Bone marrow impairment during early [177Lu]PSMA-617 radioligand therapy: Haematotoxicity or tumour progression?

Impact of baseline 18F-flotufolastat PET bone tumor volume for prognosticating severe hematologic toxicity in patients with metastatic castration-resistant prostate Cancer receiving 177Lu-PSMA-targeted radioligand therapy

PURPOSE

This retrospective analysis evaluated the prognostic value of baseline 18F-flotufolastat-PET bone tumor metrics for severe hematologic toxicity in metastatic castration-resistant prostate cancer (mCRPC) patients treated with [177Lu]Lu-PSMA-I&T.

METHODS

Data from 182 mCRPC patients with baseline 18F-flotufolastat-PET scans and complete hematologic profiles were analyzed. Bone lesions were semiautomatically delineated, and clinical parameters (e.g., pretreatments, lab results) were assessed. Hematologic adverse events (AEs) were defined per Common Terminology Criteria for Adverse Events version 5.0, with grades 3-4 considered severe. Cox regression was used to identify prognostic factors for AEs.

RESULTS

Baseline bone tumor volume prognosticated leukocytopenia (HR 1.03 per 100 ml, p = 0.036), while the number of bone lesions was prognostic for anemia (HR 1.04 per 10 lesions, p < 0.001) and severe anemia (HR per 10 lesions 1.05, p = 0.009). Higher baseline hemoglobin correlated with reduced leukocytopenia (HR 0.74, p = 0.002), thrombocytopenia (HR 0.80, p = 0.033), and severe anemia (HR 0.52, p < 0.001). Baseline kidney dysfunction was linked to anemia (HR 2.46, p = 0.002) and severe anemia (HR 3.81, p = 0.023). Prior [223Ra]Radiumdichloride treatment prognosticated severe thrombocytopenia (HR 6.43, p = 0.021).

CONCLUSION

Baseline 18F-flotufolastat-PET metrics and pretherapeutic clinical parameters are key prognostic factors for severe hematologic toxicity in mCRPC patients treated with [177Lu]Lu-PSMA-I&T.

[99mTc]Tc-antigranulocyte scintigraphy for prediction of bone marrow reserve prior to radioligand therapy in patients with metastatic castration resistant prostate cancer

PURPOSE

Radioligand therapy (RLT) targeting PSMA (prostate-specific membrane antigen) has transformed the treatment of metastatic castration-resistant prostate cancer (mCRPC). However, bone marrow depletion remains a common side effect, particularly in patients with extensive bone metastases or prior myelotoxic therapies. This study evaluated [99mTc]Tc-antigranulocyte scintigraphy to assess bone marrow reserve to guide myelotoxic treatment decisions.

METHODS

Ten mCRPC patients with extensive osseous tumor load on [18F]F-PSMA PET/CT underwent [99mTc]Tc-antigranulocyte scintigraphy to assess bone marrow reserve and RLT eligibility (interval: 26.6 ± 18.5 days). Visual comparison of both modalities evaluated tumor-bone marrow overlap. Patients without significant co-localization received RLT ([177Lu]Lu-PSMA or [225Ac]Ac-PSMA) with laboratory monitoring before and between the cycles.

RESULTS

No significant co-localization between viable bone marrow and PSMA-positive metastases was observed in 9/10 (90.0%) patients. Two other patients were excluded from RLT due to contraindications. Of the remaining seven patients, 2/7 (28.6%) underwent subsequent [177Lu]Lu-PSMA therapy, 2/7 (28.6%) [225Ac]Ac-PSMA therapy and 2/7 (28.6%) [225Ac]Ac-[177Lu]Lu-PSMA therapy. 1/7 (14.3%) received a single cycle of [177Lu]Lu-PSMA, which was then followed by [225Ac]Ac-PSMA therapy. Follow-up laboratory results showed no significant changes from baseline (p > 0.05).

CONCLUSION

[99mTc]Tc-antigranulocyte scintigraphy may support the assessment of bone marrow reserve in patients with extensive bone metastases. Spatial mismatch was associated with no significant myelotoxicity, whereas high congruence may increase the risk. No fixed thresholds currently exist; thus, this method should be regarded as a supplementary tool in complex cases and future studies are needed to define the critical bone marrow volume.

Small-scale bone marrow dosimetry study for 225Ac

BACKGROUND

Targeted alpha therapy (TAT) with 225Ac-labelled radiopharmaceuticals is a growing therapeutic option for the treatment of various cancers. Due to the short range of alpha particles in tissue, the absorbed dose can be non-uniform on a microscopic scale. Therefore, understanding bone marrow toxicity in TAT requires small-scale dosimetry.

