Extensive preclinical evaluation of lutetium-177-labeled PSMA-specific tracers for prostate cancer radionuclide therapy

Towards a 'clicked' PSMA targeting gene delivery bioconjugate-polyplex for prostate cancer

Prostate cancer is the most common cancer in men in the UK with over 50 000 new cases diagnosed each year and although therapeutic advances in surgery, anti-androgens, radio- and chemotherapy have increased survival rates, there still remains a need for new treatments to combat the most aggressive forms of the disease. Gene therapy offers promise as an alternative approach but is reliant on selective targeting to the cancer cell surface. Herein we describe the novel construction of a prostate specific membrane antigen (PSMA) binding bioconjugate-polyplex, based on a glutamate-urea peptide scaffold using 'click' chemistry, which we demonstrate is capable of targeted delivery of a GFP gene to PSMA overexpressing prostate cancer cells, and therefore may have potential future application as part of a prostate cancer gene delivery therapy.

Preclinical evaluation of CXCR4 peptides for targeted radionuclide therapy in glioblastoma

BACKGROUND

Glioblastoma (GBM), is the most fatal form of brain cancer, with a high tendency for recurrence despite combined treatments including surgery, radiotherapy, and chemotherapy with temozolomide. The C-X-C chemokine receptor 4 (CXCR4) plays an important role in tumour radioresistance and recurrence, and is considered as an interesting GBM target. TRT holds untapped potential for GBM treatment, with CXCR4-TRT being a promising strategy for recurrent GBM treatment. Our study focuses on the preclinical assessment of different 177Lu-labelled CXCR4-targeting peptides, CTCE-9908, DV1-K-DV3, and POL3026 for GBM treatment and exploring some of the radiobiological mechanisms underlying these therapies.

RESULTS

All three DOTA-conjugated peptides could be radiolabelled with 177Lu with > 95% radiochemical yield. Binding studies show high specific binding of [177Lu]Lu-DOTA-POL3026 to U87-CXCR4 + cells, with 42% of the added activity binding to the membrane at 1 nM, and 6.5% internalised into the cells. In the presence of the heterologous CXCR4 blocking agent, AMD11070, membrane binding was reduced by 95%, a result confirmed by quantitative in vitro autoradiography of orthotopic GBM xenograft sections. An activity-dependent decrease in cell viability was observed for [177Lu]Lu-DOTA-DV1-K-DV3 and [177Lu]Lu-DOTA-POL3026, along with a slight increase in the induction of apoptotic markers. Additionally, the expression of γH2AX increased in a time-and activity-dependent manner. Ex vivo biodistribution studies with [177Lu]Lu-DOTA-POL3026 show uptake in the tumour reaching a SUV of 1.9 at 24 h post-injection, with higher uptake in the kidneys, lungs, spleen, and liver. Dosimetry estimations show an absorbed dose of 0.93 Gy/MBq in the tumour. A blocking study with AMD11070 showed a 38% reduction in tumour uptake, with no significant reduction observed in µSPECT imaging. Although no brain uptake was observed in the ex vivo biodistribution study, autoradiography on U87-CXCR4 + tumour inoculated mouse brain slices shows non-specific binding in the brain, next to high specific binding to the tumour.

CONCLUSIONS

In conclusion, we compared different 177Lu-radiolabelled CXCR4-targeting peptides for their binding potential in GBM, and demonstrated their varied cytotoxic action against GBM cells in vitro, with POL3026 being the most promising, causing considerable DNA damage. Though the peptide's systemic biodistribution remains to be improved, our data demonstrate the potential of [177Lu]Lu-DOTA-POL3026 for CXCR4-TRT in the context of GBM.

A Phase 0 Study to Assess the Biodistribution and Pharmacokinetics of a Radiolabeled Antibody Targeting Human Kallikrein 2 in Participants with Metastatic Castration-Resistant Prostate Cancer

Despite the inclusion of multiple agents within the prostate cancer treatment landscape, new treatment options are needed to address the unmet need for patients with metastatic castration-resistant prostate cancer (mCRPC). Although prostate-specific membrane antigen is the only cell-surface target to yield clinical benefit in men with advanced prostate cancer, additional targets may further advance targeted immune, cytotoxic, radiopharmaceutical, and other tumor-directed therapies for these patients. Human kallikrein 2 (hK2) is a novel prostate-specific target with little to no expression in nonprostate tissues. This first-in-human phase 0 trial uses an 111In-radiolabeled anti-hK2 monoclonal antibody, [111In]-DOTA-h11B6, to credential hK2 as a potential target for prostate cancer treatment. Methods: Participants with progressive mCRPC received a single infusion of 2 mg of [111In]-DOTA-h11B6 (185 MBq of 111In), with or without 8 mg of unlabeled h11B6 to assess antibody mass effects. Sequential imaging and serial blood samples were collected to determine [111In]-DOTA-h11B6 biodistribution, dosimetry, serum radioactivity, and pharmacokinetics. Safety was assessed within a 2-wk follow-up period from the time of [111In]-DOTA-h11B6 administration. Results: Twenty-two participants received [111In]-DOTA-h11B6 and are included in this analysis. Within 6-8 d of administration, [111In]-DOTA-h11B6 visibly accumulated in known mCRPC lesions, with limited uptake in other organs. Two treatment-emergent adverse events unrelated to treatment occurred, including tumor-related bleeding in 1 patient, which led to early study discontinuation. Serum clearance, biodistribution, and tumor targeting were independent of total antibody mass (2 or 10 mg). Conclusion: This first-in-human study demonstrates that tumor-associated hK2 can be identified and targeted using h11B6 as a platform as the h11B6 antibody selectively accumulated in mCRPC metastases with mass-independent clearance kinetics. These data support the feasibility of hK2 as a target for imaging and hK2-directed agents as potential therapies in patients with mCRPC.

Prostate-Specific Membrane Antigen Targeted StarPEG Nanocarrier for Imaging and Therapy of Prostate Cancer

The tumor uptake of large non-targeted nanocarriers primarily occurs through passive extravasation, known as the enhanced permeability and retention (EPR) effect. Prior studies demonstrated improved tumor uptake and retention of 4-arm 40 kDa star polyethylene glycol (StarPEG) polymers for cancer imaging by adding prostate-specific membrane antigen (PSMA) targeting small molecule ligands. To test PSMA-targeted delivery and therapeutic efficacy, StarPEG nanodrugs with/without three copies of PSMA-targeting ligands, ACUPA, are designed and synthesized. For single-photon emission computed tomography (SPECT) imaging and therapy, each nanocarrier is labeled with 177Lu using DOTA radiometal chelator. The radiolabeled nanodrugs, [177Lu]PEG-(DOTA)1 and [177Lu]PEG-(DOTA)1(ACUPA)3, are evaluated in vitro and in vivo using PSMA+ PC3-Pip and/or PSMA- PC3-Flu cell lines, subcutaneous xenografts and disseminated metastatic models. The nanocarriers are efficiently radiolabeled with 177Lu with molar activities 10.8-15.8 MBq/nmol. Besides excellent in vitro PSMA binding affinity (kD = 51.7 nM), the targeted nanocarrier, [177Lu]PEG-(DOTA)1(ACUPA)3, demonstrated excellent in vivo SPECT imaging contrast with 21.3% ID/g PC3-Pip tumors uptake at 192 h. Single doses of 18.5 MBq [177Lu]PEG-(DOTA)1(ACUPA)3 showed complete resolution of the PC3-Pip xenografts observed up to 138 days. Along with PSMA-targeted excellent imaging contrast, these results demonstrated high treatment efficacy of [177Lu]PEG-(DOTA)1(ACUPA)3 for prostate cancer, with potential for clinical translation.

Automated radiosynthesis and preclinical evaluation of two new PSMA-617 derivatives radiolabelled via [18F]AlF2+ method

BACKGROUND

In the last decade the development of new PSMA-ligand based radiopharmaceuticals for the imaging and therapy of prostate cancer has been a highly active and important area of research. The most promising derivative in terms of interaction with the antigen and clinical properties has been found to be "PSMA-617", and its lutetium-177 radiolabelled version has recently been approved by EU and USA regulatory agencies for therapeutic purposes. For the above reasons, the development of new derivatives of PSMA-617 radiolabelled with fluorine-18 may still be of great interest. This paper proposes the comparison of two different PSMA-617 derivatives functionalized with NODA and RESCA chelators, respectively, radiolabelled via [18F]AlF2+ complexation.

RESULTS

The organic synthesis of two PSMA-617 derivatives and their radiolabelling via [18F]AlF2+ complexation resulted to proceed efficiently and successfully. Moreover, stability in solution and in plasma has been evaluated. The whole radiosynthesis procedure has been fully automated, and the final products have been obtained with radiochemical yield and purity potentially suitable for clinical studies. The biodistribution of the two derivatives was performed both in prostate cancer and glioma tumour models. Compared with the reference [18F]F-PSMA-1007 and [18F]F-PSMA-617-RESCA, [18F]F-PSMA-617-NODA derivative showed a higher uptake in both tumors, faster clearance in non-target organs, and lower uptake in salivary glands.

CONCLUSION

PSMA-617 NODA and RESCA derivatives were radiolabelled successfully via [18F]AlF2+ chelation, the former being more stable in solution and human plasma. Moreover, preclinical biodistribution studies showed that [18F]F-PSMA-617-NODA might be of potential interest for clinical applications.

