Validation of Folate Receptor-α Selectivity of 6S-5-Methyltetrahydrofolate-Based Radioconjugates: A Prerequisite for...

Area of Science:

• Nuclear medicine
• Oncology
• Radiopharmaceutical chemistry

Background:

• Folic acid-based radioconjugates are explored for imaging and therapy of folate receptor-alpha (FRα)-positive cancers.
• While folic acid binds FRα and FRβ, 6S-5-methyltetrahydrofolate (6S-5-MTHF) shows higher affinity for FRα.

Purpose of the Study:

• To investigate if 6S-5-MTHF-based radioconjugates preferentially bind to FRα over FRβ.
• To evaluate the in vitro and in vivo performance of novel 6S-5-MTHF and 6R-5-MTHF radioconjugates.

Main Methods:

• In vitro evaluation of 6R- and 6S-5-MTHF radioconjugates using FRα- and FRβ-expressing cells.
• In vivo biodistribution and SPECT/CT imaging studies in a dual xenograft mouse model.
• Immunohistochemical analysis of FRα and FRβ expression in human tissue samples (ovarian cancer, kidney, bone marrow).

Main Results:

• 6S-5-MTHF radioconjugates demonstrated 4-6 fold higher uptake and 9-29 fold increased binding affinity for FRα vs. FRβ.
• In vivo, 6S-5-MTHF radioconjugates showed significantly higher accumulation in FRα-positive xenografts (74%-100% IA/g) compared to FRβ-positive xenografts (4%-10% IA/g).
• Immunohistochemistry confirmed high FRα expression in ovarian cancer and renal tubules, with absence in bone marrow where FRβ was expressed.

Conclusions:

• 6S-5-MTHF-based radioconjugates enable selective targeting of FRα.
• High FRα expression in ovarian cancer and its absence in bone marrow suggest therapeutic and safety advantages for FRα-selective radioconjugates.
• Further optimization may be needed to reduce renal retention of 6S-5-MTHF radioconjugates.