Positron Emission Tomography (PET) Radiopharmaceuticals in Multiple Myeloma

Non-[18F]FDG PET-Radiopharmaceuticals in Oncology

Molecular imaging is a growing field, driven by technological advances, such as the improvement of PET-CT scanners through the introduction of digital detectors and scanners with an extended field of view, resulting in much higher sensitivity and a variety of new specific radiopharmaceuticals that allow the visualization of specific molecular pathways and even theragnostic approaches. In oncology, the development of dedicated tracers is crucial for personalized therapeutic approaches. Novel peptides allow the visualization of many different targets, such as PD-1 and PD-L1 expression, chemokine expression, HER expression, T-cell imaging, microenvironmental imaging, such as FAP imaging, and many more. In this article, we review recent advances in the development of non-[18F]FDG PET radiopharmaceuticals and their current clinical applications in oncology, as well as some future aspects.

Radiopharmaceuticals for Skeletal Muscle PET Imaging

The skeletal muscles account for approximately 40% of the body weight and are crucial in movement, nutrient absorption, and energy metabolism. Muscle loss and decline in function cause a decrease in the quality of life of patients and the elderly, leading to complications that require early diagnosis. Positron emission tomography/computed tomography (PET/CT) offers non-invasive, high-resolution visualization of tissues. It has emerged as a promising alternative to invasive diagnostic methods and is attracting attention as a tool for assessing muscle function and imaging muscle diseases. Effective imaging of muscle function and pathology relies on appropriate radiopharmaceuticals that target key aspects of muscle metabolism, such as glucose uptake, adenosine triphosphate (ATP) production, and the oxidation of fat and carbohydrates. In this review, we describe how [18F]fluoro-2-deoxy-D-glucose ([18F]FDG), [18F]fluorocholine ([18F]FCH), [11C]acetate, and [15O]water ([15O]H2O) are suitable radiopharmaceuticals for diagnostic imaging of skeletal muscles.

89Zr-DFO-Isatuximab for CD38-Targeted ImmunoPET Imaging of Multiple Myeloma and Lymphomas

Multiple myeloma (MM) is the second most prevalent hematological malignancy. It remains incurable despite the availability of novel therapeutic approaches, marking an urgent need for new agents for noninvasive targeted imaging of MM lesions. CD38 has proven to be an excellent biomarker due to its high expression in aberrant lymphoid and myeloid cells relative to normal cell populations. Using isatuximab (Sanofi), the latest FDA-approved CD38-targeting antibody, we have developed Zirconium-89(89Zr)-labeled isatuximab as a novel immunoPET tracer for the in vivo delineation of MM and evaluated the extension of its applicability to lymphomas. In vitro studies validated the high binding affinity and specificity of 89Zr-DFO-isatuximab for CD38. PET imaging demonstrated the high performance of 89Zr-DFO-isatuximab as a targeted imaging agent to delineate tumor burden in disseminated models of MM and Burkitt's lymphoma. Ex vivo biodistribution studies confirmed that high accumulations of the tracer in bone marrow and bone skeleton correspond to specific disease lesions as they are reduced to background in blocking and healthy controls. This work demonstrates the promise of 89Zr-DFO-isatuximab as an immunoPET tracer for CD38-targeted imaging of MM and certain lymphomas. More importantly, its potential as an alternative to 89Zr-DFO-daratumumab holds great clinical relevance.

Multitracer PET/CT with [18F]Fluorodeoxiglucose and [18F]Fluorocholine in the Initial Staging of Multiple Myeloma Patients Applying the IMPeTus Criteria: A Pilot Study