Evaluation of Prostate-Specific Membrane Antigen (PSMA) Immunohistochemical Expression in Early-Stage Breast Cancer Subtypes

Breast cancer, known for its diverse subtypes, ranks as one of the leading causes of cancer-related deaths. Prostate-specific membrane antigen (PSMA), primarily associated with prostate cancer, has also been identified in breast cancer, though its role remains unclear. This study aimed to evaluate PSMA expression across different subtypes of early-stage breast cancer and investigate its correlation with clinicopathological factors. This retrospective study included 98 breast cancer cases. PSMA expression was examined in both tumor cells and tumor-associated blood vessels. The analysis revealed PSMA expression in tumor-associated blood vessels in 88 cases and in tumor cells in 75 cases. Ki67 expression correlated positively with PSMA expression in blood vessels (p < 0.0001, RSpearman 0.42) and tumor cells (p = 0.010, RSpearman 0.26). The estrogen and progesterone receptor expression correlated negatively with PSMA levels in blood vessels (p = 0.0053, R Spearman -0.26 and p = 0.00026, R Spearman -0.347, respectively). Human epidermal growth factor receptor 2 (HER2) status did not significantly impact PSMA expression. We did not detect any statistically significant differences between breast cancer subtypes. These findings provide evidence for a heterogenous PSMA expression in breast cancer tissue and suggest its correlation with tumor aggressiveness. Despite the limited sample size, the study provides valuable insights into the potential of PSMA as a prognostic, diagnostic, and therapeutic target in the management of breast cancer.

DNA-encoded chemical libraries enable the discovery of potent PSMA-ligands with substantially reduced affinity towards the GCPIII anti-target

Prostate-specific membrane antigen (PSMA) is a tumor-associated protein that has been successfully targeted with small organic ligands and monoclonal antibodies. Pluvicto™ is a PSMA-targeted radioligand therapeutic (RLT) recently approved by the FDA for the treatment of metastatic castration-resistant prostate cancer (2022 FDA marketing authorization). Although a large Phase III clinical trial (VISION trial) demonstrated clinical benefits in patients treated with Pluvicto™, the therapeutic window of the drug is narrowed by its undesired accumulation in healthy organs. Glutamate carboxypeptidase III (GCPIII), an enzyme sharing 70% identity with PSMA, may be responsible for the off-target accumulation of PSMA-RLTs in salivary glands and kidneys. In this work, we designed and synthesized affinity and selectivity maturation DNA-encoded chemical libraries (ASM-DELs) comprising 18'284'658 compounds that were screened in parallel against PSMA and GCPIII with the aim to identify potent and selective PSMA ligands for tumor-targeting applications. Compound A70-B104 was isolated as the most potent and selective ligand (KD of 900 pM for PSMA, KD of 40 nM for GCPIII). 177Lu-A70-B104-DOTA, a radiolabeled derivative of compound A70-B104, presented selective accumulation in PSMA-positive cancer lesions (i.e., 7.4% ID g-1, 2 hour time point) after systemic administration in tumor-bearing mice. The results of autoradiography experiments showed that 177Lu-A70-B104-DOTA selectively binds to PSMA-positive cancer tissues, while negligible binding on human salivary glands was observed.

Prostate-Specific Membrane Antigen-Targeted Therapy in Prostate Cancer: History, Combination Therapies, Trials, and Future Perspective

In the last decades, the development of PET/CT radiopharmaceuticals, targeting the Prostate-Specific Membrane Antigen (PSMA), changed the management of prostate cancer (PCa) patients thanks to its higher diagnostic accuracy in comparison with conventional imaging both in staging and in recurrence. Alongside molecular imaging, PSMA was studied as a therapeutic agent targeted with various isotopes. In 2021, results from the VISION trial led to the Food and Drug Administration (FDA) approval of [177Lu]Lu-PSMA-617 as a novel therapy for metastatic castration-resistant prostate cancer (mCRPC) and set the basis for a radical change in the future perspectives of PCa treatment and the history of Nuclear Medicine. Despite these promising results, primary resistance in patients treated with single-agent [177Lu]Lu-PSMA-617 remains a real issue. Emerging trials are investigating the use of [177Lu]Lu-PSMA-617 in combination with other PCa therapies in order to cover the multiple oncologic resistance pathways and to overcome tumor heterogeneity. In this review, our aim is to retrace the history of PSMA-targeted therapy from the first preclinical studies to its future applications in PCa.

Quantification of biochemical PSA dynamics after radioligand therapy with [177Lu]Lu-PSMA-I&T using a population pharmacokinetic/pharmacodynamic model

BACKGROUND

There is an unmet need for prediction of treatment outcome or patient selection for [177Lu]Lu-PSMA therapy in patients with metastatic castration-resistant prostate cancer (mCRPC). Quantification of the tumor exposure-response relationship is pivotal for further treatment optimization. Therefore, a population pharmacokinetic (PK) model was developed for [177Lu]Lu-PSMA-I&T using SPECT/CT data and, subsequently, related to prostate-specific antigen (PSA) dynamics after therapy in patients with mCRPC using a pharmacokinetic/pharmacodynamic (PKPD) modelling approach.

METHODS

A population PK model was developed using quantitative SPECT/CT data (406 scans) of 76 patients who received multiple cycles [177Lu]Lu-PSMA-I&T (± 7.4 GBq with either two- or six-week interval). The PK model consisted of five compartments; central, salivary glands, kidneys, tumors and combined remaining tissues. Covariates (tumor volume, renal function and cycle number) were tested to explain inter-individual variability on uptake into organs and tumors. The final PK model was expanded with a PD compartment (sequential fitting approach) representing PSA dynamics during and after treatment. To explore the presence of a exposure-response relationship, individually estimated [177Lu]Lu-PSMA-I&T tumor concentrations were related to PSA changes over time.

RESULTS

The population PK model adequately described observed data in all compartments (based on visual inspection of goodness-of-fit plots) with adequate precision of parameters estimates (< 36.1% relative standard error (RSE)). A significant declining uptake in tumors (k14) during later cycles was identified (uptake decreased to 73%, 50% and 44% in cycle 2, 3 and 4-7, respectively, compared to cycle 1). Tumor growth (defined by PSA increase) was described with an exponential growth rate (0.000408 h-1 (14.2% RSE)). Therapy-induced PSA decrease was related to estimated tumor concentrations (MBq/L) using both a direct and delayed drug effect. The final model adequately captured individual PSA concentrations after treatment (based on goodness-of-fit plots). Simulation based on the final PKPD model showed no evident differences in response for the two different dosing regimens currently used.

CONCLUSIONS

Our population PK model accurately described observed [177Lu]Lu-PSMA-I&T uptake in salivary glands, kidneys and tumors and revealed a clear declining tumor uptake over treatment cycles. The PKPD model adequately captured individual PSA observations and identified population response rates for the two dosing regimens. Hence, a PKPD modelling approach can guide prediction of treatment response and thus identify patients in whom radioligand therapy is likely to fail.

Human ABC and SLC Transporters: The Culprit Responsible for Unspecific PSMA-617 Uptake?

[177Lu]Lu-PSMA-617 has recently been successfully approved by the FDA, the MHRA, Health Canada and the EMA as Pluvicto®. However, salivary gland (SG) and kidney toxicities account for its main dose-limiting side-effects, while its corresponding uptake and retention mechanisms still remain elusive. Recently, the presence of different ATP-binding cassette (ABC) transporters, such as human breast cancer resistance proteins (BCRP), multidrug resistance proteins (MDR1), multidrug-resistance-related proteins (MRP1, MRP4) and solute cassette (SLC) transporters, such as multidrug and toxin extrusion proteins (MATE1, MATE2-K), organic anion transporters (OAT1, OAT2v1, OAT3, OAT4) and peptide transporters (PEPT2), has been verified at different abundances in human SGs and kidneys. Therefore, our aim was to assess whether [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are substrates of these ABC and SLC transporters. For in vitro studies, the novel isotopologue ([α,β-3H]Nal)Lu-PSMA-617 was used in cell lines or vesicles expressing the aforementioned human ABC and SLC transporters for inhibition and uptake studies, respectively. The corresponding probe substrates and reference inhibitors were used as controls. Our results indicate that [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 are neither inhibitors nor substrates of the examined transporters. Therefore, our results show that human ABC and SLC transporters play no central role in the uptake and retention of [177Lu]Lu-PSMA-617 and [225Ac]Ac-PSMA-617 in the SGs and kidneys nor in the observed toxicities.

SENTRI: Single-Particle Energy Transducer for Radionuclide Injections for Personalized Targeted Radionuclide Cancer Therapy

PURPOSE

Targeted radionuclide therapy (TRT), whereby a tumor-targeted molecule is linked to a therapeutic beta- or alpha-emitting radioactive nuclide, is a promising treatment modality for patients with metastatic cancer, delivering radiation systemically. However, patients still progress due to suboptimal dosing, driven by the large patient-to-patient variability. Therefore, the ability to continuously monitor the real-time dose deposition in tumors and organs at risk provides an additional dimension of information during clinical trials that can enable insights into better strategies to personalize TRT.

METHODS AND MATERIALS

Here, we present a single beta-particle sensitive dosimeter consisting of a 0.27-mm3 monolithic silicon chiplet directly implanted into the tumor. To maximize the sensitivity and have enough detection area, minimum-size diodes (1 μm2) are arrayed in 64 × 64. Signal amplifiers, buffers, and on-chip memories are all integrated in the chip. For verification, PC3-PIP (prostate-specific membrane antigen [PSMA]+) and PC3-flu (PSMA-) cell lines are injected into the left and right flanks of the mice, respectively. The devices are inserted into each tumor and measure activities at 5 different time points (0-2 hours, 7-9 hours, 12-14 hours, 24-26 hours, and 48-50 hours) after 177Lu-PSMA-617 injections. Single-photon emission computed tomography/computed tomography scans are used to verify measured data.

RESULTS

With a wide detection range from 0.013 to 8.95 MBq/mL, the system is capable of detecting high tumor uptake as well as low doses delivered to organs at risk in real time. The measurement data are highly proportional (R2 > 0.99) to the 177Lu-PSMA-617 activity. The in vivo measurement data agree well with the single-photon emission computed tomography/computed tomography results within acceptable errors (±1.5%ID/mL).

CONCLUSIONS

Given the recent advances in clinical use of TRT in prostate cancer, the proposed system is verified in a prostate cancer mouse model using 177Lu-PSMA-617.

