Current status and future perspectives of PSMA-targeted therapy in Europe: opportunity knocks

Prediction of Lesion-Based Treatment Response after Two Cycles of Lu-177 Prostate Specific Membrane Antigen Treatment in Metastatic Castration-Resistant Prostate Cancer Using Machine Learning

INTRODUCTION

Lutetium-177 (Lu-177) prostate-specific membrane antigen (PSMA) therapy is a radionuclide treatment that prolongs overall survival in metastatic castration-resistant prostate cancer (MCRPC). We aimed to predict lesion-based treatment response after Lu-177 PSMA treatment using machine learning with texture analysis data obtained from pretreatment Gallium-68 (Ga-68) PSMA positron emission tomography/computed tomography (PET/CT).

METHODS

Eighty-three progressed, and 91 nonprogressed malignant foci on pretreatment Ga-68 PSMA PET/CT of 9 patients were used for analysis. Malignant foci with at least a 30% increase in Ga-68 PSMA uptake after two cycles of treatment were considered progressed lesions. All other changes in Ga-68 PSMA uptake of the lesions were considered nonprogressed lesions. The classifiers tried to predict progressed lesions.

RESULTS

Logistic regression, Naive Bayes, and k-nearest neighbors' area under the ROC curve (AUC) values in detecting progressed lesions in the training group were 0.956, 0.942, and 0.950, respectively, and their accuracy was 87%, 85%, and 89%, respectively. The AUC values of the classifiers in the testing group were 0.937, 0.954, and 0.867, respectively, and their accuracy was 85%, 88%, and 79%, respectively.

CONCLUSION

Using machine learning with texture analysis data obtained from pretreatment Ga-68 PSMA PET/CT in MCRPC predicted lesion-based treatment response after two cycles of Lu-177 PSMA treatment.

Applications of Artificial Intelligence in PSMA PET/CT for Prostate Cancer Imaging

Prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT) has emerged as an important imaging technique for prostate cancer. The use of PSMA PET/CT is rapidly increasing, while the number of nuclear medicine physicians and radiologists to interpret these scans is limited. Additionally, there is variability in interpretation among readers. Artificial intelligence techniques, including traditional machine learning and deep learning algorithms, are being used to address these challenges and provide additional insights from the images. The aim of this scoping review was to summarize the available research on the development and applications of AI in PSMA PET/CT for prostate cancer imaging. A systematic literature search was performed in PubMed, Embase and Cinahl according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A total of 26 publications were included in the synthesis. The included studies focus on different aspects of artificial intelligence in PSMA PET/CT, including detection of primary tumor, local recurrence and metastatic lesions, lesion classification, tumor quantification and prediction/prognostication. Several studies show similar performances of artificial intelligence algorithms compared to human interpretation. Few artificial intelligence tools are approved for use in clinical practice. Major limitations include the lack of external validation and prospective design. Demonstrating the clinical impact and utility of artificial intelligence tools is crucial for their adoption in healthcare settings. To take the next step towards a clinically valuable artificial intelligence tool that provides quantitative data, independent validation studies are needed across institutions and equipment to ensure robustness.

Prostate-Specific Membrane Antigen in Anaplastic and Poorly Differentiated Thyroid Cancer-A New Diagnostic and Therapeutic Target?

