177Lu-PSMA-617 Therapy in Mice, with or without the Antioxidant α1-Microglobulin (A1M), Including Kidney Damage Assessment Using 99mTc-MAG3 Imaging

Single Chelator-Minibody Theranostic Agents for 89Zr PET Imaging and 177Lu Radiopharmaceutical Therapy of PSMA-Expressing Prostate Cancer

Here we describe an anti-prostate-specific membrane antigen (PSMA) minibody (IAB2MA) conjugated to an octadentate, macrocyclic chelator based on four 1-hydroxypyridin-2-one coordinating units (Lumi804 [L804]) labeled with 89Zr (PET imaging) and 177Lu (radiopharmaceutical therapy), with the goal of developing safer and more efficacious treatment options for prostate cancer. Methods: L804 was compared with the current gold standard chelators, DOTA and deferoxamine (DFO), conjugated to IAB2MA for radiolabeling with 177Lu and 89Zr in cell binding, preclinical biodistribution, imaging, dosimetry, and efficacy studies in the PSMA-positive PC3-PIP tumor-bearing mouse model of prostate cancer. Results: Quantitative radiolabeling (>99% radiochemical yield) of L804-IAB2MA with 177Lu or 89Zr was achieved at ambient temperature in under 30 min, comparable to 89Zr labeling of DFO-IAB2MA. In contrast, DOTA-IAB2MA was radiolabeled with 177Lu for 30 min at 37°C in approximately 90% radiochemical yield, requiring further purification. Using europium(III) as a luminescent surrogate, high binding affinity of Eu-L804-IAB2MA to PSMA was demonstrated in PC3-PIP cells (dissociation constant, 4.6 ± 0.6 nM). All 4 radiolabeled constructs showed significantly higher levels of internalization after 30 min in the PC3-PIP cells than in PSMA-negative PC3-FLU cells. The accumulation of 177Lu- and 89Zr-L804-IAB2MA in PC3-PIP tumors and all organs examined (i.e., heart, liver, spleen, kidney, muscle, salivary glands, lacrimal glands, carcass, and bone) was significantly lower than that of 177Lu-DOTA-IAB2MA and 89Zr-DFO-IAB2MA at 96 and 72 h after injection, respectively. Generally, SPECT/CT and PET/CT imaging data showed no significant difference in the SUVmean of the tumors or muscle between the radiotracers. Dosimetry analysis via both organ-level and voxel-level dose calculation methods indicated significantly higher absorbed doses of 177Lu-DOTA-IAB2MA in tumors, kidney, liver, muscle, and spleen than of 177Lu-L804-IAB2MA. PC3-PIP tumor-bearing mice treated with single doses of 177Lu-L804-IAB2MA (18.4 or 22.2 MBq) exhibited significantly prolonged survival and reduced tumor volume compared with unlabeled minibody control. No significant difference in survival was observed between groups of mice treated with 177Lu-L804-IAB2MA or 177Lu-DOTA-IAB2MA (18.4 or 22.2 MBq). Treatment with 177Lu-L804-IAB2MA resulted in lower absorbed doses in tumors and less toxicity than that of 177Lu-DOTA-IAB2MA. Conclusion: 89Zr- and 177Lu-L804-IAB2MA may be a promising theranostic pair for imaging and therapy of prostate cancer.

The synergy of 177Lu-FAPI-46 with tyrosine kinase inhibitor in a sarcoma patient-derived xenograft mouse model

BACKGROUND

Given the heterogeneity and high mortality associated with metastatic soft tissue sarcoma, this study aims to evaluate the therapeutic efficacy of combining 177Lu-FAPI-46 with Pazopanib against this malignancy.

METHODS

Patient-derived xenograft (PDX)-bearing mice were randomly divided into three groups: the control group, the 177Lu-FAPI-46 monotherapy group, and the 177Lu-FAPI-46 combined with Pazopanib therapy group. Therapeutic efficacy was regularly monitored.

RESULTS

The microPET imaging showed a 0.84-fold decrease in the T/M ratio of 68Ga-FAPI-46 on day 7/8 post combination therapy, while the control group exhibited a 1.23-fold increase. Combination therapy significantly inhibited tumor proliferation, as evidenced by reduced Ki-67 and increased caspase 3 expressions. Notably, there was no significant body weight loss observed in any group.