Advancements in PSMA ligand radiolabeling for diagnosis and treatment of prostate cancer: a systematic review

Prostate cancer(PCa), a leading global health concern, profoundly impacts millions of men worldwide. Progressing through two stages, it initially develops within the prostate and subsequently extends to vital organs such as lymph nodes, bones, lungs, and the liver. In the early phases, castration therapy is often employed to mitigate androgen effects. However, when prostate cancer becomes resistant to this treatment, alternative strategies become imperative. As diagnostic and treatment methodologies for prostate cancer continually advance, radioligand therapy (RLT) has emerged as a promising avenue, yielding noteworthy outcomes. The fundamental principle of RLT involves delivering radionuclide drugs to cancerous lesions through specific carriers or technologies. Subsequently, these radionuclide drugs release radioactive energy, facilitating the destruction of cancer cell tissues. At present, the positron emission tomography (PET) targeting PSMA has been widely developed for the use of diagnosis and staging of PCa. Notably, FDA-approved prostate-specific membrane antigen (PSMA) targeting agents, such as 68Ga-PSMA-11 and 177Lu-PSMA-617, represent significant milestones in enhancing diagnostic precision and therapeutic efficacy. This review emphasizes the current research status and outcomes of various radionuclide-labeled PSMA ligands. The objective is to provide valuable insights for the continued advancement of diagnostic and therapeutic approaches in the realm of prostate cancer.

Low cost, high temporal resolution optical fiber-based γ-photon sensor for real-time pre-clinical evaluation of cancer-targeting radiopharmaceuticals

Cancer radiopharmaceutical therapies (RPTs) have demonstrated great promise in the treatment of neuroendocrine and prostate cancer, giving hope to late-stage metastatic cancer patients with currently very few treatment options. These therapies have sparked a large amount of interest in pre-clinical research due to their ability to target metastatic disease, with many research efforts focused towards developing and evaluating targeted RPTs for different cancer types in in vivo models. Here we describe a method for monitoring real-time in vivo binding kinetics for the pre-clinical evaluation of cancer RPTs. Recognizing the significant heterogeneity in biodistribution of RPTs among even genetically identical animal models, this approach offers long-term monitoring of the same in vivo organism without euthanasia in contrast to ex vivo tissue dosimetry, while providing high temporal resolution with a low-cost, easily assembled platform, that is not present in small-animal SPECT/CTs. The method utilizes the developed optical fiber-based γ-photon biosensor, characterized to have a wide linear dynamic range with Lutetium-177 (177Lu) activity (0.5-500 μCi/mL), a common radioisotope used in cancer RPT. The probe's ability to track in vivo uptake relative to SPECT/CT and ex vivo dosimetry techniques was verified by administering 177Lu-PSMA-617 to mouse models bearing human prostate cancer tumors (PC3-PIP, PC3-flu). With this method for monitoring RPT uptake, it is possible to evaluate changes in tissue uptake at temporal resolutions <1 min to determine RPT biodistribution in pre-clinical models and better understand dose relationships with tumor ablation, toxicity, and recurrence when attempting to move therapies towards clinical trial validation.

The Potential Contribution of Radiopharmaceutical Therapies in Managing Oligometastatic Disease

There is a growing understanding of the oligometastatic disease state, characterized by the presence of 5 or fewer lesions. Advanced molecular imaging techniques, such as prostate-specific membrane antigen PET, refines the ability to detect oligometastatic recurrences (oligorecurrences) early. These developments have led to the exploration of metastasis-directed therapy (MDT) in oligorecurrent disease as an alternative to or as a means of delaying systemic therapy. Unfortunately, MDT often does not provide a durable cure, and progression-particularly progression in multiple new areas-remains a concern. Simultaneously, developments in radioligand therapy (RLT) have led to studies showing overall survival benefits with α-emitting and β-emitting RLT in advanced, high-volume, metastatic castration-resistant prostate cancer. The success of RLT in late-stage disease suggests that earlier use in the disease spectrum may be impactful. Specifically, integration of RLT with MDT might reduce progression, including polymetastatic progression, in the setting of oligorecurrent disease.

177Lu-Labeled Bivalent Ligands of Prostate-Specific Membrane Antigen for Endoradiotherapy of Prostate Cancer

Recently, we developed a bivalent prostate-specific membrane antigen (PSMA) radioligand ([18F]AlF-Bi-PSMA), which showed higher tumor uptake and retention in PSMA-positive mouse models than the clinically used radioligands, [68Ga]Ga-PSMA-11 and [18F]PSMA-1007. Here, we developed two 177Lu-labeled bivalent PSMA ligands with (DOTA-Alb-Bi-PSMA) or without an albumin-binding motif (DOTA-Bi-PSMA) to enhance radiotherapeutic efficacy with minimal toxicity. The results demonstrated that both 177Lu-labeled bivalent radioligands showed good stability, high binding affinity, and PSMA-targeting specificity in vitro. Compared with [177Lu]Lu-PSMA-617, both [177Lu]Lu-Bi-PSMA and [177Lu]Lu-Alb-Bi-PSMA showed a higher area under the curve (AUC) of tumor accumulation and superior therapeutic efficacy. However, [177Lu]Lu-Alb-Bi-PSMA exhibited a dose-dependent increase in acute damage to kidneys. In terms of the radionuclide therapy efficacy and side effects, [177Lu]Lu-Bi-PSMA exhibited well-balanced action with high tumor-to-organs AUC ratios, resulting in remarkable therapeutic efficacy and negligible side effects. These promising results warrant further investigations to achieve the clinical translation of [177Lu]Lu-Bi-PSMA.

Screening and Preclinical Evaluation of Novel Radiolabeled Anti-Fibroblast Activation Protein-α Recombinant Antibodies

Background: Fibroblast activation protein-α (FAPα) is selectively overexpressed in tumor-associated fibroblasts in more than 90% of epithelial tumors, and may be a good target for anticancer treatment, for example, using an anti-FAPα recombinant antibody (rAb) labeled with radionuclides. In the present report, the radiolabeling and preclinical evaluation of novel anti-FAPα rAbs were investigated. Materials and Methods: Two novel anti-FAPα VHHs (AMS002-1 and AMS002-2) with high binding affinity to FAPα were selected from an antibody phage library. The anti-FAPα VHHs were then fused with the Fc fragment of human IgG4 to create two VHH-Fc rAbs. The VHH-Fc rAbs were radiolabeled with 89Zr and 177Lu. The radiolabeled products were evaluated by radioligand-binding assays using FAPα-expressing cells. The biodistribution and tumor-targeting properties were investigated by small-animal PET/CT. AMS002-1-Fc, which showed promising tumor-targeting properties in 89Zr-microPET imaging, was radiolabeled with 177Lu for efficacy study on HT1080 tumor-bearing mice and monitored with SPECT/CT imaging. Results: The two VHH-Fc rAbs with good affinity with KD values in low nanomolar range were identified. Both PET/CT imaging with 89Zr-AMS002-1-Fc rAb and SPECT/CT imaging with 177Lu-AMS002-1-Fc rAb demonstrated highest tumor uptakes at 72 h p.i. and long tumor retention in the preclinical models. Furthermore, ex vivo biodistribution analysis revealed high tumor uptake of 89Zr-AMS002-1-Fc at 48 h p.i. with the value of 6.91% ± 2.08% ID/g. Finally, radioimmunotherapy with 177Lu-AMS002-1-Fc rAb delayed the tumor growth without significant weight loss in mice with HT1080 xenografts. The tumor size of untreated control group was 2.59 times larger compared with the treatment group with 177Lu-AMS002-1-Fc at day 29. Conclusion: 89Zr/177Lu-AMS002-1-Fc represent a pair of promising radiopharmaceuticals for theranostics on FAPα-expressing tumors.

Advances in non-radioactive PSMA-targeted small molecule-drug conjugates in the treatment of prostate cancer

Most patients with advanced prostate cancer (PCa) will develop metastatic castration-resistant prostate cancer (mCRPC) after androgen deprivation therapy, at this time the tumor enters the end stage, and the clinical treatment is very complicated, which requires rationalization of drugs to prolong the life of patients while improving their quality of life. Prostate-specific membrane antigen (PSMA) is a promising biological target for drug delivery in mCRPC due to its high level of specific expression in PCa cell membranes and low expression in normal tissues. Non-radioactive PSMA-targeted small molecule-drug conjugates (SMDCs) are gradually becoming a heat of discovery due to their good affinity and specificity; simple synthesis steps and transport management methods. Non-radioactive PSMA-targeted SMDCs under investigation can be divided into two categories: SMDCs and dual-ligand coupled drugs, among which SMDCs are the most widespread form of this type of conjugate. SMDCs have three key components: cytotoxic load, linker, and small molecule targeting ligands. SMDCs are internalized into the cell after binding to PSMA on the cell membrane and stored in endosomes and lysosomes, where they are usually enzymatically cleaved to allow precise release of cytotoxic molecules and uniform diffusion into the tumor tissue. More than a dozen non-radioactive PSMA-targeted SMDCs have been developed, many of which have shown favorable properties in both in vitro and in vivo evaluations, demonstrating more favorable results than unmodified cytotoxic drugs. Therefore, non-radioactive PSMA-targeted SMDCs have great therapeutic potential for mCRPC as a form of targeted therapy.

Radioligand Therapy With 177 Lu-PSMA-I&T in Patients With Metastatic Prostate Cancer : Oncological Outcomes and Toxicity Profile

INTRODUCTION

This study aimed to investigate the oncological outcomes and toxicity profile of 177 Lu-PSMA-I&T radioligand therapy (RLT) in patients with metastatic castration-resistant prostate cancer (mCRPC), as well as our initial experience in metastatic hormone-sensitive prostate cancer (mHSPC).

PATIENTS AND METHODS

A total of 38 consecutive patients with metastatic prostate cancer (33 mCRPC and 5 mHSPC) received 177 Lu-PSMA-I&T RLT, with a median of 2 cycles per patient (range, 1-7). Response to RLT was evaluated based on prostate-specific antigen (PSA) changes and imaging response. Clinical progression-free survival and overall survival were used to report oncological outcomes. Toxicity was assessed using the Common Toxicity Criteria for Adverse Events criteria.