Initial staging of patients diagnosed with multiple myeloma (MM) can lead to negative results using conventional diagnostic imaging workup, including [¹⁸F]Fluorodesoxiglucose ([¹⁸F]FDG) PET/CT. The aim of this prospective pilot study was to evaluate the diagnostic efficacy of [¹⁸F]Fluorocholine ([¹⁸F]FCH) PET/CT in the initial staging of MM patients who were candidates for autologous bone marrow transplant. Materials and Methods: The inclusion criteria of our study were: (a) patients diagnosed with MM; (b) candidates for autologous bone marrow transplant (AT); and (c) studied with [¹⁸F]FCH PET/CT and [¹⁸F]FDG PET/CT for initial staging less than 4 weeks apart. Imaging analysis included the presence of: bone marrow infiltration, focal bone lesions, and para-medullary or extra-medullary disease, according to the proposed IMPeTus criteria. The analysis was performed per lesion, per patient, and per location. Results: The study population included ten patients. Globally, [¹⁸F]FCH PET/CT showed bone marrow uptake in all the patients and visualised 16 more focal lesions than [¹⁸F]FDG PET/CT. One patient presented a plasmacytoma, detected by both tracers. Extra-medullary and para-medullary disease was identified with different degrees of uptake by both tracers. In summary, [¹⁸F]FCH PET seemed to be superior to [¹⁸F]FDG PET/CT in detecting focal bone lesions. SUVmax values were slightly higher in [¹⁸F]FCH PET/CT than in [¹⁸F]FDG PET/CT. Conclusions: Taking into account the small study population, according to our results, [¹⁸F]FCH PET/CT could be a useful tool for staging MM patients.

Recent developments on the application of molecular probes in multiple myeloma: Beyond [18F]FDG

Multiple myeloma (MM) is a neoplastic plasma cell proliferative disorder characterized by various osteolytic bone destruction as a radiological morphological marker. Functional imaging, particularly nuclear medicine imaging, is a promising method to visualize disease processes before the appearance of structural changes by targeting specific biomarkers related to metabolism ability, tumor microenvironment as well as neoplastic receptors. In addition, by targeting particular antigens with therapeutic antibodies, immuno-PET imaging can support the development of personalized theranostics. At present, various imaging agents have been prepared and evaluated in MM at preclinical and clinical levels. A summary overview of molecular functional imaging in MM is provided, and commonly used radiotracers are characterized.

18F-FDG Versus Non-FDG PET Tracers in Multiple Myeloma

Multiple myeloma (MM) accounts for 0.9% of cancer diagnoses, and incidence and mortality rate have increased in previous years. 18F-fluorodeoxyglucose (FDG) PET-computed tomography (CT) is an established modality for MM evaluation. MR imaging is helpful where 18F-FDG PET-CT is lacking. To standardize PET reporting, methods like Italian Myeloma Criteria for PET Use and Deauville criteria have been studied. Tracers like 11C-acetate and 11C-choline/18F-fluoromethylcholine (FCH) have shown higher sensitivity and detected more focal lesions and diffuse involvement than 18F-FDG PET-CT. 18F-FCH showed higher maximum standardized uptake value than 18FDG. 11C-methionine appears to be the best radiopharmaceutical, apart from 18F-FDG, for evaluating MM.

Imaging for Plasma Cell Dyscrasias: What, When, and How?

Imaging plays a vital role in the diagnosis, response assessment, and follow-up of patients with plasma cell bone disease. The radiologic diagnostic paradigm has thus far evolved with developing technology and availability of better imaging platforms; however, the skewed availability of these imaging modalities in developed vis-à-vis the developing countries along with the lack of uniformity in reporting has led to a consensus on the imaging criteria for diagnosing and response assessment in plasma cell dyscrasia. Therefore, it is imperative for not only the radiologists but also the treating oncologist to be aware of the criteria and appropriate imaging modality to be used in accordance with the clinical question. The review will allow the treating oncologist to answer the following questions on the diagnostic, prognostic, and predictive abilities of various imaging modalities for plasma cell dyscrasia: a) What lesions can look like multiple myeloma (MM) but are not?; b) Does the patient have MM? To diagnose MM in a high-risk SMM patient with clinical suspicion, which modality should be used and why?; c) Is the patient responding to therapy on follow-up imaging once treatment is initiated?; d) To interpret commonly seen complications post-therapy, when is it a disease and when is the expected sequel to treatment? Fractures, red marrow reconversion?; and e) When is the appropriate time to flag a patient for further workup when interpreting MRI spine done for back pain in the elderly? How do we differentiate between commonly seen osteoporosis-related degenerative spine versus marrow infiltrative disorder?

