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Cerebral folate deficiency syndrome (CFDS) is defined as any neuropsychiatric or developmental disorder characterized by decreased CSF folate levels in the presence of normal folate status outside the nervous system. The specific clinical profile appears to be largely determined by the presence or absence of intrauterine folate deficiency as well as postnatal age at which cerebral folate deficiency occurs. The primary cause of CFDS is identified as the presence of serum folate receptor-alpha (FRα) autoantibodies impairing folate transport across the choroid plexus to the brain whereas, in a minority of cases, mitochondrial disorders, inborn errors of metabolism and loss of function mutations of the FRα (FOLR1) gene are identified. Early recognition and diagnosis of CFDS and prompt intervention is important to improve prognosis with successful outcomes. In this article we focus on FRα autoimmunity and its different age-dependent clinical syndromes, the diagnostic criteria, and treatments to be considered, including prevention strategies in this at-risk population.

Mirvetuximab soravtansine-gynx (MIRV), a first-in-class antibody-drug conjugate targeting folate receptor α (FRα), is approved for the treatment of platinum-resistant ovarian cancer in the United States. We conducted a phase 3, global, confirmatory, open-label, randomized, controlled trial to compare the efficacy and safety of MIRV with the investigator's choice of chemotherapy in the treatment of platinum-resistant, high-grade serous ovarian cancer. Participants who had previously received one to three lines of therapy and had high FRα tumor expression (≥75% of cells with ≥2+ staining intensity) were randomly assigned in a 1:1 ratio to receive MIRV (6 mg per kilogram of adjusted ideal body weight every 3 weeks) or chemotherapy (paclitaxel, pegylated liposomal doxorubicin, or topotecan). The primary end point was investigator-assessed progression-free survival; key secondary analytic end points included objective response, overall survival, and participant-reported outcomes. A total of 453 participants underwent randomization; 227 were assigned to the MIRV group and 226 to the chemotherapy group. The median progression-free survival was 5.62 months (95% confidence interval [CI], 4.34 to 5.95) with MIRV and 3.98 months (95% CI, 2.86 to 4.47) with chemotherapy (P<0.001). An objective response occurred in 42.3% of the participants in the MIRV group and in 15.9% of those in the chemotherapy group (odds ratio, 3.81; 95% CI, 2.44 to 5.94; P<0.001). Overall survival was significantly longer with MIRV than with chemotherapy (median, 16.46 months vs. 12.75 months; hazard ratio for death, 0.67; 95% CI, 0.50 to 0.89; P = 0.005). During the treatment period, fewer adverse events of grade 3 or higher occurred with MIRV than with chemotherapy (41.7% vs. 54.1%), as did serious adverse events of any grade (23.9% vs. 32.9%) and events leading to discontinuation (9.2% vs. 15.9%). Among participants with platinum-resistant, FRα-positive ovarian cancer, treatment with MIRV showed a significant benefit over chemotherapy with respect to progression-free and overall survival and objective response. (Funded by ImmunoGen; MIRASOL ClinicalTrials.gov number, NCT04209855.).

Background Folates are a family of B 9 vitamins essential for normal growth and development in the central nervous system (CNS). Transport of folates is mediated by three major transport proteins: folate receptor alpha (FRα), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC). Brain folate uptake occurs at the choroid plexus (CP) epithelium through coordinated actions of FRα and PCFT, or directly into brain parenchyma at the vascular blood–brain barrier (BBB), mediated by RFC. Impaired folate transport can occur due to loss of function mutations in FRα or PCFT, resulting in suboptimal CSF folate levels. Our previous reports have demonstrated RFC upregulation by nuclear respiratory factor-1 (NRF-1) once activated by the natural compound pyrroloquinoline quinone (PQQ). More recently, we have identified folate transporter localization at the arachnoid barrier (AB). The purpose of the present study was to further characterize folate transporters localization and function in AB cells, as well as their regulation by NRF-1/PGC-1α signaling and folate deficiency. Methods In immortalized mouse AB cells, polarized localization of RFC and PCFT was assessed by immunocytochemical analysis, with RFC and PCFT functionality examined with transport assays. The effects of PQQ treatment on changes in RFC functional expression were also investigated. Mouse AB cells grown in folate-deficient conditions were assessed for changes in gene expression of the folate transporters, and other key transporters and tight junction proteins. Results Immunocytochemical analysis revealed apical localization of RFC at the mouse AB epithelium, with PCFT localized on the basolateral side and within intracellular compartments. PQQ led to significant increases in RFC functional expression, mediated by activation of the NRF-1/PGC-1α signalling cascade. Folate deficiency led to significant increases in expression of RFC, MRP3, P-gp, GLUT1 and the tight junction protein claudin-5. Conclusion These results uncover the polarized expression of RFC and PCFT at the AB, with induction of RFC functional expression by activation of the NRF-1/PGC-1α signalling pathway and folate deficiency. These results suggest that the AB may contribute to the flow of folates into the CSF, representing an additional pathway when folate transport at the CP is impaired.