RESULTS

In mCRPC, 22 (69%), 18 (56%), and 11 (34%) patients achieved any PSA decline, PSA response of ≥30%, and PSA response of ≥50%, respectively. The clinical progression-free survival and overall survival after the first cycle of RLT were 6.3 and 21.4 months, respectively. In mHSPC, 177 Lu-PSMA-I&T RLT resulted in excellent PSA response (93.0%-99.9%) in all cases. Clinical progression and cancer-related mortality occurred in only 1 case. Toxicity profile was favorable in both mHSPC and mCRPC.

CONCLUSIONS

177 Lu-PSMA-I&T RLT demonstrated favorable PSA response (≥30%) in over half of the patients with mCRPC and excellent PSA response in all patients with mHSPC. Toxicity profile was favorable in both mHSPC and mCRPC settings. Further studies are needed to evaluate the role of 177 Lu-PSMA-I&T RLT in the management of metastatic prostate cancer.

PSMA-617 inhibits proliferation and potentiates the 177Lu-PSMA-617-induced death of human prostate cancer cells

The human prostate-specific membrane antigen (PSMA) is substantially up-regulated in metastatic prostate cancer (PCa) cells. PSMA can be targeted by 177Lu conjugated to PSMA-617, a high-affinity ligand for the PSMA. The binding of the radioligand, 177Lu-PSMA-617, results in its internalisation and delivery of β-radiation into the cancer cells. However, PSMA-617, a component of the final product in the synthesis of the radioligand, may also play a role in the pathophysiology of PCa cells. The present study aimed to clarify the effects of PSMA-617 (10, 50 and 100 nM) on the expression of PSMA in PSMA-positive LNCaP cells, their proliferation, 177Lu-PSMA-617-induced cell death by WST-1 and lactate dehydrogenase assays, immunohistochemistry, western blotting, immunofluorescence staining and uptake of 177Lu-PSMA-617. PSMA-617 at 100 nM concentration induced cell-growth arrest, down-regulated cyclin D1 and cyclin E1 (by 43 and 36%, respectively) and up-regulated the cyclin-dependent kinase inhibitor p21Waf1/Cip1 (by 48%). Immunofluorescence staining demonstrated reduced content of DNA, pointing to a lower rate of cell division. PSMA-617 (up to 100 nM) did not alter the uptake of 177Lu-PSMA-617 into the LNCaP cells. Interestingly, simultaneous treatment with 177Lu-PSMA-617 and PSMA-617 for 24 and 48 h substantially potentiated the cell-death promoting effects of the radioligand. In conclusion, the combination of impeding tumour cell proliferation by PSMA-617 and its potentiation of the radiation-induced cell death brought about by 177Lu-PSMA-617 in PCa cells may considerably improve the outcome of the radiation therapy with 177Lu-PSMA-617, especially in patients with decreased radiosensitivity of PCa cells to the radioligand.

Albumin-Binding and Conventional PSMA Ligands in Combination with 161Tb: Biodistribution, Dosimetry, and Preclinical Therapy

The favorable decay characteristics of 161Tb attracted the interest of clinicians in using this novel radionuclide for radioligand therapy (RLT). 161Tb decays with a similar half-life to 177Lu, but beyond the emission of β--particles and γ-rays, 161Tb also emits conversion and Auger electrons, which may be particularly effective to eliminate micrometastases. The aim of this study was to compare the dosimetry and therapeutic efficacy of 161Tb and 177Lu in tumor-bearing mice using SibuDAB and PSMA-I&T, which differ in their blood residence time and tumor uptake. Methods: [161Tb]Tb-SibuDAB and [161Tb]Tb-PSMA-I&T were evaluated in vitro and investigated in biodistribution, imaging, and therapy studies using PC-3 PIP tumor-bearing mice. The 177Lu-labeled counterparts served for dose calculations and comparison of therapeutic efficacy. The tolerability of RLT in mice was monitored on the basis of body mass, blood plasma parameters, blood cell counts, and the histology of relevant organs and tissues. Results: The prostate-specific membrane antigen (PSMA)-targeting radioligands, irrespective of whether labeled with 161Tb or 177Lu, showed similar in vitro data and comparable tissue distribution profiles. As a result of the albumin-binding properties, [161Tb]Tb/[177Lu]Lu-SibuDAB had an enhanced blood residence time and higher tumor uptake (62%-69% injected activity per gram at 24 h after injection) than [161Tb]Tb/[177Lu]Lu-PSMA-I&T (30%-35% injected activity per gram at 24 h after injection). [161Tb]Tb-SibuDAB inhibited tumor growth more effectively than [161Tb]Tb-PSMA-I&T, as can be ascribed to its 4-fold increased absorbed tumor dose. At any of the applied activities, the 161Tb-based radioligands were therapeutically more effective than their 177Lu-labeled counterparts, as agreed with the approximately 40% increased tumor dose of 161Tb compared with that of 177Lu. Under the given experimental conditions, no obvious adverse events were observed. Conclusion: The data of this study indicate the promising potential of 161Tb in combination with SibuDAB for RLT of prostate cancer. Future clinical studies using 161Tb-based RLT will shed light on a potential clinical benefit of 161Tb over 177Lu.

Prognostic role of early prostate specific antigen changes after [177 Lu]Lu-PSMA radioligand therapy of metastasized prostate cancer: A meta-analysis

BACKGROUND

Approximately 10%-20% of prostate cancers progress to metastatic and castration-resistant forms (mCRPC). Radioligand (RLT) therapy with [177 Lu]Lu-prostate-specific membrane antigen (PSMA) is an emerging treatment for metastasized mCRPC and its efficacy is assessed not only but also by prostate specific antigen (PSA) measurement after 12 weeks or more after treatment. Our aim was to evaluate the role of early PSA measurement after RLT in predicting overall survival (OS) of mCRPC patients.

METHODS

A systematic search on PubMed, Web Of Science and Scopus was performed from January to December 2022. PRISMA guidelines for prognostic studies was adopted. Risk of bias was assessed using quality of prognostic studies (QUIPS).

RESULTS

Twelve studies at low-intermediate risk of bias, were included in the meta-analysis (1646 patients, mean age 70 years). About 50% of patients showed a PSA decline after 1-2 of [177 Lu]Lu-PSMA, and more than 30% reported a PSA decline ≥50%. The median OS range for patients with any PSA decline was 13-20 months, while for patients with stable or increased PSA, the median OS fell to 6-12 months. The OS rate for a PSA decline after the one-two [177 Lu]Lu-PSMA cycles was 0.39 (95% CI: 0.31-0.50), while OS for a PSA decline ≥50% was 0.69 (95% CI: 0.57-0.83).

CONCLUSIONS

A PSA decline is observed in almost 50% of mCRPC patients after 1-2 [177 Lu]Lu-PSMA cycles, with a significantly longer OS compared to stable or increased PSA levels, respectively. Accordingly, any PSA decline after 1-2 cycles of therapy should be regarded as a favourable prognostic factor for OS.

Preclinical Development in Radiopharmaceutical Therapy for Prostate Cancer

Prostate cancer is a leading cause of cancer death in men worldwide. Among the various treatment options, radiopharmaceutical therapy has shown notable success in metastatic, castration-resistant disease. Radiopharmaceutical therapy is a systemic approach that delivers cytotoxic radiation doses precisely to the malignant tumors and/or tumor microenvironment. Therapeutic radiopharmaceuticals are composed of a therapeutic radionuclide and a high-affinity, tumor-targeting carrier molecule. Therapeutic radionuclides used in preclinical prostate cancer studies are primarily α-, β--, or Auger-electron-emitting radiometals or radiohalogens. Monoclonal antibodies, antibody-derived fragments, peptides, and small molecules are frequently used as tumor-targeting molecules. Over the years, several important membrane-associated proteases and receptors have been identified, validated, and subsequently used for preclinical radiotherapeutic development for prostate cancer. Prostate-specific membrane antigen (PSMA) is the most well-studied prostate cancer-associated protease in preclinical literature. PSMA-targeting radiotherapeutic agents are being investigated using high-affinity antibody- and small-molecule-based agents for safety and efficacy. Early generations of such agents were developed simply by replacing radionuclides of the imaging agents with therapeutic ones. Later, extensive structure-activity relationship studies were conducted to address the safety and efficacy issues obtained from initial patient data. Recent regulatory approval of the 177Lu-labeled low-molecular-weight agent, 177Lu-PSMA-617, is a significant accomplishment. Current preclinical experiments are focused on the structural modification of 177Lu-PSMA-617 and relevant investigational agents to increase tumor targeting and reduce off-target binding and toxicity in healthy organs. While lutetium-177 (177Lu) remains the most widely used radionuclide, radiolabeled analogs with iodine-131 (128I), yttrium-90 (89Y), copper-67 (67Cu), and terbium-161 (161Tb) have been evaluated as potential alternatives in recent years. In addition, agents carrying the α-particle-emitting radiohalogen, astatine-211 (211At), or radiometals, actinium-225 (225Ac), lead-212 (212Pb), radium-223 (223Ra), and thorium-227 (227Th), have been increasingly investigated in preclinical research. Besides PSMA-based radiotherapeutics, other prominent prostate cancer-related proteases, for example, human kallikrein peptidases (HK2 and HK3), have been explored using monoclonal-antibody-(mAb)-based targeting platforms. Several promising mAbs targeting receptors overexpressed on the different stages of prostate cancer have also been developed for radiopharmaceutical therapy, for example, Delta-like ligand 3 (DLL-3), CD46, and CUB domain-containing protein 1 (CDCP1). Progress is also being made using peptide-based targeting platforms for the gastrin-releasing peptide receptor (GRPR), a well-established membrane-associated receptor expressed in localized and metastatic prostate cancers. Furthermore, mechanism-driven combination therapies appear to be a burgeoning area in the context of preclinical prostate cancer radiotherapeutics. Here, we review the current developments related to the preclinical radiopharmaceutical therapy of prostate cancer. These are summarized in two major topics: (1) therapeutic radionuclides and (2) tumor-targeting approaches using monoclonal antibodies, small molecules, and peptides.

