Synthesis and evaluation of pteroic acid-conjugated nitroheterocyclic phosphoramidates as folate receptor-targeted alkylating agents

A Bibliometric analysis of folate receptor research

BACKGROUND

The objective of this study was to conduct a bibliometric analysis of the entire field of folate receptor research. Folate receptor is expressed on a wide variety of cancers and certain immune cells.

METHODS

A Web of Science search was performed on folate receptor or folate binding protein (1969-to June 28, 2019). The following information was examined: publications per year, overall citations, top 10 authors, top 10 institutions, top 10 cited articles, top 10 countries, co-author collaborations and key areas of research.

RESULTS

In total, 3248 documents for folate receptor or folate binding protein were retrieved for the study years outlined in the methods section search query. The range was 1 per year in 1969 to 264 for the last full year studied (2018). A total of 123,720 citations for the 3248 documents retrieved represented a mean citation rate per article of 38.09 and range of 1667 citations (range 0 to 1667). Researchers in 71 countries authored publications analyzed in this study. The US was the leader in publications and had the highest ranking institution. The top 10 articles have been cited 7270 times during the time frame of this study. The top cited article had an average citation rate of 110 citations per year. Network maps revealed considerable co-authorship among several of the top 10 authors.

CONCLUSION

Our study presents several important insights into the features and impact of folate receptor research. To our knowledge, this is the first bibliometric analysis of folate receptor.

Phytosynthesis of gold nanoparticles using Mappia foetida leaves extract and their conjugation with folic acid for delivery of doxorubicin to cancer cells

Mappia foetida leaves extract is used as bioreductant for the synthesis of gold nanoparticles and their application in the efficient delivery of doxorubicin to human cancer cells is reported here. The formation of gold nanoparticles is evident from their characteristic optical absorption at ~560 nm. X-ray diffraction pattern of gold nanoparticles confirmed their fcc structure. Fourier transform infrared spectroscopy shows the bioactive molecules from plant extract capped on the surface of gold nanoparticles and conjugation of doxorubicin along with activated folic acid as navigational molecules for targeted drug delivery. Such a conjugation of gold nanoparticles is characterized by their weight loss, ~35-40 %, due to thermal degradation of plant biomass and conjugated drug along with receptor, as observed in thermogravimetric analysis. The spherical shaped gold nanoparticles (Φ 10-20 nm) are observed by field emission scanning electron microscopy and transmission electron microscopy images and the expected elemental composition by energy dispersive X-ray spectroscopy. Gold nanoparticles conjugated with activated folic acid and doxorubicin complex is found to be toxic for human cancer cells viz., MDA-MB-231, HeLa, SiHa and Hep-G2. Furthermore, the amount of drug released was maximum at pH 5.3 (an ambient condition for intravenous cancer drugs) followed by pH 7.2 and pH 6.8.

Design, synthesis and biological evaluation of a highly-potent and cancer cell selective folate-taxoid conjugate

The folate receptor (FR) has been widely recognized as an excellent target for the tumor-selective delivery of cytotoxic agents, and four folate-drug conjugates have entered clinical evaluations for the treatment of solid tumors to date. However, most of these conjugates required structural modification of the cytotoxic warheads in order to achieve efficient drug release from the linkers. We designed and constructed a novel folate conjugate of a highly-potent next-generation taxoid, SB-T-1214, by exploiting bioorthogonal Cu-free 'click' chemistry. The synthesis was highly convergent and required no HPLC purification to obtain the final folate-taxoid conjugate 1. Conjugate 1 demonstrated highly FR-specific potency (IC₅₀ 2.1-3.5 nM) against a panel of cancer cell lines, with a >1000-fold decrease in cytotoxicity against normal human cells (IC₅₀>5000 nM). The remarkable potency and selectivity of conjugate 1 can be attributed to highly FR-specific receptor-mediated endocytosis as well as efficient release of the unmodified cytotoxic warhead using a mechanism-based self-immolative linker.