CONCLUSION

This study successfully demonstrated the reduction in FAP expression and suppression of tumor volume in sarcoma PDX following the combination therapy of 177Lu-FAPI-46 with Pazopanib.

Hematological and renal toxicity in mice after three cycles of high activity [177Lu]Lu-PSMA-617 with or without human α1-microglobulin

Radioligand therapy with [177Lu]Lu-PSMA-617 can be used to prolong life and reduce tumor burden in terminally ill castration resistant prostate cancer patients. Still, accumulation in healthy tissue limits the activity that can be administered. Therefore, fractionated therapy is used to lower toxicity. However, there might be a need to reduce toxicity even further with e.g. radioprotectors. The aim of this study was to (i). establish a preclinical mouse model with fractionated high activity therapy of three consecutive doses of 200 MBq [177Lu]Lu-PSMA-617 in which we aimed to (ii). achieve measurable hematotoxicity and nephrotoxicity and to (iii). analyze the potential protective effect of co-injecting recombinant α1-microglobulin (rA1M), a human antioxidant previously shown to have radioprotective effects. In both groups, three cycles resulted in increased albuminuria for each cycle, with large individual variation. Another marker of kidney injury, serum blood urea nitrogen (BUN), was only significantly increased compared to control animals after the third cycle. The number of white and red blood cells decreased significantly and did not reach the levels of control animals during the experiment. rA1M did reduce absorbed dose to kidney but did not show significant protection here, but future studies are warranted due to the recent clinical studies showing a significant renoprotective effect in patients.

Exploring the role of combined external beam radiotherapy and targeted radioligand therapy with [177Lu]Lu-PSMA-617 for prostate cancer - from bench to bedside

Management of prostate cancer (PC) might be improved by combining external beam radiotherapy (EBRT) and prostate-specific membrane antigen (PSMA)-targeted radioligand therapy (RLT) with lutetium-177 (177Lu)-labeled PSMA inhibitors. We hypothesized a higher efficacy of the combination due to augmentation of the radiation dose to the tumor and interactions of EBRT with PSMA expression potentially increasing radiopharmaceutical uptake. Therefore, this study analyzed the influence of radiation on PSMA expression levels in vitro. The results were translated to evaluate the efficacy of the combination of photon EBRT and [177Lu]Lu-PSMA-617 in a murine PC xenograft model. Finally, a clinical case report on a combined elective field EBRT with RLT dose escalation illustrates a proof-of-concept. Methods: PSMA gene and protein expression were assessed in human PSMA-overexpressing LNCaP cells after irradiation using reverse transcription quantitative polymerase chain reaction (RT-qPCR), flow cytometry and On-Cell Western assays. In the in vivo therapy study, LNCaP tumor-bearing BALB/c nu/nu mice were irradiated once with 2 Gy X-ray EBRT and injected with 40 MBq [177Lu]Lu-PSMA-617 after 4 h or received single or no treatment (n = 10 each). Tumor-absorbed doses by [177Lu]Lu-PSMA-617 were calculated according to the Medical Internal Radiation Dosimetry (MIRD) formalism after deriving time-activity curves using a gamma probe. An exemplified patient case is demonstrated where fractionated EBRT (54 Gy to prostate; 45 Gy to pelvic lymphatics) and three cycles of [177Lu]Lu-PSMA-617 (3.4-6.0 GBq per cycle) were sequentially combined under concurrent androgen deprivation for treating locally advanced PC. Results: At 4 h following irradiation with 2-8 Gy, LNCaP cells displayed a PSMA protein upregulation by around 18% relative to non-irradiated cells, and a stronger upregulation on mRNA level (up to 2.6-fold). This effect was reversed by 24 h when PSMA protein levels were downregulated by up to 22%. Mice treated with the combination therapy showed significantly improved outcomes regarding tumor control and median survival (p < 0.0001) as compared to single or no treatment. Relative to monotherapy with PSMA-RLT or EBRT, the tumor doubling time was prolonged 1.7- or 2.7-fold and the median survival was extended by 24% or 60% with the combination, respectively. Additionally, tumors treated with EBRT exhibited a 14% higher uptake of the radiopharmaceutical as evident from the calculated tumor-absorbed dose, albeit with high variability in the data. Concerning the patient case, the tri-modality treatment was well tolerated and the patient responded with a long-lasting complete biochemical remission for five years following end of PSMA-RLT. The patient then developed a biochemical relapse with oligo-recurrent disease on follow-up imaging. Conclusion: The present preclinical and clinical data demonstrate that the combination of EBRT with dose escalation by PSMA-RLT improves tumor control and potentially prolongs survival. This may pave the way for further clinical investigations of this approach to explore the curative potential of the combination therapy.