Synthesis and Evaluation of ePSMA-DM1: A New Theranostic Small-Molecule Drug Conjugate (T-SMDC) for Prostate Cancer

Small-molecule drug conjugates (SMDCs) are compounds in which a therapeutic payload is conjugated to a targeting vector, for specific delivery to the tumor site. This promising approach can be translated to the treatment of prostate cancer by selecting a targeting vector which binds to the prostate-specific membrane antigen (PSMA). Moreover, the addition of a bifunctional chelator to the molecule allows for the use of both diagnostic and therapeutic radionuclides. In this way, the distribution of the SMDC in the body can be monitored, and combination therapy regimes can be implemented. We combined a glutamate-urea-lysine vector to the cytotoxic agent DM1 and a DOTA chelator via an optimized linker to obtain the theranostic SMDC (T-SMDC) ePSMA-DM1. ePSMA-DM1 retained a high binding affinity to PSMA and demonstrated PSMA-specific uptake in cells. Glutathione stability assays showed that the half-life of the T-SMDC in a reducing environment was 2 h, and full drug release was obtained after 6 h. Moreover, 100 nM of ePSMA-DM1 reduced the cell viability of the human PSMA-positive LS174T cells by >85% after 72 h of incubation, which was comparable to a 10-fold higher dose of free DM1. [111In]In-ePSMA-DM1 and [177Lu]Lu-ePSMA-DM1 were both obtained in high radiochemical yields and purities (>95%), with >90% stability in PBS and >80% stability in mouse serum for up to 24 h post incubation at 37 °C. SPECT/CT imaging studies allowed for a faint tumor visualization of [111In]In-ePSMA-DM1 at 1 h p.i., and the ex vivo biodistribution showed tumor uptake (2.39 ± 0.29% ID/g) at 1 h p.i., with the compound retained in the tumor for up to 24 h. Therefore, ePSMA-DM1 is a promising T-SMDC candidate for prostate cancer, and the data obtained so far warrant further investigations, such as therapeutic experiments, after further optimization.

PSMA-targeted radioligand therapy in prostate cancer: current status and future prospects

INTRODUCTION

The prostate-specific membrane antigen (PSMA) targeted radioligand therapy (PRLT) for the treatment of metastatic castration-resistant prostate cancer (mCRPC) patients has generated significant interest among the oncologic community, with several publications documenting good response rates and survival benefits with low toxicity profiles.

AREAS COVERED

Indications, patient preparation, dose administration, post-treatment imaging, dosimetry, and side effect profiles of 177Lu-PSMA-617 are discussed in this article. We also discuss results from prospective studies, major retrospective studies, meta-analyses, clinical trials, and mentioned major ongoing clinical trials on PRLT. We have also portrayed our own experiences and future perspectives on PRLT.

EXPERT OPINION

For PRLT, PSMA-617 and PSMA-I&T molecules have revolutionized the theranostic approach in the management of advanced prostate cancer, with solid backing from several published articles showing favorable outcomes and an excellent safety profile of 177Lu-PSMA-617. Improvement in quality of life and survival was seen in the majority of mCRPC patients after 177Lu-PSMA-617 PRLT. Patients with good performance status, asymptomatic, only lymph node metastases, high PSMA expressing lesions, and no discordant FDG avid lesions have a longer survival after 177Lu-PSMA-617 PRLT than patients with poor performance status, symptomatic, hepatic, brain, and skeletal metastases, discordant PSMA, and FDG-avid lesions. Docetaxel and cabazitaxel are approved treatments for mCRPC patients. 177Lu-PSMA-617 is approved as a third-line systemic treatment for mCRPC patients with failure to respond to androgen receptor pathway inhibitors and docetaxel therapy. PRLT is a safe and effective alternative to cabazitaxel (third-line systemic treatment), but it has a higher cost. 177Lu-PSMA-617 could be a more efficient therapeutic option for mCRPC patients as first-line or combined therapy, and it may be a useful therapeutic option for the treatment of metastatic hormone-sensitive prostate cancer (mHSPC) patients. Several clinical studies and clinical trials on PRLT are currently underway. In the future, the results of these trials will be helpful in evolving treatment strategies for prostate cancer patients.

Molecular Design of 68Ga- and 89Zr-Labeled Anticalin Radioligands for PET-Imaging of PSMA-Positive Tumors

Anticalin proteins directed against the prostate-specific membrane antigen (PSMA), optionally having tailored plasma half-life using PASylation technology, show promise as radioligands for PET-imaging of xenograft tumors in mice. To investigate their suitability, the short-circulating unmodified Anticalin was labeled with ⁶⁸Ga (τ_1/2 = 68 min), using the NODAGA chelator, whereas the half-life extended PASylated Anticalin was labeled with ⁸⁹Zr (τ_1/2 = 78 h), using either the linear chelator deferoxamine (Dfo) or a cyclic derivative, fusarinine C (FsC). Different PSMA targeting Anticalin versions (optionally carrying the PASylation sequence) were produced carrying a single exposed N- or C-terminal Cys residue and site-specifically conjugated with the different radiochelators via maleimide chemistry. These protein conjugates were labeled with radioisotopes having distinct physical half-lives and, subsequently, applied for PET-imaging of subcutaneous LNCaP xenograft tumors in CB17 SCID mice. Uptake of the protein tracers into tumor versus healthy tissues was assessed by segmentation of PET data as well as biodistribution analyses. PET-imaging with both the ⁶⁸Ga-labeled plain Anticalin and the ⁸⁹Zr-labeled PASylated Anticalin allowed clear delineation of the xenograft tumor. The radioligand A3A5.1-PAS(200)-FsC·⁸⁹Zr, having an extended plasma half-life, led to a higher tumor uptake 24 h p.i. compared to the ⁶⁸Ga·NODAGA-Anticalin imaged 60 min p.i. (2.5% ID/g vs 1.2% ID/g). Pronounced demetallation was observed for the ⁸⁹Zr·Dfo-labeled PASylated Anticalin, which was ∼50% lower in the case of the cyclic radiochelator FsC (p < 0.0001). Adjusting the plasma half-life of Anticalin radioligands using PASylation technology is a viable approach to increase radioisotope accumulation within the tumor. Furthermore, ⁸⁹Zr-ImmunoPET-imaging using the FsC radiochelator is superior to that using Dfo. Our strategy for the half-life adjustment of a tumor-targeting Anticalin to match the physical half-life of the applied radioisotope illustrates the potential of small binding proteins as an alternative to antibodies for PET-imaging.

Patient-derived xenograft models in cancer therapy: technologies and applications

Patient-derived xenograft (PDX) models, in which tumor tissues from patients are implanted into immunocompromised or humanized mice, have shown superiority in recapitulating the characteristics of cancer, such as the spatial structure of cancer and the intratumor heterogeneity of cancer. Moreover, PDX models retain the genomic features of patients across different stages, subtypes, and diversified treatment backgrounds. Optimized PDX engraftment procedures and modern technologies such as multi-omics and deep learning have enabled a more comprehensive depiction of the PDX molecular landscape and boosted the utilization of PDX models. These irreplaceable advantages make PDX models an ideal choice in cancer treatment studies, such as preclinical trials of novel drugs, validating novel drug combinations, screening drug-sensitive patients, and exploring drug resistance mechanisms. In this review, we gave an overview of the history of PDX models and the process of PDX model establishment. Subsequently, the review presents the strengths and weaknesses of PDX models and highlights the integration of novel technologies in PDX model research. Finally, we delineated the broad application of PDX models in chemotherapy, targeted therapy, immunotherapy, and other novel therapies.

Kidney absorbed radiation doses for [ 177 Lu]Lu-PSMA-617 and [ 177 Lu]Lu-PSMA-I&T determined by 3D clinical dosimetry

PURPOSE

For prostate-specific membrane antigen-directed radioligand therapy (PSMA-RLT), [ 177 Lu]Lu-PSMA-617 and [ 177 Lu]Lu-PSMA-I&T are the currently preferred compounds. Recent preclinical studies suggested ~30x higher kidney absorbed dose for [ 177 Lu]Lu-PSMA-I&T compared to [ 177 Lu]Lu-PSMA-617, which may lead to an increased risk of kidney toxicity. We performed two single-centre, prospective dosimetry studies with either [ 177 Lu]Lu-PSMA-617 or [ 177 Lu]Lu-PSMA-I&T, using an identical dosimetry protocol. We evaluated the absorbed doses of both 177 Lu-labelled radioligands in human kidneys.

METHODS

3D SPECT/computed tomography (CT) imaging of the kidneys was performed after PSMA-RLT in cancer patients with PSMA-positive disease and an adequate glomerular filtration rate (≥50 mL/min). Ten metastatic hormone-sensitive prostate cancer patients (mHSPC) were treated with [ 177 Lu]Lu-PSMA-617 and 10 advanced salivary gland cancer (SGC) patients were treated with [ 177 Lu]Lu-PSMA-I&T. SPECT/CT imaging was performed at five timepoints (1 h, 24 h, 48 h, 72 h, and 168 h post-injection). In mHSPC patients, SPECT/CT imaging was performed after cycles 1 and 2 (cumulative activity: 9 GBq) and in SGC patients only after cycle 1 (activity: 7.4 GBq). Kidney absorbed dose was calculated using organ-based dosimetry.

RESULTS

The median kidney absorbed dose was 0.49 Gy/GBq (range: 0.34-0.66) and 0.73 Gy/GBq (range: 0.42-1.31) for [ 177 Lu]Lu-PSMA-617 and [ 177 Lu]Lu-PSMA-I&T, respectively (independent samples t test; P  = 0.010).

CONCLUSION

This study shows that the kidney absorbed dose for [ 177 Lu]Lu-PSMA-617 and [ 177 Lu]Lu-PSMA-I&T differs, with a ~1.5x higher median kidney absorbed dose for [ 177 Lu]Lu-PSMA-I&T. This difference in the clinical setting is considerably smaller than observed in preclinical studies and may not hamper treatments with [ 177 Lu]Lu-PSMA-I&T.