Several studies have demonstrated an expression of the prostate-specific membrane antigen (PSMA) in the cancer-related neovasculature of thyroid malignancies. Due to the poor prognosis and limited therapeutic options for patients with anaplastic (ATC) and poorly differentiated (PDTC) thyroid carcinoma, the aim of our study was to investigate the theranostic approach of PSMA expression in these patients. The PSMA uptake on Gallium-68 (68Ga)-PSMA-positron emission tomography/computed tomography (PET/CT) and glucose uptake on F-18-Fluordeoxyglucose (18F-FDG)-PET/CTs were analysed in two ATC and six PDTC patients. The PSMA expression in corresponding patients' tissue samples was detected by immunohistochemistry. In addition, various tissue sections from 22 ATC and six PDTC patients were examined concerning PSMA expression. 68Ga-PSMA-PET/CT showed heterogeneous PSMA expression among patients and lesions. Six of the eight analyzed patients (two ATC, four PDTC) showed increased glucose metabolism without increased PSMA uptake after PET/CT. In one patient (PDTC), 18F-FDG-PET/CT tracer uptake was positive and 68Ga-PSMA-PET/CT showed heterogeneous results. Another patient (PDTC) evidenced only PSMA-positive lesions and received two cycles of Lutetium-177 (177Lu)-PSMA therapy, which kept his disease stable for seven months. There was a correlation between immunohistochemical PSMA expression and uptake on 68Ga-PMSA-PET/CT in three of the examined patients. Twenty-seven of the analyzed 39 ATC and 13 of the analyzed 22 PDTC tissue sections showed a strong PSMA expression. Considering the rarity of PDTC and ATC, which is the reason for the small patient population we studied, the findings of this study confirm the high diagnostic sensitivity and superiority of 18F-FDG-PET/CT in comparison to 68Ga-PSMA-PET/CT in the diagnosis of ATC and PDTC. However, it can be suggested that 68Ga-PMSA-PET/CT can be considered as a beneficial adjunct to the well-established 18F-FDG-PET/CT for a few individual selected patients with ATC and PDTC to detect lesions not discovered by 18F-FDG-PET/CT and to determine patients' eligibility for a radioligand therapy. Radiolabelled PSMA-ligands may, in the future, represent a theranostic approach with only minor side effects for a few individual selected patients with ATC and PDTC who need alternative treatment options in case of progression when established therapies are no longer effective. However, due to the small sample size of our collective, larger studies are needed to allow for a final evaluation on the significance of PSMA-targeted diagnostic and therapy for ATC and PDTC.

Decision-support for treatment with 177Lu-PSMA: machine learning predicts response with high accuracy based on PSMA-PET/CT and clinical parameters

Background

Treatment with radiolabeled ligands to prostate-specific membrane antigen (PSMA) is gaining importance in the treatment of patients with advanced prostate carcinoma. Previous imaging with positron emission tomography/computed tomography (PET/CT) is mandatory. The aim of this study was to investigate the role of radiomics features in PSMA-PET/CT scans and clinical parameters to predict response to ¹⁷⁷Lu-PSMA treatment given just baseline PSMA scans using state-of-the-art machine learning (ML) methods.

Methods

A total of 2,070 pathological hotspots annotated in 83 prostate cancer patients undergoing PSMA therapy were analyzed. Two main tasks are performed: (I) analyzing correlation of averaged (per patient) values of radiomics features of individual hotspots and clinical parameters with difference in prostate specific antigen levels (ΔPSA) in pre- and post-therapy as a therapy response indicator. (II) ML-based classification of patients into responders and non-responders based on averaged features values and clinical parameters. To achieve this, machine learning (ML) algorithms and linear regression tests are applied. Grid search, cross validation (CV) and permutation test were performed to assure that the results were significant.

Results

Radiomics features (PET_Min, PET_Correlation, CT_Min, CT_Busyness and CT_Coarseness) and clinical parameters such as Alp1 and Gleason score showed best correlations with ΔPSA. For the treatment response prediction task, 80% area under the curve (AUC), 75% sensitivity (SE), and 75% specificity (SP) were obtained, applying ML support vector machine (SVM) classifier with radial basis function (RBF) kernel on a selection of radiomics features and clinical parameters with strong correlations with ΔPSA.

Conclusions

Machine learning based on ⁶⁸Ga-PSMA PET/CT radiomics features holds promise for the prediction of response to ¹⁷⁷Lu-PSMA treatment, given only base-line ⁶⁸Ga-PSMA scan. In addition, it was shown that, the best correlating set of radiomics features with ΔPSA are superior to clinical parameters for this therapy response prediction task using ML classifiers.

