68Ga-FAPI-46 PET/CT in a Metastatic Castration-Resistant Prostate Cancer Patient With Low PSMA Expression

Detection of tumour heterogeneity in patients with advanced, metastatic castration-resistant prostate cancer on [68Ga]Ga-/[18F]F-PSMA-11/-1007, [68Ga]Ga-FAPI-46 and 2-[18F]FDG PET/CT: a pilot study

PURPOSE

In metastatic castration-resistant prostate cancer (mCRPC), some patients show low/absent PSMA expression in tumour lesions on positron emission tomography (PET) scans, indicating heterogeneity and heightened risk of non-response to PSMA-RLT (radioligand therapy). Imaging cancer-associated fibroblasts and glucose uptake may further characterise tumour heterogeneity in mCRPC patients. Here, we aimed to evaluate tumour heterogeneity and its potential implications for management in mCRPC patients assessed for PSMA-RLT using [68Ga]Ga-FAPI-46, 2-[18F]FDG and [68Ga]Ga-/[18F]F-PSMA-11/-1007 PET.

MATERIAL AND METHODS

Patients with advanced, progressive mCRPC underwent clinical [68Ga]Ga-/[18F]F-PSMA-11/-1007, 2-[18F]FDG and [68Ga]Ga-FAPI-46 PET/CT to evaluate treatment with PSMA-directed RLT. Tumour detection/semiquantitative parameters were compared on a per-lesion/-region basis. Two phenotypes were defined: Criteria for the mixed phenotype were: (a) PSMA-negative findings for lymph node metastases ≥ 2.5 cm, any solid organ metastases ≥ 1.0 cm, or bone metastases with soft tissue component ≥ 1.0 cm, (b) low [68Ga]Ga-/[18F]F-PSMA-11/-1007 uptake and/or (c) balanced tumour uptake of all radioligands. The PSMA-dominant phenotype was assigned if the criteria were not met.

RESULTS

In ten patients, 472 lesions were detected on all imaging modalities (miTNM regions: M1b: 327 (69.3%), M1a: 95 (20.1%), N1: 26 (5.5%), M1c: 18 (3.8%), T: 5 (1.1%) and Tr: 1 (0.2%). [68Ga]Ga-/[18F]F-PSMA-11/-1007 (n = 453 (96.0%)) demonstrates the highest detection rate, followed by [68Ga]Ga-FAPI-46 (n = 268 (56.8%))/2-[18F]FDG (n = 241 (51.1%)). Semiquantitative uptake was highest for [68Ga]Ga-/[18F]F-PSMA-11/-1007 (mean SUVmax (interquartile range): 22.7 (22.5), vs. [68Ga]Ga-FAPI-46 (7.7 (3.7)) and 2-[18F]FDG (6.8 (4.7)). Seven/three patients were retrospectively assigned to the PSMA-dominant/mixed phenotype. Median overall survival was significantly longer for patients who underwent [177Lu]Lu-PSMA-617 RLT and were retrospectively assigned to the PSMA-dominant phenotype (19.7 vs. 9.3 months).

CONCLUSION

Through whole-body imaging, we identify considerable inter- and intra-patient heterogeneity of mCRPC and potential imaging phenotypes. Regarding uptake and tumour detection, [68Ga]Ga-/[18F]F-PSMA-11/-1007 was superior to [68Ga]Ga-FAPI-46 and 2-[18F]FDG, while the latter two were comparable. Patients who underwent [177Lu]Lu-PSMA-617 RLT based on clinical-decision making had a longer overall survival and could be assigned to the PSMA-dominant phenotype.

