Comparative preclinical activity of the folate-targetedVinca alkaloid conjugates EC140 and EC145

Fc Multisite Conjugation and Prolonged Delivery of the Folate-Targeted Drug Conjugate EC140

Small molecule-drug conjugate (SMDC) is a targeted drug delivery technology that develops in parallel with the antibody-drug conjugate. However, the clinical translation of SMDC faces challenges due to its limited circulating half-life in vivo. The drawback in pharmacokinetics is that it restricts the exposure time of SMDC to tumor tissues and ultimately reduces the therapeutic efficacy. In this study, we chemically conjugated a folate-targeted SMDC EC140 to the long-circulating Fc protein at multiple sites, yielding a stable and high-DAR Fc-SMDC conjugate (Fc-EC140). Fc-EC140 can bear approximately 4 molecules of EC140 per Fc protein (drug-antibody ratio = 4.1) and display enhanced potency in folate receptor (FR)-positive tumor cells compared to the SMDC comparator. In addition, Fc-EC140 retains the FcRn-mediated recycling function and displays an extended half-life of 28 h in the mice. In vivo, antitumor experiments demonstrate that intravenous administration of Fc-EC140 (Q7D × 3 at a dose of 15 mg/kg) nearly cures the KB tumors, which is far more effective than the comparator EC140 administrated at equivalent doses. This study presents a new strategy for the targeted delivery of SMDC.

In vivo activation of FAP-cleavable small molecule-drug conjugates for the targeted delivery of camptothecins and tubulin poisons to the tumor microenvironment

Small molecule-drug conjugates (SMDCs) are increasingly considered as a therapeutic alternative to antibody-drug conjugates (ADCs) for cancer therapy. OncoFAP is an ultra-high affinity ligand of Fibroblast Activation Protein (FAP), a stromal tumor-associated antigen overexpressed in a wide variety of solid human malignancies. We have recently reported the development of non-internalizing OncoFAP-based SMDCs, which are activated by FAP thanks to selective proteolytic cleavage of the -GlyPro- linker with consequent release of monomethyl auristatin E (MMAE) in the tumor microenvironment. In this article, we describe the generation and the in vivo characterization of FAP-cleavable OncoFAP-drug conjugates based on potent topoisomerase I inhibitors (DXd, SN-38, and exatecan) and an anti-tubulin payload (MMAE), which are already exploited in clinical-stage and approved ADCs. The Glycine-Proline FAP-cleavable technology was directly benchmarked against linkers found in Adcetris™, Enhertu™, and Trodelvy™ structures by means of in vivo therapeutic experiments in mice bearing tumors with cellular or stromal FAP expression. OncoFAP-GlyPro-Exatecan and OncoFAP-GlyPro-MMAE emerged as the most efficacious anti-cancer therapeutics against FAP-positive cellular models. OncoFAP-GlyPro-MMAE exhibited a potent antitumor activity also against stromal models, and was therefore selected for clinical development.

Selective delivery of clinically approved tubulin binding agents through covalent conjugation to an active targeting moiety

Abstract:

The efficacy and tolerability of tubulin binding agents are hampered by their low specificity for cancer cells, like most clinically used anticancer agents. To improve specificity, tubulin binding agents have been covalently conjugated to agents which target cancer cells to give actively targeted drug conjugates. These conjugates are designed to increase uptake of the drug by cancer cells, while having limited uptake by normal cells thereby improving efficacy and tolerability. Approaches used include attachment to small molecules, polysaccharides, peptides, proteins and antibodies that exploit the overexpression of receptors for these substances. Antibody targeted strategies have been the most successful to date with six such examples having gained clinical approval. Many other conjugate types, especially those targeting the folate receptor, have shown promising efficacy and toxicity profiles in pre-clinical models and in early-stage clinical studies. Presented herein is a discussion of the success or otherwise of the recent strategies used to form these actively targeted conjugates.

Active Targeted Nanoformulations via Folate Receptors: State of the Art and Future Perspectives

In normal tissues, the expression of folate receptors is low and limited to cells that are important for embryonic development or for folate reabsorption. However, in several pathological conditions some cells, such as cancer cells and activated macrophages, overexpress folate receptors (FRs). This overexpression makes them a potential therapeutic target in the treatment of cancer and inflammatory diseases to obtain a selective delivery of drugs at altered cells level, and thus to improve the therapeutic efficacy and decrease the systemic toxicity of the pharmacological treatments. Two strategies have been used to achieve this folate receptor targeting: (i) the use of ligands with high affinity to FRs (e.g., folic acid or anti-FRs monoclonal antibodies) linked to the therapeutic agents or (ii) the use of nanocarriers whose surface is decorated with these ligands and in which the drug is encapsulated. This manuscript analyzes the use of FRs as a target to develop new therapeutic tools in the treatment of cancer and inflammatory diseases with an emphasis on the nanoformulations that have been developed for both therapeutic and imaging purposes.

Development of Ligand-Drug Conjugates Targeting Melanoma through the Overexpressed Melanocortin 1 Receptor

Melanoma is a lethal form of skin cancer. Despite recent breakthroughs of BRAF-V600E and PD-1 inhibitors showing remarkable clinical responses, melanoma can eventually survive these targeted therapies and become resistant. To solve the drug resistance issue, we designed and synthesized ligand-drug conjugates that couple cytotoxic drugs, which have a low cancer resistance issue, with the melanocortin 1 receptor (MC1R) agonist melanotan-II (MT-II), which provides specificity to MC1R-overexpressing melanoma. The drug-MT-II conjugates maintain strong binding interactions to MC1R and induce selective drug delivery to A375 melanoma cells through its MT-II moiety in vitro. Furthermore, using camptothecin as the cytotoxic drug, camptothecin-MT-II (compound 1) can effectively inhibit A375 melanoma cell growth with an IC50 of 16 nM. By providing selectivity to melanoma cells through its MT-II moiety, this approach of drug-MT-II conjugates enables us to have many more options for cytotoxic drug selection, which can be the key to solving the cancer resistant problem for melanoma.