METHOD

We developed a voxelised trabecular bone model, based off µCT slices, with a voxel size of (37 × 37 × 37) µm3. A small-scale dosimetry study was performed to assess the marrow toxicity from uptake of unlabelled 225Ac in the trabecular bone. The Particle and Heavy Ion Transport Code System (PHITS) was used to simulate the decays and score the absorbed dose to each voxel from the alpha and beta emissions of the 225Ac decay chain.

RESULTS

For the alpha decays on the trabecular surface, 43 % of the marrow voxels were irradiated. The maximum voxel dose for the marrow was 1.1 Gy, and the mean non-zero voxel dose was 0.2 Gy (σ = 0.2 Gy). The beta-emissions from the trabecular surface irradiated all the marrow voxels, with a mean voxel dose of 3.9 mGy (σ = 1.7 mGy).

CONCLUSION

Our model demonstrated a non-uniform absorbed dose profile to the red marrow due to alpha emissions on the trabecular bone surface. The alpha emissions irradiated less than half of the marrow voxels,while the beta emissions irradiated all marrow voxels. This could potentially suggest a lower marrow toxicity from alpha-emitters compared to beta-emitters when skeletal metastases are present.

Targeted radioligand therapy: physics and biology, internal dosimetry and other practical aspects during 177Lu/225Ac treatment in neuroendocrine tumors and metastatic prostate cancer

Radioligand therapy (RLT) has garnered significant attention due to the recent emergence of innovative and effective theranostic agents, which showed promising therapeutic and prognostic results in various cancers. Moreover, understanding the interaction between different types of radiation and biological tissues is essential for optimizing therapeutic interventions These concepts directly apply to clinical RLTs and play a crucial role in determining the efficacy and toxicity profile of different radiopharmaceutical agents. Personalized dosimetry is a powerful tool that aids in estimating patient-specific absorbed doses in both tumors and normal organs. Dosimetry in RLT is an area of active investigation, as our current understanding of the relationship between absorbed dose and tissue damage is primarily derived from external-beam radiation therapy. Further research is necessary to comprehensively comprehend this relationship in the context of RLTs. In the present review, we present a thorough examination of the involvement of 177Lu/225Ac radioisotopes in the induction of direct and indirect DNA damage, as well as their influence on the initiation of DNA repair mechanisms in cancer cells of neuroendocrine tumors and metastatic prostate cancer. Current data indicate that high-energy α-emitter radioisotopes can directly impact DNA structure by causing ionization, leading to the formation of ionized atoms or molecules. This ionization process predominantly leads to the formation of irreparable and intricate double-strand breaks (DSBs). On the other hand, the majority of DNA damage caused by β-emitter radioisotopes is indirect, as it involves the production of free radicals and subsequent chemical reactions. Beta particles themselves can also physically interact with the DNA molecule, resulting in single-strand breaks (SSBs) and potentially reversible DSBs.

EANM expert opinion: How can lessons from radiobiology be applied to the design of clinical trials? Part I: back to the basics of absorbed dose-response and threshold absorbed doses

PURPOSE

This study by the EANM radiobiology working group aims to analyze the efficacy and toxicity of targeted radionuclide therapy (TRT) using radiopharmaceuticals approved by the EMA and FDA for neuroendocrine tumors and prostate cancer. It seeks to understand the correlation between physical parameters such as absorbed dose and TRT outcomes, alongside other biological factors.

METHODS

We reviewed clinical studies on TRT, focusing on the relationship between physical parameters and treatment outcomes, and applying basic radiobiological principles to radiopharmaceutical therapy to identify key factors affecting therapeutic success.

RESULTS

The analysis revealed that mean absorbed dose alone is insufficient to predict treatment response or toxicity. For absorbed doses below a certain threshold, outcomes are unpredictable, while doses above this threshold improve the likelihood of biological responses. However, even at higher absorbed doses, response plateaus indicate the need for additional parameters to explain outcome variability, including heterogeneity in target expression, anatomical disease location, (epi)genetics, DNA repair capacity, and the tumor microenvironment, aspects that will be discussed in Part II of this analysis.

CONCLUSION

Understanding radiobiology is crucial for optimizing TRT. More dosimetric data is needed to refine treatment protocols. While absorbed dose is critical, it alone does not determine TRT outcomes. Future research should integrate biological parameters with physical dosimetry to enhance efficacy and minimize toxicity.

