68Ga-FAPI-04 vs. 18F-FDG in a longitudinal preclinical PET imaging of metastatic breast cancer

Assessment of viable tumours by [68Ga]Ga-FAPI-04 PET/CT after local regional treatment in patients with hepatocellular carcinoma

PURPOSE

To investigate the efficacy of [68Ga]Ga-FAPI-04 PET/CT for assessing viable tumours (VTs) after local regional treatment (LRT) in hepatocellular carcinoma (HCC) patients. The related imaging features of HCC after LRT are preliminarily discussed.

METHODS

A cohort of 37 LRT patients with HCC (encompassing 51 lesions) was retrospectively included from a prospective parent study (ChiCTR2000039099), and sequential PET/CT using [18F]FDG and [68Ga]Ga-FAPI-04 was performed. The diagnostic accuracies of [68Ga]Ga-FAPI-04 and [18F]FDG PET/CT and multiphasic CT/MRI for detecting VTs after LRT were calculated and analysed. Pathological examination was considered the gold standard for VT diagnosis, and clinical follow-up was used as the reference standard. The SUVmax and tumour-to-background ratio (TBR) derived from [18F]FDG and [68Ga]Ga-FAPI-04 PET/CT were calculated and compared. Moreover, the SUVmax, target-to-normal liver ratio (TNR) of VT, tumour necrosis (TN), benign rim (BR), and normal liver (NL) from different imaging modalities after LRT for HCC were compared.

RESULTS

Both the sensitivity (96.0% [24/25] vs. 36.0% [9/25], p < 0.001) and accuracy (94.1%, [48/51] vs. 68.6% [35/51], p = 0.004) of [68Ga]Ga-FAPI-04 PET/CT for detecting VTs after LRT were greater than those of [18F]FDG PET/CT, whereas their specificities were comparable (92.3% [24/26] vs. 100% [26/26]). Notably, [68Ga]Ga-FAPI-04 PET/CT had a greater SUVmax (9.80 vs. 3.60) and TBR (9.93 vs. 1.57) than [18F]FDG PET/CT in VT patients (all p < 0.001). Furthermore, VT had a greater SUVmax and TNR than did TN, BR, and NL on [68Ga]Ga-FAPI-04 PET/CT (all p < 0.001) and exhibited morphologic nodular, mass-like, or irregular tracer uptake. Although no significant differences were observed for VT detection (all p > 0.05), [68Ga]Ga-FAPI-04 PET/CT and multiphasic CT/MRI complemented each other in some cases.

CONCLUSION

[68Ga]Ga-FAPI-04 PET/CT not only presented higher sensitivity and accuracy than [18F]FDG PET/CT for diagnosing VTs after LRT for HCC but also showed comparable diagnostic accuracy and complementary roles with multiphasic CT/MRI. Overall, [68Ga]Ga-FAPI-04 PET/CT may play an essential role in surveillance after LRT for HCC.

TRIAL REGISTRATION

Chinese Clinical Trial Registry ChiCTR2000039099. Registered 17 October 2020.

Diagnostic performance of [18F]FAPI-04 PET/CT in suspected recurrent hepatocellular carcinoma: prospective comparison with contrast-enhanced CT/MRI

PURPOSE

To evaluate the diagnostic performance of [18F]FAPI-04 PET/CT in differentiating hepatocellular carcinoma (HCC) recurrence from treatment-induced lesions (TILs) after therapy and compare it with contrast-enhanced CT/MRI (Ce-CT/MRI).

METHODS

Patients with suspected HCC recurrence after resection or local-regional therapy, who underwent Ce-CT/MRI and [18F]FAPI-04 PET/CT, were prospectively enrolled. For each patient or lesion, a three-point-scale (positive, negative, or equivocal) was assigned to Ce-CT/MRI and [18F]FAPI-04 PET/CT. The diagnostic performances of Ce-CT/MRI, [18F]FAPI-04 PET/CT, and their combination were compared by McNemar's test, with histopathology or radiographic follow-up as a reference standard. The maximum standardized uptake value (SUVmax) and lesion-to-background ratio (LBR) of lesions derived from [18F]FAPI-04 PET/CT were analyzed, and the cut-off points for distinguishing between HCC recurrence and TILs were calculated.