The Chemical Methods of Disulfide Bond Formation and Their Applications to Drug Conjugates

:

The disulfide bond possesses unique chemical and biophysical properties which distinguish it as one of the key structural elements of bioactive proteins and peptides, important drugs and other materials. The chemo-selective synthesis of these structures and the exploration of their function have been of longstanding interest to the chemistry community. The past decades have witnessed significant progress in both areas. This review will summarize the historically established and recently developed chemical methods in disulfide bond formation. The discussion will also be extended to the use of the disulfide linkers in small molecules, and peptide- and protein-drug conjugates. It is hoped that the combined overview of the fundamental chemistries and applications to drug discovery will inspire creative thinking and stimulate future novel uses of these versatile chemistries.

Folic acid functionalized nanoparticles as pharmaceutical carriers in drug delivery systems

Conventional chemotherapeutic approaches in cancer therapy such as surgery, chemotherapy, and radiotherapy have several disadvantages due to their nontargeted distributions in the whole body. On the other hand, nanoparticles (NPs) based therapies are remarkably progressing to solve several limitations of conventional drug delivery systems (DDSs) including nonspecific biodistribution and targeting, poor water solubility, weak bioavailability and biodegradability, low pharmacokinetic properties, and so forth. The enhanced permeability and retention effect escape from P-glycoprotein trap in cancer cells as a passive targeting mechanism, and active targeting strategies are also other most important advantages of NPs in cancer diagnosis and therapy. Folic acid (FA) is one of the biologic molecules which has been targeted overexpressed-folic acid receptor (FR) on the surface of cancer cells. Therefore, conjugation of FA to NPs most easily enhances the FR-mediated targeting delivery of therapeutic agents. Here, the recent works in FA which have been decorated NPs-based DDSs are discussed and cancer therapy potency of these NPs in clinical trials are presented.

Folate-Targeted pH and Redox Dual Stimulation-Responsive Nanocarrier for Codelivering of Docetaxel and TFPI-2 for Nasopharyngeal Carcinoma Therapy

Due to the increasing incidence of tumor metastasis and multidrug resistance, even though a combined use of chemotherapy and radiotherapy is introduced, the 5-year average survival rate of an advanced nasopharyngeal carcinoma (NPC) patient still remains low. Hence, targeted slow-release anticancer drugs represent a potential therapy for advanced NPC. In this study, pH and redox dual stimulation-responsive folate-targeted folic acid - β-cyclodextrin - hyperbranched poly(amido amine)s (FA-DS-PAAs) nanocarriers for codelivery of docetaxel (DOC) and tissue factor pathway inhibitor 2 (TFPI-2) for NPC therapy are discussed. Physical and chemical properties, in vitro DOC-release properties, folic acid (FA)-targeting, transfection, Western blotting, DOC and TFPI-2 codelivery, therapeutic properties, targeted inhibition, and biocompatibility, in vivo FA-targeting, toxicity, and therapeutic properties of FA-DS-PAAs/DOC/TFPI2 nanoparticles are evaluated. The results indicate that the 200 nm low-toxicity FA-DS-PAAs/DOC/TFPI2 nanoparticles could enhance TFPI2 gene expression, make cancer cells more sensitive to DOC, induce cell apoptosis, and reduce cell invasion more effectively compared with monochemotherapy. With respect to the targeted release of drugs (DOC and TFPI2) in tumor cells, FA-DS-PAAs/DOC/TFPI2 is associated with the slowest growth rate of tumor and the smallest volume of tumor, so this study demonstrates the best synergetic antitumor effect. We anticipate that this study is important because it not only provides a potential new therapy approach for NPC but also paves the preclinical way for potential application of FA-DS-PAAs/DOC/TFPI2.

Combinational Effects of Active Targeting, Shape, and Enhanced Permeability and Retention for Cancer Theranostic Nanocarriers

Combinatory modulation of the physical and biochemical characteristics of nanocarrier delivery systems is an emergent topic in the field of nanomedicine. Here, we studied the combined effects of incorporation of active targeting moieties into nanocarriers and their morphology affecting the enhanced permeation and retention effect for nanomedicine cancer therapy. Self-assembled lipid discoidal and vesicular nanoparticles with low-polydispersity sub-50 nm size range and identical chemical compositions were synthesized, characterized, and correlated with in vitro cancer cellular internalization, in vivo tumor accumulation and cancer treatments. The fact that folate-associated bicelle yields the best outcome is indicative of the preference for discoidal carriers over spherical carriers and the improved targeting efficacy due to the targeting ligand/receptor binding. The approach is successfully adopted to design the nanocarriers for photodynamic therapy, which yields a consistent trend in in vitro and in vivo efficacy: folate nanodiscs > folate vesicles > nonfolate nanodiscs > nonfolate vesicles. Folate discs not only have shown a higher tumor uptake and photothermal therapeutic efficiency, but also minimize skin photosensitivity side effects. The advantages of nanodiscoidal bicelles as nanocarriers, including well-defined size, robust formation, easy encapsulation of hydrophobic molecules (therapeutics and/or diagnostics), easy incorporation of targeting molecules, and low toxicity, enable the scalable manufacturing of a generalized in vivo multimodal delivery platform.

Phase I/II clinical trial of the targeted chemotherapeutic drug, folate-tubulysin, in dogs with naturally-occurring invasive urothelial carcinoma

Purpose

The purpose was to determine the safety and antitumor activity of a folate-tubulysin conjugate (EC0531) in a relevant preclinical animal model, dogs with naturally-occurring invasive urothelial carcinoma (iUC). Canine iUC is an aggressive cancer with high folate receptor (FR) expression similar to that in certain forms of human cancer.