Molecular Imaging in Multiple Myeloma-Novel PET Radiotracers Improve Patient Management and Guide Therapy

Due to its proven value in imaging of multiple myeloma (MM), including staging, prognostication, and assessment of therapy response, 2-deoxy-2-[18F]fluoro-D-glucose (FDG) positron emission tomography (PET) is utilized extensively in the clinic. However, its accuracy is hampered by imperfect sensitivity (e.g., so-called FDG-negative MM) as well as specificity (e.g., inflammatory processes), with common pitfalls including fractures and degenerative changes. Novel approaches providing a read-out of increased protein or lipid membrane syntheses, such as [11C]methionine and [11C]choline or the C-X-C motif chemokine receptor 4-targeting radiotracer [68Ga]Pentixafor, have already been shown to be suitable adjuncts or alternatives to FDG. In the present focused review, those imaging agents along with their theranostic potential in the context of MM are highlighted.

[Positron emission tomography/computed tomography (PET/CT) in multiple myeloma]

BACKGROUND

Imaging plays a pivotal role in the management of multiple myeloma (MM). Besides morphological imaging methods, such as whole-body X‑ray, computed tomography (CT) and magnetic resonance imaging (MRI), the hybrid modality positron emission tomography/CT (PET/CT) using the glucose analogue ¹⁸F‑fluorodeoxyglucose (¹⁸F‑FDG) as radiotracer is increasingly used.

OBJECTIVES

Aim of this review article is to outline the major applications of PET/CT in the diagnosis and management of MM, and to provide hints on the reading and interpretation.

MATERIALS AND METHODS

Background knowledge and guideline recommendations on imaging of MM are outlined and complemented by recent study results.

RESULTS

Although ¹⁸F‑FDG PET/CT is not currently considered a standard method for the diagnosis of MM, it is a very powerful diagnostic tool for the detection of medullary and extramedullary disease, a reliable predictor of survival and the most robust modality for treatment response evaluation. Moreover, it plays a significant role in minimal residual disease (MRD) assessment. On the other hand, practical considerations on local availability and costs limit the widespread use of PET/CT. In addition, false-negative and the seldom false-positive results and the heterogeneity of MM presentation inevitably make interpretation of PET/CT images challenging.

CONCLUSIONS

PET/CT has a high value in the diagnosis, prognosis, and assessment of treatment response in patients with MM. Therefore, the role of the modality in the management of the disease is expected to increase in the near future.

The prognostic significance of [18F]FDG PET/CT in multiple myeloma according to novel interpretation criteria (IMPeTUs)

PURPOSE

[¹⁸F]FDG PET/CT is the elective imaging modality for treatment monitoring in multiple myeloma (MM). However, MM is a heterogeneous disease from an imaging point of view, raising challenges in interpretation of PET/CT. We herein investigated the prognostic role of the novel Italian Myeloma criteria for PET Use (IMPeTUs) in MM patients undergoing high-dose chemotherapy (HDT) followed by autologous stem cell transplantation (ASCT).

METHODS

Forty-seven patients with newly diagnosed MM underwent [¹⁸F]FDG PET/CT before commencement of treatment (baseline PET/CT). Thirty-four of them (72.3%) were also examined after completion of ASCT (follow-up PET/CT). PET/CT analysis was based on the IMPeTUs criteria, which take into consideration-among others-the metabolic state of the bone marrow based on the 5-point Deauville score (DS), the number and metabolic state of focal [¹⁸F]FDG-avid lesions, as well as the presence of paramedullary disease (PMD) and extramedullary disease (EMD). We analyzed whether parameters from IMPeTUs correlate with clinically relevant parameters and patients' outcome, as assessed by progression-free survival (PFS).