There are three major folate uptake systems in human tissues and tumors, including the reduced folate carrier (RFC), folate receptors (FRs) and proton-coupled folate transporter (PCFT). We studied the functional interrelationships among these systems for the novel tumor-targeted antifolates AGF94 (transported by PCFT and FRs but not RFC) and AGF102 (selective for FRs) versus the classic antifolates pemetrexed, methotrexate and PT523 (variously transported by FRs, PCFT and RFC). We engineered HeLa cell models to express FRα or RFC under control of a tetracycline-inducible promoter with or without constitutive PCFT. We showed that cellular accumulations of extracellular folates were determined by the type and levels of the major folate transporters, with PCFT and RFC prevailing over FRα, depending on expression levels and pH. Based on patterns of cell proliferation in the presence of the inhibitors, we established transport redundancy for RFC and PCFT in pemetrexed uptake, and for PCFT and FRα in AGF94 uptake; uptake by PCFT predominated for pemetrexed and FRα for AGF94. For methotrexate and PT523, uptake by RFC predominated even in the presence of PCFT or FRα. For both classic (methotrexate, PT523) and FRα-targeted (AGF102) antifolates, anti-proliferative activities were antagonized by PCFT, likely due to its robust activity in mediating folate accumulation. Collectively, our findings describe a previously unrecognized interplay among the major folate transport systems that depends on transporter levels and extracellular pH, and that determines their contributions to the uptake and anti-tumor efficacies of targeted and untargeted antifolates.

Cyclic dinucleotides (CDNs) are ubiquitous signalling molecules in all domains of life 1 , 2 . Mammalian cells produce one CDN, 2′3′-cGAMP, through cyclic GMP–AMP synthase after detecting cytosolic DNA signals 3 – 7 . 2′3′-cGAMP, as well as bacterial and synthetic CDN analogues, can act as second messengers to activate stimulator of interferon genes (STING) and elicit broad downstream responses 8 – 21 . Extracellular CDNs must traverse the cell membrane to activate STING, a process that is dependent on the solute carrier SLC19A1 22 , 23 . Moreover, SLC19A1 represents the major transporter for folate nutrients and antifolate therapeutics 24 , 25 , thereby placing SLC19A1 as a key factor in multiple physiological and pathological processes. How SLC19A1 recognizes and transports CDNs, folate and antifolate is unclear. Here we report cryo-electron microscopy structures of human SLC19A1 (hSLC19A1) in a substrate-free state and in complexes with multiple CDNs from different sources, a predominant natural folate and a new-generation antifolate drug. The structural and mutagenesis results demonstrate that hSLC19A1 uses unique yet divergent mechanisms to recognize CDN- and folate-type substrates. Two CDN molecules bind within the hSLC19A1 cavity as a compact dual-molecule unit, whereas folate and antifolate bind as a monomer and occupy a distinct pocket of the cavity. Moreover, the structures enable accurate mapping and potential mechanistic interpretation of hSLC19A1 with loss-of-activity and disease-related mutations. Our research provides a framework for understanding the mechanism of SLC19-family transporters and is a foundation for the development of potential therapeutics. Two cyclic dinucleotide molecules bind within the cavity of human SLC19A1 as a compact dual-molecule unit, whereas folate and antifolate bind to SLC19A1 as a monomer and occupy a distinct pocket of the cavity.