Cross-reactivity to glutamate carboxypeptidase III causes undesired salivary gland and kidney uptake of PSMA-targeted small-molecule radionuclide therapeutics

PURPOSE

Recently, Pluvicto™ ([177Lu]Lu-PSMA-617), a small-molecule prostate-specific membrane antigen (PSMA) radioligand therapeutic, has been approved by the FDA in metastatic castration-resistant prostate cancer. Pluvicto™ and other PSMA-targeting radioligand therapeutics (RLTs) have shown side effects due to accumulation in certain healthy tissues, such as salivary glands and kidney. Until now, the molecular mechanism underlying the undesired accumulation of PSMA-targeting RLTs had not been elucidated.

METHODS

We compared the sequence of PSMA with the entire human proteome to identify proteins closely related to the target. We have identified glutamate carboxypeptidase III (GCPIII), N-acetylated alpha-linked acidic dipeptidase like 1 (NAALADL-1), and transferrin receptor 1 (TfR1) as extracellular targets with the highest similarity to PSMA. The affinity of compound 1 for PSMA, GCPIII, NAALADL-1, and TfR1 was measured by fluorescence polarization. The expression of the putative anti-target GCPIII was assessed by immunofluorescence on human salivary glands and kidney, using commercially available antibodies.

RESULTS

A fluorescent derivative of Pluvicto™ (compound 1) bound tightly to PSMA and to GCPIII in fluorescence polarization experiments, while no interaction was observed with NAALADL-1 and TfR1. Immunofluorescence analysis revealed abundant expression of GCPIII both in healthy human kidney and salivary glands.

CONCLUSION

We conclude that the membranous expression of GCPIII in kidney and salivary gland may be the underlying cause for unwanted accumulation of Pluvicto™ and other Glu-ureido PSMA radio pharmaceuticals in patients.

Nanobodies: a new potential for prostate cancer treatment

BACKGROUND

The current progressive increase in the cancer burden of prostate cancer requires the exploration of new diagnostic and therapeutic approaches. Nanobodies are single-domain antibodies with the advantages of small size, high stability, easy processing and modification, which are increasingly used in the treatment of many types of cancer.

METHODS

This review analyzed the relevant literature in PubMed and other databases.

RESULT

In the retrieved literature, nanobodies are widely used in the treatment of prostate cancer. The preparation of nanobodies targeting PSA or PSMA is straightforward. For diagnostic purposes, nanobodies can be used in the preparation of biosensors for more sensitive identification of prostate cancer; for therapeutic purposes, nanobodies are used in the preparation of immunotoxic and ADC drugs. Preclinical in vivo and in vitro experiments have shown that this therapeutic approach is feasible. This article is a review of the above to provide new ideas for the treatment of prostate cancer.

CONCLUSION

Compared with traditional antibodies, nano-antibodies have the advantages of small size, high stability, and high penetration. These advantages make nano-antibodies worthy to be widely used. Current studies have shown that nanobodies have advantages and future in the diagnosis and treatment of prostate cancer.

Targeted Radiation and Immune Therapies-Advances and Opportunities for the Treatment of Prostate Cancer

Prostate cancer is the most diagnosed malignancy in men in the United States and the second leading cause of cancer-related death. For localized disease, radiation therapy is a standard treatment that is often curative. For metastatic disease, radiation therapy has been primarily used for palliation, however, several newer systemic radiation therapies have been demonstrated to significantly improve patient outcomes and improve survival. In particular, several targeted radionuclide therapies have been approved for the treatment of advanced-stage cancer, including strontium-89, samarium-153, and radium-223 for bone-metastatic disease, and lutetium-177-labeled PSMA-617 for patients with prostate-specific membrane antigen (PSMA)-expressing metastatic castration-resistant prostate cancer (mCRPC). Contrarily, immune-based treatments have generally demonstrated little activity in advanced prostate cancer, with the exception of the autologous cellular vaccine, sipuleucel-T. This has been attributed to the presence of an immune-suppressive prostate cancer microenvironment. The ability of radiation therapy to not only eradicate tumor cells but also potentially other immune-regulatory cells within the tumor immune microenvironment suggests that targeted radionuclide therapies may be well poised to combine with immune-targeted therapies to eliminate prostate cancer metastases more effectively. This review provides an overview of the recent advances of targeted radiation agents currently approved for prostate cancer, and those being investigated in combination with immunotherapy, and discusses the challenges as well as the opportunities in this field.

Status of PSMA-targeted radioligand therapy in prostate cancer: current data and future trials

Metastatic prostate cancer continues to be an incurable disease. Despite all the novel therapies approved in the past two decades, overall patient outcomes remain relatively poor, and these patients die on a regular basis. Clearly, improvements in current therapies are needed. Prostate-specific membrane antigen (PSMA) is a target for prostate cancer given its increased expression on the surface of the prostate cancer cells. PSMA small molecule binders include PSMA-617 and PSMA-I&T and monoclonal antibodies such as J591. These agents have been linked to different radionuclides including beta-emitters such as lutetium-177 and alpha-emitters such as actinium-225. The only regulatory-approved PSMA-targeted radioligand therapy (PSMA-RLT) to date is lutetium-177-PSMA-617 in the setting of PSMA-positive metastatic castration-resistant prostate cancer that has failed androgen receptor pathway inhibitors and taxane chemotherapy. This approval was based on the phase III VISION trial. Many other clinical trials are evaluating PSMA-RLT in various settings. Both monotherapy and combination studies are underway. This article summarizes pertinent data from recent studies and provides an overview of human clinical trials in progress. The field of PSMA-RLT is rapidly evolving, and this therapeutic approach will likely play an increasingly important role in the years to come.

PSMA-based 18F-DCFPyL PET: a better choice than multiparametric MRI for prostate cancer diagnosis?

Owing to the high tissue contrast, multiparametric MRI (mpMRI) has already been the most widely applied imaging method for prostate cancer. Recently, prostate-specific membrane antigen (PSMA) ligands for nuclear imaging are emerging as a promising modality in prostate cancer, especially since the 2 PET/CT agents (⁶⁸Ga-PSMA-11 and ¹⁸F-DCFPy) approved by U.S. Food and Drug Administration (FDA). However, limited studies have performed the comparison of mpMRI versus recently approved ¹⁸F-DCFPyL PET/CT. In this issue of AJNMMI, Lu et al. compared the performance of ¹⁸F-DCFPyL PET/CT and pelvic mpMRI in intermediate-high risk and biochemical recurrent prostate cancer patients. The results demonstrated the two modalities have a good concordance rate for patient-based analysis, and ¹⁸F-DCFPyL PET/CT has a better diagnostic performance in detecting lymph node metastases and bone metastases for lesion-based analysis. The use of ¹⁸F-DCFPyL PET/CT provides more diagnostic confidence to better assess prostate cancer lesions.

Preparation of Patient Doses of [177Lu]Lu-DOTATATE and [177Lu]Lu-PSMA-617 with Carrier Added (CA) and No Carrier Added (NCA) 177Lu

Purpose

[177Lu]Lu-DOTATATE and [177Lu]Lu-PSMA-617 used for targeted radionuclide therapy are very often prepared in the hospital radiopharmacy. The preparation parameters vary depending upon the specific activity of the 177Lu used. The aim of this study was to develop optimized protocols to be used in the nuclear medicine department for the preparation of patient doses of the above radiopharmaceuticals.

Method

177Lu (CA and NCA) were used for radiolabeling DOTATATE and PSMA-617. Parameters studied are 177Lu of different specific activity and different peptide concentrations and two different buffer systems. Paper and thin layer chromatography systems were used for estimating the radiochemical yield as well as radiochemical purity. Solid-phase extraction was used for the purification of the labeled tracers.

Results

[177Lu]Lu-DOTATATE was prepared with CA 177Lu (n = 13) and NCA177Lu (n = 6). Four batches each of [177Lu]Lu-PSMA-617 were prepared using CA and NCA 177Lu. Radiochemical yields > 80% and final product with less than < 1% radiochemical impurity could be obtained in all batches which were used for therapy.

Conclusion

Robust protocols for the preparation of clinical doses of [177Lu]Lu-DOTATATE and [177Lu]Lu-PSMA-617 were developed and used for the preparation of clinical doses. The quality of the SPECT images of both the tracers are consistent with the expected uptake in respective diseases.

Lutetium Lu 177 vipivotide tetraxetan for metastatic castration-resistant prostate cancer

INTRODUCTION

¹⁷⁷Lu-vipivotide tetraxetan is a radiopharmaceutical that selectively targets prostate-specific membrane antigen (PSMA) and delivers beta-radiations to kill prostate cancer cells.

AREAS COVERED

Extensive experience outside the United States as well as randomized phase II and phase III data demonstrate that ¹⁷⁷Lu-vipivotide tetraxetan is a safe, generally well tolerated, and effective therapy for men with mCRPC. ¹⁷⁷Lu-vipivotide tetraxetan was approved by the FDA in March 2022 for the treatment of PSMA-positive metastatic castration-resistant prostate cancer (mCRPC) after androgen receptor pathway inhibition and taxane-based chemotherapy based on the results of the VISION trial.

EXPERT OPINION

This review discusses the development and studies leading to the approval of ¹⁷⁷Lu-vipivotide tetraxetan. In all, ¹⁷⁷Lu-vipivotide tetraxetan is an exciting new tool in the arsenal for men with mCRPC after novel androgen pathway inhibitors and at least one taxane chemotherapy. Optimal selection of patients, sequencing of ¹⁷⁷Lu-vipivotide tetraxetan with the other agents available to treat mCRPC, and the use of dosimetry are current areas of interest with great potential and opportunities for further individual patient optimization using the tools of theranostics.

