Pre-Therapeutic Dosimetry Employing Scandium-44 for Radiolabeling PSMA-617 Pre-Therapeutic Dosimetry Employing Scandium-44 for Radiolabeling PSMA-617

In recent years, the positron emitter scandium-44 moved into the focus of research pro- viding favorable nuclide properties for an application in nuclear medicine. Radiolabeling of PSMA-617 with scandium-44 as diagnostic match for [ 177 Lu]Lu-PSMA-617 instead of gallium-68 would enable pre-therapeutic dosimetry in clinical setting. Due to the chemical similarities of scandium and lutetium, the in vitro and in vivo characteristics of [ 177 Lu]Lu-PSMA-617 are more similar to [ 44 Sc]Sc-PSMA-617 than to the 68 Ga-compounds [ 68 Ga]Ga-PSMA-617 or [ 68 Ga]Ga-PSMA-11. [ 44 Sc]Sc-PSMA-617 showed its potential in a clinical setting as a PET imaging agent of prostate cancer providing several advan- tages over gallium-68 labeled tracers. The longer half-life of the nuclide would allow, for example, an optimized patient management and treatment, long-term or late time point imaging as well as transportation to more distant PET centers. However, especially clinical applications like individual dosimetry or intraoperative applications are still under investigation.


Introduction
Prostate carcinoma is the fourth most common cancer in both sexes combined, the second most common cancer in men, and with an estimated 307,000 deaths in 2012, it is the fifth leading cause of death from cancer in men [1]. While prognosis of prostate carcinoma is good at an early stage, the 5-year survival of patients in advanced stages decreases to 31% [2,3]. Consequently, a number of studies were conducted developing new strategies against the disease.
As the prostate-specific membrane antigen (PSMA) is overexpressed on prostate carcinoma and the neovasculature of most of the solid tumors but not of normal tissue, it is an attractive target for imaging and therapy [4]. Consequently, the development and the evaluation of small ligands targeting PSMA are the objectives of various studies.

Part I: Radiochemistry
Currently, [ 68 Ga]Ga-PSMA-11 is the most frequently used PET tracer, targeting the prostatespecific membrane antigen, worldwide [22,23]. Gallium-68 has for PET imaging appropriate decay properties; nevertheless, its disadvantages limit its application.
Its high positron energy compared to fluorine-18 (cf. Table 1) leads to images tending to be noisier while its short physical half-life only covers imaging periods of a few hours. Moreover, the differences in coordination chemistry between gallium-68 and lutetium-177 lead to deviations in pharmacokinetics [20]. As a consequence, gallium-68 is not the nuclide of choice for late time imaging, extended dosimetric evaluations as well as intraoperative applications several hours post-injection (p.i.).
Scandium-44 can be quantitatively detected via its 511 keV emission. High radioactivities of scandium-44 can be measured in a dose calibrator applying the 18 F-setting. But due to different radionuclide characteristics, a multiplication factor has to be used, which is depending on the dose calibrator.
Since the 1980s, several radiolabeling studies with scandium radionuclides have been published [20,21,25,28,38,[41][42][43][44]. Chemically, scandium is similar to Y 3+ and lanthanides. However, the ionic radius of Sc 3+ is smaller than that of lanthanides for the coordination number 6 while at the same time, it is larger than any trivalent 3d transition metal cation. The most common coordination number of Sc 3+ is six; nevertheless, examples for coordination numbers between three and nine exist [45,46].
In vivo stability of a radiopharmaceutical is a crucial factor for clinical application; macrocyclic ligands are the ligands of choice forming thermodynamically and kinetically stable complexes with trivalent hard metal cations. Chemical and, at the end, biological behavior of the complex and consequently of the radiopharmaceutical depend on structural factors, for example, rigidity, cavity size and nature and number of the donor atoms chelating the metal cation [47]. Due to the similarity between Sc 3+ and Ga 3+ , Y 3+ or trivalent lanthanides, DOTA, a common ligand in nuclear medicine, was evaluated with regard of its usability [48]. The study revealed that the stability constant of [Sc-DOTA] is comparable with those for Y 3+ or the heaviest lanthanides and higher than those for In 3+ and Ga 3+ as well as the eight-coordination geometry of the complex in solution [48].
Together with its four times longer half-life than gallium-68 and its coordination chemistry similar to lutetium-177, scandium-44 enables longer imaging periods covering up to 24 h post injection as well as improved pre-therapeutic dosimetry.