RESULTS

A total of 44 patients with 129 lesions were analyzed, of which 31 patients (91 lesions) were proved to be recurrent HCC and 38 lesions were TILs. On lesion-based analysis, the combination of [18F]FAPI-04 PET/CT with Ce-CT/MRI demonstrated superior sensitivity (86.8% vs. 70.3% vs. 72.5%), specificity (89.5% vs. 42.1% vs. 52.6%), and accuracy (87.6% vs. 62.0% vs. 66.7%) compared to Ce-CT/MRI and [18F]FAPI-04 PET/CT alone (all P < 0.001). The combination altered therapy management in 22.7% of patients. On semiquantitative analysis, the SUVmax and LBR of [18F]FAPI-04 PET in HCC recurrence (n = 91) were significantly higher than those in TILs (n = 38) (SUVmax: 8.1 vs. 3.65, P < 0.001; LBR: 5.4 vs. 1.9, P < 0.001). Using cutoff points of 5.1 for SUVmax and 2.3 for LBR, [18F]FAPI-04 PET/CT exhibited high sensitivity and specificity in differentiating recurrent HCC from TILs, with 83.5%, 78.9% and 96.2%, 63.2%, respectively.

CONCLUSIONS

The combination of [18F]FAPI-04 PET/CT with Ce-CT/MRI significantly improved diagnostic sensitivity, specificity, and accuracy in differentiating recurrent HCC from TILs. [18F]FAPI-04 PET may be a promising imaging modality in detecting recurrent HCC.

TRIAL REGISTRATION

clinicaltrials.gov: NCT05485792.

Exploring currently available fibroblast activation protein targeting molecules in adrenocortical carcinoma: Navigating theranostic pathways

INTRODUCTION

Cancer-associated fibroblasts (CAFs) expressing fibroblast activation protein (FAP) in the adrenocortical carcinoma (ACC) microenvironment may be used as potential therapeutic targets. This study investigated the diagnostic potential of four FAPi derivatives i.e. DOTA-FAPi-46 (FAPi46), DOTA.SA.FAPi (SA.FAPi), DATA5m.SA.FAPi (DATA.FAPi) and DATA5m.C4.FAPi (C4.FAPi) and compared with standard-of-care 18F-FDG (FDG) in ACC.

METHODS

Thirty histopathological proven cases of localized or metastatic ACC were recruited for both FDG and FAPi PET (number of patients (n) = 5 for SA.FAPi, n = 5 for DATA.FAPi, n = 5 for C4.FAPi and n = 15 for FAPi46). For biodistribution, standardized uptake values (SUV's) were computed by delineating region-of-interest on various body organs. For comparative analysis in disease identification, lesion tracer uptake was quantified using standardized uptake values corrected for lean body mass (SUL), tumor-to-background ratio (TBR), total lesion glycolysis (TLG for FDG) and total lesion FAP expression (TLF for FAPi).

RESULTS

In overall analysis, both FAPi and FDG PET exhibited comparable mean SULpeak [FAPi 4.3 (8.0-1.7) vs FDG 3.9 (8.1-2.5), p-0.271], mean SULavg [2.2 (4.3-1.2) vs 2.2 (3.4-1.3), p-0.897] and mean TBR [1.8 (3.2-1.2) vs 1.9 (2.7-1.2), p-0.696]. In volumetric analysis, comparable mean TLF and mean TLG was noted for the cohort [9.3 (53.7-4.5) vs 11.8 (33.0-4.3), p-0.107]. Sub-categorical analysis demonstrated complete concordant findings for both radiotracers in detection of all primary lesions, nodal lesions and distant metastases in lung and peritoneum with discordant findings in liver (22%) and skeletal lesions (33%). For lesion detection, DATA.FAPi and FAPi46 showed 100% concordance with FDG scan findings in metastatic disease. SA.FAPi exhibited 33% discordance by detecting an additional skeletal lesion, while C4.FAPi had 10% discordance, missing one liver lesion identified by FDG. Three 68 Ga-FAP derivatives (SA.FAPi, DATA.FAPi, and C4.FAPi) exhibited similar biodistribution, with uptake in the salivary glands, thyroid, liver, pancreas, muscles, and kidneys, and variable uptake in the lacrimal glands, extra-ocular muscles, oral mucosa, and uterus. In contrast, FAPi46 physiological expression was noted in salivary glands and muscles, with no uptake in other organs. Pancreatic uptake was highest for SA.FAPi (SUVmean 11.8), DATA.FAPi (12.1), and C4.FAPi (10.8), while FAPi46 had the lowest (1.7). Conversely, FAPi46 exhibited the highest muscle uptake (SUVmean 4.3) compared to SA.FAPi (1.7), DATA.FAPi (1.4), and C4.FAPi (1.0).

CONCLUSION

All the existing FAP inhibitor molecules were comparable to FDG PET for mapping disease spread and appeared as potential theranostic targets for the management of ACC.