RESULTS

Median follow-up from baseline and follow-up PET/CT were 85.1 months and 76.7 months, respectively. The number of focal, [¹⁸F]FDG-avid lesions significantly correlated with the bone marrow infiltration rate and the R-ISS stage, while the presence of PMD was associated with LDH. After univariate survival analysis, the number of focal, [¹⁸F]FDG-avid lesions both before and after therapy as well as the presence of PMD and EMD before therapy adversely affected PFS. Multivariate survival analysis for baseline parameters confirmed that the number of focal, [¹⁸F]FDG-avid lesions and the presence of EMD are associated with adverse prognosis, irrespective of the ISS stage and/or the presence of high-risk cytogenetic abnormalities. The 5-point DS of [¹⁸F]FDG uptake in reference bone marrow and focal lesions showed a significant decrease as response to treatment, but it did not affect PFS.

CONCLUSION

Several parameters utilized in IMPeTUs predict PFS in MM patients, suggesting the potentially significant role of the new criteria in patient stratification and response assessment. Additional studies are warranted for the further evaluation of IMPeTUs in the direction of establishment of robust cut-off values with a prognostic significance in the disease.

Case series: MRD negativity assessment using 11C-Acetate PET with 3-weekly daratumumab-based quadruplet induction in newly diagnosed multiple myeloma

Complete response (CR) is an important favorable factor for survival in multiple myeloma (MM). However, CR patients continue to relapse, especially in the presence of minimal residual disease (MRD). Bone marrow (BM) MRD is predictive of progression-free survival (PFS) in MM. However, myeloma outside the BM aspiration site may result in subsequent relapse despite MRD-negativity. Therefore, positron emission tomography-computed tomography (PET-CT) based on F-fluorodeoxyglucose (FDG) is a complementary tool to monitor residual disease in MM. However, FDG may miss myeloma lesions that are not FDG-avid. On the other hand, ¹¹C-Acetate (ACT) has been found to be a more sensitive and specific tracer than FDG in MM. Recently, the addition of daratumumab to bortezomib, thalidomide, dexamethasone (VTd) or bortezomib, lenalidomide, dexamethasone (VRd) backbone has been proven to improve outcomes. Herein, we report three newly-diagnosed MM patients achieving deep responses with imaging CR using ACT PET in addition to conventional immunofixation CR and MRD-negative CR after a 3-weekly daratumumab-based quadruplet induction regimen.

In vivo quantitative assessment of therapeutic response to bortezomib therapy in disseminated animal models of multiple myeloma with [18F]FDG and [64Cu]Cu-LLP2A PET

Background

Multiple myeloma (MM) is a disease of cancerous plasma cells in the bone marrow. Imaging-based timely determination of therapeutic response is critical for improving outcomes in MM patients. Very late antigen-4 (VLA4, CD49d/CD29) is overexpressed in MM cells. Here, we evaluated [¹⁸F]FDG and VLA4 targeted [⁶⁴Cu]Cu-LLP2A for quantitative PET imaging in disseminated MM models of variable VLA4 expression, following bortezomib therapy.

Methods

In vitro and ex vivo VLA4 expression was evaluated by flow cytometry. Human MM cells, MM.1S-CG and U266-CG (C: luciferase and G: green fluorescent protein), were injected intravenously in NOD-SCID gamma mice. Tumor progression was monitored by bioluminescence imaging (BLI). Treatment group received bortezomib (1 mg/kg, twice/week) intraperitoneally. All cohorts (treated, untreated and no tumor) were longitudinally imaged with [¹⁸F]FDG (7.4–8.0 MBq) and [⁶⁴Cu]Cu-LLP2A (2–3 MBq; Molar Activity: 44.14 ± 1.40 MBq/nmol) PET, respectively.