Experimental Design

A 3+3 dose escalation study of EC0531 (starting dose 0.2 mg/kg given intravenously at two-week intervals) was performed in dogs with iUC expressing high levels of FRs (>50% positive tumor cells). Pharmacokinetic (PK) analysis was performed, and the maximum tolerated dose (MTD) was determined. The dose cohort at the MTD was expanded to determine antitumor activity.

Results

The MTD of EC0531 was 0.26 mg/kg every two weeks, with grade 3-4 neutropenia and gastrointestinal toxicity observed at higher doses. Treatment at the MTD was well tolerated. Clinical benefit was found in 20 of 28 dogs (71%), including three dogs with partial remission and 17 dogs with stable disease. Plasma EC0531 concentrations in the dogs far exceeded those required to inhibit proliferation of FR-expressing cell in vitro. Unlike human neutrophils, canine neutrophils were found to express FRs, which contributes to the neutropenia at higher doses of EC0531 in dogs.

Conclusion

EC0531 was well tolerated and had good antitumor activity in dogs with iUC. It is likely that humans will tolerate higher, potentially more effective doses of folate-tubulysin without myelotoxicity because of the absence of FRs on human neutrophils. The results clearly justify the evaluation of folate-tubulysin in human clinical trials.

Carbon nanotubes functionalized with folic acid attached via biomimetic peptide linker

AIM

Anchoring folic acid (FA) with a biomimetic peptidic linker resistant to proteolytic degradation to act as a homing device on functionalized carbon nanotubes.

MATERIALS & METHODS

Ethylenediamine was attached to oxidized multiwalled carbon nanotubes (MWNTs) using 4-(4,6-dimethoxy-[1,3,5]triazin-2-yl)-4-methylmorpholinium tetrafluoroborate. FA was coupled with 6-aminohexanoic acid and derivatives of β-alanine, affording four intermediates, which connected to the MWNTs via peptidic linkers of various lengths.

RESULTS

Biomimetic nanomaterials were produced with FA as a homing molecule. The structure and properties of the nanomaterials were analyzed, confirming the versatility of the peptides used as linkers.

CONCLUSION

Conjugates of FA attached to MWNTs via peptide linkers prepared from β-alanine residues are resistant to proteolytic degradation. Viability in colon cancer cells and normal colonocytes confirmed their lack of cytotoxicity.

Folate Receptor-Positive Gynecological Cancer Cells: In Vitro and In Vivo Characterization

The folate receptor alpha (FR) is expressed in a variety of gynecological cancer types. It has been widely used for tumor targeting with folic acid conjugates of diagnostic and therapeutic probes. The cervical KB tumor cells have evolved as the standard model for preclinical investigations of folate-based (radio) conjugates. In this study, a panel of FR-expressing human cancer cell lines—including cervical (HeLa, KB, KB-V1), ovarian (IGROV-1, SKOV-3, SKOV-3.ip), choriocarcinoma (JAR, BeWo) and endometrial (EFE-184) tumor cells—was investigated in vitro and for their ability to grow as xenografts in mice. FR-expression levels were compared in vitro and in vivo and the cell lines were characterized by determination of the sensitivity towards commonly-used chemotherapeutics and the expression of two additional, relevant tumor markers, HER2 and L1-CAM. It was found that, besides KB cells, its multiresistant KB-V1 subclone as well as the ovarian cancer cell lines, IGROV-1 and SKOV-3.ip, could be used as potentially more relevant preclinical models. They would allow addressing specific questions such as the therapeutic efficacy of FR-targeting agents in tumor (mouse) models of multi-resistance and in mouse models of metastases formation.

Accessing Structurally Diverse Near-Infrared Cyanine Dyes for Folate Receptor-Targeted Cancer Cell Staining

Folate receptor (FR) targeting is one of the most promising strategies for the development of small-molecule-based cancer imaging agents considering that the FR is highly overexpressed on the surface of many cancer cell types. FR-targeted conjugates of near-infrared (NIR) emissive cyanine dyes are in advanced clinical trials for fluorescence-guided surgery and are valuable research tools for optical molecular imaging in animal models. Only a small number of promising conjugates has been evaluated so far. Analysis of structure-performance relations to identify critical factors modulating the performance of targeted conjugates is essential for successful further optimization. This contribution addresses the need for convenient synthetic access to structurally diverse NIR-emissive cyanine dyes for conjugation with folic acid. Structural variations were introduced to readily available cyanine precursors in particular via C-C-coupling reactions including Suzuki and (for the first time with these types of dyes) Sonogashira cross-couplings. Photophysical properties such as absorbance maxima, brightness, and photostability are highly dependent on the molecular structure. Selected modified cyanines were conjugated to folic acid for cancer cell targeting. Several conjugates display a favorable combination of high fluorescence brightness and photostability with high affinity to FR-positive cancer cells, and enable the selective imaging of these cells with low background.

Small Molecules for Active Targeting in Cancer

For the purpose of this review, active targeting in cancer research encompasses strategies wherein a ligand for a cell surface receptor expressed on tumor cells is used to deliver a cytotoxic or imaging cargo. This area of research is more than two decades old, but in those 20 and more years, how many receptors have been studied extensively? What kinds of the ligands are used for active targeting? Are they mostly naturally occurring molecules such as folic acid, or synthetic substances developed in campaigns for medicinal chemistry efforts? This review outlines the most important receptor or ligand combinations that have been used in active targeting to answer these questions, and therefore to address the most important one of all: is research in active targeting affording diminishing returns, or is this an area for which the potential far exceeds progress made so far?

