Tryptophan metabolism in breast cancers: molecular imaging and immunohistochemistry studies

Tryptophan oxidation via the kynurenine pathway is an important mechanism of tumoral immunoresistance. Increased tryptophan metabolism via the serotonin pathway has been linked to malignant progression in breast cancer. In this study, we combined quantitative positron emission tomography (PET) with tumor immunohistochemistry to analyze tryptophan transport and metabolism in breast cancer.

Dynamic α-[(11)C]methyl-l-tryptophan (AMT) PET was performed in nine women with stage II-IV breast cancer. PET tracer kinetic modeling was performed in all tumors. Expression of L-type amino acid transporter 1 (LAT1), indoleamine 2,3-dioxygenase (IDO; the initial and rate-limiting enzyme of the kynurenine pathway) and tryptophan hydroxylase 1 (TPH1; the initial enzyme of the serotonin pathway) was assessed by immunostaining of resected tumor specimens.

Tumor AMT uptake peaked at 5-20 min postinjection in seven tumors; the other two cases showed protracted tracer accumulation. Tumor standardized uptake values (SUVs) varied widely (2.6-9.8) and showed a strong positive correlation with volume of distribution values derived from kinetic analysis (P<.01). Invasive ductal carcinomas (n=6) showed particularly high AMT SUVs (range, 4.7-9.8). Moderate to strong immunostaining for LAT1, IDO and TPH1 was detected in most tumor cells.

Breast cancers show differential tryptophan kinetics on dynamic PET. SUVs measured 5-20 min postinjection reflect reasonably the tracer's volume of distribution. Further studies are warranted to determine if in vivo AMT accumulation in these tumors is related to tryptophan metabolism via the kynurenine and serotonin pathways.