Synthesis and Evaluation of (99m)Tc-Labeled Folate-Tripeptide Conjugate as a Folate Receptor-Targeted Imaging Agent in a Tumor-Bearing Mouse Model

Vitamin-based radiopharmaceuticals for tumor imaging

Advances in nuclear medicine, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET), are among the most sensitive methods that can be used in diagnostic imaging and radionuclide therapy with higher efficiency and reduced toxicity benefits. In order to improve the success of treatment, it is also important to develop methods that can be used when lesions can be detected at the earliest stages. Vitamins are macromolecules that play a crucial role in numerous biological processes in both animals and humans. Escalating development of vitamin-based radiopharmaceuticals for application in the diagnosis and treatment of cancers. In this review, we aimed to discuss about recent research utilizing radio-labeled vitamins for targeted tumor imaging.

Dietary folic acid deficiency impacts hippocampal morphology and cortical acetylcholine metabolism in adult male and female mice

OBJECTIVE

One-carbon (1C) metabolism is a metabolic network that integrates nutritional signals with biosynthesis, redox homeostasis, and epigenetics. There are sex differences in hepatic 1C metabolism, however, it is unclear whether sex differences in 1C impact the brain. The aim of this study was to investigate if sex modulates the effects of dietary folic acid deficiency, the main component of 1C, in brain tissue using a mouse model.

METHODS

Male and female C57Bl/6J mice were placed on a folic acid deficient (FD) or control diet (CD) at six weeks until six months of aged. After which brain tissue and serum were collected for analysis. In brain tissue, hippocampal volume, morphology, and apoptosis as well as cortical acetylcholine metabolism were measured.

RESULTS

Male and female FD mice had reduced serum levels of folate. Both males and females maintained on a FD showed a decrease in the thickness of the hippocampal CA1-CA3 region. Interestingly, there was a sex difference in the levels of active caspase-3 within the CA3 region of the hippocampus. In cortical tissue, there were increased levels of neuronal ChAT and reduced levels of AChE in FD females and male mice.

CONCLUSIONS

The results indicated that FD impacts hippocampal morphology and cortical neuronal acetylcholine metabolism. The data from our study indicate that there was only one sex difference and that was in hippocampal apoptosis. Our study provides little evidence that sex modulates the effects of dietary folate deficiency on hippocampal morphology and cortical neuronal acetylcholine metabolism.

Synthesis and Evaluation of (99m)Tc-Labeled Dimeric Folic Acid for FR-Targeting

The folate receptor (FR) is overexpressed in a wide variety of human tumors. In our study, the multimeric concept was used to synthesize a dimeric folate derivative via a click reaction. The novel folate derivative (HYNIC-D₁-FA₂) was radiolabeled with ^99mTc using tricine and trisodium triphenylphosphine-3,3′,3″-trisulfonate (TPPTS) as coligands (^99mTc-HYNIC-D₁-FA₂) and its in vitro physicochemical properties, ex vivo biodistribution and in vivo micro-SPECT/CT imaging as a potential FR targeted agent were evaluated. It is a hydrophilic compound (log P = −2.52 ± 0.13) with high binding affinity (IC₅₀ = 19.06 nM). Biodistribution in KB tumor-bearing mice showed that ^99mTc-HYNIC-D₁-FA₂ had high uptake in FR overexpressed tumor and kidney at all time-points, and both of them could obviously be inhibited when blocking with free FA in the blocking studies. From the in vivo micro-SPECT/CT imaging results, good tumor uptake of ^99mTc-HYNIC-D₁-FA₂ was observed in KB tumor-bearing mice and it could be blocked obviously. Based on the results, this new radiolabeled dimeric FA tracer might be a promising candidate for FR-targeting imaging with high affinity and selectivity.

Profile of farletuzumab and its potential in the treatment of solid tumors

Folate receptor (FR) α expression in normal tissues is restricted to a subpopulation of epithelial cells. In contrast, FRα is overexpressed in epithelial ovarian cancer (EOC) and non-small-cell lung carcinoma. Therefore, FRα is considered a promising therapeutic target for EOC and non-small-cell lung carcinoma. Farletuzumab (MORAb-003) is a humanized monoclonal antibody of immunoglobulin G subtype 1 kappa, targeting human FRα. To date, Phase I/II clinical trials have clearly demonstrated the feasibility and safety of farletuzumab as a treatment option against solid tumors. However, in Phase III clinical trial that was conducted to verify the combined effect of paclitaxel-carboplatin combination therapy and farletuzumab for patients with recurrent EOC, improvement in progression-free survival was not statistically significant. This result might be owing to the fact that the eligibility criteria for these studies did not include FRα expression. The significance of FRα as a predictive/prognostic biomarker remains unclear. In addition, there is currently no established biomarker to predict the response and toxicities among patients receiving farletuzumab therapy. Furthermore, the primary mechanism of action of farletuzumab has not yet been identified. Therefore, further research to identify the mechanism of farletuzumab in tumor suppression is necessary to clarify the full potential of this chemotherapeutic agent.