Bioconjugate Chem. 2000, 11, 253−257
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Synthesis of [99mTc]DTPA-Folate and Its Evaluation as a Folate-Receptor-Targeted Radiopharmaceutical Carla J. Mathias, David Hubers, Philip S. Low,† and Mark A. Green* Department of Medicinal Chemistry and Molecular Pharmacology, 1333 Pharmacy Building, and Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1333. Received October 22, 1999; Revised Manuscript Received December 22, 1999
A DTPA-folate conjugate was radiolabeled with 99mTc by stannous chloride reduction of [99mTc]sodium pertechnetate in an aqueous solution of DTPA-folate. The radiochemical purity of the product consistently exceeded 97%, as assessed by thin-layer chromatography employing conditions analogous to those for radiochemical quality control of the radiopharmaceutical [99mTc]DTPA. HPLC demonstrated that the radiolabeled product resulted from the intact DTPA-folate conjugate and not unconjugated DTPA. The ability of [99mTc]DTPA-folate to target folate receptors in vivo was assessed in biodistribution studies with athymic mice bearing subcutaneous folate-receptor-positive human KB cell tumors. As an internal control, previously studied [111In]DTPA-folate was coinjected with the [99mTc]DTPA-folate, along with varying amounts of DTPA-folate (0.38 mg/kg, 1.6 mg/kg, or 14 mg/ kg). At each DTPA-folate dose, [99mTc]DTPA-folate exhibited tumor uptake comparable to that of the coadministered [111In]DTPA-folate, with radiotracer levels declining at the higher DTPA-folate doses due to competitive receptor binding of the unlabeled conjugate. Tumor uptake of both tracers was also competitively blocked by preadministered folic acid dihydrate (2.9 mg/kg). Tumor-tobackground tissue contrast obtained with [99mTc]DTPA-folate was generally similar to that obtained with [111In]DTPA-folate. The 99mTc-labeled DTPA-folate conjugate may have utility as a targeted radiopharmaceutical for imaging neoplastic tissues known to overexpress the folate receptor.
INTRODUCTION
A tumor-cell-membrane-associated receptor for the vitamin folic acid appears viable as a molecular target for tumor-selective delivery of drugs and radiopharmaceuticals (1-9). The folate receptor (or folate binding protein, FBP) is overexpressed by a variety of neoplastic tissues, including breast, cervical, ovarian, colorectal, renal, and nasopharyngeal tumors, while being highly restricted in most normal tissues (10-17). It has been previously shown that this receptor system can be targeted with low-molecular-weight folate-chelate conjugates, such as [67Ga]deferoxamine-folate and [111In]DTPA-folate (5-9). The biokinetics of [111In]DTPAfolate suggest that selective folate-receptor-mediated tumor targeting might be feasible with shorter-lived, and less expensive, Tc-99m. We report here conditions for the preparation of [99mTc]DTPA-folate and evaluation of this agent in an athymic mouse tumor model (18). EXPERIMENTAL PROCEDURES
General. A radionuclide dose calibrator was used for assays of radioactivity in the microcurie to millicurie range, while low level (
