Imaging tumor folate receptors using 111In-DTPA-methotrexate

It is known that membrane folic acid receptors are responsible for cellular accumulation of folate and folate analogs, such as methotrexate, and overexpressed on various tumor cells. This study was aimed to develop an 111In labelled DTPA-methotrexate (DTPA-MTX) to image tumor folate receptors in vivo. DTPA-MTX was synthesized by reacting ethylenediamine with MTX. The resulting amino analogue of MTX was reacted with DTPA dianhydride in basic aqueous solution followed by dialysis. Tissue distribution was determined in breast tumor-bearing rats at 0.5, 2, 24, and 48 h (n = 3/time interval). To determine receptor-mediated process 111In-DTPA-MTX was co-administrated with varying blocking doses of cold folate to tumor-bearing rats. Planar imaging and whole-body autoradiograms were performed. The data was compared to that using 111In-DTPA. In animal studies, tumor/blood count density ratios at 0.5-48 h gradually increased from 0.8 +/- 0.32 to 2.2 +/- 0.41 with 111In-DTPA-MTX. Conversely, these values showed time-dependent decrease from 1.19 +/- 0.69 to 0.56 +/- 0.10 with 111In-DTPA in the same time period. Tumor/muscle and tumor/blood count density ratios significantly decreased with high doses of folic acid co-administration. Planar images and autoradiograms confirmed that the tumors could be visualized acceptably with 111In-DTPA-MTX. The results indicate the feasibility of using 111In-DTPA-MTX to image tumors through a folate receptor-mediated process.