Preclinical evaluation of 5-methyltetrahydrofolate-based radioconjugates—new perspectives for folate receptor–targeted radionuclide therapy

PURPOSE: The folate receptor (FR) is frequently overexpressed in a variety of tumor types and, hence, an interesting target for radionuclide therapy. The aim of this study was to evaluate a new class of albumin-binding radioconjugates comprising 5-methyltetrahydrofolate (5-MTHF) as a targeting agent and to compare their properties with those of the previously established folic acid-based [(177)Lu]Lu-OxFol-1. METHODS: [(177)Lu]Lu-6R-RedFol-1 and [(177)Lu]Lu-6S-RedFol-1 were investigated in vitro using FR-positive KB tumor cells. Biodistribution studies were performed in KB tumor-bearing mice, and the areas under the curve (AUC(0 → 120h)) were determined for the uptake in tumors and kidneys. [(177)Lu]Lu-6R-RedFol-1 was compared with [(177)Lu]Lu-OxFol-1 in a therapy study over 8 weeks using KB tumor-bearing mice. RESULTS: Both radioconjugates demonstrated similar in vitro properties as [(177)Lu]Lu-OxFol-1; however, the tumor uptake of [(177)Lu]Lu-6R-RedFol-1 and [(177)Lu]Lu-6S-RedFol-1 was significantly increased in comparison with [(177)Lu]Lu-OxFol-1. In the case of [(177)Lu]Lu-6S-RedFol-1, also the kidney uptake was increased; however, renal retention of [(177)Lu]Lu-6R-RedFol-1 was similar to that of [(177)Lu]Lu-OxFol-1. This led to an almost 4-fold increased tumor-to-kidney AUC(0 → 120h) ratio of [(177)Lu]Lu-6R-RedFol-1 as compared with [(177)Lu]Lu-6S-RedFol-1 and [(177)Lu]Lu-OxFol-1. At equal activity, the therapeutic effect of [(177)Lu]Lu-6R-RedFol-1 was better than that of [(177)Lu]Lu-OxFol-1, reflected by a slower tumor growth and, consequently, an increased median survival time (49 days vs. 34 days). CONCLUSION: This study demonstrated the promising potential of 5-MTHF-based radioconjugates for FR-targeting. Application of [(177)Lu]Lu-6R-RedFol-1 resulted in unprecedentedly high tumor-to-kidney ratios and, as a consequence, a superior therapeutic effect as compared with [(177)Lu]Lu-OxFol-1. These findings, together with the absence of early side effects, make [(177)Lu]Lu-6R-RedFol-1 attractive in view of a future clinical translation. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (10.1007/s00259-020-04980-y) contains supplementary material, which is available to authorized users.