Abstract
An observational study was set up to assess the feasibility of FPRG PET/CT for imaging patients with multiple myeloma (MM) and to compare its detection rate with low dose CT alone and combined NaF/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 FPRG. The obtained images were compared with results of low dose CT and already available results of a combined NaF/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 FPRG PET/CT compared to low dose CT (98%) or NaF/FDG PET/CT (70%) and all FLs detected with FPRG PET were associated with an underlying bone lesion. In one newly diagnosed patient, more FPRG positive lesions were seen than NaF/FDG positive lesions. This study suggests that FPRG PET/CT might be less useful for the detection of myeloma lesions in patients with advanced disease as all FLs with FPRG uptake were already detected with CT alone.
1. 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 fluorodeoxyglucose (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 -FB-mini-PEG-E[c(RGDyK)]2 (FPRG), a validated radiopharmaceutical with high binding affinity for integrin , seems attractive for the detection of MM lesions [4–8]. The RGD-based radiopharmaceuticals were initially developed to accelerate the development of therapies targeting integrin [9]. The high expression of integrin by activated endothelial cells during angiogenesis aroused keen interest in RGD-based radiopharmaceuticals for imaging of tumor angiogenesis [10, 11]. Nevertheless, the integrin 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 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 [13–17]. Our group previously studied the use of FPRG in rectal and renal cancers, where a correlation between integrin expression and tracer uptake was shown [7, 8]. Since multiple players within the myeloma microenvironment express the integrin, we hypothesized that FPRG PET/CT could be an effective imaging technique for the detection of myeloma lesions.
The combination of NaF and FDG for PET/CT is another strategy to improve the detection of bone metastases and was first introduced by Iagaru et al. [18, 19]. The rationale for the use of both NaF, allowing the detection of bone metastases with bone formation, and FDG, enabling the detection of metastases with increased rate of glucose metabolism, was to improve the sensitivity for detecting metastatic lesions. A prospective clinical trial evaluating combined NaF and FDG for PET/CT in patients with MM is currently under investigation (EudraCT 2013-004807-38), aiming at comparing its capacity to detect MM lesions with the capacities of magnetic resonance imaging, CT alone, and whole-body X-rays [20].
The current observational study was set up to assess the feasibility of FPRG PET/CT to identify myeloma lesions. Secondarily, the detection rate of FPRG PET/CT was compared to CT alone. Additionally, FPRG PET/CT images were compared to combined NaF/FDG PET/CT images, available for those patients that were also included in the above-mentioned trial [20].
2. 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 FPRG 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 μg (±1.6 μg) [7].
Every patient underwent whole-body (WB) scans, from vertex to toes, using FPRG PET/CT and combined NaF/FDG PET/CT (median delay between scans: 4 days; range: 3–5 d). PET/CT scans were acquired in a Gemini TF scanner after injection of MBq FPRG (median uptake time: 62 min) or MBq NaF and MBq FDG (median delay between FDG and NaF injections: 2 min and uptake time: 66 min). All patients fasted for 6 h prior to radiopharmaceutical injection (glycemia < 120 μg/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 (). The maximum diameter of the osteolytic lesions, when present, was also measured. The results are presented as means ± standard deviation (SD).
3. Results
Four patients with MM were included, with newly diagnosed MM and with relapsed MM (Table 1). Based on the low dose CT images, the pattern of bone marrow involvement was focal () or combined diffuse and focal (). Per patient, ≤3 FL () or >10 FLs () were detected. No extramedullary disease was detected. Overall, 81 FLs were detected with PET/CT with underlying bone destruction on CT images (; 89%) or fractures (; 10%; vertebra ; rib ) and one FL (1%) detected with NaF/FDG PET in the femur did not show any abnormality on CT images. Overall, the detection rate of FPRG PET was lower than NaF/FDG PET, whatever the FL location, and the mean uptake () of FPRG was overall lower than NaF/FDG (Table 2). Out of the 72 osteolytic FLs detected with the CT of the PET, only 50% (36/72) showed FPRG uptake (Figure 1). Nonetheless, in one patient with newly diagnosed MM (Figure 1: patient #1), five FLs showed FPRG uptake but no NaF/FDG uptake (Figure 2). In patient # 2 (Figure 1), both FPRG and NaF/FDG PET/CT detected one rib osteolytic FL, while 2 additional osteolytic FLs were detected with CT. In patient #3 (Figure 1), the detection rate of FPRG PET was much lower than NaF/FDG PET (Figure 3). In patient #4 (Figure 1), FPRG PET/CT overlooked one 5 mm osteolytic FL of the cortical bone of a femur that was detected with NaF/FDG PET/CT. In the contingency Table 3, the obtained results in patients with newly diagnosed disease are compared to those of patients with relapsing disease. FPRG positive lesions without concomitant NaF/FDG uptake were observed in one patient with newly diagnosed disease, while patient #3 (with a disease relapse) showed NaF/FDG positive lesions without FPRG uptake.