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.

Drug Delivery Devices and Targeting Agents for Platinum(II) Anticancer Complexes

An ideal platinum-based delivery device would be one that selectively targets cancerous cells, can be systemically delivered, and is non-toxic to normal cells. It would be beneficial to provide drug delivery devices for platinum-based anticancer agents that exhibit high drug transport capacity, good water solubility, stability during storage, reduced toxicity, and enhanced anticancer activity in vivo. However, the challenges for developing drug delivery devices include carrier stability in vivo, the method by which extracellular or intracellular drug release is achieved, overcoming the various mechanisms of cell resistance to drugs, controlled drug release to cancer cells, and platinum drug bioavailability. There are many potential candidates under investigation including cucurbit[n]urils, cyclodextrins, calix[n]arenes, and dendrimers, with the most promising being those that are synthetically adaptable enough to attach to targeting agents.

Chemical modification of therapeutic drugs or drug vector systems to achieve targeted therapy: looking for the grail

Most therapeutic drugs distribute to the whole body, which results in general toxicity and poor acceptance of the treatments by patients. The targeted delivery of chemotherapeutics to defined cells, either stromal or cancer cells in cancer lesions, or defined inflammatory cells in immunological disorders, is one of the main challenges and a very active field of research in the development of treatment strategies to minimize side-effects of drugs. Disease-associated cells express molecules, including proteases, receptors, or adhesion molecules, that are different or differently expressed than their normal counterparts. Therefore one goal in the field of targeted therapies is to develop chemically derivatized drugs or drug vectors able to target defined cells via specific recognition mechanisms and also able to overcome biological barriers. This article will review the approaches which have been explored to achieve these goals and will discuss in more detail three examples (i) the use of nanostructures to take advantage of increased vascular permeability in some human diseases, (ii) the targeting of therapeutic drugs to an organ, the brain, protected against foreign molecules by the blood-brain barrier, and (iii) the use of the folate receptor to target either tumor cells or activated macrophages.

Improved biochemical strategies for targeted delivery of taxoids

Paclitaxel (Taxol) and docetaxel (Taxotere) are very important anti-tumor drugs in clinical use for cancer. However, their clinical utility is limited due to systemic toxicity, low solubility and inactivity against drug resistant tumors. To improve chemotherapeutic levels of these drugs, it would be highly desirable to design strategies which bypass the above limitations. In this respect various prodrug and drug targeting strategies have been envisioned either to improve oral bioavailability or tumor specific delivery of taxoids. Abnormal properties of cancer cells with respect to normal cells have guided in designing of these protocols. This review article records the designed biochemical strategies and their biological efficacies as potential taxoid chemotherapeutics.

Synthesis and biological properties of bioreductively targeted nitrothienyl prodrugs of combretastatin A-4

Nitrothienylprop-2-yl ether formation on the 3'-phenolic position of combretastatin A-4 (1) abolishes the cytotoxicity and tubulin polymerization-inhibitory effects of the drug. 5-Nitrothiophene derivatives of 1 were synthesized following model kinetic studies with analogous coumarin derivatives, and of these, compound 13 represents a promising new lead in bioreductively targeted cytotoxic anticancer therapies. In this compound, optimized gem-dimethyl alpha-carbon substitution enhances both the aerobic metabolic stability and the efficiency of hypoxia-mediated drug release. Only the gem-substituted derivative 13 released 1 under anoxia in either in vitro whole-cell experiments or supersomal suspensions. The rate of release of 1 from the radical anions of these prodrugs is enhanced by greater methyl substitution on the alpha-carbon. Cellular and supersomal studies showed that this alpha-substitution pattern controls the useful range of oxygen concentrations over which 1 can be effectively released by the prodrug.

