(11)C-methylaminoisobutyric acid (MeAIB) PET for evaluation of prostate cancer: compared with (18)F-fluorodeoxyglucose PET

Sulfur metabolism and its contribution to malignancy

Metabolic dysregulation is an appreciated hallmark of cancer and a target for therapeutic intervention. Cellular metabolism involves a series of oxidation/reduction (redox) reactions that yield the energy and biomass required for tumor growth. Cells require diverse molecular species with constituent sulfur atoms to facilitate these processes. For humans, this sulfur is derived from the dietary consumption of the proteinogenic amino acids cysteine and methionine, as only lower organisms (e.g., bacteria, fungi, and plants) can synthesize them de novo. In addition to providing the sulfur required to sustain redox chemistry, the metabolism of these sulfur-containing amino acids yield intermediate metabolites that constitute the cellular antioxidant system, mediate inter- and intracellular signaling, and facilitate the epigenetic regulation of gene expression, all of which contribute to tumorigenesis.

Differential Diagnosis between Low-Grade and High-Grade Astrocytoma Using System A Amino Acid Transport PET Imaging with C-11-MeAIB: A Comparison Study with C-11-Methionine PET Imaging

Introductions

[N-methyl-C-11]α-Methylaminoisobutyric acid (MeAIB) is an artificial amino acid radiotracer used for PET study, which is metabolically stable in vivo. In addition, MeAIB is transported by system A neutral amino acid transport, which is observed ubiquitously in all types of mammalian cells. It has already been shown that MeAIB-PET is useful for malignant lymphoma, head and neck cancers, and lung tumors. However, there have been no reports evaluating the usefulness of MeAIB-PET in the diagnosis of brain tumors. The purpose of this study is to investigate the efficacy of system A amino acid transport PET imaging, MeAIB-PET, in clinical brain tumor diagnosis compared to [S-methyl-C-11]-L-methionine (MET)-PET.

Methods

Thirty-one consecutive patients (male: 16, female: 15), who were suspected of having brain tumors, received both MeAIB-PET and MET-PET within a 2-week interval. All patients were classified into two groups: Group A as a benign group, which included patients who were diagnosed as low-grade astrocytoma, grade II or less, or other low-grade astrocytoma (n=12) and Group B as a malignant group, which included patients who were diagnosed as anaplastic astrocytoma, glioblastoma multiforme (GBM), or recurrent GBM despite prior surgery or chemoradiotherapy (n=19). PET imaging was performed 20 min after the IV injection of MeAIB and MET, respectively. Semiquantitative analyses of MeAIB and MET uptake using SUVmax and tumor-to-contralateral normal brain tissue (T/N) ratio were evaluated to compare these PET images. ROC analyses for the diagnostic accuracy of MeAIB-PET and MET-PET were also calculated.

Results

In MeAIB-PET imaging, the SUVmax was 1.20 ± 1.29 for the benign group and 2.94 ± 1.22 for the malignant group (p < 0.005), and the T/N ratio was 3.77 ± 2.39 for the benign group and 16.83 ± 2.39 for the malignant group (p < 0.001). In MET-PET, the SUVmax was 3.01 ± 0.94 for the benign group and 4.72 ± 1.61 for the malignant group (p < 0.005), and the T/N ratio was 2.64 ± 1.40 for the benign group and 3.21 ± 1.14 for the malignant group (n.s.). For the analysis using the T/N ratio, there was a significant difference between the benign and malignant groups with MeAIB-PET with p < 0.001. The result of ROC analysis using the T/N ratio indicated a better diagnosis accuracy for MeAIB-PET for brain tumors than MET-PET (p < 0.01).

Conclusions

MeAIB, a system A amino acid transport-specific radiolabeled agents, could provide better assessments for detecting malignant type brain tumors. In a differential diagnosis between low-grade and high-grade astrocytoma, MeAIB-PET is a useful diagnostic imaging tool, especially in evaluations using the T/N ratio.

Clinical trial registration

This trial was registered with UMIN000032498.

Relationship between [14C]MeAIB uptake and amino acid transporter family gene expression levels or proliferative activity in a pilot study in human carcinoma cells: Comparison with [3H]methionine uptake

INTRODUCTION

To clarify the difference between system A and L amino acid transport imaging in PET clinical imaging, we focused on the use of α-[N-methyl-¹¹C]-methylaminoisobutyric acid ([¹¹C]MeAIB), and compared it with [S-methyl-¹¹C]-L-methionine ([¹¹C]MET). The aim of this study was to assess the correlation of accumulation of these two radioactive amino acid analogs with expression of amino acid transporters and cell proliferative activity in carcinoma cells.

METHODS

Amino acid uptake inhibitor studies were performed in four human carcinoma cells (epidermal carcinoma A431, colorectal carcinoma LS180, and lung carcinomas PC14/GL and H441/GL) using the radioisotope analogs [³H]MET and [¹⁴C]MeAIB. MeAIB was used to inhibit the A system and 2-amino-2-norbornane-carboxylic acid (BCH) was used to inhibit the L system. The carcinoma gene expression levels of a number of amino acid transporters were measured by microarray and quantitative polymerase chain reaction. Carcinoma proliferative activity was assessed using accumulation of [methyl-³H]-3'-deoxy-3'-fluorothymidine ([³H]FLT).

RESULTS AND CONCLUSION

[¹⁴C]MeAIB uptake occurred principally via a Na⁺-dependent A type mechanism whereas [³H]MET uptake occurred predominantly via a Na⁺-independent L type mechanism although other transporters were also utilized depending on cell type. There was no correlation between [³H]MET uptake and total system L amino acid transporter (LAT) expression. In contrast, [¹⁴C]MeAIB uptake strongly correlated with total system A amino acid transporter (SNAT) expression and proliferative activity in this preliminary study using four human carcinoma cell lines. Carcinoma proliferative activity also correlated with total SNAT expression. Advances in Knowledge and Implications for Patient Care: Because there is a significant correlation between the accumulation of [¹⁴C]MeAIB and the gene expression level of total SNAT as well as the accumulation of [³H]FLT, it is suggested that use of the analog [¹¹C]MeAIB in PET may provide an indication of tumor cell proliferative activity. [¹¹C]MeAIB is therefore expected to be very useful in PET imaging.

11C-Choline-Avid but 18F-FDG-Nonavid Prostate Cancer with Lymph Node Metastases on Positron Emission Tomography

Choline is a new positron emission tomography (PET) tracer useful for detection of prostate cancer and metastatic lesions. We report a 70-year-old man with prostate cancer and multiple abdominal, pelvic, and inguinal node metastases. PET scans demonstrated accumulation of ¹¹C-choline in the primary tumor and lymph node metastases but no accumulation of ¹⁸F-FDG. Choline PET/computed tomography may be useful for diagnosis of advanced prostate cancer with suspected metastatic lesions and treatment planning.

PET imaging of recurrent and metastatic prostate cancer with novel tracers

Early detection of recurrent prostate cancer (PCa) is of paramount importance to deliver prompt and accurate therapy reducing the chance of progression to metastatic disease. However, current imaging modalities such as conventional computed tomography, MRI and PET scanning do not provide sufficient sensitivity, especially at lower prostate-specific antigen values. Moreover, biological characterization of PCa has become increasingly important to provide patient-specific therapy and current imaging poorly characterizes disease aggressiveness. The current uprise of novel PET tracers in recurrent and metastatic PCa shows promising, yet variable sensitivities and specificities in detection, indicating the need for further studies. In this review, we highlight current and new PET tracers that have been developed to improve the detection of recurrent and metastatic PCa.