Imaging with radiolabelled bisphosphonates

Abstract

Radiolabeled bisphosphonates were developed in the 1970s for scintigraphic functional imaging of the skeleton in benign and malignant disease. Tracers such as ^99mTc-methylene diphosphonate, that map focal or global changes in mineralization in the skeleton qualitatively and quantitatively, have been the backbone of nuclear medicine imaging for decades. While competing technologies are evolving, new indications and improvements in scanner hardware, in particular hybrid imaging (e.g. single photon emission computed tomography combined with computed tomography), have allowed improved diagnostic accuracy and a continued role for radiolabeled bisphosphonate imaging in current practice.

1. History and basis of bisphosphonate imaging

A number of radiotracers that act as markers of skeletal mineralization have been described and compared [1]. However, the ability to radiolabel with the commonly available radionuclide ^99mTc-technetium (^99mTc), that has ideal physical characteristics for gamma camera imaging (half-life 6 h, photon energy 140 keV), is a distinct advantage leading to the ubiquitous use of radiolabeled bisphosphonates for bone scintigraphy.

^99mTc-labeled pyrophosphates were initially used with some success in skeletal imaging [2]. However, the replacement of P-O-P in pyrophosphates with P-C-P, results in bisphosphonates (commonly named diphosphonates in nuclear medicine) that act as analogues that are more stable and resist in-vivo metabolism, and therefore more suitable for radiolabeling and intravenous injection, for imaging. Labeling, with the commonly available radionuclide ^99mTc, allows gamma camera imaging of focal or global disturbances in skeletal mineralization, for which these compounds act as biomarkers.

In the 1970s, Subramanian et al. [3] developed ^99mTc-labeled methylene diphosphonate (MDP) which showed superior imaging characteristics to other ^99mTc-labeled bone-seeking tracers, including pyrophosphate, polyphosphate and another diphosphonate (ethane-1-hydroxyl-1, 1-diphosphonate). ^99mTc-MDP was associated with more rapid clearance from blood and soft tissues with good image contrast as early as 2 h post injection and with an acceptable radiation dose. Structurally-related hydroxymethylene diphosphonate (HMDP) also shows favourable properties with higher skeletal uptake and comparable image quality [4,5]. Ignac Fogelman [6] and his collaborators and colleagues of the time were instrumental in the early and subsequent development and refinement of nuclear medicine imaging of the skeleton with bisphosphonates in benign and malignant disease [7–9] and for 50 years ^99mTc-MDP, or similar compounds, have been used for scintigraphic imaging of a wide variety of benign and malignant clinical indications worldwide (Fig. 1).

Fig. 1.

Structures of pyrophosphate (top left), methylene diphosphonate (MDP) (top right), ethane-1-hydroxy-1,1-dphosphonate (EHDP) (bottom left), hydro-xymethane diphosphonate (HMDP) bottom right.

2. Mechanisms and quantification of skeletal metabolism with radiolabeled bisphosphonates

After intravenous injection, between 20 and 60% of radiolabeled bisphosphonates are cleared to the skeleton over the following 24 h. There is significant protein binding that increases with time, but the free tracer is excreted through the urinary tract in relation to the glomerular filtration rate [10,11]. The resulting images, which can be acquired with good contrast between bone and soft tissue between 2 and 4 h post injection, are a map of the regional clearance of tracer in the skeleton related to regional blood flow and osteoblastic activity [3,11] (Fig. 2) although work with fluorescent bisphosphonates has also shown labeling at the resorptive surfaces associated with osteoclasts in animal models [12,13]. Indeed, bisphosphonates remain fixed in bone until the bone eventually undergoes resorption [14].

Fig. 2. A planar anterior and posterior ^99mTc-MDP bone scan of a male with metastatic prostate cancer.