Humanized Mice as a Model to Assess the Response of Human Hematopoietic Stem Cells to Irradiation

NOD SCID mice were humanized by transplanting human hematopoietic cells isolated from umbilical cord blood. A dose-dependent death of hematopoietic cells and their subsequent recovery were shown after acute external γ-irradiation in the model of humanized mice. The proposed approach can be used for preclinical studies of radioprotective agents and for assessment of the impact of adverse factors on the survival rate and functional properties of human hematopoietic stem cells in vivo.

Tumor Control Probability and Small-Scale Monte Carlo Dosimetry: Effects of Heterogenous Intratumoral Activity Distribution in Radiopharmaceutical Therapy

In radiopharmaceutical therapy, intratumoral uptake of radioactivity usually leads to heterogeneous absorbed dose distribution. The likelihood of treatment success can be estimated with the tumor control probability (TCP), which requires accurate dosimetry, estimating the absorbed dose rate per unit activity to individual tumor cells. Methods: Xenograft cryosections of the prostate cancer cell line LNCaP treated with [177Lu]Lu-PSMA-617 were evaluated with digital autoradiography and stained with hematoxylin and eosin. The digital autoradiography images were used to define the source in a Monte Carlo simulation of the absorbed dose, and the stained sections were used to detect the position of cell nuclei to relate the intratumoral absorbed dose heterogeneity to the cell density. Simulations were performed for 225Ac, 177Lu, and 90Y. TCP was calculated to estimate the mean necessary injected activity for a high TCP. A hypothetical case of activity mainly taken up on the tumor borders was generated and used to simulate the absorbed dose. Results: The absorbed dose per decay to tumor cells was calculated from the staining and simulation results to avoid underestimating the tumor response from low absorbed doses in tumor regions with low cell density. The mean of necessary injected activity to reach a 90% TCP for 225Ac, 177Lu, and 90Y was found to be 18.3 kBq (range, 18-22 kBq), 24.3 MBq (range, 20-29 MBq), and 5.6 MBq (range, 5-6 MBq), respectively. Conclusion: To account for the heterogeneous absorbed dose generated from nonuniform intratumoral activity uptake, dosimetry models can estimate the mean necessary activity to reach a sufficient TCP for treatment response. This approach is necessary to accurately evaluate the efficacy of suggested radiopharmaceuticals for therapy.

No Evidence for Myocarditis or Other Organ Affection by Induction of an Immune Response against Critical SARS-CoV-2 Protein Epitopes in a Mouse Model Susceptible for Autoimmunity

After Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) developed into a global pandemic, not only the infection itself but also several immune-mediated side effects led to additional consequences. Immune reactions such as epitope spreading and cross-reactivity may also play a role in the development of long-COVID, although the exact pathomechanisms have not yet been elucidated. Infection with SARS-CoV-2 can not only cause direct damage to the lungs but can also lead to secondary indirect organ damage (e.g., myocardial involvement), which is often associated with high mortality. To investigate whether an immune reaction against the viral peptides can lead to organ affection, a mouse strain known to be susceptible to the development of autoimmune diseases, such as experimental autoimmune myocarditis (EAM), was used. First, the mice were immunized with single or pooled peptide sequences of the virus's spike (SP), membrane (MP), nucleocapsid (NP), and envelope protein (EP), then the heart and other organs such as the liver, kidney, lung, intestine, and muscle were examined for signs of inflammation or other damage. Our results showed no significant inflammation or signs of pathology in any of these organs as a result of the immunization with these different viral protein sequences. In summary, immunization with different SARS-CoV-2 spike-, membrane-, nucleocapsid-, and envelope-protein peptides does not significantly affect the heart or other organ systems adversely, even when using a highly susceptible mouse strain for experimental autoimmune diseases. This suggests that inducing an immune reaction against these peptides of the SARS-CoV-2 virus alone is not sufficient to cause inflammation and/or dysfunction of the myocardium or other studied organs.