Design and synthesis of phosphotyrosine peptidomimetic prodrugs

A novel approach to the intracellular delivery of aryl phosphates has been developed that utilizes a phosphoramidate-based prodrug approach. The prodrugs contain an ester group that undergoes reductive activation intracellularly with concomitant expulsion of a phosphoramidate anion. This anion undergoes intramolecular cyclization and hydrolysis to generate aryl phosphate exclusively with a t(1/2) = approximately 20 min. Phosphoramidate prodrugs (8-10) of phosphate-containing peptidomimetics that target the SH2 domain were synthesized. Evaluation of these peptidomimetic prodrugs in a growth inhibition assay and in a cell-based transcriptional assay demonstrated that the prodrugs had IC50 values in the low micromolar range. Synthesis of phosphorodiamidate analogues containing a P-NH-Ar linker (16-18) was also carried out in the hope that the phosphoramidates released might be phosphatase-resistant. Comparable activation rates and cell-based activities were observed for these prodrugs, but the intermediate phosphoramidate dianion underwent spontaneous hydrolysis with a t(1/2) = approximately 30 min.

Novel class of cyclophosphamide prodrug: Cyclophosphamide spiropiperaziniums (CPSP)

A novel class of cyclophosphamide spiropiperaziniums was synthesized and evaluated for their in vivo anti-cancer activities against S180 and H22. Most of them exhibited definite activities. Especially, compounds 8b and 8k showed good anti-cancer activities, meanwhile, 8k also showed much lower toxicity than CP. Several interesting structure-activity relationships were revealed.

Folate Receptor-Mediated Drug Targeting: From Therapeutics to Diagnostics

Folate targeted drug delivery has emerged as an alternative therapy for the treatment and imaging of many cancers and inflammatory diseases. Due to its small molecular size and high binding affinity for cell surface folate receptors (FR), folate conjugates have the ability to deliver a variety of molecular complexes to pathologic cells without causing harm to normal tissues. Complexes that have been successfully delivered to FR expressing cells, to date, include protein toxins, immune stimulants, chemotherapeutic agents, liposomes, nanoparticles, and imaging agents. This review will summarize the applications of folic acid as a targeting ligand and highlight the various methods being developed for delivery of therapeutic and imaging agents to FR-expressing cells.

Synthesis and Biological Evaluation of a Cytarabine Phosphoramidate Prodrug

Recently, we reported a novel approach for the intracellular delivery of the anti-cancer nucleotide 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP) using phosphoramidate-based prodrugs. These phosphoramidate prodrugs contain an ester group that undergoes intracellular activation, liberating phosphoramidate anion, which in turn undergoes spontaneous cyclization and P-N bond cleavage to yield the nucleoside monophosphate quantitatively. This approach has now been extended to cytarabine [1-beta-D-arabinofuranosylcytosine (Ara-C)], an anti-cancer nucleoside that is limited in its utility because of poor intracellular transport characteristics and weak activity as a substrate for tumor cell kinases. The cytarabine phosphoramidate prodrug 1 has been synthesized and evaluated in comparison with cytarabine for growth inhibitory activity against wild-type, nucleoside transport-deficient, and nucleoside kinase-deficient CEM leukemia cell lines. The prodrug was comparable in growth inhibitory activity (IC50 = 32 nM) to cytarabine (IC50 = 16 nM) in wild-type CCRF-CEM cells following drug treatment for 72 h. The nucleoside transport-deficient CEM/AraC8C exhibited a high level of resistance (6400-fold) to cytarabine but was more sensitive (210-fold resistant vs CCRF-CEM cells) to prodrug 1. Similarly, the deoxycytidine kinase-deficient cell line (CEM/dCK-) was highly resistant to cytarabine (13900-fold) but more sensitive (106-fold resistant vs CCRF-CEM cells) to prodrug 1. These results indicate that prodrug 1 is significantly more potent than cytarabine against transport- and kinase-deficient cell lines and are consistent with a mechanism involving intracellular delivery of cytarabine 5'-monophosphate.