Inhibition of purple acid phosphatase with α-alkoxynaphthylmethylphosphonic acids

Optimization of an α-aminonaphthylmethylphosphonic acid inhibitor of purple acid phosphatase using rational structure-based design approaches

Purple acid phosphatases (PAPs) are ubiquitous binuclear metallohydrolases that have been isolated from various animals, plants and some types of fungi. In humans and mice, elevated PAP activity in osteoclasts is associated with osteoporosis, making human PAP an attractive target for the development of anti-osteoporotic drugs. Based on previous studies focusing on phosphonate scaffolds, as well as a new crystal structure of a PAP in complex with a derivative of a previously synthesized α-aminonaphthylmethylphosphonic acid, phosphonates 24-40 were designed as new PAP inhibitor candidates. Subsequent docking studies predicted that all of these compounds are likely to interact strongly with the active site of human PAP and most are likely to interact strongly with the active site of pig PAP. The seventeen candidates were synthesized with good yields and nine of them (26-28, 30, 33-36 and 38) inhibit in the sub-micromolar to nanomolar range against pig PAP, with 28 and 35 being the most potent mammalian PAP inhibitors reported with K_i values of 168 nM and 186 nM, respectively. This study thus paves the way for the next stage of drug development for phosphonate inhibitors of PAPs as anti-osteoporotic agents.

Regiospecific Photochemical Synthesis of Methylchrysenes

Methylated polycyclic aromatic hydrocarbons (PAHs) are suspected to be some of the toxic compounds in crude oil towards marine life and are needed as single compounds for environmental studies. 1-, 3- and 6-methylchrysene (3a,b,c) were prepared as single isomers by photochemical cyclization of the corresponding stilbenoids in the Mallory reaction using stoichiometric amounts of iodine in 82-88% yield. 2-methylchrysene (3d) was prepared by photochemical cyclization where the regioselectivity was controlled by elimination of an ortho-methoxy group under acidic oxygen free conditions in 72% yield. These conditions failed to form 4-methylchrysene from the corresponding stilbenoid. All stilbenoids were made from a common naphthyl Wittig salt and suitably substituted benzaldehydes. We have also demonstrated that methylchrysenes can be oxidized to the corresponding chrysenecarboxylic acids by KMnO₄ in modest yields.

Rational Design of Potent Inhibitors of a Metallohydrolase Using a Fragment-Based Approach

Metallohydrolases form a large group of enzymes that have fundamental importance in a broad range of biological functions. Among them, the purple acid phosphatases (PAPs) have gained attention due to their crucial role in the acquisition and use of phosphate by plants and also as a promising target for novel treatments of bone-related disorders and cancer. To date, no crystal structure of a mammalian PAP with drug-like molecules bound near the active site is available. Herein, we used a fragment-based design approach using structures of a mammalian PAP in complex with the Maybridge^TM fragment CC063346, the amino acid L-glutamine and the buffer molecule HEPES, as well as various solvent molecules to guide the design of highly potent and efficient mammalian PAP inhibitors. These inhibitors have improved aqueous solubility when compared to the clinically most promising PAP inhibitors available to date. Furthermore, drug-like fragments bound in newly discovered binding sites mapped out additional scaffolds for further inhibitor discovery, as well as scaffolds for the design of inhibitors with novel modes of action.

Synthesis and biological activities of drugs for the treatment of osteoporosis

Osteoporosis is an asymptomatic progressive disease. With the improvement of people's living standard and the aging of population, osteoporosis and its fracture have become one of the main diseases threatening the aging society. The serious medical and social burden caused by this has aroused wide public concern. Osteoporosis is listed as one of the three major diseases of the elderly. At present, the drugs for osteoporosis include bone resorption inhibitors and bone formation promoters. The purpose of these anti-osteoporosis drugs is to balance osteoblast bone formation and osteoclast bone resorption. With the development of anti-osteoporosis drugs, new anti osteoporosis drugs have been designed and synthesized. There are many kinds of new compounds with anti osteoporosis activity, but most of them are concentrated on the original drugs with anti osteoporosis activity, or the natural products with anti-osteoporosis activity are extracted from the natural products for structural modification to obtain the corresponding derivatives or analogues. These target compounds showed good ALP activity in vitro and in vivo, promoted osteoblast differentiation and mineralization, or had anti TRAP activity, inhibited osteoclast absorption. This work attempts to systematically review the studies on the synthesis and bioactivity of anti-osteoporosis drugs in the past 10 years. The structure-activity relationship was discussed, which provided a reasonable idea for the design and development of new anti-osteoporosis drugs.

