Metastases expressing tumor-specific receptors can be targeted and treated by binding of radiolabeled peptides (peptide receptor radionuclide therapy or PRRT). For example, patients with metastasized somatostatin receptor-positive neuroendocrine tumors (NETs) can be treated with radiolabeled somatostatin analogues, resulting in strongly increased progression-free survival and quality of life. There is nevertheless still room for improvement, as very few patients can be cured at this stage of disease. We aimed to specifically sensitize replicating tumor cells without further damage to healthy tissues. Thereto we investigated the DNA damaging effects of PRRT with the purpose to enhance these effects through modulation of the DNA damage response. Although PRRT induces DNA double strand breaks (DSBs), a larger fraction of the induced lesions are single strand breaks (expected to be similar to those induced by external beam radiotherapy) that require poly-[ADP-ribose]-polymerase 1 (PARP-1) activity for repair. If these breaks cannot be repaired, they will cause replication fork arrest and DSB formation during replication. Therefore, we used the PARP-1 inhibitor Olaparib to increase the number of cytotoxic DSBs. Here we show that this new combination strategy synergistically sensitized somatostatin receptor expressing cells to PRRT. We observed increased cell death and reduced cellular proliferation compared to the PRRT alone. The enhanced cell death was caused by increased numbers of DSBs that are repaired with remarkably slow kinetics, leading to genome instability. Furthermore, we validated the increased DSB induction after PARP inhibitor addition in the clinically relevant model of living human NET slices. We expect that this combined regimen can thus augment current PRRT outcomes.

Peptide receptor scintigraphy (PRS) and peptide receptor radionuclide therapy (PRRT) using radiolabeled somatostatin receptor (SSTR) agonists are successfully being applied in the clinic for imaging and treatment of neuroendocrine tumors. Contrary to the paradigm that internalization and the resulting accumulation of radiotracers in cells is necessary for efficient tumor targeting, recent studies have demonstrated superiority of radiolabeled SSTR antagonists for imaging purposes, despite little to no internalization in cells. However, studies comparing the therapeutic anti-tumor effects of radiolabeled SSTR agonists versus antagonists are lacking. The aim of this study was to directly compare the therapeutic effect of ¹⁷⁷Lu-DOTA-octreotate, an SSTR agonist, and ¹⁷⁷Lu-DOTA-JR11, an SSTR antagonist. Methods: We analyzed radiotracer uptake (both membrane-bound and internalized fractions) and the produced DNA double strand breaks, by determining the number of p53 binding protein 1 (53BP1) foci, after incubating SSTR2 positive cells with ¹⁷⁷Lu-DTPA, ¹⁷⁷Lu-DOTA-octreotate or ¹⁷⁷Lu-DOTA-JR11. Also, biodistribution studies were performed in tumor xenografted mice to determine the optimal dose for therapy experiments. Afterwards in vivo therapy experiments comparing the effect of ¹⁷⁷Lu-DOTA-octreotate and ¹⁷⁷Lu-DOTA-JR11 were performed in this same animal model. Results: We found a 5 times higher uptake of ¹⁷⁷Lu-DOTA-JR11 compared to that of ¹⁷⁷Lu-DOTA-octreotate. The major part (88±1%) of the antagonist uptake was membrane bound, while 74±3% of the total receptor agonist uptake was internalized. Cells treated with ¹⁷⁷Lu-DOTA-JR11 showed 2 times more 53BP1 foci compared to cells treated with ¹⁷⁷Lu-DOTA-octreotate. Biodistribution studies with ¹⁷⁷Lu-DOTA-JR11 (0.5 μg/30 MBq) resulted in the highest tumor radiation dose of 1.8±0.7 Gy/MBq, 4.4 times higher than the highest tumor radiation dose found for ¹⁷⁷Lu-DOTA-octreotate. In vivo therapy studies with ¹⁷⁷Lu-DOTA-octreotate and ¹⁷⁷Lu-DOTA-JR11 resulted in a tumor growth delay time of 18±5 d and 26±7 d, respectively. Median survival rates were 43.5 d, 61 d and 71 d for the control group, the ¹⁷⁷Lu-DOTA-octreotate and the ¹⁷⁷Lu-DOTA-JR11 treated group, respectively. Conclusion: Based on these results we conclude that the use of radiolabeled SSTR antagonists such as JR11 might enhance PRS and PRRT of neuroendocrine tumors and provide successful imaging and therapeutic strategies for cancer types with relatively low SSTR expression.

DNA damage-inducing therapies are of tremendous value for cancer treatment and function by the direct or indirect formation of DNA lesions and subsequent inhibition of cellular proliferation. Of central importance in the cellular response to therapy-induced DNA damage is the DNA damage response (DDR), a protein network guiding both DNA damage repair and the induction of cancer-eradicating mechanisms such as apoptosis. A detailed understanding of DNA damage induction and the DDR has greatly improved our knowledge of the classical DNA damage-inducing therapies, radiotherapy and cytotoxic chemotherapy, and has paved the way for rational improvement of these treatments. Moreover, compounds targeting specific DDR proteins, selectively impairing DNA damage repair in cancer cells, form a promising novel therapy class that is now entering the clinic. In this review, we give an overview of the current state and ongoing developments, and discuss potential avenues for improvement for DNA damage-inducing therapies, with a central focus on the role of the DDR in therapy response, toxicity and resistance. Furthermore, we describe the relevance of using combination regimens containing DNA damage-inducing therapies and how they can be utilized to potentiate other anticancer strategies such as immunotherapy.

