Metabolic magnetic resonance imaging (MRI) using hyperpolarized (HP) pyruvate is becoming a non‐invasive technique for diagnosing, staging, and monitoring response to treatment in cancer and other diseases. The clinically established method for producing HP pyruvate, dissolution dynamic nuclear polarization, however, is rather complex and slow. Signal Amplification By Reversible Exchange (SABRE) is an ultra‐fast and low‐cost method based on fast chemical exchange. Here, for the first time, we demonstrate not only in vivo utility, but also metabolic MRI with SABRE. We present a novel routine to produce aqueous HP [1‐¹³C]pyruvate‐d₃ for injection in 6 minutes. The injected solution was sterile, non‐toxic, pH neutral and contained ≈30 mM [1‐¹³C]pyruvate‐d₃ polarized to ≈11 % (residual 250 mM methanol and 20 μM catalyst). It was obtained by rapid solvent evaporation and metal filtering, which we detail in this manuscript. This achievement makes HP pyruvate MRI available to a wide biomedical community for fast metabolic imaging of living organisms.

Cathepsin D (CTSD) is a lysosomal protease and a marker of poor prognosis in breast cancer. However, the cells responsible for this association and the function of CTSD in cancer are still incompletely understood. By using a conditional CTSD knockout mouse crossed to the transgenic MMTV-PyMT breast cancer model we demonstrate that CTSD deficiency in the mammary epithelium, but not in myeloid cells, blocked tumor development in a cell-autonomous manner. We show that lack of CTSD impaired mechanistic Target of Rapamycin Complex 1 (mTORC1) signaling and induced reversible cellular quiescence. In line, CTSD-deficient tumors started to grow with a two-month delay and quiescent Ctsd^-/- tumor cells re-started proliferation upon long-term culture. This was accompanied by rewiring of oncogenic gene expression and signaling pathways, while mTORC1 signaling remained permanently disabled in CTSD-deficient cells. Together, these studies reveal a tumor cell-autonomous effect of CTSD deficiency, and establish a pivotal role of this protease in the cellular response to oncogenic stimuli.

Objectives: To evaluate the potential of diffusion-weighted imaging (DWI) for monitoring dose-dependent tumor response in a mouse-xenograft model of Ewing sarcoma after administration of treosulfan in different dosages. Materials and Methods: Ewing sarcoma implanted orthotopically into the left thigh of non-obese diabetic/severe combined immunodeficiency (NOD/SCID) mice were evaluated with a 9.4 T MR unit by using a specific mouse whole body coil. Transverse T2-weighted fast spin-echo sequences, T1-weighted spin-echo sequences, and transverse echo-planar DWI examinations were performed at baseline, 24 hours and 72 hours after intraperitoneal injection of treosulfan in 2 different doses. The apparent diffusion coefficient (ADC) in the viable parts of the tumor was automatically calculated from DWI imaging findings. These findings were correlated with histopathologic results at each time point. Volumetric measurements were performed by summing up the regions of interest in consecutive slices. Results: T1- and T2-weighted images before administration of treosulfan showed viable tumor tissue with small necrotic areas. At 24 hours after treosulfan injection, a significantly higher increase in ADC in the viable parts of the tumor could be observed in tumors of mice that had been injected with the higher dose of treosulfan compared with mice injected with the lower dose treosulfan, whereas no significant volumetric differences between the 2 different doses could be observed. Seventy-two hours after injection of treosulfan the ADC values in the viable parts of the tumor of mice treated with the high dose of treosulfan further increased and the tumor volume in the high-dose group was now significantly lower than in the low-dose group. Conclusion: Compared with volumetric measurements, DWI of the viable tumor parts could be used to discriminate between the effects of 2 different dosages at an earlier time point than volumetric measurements in an animal model in vivo. This method could be especially useful for monitoring drug effects in phase I/II clinical trials.

Despite recent advances in therapy, liver metastasis from melanoma is still associated with poor prognosis. Although targeting the mTOR signaling pathway exerts potent anti-tumor activity, little is known about specific mTORC2 inhibition regarding liver metastasis. Using the novel mTORC2 specific inhibitor JR-AB2-011, we show significantly reduced migration and invasion capacity by impaired activation of MMP2 in melanoma cells. In addition, blockade of mTORC2 induces cell death by non-apoptotic pathways and reduces tumor cell proliferation rate dose-dependently. Furthermore, a significant reduction of liver metastasis was detected in a syngeneic murine metastasis model upon therapy with JR-AB2-011 as determined by in vivo imaging and necropsy. Hence, our study for the first time highlights the impact of the pharmacological blockade of mTORC2 as a potent novel anti-cancer approach for liver metastasis from melanoma.