Synthesis and evaluation of novel purple acid phosphatase inhibitors

Transgenic studies in animals have demonstrated a direct association between the level of expression of purple acid phosphatase (PAP; also known as tartrate-resistant acid phosphatase) and the progression of osteoporosis. Consequently, PAP has emerged as a promising target for the development of novel therapeutic agents to treat this debilitating disorder. PAPs are binuclear hydrolases that catalyse the hydrolysis of phosphorylated substrates under acidic to neutral conditions. A series of phenyltriazole carboxylic acids, prepared by the reactions of azide derivatives with propiolic acid through copper(i)-catalysed azide-alkyne cycloaddition click reactions, has been assessed for their inhibitory effect on the catalytic activity of pig and red kidney bean PAPs. The binding mode of most of these compounds is purely uncompetitive with K _iuc values as low as ∼23 μM for the mammalian enzyme. Molecular modelling has been used to examine the binding modes of these triazole compounds in the presence of a substrate in the active site of the enzyme in order to rationalise their activities and to design more potent and specific derivatives.

Synthesis and Reactions of α-Hydroxyphosphonates

This review summarizes the main synthetic routes towards α-hydroxyphosphonates that are known as enzyme inhibitors, herbicides and antioxidants, moreover, a number of representatives express antibacterial or antifungal effect. Special attention is devoted to green chemical aspects. α-Hydroxyphosphonates are also versatile intermediates for other valuable derivatives. O-Alkylation and O-acylation are typical reactions to afford α-alkoxy-, or α-acyloxyphosphonates, respectively. The oxidation of hydroxyphosphonates leads to ketophosphonates. The hydroxy function at the α carbon atom of hydroxyphosphonates may be replaced by a halogen atom. α-Aminophosphonates formed in the nucleophilic substitution reaction of α-hydroxyphosphonates with primary or secondary amines are also potentially bioactive compounds. Another typical reaction is the base-catalyzed rearrangement of α-hydroxy-phosphonates to phosphates. Hydrolysis of the ester function of hydroxyphosphonates leads to the corresponding phosphonic acids.

Identification of inhibitors of Tartrate-resistant acid phosphatase (TRAP/ACP5) activity by small-molecule screening

Tartrate-resistant acid phosphatase (TRAP/ACP5) occurs as two isoforms-TRAP 5a with low enzymatic activity due to a loop interacting with the active site and the more active TRAP isoform 5b generated upon proteolytic cleavage of this loop. TRAP has been implicated in several diseases, including cancer. Thus, this study set out to identify small-molecule inhibitors of TRAP activity. A microplate-based enzymatic assay for TRAP 5b was applied in a screen of 30,315 compounds, resulting in the identification of 90 primary hits. After removal of promiscuous compounds, unwanted groups, and false positives by orthogonal assays and three-concentration validation, the properties of 52 compounds were further investigated to better understand their mechanism of action. Full-concentration-response curves for these compounds were established under different enzyme concentrations and (pre)incubation times to remove compounds with inconsistent results and low potencies. Full-concentration-response curves were also performed for both isoforms, to examine isoform prevalence. Filtering led to six prioritized compounds, representing different clusters. One of these, CBK289001 or (6S)-6-[3-(2H-1,3-benzodioxol-5-yl)-1,2,4-oxadiazol-5-yl]-N-(propan-2-yl)-1H,4H,5H,6H,7H-imidazo[4,5-c]pyridine-5-carboxamide, demonstrated efficacy in a migration assay and IC₅₀ values from 4 to 125 μm. Molecular docking studies and analog testing were performed around CBK289001 to provide openings for further improvement toward more potent blockers of TRAP activity.

Diethylalkylsulfonamido(4-methoxyphenyl)methyl)phosphonate/phosphonic acid derivatives act as acid phosphatase inhibitors: synthesis accompanied by experimental and molecular modeling assessments

Purple acid phosphatases (PAPs) are binuclear metallo-hydrolases that have been isolated from various mammals, plants, fungi and bacteria. In mammals, PAP activity is associated with bone resorption and can lead to bone metabolic disorders such as osteoporosis; thus human PAP is an attractive target to develop anti-osteoporotic drugs. The aim of the present study was to investigate inhibitory effect of synthesized diethylalkylsulfonamido(4-methoxyphenyl)methyl)phosphonate/phosphonic acid derivatives as potential red kidney bean PAP (rkbPAP) inhibitors accompanied by experimental and molecular modeling assessments. Enzyme kinetic data showed that they are good rkbPAP inhibitors whose potencies improve with increasing alkyl chain length. Hexadecyl derivatives, as most potent compounds (K_i = 1.1 µM), inhibit rkbPAP in the mixed manner, while dodecyl derivatives act as efficient noncompetitive inhibitor. Also, analysis by molecular modeling of the structure of the rkbPAP-inhibitor complexes reveals factors, which may be important for the determination of inhibition specificity.