Production of scandium-44
Scandium-44 can be produced via 44 Ti/ 44 Sc-generator [30,31,40]. Despite the advantages of the radionuclide generator system prevents the availability of titanium-44 the production of this generator. Titanium-44 with its half-life of 60 years is only producible with limited yields and at high costs by a small number of facilities [49]. Accordingly, accessibility of the daughter scandium-44 by cyclotron production is an alternative as it provides scandium-44 in sufficiently high yields with radionuclidic purities >99% avoiding the problem of 44 Ti-waste management.
Recently, the accessibility of scandium-44 via proton irradiation of natural calcium targets was described [53] as well as the employment of enriched calcium targets optimizing radionuclidic purity of the radionuclide produced [52].
Similar experiments performed by bombarding natural calcium targets with protons were reported [53,55], yielding more than 650 MBq scandium-44 with 95.8% radionuclidic purity [53]. As this method leads to co-production of long-living radionuclidic impurities accounting for unnecessary doses for the patient its usability is limited. To obtain scandium-44 of higher radionuclidic purity enriched [ 44 Ca]CaCO 3 target material was found to be optimal [59]. This study also confirmed an optimal ratio of scandium-44m to scandium-44 by irradiating the targets with 9 MeV protons and the possibility to achieve yields in the GBq range utilizing this method [59]. Further refinement leads to reproducible production of GBq-activities of scandium-44 at a cyclotron in excellent quality [56]. As a result of all these investigations towards scandium-44 production, the basis for the introduction of scandium-44 into clinical routine for PET imaging may have been created.
Nuclide production via cyclotron is in need for an efficient separation strategy of the produced radionuclide from the target material. This is necessary to remove bulk metal, which disturbs eventual radiolabeling of PET tracers, to reduce the volume and to recover target material. For this purpose, different methods such as filtration [53] or ion exchange employing chelating resins were investigated [26,55,56,59].

The 44 Ti/ 44 Sc-generator
Radionuclide generators are an alternative production route to reactors and cyclotron. They exploit radiochemical equilibria (transient or secular) between mother and daughter isotope. This means that the mother isotope has a half-life much greater than or approximately equal to 10 times longer than the half-life of the daughter usable for imaging. As mother and daughter are isotopes of different elements, they are present in different chemical forms and can be relatively easily separated chemically.
Beside the 99 Mo/ 99m Tc-generator, which is still the working horse in nuclear medicine, the relevance of the 68 Ge/ 68 Ga-generator continues to increase with recent developments of new potent 68 Ga-radiopharmaceuticals for PET imaging. Apart from the cyclotron, scandium-44 can also be produced via 44 Ti/ 44 Sc-generator system. Just like the 68 Ge/ 68 Ga-generator, there is a secular equilibrium between the long-living mother and the short-living daughter nuclide. Titanium-44 decays via electron capture (t 1/2 = 59 ± 2 a) [60] into the ground state of scandium-44 which transforms to the stable calcium isotope calcium-44 emitting a positron.
First studies on the design of a 44 Ti/ 44 Sc-generator were conducted in the 1960ies and 70ies excluding pharmaceutical aspects [32,35,61,62]. A first 185 MBq 44 Ti/ 44 Sc-generator designed for radiopharmaceutical use was described in the last decade [31] as well as a suitable postprocessing [40]. An initial preclinical proof of concept study could show that scandium-44 is able to radiolabel a clinical relevant precursor (DOTA-TOC) leading to a stable radiopharmaceutical in good yields as well as the suitability of the generator and post-processing for this purpose [38]. Furthermore, a first clinical application of [ 44 Sc]Sc-DOTA-TOC, radiolabeled with generator-derived scandium-44, was conducted to proof the high potential of the radionuclide for PET imaging [30].
First challenge in the development of the 44 Ti/ 44 Sc-generator is the high-yield production of titantium-44 via accelerated particles. Up to now, all attempts building a 44 Ti/ 44 Sc-generator described in the literature use the 45 Sc(p,2n) 44 Ti-process, although cyclotrons of high positron flux are necessary, to obtain titanium-44 in relatively low radioactivity yields [31,32,35,[61][62][63]. Before titanium-44 can be used separation from the target material and subsequent purification from residual metallic contaminants is mandatory.
Generally, for the design of a radionuclide generator, several critical radiochemical parameters have to be considered, such as separation strategy, stability of the generator and type of eluate.
In context with the 44 Ti/ 44 Sc-generator, this means a separation strategy is needed which provides high 44 Sc-elution yields combined with low 44 Ti-breakthrough employing an eluate which is suitable for subsequent radiolabeling in terms of pH, volume and purity. Additionally, this separation strategy should guarantee high long-term stability of the generator. This is of particular importance for the 44 [31]. Long-term stability of the generator is ensured by a reverse elution mode which is needed to provide high retention of titanium-44 on the column [31]. This concept leads to a generator design providing scandium-44 in stable yields without significant 44 Ti-breakthrough since approximate 10 years.
As volume, pH and eluent composition of the 180 MBq 44 Ti/ 44 Sc generator are not suitable for subsequent radiolabeling, for example, peptides for clinical application, an efficient postprocessing strategy in analogy to the post-processing approach of 68 Ge/ 68 Ga generators was developed [40,64,65]. This post-processing includes reduction of the volume of 44 Sc solution, optimization of pH for subsequent radiolabeling as well as further purification from metal contaminants disturbing the complex formation by utilizing a cation exchange column. Finally, ~ 90% of chemically and radiochemically highly pure scandium-44 can be recovered in 3 ml 0.25 M ammonium acetate (pH = 4) with less than 7 Bq 44 Ti-breakthrough within 10 min ready for following radiolabeling reactions [21,38,40].

