A First Report on [18F]FPRGD2 PET/CT Imaging in Multiple Myeloma

An observational study was set up to assess the feasibility of [18F]FPRGD2 PET/CT for imaging patients with multiple myeloma (MM) and to compare its detection rate with low dose CT alone and combined [18F]NaF/[18F]FDG PET/CT images. Four patients (2 newly diagnosed patients and 2 with relapsed MM) were included and underwent whole-body PET/CT after injection of [18F]FPRGD2. The obtained images were compared with results of low dose CT and already available results of a combined [18F]NaF/[18F]FDG PET/CT. In total, 81 focal lesions (FLs) were detected with PET/CT and an underlying bone destruction or fracture was seen in 72 (89%) or 8 (10%) FLs, respectively. Fewer FLs (54%) were detected by [18F]FPRGD2 PET/CT compared to low dose CT (98%) or [18F]NaF/[18F]FDG PET/CT (70%) and all FLs detected with [18F]FPRGD2 PET were associated with an underlying bone lesion. In one newly diagnosed patient, more [18F]FPRGD2 positive lesions were seen than [18F]NaF/[18F]FDG positive lesions. This study suggests that [18F]FPRGD2 PET/CT might be less useful for the detection of myeloma lesions in patients with advanced disease as all FLs with [18F]FPRGD2 uptake were already detected with CT alone.


Background
The introduction of efficient and less toxic treatments caused a paradigm shift in the management of multiple myeloma (MM) towards an earlier diagnosis and treatment [1,2]. To detect early signs of bone disease and to identify those patients for whom treatment is needed, highly sensitive imaging techniques are required. Positron emission tomography combined with computed tomography (PET/CT) using [ 18 F]fluorodeoxyglucose ([ 18 F]FDG) has already proven to be a sensitive technique for the detection of metabolically active MM lesions and was recently incorporated in the diagnostic work-up of MM by the International Myeloma Working Group (IMWG) accordingly [3].
Alternatively, the 18 F-FB-mini-PEG-E[c(RGDyK)] 2 ([ 18 F]FPRGD 2 ), a validated radiopharmaceutical with high binding affinity for integrin V 3 , seems attractive for the detection of MM lesions [4][5][6][7][8]. The RGD-based radiopharmaceuticals were initially developed to accelerate the development of therapies targeting integrin V 3 [9]. The high expression of integrin V 3 by activated endothelial cells during angiogenesis aroused keen interest in RGD-based radiopharmaceuticals for imaging of tumor angiogenesis [10,11]. Nevertheless, the integrin V 3 is not solely expressed by activated endothelial cells; it can be overexpressed by many types of cancer cells, regulating cell survival, metastases, and drug resistance [12]. In the case of myeloma, the integrin V 3 is expressed by activated endothelial cells but it can also be overexpressed by myeloma tumor cells and other cell types of the tumor microenvironment such as osteoclasts [13][14][15][16][17]. Our group previously studied the use of [ 18 F]FPRGD 2 in rectal and renal cancers, where a correlation between integrin V 3 expression and tracer uptake was shown [7,8]

Materials and Methods
Patients with newly diagnosed or relapsed MM were prospectively included. This study was registered as EudraCT #2013-004807-38 and was approved by the Ethics Committee of the academic hospital (CHU of Liege). All subjects provided written informed consent for this study.
The radiosynthesis of [ 18 F]FPRGD 2 was performed as previously reported and in compliance with current good manufacturing practice regulations [5,7]. The mean (±standard deviation) injected mass of the active pharmaceutical ingredient was 11.1 g (±1.6 g) [7].
Every patient underwent whole-body (WB) scans, from vertex to toes, using F]NaF injections: 2 min and uptake time: 66 min). All patients fasted for 6 h prior to radiopharmaceutical injection (glycemia < 120 g/ml in all patients). A low dose CT (3 mm slice thickness; 120 kV and 50 to 80 mAs depending on patient's weight) followed by the PET emission scan of 90 seconds per bed position was performed.
The PET/CT images were reviewed by 2 experienced nuclear medicine physicians and 2 radiologists to detect focal lesions (FLs) and/or diffuse bone marrow involvement. Areas of tracers' uptake corresponding to degenerative changes were excluded. Focal areas of increased uptake, regardless of the presence of bone abnormality on CT images, and hypoactive FLs with underlying bone destruction on CT images and suspected of being associated with myeloma lesions were considered PET MM FLs. The FLs were classified according to their location in 7 regions of the body: pelvis, skull, superior limbs, inferior limbs, spine, ribs, and one location including the sternum, scapula, and clavicles. A 1.2 ml volume of interest was drawn in the focal area of radiopharmaceutical's uptake to estimate the maximum standardized uptake value (SUV max ). The maximum diameter of the osteolytic lesions, when present, was also measured. The results are presented as means ± standard deviation (SD).

Results
Four patients with MM were included, = 2 with newly diagnosed MM and = 2 with relapsed MM (Table 1). Based on the low dose CT images, the pattern of bone marrow involvement was focal ( = 2) or combined diffuse and focal ( = 2  (Figure 1). Nonetheless, in one patient with newly diagnosed MM (Figure 1: patient #1), five  In patient #4 (Figure 1)

Discussion
Our purpose was to explore the detection capabilities of [ 18 F]FPRGD 2 PET/CT and to assess its feasibility in MM disease. In the studied patients, the detection rate of [ 18 F]FPRGD 2 PET was lower than the detection rate of low dose CT alone (Figure 1 Figure 4). Our report included 2 patients with relapsed MM and thus with possible long-lasting healed lesions. In one of these patients (patient #3; Figure 3), some of the osteolytic lesions did not show uptake of 18 F]FDG PET showed tracer's uptake in all these lesions, indicating residual activity. However, whether the uptake was related to [ 18 F]FDG in the presence of residual metabolically active tumor and/or whether it was related to [ 18 F]NaF due to bone turnover in the long-lasting healing process of bone lesions after treatment is unknown [21]. Moreover, we excluded patients with a short treatment-free interval before inclusion to avoid PET-negativity induced by a recent chemotherapy.
As mentioned in the introduction, both imaging techniques highlight different biological aspects.  tumor cell metabolism and/or bone formation. The heterogeneous uptake of [ 18 F]FPRGD 2 can be explained by biological phenomena and previously received treatments. The myeloma-induced angiogenesis appears after an "angiogenic switch" due to the release of angiogenic factors by subsets of myeloma cells or can be directly in proportion to the tumor infiltration inside the bone marrow [22]. This angiogenesis is counteracted by targeted treatments such as thalidomide and bortezomib which could explain reduced uptake in relapsing patients. Decreased uptake of [ 18 F]FDG was recently found to be associated with reduced expression of hexokinase-2, responsible for the first step of glycolysis [23].  Even though this case report suggests that [ 18 F]FPRGD 2 PET/CT might not be appropriate for detection of MM lesions, it may be of use in the assessment of integrin V 3 expression in MM lesions, especially in clinical trials evaluating inhibitors targeting V 3 integrins, as recently investigated by Tucci et al. [24]. In addition, our study focused on patients with symptomatic myeloma disease, while [ 18 F]FPRGD 2 PET/CT might be useful to detect bone marrow infiltration in precursor states of the disease (smoldering multiple myeloma or monoclonal gammopathy of undetermined significance).

Conclusions
In this case report,

Ethical Approval
All procedures performed in this report were in accordance with the ethical standards of the institutional research

Consent
Every enrolled patient signed specific informed consent.

Conflicts of Interest
The authors declare that they have no conflicts of interest.