Cerebral folate deficiency (CFD) syndrome is a neurodevelopmental disorder typically caused by folate receptor autoantibodies (FRAs) that interfere with folate transport across the blood–brain barrier. Autism spectrum disorders (ASDs) and improvements in ASD symptoms with leucovorin (folinic acid) treatment have been reported in some children with CFD. In children with ASD, the prevalence of FRAs and the response to leucovorin in FRA-positive children has not been systematically investigated. In this study, serum FRA concentrations were measured in 93 children with ASD and a high prevalence (75.3%) of FRAs was found. In 16 children, the concentration of blocking FRA significantly correlated with cerebrospinal fluid 5-methyltetrahydrofolate concentrations, which were below the normative mean in every case. Children with FRAs were treated with oral leucovorin calcium (2 mg kg −1 per day; maximum 50 mg per day). Treatment response was measured and compared with a wait-list control group. Compared with controls, significantly higher improvement ratings were observed in treated children over a mean period of 4 months in verbal communication, receptive and expressive language, attention and stereotypical behavior. Approximately one-third of treated children demonstrated moderate to much improvement. The incidence of adverse effects was low. This study suggests that FRAs may be important in ASD and that FRA-positive children with ASD may benefit from leucovorin calcium treatment. Given these results, empirical treatment with leucovorin calcium may be a reasonable and non-invasive approach in FRA-positive children with ASD. Additional studies of folate receptor autoimmunity and leucovorin calcium treatment in children with ASD are warranted.

Mirvetuximab soravtansine-gynx (MIRV) is a first-in-class antibody–drug conjugate targeting folate receptor α (FRα), approved by the US Food and Drug Administration for the treatment of platinum-resistant ovarian cancer in the USA. Here, we report patient-reported outcomes for participants treated with MIRV compared with investigator's choice of chemotherapy from the phase 3 MIRASOL trial, which met its primary endpoint of progression-free survival and key secondary endpoints of objective response rate and overall survival. The MIRASOL trial was a confirmatory, phase 3, randomised, controlled, open-label trial, building on the phase 2 SORAYA trial which had previously demonstrated the safety and efficacy of MIRV in platinum-resistant ovarian cancer. Patients 18 years or older with a confirmed platinum-resistant, recurrent high-grade serous epithelial ovarian cancer diagnosis were recruited from 253 sites including hospitals, academic centres, and community centres in 21 countries. Patients must have received one to three previous systemic anticancer therapies, and have high FRα tumour expression (≥75% tumour cells with an immunohistochemistry score of ≥2+ membrane staining using the PS2+ scoring method), one or more lesions with measurable disease, and an Eastern Cooperative Oncology Group performance status of 0 or 1. Patients were randomly assigned (1:1) to MIRV or investigator's choice of chemotherapy, stratified by number of previous therapy lines and the type of investigator's choice of chemotherapy. Therapies were administered in an open-label manner; MIRV was administered intravenously at 6 mg/kg of adjusted ideal bodyweight every 3 weeks. The primary endpoint was progression-free survival. Key secondary endpoints were objective response rate, overall survival, and a 15·0-point or greater improvement at week 8 or 9 in abdominal and gastrointestinal symptoms using the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire-Ovarian Cancer Module (EORTC QLQ-OV28) in the intention-to-treat population. The MIRASOL trial was registered at ClinicalTrials.gov (NCT04209855), the Gynecologic Oncology Group (GOG 3045), and the European Network of Gynaecological Oncological Trial Groups (ENGOT-ov55), and is complete. Between Feb 3, 2020, and Aug 3, 2022, 453 patients were enrolled and randomly assigned to treatment (227 to the MIRV group and 226 to the investigator's choice of chemotherapy group). All patients were female; 301 (66%) participants were White, 53 (12%) were Asian, 13 (3%) were Black, and 86 (19%) were of another race or not reported; 27 (6%) were Hispanic or Latino. The median follow-up for the study, determined by the reverse Kaplan–Meier method, was 13·1 months (95% CI 12·1–14). QLQ-OV28 completion rates were 86% (365 of 425) at baseline and 81% (282 of 349) at week 8 or 9. 34 (21·0%; 95% CI 15·0–28·1) of 162 patients treated with MIRV reported improvement in QLQ-OV28 abdominal and gastrointestinal scores, compared with 23 (15·3%; 10·0–22·1) of 150 patients treated with the investigator's choice of chemotherapy. These differences were not statistically significant (odds ratio 1·5 [95% CI 0·8–2·6]; p=0·26). MIRV did not seem to impair or improve patient quality of life compared with investigator's choice of chemotherapy. The similar quality-of-life outcomes in the two treatment groups, combined with the previously reported higher efficacy of MIRV compared with single-agent chemotherapy, support MIRV as new treatment option for FRα-positive platinum-resistant ovarian cancer. AbbVie.

