Diastereomerically Pure 6R- and 6S-3'-Aza-2'-18F-Fluoro-5-Methyltetrahydrofolates Show Unprecedentedly High Uptake in Folate Receptor-Positive KB Tumors

Investigations Using Albumin Binders to Modify the Tissue Distribution Profile of Radiopharmaceuticals Exemplified with Folate Radioconjugates

Introducing an albumin-binding entity into otherwise short-lived radiopharmaceuticals can be an effective means to improve their pharmacokinetic properties due to enhanced blood residence time. In the current study, DOTA-derivatized albumin binders based on 4-(p-iodophenyl)butanoate (DOTA-ALB-1 and DOTA-ALB-3) and 5-(p-iodophenyl)pentanoate entities (DOTA-ALB-24 and DOTA-ALB-25) without and with a hydrophobic 4-(aminomethyl)benzoic acid (AMBA) linker unit, respectively, were synthesized and labeled with lutetium-177 for in vitro and in vivo comparison. Overall, [177Lu]Lu-DOTA-ALB-1 demonstrated ~3-fold stronger in vitro albumin-binding affinity and a longer blood residence time (T50%IA ~8 h) than [177Lu]Lu-DOTA-ALB-24 (T50%IA ~0.8 h). Introducing an AMBA linker enhanced the albumin-binding affinity, resulting in a T50%IA of ~24 h for [177Lu]Lu-DOTA-ALB-3 and ~2 h for [177Lu]Lu-DOTA-ALB-25. The same albumin binders without or with the AMBA linker were incorporated into 6R- and 6S-5-methyltetrahydrofolate-based DOTA-conjugates (177Lu-RedFols). Biodistribution studies in mice performed with both diastereoisomers of [177Lu]Lu-RedFol-1 and [177Lu]Lu-RedFol-3, which comprised the 4-(p-iodophenyl)butanoate moiety, demonstrated a slower accumulation in KB tumors than those of [177Lu]Lu-RedFol-24 and [177Lu]Lu-RedFol-25 with the 5-(p-iodophenyl)pentanoate entity. In all cases, the tumor uptake was high (30-45% IA/g) 24 h after injection. Both diastereoisomers of [177Lu]Lu-RedFol-1 and [177Lu]Lu-RedFol-3 demonstrated high blood retention (3.8-8.7% IA/g, 24 h p.i.) and a 2- to 4-fold lower kidney uptake than the corresponding diastereoisomers of [177Lu]Lu-RedFol-24 and [177Lu]Lu-RedFol-25, which were more rapidly cleared from the blood (<0.2% IA/g, 24 h after injection). Kidney retention of the 6S-diastereoisomers of all 177Lu-RedFols was consistently higher than that of the respective 6R-diastereoisomers, irrespective of the albumin binder and linker unit used. It was demonstrated that the blood clearance data obtained with 177Lu-DOTA-ALBs had predictive value for the blood retention times of the respective folate radioconjugates. The use of these albumin-binding entities without or with an AMBA linker may serve for fine-tuning the blood retention of folate radioconjugates and also other radiopharmaceuticals and, hence, optimize their tissue distribution profiles. Dosimetry estimations based on patient data obtained with one of the most promising folate radioconjugates will be crucial to identify the dose-limiting organ, which will allow for selecting the most suitable folate radioconjugate for therapeutic purposes.

A multi-functional nano-system combining PI3K-110α/β inhibitor overcomes P-glycoprotein mediated MDR and improves anti-cancer efficiency

P-glycoprotein (P-gp/ABCB1)-mediated multidrug resistance (MDR) in cancers severely limit chemotherapeutic efficacy. We recently reported that phosphatidylinositol-3-kinase (PI3K) 110α and 110β subunits can be novel targets for reversal of P-gp mediated MDR in cancers, and BAY-1082439 as an inhibitor specific for PI3K 110α and 110β subunits could reverse P-gp-mediated MDR by downregulating P-gp expression in cancer cells. However, BAY-1082439 has very low solubility, short half-life and high in-vivo clearance rate. Till now, nano-system with the functions to target PI3K P110α and P110β and reverse P-gp mediated MDR in cancers has not been reported. In our study, a tumor targeting drug delivery nano-system PBDF was established, which comprised doxorubicin (DOX) and BAY-1082439 respectively encapsulated by biodegradable PLGA-SH nanoparticles (NPs) that were grafted to gold nanorods (Au NRs) modified with FA-PEG-SH, to enhance the efficacy to reverse P-gp mediated MDR and to target tumor cells, further, to enhance the efficiency to inhibit MDR tumors overexpressing P-gp. In-vitro experiments indicated that PBDF NPs greatly enhanced uptake of DOX, improved the activity to reverse MDR, inhibited the cell proliferation, and induced S-phase arrest and apoptosis in KB-C2 cells, as compared with free DOX combining free BAY-1082439. In-vivo experiments further demonstrated that PBDF NPs improved the anti-tumor ability of DOX and inhibited development of KB-C2 tumors. Notably, the metastasis of KB-C2 cells in livers and lungs of nude mice were inhibited by treatment with PBDF NPs, which showed no obvious in-vitro or in-vivo toxicity.