A Systematic Review on the Diagnostic Value of Fibroblast Activation Protein Inhibitor PET/CT in Genitourinary Cancers

In contemporary oncologic diagnostics, molecular imaging modalities are pivotal for precise local and metastatic staging. Recent studies identified fibroblast activation protein as a promising target for molecular imaging across various malignancies. Therefore, we aimed to systematically evaluate the current literature on the utility of fibroblast activation protein inhibitor (FAPI) PET/CT for staging patients with genitourinary malignancies. Methods: A systematic Embase and Medline search was conducted, according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) process, on August 1, 2023. Relevant publications reporting on the diagnostic value of FAPI PET/CT in genitourinary malignancies were identified and included. Studies were critically reviewed using a modified version of a tool for quality appraisal of case reports. Study results were summarized using a narrative approach. Results: We included 22 retrospective studies with a cumulative total of 69 patients, focusing on prostate cancer, urothelial carcinoma of the bladder and of the upper urinary tract, renal cell carcinoma, and testicular cancer. FAPI PET/CT was able to visualize both local and metastatic disease, including challenging cases such as prostate-specific membrane antigen (PSMA)-negative prostate cancer. Compared with radiolabeled 18F-FDG and PSMA PET/CT, FAPI PET/CT showed heterogeneous performance. In selected cases, FAPI PET/CT demonstrated superior tumor visualization (i.e., better tumor-to-background ratios and visualization of small tumors or metastatic deposits visible in no other way) over 18F-FDG PET/CT in detecting local or metastatic disease, whereas comparisons with PSMA PET/CT showed both superior and inferior performances. Challenges in FAPI PET/CT arise from physiologic urinary excretion of most FAPI radiotracers, hindering primary-lesion visualization in the bladder and upper urinary tract, despite generally providing high tumor-to-background ratios. Conclusion: The current findings suggest that FAPI PET/CT may hold promise as a future tool to aid clinicians in detecting genitourinary malignancies. Given the substantial heterogeneity among the included studies and the limited number of patients, caution in interpreting these findings is warranted. Subsequent prospective and comparative investigations are anticipated to delve more deeply into this innovative imaging modality and elucidate its role in clinical practice.

Diagnostic Accuracy of 68Ga-FAPI Versus 18F-FDG PET in Patients with Various Malignancies

To assess the diagnostic accuracy of 68Ga-labeled fibroblast activation protein inhibitor (FAPI) and 18F-labeled FDG PET for the detection of various tumors, we performed a head-to-head comparison of both imaging modalities across a range of tumor entities as part of our ongoing 68Ga-FAPI PET observational trial. Methods: The study included 115 patients with 8 tumor entities who received imaging with 68Ga-FAPI for tumor staging or restaging between October 2018 and March 2022. Of those, 103 patients received concomitant imaging with 68Ga-FAPI and 18F-FDG PET and had adequate lesion validation for accuracy analysis. Each scan was evaluated for the detection of primary tumor, lymph nodes, and visceral and bone metastases. True or false positivity and negativity to detected lesions was assigned on the basis of histopathology from biopsies or surgical excision, as well as imaging validation. Results: 68Ga-FAPI PET revealed higher accuracy than 18F-FDG PET in the detection of colorectal cancer (n = 14; per-patient, 85.7% vs. 78.6%; per-region, 95.6% vs. 91.1%) and prostate cancer (n = 22; per-patient, 100% vs. 90.9%; per-region, 96.4% vs. 92.7%). 68Ga-FAPI PET and 18F-FDG PET had comparable per-patient accuracy in detecting breast cancer (n = 16, 100% for both) and head and neck cancers (n = 10, 90% for both modalities). 68Ga-FAPI PET had lower per-patient accuracy than 18F-FDG PET in cancers of the bladder (n = 12, 75% vs. 100%) and kidney (n = 10, 80% vs. 90%), as well as lymphoma (n = 9, 88.9% vs. 100%) and myeloma (n = 10, 80% vs. 90%). Conclusion: 68Ga-FAPI PET demonstrated higher diagnostic accuracy than 18F-FDG PET in the diagnosis of colorectal cancer and prostate cancer, as well as comparable diagnostic performance for cancers of the breast and head and neck. Accuracy and impact on management will be further assessed in an ongoing prospective interventional trial (NCT05160051).

Contribution of 68 Ga-DOTA-FAPI-04 PET/CT to Prostate Cancer Imaging : Complementary Role in PSMA-Negative Cases

PURPOSE

Prostate-specific membrane antigen (PSMA)-targeted PET/CT is a well-established imaging method in prostate cancer (PC) for both staging and restaging, and also for theranostic applications. An alternative imaging method is crucial for 15% PSMA-negative cases. We aimed to investigate the contribution of 68 Ga-DOTA-FAPI-04 PET/CT to PC imaging.