Serum folate receptor alpha as a biomarker for ovarian cancer: Implications for diagnosis, prognosis and predicting its local tumor expression

Folate receptor alpha (FRA) is a GPI-anchored glycoprotein and encoded by the FOLR1 gene. High expression of FRA is observed in specific malignant tumors of epithelial origin, including ovarian cancer, but exhibits very limited normal tissue expression, making it as an attractive target for the ovarian cancer therapy. FRA is known to shed from the cell surface into the circulation which allows for its measurement in the serum of patients. Recently, methods to detect the soluble form of FRA have been developed and serum FRA (sFRA) is considered a highly promising biomarker for ovarian cancer. We prospectively investigated the levels of sFRA in patients clinically suspected of having malignant ovarian tumors. A total of 231 patients were enrolled in this study and analyzed for sFRA as well as tumor expression of FRA by immunohistochemistry. High sFRA was predominantly observed in epithelial ovarian cancer patients, but not in patients with benign or borderline gynecological disease or metastatic ovarian tumors from advanced colorectal cancers. Levels of sFRA were highly correlated to clinical stage, tumor grade and histological type and demonstrated superior accuracy for the detection of ovarian cancer than did serum CA125. High sFRA was significantly associated with shorter progression-free survival in both early and advanced ovarian cancer patients. Finally, tumor FRA expression status was strongly correlated with sFRA levels. Taken together, these data suggest that sFRA might be a useful noninvasive serum biomarkers for future clinical trials assessing FRA-targeted therapy.

Peptide Anchor for Folate-Targeted Liposomal Delivery

Specific folate receptors are abundantly overexpressed in chronically activated macrophages and in most cancer cells. Directed folate receptor targeting using liposomes is usually achieved using folate linked to a phospholipid or cholesterol anchor. This link is formed using a large spacer like polyethylene glycol. Here, we report an innovative strategy for targeted liposome delivery that uses a hydrophobic fragment of surfactant protein D linked to folate. Our proposed spacer is a small 4 amino acid residue linker. The peptide conjugate inserts deeply into the lipid bilayer without affecting liposomal integrity, with high stability and specificity. To compare the drug delivery potential of both liposomal targeting systems, we encapsulated the nuclear dye Hoechst 34580. The eventual increase in blue fluorescence would only be detectable upon liposome disruption, leading to specific binding of this dye to DNA. Our delivery system was proven to be more efficient (2-fold) in Caco-2 cells than classic systems where the folate moiety is linked to liposomes by polyethylene glycol.

Vintafolide: a novel targeted therapy for the treatment of folate receptor expressing tumors

Despite advances in the development of molecularly targeted therapies, limited improvements in overall survival have been noted among many cancer patients with solid tumors, primarily due to development of drug resistance. Accordingly, there is an unmet need for new targeted therapies and treatment approaches for cancer, especially for overcoming resistance. Expression of the folate receptor is upregulated in many tumor types and thus represents an ideal target for cancer treatment. Several folate receptor targeted therapies are in development, including the small molecule drug conjugate vintafolide, the monoclonal antibody farletuzumab, and the antibody-drug conjugate IMGN853. The role of the folate receptor as a target in cancer progression and resistance as well as emerging preclinical and clinical data from studies on those folate receptor targeted agents that are in development with a focus on vintafolide are reviewed. The folate receptor has several unique properties, such as high expression in several tumor types, that make it a rational target for cancer treatment, and allow for selective delivery of folate receptor targeted agents. Early-stage clinical data in lung and ovarian cancer suggest that vintafolide has the potential for combination with other standard approved agents.

Selective Tumor Targeting of Desacetyl Vinblastine Hydrazide and Tubulysin B via Conjugation to a Cholecystokinin 2 Receptor (CCK2R) Ligand

As the delivery of selectively targeted cytotoxic agents via antibodies or small molecule ligands to malignancies has begun to show promise in the clinic, the need to identify and validate additional cellular targets for specific therapeutic delivery is critical. Although a multitude of cancers have been targeted using the folate receptor, PSMA, bombesin receptor, somatostatin receptor, LHRH, and α_vβ₃, there is a notable lack of specific small molecule ligand/receptor pairs to cellular targets found within cancers of the GI tract. Because of the selective GI tract expression of the cholecystokinin 2 receptor (CCK2R), we undertook the creation of conjugates that would deliver microtubule-disrupting drugs to malignancies through the specific targeting of CCK2R via a high affinity small molecule ligand. The cytotoxic activity of these conjugates were shown to be receptor mediated in vitro and in vivo with xenograft mouse models exhibiting delayed growth or regression of tumors that expressed CCK2R. Overall, this work demonstrates that ligands to CCK2R can be used to create selectively targeted therapeutic conjugates.

Folate receptor-targeted radionuclide therapy: preclinical investigation of anti-tumor effects and potential radionephropathy

INTRODUCTION

Application of therapeutic folate radioconjugates is a promising option for the treatment of folate receptor (FR)-positive tumors, although high uptake of radiofolates in the kidneys remains a critical issue. Recently, it was shown that enhancing the blood circulation of radiofolates results in increased tumor uptake and reduced retention of radioactivity in the kidneys. In this study, we investigated and compared the anti-tumor effects and potential long-term damage to the kidneys after application of an albumin-binding ((177)Lu-cm09), and a conventional ((177)Lu-EC0800) folate radioconjugate.

METHODS

In vivo studies were performed with KB tumor-bearing nude mice. (177)Lu-EC0800 and (177)Lu-cm09 were applied at variable quantities (10-30 MBq/mouse), and the tumor growth was monitored over time. Mice without tumors were injected with the same radiofolates and investigated over eight months by determination of creatinine and blood urea nitrogen plasma levels and by measuring renal uptake of (99m)Tc-DMSA using SPECT. At the study end, the morphological changes were examined on renal tissue sections using variable staining methods.