Autoradiography on deparaffinized tissue sections - A feasibility study with 68Ga-labeled PET-tracers

Tissue available for retrospective research questions is often already paraffin-embedded for better preservation. However, in vitro autoradiography (AURA) is normally performed on cryopreserved tissue sections. We hypothesized a) that it would also be feasible with deparaffinized tissue sections, enabling the use of human paraffin-embedded tissue for in vitro AURA and b) that the results would be comparable to those obtained with corresponding cryosections. For that purpose, the clinically relevant oncological targets CXCR4, SSTR and PSMA were evaluated. In vitro AURA on deparaffinized tissue sections was feasible, but only with the two receptor ligands [⁶⁸Ga]Ga-PentixaFor and [⁶⁸Ga]Ga-DOTANOC. [⁶⁸Ga]Ga-PSMA-11 did not show any binding on deparaffinized tissue sections, suggesting that native tissue is required for an interaction between this inhibitor and the enzyme.

In vitro dose effect relationships of actinium-225- and lutetium-177-labeled PSMA-I&T

PURPOSE

Targeting the prostate-specific membrane antigen (PSMA) using lutetium-177-labeled PSMA-specific tracers has become a very promising novel therapy option for prostate cancer (PCa). The efficacy of this therapy might be further improved by replacing the β-emitting lutetium-177 with the α-emitting actinium-225. Actinium-225 is thought to have a higher therapeutic efficacy due to the high linear energy transfer (LET) of the emitted α-particles, which can increase the amount and complexity of the therapy induced DNA double strand breaks (DSBs). Here we evaluated the relative biological effectiveness of [²²⁵Ac]Ac-PSMA-I&T and [¹⁷⁷Lu]Lu-PSMA-I&T by assessing in vitro binding characteristics, dosimetry, and therapeutic efficacy.

METHODS AND RESULTS

The PSMA-expressing PCa cell line PC3-PIP was used for all in vitro assays. First, binding and displacement assays were performed, which revealed similar binding characteristics between [²²⁵Ac]Ac-PSMA-I&T and [¹⁷⁷Lu]Lu-PSMA-I&T. Next, the assessment of the number of 53BP1 foci, a marker for the number of DNA double strand breaks (DSBs), showed that cells treated with [²²⁵Ac]Ac-PSMA-I&T had slower DSB repair kinetics compared to cells treated with [¹⁷⁷Lu]Lu-PSMA-I&T. Additionally, clonogenic survival assays showed that specific targeting with [²²⁵Ac]Ac-PSMA-I&T and [¹⁷⁷Lu]Lu-PSMA-I&T caused a dose-dependent decrease in survival. Lastly, after dosimetric assessment, the relative biological effectiveness (RBE) of [²²⁵Ac]Ac-PSMA-I&T was found to be 4.2 times higher compared to [¹⁷⁷Lu]Lu-PSMA-I&T.

CONCLUSION

We found that labeling of PSMA-I&T with lutetium-177 or actinium-225 resulted in similar in vitro binding characteristics, indicating that the distinct biological effects observed in this study are not caused by a difference in uptake of the two tracers. The slower repair kinetics of [²²⁵Ac]Ac-PSMA-I&T compared to [¹⁷⁷Lu]Lu-PSMA-I&T correlates to the assumption that irradiation with actinium-225 causes more complex, more difficult to repair DSBs compared to lutetium-177 irradiation. Furthermore, the higher RBE of [²²⁵Ac]Ac-PSMA-I&T compared to [¹⁷⁷Lu]Lu-PSMA-I&T underlines the therapeutic potential for the treatment of PCa.

Update of PSMA Theranostics in Prostate Cancer: Current Applications and Future Trends

There is now an increasing trend for targeting cancers to go beyond early diagnosis and actually improve Progression-Free Survival and Overall Survival. Identifying patients who might benefit from a particular targeted treatment is the main focus for Precision Medicine. Radiolabeled ligands can be used as predictive biomarkers which can confirm target expression by cancers using positron emission tomography (PET). The same ligand can subsequently be labeled with a therapeutic radionuclide for targeted radionuclide therapy. This combined approach is termed “Theranostics”. The prostate-specific membrane antigen (PSMA) has emerged as an attractive diagnostic and therapeutic target for small molecule ligands in prostate cancer. It can be labeled with either positron emitters for PET-based imaging or beta and alpha emitters for targeted radionuclide therapy. This review article summarizes the important concepts for Precision Medicine contributing to improved diagnosis and targeted therapy of patients with prostate cancer and we identify some key learning points and areas for further research.

Matched-pair analysis of [177Lu]Lu-PSMA I&T and [177Lu]Lu-PSMA-617 in patients with metastatic castration-resistant prostate cancer

Background

Labelled with lutetium-177, the urea-based small molecules PSMA I&T and PSMA-617 are the two agents most frequently used for radioligand therapy (RLT) in patients with advanced metastatic castration-resistant and prostate-specific membrane antigen (PSMA) expressing prostate cancer (mCRPC). In this matched-pair analysis, we aimed to compare the toxicity and efficacy of both agents for PSMA-directed RLT.

Materials and methods

A total of 110 mCRPC patients from two centres were accrued, 55 individuals treated with [¹⁷⁷Lu]Lu-PSMA I&T, and a matched cohort of 55 patients treated with [¹⁷⁷Lu]Lu-PSMA-617. Matching criteria included age at the first cycle, Gleason score, prostate-specific antigen (PSA) values, and previous taxane-based chemotherapy. Using common terminology criteria for adverse events (CTCAE v. 5.0), toxicity profiles were investigated (including bone marrow and renal toxicity). Overall survival (OS) between both groups was compared.

Results

Toxicity assessment revealed grade III anaemia in a single patient (1.8%) for [¹⁷⁷Lu]Lu-PSMA I&T and five (9.1%) for [¹⁷⁷Lu]Lu-PSMA-617. In addition, one (1.9%) grade III thrombopenia for [¹⁷⁷Lu]Lu-PSMA-617 was recorded. Apart from that, no other grade III/IV toxicities were present. A median OS of 12 months for patients treated with [¹⁷⁷Lu]Lu-PSMA I&T did not differ significantly when compared to patients treated with [¹⁷⁷Lu]Lu-PSMA-617 (median OS, 13 months; P = 0.89).

Conclusion

In this matched-pair analysis of patients receiving one of the two agents most frequently applied for PSMA RLT, the rate of clinically relevant toxicities was low for both compounds. In addition, no relevant differences for OS were observed.

89Zr-labeled PSMA ligands for pharmacokinetic PET imaging and dosimetry of PSMA-617 and PSMA-I&T: a preclinical evaluation and first in man

RATIONALE

Prolonged in vivo evaluation of PSMA tracers could improve tumor imaging and patient selection for ¹⁷⁷Lu-PSMA-617 and ¹⁷⁷Lu-PSMA-I&T. In this study, we present the radiolabeling method of PSMA-617 and PSMA-I&T with the long-lived positron emitter ⁸⁹Zr to enable PET imaging up to 7 days post-injection. We compared the biodistribution of ⁸⁹Zr-PSMA-617 and ⁸⁹Zr-PSMA-I&T to those of ¹⁷⁷Lu-PSMA-617 and ¹⁷⁷Lu-PSMA-I&T, respectively, in a PSMA⁺ xenograft model. Moreover, we provide the first human ⁸⁹Zr-PSMA-617 images.

MATERIALS AND METHODS

PSMA ligands were labeled with 50-55 MBq [⁸⁹Zr]ZrCl₄ using a two-step labeling protocol. For biodistribution, BALB/c nude mice bearing PSMA⁺ and PSMA^- xenografts received 0.6 µg (0.6-1 MBq) of ⁸⁹Zr-PSMA-617, ⁸⁹Zr-PSMA-I&T, ¹⁷⁷Lu-PSMA-617, or ¹⁷⁷Lu-PSMA-I&T intravenously. Ex vivo biodistribution and PET/SPECT imaging were performed up to 168 h post-injection. Dosimetry was performed from the biodistribution data. The patient received 90.5 MBq ⁸⁹Zr-PSMA-617 followed by PET/CT imaging.

RESULTS

⁸⁹Zr-labeled PSMA ligands showed a comparable ex vivo biodistribution to its respective ¹⁷⁷Lu-labeled counterparts with high tumor accumulation in the PSMA⁺ xenografts. However, using a dose estimation model for ¹⁷⁷Lu, absorbed radiation dose in bone and kidneys differed among the ¹⁷⁷Lu-PSMA and ⁸⁹Zr-PSMA tracers. ⁸⁹Zr-PSMA-617 PET in the first human patient showed high contrast of PSMA expressing tissues up to 48 h post-injection.

CONCLUSION

PSMA-617 and PSMA-I&T were successfully labeled with ⁸⁹Zr and demonstrated high uptake in PSMA⁺ xenografts, which enabled PET up to 168 h post-injection. The biodistribution of ⁸⁹Zr-PSMA-I&T and ⁸⁹Zr-PSMA-617 resembled that of ¹⁷⁷Lu-PSMA-I&T and ¹⁷⁷Lu-PSMA-617, respectively. The first patient ⁸⁹Zr-PSMA-617 PET images were of high quality warranting further clinical investigation.

Radiation Dosimetry in 177Lu-PSMA-617 Therapy

Radionuclide therapy using the small molecule PSMA bound to the beta-emitting radionuclide, Lutetium-177 (¹⁷⁷Lu-PSMA) has demonstrated efficacy and survival benefit castrate resistant metastatic disease and represents a novel new line of therapy. Whilst dosimetry was critical for early development, it was not incorporated into either the TheraP or VISION randomized studies, highlighting the difficulty of adopting dosimetry in routine clinical practice. Accumulated clinical experience has also shown that the common (and generally low grade) toxicities such as nausea, xerostomia, and cytopenias are not readily predicted on the basis of dosimetry estimates. The majority of dosimetry and clinical literature deals with the radiopharmaceutical ¹⁷⁷Lu-PSMA-617 which displays relatively consistent patterns of retention among normal tissues and high specificity for metastatic prostate cancer phenotypes. Population dosimetry incorporating estimates to the kidneys, salivary glands, and bone marrow have been widely reported the typical range of doses is becoming well established. There is growing interest on tumor dosimetry in ¹⁷⁷Lu-PSMA-617 therapy as an overall modest side-effect profile from primary organ retention has been observed. A focus away from normal organ dosimetry to whole body tumor dosimetry may enable early prediction of treatment failure. Given the safety of ¹⁷⁷Lu-PSMA there is also potential to escalate administered radioactivity to further improve outcomes. Importantly, the variability of uptake between individuals, both to tumor and normal organs, has also been highlighted which provides some rationale for the utility of personalized radiation analysis to optimize treatment based on potential toxicity thresholds or tumor control. Methods to perform dosimetry using serial post treatment imaging may incorporate planar, 3D SPECT, or hybrid datasets. Reliable measurements may be obtained through either method, however, continued developments in computational analysis are better suited to fully 3D imaging; particularly in conjunction with volumetric CT to assist with alignment and contouring. Dose analysis over sequential treatment cycles is vital to understand the radiobiology of these treatments which is unique compared to external beam therapy due to dose rate, fractionation scheme, and potential for intratumoral nonuniformity.