Gamma cameras detect the emitted 140 keV photons from the ^99mTc-MDP which are collimated before interaction with a scintigraphic crystal such as sodium iodide. The light scintillations are then collected, and the signal is amplified through photomultiplier tubes to produce a digital image representation of the distribution within the body. Multiple overlapping views of the skeleton can be acquired on a single-headed gamma camera or whole-body sweeps, with anterior and posterior whole-body images, can be acquired on dual-headed scanners (Fig. 2). The gamma camera heads, typically around 40 cm in diameter, can rotate around an area of the skeleton to acquire data that can be processed to make tomographic images that can be displayed in any plane, but typically axial, coronal and sagittal (Fig. 3). Tomographic images (single-photon emission computed tomography, SPECT) give a better 3-dimensional representation of the tracer distribution that can improve localization and diagnostic specificity. Although spatial resolution is inferior to non-tomographic planar images (e.g. 0.8–1.2 cm vs 0.5–1.0 cm), the contrast resolution is improved, thereby increasing sensitivity for lesion detection. If knowledge of the vascularity of a lesion is required, then early post injection scans can be acquired, e.g. 0–1 min for blood flow and 3–5 min for blood pool.

Fig. 3. Planar (a) and SPECT (b) ^99mTc-MDP bone scan images in a patient with prostate cancer.

A focal metastatic lesion in L1 localizes to the vertebral body on the tomographic images.

As well as providing qualitative diagnostic imaging, it is possible to quantify either regional or global parameters of skeletal metabolism with bone-specific tracers such as ^99mTc-MDP.

In particular, global disorders of skeletal metabolism may not be visibly abnormal to the eye without more sensitive quantification.

Fogelman pioneered the 24 h whole-body retention test [15–20]. By comparing whole-body counts at 24 h to the initial counts at 5 min after injection, the ratio is related to the clearance of tracer to the skeleton compared to the total clearance of tracer by the skeleton and kidneys. It is assumed that any subsequent release of tracer from the bone mineral compartment is negligible at 24 h and that retention in soft tissues at 24 h is low. Measurements can be performed on a gamma camera or a whole-body counter.

Fogelman reported 24 h whole-body retention results in normals related to age [20] and in patients with a variety of metabolic bone diseases, including osteoporosis, hyperparathyroidism, Paget's disease, osteomalacia and renal osteodystrophy [15,19]. At 24 h, retention is < 20% in normals when using ^99mTc-HEDP but as high as nearly 90% in those with renal osteodystrophy [15]. Some variation was noted when using different bisphosphonates [4].

In a study of 250 healthy adults, a rise in 24 h whole-body retention was noted at the menopause as a result of increased skeletal turnover [20], a finding confirmed by other investigators [21]. Fogelman also reported 24 h whole-body retention results in women post oophorectomy showing those on estrogen replacement had significantly lower results than those on placebo and that results correlated with dose and bone loss measured by photon absorptiometry [18].

Refinements to the 24 h whole-body retention methodology by Nisbet allowed quantification during routine clinical bone scanning using ⁵¹Cr-EDTA as a glomerular filtration co-tracer [22] and a further method that employs deconvolution analysis for quantifying skeletal metabolism, has also been described [23]. More recently, quantitative SPECT gamma camera data has been used for measurement of regional skeletal indices [24]. This method allowed differences between trabecular and cortical sites of the skeleton to be measured and showed higher values in cortical bone in osteoporotic women compared to normal women [25]. The same group observed increased skeletal metabolism, particularly in the femoral shaft, in patients with hyperparathyroidism and thyrotoxicosis [26]. The majority of both groups of patients showed a reduction in bone turnover after 3 months of appropriate treatment.

3. Diagnostic imaging approaches using radiolabeled bisphosphonates

Several metabolic and non-malignant orthopedic conditions are investigated routinely with bone scintigraphy using radiolabeled bisphosphonates such as ^99mTc-MDP. These include Paget’s disease for diagnosis and response assessment [27,28], assessment and follow up of fractures, joint prosthesis infection and loosening, amongst others [29]. In addition, ^99mTc-MDP bone scintigraphy has been a routine staging procedure for cancers that are commonly associated with bone metastases, such as breast and prostate cancers [30]. While the latter indication is still common, there are competing technologies such as positron emission tomography (PET) and whole-body magnetic resonance imaging with diffusion-weighted imaging (DW-MRI), that offer better diagnostic accuracy [30,31].