Synthesis of [ 44 Sc]Sc-PSMA-617
DOTA is used as bifunctional chelator in PSMA-617 (cf. Figure 1) requiring elevated temperatures for complex formation. Commonly DOTA-based radiopharmaceuticals are prepared using 95°C; therefore, it was evident to choose this as radiolabeling temperature for generator as well as for cyclotron produced scandium-44 [21,25].
Due to the low activity obtained from the 44 Ti/ 44 Sc-generator, evaluation of the influence of precursor amount and reaction time on radiochemical yield resulted in apparent molar activities of 6.50 ± 0.76 MBq/nmol [21] while values of 5-10 MBq/nmol using cyclotron produced scandium-44 are possible [20].
With regard to the reported radiochemical yields of >97% [20,21], it seems not necessary to evaluate a purification method. Nevertheless, removal of unwanted ions (e.g., acetate ions, uncomplexed 44 Sc 3+ ) from the crude product solution is of interest especially with a view to clinical application. The purification method of choice is solid phase extraction. This cheap and easy method is commonly used when it is necessary to purify radiopharmaceuticals. Solid phase extraction with C-18 cartridges was suitable for further purification. After equilibration of the cartridge, almost quantitative retention of [ 44 Sc]Sc-PSMA-617 on the cartridge and product recovery with >90% efficacy is possible [21].

Preclinical evaluation
The evaluation of the logD values of the 68 Ga-, 44 [20]. The study confirmed comparable in vitro behavior, which was expected due to similar coordination behavior of scandium-44 and lutetium-177 [20,48]. The similar chemical behavior of the two nuclides is also evident in vivo in the pharmacokinetics of the radiopharmaceuticals.

Synthesis and quality control for human use
The pharmacopeia contains recognized pharmaceutical rules on the quality, testing, storage and labeling of medicinal products and the substances, materials and methods used in their manufacture and testing. It is legally binding [21].
As scandium-44 is a new isotope for human PET application, there is no monograph in the European or another pharmacopeia available for the preparation of scandium-44 or 44 Sc-radiopharmaceuticals. Therefore, quality control was performed based on the monograph for [ 68 Ga]Ga-DOTATOC of the European Pharmacopeia [67].
With respect to the use of generator-derived scandium-44, special attention has to be paid to the quality control of the titanium-44 content in the final formulation.
To ensure the quality of [ 44 Sc]Sc-PSMA-617, the radiolabeling procedure was modified for patient application. Since only a maximum of 180 MBq scandium-44 is available via the generator per elution and the time from the beginning of the generator elution to the injection to the patient is 3-4 h, it was necessary to guarantee high and stable radiochemical yields. To achieve this, the amount of precursor was increased to 38.4 nmol, and 9 vol% ethanol

Log D
Relative PSMA-binding affinity  Table 4. Log D (n = 3-5) and relative PSMA-binding affinity as the inverse molar ratio of the average K D values as determined in cell studies with LNCaP cells [21] and PC-3 PIP cells [20] according to Reddy et al. [66].

LNCaP cells PC-3 PIP cells
was added to the radiolabeling mixture. Ethanol has two tasks: to improve radiolabeling efficacy [68] and to prevent radiolysis in the initial radiolabeling mixture. Its use as scavenger is very important to ensure radiochemical purity as radiolysis by-products can cause undesired and serious side effects while their removal is time-consuming and complicated. Additionally, C-18 purification was performed by default. This step removes potentially remaining 44 Ti-breakthrough, uncomplexed scandium-44 as well as ammonium acetate buffer prior to final formulation of the radiopharmaceutical. Although this step extends synthesis time, its contribution to ensure radiochemical and especially radionuclidic purity is very important. With respect to the use of generator-derived scandiuim-44, the 44 Ti-breakthrough was of major interest. During process set-up, it was even tested twice, in the radiolabeling mixture and final formulation. It was measured not earlier than 120 h after synthesis in a γ-spectrometer at 67.9 and 78.3 keV. Titanium-44 was not traceable in any of the quality control samples.
Due to the limited activity derived from the 44  Parameters checked during the quality control procedure were listed in Table 5.
Due to the nature of radiopharmaceuticals sterility, breakthrough and content of long living radionuclides could not be determined before release of the final radiopharmaceutical. Therefore, only a preliminary release was possible. Final release of the respective batch was performed with receipt of the last test results.  Table 6. Details of study population [21,69,70].