This review considers the “promise” of exploiting the proton-coupled folate transporter (PCFT) for selective therapeutic targeting of cancer. PCFT was discovered in 2006 and was identified as the principal folate transporter involved in the intestinal absorption of dietary folates. The recognition that PCFT was highly expressed in many tumors stimulated substantial interest in using PCFT for cytotoxic drug targeting, taking advantage of its high level transport activity under the acidic pH conditions that characterize many tumors. For pemetrexed, among the best PCFT substrates, transport by PCFT establishes its importance as a clinically important transporter in malignant pleural mesothelioma and non-small cell lung cancer. In recent years, the notion of PCFT-targeting has been extended to a new generation of tumor-targeted 6-substituted pyrrolo[2,3-d]pyrimidine compounds that are structurally and functionally distinct from pemetrexed, and that exhibit near exclusive transport by PCFT and potent inhibition of de novo purine nucleotide biosynthesis. Based on compelling preclinical evidence in a wide range of human tumor models, it is now time to advance the most optimized PCFT-targeted agents with the best balance of PCFT transport specificity and potent antitumor efficacy to the clinic to validate this novel paradigm of highly selective tumor targeting.

Folate receptor-α (FRα) is overexpressed in many cancer cells and is therefore an important therapeutic target: here the X-ray crystal structure of folate-bound FRα is presented, revealing details of the ligand-binding pocket that may be useful in the development of small-molecule inhibitors for anticancer therapy. Folic acid receptor structure Folic acid, or folate, is an essential vitamin that is needed for many biological processes, including DNA synthesis, DNA repair and cell division. 'Normal' cells express relatively low amounts of the three folate receptors α, β and γ, but they are commonly overexpressed in cancer cell lines; for this reason, they are potential targets for new chemotherapeutics and cancer-imaging reagents. In this manuscript, the authors solve the X-ray crystal structure of the folate-bound form of human folate receptor α, which mediates folate uptake into cells. The authors map the ligand-binding pocket, providing data that should be useful for the development of new small molecules to target the receptor. Folate receptors (FRα, FRβ and FRγ) are cysteine-rich cell-surface glycoproteins that bind folate with high affinity to mediate cellular uptake of folate. Although expressed at very low levels in most tissues, folate receptors, especially FRα, are expressed at high levels in numerous cancers to meet the folate demand of rapidly dividing cells under low folate conditions 1 , 2 , 3 . The folate dependency of many tumours has been therapeutically and diagnostically exploited by administration of anti-FRα antibodies, high-affinity antifolates 4 , 5 , folate-based imaging agents and folate-conjugated drugs and toxins 6 , 7 , 8 . To understand how folate binds its receptors, we determined the crystal structure of human FRα in complex with folic acid at 2.8 Å resolution. FRα has a globular structure stabilized by eight disulphide bonds and contains a deep open folate-binding pocket comprised of residues that are conserved in all receptor subtypes. The folate pteroate moiety is buried inside the receptor, whereas its glutamate moiety is solvent-exposed and sticks out of the pocket entrance, allowing it to be conjugated to drugs without adversely affecting FRα binding. The extensive interactions between the receptor and ligand readily explain the high folate-binding affinity of folate receptors and provide a template for designing more specific drugs targeting the folate receptor system.

Cellular metabolism is crucial for various physiological processes, with folate-dependent one-carbon (1C) metabolism playing a pivotal role. Folate, a B vitamin, is a key cofactor in this pathway, supporting DNA synthesis, methylation processes, and antioxidant defenses. In dividing cells, folate facilitates nucleotide biosynthesis, ensuring genomic stability and preventing carcinogenesis. Additionally, in neurodevelopment, folate is essential for neural tube closure and central nervous system formation. Thus, dysregulation of folate metabolism can contribute to pathologies such as cancer, severe birth defects, and neurodegenerative diseases. Epidemiological evidence highlights folate’s impact on disease risk and its potential as a therapeutic target. In cancer, antifolate drugs that inhibit key enzymes of folate-dependent 1C metabolism and strategies targeting folate receptors are current therapeutic options. However, folate’s impact on cancer risk is complex, varying among cancer types and dietary contexts. In neurodegenerative conditions, including Alzheimer’s and Parkinson’s diseases, folate deficiency exacerbates cognitive decline through elevated homocysteine levels, contributing to neuronal damage. Clinical trials of folic acid supplementation show mixed outcomes, underscoring the complexities of its neuroprotective effects. This review integrates current knowledge on folate metabolism in cancer and neurodegeneration, exploring molecular mechanisms, clinical implications, and therapeutic strategies, which can provide crucial information for advancing treatments.