Results

Flow cytometry confirmed high expression of CD49d in U266 cells (> 99%) and moderate expression in MM.1S cells (~ 52%). BLI showed decrease in total body flux in treated mice. In MM.1S-CG untreated versus treated mice, [⁶⁴Cu]Cu-LLP2A localized with a significantly higher SUV_mean in spine (0.58 versus 0.31, p < 0.01) and femur (0.72 versus 0.39, p < 0.05) at week 4 post-tumor inoculation. There was a four-fold higher uptake of [⁶⁴Cu]Cu-LLP2A (SUV_mean) in untreated U266-CG mice compared to treated mice at 3 weeks post-treatment. Compared to [⁶⁴Cu]Cu-LLP2A, [¹⁸F]FDG PET detected treatment-related changes at later time points.

Conclusion

[⁶⁴Cu]Cu-LLP2A is a promising tracer for timely in vivo assessment of therapeutic response in disseminated models of MM.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13550-021-00840-4.

Quantitative, Dynamic 18F-FDG PET/CT in Monitoring of Smoldering Myeloma: A Case Report

We report on a 52-year-old patient with an initial diagnosis of smoldering myeloma (SMM), who was monitored by means of dynamic and static positron emission tomography/computed tomography (PET/CT) with the radiotracer ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). Baseline PET/CT revealed no pathological signs. Six months later, a transition to symptomatic, multiple myeloma (MM) was diagnosed. The transition was not accompanied by focal, hypermetabolic lesions on PET/CT. However, a diffusely increased ¹⁸F-FDG uptake in the bone marrow, accompanied by a marked increase of semi-quantitative (standardized uptake value, SUV) and quantitative, pharmacokinetic ¹⁸F-FDG parameters, was demonstrated. After successful treatment, including tandem autologous transplantation, the diffuse uptake in the bone marrow as well as the semi-quantitative and quantitative parameters showed a marked remission. This response was also confirmed by the clinical follow-up of the patient. These findings suggest that in MM a diffuse ¹⁸F-FDG uptake in the bone marrow may indeed reflect an actual bone marrow infiltration by plasma cells. Moreover, SUV values and kinetic parameters, not only from myeloma lesions but also from random bone marrow samples, may be used for MM monitoring. This could be particularly helpful in the follow-up of myeloma patients negative for ¹⁸F-FDG-avid focal lesions.

Can 18F-NaF PET/CT before Autologous Stem Cell Transplantation Predict Survival in Multiple Myeloma?

There is an unmet need for positron emission tomography (PET) radiotracers that can image bone disease in multiple myeloma (MM) in a more sensitive and specific way than the widely used ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG). Sodium fluoride (¹⁸F-NaF) is a highly sensitive tracer of bone reconstruction, evolving as an important imaging agent for the assessment of malignant bone diseases. We attempted to investigate for the first time the prognostic significance of ¹⁸F-NaF PET/CT in newly diagnosed, symptomatic MM patients planned for autologous stem cell transplantation (ASCT). Forty-seven patients underwent dynamic and static PET/CT with ¹⁸F-NaF before treatment. After correlation with the respective findings on CT and ¹⁸F-FDG PET/CT that served as reference, the ¹⁸F-NaF PET findings were compared with established factors of high-risk disease, like cytogenetic abnormalities as well as bone marrow plasma cell infiltration rate. Furthermore, the impact of ¹⁸F-NaF PET/CT on progression-free survival (PFS) was analyzed. Correlation analysis revealed a moderate, significant correlation of the ¹⁸F-NaF parameters SUV_average and K₁ in reference tissue with bone marrow plasma cell infiltration rate. However, no significant correlation was observed regarding all other ¹⁸F-NaF PET parameters. Survival analysis revealed that patients with a pathologic ¹⁸F-NaF PET/CT have a shorter PFS (median = 36.2 months) than those with a physiologic scan (median = 55.6 months) (p = 0.02). Nevertheless, no quantitative ¹⁸F-NaF parameter could be shown to adversely affect PFS. In contrast, the respective analysis for quantitative dynamic ¹⁸F-FDG PET/CT revealed that the parameters SUV_max, fractional blood volume (V_B), k₃ and influx from reference tissue as well as SUV_average from MM lesions had a significant negative impact on patient survival. The herein presented findings highlight the rather limited role of ¹⁸F-NaF PET/CT as a single PET approach in MM.