PATIENTS AND METHODS

Thirty-six patients diagnosed with PC were included. Patients underwent both 68 Ga-PSMA PET/CT and 68 Ga-DOTA-FAPI-04 PET/CT imaging within 1 week. In staging group, primary tumor uptake values were compared, and also correlations were done with histopathological findings, MRI findings, and total PSA levels. In biochemical recurrence group, the uptake values in prostatic region and metastases were evaluated to define the local recurrence or metastatic disease.

RESULTS

In staging group, PSMA PET showed increased uptake in the primary lesion area in 14/27 (52%) patients, whereas 20/27 (74%) patients were positive in FAPI-04 PET. FAPI-04 positivity was found to be quite high, such as 54%, in PSMA-negative patients. A significant difference was observed between ISUP grade 1-3 patients and ISUP grade 4-5 patients in FAPI-04 PET ( P = 0.03). Local recurrence was detected in 3 patients, pelvic lymph node metastasis in 1 patient, and sacrum metastasis in 1 patient in biochemical recurrence group, and all of the lesions had more intense uptake in PSMA PET than FAPI-04 PET.

CONCLUSIONS

FAPI PET imaging seems to have a potential to contribute PSMA PET imaging with FAPI positivity in more than half of PSMA-negative cases. Also, FAPI-targeted radionuclide therapy may be a promising method in patients resistant to PSMA-targeted therapy.

Future Imaging of Prostate Cancer: Do We Need More Than PSMA PET/CT?

In the setting of prostate cancer (PCa), many different imaging modalities are available to correctly assess staging, restaging, treatment response and radio-ligand therapy recruitment. The introduction of fluoride or gallium-labelled prostate specific membrane antigen (PSMA) made a revolution in PCa management, also due to its possible theragnostic use. Nowadays PSMA-PET/CT is a fundamental tool for staging and restaging PCa. This review discusses the latest findings in PSMA imaging in PCa patients and the impact of PSMA imaging on the patients' management in primary staging, biochemical recurrence and in advanced prostate cancer, always keeping in mind the important theragnostic role of PSMA. This review tries also to assess the current role of other radiopharmaceuticals as Choline, FACBC or other radiotracers like gastrin-releasing peptide receptor targeting tracers and FAPI in different PCa settings.

Fibroblast Activation Protein-Targeted PET/CT Imaging in a Treatment-Naive Prostate Cancer Patient With Low PSMA Expression

ABSTRACT

Prostate-specific membrane antigen (PSMA) PET/CT has become increasingly accepted for imaging prostate cancer (PCa), including its recent use in primary staging. In this case report, we present the case of a 76-year-old man with newly diagnosed PCa. 18 F-PSMA-1007 PET/CT showed minimal PSMA activity in the primary tumor and metastases. However, 18 F-FAPI-04 PET/CT revealed more avid lesions in primary tumor, metastatic lymph nodes, and bones. Subsequent histopathologic examination confirmed the diagnosis of PCa. These findings suggest that 18 F-FAPI-04 may have a potential role in the evaluation of PCa with low PSMA expression in treatment-naive patients.

68 Ga-FAPI-04 PET/CT In Primary Signet Ring-Like Cell Carcinoma of Prostate With Bone Metastases

ABSTRACT

We presented a 69-year-old man with liver cirrhosis who underwent 68 Ga-fibroblast activation protein (FAPI)-04 PET/CT imaging for an ongoing study. In addition to the significant activity in liver cirrhosis on FAPI PET, a primary prostate tumor was incidentally detected with bone metastases. Further, FAPI-negative metastases were not observed on CT evaluation. In this context, we think that FAPI may be helpful with the potential theranostic aspect, especially in selected patients when current treatment options are not applicable or unresponsive. However, we should await the results of prospective studies investigating both prognostic and theranostic use of FAPI and demonstrating its effectiveness in cancer therapy.