RESULTS

Compared to untreated controls, dose-dependent tumor growth inhibition and prolonged survival was observed in all treated mice. In line with the resulting absorbed dose, the treatment was more effective with (177)Lu-cm09 than with (177)Lu-EC0800, enabling complete tumor remission after application of ≥20MBq (≥28Gy). Application of radiofolates with an absorbed renal dose ≥23 Gy showed increased levels of renal plasma parameters and reduced renal uptake of (99m)Tc-DSMA. Morphological changes observed on tissue sections confirmed radionephropathy of variable stages.

CONCLUSIONS

(177)Lu-cm09 showed more favorable anti-tumor effects and significantly less damage to the kidneys compared to (177)Lu-EC0800 as was expected based on improved tumor-to-kidney ratios. It was demonstrated that enhancing the blood circulation time of radiofolates was favorable regarding the risk-benefit profile of a therapeutic application. These results hold promise for future translation of the albumin-binder concept to the clinics, potentially enabling FR-targeted radionuclide therapy in patients.

Folate receptor-α (FOLR1) expression and function in triple negative tumors

Folate receptor alpha (FOLR1) has been identified as a potential prognostic and therapeutic target in a number of cancers. A correlation has been shown between intense overexpression of FOLR1 in breast tumors and poor prognosis, yet there is limited examination of the distribution of FOLR1 across clinically relevant breast cancer subtypes. To explore this further, we used RNA-seq data from multiple patient cohorts to analyze the distribution of FOLR1 mRNA across breast cancer subtypes comprised of estrogen receptor positive (ER+), human epidermal growth factor receptor positive (HER2+), and triple negative (TNBC) tumors. FOLR1 expression varied within breast tumor subtypes; triple negative/basal tumors were significantly associated with increased expression of FOLR1 mRNA, compared to ER+ and HER2+ tumors. However, subsets of high level FOLR1 expressing tumors were observed in all clinical subtypes. These observations were supported by immunohistochemical analysis of tissue microarrays, with the largest number of 3+ positive tumors and highest H-scores of any subtype represented by triple negatives, and lowest by ER+ tumors. FOLR1 expression did not correlate to common clinicopathological parameters such as tumor stage and nodal status. To delineate the importance of FOLR1 overexpression in triple negative cancers, RNA-interference was used to deplete FOLR1 in overexpressing triple negative cell breast lines. Loss of FOLR1 resulted in growth inhibition, whereas FOLR1 overexpression promoted folate uptake and growth advantage in low folate conditions. Taken together, our data suggests patients with triple negative cancers expressing high FOLR1 expression represent an important population of patients that may benefit from targeted anti-FOLR1 therapy. This may prove particularly helpful for a large number of patients who would typically be classified as triple negative and who to this point have been left without any targeted treatment options.

Folate receptor-mediated enhanced and specific delivery of far-red light-activatable prodrugs of combretastatin A-4 to FR-positive tumor

We examined the concept of a novel prodrug strategy in which anticancer drug can be locally released by visible/near IR light, taking advantage of the photodynamic process and photo-unclick chemistry. Our most recently formulated prodrug of combretastatin A-4, Pc-(L-CA4)₂, showed multifunctionality for fluorescence imaging, light-activated drug release, and the combined effects of PDT and local chemotherapy. In this formulation, L is a singlet oxygen cleavable linker. Here, we advanced this multifunctional prodrug by adding a tumor-targeting group, folic acid (FA). We designed and prepared four FA-conjugated prodrugs 1–4 (CA4-L-Pc-PEG_n-FA: n = 0, 2, 18, ∼45) and one non-FA-conjugated prodrug 5 (CA4-L-Pc-PEG₁₈-boc). Prodrugs 3 and 4 had a longer PEG spacer and showed higher hydrophilicity, enhanced uptake to colon 26 cells via FR-mediated mechanisms, and more specific localization to SC colon 26 tumors in Balb/c mice than prodrugs 1 and 2. Prodrug 4 also showed higher and more specific uptake to tumors, resulting in selective tumor damage and more effective antitumor efficacy than non-FA-conjugated prodrug 5. FR-mediated targeting seemed to be an effective strategy to spare normal tissues surrounding tumors in the illuminated area during treatment with this prodrug.

Vintafolide (EC145) for the treatment of folate-receptor-α positive platinum-resistant ovarian cancer

Seminal advances in the treatment of cancer have been achieved because of drug development in ovarian cancer; notably the developments of platinums and taxanes. However, no new drug has been FDA approved for ovarian cancer since 2006, and the approval of an antiangiogenic agent for ovarian cancer in the US has stalled. Predicting the next breakthrough is a high risk and highly expensive venture. One of the most promising prospects is folate-receptor (FR)-targeted therapy, given the high expression in FR ovarian cancer. We review the development of vintafolide (EC145), a folic acid-desacetylvinblastine conjugate, the predictive utility of a FR-targeted imaging agent, technetium-(99)m-etarfolatide (EC20), the challenges in proving survival advantage, and other approaches to exploiting FR as a target in ovarian cancer.

"Click and go": simple and fast folic acid conjugation

Folic acid targeting by functionalization of the terminal γ-carboxylic acid is one of the most important strategies to selectively deliver chemotherapeutics and dyes to cancer cells which overexpress folate receptors. However, conjugation of folic acid is limited by its unique solubility and by selectivity issues imposing the need for expensive preparative reverse-phase chromatographic purification to isolate γ-folate conjugates. Herein is provided a novel synthetic tool for the synthesis of new folic acid conjugates with excellent γ-purity based on strain-promoted alkyne-azide cycloadditions with a γ-folate-cyclooctyne conjugate 3. To demonstrate the potential of this methodology several new folate conjugates were synthesized with high γ-purity and without using any type of chromatographic purification by reacting conjugate 3 with several fluorescent probes, polymers and siliceous materials bearing azide. In addition, the cycloaddition reaction between conjugate 3 and an azido-derived fluorescent dye was successfully performed in cellular media leading to an increase of fluorescence in the cells which overexpress folate receptors (NCI-H460).