Identification of a PET Radiotracer for Imaging of the Folate Receptor-α: A Potential Tool to Select Patients for Targeted Tumor Therapy

The aim of this study was to identify a folate receptor-α (FRα)-selective PET agent potentially suitable for the selection of patients who might profit from FRα-targeted therapies. The 6R and 6S isomers of ¹⁸F-aza-5-methyltetrahydrofolate (MTHF) were assessed regarding their binding to FRα and FRβ, expressed on cancer and inflammatory cells, respectively, and compared with ¹⁸F-AzaFol, the folic acid-based analog. Methods: FR selectivity was investigated using FRα-transfected (RT16) and FRβ-transfected (D4) CHO cells. The cell uptake of ¹⁸F-folate tracers was investigated, and receptor-binding affinities were determined with the nonradioactive analogs. In vitro autoradiography of the ¹⁸F-folate tracers was performed using RT16 and D4 tissue sections. Biodistribution studies and PET/CT imaging of the radiotracers were performed on mice bearing RT16 and D4 xenografts. Results: The uptake of ¹⁸F-6R-aza-5-MTHF was high when using RT16 cells (62% ± 10% of added activity) but much lower when using D4 cells (5% ± 2%). The FRα selectivity of ¹⁸F-6R-aza-5-MTHF was further demonstrated by its approximately 43-fold higher binding affinity to FRα (half-maximal inhibitory concentration [IC₅₀], 1.8 ± 0.1 nM) than to FRβ (IC₅₀, 77 ± 27 nM). The uptake of ¹⁸F-6S-aza-5-MTHF and ¹⁸F-AzaFol was equal in both cell lines (52%-70%), with similar affinities to FRα (IC₅₀, 2.1 ± 0.4 nM and 0.6 ± 0.3 nM, respectively) and FRβ (0.8 ± 0.2 nM and 0.3 ± 0.1 nM, respectively). The autoradiography signal obtained with ¹⁸F-6R-aza-5-MTHF was 11-fold more intense for RT16 than for D4 tissue sections. Biodistribution data showed high uptake of ¹⁸F-6R-aza-5-MTHF in RT16 xenografts (81% ± 20% injected activity per gram [IA]/g 1 h after injection) but significantly lower accumulation in D4 xenografts (7.3% ± 2.1% IA/g 1 h after injection), which was also visualized using PET. The uptake of ¹⁸F-6S-aza-5-MTHF and ¹⁸F-AzaFol was similar in RT16 (53% ± 10% IA/g and 45% ± 2% IA/g, respectively) and D4 xenografts (77% ± 10% IA/g and 52% ± 7% IA/g, respectively). Conclusion: This study demonstrated FRα selectivity for ¹⁸F-6R-aza-5-MTHF but not for ¹⁸F-6S-aza-5-MTHF or ¹⁸F-AzaFol. This characteristic, together with its favorable tissue distribution, makes ¹⁸F-6R-aza-5-MTHF attractive for clinical translation to enable detection of FRα-positive cancer while preventing undesired accumulation in FRβ-expressing inflammatory cells.

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 [¹⁷⁷Lu]Lu-OxFol-1.

Methods

[¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]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. [¹⁷⁷Lu]Lu-6R-RedFol-1 was compared with [¹⁷⁷Lu]Lu-OxFol-1 in a therapy study over 8 weeks using KB tumor-bearing mice.