Recombinant α1-Microglobulin (rA1M) Protects against Hematopoietic and Renal Toxicity, Alone and in Combination with Amino Acids, in a 177Lu-DOTATATE Mouse Radiation Model

¹⁷⁷Lu-DOTATATE peptide receptor radionuclide therapy (PRRT) is used clinically to treat metastasized or unresectable neuroendocrine tumors (NETs). Although ¹⁷⁷Lu-DOTATATE is mostly well tolerated in patients, bone marrow suppression and long-term renal toxicity are still side effects that should be considered. Amino acids are often used to minimize renal radiotoxicity, however, they are associated with nausea and vomiting in patients. α₁-microglobulin (A1M) is an antioxidant with heme- and radical-scavenging abilities. A recombinant form (rA1M) has previously been shown to be renoprotective in preclinical models, including in PRRT-induced kidney damage. Here, we further investigated rA1M's renal protective effect in a mouse ¹⁷⁷Lu-DOTATATE model in terms of administration route and dosing regimen and as a combined therapy with amino acids (Vamin). Moreover, we investigated the protective effect of rA1M on peripheral blood and bone marrow cells, as well as circulatory biomarkers. Intravenous (i.v.) administration of rA1M reduced albuminuria levels and circulatory levels of the oxidative stress-related protein fibroblast growth factor-21 (FGF-21). Dual injections of rA1M (i.e., at 0 and 24 h post-¹⁷⁷Lu-DOTATATE administration) preserved bone marrow cellularity and peripheral blood reticulocytes. Administration of Vamin, alone or in combination with rA1M, did not show any protection of bone marrow cellularity or peripheral reticulocytes. In conclusion, this study suggests that rA1M, administered i.v. for two consecutive days in conjunction with ¹⁷⁷Lu-DOTATATE, may reduce hematopoietic and kidney toxicity during PRRT with ¹⁷⁷Lu-DOTATATE.

Co-administration with A1M does not influence apoptotic response of 177Lu-octreotate in GOT1 neuroendocrine tumors

Recombinant α₁-microglobulin (A1M) is a proposed radioprotector during ¹⁷⁷Lu-octreotate therapy of neuroendocrine tumors (NETs). To ensure a maintained therapeutic effect, we previously demonstrated that A1M does not affect the ¹⁷⁷Lu-octreotate induced decrease in GOT1 tumor volume. However, the underlying biological events of these findings are still unknown. The aim of this work was to examine the regulation of apoptosis-related genes in GOT1 tumors short-time after i.v. administration of ¹⁷⁷Lu-octreotate with and without A1M or A1M alone. Human GOT1 tumor-bearing mice received 30 MBq ¹⁷⁷Lu-octreotate or 5 mg/kg A1M or co-treatment with both. Animals were sacrificed after 1 or 7 days. Gene expression analysis of apoptosis-related genes in GOT1 tissue was performed with RT-PCR. In general, similar expression patterns of pro- and anti-apoptotic genes were found after ¹⁷⁷Lu-octreotate exposure with or without co-administration of A1M. The highest regulated genes in both irradiated groups compared to untreated controls were FAS and TNFSFRS10B. Administration of A1M alone only resulted in significantly regulated genes after 7 days. Co-administration of A1M did not negatively affect the transcriptional apoptotic response of ¹⁷⁷Lu-octreotate in GOT1 tumors.