Nanobodies: a tool to open new horizons in diagnosis and treatment of prostate cancer

Background

Prostate cancer is one of the most common cancers in men and its incidence has increased dramatically in the last decade. This increase in the detection of this type of cancer is based more on the detection of PSA or PSMA antigens as the most important specific antigens of this cancer, and this early detection has greatly helped in the more optimal treatment of patients.

Main body

Many methods have been proposed by researchers for early detection of prostate cancer, but most of the methods used today to detect this type of cancer have been using classical antibodies. Although classical antibodies are able to detect tumor cell markers, but instability, large size, costly and laborious production, and random immobility characteristics, causes many problems. Nanobodies or VHHs, which are derived from camel heavy chain antibodies, have special advantages and have eliminated the disadvantages of classical antibodies which makes them attractive to use in biosensors and cancer diagnostic kits. The research that has been done so far shows that the introduced nanobodies are created for the purpose of targeting, detecting and sensing prostate cancer cells with two main purposes. The first is the efficient identification of prostate cancer and the second is the elimination of cancer cells.

Conclusion

Research shows the use of specific nanobodies against prostate cancer antigens in the design of biosensors and target therapy will be very interesting. In this review article, these nanobodies are introduced and categorized based on their performance.

Tune the channel: TRPM8 targeting in prostate cancer

The therapeutic landscape of cancer treatments is quickly evolving thanks to the advent of precision oncology. Discovery of novel druggable targets and more reliable biomarkers is a primary objective towards personalized strategies of cancer treatment. Highly expressed in the prostate epithelium within the human body, Transient Receptor Potential subfamily M member 8 (TRPM8) levels rise in primary and hormone naïve metastatic prostate cancer (PCa) lesions, which makes this channel an interesting prototype of molecular target. Recently, by combining a multidisciplinary approach to an in vitro genetic platform, we demonstrated that the combination of potent TRPM8 agonists with X-rays induces a massive apoptotic response in radioresistant pre-malignant and malignant models of primary prostate lesions. As well, TRPM8 activation enhances the efficacy of docetaxel or enzalutamide in eradicating hormone naïve metastatic PCa cells. Overall, our findings provide a solid rationale for pursuing the pre-clinical and clinical study of TRPM8 as a valuable target for future approaches of precise oncology in PCa.

PSMA Theranostics: Current Landscape and Future Outlook

INTRODUCTION

Prostate-specific membrane antigen (PSMA) is a promising novel molecular target for imaging diagnostics and therapeutics (theranostics). There has been a growing body of evidence supporting PSMA theranostics approaches in optimizing the management of prostate cancer and potentially altering its natural history.

METHODS

We utilized PubMed and Google Scholar for published studies, and clinicaltrials.gov for planned, ongoing, and completed clinical trials in PSMA theranostics as of June 2021. We presented evolving evidence for various PSMA-targeted radiopharmaceutical agents in the treatment paradigm for prostate cancer, as well as combination treatment strategies with other targeted therapy and immunotherapy. We highlighted the emerging evidence of PSMA and fluorodeoxyglucose (FDG) PET/CT as a predictive biomarker for PSMA radioligand therapy. We identified seven ongoing clinical trials in oligometastatic-directed therapy using PSMA PET imaging. We also presented a schematic overview of 17 key PSMA theranostic clinical trials throughout the various stages of prostate cancer.

CONCLUSIONS

In this review, we presented the contemporary and future landscape of theranostic applications in prostate cancer with a focus on PSMA ligands. As PSMA theranostics will soon become the standard of care for the management of prostate cancer, we underscore the importance of integrating nuclear medicine physicians into the multidisciplinary team.

Nomograms to predict outcomes after 177Lu-PSMA therapy in men with metastatic castration-resistant prostate cancer: an international, multicentre, retrospective study

BACKGROUND

Lutetium-177 (¹⁷⁷Lu) prostate-specific membrane antigen (¹⁷⁷Lu-PSMA) is a novel targeted treatment for patients with metastatic castration-resistant prostate cancer (mCRPC). Predictors of outcomes after ¹⁷⁷Lu-PSMA to enhance its clinical implementation are yet to be identified. We aimed to develop nomograms to predict outcomes after ¹⁷⁷Lu-PSMA in patients with mCRPC.

METHODS

In this multicentre, retrospective study, we screened patients with mCRPC who had received ¹⁷⁷Lu-PSMA between Dec 10, 2014, and July 19, 2019, as part of the previous phase 2 trials (NCT03042312, ACTRN12615000912583) or compassionate access programmes at six hospitals and academic centres in Germany, the USA, and Australia. Eligible patients had received intravenous 6·0-8·5 GBq ¹⁷⁷Lu-PSMA once every 6-8 weeks, for a maximum of four to six cycles, and had available baseline [⁶⁸Ga]Ga-PSMA-11 PET/CT scan, clinical data, and survival outcomes. Putative predictors included 18 pretherapeutic clinicopathological and [⁶⁸Ga]Ga-PSMA-11 PET/CT variables. Data were collected locally and centralised. Primary outcomes for the nomograms were overall survival and prostate-specific antigen (PSA)-progression-free survival. Nomograms for each outcome were computed from Cox regression models with LASSO penalty for variable selection. Model performance was measured by examining discrimination (Harrell's C-index), calibration (calibration plots), and utility (patient stratification into low-risk vs high-risk groups). Models were validated internally using bootstrapping and externally by calculating their performance on a validation cohort.

FINDINGS

Between April 23, 2019, and Jan 13, 2020, 414 patients were screened; 270 (65%) of whom were eligible and were divided into development (n=196) and validation (n=74) cohorts. The median duration of follow-up was 21·5 months (IQR 13·3-30·7). Predictors included in the nomograms were time since initial diagnosis of prostate cancer, chemotherapy status, baseline haemoglobin concentration, and [⁶⁸Ga]Ga-PSMA-11 PET/CT parameters (molecular imaging TNM classification and tumour burden). The C-index of the overall survival model was 0·71 (95% CI 0·69-0·73). Similar C-indices were achieved at internal validation (0·71 [0·69-0·73]) and external validation (0·72 [0·68-0·76]). The C-index of the PSA-progression-free survival model was 0·70 (95% CI 0·68-0·72). Similar C-indices were achieved at internal validation (0·70 [0·68-0·72]) and external validation (0·71 [0·68-0·74]). Both models were adequately calibrated and their predictions correlated with the observed outcome. Compared with high-risk patients, low-risk patients had significantly longer overall survival in the validation cohort (24·9 months [95% CI 16·8-27·3] vs 7·4 months [4·0-10·8]; p<0·0001) and PSA-progression-free survival (6·6 months [6·0-7·1] vs 2·5 months [1·2-3·8]; p=0·022).

INTERPRETATION

These externally validated nomograms that are predictive of outcomes after ¹⁷⁷Lu-PSMA in patients with mCRPC might help in clinical trial design and individual clinical decision making, particularly at institutions where ¹⁷⁷Lu-PSMA is introduced as a novel therapeutic option.

FUNDING

Prostate Cancer Foundation.

Radiolabeled nanobodies for tumor targeting: From bioengineering to imaging and therapy

So far, numerous molecules and biomolecules have been evaluated for tumor targeting purposes for radionuclide-based imaging and therapy modalities. Due to the high affinity and specificity against tumor antigens, monoclonal antibodies are appropriate candidates for tumor targeting. However, their large size prevents their comprehensive application in radionuclide-based tumor imaging or therapy, since it leads to their low tumor penetration, low blood clearance, and thus inappropriate tumor-to-background ratio. Nowadays, the variable domain of heavy-chain antibodies from the Camelidae family, known as nanobodies (Nbs), turn into exciting candidates for medical research. Considering several innate advantages of these new tumor-targeting agents, including excellent affinity and specificity toward antigen, high solubility, high stability, fast washout from blood, convenient production, ease of selection, and low immunogenicity, it assumes that they may overcome generic problems of monoclonal antibodies, their fragments, and other vectors used for tumor imaging/therapy. After three decades of Nbs discovery, the increasing number of their preclinical and clinical investigations, which have led to outstanding results, confirm their application for tumor targeting purposes. This review describes Nbs characteristics, the diagnostic and therapeutic application of their radioconjugates, and their recent advances.

Preclinical and Clinical Status of PSMA-Targeted Alpha Therapy for Metastatic Castration-Resistant Prostate Cancer

Bone, lymph node, and visceral metastases are frequent in castrate-resistant prostate cancer patients. Since such patients have only a few months' survival benefit from standard therapies, there is an urgent need for new personalized therapies. The prostate-specific membrane antigen (PSMA) is overexpressed in prostate cancer and is a molecular target for imaging diagnostics and targeted radionuclide therapy (theragnostics). PSMA-targeted α therapies (PSMA-TAT) may deliver potent and local radiation more selectively to cancer cells than PSMA-targeted β- therapies. In this review, we summarize both the recent preclinical and clinical advances made in the development of PSMA-TAT, as well as the availability of therapeutic α-emitting radionuclides, the development of small molecules and antibodies targeting PSMA. Lastly, we discuss the potentials, limitations, and future perspectives of PSMA-TAT.