Preclinical evaluation of 64Cu-labeled cetuximab in immuno-PET for detecting sentinel lymph node metastasis in epidermal growth factor receptor-positive breast cancer

BACKGROUND

Despite advances in breast cancer imaging, reliable detection of sentinel lymph node (SLN) metastasis remains challenging. This study aimed to determine the ability of immuno-positron emission tomography (PET) using 64Cu-labeled cetuximab to detect SLN metastasis in a model of epidermal growth factor receptor (EGFR)-positive breast cancer.

METHODS

The SLN metastasis model was established using the EGFR-strongly-expressing MDA-MB-468 breast cancer cell line. In this xenograft model, [64Cu]Cu-PCTA-cetuximab was administered intravenously (5.8 ± 0.9 MBq; n = 12) or both intradermally and subdermally into the parapapillary region of the tumor-containing mammary gland (4.3 ± 0.4 MBq; n = 11), after which PET was performed. 18F-FDG PET was also performed intravenously (9.1 ± 1.4 MBq; n = 4) or intradermally/subdermally (5.4 ± 2.2 MBq; n = 3) in the same cohort before [64Cu]Cu-PCTA-cetuximab PET. PET/computed tomography was performed 60 min after administration of 18F-FDG and 24 h after administration of [64Cu]Cu-PCTA-cetuximab. Delayed PET/CT scans were conducted 48 h after administration for all mice in the intradermally/subdermally administered [64Cu]Cu-PCTA-cetuximab group and for four of the 12 mice in the intravenously administered [64Cu]Cu-PCTA-cetuximab group. SLNs were identified using blue dye, and PET and pathological evaluations of the resected SLN were performed to confirm metastases.

RESULTS

After intravenous administration of [64Cu]Cu-PCTA-cetuximab (n = 12), accumulation was detected in the primary tumor in all mice and in the axilla of eight mice (67%, SUVmax 1.24 ± 0.51), all of which were found to have SLNs with histologically confirmed metastasis. The sensitivity, specificity, accuracy, and negative and positive predictive values for PET with intravenously administered [64Cu]Cu-PCTA-cetuximab were 89%, 100%, 92%, 75%, and 100%, respectively. In contrast, all mice with intradermal/subdermal administration (n = 11) showed high accumulation in both the primary tumor and axillary lymph nodes (SUVmax 4.28 ± 1.19), with six mice (55%, SUVmax 5.01 ± 1.12) having histologically confirmed metastasis. The sensitivity, specificity, accuracy, and positive predictive values for PET with intradermally/subdermally administered [64Cu]Cu-PCTA-cetuximab were 100%, 0%, 55% and 55%, respectively. SLN metastasis was not detectable by intravenous or intradermal/subdermal 18F-FDG PET.

CONCLUSIONS

PET with intravenously administered [64Cu]Cu-PCTA-cetuximab demonstrated high precision for diagnosis of SLN metastasis in a xenograft model of EGFR-positive human breast cancer. Although further evaluation is necessary, intradermal/subdermal administration could be a useful therapeutic approach owing to its high accumulation in SLNs.

Current Advances in PARP1-Targeted Theranostics

Poly (ADP-ribose) polymerase 1 (PARP1) plays critical roles in DNA repair, chromatin regulation, and cellular equilibrium, positioning it as a pivotal target for therapeutic interventions in cancer and central nervous system (CNS) disorders. PARP1 responds to oxidative stress and DNA damage through PARylation, influencing energy depletion, survival, inflammation, and genomic regulation in many biological scenarios. PARP inhibitors (PARPis) have demonstrated efficacy against cancers harboring defective homologous recombination repair pathways, notably those linked to BRCA mutations. PARP1-targeted PET imaging enables patient stratification, treatment assessment, and PARPi pharmacodynamic evaluation in cancers and other pathophysiological conditions. Importantly, PARP1-targeted theranostics have emerged for both diagnostic imaging and therapeutic applications in multiple types of cancers, representing a pivotal advancement in personalized oncology. However, its application in brain tumors is limited by the heterogeneous integrity of the blood brain barrier (BBB) and the blood-tumor barrier. Thus, the development of BBB-penetrant PARP1 tracers remains an unmet need for imaging brain cancers. This review summarizes the current landscape of radiopharmaceuticals and radioligands targeting PARP1, detailing their pharmacological characteristics and potential clinical uses. Furthermore, this review discusses PARP1 tracers that can cross the BBB, underscoring their potential applications in neurooncology and other neurological disorders.