Part II: Dosimetry
Theranostics and personalized medicine in oncology are in need for highly sensitive and specific diagnostic PET probes that may be radiolabeled with therapeutic radionuclides [18]. It is assumed that diagnostic PET agent distribution is more appropriate for prediction of therapeutic dose increasing therapeutic outcome [18]. Among the several matched pairs for imaging and therapy used in nuclear medicine, focus is on the PET nuclides gallium-68 and scandium-44 as imaging counterpart for lutetium-177.
The study protocol stipulates PET/CT imaging starting with a dynamic PET scan of abdomen with kidneys in the field of view (FOV) followed by a low dose CT scan and three static wholebody scans from skull to mid-thigh acquired 45 minutes, 2 h and 19.5 h post injection with preceding low-dose CT. Quantitative analysis was performed visually to identify organs of increased tracer uptake as source organs for further dosimetric calculations. Residence times, organ-absorbed doses (mSv/MBq) as well as effective doses were calculated during quantitative analysis [21,69,70] and the maximum permissible activity as well as the maximum number of therapy cycles (6 GBq per cycle) which can be administered were determined [70].

First in-human studies
Following the promising preclinical results, Eppard et al. conducted a first-in-human application [21,69,70].
In all patients, PSMA-positive metastases were detectable by [ 44 Figure 2.
Due to the longer half-life of scandium-44, patient management could become more flexible through its use allowing PET/CT imaging several hours post injection (Figure 3) [20]. Indeed using low doses still and late time point imaging still enables detection of lesions while accumulated activity in urinary tract or kidney is no longer observed [21]. Qualitative detection of PSMApositive lesions is feasible due to increased tumor-to-background ratios and resulting improved image contrast [21].
Khawar et al. reported estimated residence times (MBq-h/MBq) to be prolonged in the liver followed by the kidneys, urinary bladder, bone marrow and rest of organs compared with [ 68 Ga]Ga-PSMA-617 [69]. Also, the study revealed that kidneys (3.19E-01 mSv/MBq;  range: 1.78 E-01-4.88E-01 mSv/MBq) are the critical organs at risk receiving the highest dose followed by the urinary bladder wall, spleen, salivary glands, liver and small intestine while bone marrow dose was less and consequently not included in organs at risk for therapeutic application [69]. These findings are consistent with the results for small PSMA ligands of previous studies [71,72]. Overall, the study confirmed absorbed doses to be higher for [ 44 [20,70]. Total activity (MBq) in source organs and whole body from reconstructed images of dynamic data, and three static whole body PET/CT images were decay corrected back to time of injection using scandium-44 half-life and then forward decay corrected using half-life of lutetium-177 for calculation [70]. Also for [ 177 Lu]Lu-PSMA-617, kidneys appeared to be the organ at risk (mean absorbed dose 0.44 mSv/MBq) followed by the salivary glands, liver, small intestine, spleen and urinary bladder wall [70]. The mean bone marrow absorbed dose was reported to be 0.05 mSv/MBq, and the mean whole body dose was 0.08 mSv/MBq [70]. These findings are comparable with literature [11,13,17,19]. Total dose (Gy) per cycle administered lies in a range from 2 till 3.26 Gy although applying the same therapeutic activities [70]. Due to the use of 3D instead of usual 2 D dosimetric analysis, it was found that it is possible to administer a mean dose of 52 Gy to reach a dose limit of 23 Gy [70] which is significantly higher than reported before with 30 Gy [13].
All together both studies proved that dosimetry using [ 44 Sc]Sc-PSMA-617 PET/CT is possible applying a protocol which could be implemented in clinical daily routine.

Conclusion
Recent studies demonstrated the high potential of [ 44 Sc]Sc-PSMA-617 for PET imaging in a preclinical as well as a clinical setting where it revealed more similar characteristics to [ 177 Lu]Lu-PSMA-617 than the routinely used [ 68 Ga]Ga-PSMA-11 [20,21].
While images at early time points are comparable with those of [ 68 Ga]Ga-PSMA-11, the advantages of scandium-44 over gallium-68 show up at late time points due to its longer half-life. Enabling delayed image acquisition would simplify patient management at improved image quality and allows improved pre-therapeutic dosimetry for therapy with [ 177 Lu]Lu-PSMA-617. Especially for pre-therapeutic dosimetry scandium-44 would be beneficial as implementation in the clinical setting is uncomplicated, and there is no need for patient hospitalization. Together with the possibility transporting scandium-44 and 44 Sc-radiopharmaceuticals further routes to radiopharmaceutical institutions without option for in-house production scandium-44 could make a significant contribution to patient care even in remote areas. Author details