Contributors: Tristan Barrett and Ferdia Gallagher
Prostate cancer (PCa) is the second commonest and the fifth deadliest male cancer worldwide. The key diagnostic challenge in PCa is differentiating indolent from clinically significant disease, with the latter requiring more stringent patient follow-up and/or immediate therapeutic intervention. One major breakthrough towards addressing this challenge has been the recent introduction of pre-biopsy multiparametric magnetic resonance imaging (mpMRI) as the first-line PCa diagnostic tool. mpMRI accurately rules out PCa in >90% cases1, halves the number of unnecessary biopsies2, and reduces the over-detection of indolent disease by ~13%3. However, mpMRI has a poor positive predictive value for detecting clinically significant PCa of ~35%4, which still necessitates the invasive step of tissue biopsy. Outside the diagnostic setting, mpMRI is also limited in its ability to monitor treatment response in advanced disease, making it difficult to visualise residual PCa foci within an ablated organ that could otherwise be treated using focal therapy.
Imaging metabolic alterations that occur throughout tumorigenesis is a promising approach for improving the diagnostic potential of standard-of-care mpMRI. Hyperpolarised [1-13C]pyruvate MRI (HP-13C-MRI) is an emerging clinical imaging technique5 that can visualise real-time pyruvate-to-lactate conversion that is a known hallmark of cancer metabolism. Prior studies in PCa have demonstrated the relationship between Gleason grade and increased [1-13C]lactate labelling6, with the latter being decreased following androgen deprivation therapy7 and immunotherapy8. Despite the promising nature of these early data, both the speed and direction of HP-13C-MRI clinical translation depend on addressing several clinical and biological questions that we aimed to dissect as part of the present study9.
Firstly, in order to ascertain the potential diagnostic value of HP-13C-MRI, we correlated tumour [1-13C]lactate labelling with standard-of-care pathological and imaging biomarkers of clinically significant PCa. We showed that [1-13C]lactate labelling correlated significantly with the percentage of Gleason pattern 4 (%GP4) disease, which is a more granular continuous histopathological metric of PCa aggressiveness compared to Gleason grade. Clinically, biopsy-derived %GP4 is used to guide both the baseline selection and follow-up assessment of patients enrolled on PCa active surveillance. HP-13C-MRI can, therefore, offer a non-invasive quantitative alternative for its accurate evaluation, which presents an important avenue for the clinical translation of the technique. In addition, we confirmed previous reports of the ability of HP-13C-MRI to visualise mpMRI-occult lesions, which is critical for assessing the burden of multifocal disease and making appropriate management decisions, e.g. when considering focal therapy.
Secondly, to better understand the biological processes underpinning [1-13C]lactate labelling in PCa, we quantified the tissue-based expression of the key pyruvate and lactate transporters (MCT1 and MCT4) alongside mRNA expression of lactate dehydrogenase (LDHA and LDHB). We showed that [1-13C]lactate labelling correlates strongly with the combined epithelial LDH expression (LDHA + LDHB) rather than individual isoenzyme expression. We also observed a specific MCT staining pattern, whereby pyruvate-importing MCT1 was primarily expressed on tumour epithelial cells and lactate-exporting MCT4 was seen almost exclusively in the stromal compartment. In more aggressive lesions MCT4, appeared on epithelial cells as well, which explains the strong positive correlation noted between [1-13C]lactate labelling and epithelium-to-stroma MCT4 ratio. Finally, we showed that [1-13C]lactate labelling correlates significantly with the number of tumour epithelial cells that overexpress MCT1, which prompted us to conclude that HP-13C-MRI specifically measures epithelial cell metabolism. These findings may provide a mechanistic explanation for the previously reported post-treatment HP-13C-MRI changes7,8 as both androgen deprivation therapy and immunotherapy primarily target tumour epithelial cell compartment. These data also support the use of HP-13C-MRI as a treatment response assessment tool for therapeutics that target LDH or MCT1/4 expression.
Thirdly, to move from correlative to comparative analysis, we analysed the epithelial expression of the above markers in tumours with low (<10%) and high (>10%) %GP4. We saw that high %GP4 lesions showed significantly higher combined epithelial LDH expression and epithelial-to-stroma MCT4 ratio, supporting higher [1-13C]lactate labelling seen in more aggressive disease on HP-13C-MRI. We also reported a significantly higher LDHA/PDHA1 ratio in high %GP4 disease, suggesting a metabolic shift from oxidative phosphorylation (pyruvate converted to acetyl-CoA via PDH complex) to glycolysis (pyruvate converted to lactate via LDHA). The same trend was also observed in individual Gleason pattern 3, 4, and 5 malignant glands, which supports the presence of a continuous oxidative phosphorylation to glycolysis shift throughout PCa tumorigenesis at all levels. These findings not only offer a potential mechanistic explanation for differential [1-13C]lactate labelling in PCa of different aggressiveness but also provide evidence for the use of the above molecules as part of PCa tissue-based metabolic biomarker panels.
These results demonstrate the ability of HP-13C-MRI to non-invasively probe %GP4 and provide additional information to grade-dependent disease differentiation used as part of the standard of care. We also provide evidence that increasing [1-13C]lactate labelling is a reflection of tumour epithelial cell metabolism, as opposed to stromal metabolism, as demonstrated by an increase in the combined tumour epithelial LDH expression and epithelium-to-stroma MCT4 ratio. At the tissue level, we linked these findings to a possible reciprocal relationship between tumour glycolysis and oxidative phosphorylation, establishing RNAscope-derived LDHA/PDHA1 ratio as a potential biomarker of PCa aggressiveness. Our future work will be aimed at validating these findings in larger patient cohorts by applying them to the specific clinical scenarios of treatment response assessment and active surveillance management, as well as to tissue-based PCa metabolic biomarker development.
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- Barrett, T. et al. Three-year experience of a dedicated prostate mpMRI pre-biopsy programme and effect on timed cancer diagnostic pathways. Clin. Radiol. 74, (2019).
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- Chen, H. Y. et al. Hyperpolarized 13C-pyruvate MRI detects real-time metabolic flux in prostate cancer metastases to bone and liver: a clinical feasibility study. Prostate Cancer Prostatic Dis. 23, 269–276 (2020).
- Kouchkovsky, I. de et al. Hyperpolarized 1-[13C]-Pyruvate Magnetic Resonance Imaging Detects an Early Metabolic Response to Immune Checkpoint Inhibitor Therapy in Prostate Cancer. Eur. Urol. 0, (2021).
- Sushentsev, N. et al. Hyperpolarised 13C-MRI identifies the emergence of a glycolytic cell population within intermediate-risk human prostate cancer. Nat. Commun. 2022 131 13, 1–12 (2022).