Preliminary Findings of the Role of FAPi in Prostate Cancer Theranostics

Prostate cancer (PCa) is the most frequently diagnosed cancer worldwide and the second most common cause of cancer-related deaths among men. Progress in molecular imaging has magnified its clinical management; however, an unmet clinical need involves the identification of new imaging biomarkers that complement the gold standard of prostate-specific membrane antigen (PSMA) positron emission tomography (PET) in cases of clinically significant PCa that do not express PSMA. Fibroblast activation protein (FAP) is a type II transmembrane serine overexpressed in many solid cancers that can be imaged through quinoline-based PET tracers derived from an FAP inhibitor (FAPi). Preliminary results of FAPi application in PCa (in PSMA-negative lesions, and in comparison with fluorodeoxyglucose—FDG) are now available in the literature. FAP-targeting ligands for PCa are not limited to detection, but could also include therapeutic applications. In this preliminary review, we provide an overview of the clinical applications of FAPi ligands in PCa, summarising the main results and highlighting contemporary strengths and limitations.

68Ga-PSMA and 68Ga-FAPI-04 PET/CT Findings With 18F-FDG PET/CT in a Patient With Recurrent Prostate Cancer

ABSTRACT

A 79-year-old man with prostate cancer was evaluated with 68Ga-prostate-specific membrane antigen (PSMA), 68Ga-FAPI-04, and 18F-FDG PET/CT scans since a rise in prostate-specific antigen was seen at last control while receiving chemotherapy. Abnormal FDG uptakes were observed in the tumoral areas of the left half of the prostate and walls of the bladder, although PSMA and FAPI activity were absent or lower in the relevant tumoral areas. Restaging 68Ga-PSMA and 18F-FDG PET/CT scans were obtained on the patient whose progression continued after transurethral tumor resection. Herein, we present a case proving intraindividual tumor heterogeneity in prostate cancer underwent PET/CT imaging with 3 different radiopharmaceuticals.

Fibroblast Activation Protein Inhibitor (FAPI)-Based Theranostics-Where We Are at and Where We Are Heading: A Systematic Review

Cancer is the leading cause of death around the globe, followed by heart disease and stroke, with the highest mortality to this day. We have reached great levels of understanding of how these various types of cancer operate at a cellular level and this has brought us to what we call "precision medicine" where every diagnostic examination and the therapeutic procedure is tailored to the patient. FAPI is among the new tracers that can be used to assess and treat many types of cancer. The aim of this review was to gather all the known literature on FAPI theranostics. A MEDLINE search was conducted on four web libraries, PUBMED, Cochrane, Scopus, and Web of Sciences. All of the available articles that included both diagnoses and therapy with FAPI tracers were collected and put through the CASP (Critical Appraisal Skills Programme) questionnaire for systematic reviewing. A total of 8 records were deemed suitable for CASP review, ranging from 2018 to November 2022. These studies were put through the CASP diagnostic checklist, in order to assess the goal of the study, diagnostic and reference tests, results, descriptions of the patient sample, and future applications. Sample sizes were heterogeneous, both for size as well as for tumor type. Only one author studied a single type of cancer with FAPI tracers. Progression of disease was the most common outcome, and no relevant collateral effects were noted. Although FAPI theranostics is still in its infancy and lacks solid grounds to be brought into clinical practice, it does not show any collateral effects that prohibit administration to patients, thus far, and has good tolerability profiles.

Clinical advancement of precision theranostics in prostate cancer

Theranostic approaches with positron emission tomography/computed tomography (PET/CT) or PET/magnetic resonance imaging (PET/MRI) molecular imaging probes are being implemented clinically in prostate cancer (PCa) diagnosis and imaging-guided precision surgery. This review article provides a comprehensive summary of the rapidly expanding list of molecular imaging probes in this field, including their applications in early diagnosis of primary prostate lesions; detection of lymph node, skeletal and visceral metastases in biochemical relapsed patients; and intraoperative guidance for tumor margin detection and nerve preservation. Although each imaging probe shows preferred efficacy in some applications and limitations in others, the exploration and research efforts in this field will eventually lead to improved precision theranostics of PCa.