Phase II study of treatment of advanced ovarian cancer with folate-receptor-targeted therapeutic (vintafolide) and companion SPECT-based imaging agent (99mTc-etarfolatide)

BACKGROUND

This report examines (99m)Tc-etarfolatide imaging to identify the presence of folate receptor (FR) on tumors of women with recurrent/refractory ovarian or endometrial cancer and correlates expression with response to FR-targeted therapy (vintafolide).

PATIENTS AND METHODS

In this phase II, single-arm, multicenter study, patients with advanced ovarian cancer were imaged with (99m)Tc-etarfolatide before vintafolide treatment. Up to 10 target lesions (TLs) were selected based on Response Evaluation Criteria In Solid Tumors criteria using computed tomography scans. Single-photon emission computed tomography images of TLs were assessed for (99m)Tc-etarfolatide uptake as either FR positive or negative. Patients were categorized by percentage of TLs positive and grouped as FR(100%), FR(10%-90%), and FR(0%). Lesion and patient response were correlated with etarfolatide uptake.

RESULTS

Forty-nine patients were enrolled; 43 were available for analysis. One hundred thirty-nine lesions were (99m)Tc-etarfolatide evaluable: 110 FR positive and 29 FR negative. Lesion disease control rate (DCR = stable or response) was observed in 56.4% of FR-positive lesions versus 20.7% of FR-negative lesions (P < 0.001). Patient DCR was 57%, 36%, and 33% in FR(100%), FR(10%-90%), and FR(0%) patients, respectively. Median overall survival was 14.6, 9.6, and 3.0 months in FR(100%), FR(10%-90%), and FR(0%) patients, respectively.

CONCLUSIONS

Overall response to FR-targeted therapy and DCR correlate with FR positivity demonstrated by (99m)Tc-etarfolatide imaging.

CLINICAL TRIAL NUMBER

NCT00507741.

Folate-vinca alkaloid conjugates for cancer therapy: a structure-activity relationship

Vintafolide is a potent folate-targeted vinca alkaloid small molecule drug conjugate (SMDC) that has shown promising results in multiple clinical oncology studies. Structurally, vintafolide consists of 4 essential modules: (1) folic acid, (2) a hydrophilic peptide spacer, (3) a disulfide-containing, self-immolative linker, and (4) the cytotoxic drug, desacetylvinblastine hydrazide (DAVLBH). Here, we report a structure-activity study evaluating the biological impact of (i) substituting DAVLBH within the vintafolide molecule with other vinca alkaloid analogues such as vincristine, vindesine, vinflunine, or vinorelbine; (ii) substituting the naturally (S)-configured Asp-Arg-Asp-Asp-Cys peptide with alternative hydrophilic spacers of varied composition; and (iii) varying the composition of the linker module. A series of vinca alkaloid-containing SMDCs were synthesized and purified by HPLC and LCMS. The SMDCs were screened in vitro against folate receptor (FR)-positive cells, and anti-tumor activity was tested against well-established subcutaneous FR-positive tumor xenografts. The cytotoxic and anti-tumor activity was directly compared to that produced by vintafolide. Among all the folate vinca alkaloid SMDCs tested, DAVLBH-containing SMDCs were active, while those constructed with vincristine, vindesine, or vinorelbine analogues failed to produce meaningful biological activity. Within the DAVLBH series, having a bioreleasable, self-immolative linker system was found to be critical for activity since multiple analogues constructed with thioether-based linkers all failed to produce meaningful activity both in vitro and in vivo. Substitutions of some or all of the natural amino acids within vintafolide's hydrophilic spacer module did not significantly change the in vitro or in vivo potency of the SMDCs. Vintafolide remains one of the most potent folate-vinca alkaloid SMDCs produced to date, and continued clinical development is warranted.

Targeted treatment of folate receptor-positive platinum-resistant ovarian cancer and companion diagnostics, with specific focus on vintafolide and etarfolatide

Among the gynecological malignancies, ovarian cancer is the leading cause of mortality in developed countries. Treatment of ovarian cancer is based on surgery integrated with chemotherapy. Platinum-based drugs (cisplatin and carboplatin) comprise the core of first-line chemotherapy for patients with advanced ovarian cancer. Platinum-resistant ovarian cancer can be treated with cytotoxic chemotherapeutics such as paclitaxel, topotecan, PEGylated liposomal doxorubicin, or gemcitabine, but many patients eventually relapse on treatment. Targeted therapies based on agents specifically directed to overexpressed receptors, or to selected molecular targets, may be the future of clinical treatment. In this regard, overexpression of folate receptor-α on the surface of almost all epithelial ovarian cancers makes this receptor an excellent "tumor-associated antigen". With appropriate use of spacers/linkers, folate-targeted drugs can be distributed within the body, where they preferentially bind to ovarian cancer cells and are released inside their target cells. Here they can exert their desired cytotoxic function. Based on this strategy, 12 years after it was first described, a folate-targeted vinblastine derivative has now reached Phase III clinical trials in ovarian cancer. This review examines the importance of folate targeting, the state of the art of a vinblastine folate-targeted agent (vintafolide) for treating platinum-resistant ovarian cancer, and its diagnostic companion (etarfolatide) as a prognostic agent. Etarfolatide is a valuable noninvasive diagnostic imaging agent with which to select ovarian cancer patient populations that may benefit from this specific targeted therapy.

