easy transport to distant laboratories without an on-site cyclotron facilitates the integration of ⁶⁴Cu-appended radiopharmaceuticals into diagnostics. Moreover, the 12.7 h half-life of the radiometal can be easily tailored to biomarker-targeting small and large molecules, peptides, antibodies, and nanomolecules with prolonged elimination kinetics [60]. Additionally, its longer half-life makes ⁶⁴Cu-based radiochemical procedures facile [60]. The coordination chemistry of the radiometal enables complexation with various chelating agents, which further supports the frictionless performance of radiolabelling [60]. Therefore, ⁶⁴Cu is extremely useful in the establishment of a broad set of radiotracers for diagnostic purposes. However, the use of ⁶⁴Cu is not without shortcomings. Its concomitant β⁻ emission (β⁻ = 39.0%, E = 190.2 keV) and relatively low positron-branching ratio (17.6%) mean a meaningful extra radiation danger for patients ^[58][61][62][58,61,62]. Furthermore, in case of complexation, chelators must be customized to the complex redox chemistry of ⁶⁴Cu [54].

Beyond ⁴⁴Sc, the use of zirconium-89 (⁸⁹Zr) has also been exhaustively investigated in PET imaging [63]. Given its favourable decay half-life (T_1/2 = 3.3 days; 78.4 h), appropriate radiochemistry, and the accessibility of different chelating agents for complexation with the radioisotope, ⁸⁹Zr is welcomed for the radiolabelling of PET-based imaging molecules [63]. The relatively long half-life of the radiometal could be easily adjusted to the pharmacokinetic profile of monoclonal antibodies (mABs); therefore, ⁸⁹Zr seems to be well suited for the radiolabelling of mABs and for PET immunoimaging applications ^[63][64][65][63,64,65]. Several mAbs, including anti-human epidermal growth factor receptor 2 (HER2) trastuzumab, anti-epidermal growth factor receptor (EGFR) mAb cetuximab, anti-PSMA mAb J591, and anti-vascular endothelial growth factor (VEGF) bevacizumab, were successfully labelled with ⁸⁹Zr for the PET imaging of breast cancer, squamous-cell carcinoma, prostate tumours, and ovarian tumours, respectively, at both preclinical and clinical levels ^{[66][67][68][69]}[66,67,68,69].

Due to their decay characteristics (positron emission: E_max and E_average, 897 keV and 396.9 keV, respectively), PET images obtained with ⁸⁹Zr-labelled compounds are of adequate spatial resolution ^[70][71][72][70,71,72]. However, similar to ⁴⁴Sc, ⁸⁹Zr has spontaneous high-energy gamma radiation of 908.97 keV, which accounts for a major drawback of its usage [70]. Although the concomitant gamma radiation does not influence either the quality or the quantification of the PET scans, it needs to be taken into account when patient doses are determined ^[73][74][73,74]. Another potential disadvantage could be the limited availability of the radiometal [63].

Taking the abovementioned facts into account, ⁴⁴Sc may therefore be a valuable substitute for the currently applied ⁶⁸Ga and ⁶⁴Cu in the establishment of peptide-based targeted PET radiopharmaceuticals. RIn this researchers view, we provide a comprehensive overview of the role of ⁴⁴Sc-labelled peptide-based radiopharmaceuticals in the molecular imaging of cancer-related angiogenesis (Figure 1). Table 1 summarises the preclinical studies with ⁴⁴Sc-labelled PET radiotracers that selectively target angiogenic biomarkers. In Table 2 the most important physical characteristics of the discussed PET radioisotopes are displayed.

AMBA: aminobenzoyl–bombesin analogue; APN/CD13: aminopeptidase N; BBN: bombesin; GRPR: gastrin-releasing peptide receptor; NGR: asparagine–glycine–arginine; PET: positron emission tomography; RGD: Arg-Gly-Asp; ⁴⁴Sc: scandium-44; ⁴⁷Sc: scandium-47.

AAZTA: (1,4-bis(carboxymethyl)-6-[bis(carboxymethyl)]amino-6-methylperhydro-1,4-diazepine) AMBA: aminobenzoyl–bombesin analogue; AR42J: rat exocrine pancreatic tumour; BBN: bombesin; BN: bombesin; c(RGD)₂: dimeric cyclic arginine–glycine–aspartic acid; c(RGDfK): cyclo(-Arg-Gly-Asp-d-Phe-Lys); DOTA: 1,4,7,10-teraazacyclododecane-N,N′,N″,N″′-teraacetic acid; ELISA: enzyme-linked immunosorbent assay; ⁶⁸Ga: gallium-68; GRPR: gastrin-releasing peptide receptor; HaCaT: human immortal keratinocyte; ¹²⁵I: iodine-125; KB: human epidermal carcinoma; NI: nitroimidazole; NODAGA: 1,4,7-triazacyclononane-1-glutaric acid-4,7-diacetic acid; NRP-1: neuropilin-1 co-receptor; PCa PC-3: prostate cancer; PET/CT: positron emission tomography/computed tomography; PET/MRI: positron emission tomography/magnetic resonance imaging; RGD: Arg-Gly-Asp; ⁴⁴Sc: scandium-44; SCID: severe combined immunodeficient; 4T1: mouse breast cancer; U87MG: human glioblastoma; VEGF: vascular endothelial growth factor.