While the standard clinical bone tracers have not changed for several decades, there has been a significant improvement in quality and complexity of scanner hardware. Hybrid imaging, that combines morphological modalities such as CT and MRI with functional imaging, including SPECT and PET, is now routinely used in many hospitals. SPECT/CT and PET/CT are routinely available and PET/MRI scanners are available in some centers. SPECT/CT and PET/CT scans have the 2 modalities acquired consecutively with resultant images being available to view separately or overlaid (Fig. 4). The two components of PET/MRI can be performed simultaneously.

Fig. 4. Planar (a) and SPECT/CT (b) ^99mTc-MDP images of a patient with a history of breast cancer with back pain.

Planar images show areas of increased activity in the lumbar spine. On SPECT/CT images (top row SPECT, bottom row SPECT/CT) it can clearly be seen that the abnormal activity originates in the facet joints and is therefore likely to represent benign arthrosis rather than metastases.

A weakness of bone scintigraphy is that uptake of ^99mTc-MDP is non-specific and can occur in any disease that causes focal or global changes in mineralization. Tomographic SPECT or PET images on their own may complement planar images and increase sensitivity by increasing contrast resolution and increase specificity by allowing better anatomical and morphological analysis. However, with the addition of CT (or MRI) the specificity can be increased further as the morphological features can more readily differentiate benign from malignant causes of tracer uptake in the skeleton (Fig. 4) [32].

Planar bone scintigraphy has been losing favor as it has long been recognized that it has limited specificity in the diagnosis of bone metastases, and more recently sensitivity has been questioned, as modalities that can detect cancer in the marrow space before an osteoblastic bone reaction has occurred (e.g. MRI, PET), have shown superiority. Sensitivity is also poor in diseases that are predominantly osteolytic in nature such as myeloma where osteoblastic activity is suppressed [33]. However, the availability of hybrid SPECT/CT scanners has vastly improved sensitivity and specificity, notwithstanding the poor sensitivity for marrow-based and osteolytic lesions.

Another disadvantage of tracers that rely on osteoblastic activity for uptake is the flare phenomenon during systemic therapy for bone metastases. This phenomenon causes an increase in uptake of previous lesions, or visualization of previously occult lesions, as a result of osteoblastic healing after successful treatment and cannot be readily differentiated from an increase in uptake secondary to progressive, non-responding disease, for several weeks or months. If a flare is recognized, then it is a good prognostic sign [34]. To overcome this limitation the Prostate Cancer Working Group have described criteria that primarily aim to determine disease progression, requiring at least 2 new lesions on the first assessment following a baseline bone scan and then at least a further 2 lesions on a subsequent confirmatory scan before progressive disease is confirmed [35].

The flare phenomenon can be used to increase diagnostic sensitivity and specificity in bone scintigraphy. Patients with high-risk prostate cancer treated with hormones can show an increase in lesion activity or number six weeks after commencing treatment. In a prospective study of 99 patients with new diagnoses of prostate cancer, 41% with unequivocal metastases showed a 6-week flare, 11% with a normal baseline scan became positive and 20% of those with equivocal lesions showed a flare that was specific for metastases on follow up [36].

Another area that may continue to require radiolabeled bisphosphonate imaging is the planning and follow up of targeted radionuclide therapy for bone metastases. The therapeutic agents such as ⁸⁹Sr-chloride, ¹⁵³Sm-EDTMP, ^186/188Re-HEDP and ²²³Ra-chloride, have the same or similar mechanisms of accumulation in skeletal metastases and so ^99mTc-MDP scintigraphy can be used to confirm targeting prior to treatment and subsequent treatment response [37]. While radiolabeled bisphosphonates have been almost exclusively associated with SPECT radionuclides, there is some interest in the development of PET radiolabeling, allowing higher resolution and quantitative bone imaging with tracers such as ⁶⁸Ga-pamidronate that is in preclinical development [38].