Rational combination therapy of vintafolide (EC145) with commonly used chemotherapeutic drugs

PURPOSE

When evaluated in patients with ovarian and other cancer, vintafolide (EC145), a potent folate-targeted vinca alkaloid conjugate, displayed a toxicity profile that seemed to be nonoverlapping with many standard-of-care cancer therapeutics. It was, therefore, hypothesized that combining vintafolide with certain approved anticancer drugs may afford greater therapeutic efficacy compared with single-agent therapy. To explore this concept, vintafolide was evaluated in combination with pegylated liposomal doxorubicin (PLD; DOXIL), cisplatin, carboplatin, paclitaxel, docetaxel, topotecan, and irinotecan against folate receptor (FR)-positive models.

EXPERIMENTAL DESIGN

FR-expressing KB, M109, IGROV, and L1210 cells were first exposed to graded concentrations of vintafolide, either alone or in combination with doxorubicin (active ingredient in PLD), and isobologram plots and combination index values generated. The vintafolide combinations were also studied in mice bearing various FR-expressing tumors.

RESULTS

Vintafolide displayed strong synergistic activity against KB cells when combined with doxorubicin, and no less-than-additive effects resulted when tested against M109, IGROV, and L1210 cells. In contrast, when either desacetylvinblastine hydrazide (DAVLBH; the vinca alkaloid moiety in vintafolide) or vindesine (the vinca alkaloid most structurally similar to DAVLBH) were tested in combination with doxorubicin, less-than-additive antitumor effects were observed. In vivo, all vintafolide drug combinations produced far greater antitumor effect (complete responses and cures) compared with the single agents alone, without significant increase in overall toxicity. Importantly, these benefits were not observed with combinations of PLD and DAVLBH or vindesine.

CONCLUSIONS

On the basis of these encouraging preclinical results, clinical studies to evaluate vintafolide drug combination therapies are now under way.

177Lu-EC0800 combined with the antifolate pemetrexed: preclinical pilot study of folate receptor targeted radionuclide tumor therapy

Targeted radionuclide therapy has shown impressive results for the palliative treatment of several types of cancer diseases. The folate receptor has been identified as specifically associated with a variety of frequent tumor types. Therefore, it is an attractive target for the development of new radionuclide therapies using folate-based radioconjugates. Previously, we found that pemetrexed (PMX) has a favorable effect in reducing undesired renal uptake of radiofolates. Moreover, PMX also acts as a chemotherapeutic and radiosensitizing agent on tumors. Thus, the aim of our study was to investigate the combined application of PMX and the therapeutic radiofolate (177)Lu-EC0800. Determination of the combination index (CI) revealed a synergistic inhibitory effect of (177)Lu-EC0800 and PMX on the viability of folate receptor-positive cervical (KB) and ovarian (IGROV-1) cancer cells in vitro (CI < 0.8). In an in vivo study, tumor-bearing mice were treated with (177)Lu-EC0800 (20 MBq) and a subtherapeutic (0.4 mg) or therapeutic amount (1.6 mg) of PMX. Application of (177)Lu-EC0800 with PMXther resulted in a two- to four-fold enhanced tumor growth delay and a prolonged survival of KB and IGROV-1 tumor-bearing mice, as compared to the combination with PMXsubther or untreated control mice. PMXsubther protected the kidneys from undesired side effects of (177)Lu-EC0800 (20 MBq) by reducing the absorbed radiation dose. Intact kidney function was shown by determination of plasma parameters and quantitative single-photon emission computed tomography using (99m)Tc-DMSA. Our results confirmed the anticipated dual role of PMX. Its unique features resulted in an improved antitumor effect of folate-based radionuclide therapy and prevented undesired radio-nephrotoxicity.

Development of tumor-targeted near infrared probes for fluorescence guided surgery

Complete surgical resection of malignant disease is the only reliable method to cure cancer. Unfortunately, quantitative tumor resection is often limited by a surgeon's ability to locate all malignant disease and distinguish it from healthy tissue. Fluorescence-guided surgery has emerged as a tool to aid surgeons in the identification and removal of malignant lesions. While nontargeted fluorescent dyes have been shown to passively accumulate in some tumors, the resulting tumor-to-background ratios are often poor, and the boundaries between malignant and healthy tissues can be difficult to define. To circumvent these problems, our laboratory has developed high affinity tumor targeting ligands that bind to receptors that are overexpressed on cancer cells and deliver attached molecules selectively into these cells. In this study, we explore the use of two tumor-specific targeting ligands (i.e., folic acid that targets the folate receptor (FR) and DUPA that targets prostate specific membrane antigen (PSMA)) to deliver near-infrared (NIR) fluorescent dyes specifically to FR and PSMA expressing cancers, thereby rendering only the malignant cells highly fluorescent. We report here that all FR- and PSMA-targeted NIR probes examined bind cultured cancer cells in the low nanomolar range. Moreover, upon intravenous injection into tumor-bearing mice with metastatic disease, these same ligand-NIR dye conjugates render receptor-expressing tumor tissues fluorescent, enabling their facile resection with minimal contamination from healthy tissues.