Current status of FAP-directed cancer theranostics: a bibliometric analysis

Fibroblast activation protein (FAP) is a key molecule in the field of oncology, with significant impacts on tumor diagnosis and treatment. Importantly, it has paved the way for the development of radiotracers for quinoline-based FAP inhibitors (FAPIs), which are currently among the most promising radiotracers for PET imaging in cancer. We performed a bibliometric analysis of scientific publications related to FAP and FAPI-based radiotracers, which included the quantification and visualization of current research trends and prospects based on various bibliometric indicators. In our survey of FAP-related studies in the Web of Science Core Collection databases, R and VOSviewer were used for visualization and bibliometric analyses based on country, institute, author, journal, and keywords. We also examined the methodology, radionuclide type, imaging instruments, and major diseases associated with studies on FAPI-based radiotracers. The results revealed 2,664 FAP-related publications from 1992 to the present. Germany, the USA, and China dominated paper publications, multinational collaborations, and societal impacts on FAP research. Southwest Medical University was the most productive institute, while Haberkorn Uwe authored the most cited papers and the highest H-index. The European Journal of Nuclear Medicine and Molecular Imaging and the Journal of Nuclear Medicine were the most influential periodicals. Keywords "FAP", "68Ga-FAPI", and "PET/CT" emerged as the most significant in this field. This study may help elucidate current research trends, hotspots, and directions for future research.

Addressing Biological Questions with Preclinical Cancer Imaging

The broad application of noninvasive imaging has transformed preclinical cancer research, providing a powerful means to measure dynamic processes in living animals. While imaging technologies are routinely used to monitor tumor growth in model systems, their greatest potential lies in their ability to answer fundamental biological questions. Here we present the broad range of potential imaging applications according to the needs of a cancer biologist with a focus on some of the common biological processes that can be used to visualize and measure. Topics include imaging metastasis; biophysical properties such as perfusion, diffusion, oxygenation, and stiffness; imaging the immune system and tumor microenvironment; and imaging tumor metabolism. We also discuss the general ability of each approach and the level of training needed to both acquire and analyze images. The overall goal is to provide a practical guide for cancer biologists interested in answering biological questions with preclinical imaging technologies.

[68Ga]Ga-FAPI PET/CT in brain tumors: comparison with [18F]F-FDG PET/CT

Purpose

To investigate the feasibility of [68Ga]Ga-FAPI PET/CT in brain tumor imaging and to compare it with [18F]F-FDG PET/CT.

Methods

25 patients with MRI-suspected brain tumors were included in the study. They underwent whole body [18F]F-FDG PET/CT and [68Ga]Ga-FAPI PET/CT and brain scans. The target-to-background ratio (TBR) of brain tumors was calculated with the background of surrounding normal brain tissues uptake. The SUVmax and TBR of [18F]F-FDG PET/CT and [68Ga]Ga-FAPI PET/CT were compared. Additionally, the correlation between the uptake of the tracer by lesions with the greatest diameter of the lesion, the breadth of the oedema band, and the enhancement scores of the MRI enhancement scans was analyzed.

Result

[68Ga]Ga-FAPI PET/CT was superior to [18F]F-FDG PET/CT for lesion detection, especially for brain metastases. Among gliomas, only high-grade gliomas uptake [68Ga]Ga-FAPI. Compared with [18F]F-FDG PET/CT, [68Ga]Ga-FAPI PET/CT had a lower SUVmax but a significantly better TBR. On [68Ga]Ga-FAPI PET/CT, the TBR may be associated with brain tumor blood-brain barrier disruption.

Conclusions

[68Ga]Ga-FAPI PET/CT is a promising imaging tool for the assessment of brain tumors. Lack of physiological uptake of [68Ga]Ga-FAPI in normal brain parenchyma results in high TBR values, leading to better visualization of lesions and contributing to subsequent targeted therapy studies.

Advances in knowledge

Clinical utility of [68Ga]Ga-FAPI PET/CT in brain tumors remains unclear, and there aren't many similar studies in the literature. We evaluated the role of [68Ga]Ga-FAPI PET/CT in diagnosing brain tumors.

Applications of FAPI PET/CT in the diagnosis and treatment of breast and the most common gynecologic malignancies: a literature review

The fibroblast activating protein (FAP) is expressed by some fibroblasts found in healthy tissues. However, FAP is overexpressed in more than 90% of epithelial tumors, including breast and gynecological tumors. As a result, the FAP ligand could be used as a target for diagnosis and treatment purposes. Positron emission tomography/computed tomography (PET/CT) is a hybrid imaging technique commonly used to locate and assess the tumor's molecular and metabolic functions. PET imaging involves the injection of a radiotracer that tends to accumulate more in metabolically active lesions such as cancer. Several radiotracers have been developed to target FAP in PET/CT imaging, such as the fibroblast-activation protein inhibitor (FAPI). These tracers bind to FAP with high specificity and affinity, allowing for the non-invasive detection and quantification of FAP expression in tumors. In this review, we discussed the applications of FAPI PET/CT in the diagnosis and treatment of breast and the most common gynecologic malignancies. Radiolabeled FAPI can improve the detection, staging, and assessment of treatment response in breast and the most common gynecologic malignancies, but the problem with normal hormone-responsive organs remains insurmountable. Compared to the diagnostic applications of FAPI, further research is needed for future therapeutic applications.