Correlation of an Index-Lesion-Based SPECT Dosimetry Method with Mean Tumor Dose and Clinical Outcome after 177Lu-PSMA-617 Radioligand Therapy

Background: Dosimetry can tailor prostate-specific membrane-antigen-targeted radioligand therapy (PSMA-RLT) for metastatic castration-resistant prostate cancer (mCRPC). However, whole-body tumor dosimetry is challenging in patients with a high tumor burden. We evaluate a simplified index-lesion-based single-photon emission computed tomography (SPECT) dosimetry method in correlation with clinical outcome. Methods: 30 mCRPC patients were included (median 71 years). The dosimetry was performed for the first cycle using quantitative ¹⁷⁷Lu-SPECT. The response was evaluated using RECIST 1.1 and PERCIST criteria, as well as changes in PSMA-positive tumor volume (PSMA-TV) in post-therapy PSMA-PET and biochemical response according to PSA changes after two RLT cycles. Results: Mean tumor doses as well as index-lesion doses were significantly higher in PERCIST responders compared to non-responders (10.2 ± 12.0 Gy/GBq vs. 4.0 ± 2.9 Gy/GBq, p = 0.03 and 13.7 ± 14.2 Gy/GBq vs. 5.9 ± 4.4 Gy/GBq, p = 0.04, respectively). No significant differences in mean tumor and index lesion doses were observed between responders and non-responders according to RECIST 1.1, PSMA-TV, and biochemical response criteria. Conclusion: Compared to mean tumor doses on a patient level, single index-lesion-based SPECT dosimetry correlates equally well with the response to PSMA-RLT according to PERCIST criteria and may represent a fast and feasible dosimetry approach for clinical routine.

Coordination chemistry of [Y(pypa)]- and comparison immuno-PET imaging of [44Sc]Sc- and [86Y]Y-pypa-phenyl-TRC105

Both scandium-44 and yttrium-86 are popular PET isotopes with appropriate half-lives for immuno-positron emission tomography (immuno-PET) imaging. Herein, a new bifunctional H4pypa ligand, H4pypa-phenyl-NCS, is synthesized, conjugated to a monoclonal antibody, TRC105, and labeled with both radionuclides to investigate the long-term in vivo stability of each complex. While the 44Sc-labeled radiotracer exhibited promising pharmacokinetics and stability in 4T1-xenograft mice (n = 3) even upon prolonged interactions with blood serum proteins, the progressive bone uptake of the 86Y-counterpart indicated in vivo demetallation, obviating H4pypa as a suitable chelator for Y3+ ion in vivo. The solution chemistry of [natY(pypa)]- was studied in detail and the complex found to be thermodynamically stable in solution with a pM value 22.0, ≥3 units higher than those of the analogous DOTA- and CHX-A''-DTPA-complexes; the 86Y-result in vivo was therefore most unexpected. To explore further this in vivo lability, Density Functional Theory (DFT) calculation was performed to predict the geometry of [Y(pypa)]- and the results were compared with those for the analogous Sc- and Lu-complexes; all three adopted the same coordination geometry (i.e. distorted capped square antiprism), but the metal-ligand bonds were much longer in [Y(pypa)]- than in [Lu(pypa)]- and [Sc(pypa)]-, which could indicate that the size of the binding cavity is too small for the Y3+ ion, but suitable for both the Lu3+ and Sc3+ ions. Considered along with results from [86Y][Y(pypa-phenyl-TRC105)], it is noted that when matching chelators with radionuclides, chemical data such as the thermodynamic stability and in vitro inertness, albeit useful and necessary, do not always translate to in vivo inertness, especially with the prolonged blood circulation of the radiotracer bound to a monoclonal antibody. Although H4pypa is a nonadentate chelator, which theoretically matches the coordination number of the Y3+ ion, we show herein that its binding cavity, in fact, favors smaller metal ions such as Sc3+ and Lu3+ and further exploitation of the Sc-pypa combination is desired.

Biodistribution and post-therapy dosimetric analysis of [177Lu]Lu-DOTAZOL in patients with osteoblastic metastases: first results

Background

Preclinical biodistribution and dosimetric analysis of [¹⁷⁷Lu]Lu-DOTA^ZOL suggest the bisphosphonate zoledronate as a promising new radiopharmaceutical for therapy of bone metastases. We evaluated biodistribution and normal organ absorbed doses resulting from therapeutic doses of [¹⁷⁷Lu]Lu-DOTA^ZOL in patients with metastatic skeletal disease.