Folic acid modified cationic γ-cyclodextrin-oligoethylenimine star polymer with bioreducible disulfide linker for efficient targeted gene delivery

For an efficient folate-targeted delivery, while the interaction between the folate on the carriers and the folate receptor (FR) on the cells is necessary, the recovering and recycling of FR to maintain a high density level of FR on the cellular membrane is also important. Herein, we demonstrate a design and synthesis of a new star-shaped cationic polymer containing a γ-cyclodextrin (γ-CD) core and multiple oligoethylenimine (OEI) arms with folic acid (FA) linked by a bioreducible disulfide bond for efficient targeted gene delivery. The newly synthesized cationic polymer, named γ-CD-OEI-SS-FA, could be cleaved efficiently, and FA was readily released under reductive condition similar to intracellular environment. The γ-CD-OEI-SS-FA polymer was well-characterized and studied in terms of its gene delivery properties in FR-positive KB cells and FR-negative A549 cells under various conditions, in comparison with cationic polymers such as high molecular weight branched polyethylenimine (PEI), γ-CD-OEI star-shaped cationic polymer, γ-CD-OEI-FA polymer where FA was directed linked to the star polymer without disulfide linker. Our data have demonstrated that the new γ-CD-OEI-SS-FA gene carrier had low cytotoxicity and possessed capacity to target and deliver DNA to specific tumor cells that overexpress FRs, as well as functions to recover and recycle FRs onto cellular membranes to facilitate continuous FR-mediated endocytosis to achieve very high levels of gene expression. This study has expanded the strategy of FA-targeted delivery by combining the smart FR-recycling function to achieve the significant enhancement of gene expression. The new FA-targeted and bioreducible carrier may be a promising efficient gene delivery system for potential cancer gene therapy.

Synthesis and biological evaluation of a folate-targeted rhaponticin conjugate

To improve the therapeutic effect of rhaponticin (RHA), a folate receptor (FR) targeted RHA conjugate was synthesized by utilizing a hydrophilic peptide spacer linked to folic acid (FA) via a releasable disulfide linker. This water-soluble conjugate was found to retain high affinity for FR-positive cells, and it produced specific, dose-responsive activity in vitro. Treatment of FRHA with a reducing agent indicated that the amino-reactive derivative of RHA would be released spontaneously following disulfide bond reduction within the endosomes. FRHA also proved to be active predominantly specific against FR-positive syngeneic and xenograft models in vivo, and possible curative activity resulted with minimal to moderate toxicity. The FRHA conjugate greatly enhanced the therapeutic effects and reduced the toxicity of RHA. In conclusion, FRHA represents a folate-targeted chemotherapeutic that can produce potent activity against established sc tumors. Hence, this report has a great significance in pharmacology and clinical medicine as well as methodology.

Phase I study of folate conjugate EC145 (Vintafolide) in patients with refractory solid tumors

PURPOSE

EC145 (vintafolide), a conjugate of folic acid and the vinca alkaloid desacetylvinblastine hydrazide (DAVLBH), is a ligand for the folate receptor (FR), with activity against FR-positive tumor xenografts in vivo. This phase I study determined the maximum-tolerated dose (MTD) of EC145 administered as a bolus intravenous injection or 1-hour infusion in patients with refractory solid tumors.

PATIENTS AND METHODS

EC145 was administered as a bolus injection or 1-hour infusion on days 1, 3, and 5 and days 15, 17, and 19 of each 28-day cycle with dose escalation in cohorts of three to six patients until the MTD was identified. Plasma pharmacokinetics were determined on days 1 and 3 of the first cycle.

RESULTS

The MTD of EC145 was 2.5 mg when administered as either a bolus injection or 1-hour infusion. Constipation was the dose-limiting toxicity with both routes. Constipation, nausea, fatigue, and vomiting were the most commonly reported adverse events. One partial response to therapy was observed in a patient with metastatic ovarian cancer.

CONCLUSION

EC145 administered by bolus injection or as a 1-hour infusion at a dose of 2.5 mg on days 1, 3, and 5 and days 15, 17, and 19 of a 28-day cycle has an acceptable safety profile in patients with advanced cancer. On the basis of these findings, phase II studies of EC145 have been initiated in patients with advanced epithelial ovarian cancer and non-small-cell lung cancer.

Engineering folate-drug conjugates to target cancer: from chemistry to clinic

The folate receptor (FR) is a potentially useful biological target for the management of many human cancers. This membrane protein binds extracellular folates with very high affinity and, through an endocytic process, physically delivers them inside the cell for biological consumption. There are now many examples of how this physiological system can be exploited for the targeted delivery of biologically active molecules to cancer. In fact, strong preclinical as well as emerging clinical evidence exists showing how FR-positive cancers can be (i) anatomically identified using folate conjugates of radiodiagnostic imaging agents and (ii) effectively treated with companion folate-targeted chemotherapies. While the biological results are compelling, it is of equal importance to understand the conjugation chemistries that were developed to produce these active molecules. Therefore, this review will focus on the methods utilized to construct folate-based small-molecule drug conjugates (SMDCs), with particular attention focused on modular design, hydrophilic spacers, and self-immolative linkers.

Characterization of in vivo disulfide-reduction mediated drug release in mouse kidneys

Due to the overexpression of a folate receptor (FR) on many malignant cells, folate-targeted drugs have been developed to improve the cancer specificity of chemotherapeutic agents. Therapeutic index is further enhanced with the use of self-immolative linkers that efficiently release the attached drug upon cellular internalization of the folate-drug conjugate. Because FR is also abundant in normal kidney proximal tubule (PT) cells, we sought to examine in real time the trafficking and release of folate-targeted drugs in the kidney in vivo. Thus, we conducted two-photon kidney imaging studies in mice utilizing a Förster resonance energy transfer (FRET) based folate conjugate that undergoes a color shift from red to green upon reduction of the disulfide bond linking folate to a surrogate drug molecule. Following infusion via intravenous injection, folate-FRET reached the kidney in its intact unreduced form. The folate-FRET conjugate was then filtered into the lumen of PT, where it was efficiently captured by FR. As FR transcytosed across PT, some disulfide reduction occurred, with reduced folate-FRET detectable in PT vesicles 30 min postinjection. Prolonged monitoring of folate-FRET in mice showed modest progression of reduction in PT cells over time. Moreover, inhibition of FR trafficking in PT cells by colchicine did not significantly affect the rate or extent of folate-FRET reduction. Finally, the lack of cytosolic accumulation of released drug surrogate in the PT suggests that drug release via disulfide bond reduction should cause little kidney toxicity.