DOTA conjugate with an albumin-binding entity enables the first folic acid-targeted 177Lu-radionuclide tumor therapy in mice

Abstract

The folate receptor (FR) has proven a valuable target for nuclear imaging using folic acid radioconjugates. However, using folate-based radiopharmaceuticals for therapy has long been regarded as an unattainable goal because of their considerable renal accumulation. Herein, we present a novel strategy in which a DOTA-folate conjugate with an albumin-binding entity (cm09) was designed with the aim of prolonging circulation in the blood and therewith potentially improving tumor-to-kidney ratios. Methods: The folate conjugate cm09 was radiolabeled with 177LuCl3, and stability experiments were performed in plasma. Cell uptake studies were performed on FR-positive KB tumor cells, and an ultrafiltration assay was used to determine the plasma protein-binding properties of 177Lu-cm09. In vivo, 177Lu-cm09 was tested in KB tumor-bearing mice using SPECT/CT. The therapeutic anticancer effect of 177Lu-cm09 (20 MBq) applied as a single injection or as fractionated injections was investigated in different groups of mice (n = 5) by monitoring tumor size and the survival time of treated mice, compared with untreated controls. Results: Compound cm09 was radiolabeled at a specific activity of 40 MBq/nmol, a radiochemical yield of more than 98%, and a stability of more than 99% over 5 d in plasma. Ultrafiltration revealed significant binding of 177Lu-cm09 to serum proteins (∼91%) in plasma, compared with folate radio-conjugate without an albumin-binding entity. Cell uptake and internalization of 177Lu-cm09 was FR-specific and comparable to other folate radioconjugates. In vivo studies resulted in high tumor uptake (17.56 percentage injected dose per gram [%ID/g] at 4 h after injection), which was almost completely retained for at least 72 h. Renal accumulation was significantly reduced (28% ID/g at 4 h after injection), compared with folate conjugates that lack an albumin-binding entity (∼70%ID/g at 4 h after injection). These circumstances enabled SPECT imaging of excellent quality. Radionuclide therapy (1 x 20 MBq) revealed complete remission of tumors in 4 of 5 cases and a significantly prolonged survival time, compared with untreated controls. Conclusion: The modification of a folate radioconjugate with an albumin-binding entity resulted in a significant increase of the tumor-tokidney ratio of radioactivity, enabling for the first time, to our knowledge, the preclinical application of folic acid-targeted radionuclide therapy in mice. COPYRIGHT © 2013 by the Society of Nuclear Medicine and Molecular Imaging, Inc.