Synthesis and Preliminary Study of 99mTc-Labeled HYNIC-FAPi for Imaging of Fibroblast Activation Proteins in Tumors

Fibroblast activation protein (FAP) is an emerging target for cancer diagnosis. Different types of FAP inhibitor (FAPI)-based radiotracers have been developed and applied for tumor imaging. However, few FAPI tracers for single photon emission computed tomography (SPECT) imaging have been reported. SPECT imaging is less expensive and more widely distributed than positron emission tomography (PET), and thus, 99mTc-labeled FAPIs would be more available to patients in developing regions. Herein, we developed a FAPI-04-derived radiotracer, HYNIC-FAPi-04 (HFAPi), for SPECT imaging. 99mTc-HFAPi, with a radiochemical purity of >98%, was prepared using a kit formula within 30 min. The specificity of 99mTc-HFAPi for FAP was validated by a cell binding assay in vitro and SPECT/CT imaging in vivo. The binding affinity (Kd value) of 99mTc-HFAPi for human FAP and murine FAP was 4.49 and 2.07 nmol/L, respectively. SPECT/CT imaging in HT1080-hFAP tumor-bearing mice showed the specific FAP targeting ability of 99mTc-HFAPi in vivo. In U87MG tumor-bearing mice, 99mTc-HFAPi had a higher tumor uptake compared with that of HT1080-hFAP and 4T1-mFAP tumor models. Interestingly, 99mTc-HFAPi showed a relatively high uptake in some murine joints. 99mTc-HFAPi accumulated in tumor lesions with a high tumor-to-background ratio. A preliminary clinical study was also performed in breast cancer patients. Additionally, 99mTc-HFAPi exhibited an advantage over 18F-FDG in the detection of lymph node metastatic lesions in breast cancer patients, which is helpful in improving treatment strategies. In short, 99mTc-HFAPi showed excellent affinity and specificity for FAP and is a promising SPECT radiotracer for (re)staging and treatment planning of breast cancers.

Imaging Molecular Targets and Metabolic Pathways in Breast Cancer for Improved Clinical Management: Current Practice and Future Perspectives

Breast cancer is the most frequently diagnosed cancer and leading cause of cancer-related deaths worldwide. Timely decision-making that enables implementation of the most appropriate therapy or therapies is essential for achieving the best clinical outcomes in breast cancer. While clinicopathologic characteristics and immunohistochemistry have traditionally been used in decision-making, these clinical and laboratory parameters may be difficult to ascertain or be equivocal due to tumor heterogeneity. Tumor heterogeneity is described as a phenomenon characterized by spatial or temporal phenotypic variations in tumor characteristics. Spatial variations occur within tumor lesions or between lesions at a single time point while temporal variations are seen as tumor lesions evolve with time. Due to limitations associated with immunohistochemistry (which requires invasive biopsies), whole-body molecular imaging tools such as standard-of-care [18F]FDG and [18F]FES PET/CT are indispensable in addressing this conundrum. Despite their proven utility, these standard-of-care imaging methods are often unable to image a myriad of other molecular pathways associated with breast cancer. This has stimulated interest in the development of novel radiopharmaceuticals targeting other molecular pathways and processes. In this review, we discuss validated and potential roles of these standard-of-care and novel molecular approaches. These approaches' relationships with patient clinicopathologic and immunohistochemical characteristics as well as their influence on patient management will be discussed in greater detail. This paper will also introduce and discuss the potential utility of novel PARP inhibitor-based radiopharmaceuticals as non-invasive biomarkers of PARP expression/upregulation.

Physiological Uptake Characteristics of Breast on 68Ga-FAPI-04 PET/CT

PURPOSE

Gallium 68 (68Ga)-labeled fibroblast activation protein inhibitor (FAPI) PET/CT is sensitive on breast cancer staging, but its clinical utility may be limited by the high physiological 68Ga-FAPI-04 uptake in normal breast tissue that can obscure primary tumors. The aim of this study was to elucidate the characteristics of physiological 68Ga-FAPI-04 uptake in normal breast.