Advancing Cancer Theranostics Through Integrin αVβ3-Targeted Peptidomimetic IAC: From Bench to Bedside

Introduction: The expression of alpha-five beta-three (αVβ3) integrins is upregulated in various malignancies undergoing angiogenesis. The development of integrin antagonists as diagnostic probes makes the αVβ3 integrin a suitable candidate for targeting tumor angiogenesis. The goal of this study was to optimize the radiolabeling and evaluate the potential of conjugated integrin antagonist carbamate (IAC), a peptidomimetic, as a theranostic radiopharmaceutical for targeting tumor angiogenesis. Methodology: Radiolabeling of DOTAGA [2,2',2"-{10-(2,6-dioxotetrahydro-2H-pyran-3-yl)-1,4,7,10-tetraazacyclododecane-1,4,7-triyl} triacetic-acid]-IAC with [68Ga]Ga, [177Lu]Lu, and [225Ac]Ac was optimized. The binding affinity (Kd) of DOTAGA-IAC for the αVβ3 receptor and cancer cell lines was quantified. The biodistribution studies were conducted in healthy Wistar rats. Dosimetry analysis was performed on [177Lu]Lu-DOTAGA-IAC distribution data. A pilot study of [68Ga]Ga-DOTAGA-IAC and [18F]FDG Positron Emission Tomography (PET/CT) imaging was performed in five patients with histopathologically confirmed breast cancer. PET/CT findings were compared between [68Ga]Ga-DOTAGA-IAC and [18F]FDG in these patients. Results: Radiopharmaceuticals were prepared with high radiochemical purity (>99.9%). Kd and Bmax measurements were 15.02 nM and 417 fmol for αVβ3 receptor protein: 115.7 nM and 295.3 fmol for C6 glioma cells. Biodistribution studies in rats suggested the excretion via kidneys and partially through the hepatobiliary route. The effective dose of [177Lu]Lu-DOTAGA-IAC was found to be 0.17 mSv/MBq. The dynamic study in patients revealed the optimal imaging time to be 30-35 mins postadministration. Out of the cohort, [68Ga]Ga-DOTAGA-IAC detected the primary lesions in all five patients with a mean standard uptake value (SUVmax) of 3.94 ± 0.58 compared with [18F]FDG (SUVmax 13.8 ± 6.53). Conclusion: The study demonstrates that DOTAGA-IAC exhibits strong binding to αVβ3 integrin, positioning it as a promising PET agent for assessing primary and metastatic cancers. The outcomes from the pilot study suggest the potential of [68Ga]Ga-DOTAGA-IAC PET/CT in breast carcinoma diagnosis. While recognizing the theranostic potential of DOTAGA-IAC for αVβ3 integrin-expressing tumors, further clinical investigations are warranted to comprehensively assess therapeutic efficacy.

A review of 177Lu dosimetry workflows: how to reduce the imaging workloads?

177 Lu radiopharmaceutical therapy is a standardized systemic treatment, with a typical dose of 7.4 GBq per injection, but its response varies from patient to patient. Dosimetry provides the opportunity to personalize treatment, but it requires multiple post-injection images to monitor the radiopharmaceutical's biodistribution over time. This imposes an additional imaging burden on centers with limited resources. This review explores methods to lessen this burden by optimizing acquisition types and minimizing the number and duration of imaging sessions. After summarizing the different steps of dosimetry and providing examples of dosimetric workflows for177 Lu -DOTATATE and177 Lu -PSMA, we examine dosimetric workflows based on a reduced number of acquisitions, or even just one. We provide a non-exhaustive description of simplified methods and their assumptions, as well as their limitations. Next, we detail the specificities of each normal tissue and tumors, before reviewing dose-response relationships in the literature. In conclusion, we will discuss the current limitations of dosimetric workflows and propose avenues for improvement.

Short-Term Biological Toxicity Prediction of [177Lu]Lutetium-Oxodotreotide: An Original Retrospective Analysis