Results

Both radioconjugates demonstrated similar in vitro properties as [¹⁷⁷Lu]Lu-OxFol-1; however, the tumor uptake of [¹⁷⁷Lu]Lu-6R-RedFol-1 and [¹⁷⁷Lu]Lu-6S-RedFol-1 was significantly increased in comparison with [¹⁷⁷Lu]Lu-OxFol-1. In the case of [¹⁷⁷Lu]Lu-6S-RedFol-1, also the kidney uptake was increased; however, renal retention of [¹⁷⁷Lu]Lu-6R-RedFol-1 was similar to that of [¹⁷⁷Lu]Lu-OxFol-1. This led to an almost 4-fold increased tumor-to-kidney AUC_{0 → 120h} ratio of [¹⁷⁷Lu]Lu-6R-RedFol-1 as compared with [¹⁷⁷Lu]Lu-6S-RedFol-1 and [¹⁷⁷Lu]Lu-OxFol-1. At equal activity, the therapeutic effect of [¹⁷⁷Lu]Lu-6R-RedFol-1 was better than that of [¹⁷⁷Lu]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 [¹⁷⁷Lu]Lu-6R-RedFol-1 resulted in unprecedentedly high tumor-to-kidney ratios and, as a consequence, a superior therapeutic effect as compared with [¹⁷⁷Lu]Lu-OxFol-1. These findings, together with the absence of early side effects, make [¹⁷⁷Lu]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.

Development of Folate Receptor-Targeted PET Radiopharmaceuticals for Tumor Imaging-A Bench-to-Bedside Journey

The folate receptor-α (FR-α) is overexpressed in many epithelial cancers, including ovary, uterus, kidneys, breast, lung, colon and prostate carcinomas, but shows limited expression in normal tissues such as kidneys, salivary glands, choroid plexus and placenta. FR-α has therefore emerged as a promising target for the delivery of therapeutic and imaging agents to FR-positive tumors. A series of folate-based PET (positron emission tomography) radiopharmaceuticals have been developed for the selective targeting of FR-positive malignancies. This review provides an overview on the research progress made so far regarding the design, radiosynthesis and the utility of the folate-derived PET radioconjugates for targeting FR-positive tumors. For the most part, results from folate radioconjugates labeled with fluorine-18 (t_1/2 = 109.8 min) and gallium-68 (t_1/2 = 67.7 min) have been presented but folates labeled with "exotic" and new PET radionuclides such as copper-64 (t_1/2 = 12.7 h), terbium-152 (t_1/2 = 17.5 h), scandium-44 (t_1/2 = 3.97 h), cobalt-55 (t_1/2 = 17.5 h) and zirconium-89 (t_1/2 = 78.4 h) are also discussed. For tumor imaging, none of the reported PET radiolabeled folates reported to date has made the complete bench-to-bedside journey except [¹⁸F]AzaFol, which made it to patients with metastatic ovarian and lung cancers in a multicenter first-in-human trial. In the near future, however, we expect more clinical trials with folate-based PET radiopharmaceuticals given the increasing clinical interest in imaging and the treatment of FR-related malignancies.

18F-AzaFol for Detection of Folate Receptor-β Positive Macrophages in Experimental Interstitial Lung Disease-A Proof-of-Concept Study

Background: Interstitial lung disease (ILD) is a common and severe complication in rheumatic diseases. Folate receptor-β is expressed on activated, but not resting macrophages which play a key role in dysregulated tissue repair including ILD. We therefore aimed to pre-clinically evaluate the potential of 18F-AzaFol-based PET/CT (positron emission computed tomography/computed tomography) for the specific detection of macrophage-driven pathophysiologic processes in experimental ILD. Methods: The pulmonary expression of folate receptor-β was analyzed in patients with different subtypes of ILD as well as in bleomycin (BLM)-treated mice and respective controls using immunohistochemistry. PET/CT was performed at days 3, 7, and 14 after BLM instillation using the 18F-based folate radiotracer 18F-AzaFol. The specific pulmonary accumulation of the radiotracer was assessed by ex vivo PET/CT scans and quantified by ex vivo biodistribution studies. Results: Folate receptor-β expression was 3- to 4-fold increased in patients with fibrotic ILD, including idiopathic pulmonary fibrosis and connective tissue disease-related ILD, and significantly correlated with the degree of lung remodeling. A similar increase in the expression of folate receptor-β was observed in experimental lung fibrosis, where it also correlated with disease extent. In the mouse model of BLM-induced ILD, pulmonary accumulation of 18F-AzaFol reflected macrophage-related disease development with good correlation of folate receptor-β positivity with radiotracer uptake. In the ex vivo imaging and biodistribution studies, the maximum lung accumulation was observed at day 7 with a mean accumulation of 1.01 ± 0.30% injected activity/lung in BLM-treated vs. control animals (0.31 ± 0.06% % injected activity/lung; p < 0.01). Conclusion: Our preclinical proof-of-concept study demonstrated the potential of 18F-AzaFol as a novel imaging tool for the visualization of macrophage-driven fibrotic lung diseases.