PROCEDURES

A total of 143 consecutive women with 68Ga-FAPI-04 PET/CT data were reviewed retrospectively. SUVmax, density and thickness of breast, as well as SUVmax of nipple, were measured. Univariate and multivariate regression analyses were used to identify factors related to the breast and nipple SUVs.

RESULTS

Twenty-eight premenopausal, 62 menopausal and 10 post-operative (after bilateral adnexectomy) women with 103 examinations were included. All had a diffuse, symmetrical uptake in breast. There was no difference in 68Ga-FAPI-04 uptake between bilateral breasts (right: median, 1.14[IQR, 0.85-1.54] vs. left: median, 1.09[IQR, 0.86-1.54]; P = 0.253). Patients in menstrual status with expected high estrogen level (late follicular, ovulatory and mid luteal phases) had higher breast SUVs (median SUV, 3.91 [IQR, 2.85-4.35]) than those with expected moderate (early follicular, early luteal and late luteal phases; median SUV, 1.57 [IQR, 1.39-2.08]; P < 0.001) or low level (menopause and post-operation; median SUV, 0.98 [IQR, 0.83-1.21]; P < 0.001). Menstrual status was an independent predictors of breast SUV (r2 = 0.689, P < 0.001). All the patients had a focal, symmetrical uptake in nipples. Nipple SUV did not correlate with menstrual status (P = 0.913).

FAPI-Negative FDG-Avid Metastatic Relapse of Breast Cancer on PET/CT

ABSTRACT

A 69-year-old woman with breast cancer underwent 18F-FDG PET/CT because of the increased CA-15-3 level. 18F-FDG PET/CT showed multiple hypermetabolic lymph nodes (LNs) in the neck and mediastinum. The patient was also referred for 68Ga-fibroblast activation protein inhibitor (FAPI) 04 PET/CT for further evaluation. However, 18F-FDG-avid LNs were FAPI-negative on 68Ga-FAPI-04 PET/CT. Supraclavicular LN biopsy confirmed the metastasis of breast cancer. Recent reports have focused on the potential of FAPI PET imaging in breast cancer; however, this case highlights that false-negative 68Ga-FAPI-04-PET/CT findings should also be considered while evaluating metastatic spread.

[68Ga]Ga-HBED-CC-FAPI Derivatives with Improved Radiolabeling and Specific Tumor Uptake

Fibroblast activation protein (FAP) is selectively expressed in tumors and highly important for maintaining the microenvironment in malignant tumors. Radioisotope-labeled FAP inhibitors (FAPIs) were proven to be useful for diagnosis and radionuclide therapy of cancer and are under active clinical investigations. Ga-HBED complex displays a higher in vivo stability constant (log K_GaL: 38.5), compared to that of Ga-DOTA (log K_GaL: 21.3). Such advantage in stability constant suggests that it may be useful for development of alternative FAPI imaging agents. In this study, previously reported [⁶⁸Ga]Ga-DOTA-FAPI-02 and -04 were converted to the corresponding [⁶⁸Ga]Ga-HBED-CC-FAPI-02 and -04 derivatives ([⁶⁸Ga]Ga-4, [⁶⁸Ga]Ga-5, [⁶⁸Ga]Ga-6, and [⁶⁸Ga]Ga-7). It was found that substituting the DOTA chelating group with HBED-CC led to several unique and desirable tumor-targeting properties: (1) robust, fast, and high yield labeling─readily adaptable to a kit formulation; (2) high stabilities in vitro; (3) excellent FAP binding affinities (IC₅₀ ranging between 4 and 7 nM) and improved cell uptake and retention (in HT1080 (FAP+) cells); and (4) excellent selective in vivo tumor uptake in nude mice bearing U87MG tumor. It appeared that Ga(III) chelation with HBED-CC improved the in vivo kinetics favoring higher tumor uptake and retention compared to the corresponding Ga-DOTA complex. Out of the four tested ligands the new [⁶⁸Ga]Ga-HBED-CC-FAPI dimer, [⁶⁸Ga]Ga-6, displayed the best tumor localization properties, and further studies are warranted to demonstrate that it is an alternative FAP imaging agent for cancer patients.

