The prostate-specific membrane antigen holds potential as a vascular target for endogenous radiotherapy with [177Lu]Lu-PSMA-I&T for triple-negative breast cancer.

INTRODUCTION: Overexpression of prostate-specific membrane antigen (PSMA) on the vasculature of triple-negative breast cancer (TNBC) presents a promising avenue for targeted endogenous radiotherapy with [177Lu]Lu-PSMA-I&T. This study aimed to assess and compare the therapeutic efficacy of a single dose with a fractionated dose of [177Lu]Lu-PSMA-I&T in an orthotopic model of TNBC. METHODS: Rj:NMRI-Foxn1nu/nu mice were used as recipients of MDA-MB-231 xenografts. The single dose group was treated with 1 × 60 ± 5 MBq dose of [177Lu]Lu-PSMA-I&T, while the fractionated dose group received 4 × a 15 ± 2 MBq dose of [177Lu]Lu-PSMA-I&T at 7 day intervals. The control group received 0.9% NaCl. Tumor progression was monitored using [18F]FDG-PET/CT. Ex vivo analysis encompassed immunostaining, TUNEL staining, H&E staining, microautoradiography, and autoradiography. RESULTS: Tumor volumes were significantly smaller in the single dose (p < 0.001) and fractionated dose (p < 0.001) groups. Tumor growth inhibition rates were 38% (single dose) and 30% (fractionated dose). Median survival was notably prolonged in the treated groups compared to the control groups (31d, 28d and 19d for single dose, fractionated dose and control, respectively). [177Lu]Lu-PSMA-I&T decreased the size of viable tumor areas. We further demonstrated, that [177Lu]Lu-PSMA-I&T binds specifically to the tumor-associated vasculature. CONCLUSION: This study highlights the potential of [177Lu]Lu-PSMA-I&T for endogenous radiotherapy of TNBC.

Differential effects of familial Alzheimer's disease-causing mutations on amyloid precursor protein (APP) trafficking, proteolytic conversion, and synaptogenic activity.

The amyloid precursor protein (APP) is a key player in Alzheimer`s disease (AD) and the precursor of the Aß peptide, which is generated by consecutive cleavages of ß- and γ-secretases. Familial Alzheimer's disease (FAD) describes a hereditary subgroup of AD that represents a low percentage of AD cases with an early onset of the disease. Different APP FAD mutations are thought to have qualitatively different effects on its proteolytic conversion. However, few studies have explored the pathogenic and putative physiological differences in more detail. Here, we compared different FAD mutations, located at the ß- (Swedish), α- (Flemish, Arctic, Iowa) or γ-secretase (Iberian) cleavage sites. We examined heterologous expression of APP WT and FAD mutants in non-neuronal cells and their impact on presynaptic differentiation in contacting axons of co-cultured neurons. To decipher the underlying molecular mechanism, we tested the subcellular localization, the endocytosis rate and the proteolytic processing in detail by immunoprecipitation-mass spectrometry. Interestingly, we found that only the Iberian mutation showed altered synaptogenic function. Furthermore, the APP Iowa mutant shows significantly decreased α-secretase processing which is in line with our results that APP carrying the Iowa mutation was significantly increased in early endosomes. However, most interestingly, immunoprecipitation-mass spectrometry analysis revealed that the amino acid substitutions of APP FAD mutants have a decisive impact on their processing reflected in altered Aß profiles. Importantly, N-terminally truncated Aß peptides starting at position 5 were detected preferentially for APP Flemish, Arctic, and Iowa mutants containing amino acid substitutions around the α-secretase cleavage site. The strongest change in the ratio of Aß40/Aß42 was observed for the Iberian mutation while APP Swedish showed a substantial increase in Aß1-17 peptides. Together, our data indicate that familial AD mutations located at the α-, ß-, and γ-secretase cleavage sites show considerable differences in the underlying pathogenic mechanisms.

The Potential of PSMA as a Vascular Target in TNBC.

Recent studies proving prostate-specific membrane antigen (PSMA) expression on triple-negative breast cancer (TNBC) cells and adjacent endothelial cells suggest PSMA as a promising target for therapy of until now not-targetable cancer entities. In this study, PSMA and its isoform expression were analyzed in different TNBC cells, breast cancer stem cells (BCSCs), and tumor-associated endothelial cells. PSMA expression was detected in 91% of the investigated TNBC cell lines. The PSMA splice isoforms were predominantly found in the BCSCs. Tumor-conditioned media from two TNBC cell lines, BT-20 (high full-length PSMA expression, PSMAΔ18 expression) and Hs578T (low full-length PSMA expression, no isoform expression), showed significant pro-angiogenic effect with induction of tube formation in endothelial cells. All TNBC cell lines induced PSMA expression in human umbilical vein endothelial cells (HUVEC). Significant uptake of radiolabeled ligand [68Ga]Ga-PSMA was detected in BCSC1 (4.2%), corresponding to the high PSMA expression. Moreover, hypoxic conditions increased the uptake of radiolabeled ligand [177Lu]Lu-PSMA in MDA-MB-231 (0.4% vs. 3.4%, under hypoxia and normoxia, respectively) and MCF-10A (0.3% vs. 3.0%, under normoxia and hypoxia, respectively) significantly (p < 0.001). [177Lu]Lu-PSMA-induced apoptosis rates were highest in BT-20 and MDA-MB-231 associated endothelial cells. Together, these findings demonstrate the potential of PSMA-targeted therapy in TNBC.

Development of Radiotracers for Breast Cancer-The Tumor Microenvironment as an Emerging Target.

Molecular imaging plays an increasingly important role in the diagnosis and treatment of different malignancies. Radiolabeled probes enable the visualization of the primary tumor as well as the metastases and have been also employed in targeted therapy and theranostic approaches. With breast cancer being the most common malignancy in women worldwide it is of special interest to develop novel targeted treatments. However, tumor microenvironment and escape mechanisms often limit their therapeutic potential. Addressing tumor stroma associated targets provides a promising option to inhibit tumor growth and angiogenesis and to disrupt tumor tissue architecture. This review describes recent developments on radiolabeled probes used in diagnosis and treatment of breast cancer especially in triple negative type with the focus on potential targets offered by the tumor microenvironment, like tumor associated macrophages, cancer associated fibroblasts, and endothelial cells.

The Rab5 activator RME-6 is required for amyloid precursor protein endocytosis depending on the YTSI motif.

Endocytosis of the amyloid precursor protein (APP) is critical for generation of ß-amyloid, aggregating in Alzheimer's disease. APP endocytosis depending on the intracellular NPTY motif is well investigated, whereas involvement of the YTSI (also termed BaSS) motif remains controversial. Here, we show that APP lacking the YTSI motif (ΔYTSI) displays reduced localization to early endosomes and decreased internalization rates, similar to APP ΔNPTY. Additionally, we show that the YTSI-binding protein, PAT1a interacts with the Rab5 activator RME-6, as shown by several independent assays. Interestingly, knockdown of RME-6 decreased APP endocytosis, whereas overexpression increased the same. Similarly, APP ΔNPTY endocytosis was affected by PAT1a and RME-6 overexpression, whereas APP ΔYTSI internalization remained unchanged. Moreover, we could show that RME-6 mediated increase of APP endocytosis can be diminished upon knocking down PAT1a. Together, our data identify RME-6 as a novel player in APP endocytosis, involving the YTSI-binding protein PAT1a.