Introduction: [177Lu]Lutetium (Lu)-oxodotreotide is a radiopharmaceutical drug used as peptide receptor radionuclide therapy (PRRT) for somatostatin receptor-expressing neuroendocrine neoplasms. It provides an additional effective alternative treatment for these rare cancers. Although well tolerated, its safety profile must continue to be characterized to support its use as a first-line treatment or for additional cycles. This study evaluated factors associated with the occurrence of [177Lu]Lu-oxodotreotide induced short-term toxicity. Materials and Methods: A retrospective observational monocentric study was carried out from July 2013 to October 2021. Inclusion criteria were defined as follows: patients who received at least four cycles of [177Lu]Lu-oxodotreotide and were followed up for 6 months after the last injection. Graduated toxicity was defined using the National Cancer Institute Common Terminology Criteria for Adverse Events 5.0. Cox regression was used in the analysis. Results: Forty patients were included. The most frequent toxicities occurred during the first cycle and were graded as G1 or G2. As expected, toxicities were predominantly hematological and hepatic, with incomplete reversibility after each cycle. The following factors were significantly related to the occurrence of hematological or hepatic toxicity during PRRT: gastrointestinal primary tumor diagnosis, bone metastases, peritoneal metastases, pancreatic metastases or pulmonary metastases, and high tumor grade. Conclusion: Knowledge and consideration of these factors in adjusting [177Lu]Lu-oxodotreotide treatment regimen could help prevent or reduce the severity of these toxicities. Further studies are still warranted to refine these results and improve treatment management.

Recent advances and applications of peptide-agent conjugates for targeting tumor cells

BACKGROUND

Cancer, being a complex disease, presents a major challenge for the scientific and medical communities. Peptide therapeutics have played a significant role in different medical practices, including cancer treatment.

METHOD

This review provides an overview of the current situation and potential development prospects of anticancer peptides (ACPs), with a particular focus on peptide vaccines and peptide-drug conjugates for cancer treatment.

RESULTS

ACPs can be used directly as cytotoxic agents (molecularly targeted peptides) or can act as carriers (guiding missile) of chemotherapeutic agents and radionuclides by specifically targeting cancer cells. More than 60 natural and synthetic cationic peptides are approved in the USA and other major markets for the treatment of cancer and other diseases. Compared to traditional cancer treatments, peptides exhibit anticancer activity with high specificity and the ability to rapidly kill target cancer cells. ACP's target and kill cancer cells via different mechanisms, including membrane disruption, pore formation, induction of apoptosis, necrosis, autophagy, and regulation of the immune system. Modified peptides have been developed as carriers for drugs, vaccines, and peptide-drug conjugates, which have been evaluated in various phases of clinical trials for the treatment of different types of solid and leukemia cancer.

CONCLUSIONS

This review highlights the potential of ACPs as a promising therapeutic option for cancer treatment, particularly through the use of peptide vaccines and peptide-drug conjugates. Despite the limitations of peptides, such as poor metabolic stability and low bioavailability, modified peptides show promise in addressing these challenges. Various mechanism of action of anticancer peptides. Modes of action against cancer cells including: inducing apoptosis by cytochrome c release, direct cell membrane lysis (necrosis), inhibiting angiogenesis, inducing autophagy-mediated cell death and immune cell regulation.

Long-term survival outcomes of salvage [225Ac]Ac-PSMA-617 targeted alpha therapy in patients with PSMA-expressing end-stage metastatic castration-resistant prostate cancer: a real-world study

PURPOSE

Despite the existence of various treatment options, the prognosis for patients with metastatic castration-resistant prostate cancer (mCRPC) remains unfavorable. One potential therapeutic approach is the use of [225Ac]Ac-PSMA-617, a targeted alpha therapy (TAT) that administers alpha-particle radiation specifically to prostate cancer cells expressing PSMA. In this study, we report the long-term survival outcomes of this novel therapy in a series of patients with mCRPC who have exhausted all standard treatment options.

METHODS

The study enrolled patients with mCRPC who had shown resistance to standard lines of therapies, including next-generation anti-androgen therapies and taxane-based chemotherapies. These eligible patients received treatment with [225Ac]Ac-PSMA-617 at 100-150 kBq/kg doses administered every 8 weeks. The primary objective of the study was to assess overall survival (OS), while secondary objectives included evaluating radiological progression-free survival (rPFS), monitoring serum prostate-specific antigen (PSA) levels as a measure of biochemical response, and assessing adverse events using the CTCAE v5.0 grading system.

RESULTS

Among the 63 initially enrolled patients, a total of 56 patients who had completed at least two cycles of [225Ac]Ac-PSMA-617 were included in this study. The mean age was 67 years (range, 39-87) and patients received a total of 204 cycles of [225Ac]Ac-PSMA-617 TAT. 91% of patients exhibited any PSA decline, with 67.8% experiencing a decline of 50% or more. The median follow-up was of 22 months (range: 6-59 months). Imaging-based disease progression was observed in 68% of patients, and 66% of patients succumbed to the disease. The median OS was 15 months (95% CI: 10-19). In univariate analysis, factors such as lack of >50% PSA decline (P=0.031), Eastern Cooperative Oncology Group (ECOG) performance status of 2 or higher (P=0.048), and radiological progression (rPD) (P<0.001) were found to be predictors of poor OS. However, in multivariate analysis, only rPD emerged as an independent prognostic factor with a hazard ratio (HR) of 8.264 (95% CI: 1.429-16.497, P=0.004). The estimated median rPFS was 9 months (95% CI: 7-15). Moreover, patients who demonstrated any PSA decline had a median rPFS of 10 months compared to only 3 months in patients without any PSA decline (multivariate HR: 6.749; 95% CI: 1.949-23.370; P=0.002). Fatigue was one of the most common treatment-emergent adverse events, with grades 1/2 occurring in 70% of patients and grades 3 or higher in 3.5% of patients. This fatigue was transient and resolved before the next treatment cycle. Additionally, approximately one-third of patients experienced xerostomia (grades 1/2: 32.1%).