[68Ga]Ga-FAPI PET for the evaluation of digestive system tumors: systematic review and meta-analysis

PURPOSE

Digestive system tumors are a group of tumors with high incidence in the world nowadays. The assessment of digestive system tumor metastasis by conventional imaging seems to be unsatisfactory. [⁶⁸Ga]Ga-FAPI, which has emerged in recent years, seems to be able to evaluate digestive system tumor metastasis. We aimed to summarize the current evidence of [⁶⁸Ga]Ga-FAPI PET/CT or PET/MR for the assessment of primary tumors, lymph node metastases, and distant metastases in digestive system tumors. Besides, we also aimed to perform a meta-analysis of the sensitivity of [⁶⁸Ga]Ga-FAPI PET diagnosis to discriminate between digestive system tumors, primary lesions, and non-primary lesions (lymph node metastases and distant metastases).

MATERIALS AND METHODS

PubMed, MEDLINE and Cochrane Library databases were searched from the beginning of the database build to August 12, 2022. All studies undergoing [⁶⁸Ga]Ga-FAPI PET for the evaluation of digestive tumors were included in the screening and review. Screening and full text review was performed by 3 investigators and data extraction was performed by 2 investigators. Risk of bias was examined with the QUADAS-2 criteria. Diagnostic test meta-analysis was performed with a random-effects model.

RESULTS

A total of 541 studies were retrieved. Finally, 22 studies were selected for the systematic review and 18 studies were included in the meta-analysis. In the 18 publications, a total of 524 patients with digestive system tumors, 459 primary tumor lesions of digestive system tumors, and 1921 metastatic lesions of digestive system tumors were included in the meta-analysis. Based on patients, the sensitivity of [⁶⁸Ga]Ga-FAPI PET for the diagnosis of digestive system tumors was 0.98 (95% CI: 0.94-0.99). Based on lesions, the sensitivity of [⁶⁸Ga]Ga-FAPI PET for the diagnostic evaluation of primary tumor lesions of the digestive system was 0.97 (95% CI: 0.93-0.99); the sensitivity of [⁶⁸Ga]Ga-FAPI PET for the diagnostic evaluation of non-primary lesions (lymph node metastases and distant metastases) of the digestive system tumors was 0.94 (95% CI: 0.79-0.99).

CONCLUSION

[⁶⁸Ga]Ga-FAPI PET has high accuracy and its sensitivity is at a high level for the diagnostic evaluation of digestive system tumors. Clinicians, nuclear medicine physicians, and radiologists may consider using [⁶⁸Ga]Ga-FAPI PET/CT or PET/MR in the evaluation of primary tumors, lymph node metastases, and distant metastases in digestive system tumors.

Advances in PET/CT Imaging for Breast Cancer Patients and Beyond

Breast cancer is the most common cancer in women around the world and the fifth leading cause of cancer-related death [...].

Imaging of Bone Metastases in Breast Cancer

Bone metastases are a common site of spread in advanced breast cancer and responsible for morbidity and high health care costs. Imaging contributes to staging and response assessment of the skeleton and has been instrumental in guiding patient management for several decades. Historically this has been with radiographs, computed tomography and bone scans. More recently, molecular and hybrid imaging methods have undergone significant development, including the addition of single photon emission computed tomography/computed tomography to the bone scan, positron emission tomography, with bone-specific and tumor-specific tracers, and magnetic resonance imaging with complementary functional diffusion-weighted imaging. These have allowed different aspects of the abnormal biology associated with bone metastases to be explored. There is ability to interrogate the bone microenvironment with bone-specific tracers and cancer cell characteristics with tumor-specific methods that complement morphological appearances on computed tomography or magnetic resonance imaging. Alongside the advent of novel, more effective and nuanced therapies for bone metastases in breast cancer, there is accumulating evidence that the developments in imaging allow more sensitive and specific detection of bone metastases as well as more accurate and earlier assessment of treatment response leading to improvements in patient management.

64CuCl2 PET Imaging of 4T1-Related Allograft of Triple-Negative Breast Cancer in Mice

64CuCl2 is an economic radiotracer for oncologic PET investigations. In the present study, we characterized the uptake of 64CuCl2 in vivo by µPET/CT in an allograft 4T1-related mouse model (BALB/c) of advanced breast cancer. 18F-FDG was used as a comparator. Twenty-two animals were imaged 7-9 days following 4T1-cell implantation inside mammary glands. Dynamic 64CuCl2 µPET/CT acquisition or iterative static images up to 8 h p.i. were performed. Animal biodistribution and tumor uptake were first evaluated in vivo by µPET analysis and then assessed on tissue specimens. Concerning 18F-FDG µPET, a static acquisition was performed at 15 min and 60 min p.i. Tumor 64CuCl2 accumulation increased from 5 min to 4 h p.i., reaching a maximum value of 5.0 ± 0.20 %ID/g. Liver, brain, and muscle 64CuCl2 accumulation was stable over time. The tumor-to-muscle ratio remained stable from 1 to 8 h p.i., ranging from 3.0 to 3.7. Ex vivo data were consistent with in vivo estimations. The 18F-FDG tumor accumulation was 8.82 ± 1.03 %ID/g, and the tumor-to-muscle ratio was 4.54 ± 1.11. 64CuCl2 PET/CT provides good characterization of the 4T1-related breast cancer model and allows for exploration of non-glycolytic cellular pathways potentially of interest for theragnostic strategies.