Method

Four patients with metastatic skeletal disease (age range, 64–83 years) secondary to metastatic castration-resistant prostate carcinoma or bronchial carcinoma were treated with a mean dose of 5968 ± 64 MBq (161.3 mCi) of [¹⁷⁷Lu]Lu-DOTA^ZOL. Biodistribution was assessed with serial planar whole body scintigraphy at 20 min and 3, 24, and 167 h post injection (p.i.) and blood samples at 20 min and 3, 8, 24, and 167 h p.i. Percent of injected activity in the blood, kidneys, urinary bladder, skeleton, and whole body was determined. Bone marrow self-dose was determined by an indirect blood-based method. Urinary bladder wall residence time was calculated using Cloutier’s dynamic urinary bladder model with a 4-h voiding interval. OLINDA/EXM version 2.0 (Hermes Medical Solutions, Stockholm, Sweden) software was used to determine residence times in source organs by applying biexponential curve fitting and to calculate organ absorbed dose.

Results

Qualitative biodistribution analysis revealed early and high uptake of [¹⁷⁷Lu]Lu-DOTA^ZOL in the kidneys with fast clearance showing minimal activity by 24 h p.i. Activity in the skeleton increased gradually over time. Mean residence times were found to be highest in the skeleton followed by the kidneys. Highest mean organ absorbed dose was 3.33 mSv/MBq for osteogenic cells followed by kidneys (0.490 mSv/MBq), red marrow (0.461 mSv/MBq), and urinary bladder wall (0.322 mSv/MBq). The biodistribution and normal organ absorbed doses of [¹⁷⁷Lu]Lu-DOTA^ZOL are consistent with preclinical data.

Conclusion

[¹⁷⁷Lu]Lu-DOTA^ZOL shows maximum absorbed doses in bone and low kidney doses, making it a promising agent for radionuclide therapy of bone metastasis. Further studies are warranted to evaluate the efficacy and safety of radionuclide therapy with [¹⁷⁷Lu]Lu-DOTA^ZOL in the clinical setting.

Prostate-specific membrane antigen (PSMA) expression in breast cancer and its metastases

The present study was undertaken to investigate the expression of prostate-specific membrane antigen (PSMA) in normal breast tissues, in cancerous breast tissues and in distant metastases from patients with breast cancer. Immunohistochemical analysis was performed to determine PSMA expression and angiogenic activity using anti-PSMA mAb and anti-CD31 mAb respectively. Immunofluorescence staining was applied to confirm the exact co-localization of PSMA and CD31. We observed different patterns of PSMA expression between normal and cancerous tissues. Normal breast tissues showed PSMA expression only in normal glandular cells. However, primary breast tumors and distant metastases showed PSMA expression in tumor cells and in tumor-associated neovasculature. PSMA score group status in primary breast tumors was significantly associated with histologic type and tumor grade (p = 0.026 and p = 0.004 respectively). Distant metastases showed higher PSMA expression in tumor-associated neovasculature comparing with primary tumors. Moreover, brain tumor-associated neovasculture had significantly higher expression of PSMA comparing with bone tumor-associated neovasculture. The localized binding of PSMA mAb to the neovasculature endothelium was confirmed with the double Immunofluorescence staining. ⁶⁸Ga-PSAM imaging of a patient with metastatic breast cancer showed strong tracer uptake in all known skeletal metastases. To the best of our knowledge, this study is the second one that has assessed PSMA expression in a large number of breast cancer patients. Our findings showed that PSMA is particularly expressed in tumor-associated neovasculature of breast tumors and its distant metastases, thus enhancing the evidence on the potential usefulness of PSMA as a therapeutic vascular target.

Individualized Dosimetry for Theranostics: Necessary, Nice to Have, or Counterproductive?

In 2005, the term theragnostics (theranostics) was introduced for describing the use of imaging for therapy planning in radiation oncology. In nuclear medicine, this expression describes the use of tracers for predicting the absorbed doses in molecular radiotherapy and, thus, the safety and efficacy of a treatment. At present, the most successful groups of isotopes for this purpose are ¹²³I/¹²⁴I/¹³¹I, ⁶⁸Ga/¹⁷⁷Lu, and ¹¹¹In/⁸⁶Y/⁹⁰Y. The purpose of this review is to summarize available data on the dosimetry and dose-response relationships of several theranostic compounds, with a special focus on radioiodine therapy for differentiated thyroid cancer and peptide receptor radionuclide therapy. These are treatment modalities for which dose-response relationships for healthy tissues and tumors have been demonstrated. In addition, available data demonstrate that posttherapeutic dosimetry after a first treatment cycle predicts the absorbed doses in further cycles. Both examples show the applicability of the concept of theranostics in molecular radiotherapies. Nevertheless, unanswered questions need to be addressed in clinical trials incorporating dosimetry-related concepts for determining the amount of therapeutic activity to be administered.