4. Alternative tracers and methods for functional bone imaging

¹⁸F-fluoride was first described as a bone-specific PET tracer over 50 years ago [39] but it was not until the development of modern PET scanners that it was possible to obtain high quality images of the skeleton (Fig. 5). The mechanism of uptake of ¹⁸F-Fluoride is similar to bisphosphonates in that it is related to local blood flow and osteoblastic mineralization activity [11]. Advantages over SPECT imaging with radiolabeled bisphosphonates include a more rapid background clearance, allowing imaging at 1 h post injection, higher spatial resolution and inherent quantitative accuracy. The ability to quantify skeletal kinetics in meaningful parameters such as K₁ (related to local blood flow) and K_i (local clearance of tracer to the bone mineral compartment) has led to the method being used to evaluate metabolic bone disorders such as Paget's disease, renal osteodystrophy and osteoporosis [40–42]. The occasional shortage in availability of ^99mTc as a radionuclide has also led to the replacement of ^99mTc-MDP bone scintigraphy with ¹⁸F-fluoride PET/CT in some centers with evidence of resultant changes in management in the diagnosis and follow up of skeletal metastases [43–45]. ¹⁸F-fluoride, as an osteoblastic tracer, still suffers from the flare phenomenon, however [46].

Fig. 5.

¹⁸F-Fluoride PET images of a woman with metastatic breast cancer before (left) and 8 weeks after endocrine therapy (right) showing a reduction in activity in keeping with response to treatment (from [46]).

¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) is a commonly used PET tracer in oncology that exploits the preferential metabolism of glucose in many cancers. As well as showing high uptake in nodal and visceral cancer, most cancers, with the exception of prostate cancer, show high uptake in skeletal metastases. ¹⁸F-FDG PET/CT has been shown as a sensitive method for detecting bone metastases compared to bone scintigraphy [47] and a better method to measure response as the signal relates to tumor cell viability rather than secondary osteoblastic activity and so the flare phenomenon is not seen (Fig. 6) [46,48].

Fig. 6.

¹⁸F-FDG PET images of the same patients as Fig. 4 showing a reduction in activity in bone metastases indicating response to treatment (from [46]).

Other SPECT and PET tracers that are tumor-specific rather than bone-specific may also be useful in detecting skeletal metastases, and like ¹⁸F-FDG PET, may have a higher sensitivity for detecting disease at the marrow stage before an osteoblastic response has occurred. Examples include ¹³¹I-iodine SPECT for thyroid cancer [49], ⁶⁸Gaprostate-specific membrane antigen for prostate cancer [50] and ⁶⁸Gadotatate for neuroendocrine tumor imaging [51].

Another tumor-specific imaging method that is showing increasing evidence of efficacy in detection of skeletal metastases and ability to determine treatment response quantitatively is whole-body DW-MRI (Fig. 7) [46,52]. This method uses standard morphological MRI sequences that can detect marrow-based disease with great sensitivity in conjunction with diffusion-weighted imaging that measures the restriction of water molecule motion in highly cellular tissue such as metastases and can be quantified by measurement of the apparent diffusion coefficient (ADC) for monitoring treatment response.

Fig. 7.

DW-MRI images of the same patients as Figs. 4 and 5 showing a decrease in signal in bone metastases in keeping with response to treatment (from [46]).

A novel aspect of radionuclide imaging with PET and SPECT is to target osteoclasts as these cells are implicated in the pathogenesis of many benign and malignant bone diseases. The integrin α_vβ₃ allows osteoclast adhesion to the bone matrix before resorption and is expressed in high concentrations on these cells [53]. PET and SPECT tracers that include the Arg-Gly-Asp (RGD) motif that binds α_vβ₃ integrin with high affinity have been developed primarily for imaging tumor-associated neoangiogenesis but have also been investigated as osteoclast biomarkers. Preclinical work has confirmed specific osteoclast-related uptake in bone metastasis models [54,55]. In man, uptake has been seen in prostate cancer skeletal metastases with change in activity predicting treatment response to systemic therapy (Fig. 8) [56].

Fig. 8.

^99mTc-maraciclatide SPECT (top row) and SPECT/CT (bottom row) images of a patient with prostate cancer showing α_vβ₃ expression at sites of metastatic disease.

5. Conclusions

Radiolabeled bisphosphonate imaging has been central to nuclear medicine practice for several decades. While there are now competing technologies for imaging the skeleton, including PET and DW-MRI, new indications and improvements in scanner hardware mean that there is still a place for these compounds in current clinical practice.