Head-to-head evaluation of [18F]FDG and [68 Ga]Ga-DOTA-FAPI-04 PET/CT in recurrent soft tissue sarcoma

Purpose

We aimed to evaluate the value of [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT for the diagnosis of recurrent soft tissue sarcoma (STS), compared with [¹⁸F]FDG PET/CT.

Methods

A total of 45 patients (21 females and 24 males; median age, 46 years; range, 18–71 years) with 13 subtypes of STS underwent [¹⁸F]FDG and [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT examination within 1 week for assessment local relapse or distant metastasis. Positive lesions on PET/CT images were verified by biopsy or 3-month follow-up. Wilcoxon matched-pairs signed-rank test was used to compare the semiquantitative values (SUV_max and TBR) of [¹⁸F]FDG and [⁶⁸ Ga]Ga-DOTA-FAPI-04 in tumor lesions, and McNemar test was applied to test for differences of both tracers.

Results

Among the 45 patients, 282 local relapses and distant metastases were identified. Compared to [¹⁸F]FDG, [⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT detected more lesions (275 vs. 186) and outperformed in sensitivity, specificity, PPV, NPV, and accuracy for the diagnosis of recurrent lesions (P < 0.001). [⁶⁸ Ga]Ga-DOTA-FAPI-04 demonstrated significantly higher values of SUV_max and TBR than [¹⁸F]FDG PET/CT in liposarcoma (P = 0.011 and P < 0.001, respectively), malignant solitary fibrous tumor (MSFT) (P < 0.001 and P < 0.001, respectively), and interdigitating dendritic cell sarcoma (IDCS) (P < 0.001and P < 0.001, respectively). While mean SUV_max and TBR presented favorable uptake of [¹⁸F]FDG over [⁶⁸ Ga]Ga-DOTA-FAPI-04 in undifferentiated pleomorphic sarcoma (UPS) (P = 0.003 and P < 0.001, respectively) and rhabdomyosarcoma (RMS) (P < 0.001 and P < 0.001, respectively).

Conclusion

[⁶⁸ Ga]Ga-DOTA-FAPI-04 PET/CT is a promising new imaging modality for recurrent surveillance of STS, and compares favorably with [¹⁸F]FDG for identifying recurrent lesions of liposarcoma, MSFT, and IDCS.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00259-022-05700-4.

Radioligands Targeting Fibroblast Activation Protein (FAP)

Simple Summary

FAP-targeted radiotracers, recently introduced in cancer treatment, accumulate in Cancer-Associated Fibroblasts (CAFs). CAFs are present in tumor lesions but do not correspond to genuine cancer cells, although they behave in an abnormal and disease-promoting manner. One of their characteristic features, the expression of the surface protein FAP, can be utilized to discriminate between cancerous and healthy tissues. By the choice of an appropriate radionuclide, FAP-targeted tracers can be used for imaging or therapy in many cancer types. Therefore, the first successful application of FAP-targeted imaging has led to an enormous and growing interest in nuclear medicine and radiopharmacy.

Abstract

Targeting fibroblast activation protein (FAP) in cancer-associated fibroblasts (CAFs) has attracted significant attention in nuclear medicine. Since these cells are present in most cancerous tissues and FAP is rarely expressed in healthy tissues, anti-FAP tracers have a potential as pan-tumor agents. Compared to the standard tumor tracer [¹⁸F]FDG, these tracers show better tumor-to-background ratios (TBR) in many indications. Unlike [¹⁸F]FDG, FAP-targeted tracers do not require exhausting preparations, such as dietary restrictions on the part of the patient, and offer the possibility of radioligand therapy (RLT) in a theragnostic approach. Although a radiolabeled antibody was clinically investigated as early as the 1990s, the breakthrough event for FAP-targeting in nuclear medicine was the introduction and clinical application of the so-called FAPI-tracers in 2018. From then, the development and application of FAP-targeted tracers became hot topics for the radiopharmaceutical and nuclear medicine community, and attracted the interest of pharmaceutical companies. The aim of this review is to provide a comprehensive overview of the development of FAP-targeted radiopharmaceuticals and their application in nuclear medicine.