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4. Discussion
Our purpose was to explore the detection capabilities of FPRG PET/CT and to assess its feasibility in MM disease. In the studied patients, the detection rate of FPRG PET was lower than the detection rate of low dose CT alone (Figure 1). Every FL showing FPRG uptake corresponded to an osteolytic lesion or a fracture on low dose CT images. Although the integrin is expressed by multiple cells in tumor microenvironment such as MM tumor cells, osteoclasts, and activated endothelial cells during angiogenesis, our clinical observation suggests that FPRG PET/CT does not allow a higher detection rate of MM bone lesions than low dose CT alone. The detection rate of FPRG PET was overall lower than NaF/FDG PET (patient #3; Figures 1 and 3) but in one patient, more lesions were visible on the FPRG scan (patient #1; Figures 1 and 2). The prognostic value of FPRG positive lesions and the value of FPRG PET/CT in patients with asymptomatic disease (and thus without bone lesions) were not studied and could be of interest. On the other hand, the high bone marrow background activity related to NaF uptake may explain why some of the FLs detected with FPRG PET/CT were not seen with NaF/FDG. Diffuse bone marrow infiltration was not reliably estimated with NaF/FDG PET due to high NaF bone uptake while it was suspected with FPRG PET/CT in 2 of the 4 patients (Figure 4).
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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 FPRG while NaF/FDG PET showed tracer’s uptake in all these lesions, indicating residual activity. However, whether the uptake was related to FDG in the presence of residual metabolically active tumor and/or whether it was related to 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. FPRG allows the estimation of integrin expression by endothelial cells (and thus neovascularization), tumor cells, and activated osteoclasts, while NaF/FDG uptake reflects tumor cell metabolism and/or bone formation. The heterogeneous uptake of FPRG 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 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 FPRG PET/CT might not be appropriate for detection of MM lesions, it may be of use in the assessment of integrin expression in MM lesions, especially in clinical trials evaluating inhibitors targeting integrins, as recently investigated by Tucci et al. [24]. In addition, our study focused on patients with symptomatic myeloma disease, while FPRG 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).
5. Conclusions
In this case report, FPRG PET/CT detected only 50% of the FLs detected by CT suggesting that the clinical utility of FPRG PET/CT is rather limited for the detection of overt MM lesions. However, the clinical and possibly prognostic relevance of FPRG positive MM lesions needs further investigation.
Abbreviations
| PET: | Positron emission tomography |
| CT: | Computed tomography |
| FDG: | Fluorodeoxyglucose |
| FL: | Focal lesion |
| MM: | Multiple myeloma |
| WB: | Whole-body |
| IMWG: | International Myeloma Working Group |
| SUV: | Standardized uptake value |
| SD: | Standard deviation. |
Ethical Approval
All procedures performed in this report were in accordance with the ethical standards of the institutional research committee and with the principles of the 1964 Declaration of Helsinki and its later amendments or comparable ethical standards. This Belgian monocentric prospective protocol (EudraCT 2013-004807-38) was approved by the Ethics Committee of the University Hospital of Liege and the Federal Agency for Medicines and Health Products (FAMHP).
Consent
Every enrolled patient signed specific informed consent.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Authors’ Contributions
Nadia Withofs and Roland Hustinx supervised PET/CT image acquisition and analyzed the PET/CT images. François Cousin analyzed the CT images. Sanjiv S. Gambhir shared his expertise in FPRG PET/CT. Jo Caers and Yves Beguin designed the study and recruited and informed patients. Bernard De Prijck and Christophe Bonnet informed and recruited patients. Every author contributed to the writing of the manuscript.
Acknowledgments
The authors thank Frederic Mievis, Fabrice Giacomelli, and Christine Mella (Cyclotron Research Centre, University of Liege, Belgium) for the syntheses of FPRG and NaF. This work has been supported by grants from the Foundation against Cancer, Fonds de la Recherche Scientifique Médicale, FIRS Research Fund (CHU de Liège), the Fonds National de la Recherche Scientifique (FNRS, Belgium), and the Fonds Spéciaux de la Recherche (University of Liege).