Clinical History of the Theranostic Radionuclide Approach to Neuroendocrine Tumors and Other Types of Cancer: Historical Review Based on an Interview of Eric P. Krenning by Rachel Levine

In nuclear medicine, the term theranostics describes the combination of therapy and diagnostic imaging. In practice, this concept dates back more than 50 years; however, among the most successful examples of theranostics are peptide receptor scintigraphy and peptide receptor radionuclide therapy of neuroendocrine tumors. The development of these modalities through the radiolabeling of somatostatin analogs with various radionuclides has led to a revolution in patient management and established a foundation for expansion of the theranostic principle into other oncology indications. This article provides a review of the evolution and development of the theranostic radionuclide approach to the management of neuroendocrine tumors, as described by the inventor of this technique, Eric P. Krenning, in an interview with Rachel Levine.

Prostate-specific membrane antigen positron emission tomography in prostate cancer: a step toward personalized medicine

PURPOSE OF REVIEW

Increasing attention is being given to personalized medicine in oncology, where therapies are tailored to the particular characteristics of the individual cancer patient. In recent years, there has been greater focus on prostate-specific membrane antigen (PSMA) in prostate cancer (PCa) as a target for imaging and therapy with radionuclides. This review highlights the recent advancements in PSMA positron emission tomography (PET) in PCa during the past year.

RECENT FINDINGS

Several reports on PSMA PET/computed tomography (CT) in PCa patients are demonstrating promising results, especially for detection of biochemical recurrence. F-PSMA PET/CT may be superior to Ga-PSMA PET/CT. The detection rate of PSMA PET is influenced by prostate-specific antigen level. PSMA PET/CT may have a higher detection rate than choline PET/CT. Only a few reports have been published on PSMA PET/magnetic resonance imaging (MRI), and this modality remains to be elucidated further.

SUMMARY

Molecular imaging with PSMA PET is paving the way for personalized medicine in PCa. However, large prospective clinical studies are needed to further evaluate the role of PSMA PET/CT and PET/MRI in the clinical workflow of PCa. PSMA is an excellent target for imaging and therapy with radionuclides, and the 'image and treat' strategy has the potential to become a milestone in the management of PCa patients.

Investigating the Effect of Ligand Amount and Injected Therapeutic Activity: A Simulation Study for 177Lu-Labeled PSMA-Targeting Peptides

In molecular radiotherapy with ¹⁷⁷Lu-labeled prostate specific membrane antigen (PSMA) peptides, kidney and/or salivary glands doses limit the activity which can be administered. The aim of this work was to investigate the effect of the ligand amount and injected activity on the tumor-to-normal tissue biologically effective dose (BED) ratio for ¹⁷⁷Lu-labeled PSMA peptides. For this retrospective study, a recently developed physiologically based pharmacokinetic model was adapted for PSMA targeting peptides. General physiological parameters were taken from the literature. Individual parameters were fitted to planar gamma camera measurements (¹⁷⁷Lu-PSMA I&T) of five patients with metastasizing prostate cancer. Based on the estimated parameters, the pharmacokinetics of tumor, salivary glands, kidneys, total body and red marrow was simulated and time-integrated activity coefficients were calculated for different peptide amounts. Based on these simulations, the absorbed doses and BEDs for normal tissue and tumor were calculated for all activities leading to a maximal tolerable kidney BED of 10 Gy_2.5/cycle, a maximal salivary gland absorbed dose of 7.5 Gy/cycle and a maximal red marrow BED of 0.25 Gy₁₅/cycle. The fits yielded coefficients of determination > 0.85, acceptable relative standard errors and low parameter correlations. All estimated parameters were in a physiologically reasonable range. The amounts (for 2⁵−2⁹ nmol) and pertaining activities leading to a maximal tumor dose, considering the defined maximal tolerable doses to organs of risk, were calculated to be 272±253 nmol (452±420 μg) and 7.3±5.1 GBq. Using the actually injected amount (235±155 μg) and the same maximal tolerable doses, the potential improvement for the tumor BED was 1–3 fold. The results suggest that currently given amounts for therapy are in the appropriate order of magnitude for many lesions. However, for lesions with high binding site density or lower perfusion, optimizing the peptide amount and activity might improve the tumor-to-kidney and tumor-to-salivary glands BED ratio considerably.