The biogenesis of ribosomes and their coordination of protein translation consume an enormous amount of cellular energy. As such, it has been established that the inhibition of either process can extend eukaryotic lifespan. Here, we used next-generation sequencing to compare ribosome-associated RNAs from normal strains of Caenorhabditis elegans to those carrying the life-extending daf-2 mutation. We found a long noncoding RNA (lncRNA), transcribed telomeric sequence 1 (tts-1), on ribosomes of the daf-2 mutant. Depleting tts-1 in daf-2 mutants increases ribosome levels and significantly shortens their extended lifespan. We find tts-1 is also required for the longer lifespan of the mitochondrial clk-1 mutants but not the feeding-defective eat-2 mutants. In line with this, the clk-1 mutants express more tts-1 and fewer ribosomes than the eat-2 mutants. Our results suggest that the expression of tts-1 functions in different longevity pathways to reduce ribosome levels in a way that promotes life extension.

Purpose: Various radiolabeled prostate-specific membrane antigen (PSMA)–targeting tracers are clinically applied for prostate cancer (PCa) imaging and targeted radionuclide therapy. The PSMA binding affinities, biodistribution, and DNA-damaging capacities of these radiotracers have not yet been compared in detail. A major concern of PSMA-targeting radiotracers is the toxicity in other PSMA-expressing organs, such as the salivary glands, thus demanding careful evaluation of the most optimal and safest radiotracer. In this extensive preclinical study, we evaluated the clinically applied PSMA-targeting small molecule inhibitors DOTA-PSMA-617 (PSMA-617) and DOTAGA-PSMA-I&T (PSMA-I&T) and the PSMA nanobody DOTA-JVZ-007 (JVZ-007) using PSMA-expressing cell lines, a unique set of PCa patient-derived xenografts (PDX) and healthy human tissues. Methods and results: In vitro displacement studies on PSMA-expressing cells and cryosections of a PSMA-positive PDX revealed high and specific binding affinity for all three tracers labeled with lutetium-177 with IC₅₀ values in the nanomolar range. Interestingly, [¹⁷⁷Lu]Lu-JVZ-007 could not be displaced by PSMA-617 or PSMA-I&T, suggesting that this tracer targets an alternative binding site. Autoradiography assays on cryosections of human salivary and renal tissues revealed [¹⁷⁷Lu]Lu-PSMA-617 to have the lowest binding to these healthy organs compared with [¹⁷⁷Lu]Lu-PSMA-I&T. In vivo biodistribution assays confirmed the in vitro results with comparable tumor uptake of [¹⁷⁷Lu]Lu-PSMA-617 and [¹⁷⁷Lu]Lu-PSMA-I&T at all timepoints, resulting in induction of similar levels of DNA double-strand breaks in the tumors. However, [¹⁷⁷Lu]Lu-PSMA-I&T demonstrated approximately 40× higher renal uptake at 4 and 8 h post injection resulting in an unfavorable tumor-to-kidney ratio. Conclusion: [¹⁷⁷Lu]Lu-PSMA-617 has the most favorable biodistribution in mice as well as more favorable binding characteristics in vitro in PSMA-positive cells and human kidney and salivary gland specimens compared with [¹⁷⁷Lu]Lu-PSMA-I&T and [¹⁷⁷Lu]Lu-JVZ-007. Based on our preclinical evaluation, [¹⁷⁷Lu]Lu-PSMA-617 is the best performing tracer to be taken further into clinical evaluation for PSMA-targeted radiotherapeutic development although with careful evaluation of the tracer binding to PSMA-expressing organs.

Background: Clinical and preclinical studies have revealed that alterations in DNA damage response (DDR) pathways may play an important role in prostate cancer (PCa) etiology and progression. These alterations can influence PCa responses to radiotherapy and anti-androgen treatment. The identification of DNA repair gene aberrations in PCa has driven the interest for further evaluation whether these genetic changes may serve as biomarkers for patient stratification. Methods: In this review, we summarize the current knowledge on DDR alterations in PCa, their potential impact on clinical interventions and prospects for improved management of PCa. We particularly focus on the influence of DDR gene mutations on PCa initiation and progression and describe the underlying mechanisms. Results and Conclusions: A better understanding of these mechanisms, will contribute to better disease management as treatment strategies can be chosen based on the specific disease properties, since a growing number of treatments are targeting DDR pathway alterations (such as Poly(ADP-ribose) polymerase inhibitors). Furthermore, the recently discovered crosstalk between the DDR and androgen receptor signaling opens a new array of possible strategies to optimize treatment combinations. We discuss how these recent and ongoing studies will help to improve diagnostic, prognostic and therapeutic approaches for PCa management.