CONCLUSION

[225Ac]Ac-PSMA-617 targeted alpha therapy, was found to be well-tolerated with acceptable adverse events and effective in the treatment of patients with end-stage mCRPC.

Peptide-drug conjugates (PDCs): a novel trend of research and development on targeted therapy, hype or hope?

Peptide-drug conjugates (PDCs) are the next generation of targeted therapeutics drug after antibody-drug conjugates (ADCs), with the core benefits of enhanced cellular permeability and improved drug selectivity. Two drugs are now approved for market by US Food and Drug Administration (FDA), and in the last two years, the pharmaceutical companies have been developing PDCs as targeted therapeutic candidates for cancer, coronavirus disease 2019 (COVID-19), metabolic diseases, and so on. The therapeutic benefits of PDCs are significant, but poor stability, low bioactivity, long research and development time, and slow clinical development process as therapeutic agents of PDC, how can we design PDCs more effectively and what is the future direction of PDCs? This review summarises the components and functions of PDCs for therapeutic, from drug target screening and PDC design improvement strategies to clinical applications to improve the permeability, targeting, and stability of the various components of PDCs. This holds great promise for the future of PDCs, such as bicyclic peptide‒toxin coupling or supramolecular nanostructures for peptide-conjugated drugs. The mode of drug delivery is determined according to the PDC design and current clinical trials are summarised. The way is shown for future PDC development.

Extended therapy with [177Lu]Lu-PSMA-617 in responding patients with high-volume metastatic castration-resistant prostate cancer

PURPOSE

The currently used scheme for radioligand therapy (RLT) of patients with metastatic castration-resistant prostate cancer (mCRPC) consists of 4-6 cycles of 6.0-7.4 GBq [¹⁷⁷Lu]Lu-PSMA-617 each. This standard treatment scheme has proved safe and effective resulting in objective response in most patients with no significant toxicity. Many patients, however, show high-volume residual tumor burden after the sixth cycle and may benefit from treatment continuation. Extended treatment with additional cycles has been withheld due to concerns on potential increased toxicity.

METHODS

Twenty-six patients with high-volume residual tumor burden (according to CHAARTED) after standard RLT with [¹⁷⁷Lu]Lu-PSMA-617 and no alternative treatment option received additional RLT cycles reaching a median of 10 (range 7-16) cycles with a mean activity of 7.4 ± 0.9 GBq per cycle. Response assessment with [⁶⁸Ga]Ga-PSMA-11 PET/CT was done every 2-3 cycles or if disease progression was clinically suspected or based on change in PSA value (according to the PCWG3 criteria). Toxicity was measured using routine blood work up including blood counts, liver and renal function, and was graded according to CTCAE v5.0 criteria. Survival outcome was calculated based on the Kaplan-Meier method.

RESULTS

Further PSA decline of 33 ± 28% during the extended treatment was observed in 21/26 (81%) patients, whereas 5/26 (19%) patients showed a PSA increase; correspondingly in 11/21 patients with an initial response (PR or SD) to extended cycles, treatment was discontinued due to progressive disease, whereas six (23%) patients achieved low-volume residual disease. Two (8%) patients died without showing progression, and two (8%) patients are still under therapy. The median progression-free survival was 19 (95% CI: 15-23) months, and the overall survival was 29 (95% CI: 18-40) months. Grade ≥ 3 hematological toxicities occurred in 4/26 (15%) patients during treatment extension, and nephrotoxicity (grade ≥ 3) was observed in 1/26 (4%) patient during the follow-up.

CONCLUSION

Extended radioligand therapy is a feasible treatment option in patients with high-volume residual tumor after the completion of standard treatment with six cycles of [¹⁷⁷Lu]Lu-PSMA-617. Improved survival and the acceptable safety profile warrant further investigation of the concept of additional cycles in selected patients.