Design and Fabrication of Kidney Phantoms for Internal Radiation Dosimetry Using 3D Printing Technology

Currently, the validation of multimodal quantitative imaging and absorbed dose measurements is impeded by the lack of suitable, commercially available anthropomorphic phantoms of variable sizes and shapes. To demonstrate the potential of 3-dimensional (3D) printing techniques for quantitative SPECT/CT imaging, a set of kidney dosimetry phantoms and their spherical counterparts was designed and manufactured with a fused-deposition-modeling 3D printer. Nuclide-dependent SPECT/CT calibration factors were determined to assess the accuracy of quantitative imaging for internal renal dosimetry.

METHODS

A set of 4 single-compartment kidney phantoms with filling volumes between 8 and 123 mL was designed on the basis of the outer kidney dimensions provided by MIRD pamphlet 19. After the phantoms had been printed, SPECT/CT acquisitions of 3 radionuclides (^99mTc, ¹⁷⁷Lu, and ¹³¹I) were obtained and calibration constants determined for each radionuclide-volume combination. A set of additionally manufactured spheres matching the kidney volumes was also examined to assess the influence of phantom shape and size on the calibration constants.

RESULTS

A set of refillable, waterproof, and chemically stable kidneys and spheres was successfully manufactured. Average calibration factors for ^99mTc, ¹⁷⁷Lu, and ¹³¹I were obtained in a large source measured in air. For the largest phantom (122.9 mL), the volumes of interest had to be enlarged by 1.2 mm for ^99mTc, 2.5 mm for ¹⁷⁷Lu, and 4.9 mm for ¹³¹I in all directions to obtain calibration factors comparable to the reference. Although partial-volume effects were observed for decreasing phantom volumes (percentage difference of up to 9.8% for the smallest volume [8.6 mL]), the difference between corresponding sphere-kidney pairs was small (<1.1% for all volumes).

CONCLUSION

3D printing is a promising prototyping technique for geometry-specific calibration of SPECT/CT systems. Although the underlying radionuclide and the related collimator have a major influence on the calibration, no relevant differences between kidney-shaped and spherically shaped uniform-activity phantoms were observed. With comparably low costs and submillimeter resolution, 3D printing techniques hold the potential for manufacturing individualized anthropomorphic phantoms in many clinical applications in nuclear medicine.

Heterogeneous PSMA expression on circulating tumor cells: a potential basis for stratification and monitoring of PSMA-directed therapies in prostate cancer

The prostate specific membrane antigen (PSMA) is the only clinically validated marker for therapeutic decisions in prostate cancer (PC). Characterization of circulating tumor cells (CTCs) obtained from the peripheral blood of PC patients might provide an alternative to tissue biopsies called "liquid biopsy". The aim of this study was to develop a reliable assay for the determination of PSMA on CTCs. PSMA expression was analyzed on tissue samples (cohort one, n = 75) and CTCs from metastatic PC patients (cohort two, n = 29). Specific signals for the expression of PSMA could be seen for different prostate cancer cell line cells (PC3, LaPC4, 22Rv1, and LNCaP) by Western blot, immunohistochemistry (IHC), immunocytochemistry (ICC), and FACS. PSMA expression was found to be significantly increased in patients with higher Gleason grade (p = 0.0011) and metastases in lymph nodes (p = 0.0000085) or bone (p = 0.0020) (cohort one). In cohort two, CTCs were detectable in 20 out of 29 samples (69 %, range from 1 - 1000 cells). Twelve out of 20 CTC-positive patients showed PSMA-positive CTCs (67 %, score 1+ to 3+). We found intra-patient heterogeneity regarding the PSMA status between CTCs and the corresponding primary tumors. The results of our study could help to address the question whether treatment decisions based on CTC PSMA profiling will lead to a measurable benefit in clinical outcome for prostate cancer patients in the near future.