In the light of up regulation of glycolysis in tumors of the Warburg type and altered intracellular pathways, hyperpolarized ¹³C-labeled metabolic tracers enable new possibilities to probe fast metabolic pathways in real-time. So far only few studies have focused on the potential benefits of applying this method in the setting of therapy monitoring.^1,2 As a key enzyme in cancer cells elevated pyruvate-dehydrogenase-activity maintains the energy demand of proliferating cancer cells. Hence using hyperpolarized ¹³C-pyruvate, metabolic activity can be investigated through pyruvate to lactate conversion. One aim of this study was to assess the treatment response of malignant tumor cells with hyperpolarized [1-¹³C]-pyruvate to treatment with lipoic acid – a substance that is known to inhibit pyruvate dehydrogenase (PDH) in cells of the Warburg type. Inhibition of PDH would revert the glycolytic phenotype and stimulate cells to generate ATP via oxidative phosphorylation thus reducing cell proliferation/viability.³ Beyond we investigated if treatment with a conjugated ²¹³Bi EGFR targeting antibody (²¹³Bi-anti-EGFR-MAb) results in altered cell metabolism.In LN18 glioblastoma cells measurements were carried out after 24 h of incubation with normal cell medium or treatment with 2,5 mM alpha-lipoic acid. Human bladder cancer cells EJ28 Luc (cultivated in RPMI supplemented with 10% FCS, 1% NEA) were either left untreated or were treated with an alpha radiation emitter (²¹³Bi) conjugated with an EGFR targeting MAb for 3h. Measurements were performed 48 h thereafter. Cells were harvested immediately before the experiment. A [1-¹³C]-labeled pyruvate solution was polarized at ~1.4 K using a Hypersense dynamic nuclear polarisation (DNP) polarizer (Oxford Instruments) for approx. 1h. After dissolution, approx. 330 µl oft the hyperpolarized pyruvate solution was injected into a 5 mm tube containing LN18 glioblastoma cells and medium (RPMI + 10% FCS). The hyperpolarized label from pyruvate is intracellularly exchanged to lactate by LDH activity (TR 3s, flip angle 10°, Two-Site-Exchange Model Fit). NMR data were acquired on a Magritek spectrometer system using sequences optimized for hyperpolarized ¹³C-pyruvate detection. ¹³C-data were processed using MestReNova software (Mestrelab Research S.L.). Beyond proliferation/viability of cells (treated and untreated) was investigated using a WST-1 assay (Roche).Treatment of LN18 glioblastoma cells with 2,5 mM LPA resulted in decreased proliferation/viability compared to untreated cells by ≈ 42% (WST-1 assay). In LN18 cells hyperpolarized [1-¹³C]-labeled pyruvate measurements detected the presence of small amounts of lactate. Treatment with LPA resulted in almost no lactate signal in the spectra of these cells (Fig 1) Further analysis of the conversion rate revealed that the kinetic rate showed no significant difference between untreated and treated cells. In EJ28 Luc cells treated with ²¹³Bi-anti-EGFR-MAb more lactate was present and the metabolic conversion was higher compared to untreated controls (Fig. 2).In this study we showed that therapy response can be evaluated with hyperpolarized [1-¹³C]-pyruvate measurements. Treatment of LN18 glioblastoma cells with LPA resulted in decreased proliferation/viability and reduced lactate export pointing to the fact that cells responded to treatment. Since the kinetic exchange rates in these cells did not differ significantly between groups altered PDH activity might not be the only cause of this effect. Beyond, NMR of hyperpolarized [1-¹³C]-pyruvate proved to be adequate for monitoring the response of bladder carcinoma cells to treatment with ²¹³Bi-anti-EGFR-MAb. Cells responded with up regulated PDH activity as indicated by elevated metabolic conversion of pyruvate to lactate.Using hyperpolarized [1-¹³C]-pyruvate proved to be suitable in the light of treatment response evaluation notably in bladder carcinoma cells treated with ²¹³Bi-anti-EGFR-MAb as indicated by up regulated metabolic conversion.