Hematological Toxicity in Mice after High Activity Injections of 177Lu-PSMA-617

Prostate cancer (PC) is one of the most common malignancies affecting men, with poor prognosis after progression to metastatic castration-resistant prostate cancer (mCRPC). Radioligand therapy (RLT) targeting the overexpressed PSMA on PC cells, with, e.g., ¹⁷⁷Lu-PSMA-617, has been effective in reducing tumor burden and prolonging survival in mCRPC. However, it is not a curative method with kidney and bone marrow toxicity limiting the activity given to patients. Previous preclinical models have reported transient hematotoxicity for up to 120 MBq. This activity may still be too low to investigate the effect on renal function since it corresponds to an absorbed dose below 10 Gy, whereas the kidneys in a clinical setting usually receive an absorbed dose more than double. Here we investigated the hematotoxicity and recovery after administered activities of 120, 160, and 200 MBq in a ¹⁷⁷Lu-PSMA-617 BALB/cAnNRj mouse model. The animals had an initial drop in white blood cells (WBC) starting 4 days post injection, which recovered after 21 days. The effect on red blood cells (RBC) and platelets was detected later; 17 days post-injection levels decreased compared to the control group. The reduction was restored again 32 days post injection. No correlation between injected activity and hematotoxicity was found. Our results suggest that activities up to 200 MBq of ¹⁷⁷Lu-PSMA-617 give transient hematotoxicity from which animals recover within a month and no radiation-related deaths. Injecting these high activities could allow animal studies with increased clinical relevance when studying renal toxicity in animal models.

Kidney Protection with the Radical Scavenger α1-Microglobulin (A1M) during Peptide Receptor Radionuclide and Radioligand Therapy

α₁-Microglobulin (A1M) is an antioxidant found in all vertebrates, including humans. It has enzymatic reductase activity and can scavenge radicals and bind free heme groups. Infused recombinant A1M accumulates in the kidneys and has therefore been successful in protecting kidney injuries in different animal models. In this review, we focus on A1M as a radioprotector of the kidneys during peptide receptor radionuclide/radioligand therapy (PRRT/RLT). Patients with, e.g., neuroendocrine tumors or castration resistant prostate cancer can be treated by administration of radiolabeled small molecules which target and therefore enable the irradiation and killing of cancer cells through specific receptor interaction. The treatment is not curative, and kidney toxicity has been reported as a side effect since the small, radiolabeled substances are retained and excreted through the kidneys. In recent studies, A1M was shown to have radioprotective effects on cell cultures as well as having a similar biodistribution as the somatostatin analogue peptide ¹⁷⁷Lu-DOTATATE after intravenous infusion in mice. Therefore, several animal studies were conducted to investigate the in vivo radioprotective potential of A1M towards kidneys. The results of these studies demonstrated that A1M co-infusion yielded protection against kidney toxicity and improved overall survival in mouse models. Moreover, two different mouse studies reported that A1M did not interfere with tumor treatment itself. Here, we give an overview of radionuclide therapy, the A1M physiology and the results from the radioprotector studies of the protein.

Evaluation of the PSMA-Binding Ligand 212Pb-NG001 in Multicellular Tumour Spheroid and Mouse Models of Prostate Cancer

Radioligand therapy targeting the prostate-specific membrane antigen (PSMA) is rapidly evolving as a promising treatment for metastatic castration-resistant prostate cancer. The PSMA-targeting ligand p-SCN-Bn-TCMC-PSMA (NG001) labelled with ²¹²Pb efficiently targets PSMA-positive cells in vitro and in vivo. The aim of this preclinical study was to evaluate the therapeutic potential of ²¹²Pb-NG001 in multicellular tumour spheroid and mouse models of prostate cancer. The cytotoxic effect of ²¹²Pb-NG001 was tested in human prostate C4-2 spheroids. Biodistribution at various time points and therapeutic effects of different activities of the radioligand were investigated in male athymic nude mice bearing C4-2 tumours, while long-term toxicity was studied in immunocompetent BALB/c mice. The radioligand induced a selective cytotoxic effect in spheroids at activity concentrations of 3–10 kBq/mL. In mice, the radioligand accumulated rapidly in tumours and was retained over 24 h, while it rapidly cleared from nontargeted tissues. Treatment with 0.25, 0.30 or 0.40 MBq of ²¹²Pb-NG001 significantly inhibited tumour growth and improved median survival with therapeutic indexes of 1.5, 2.3 and 2.7, respectively. In BALB/c mice, no signs of long-term radiation toxicity were observed at activities of 0.05 and 0.33 MBq. The obtained results warrant clinical studies to evaluate the biodistribution, therapeutic efficacy and toxicity of ²¹²Pb-NG001.