The small chemical enzyme inhibitor 5-phenylnicotinic acid/CD13 inhibits cell migration and invasion of tartrate-resistant acid phosphatase/ACP5-overexpressing MDA-MB-231 breast cancer cells

Tartrate-resistant acid phosphatase (TRAP/ACP5/uteroferrin/purple acid phosphatase/PP5) has received considerable attention as a newly discovered proinvasion metastasis driver associated with different malignancies. This renders TRAP an interesting target for novel anti-cancer therapy approaches. TRAP exists as two isoforms, 5a and 5b, where the 5a isoform represents an enzymatically less active monomeric precursor to the more enzymatically active 5b isoform generated by proteolytic excision of a repressive loop domain. Recently, three novel lead compounds were identified by fragment-based screening and demonstrated to be efficient TRAP enzyme inhibitors in vitro. We conclude that one of the three compounds i.e. 5-phenylnicotinic acid (CD13) was efficient as a TRAP inhibitor with Kic values in the low micromolar range towards the TRAP 5b isoform, but was not able to inhibit the TRAP 5a isoform. Structure-based docking revealed similar interactions of CD13 with the active site in both TRAP isoforms. In stably TRAP-overexpressing MDA-MB-231 breast cancer cells, CD13 inhibited intracellular TRAP activity and showed no cytotoxicity at 200 µM. Furthermore, CD13 selectively blocked the TRAP 5b isoform compared to the TRAP 5a in cultured cells, indicating the usefulness of CD13 for assessing the different biological functions of the two TRAP isoforms 5a and 5b in cell systems. Moreover, inhibition of cell migration and invasion of stably TRAP-overexpressing MDA-MB-231 by CD13 was observed. These data establish a proof of principle that a small chemical inhibitor of the TRAP enzyme can block TRAP-dependent functions in cancer cells.

Binuclear metallohydrolases: complex mechanistic strategies for a simple chemical reaction

Binuclear metallohydrolases are a large family of enzymes that require two closely spaced transition metal ions to carry out a plethora of hydrolytic reactions. Representatives include purple acid phosphatases (PAPs), enzymes that play a role in bone metabolism and are the only member of this family with a heterovalent binuclear center in the active form (Fe(3+)-M(2+), M = Fe, Zn, Mn). Other members of this family are urease, which contains a di-Ni(2+) center and catalyzes the breakdown of urea, arginase, which contains a di-Mn(2+) center and catalyzes the final step in the urea cycle, and the metallo-β-lactamases, which contain a di-Zn(2+) center and are virulence factors contributing to the spread of antibiotic-resistant pathogens. Binuclear metallohydrolases catalyze numerous vital reactions and are potential targets of drugs against a wide variety of human disorders including osteoporosis, various cancers, antibiotic resistance, and erectile dysfunctions. These enzymes also tend to catalyze more than one reaction. An example is an organophosphate (OP)-degrading enzyme from Enterobacter aerogenes (GpdQ). Although GpdQ is part of a pathway that is used by bacteria to degrade glycerolphosphoesters, it hydrolyzes a variety of other phosphodiesters and displays low levels of activity against phosphomono- and triesters. Such a promiscuous nature may have assisted the apparent recent evolution of some binuclear metallohydrolases to deal with situations created by human intervention such as OP pesticides in the environment. OP pesticides were first used approximately 70 years ago, and therefore the enzymes that bacteria use to degrade them must have evolved very quickly on the evolutionary time scale. The promiscuous nature of enzymes such as GpdQ makes them ideal candidates for the application of directed evolution to produce new enzymes that can be used in bioremediation and against chemical warfare. In this Account, we review the mechanisms employed by binuclear metallohydrolases and use PAP, the OP-degrading enzyme from Agrobacterium radiobacter (OPDA), and GpdQ as representative systems because they illustrate both the diversity and similarity of the reactions catalyzed by this family of enzymes. The majority of binuclear metallohydrolases utilize metal ion-activated water molecules as nucleophiles to initiate hydrolysis, while some, such as alkaline phosphatase, employ an intrinsic polar amino acid. Here we only focus on catalytic strategies applied by the former group.

α-Substituted β-oxa isosteres of fosmidomycin: synthesis and biological evaluation

Specific inhibition of enzymes of the non-mevalonate pathway is a promising strategy for the development of novel antiplasmodial drugs. α-Aryl-substituted β-oxa isosteres of fosmidomycin with a reverse orientation of the hydroxamic acid group were synthesized and evaluated for their inhibitory activity against recombinant 1-deoxy-d-xylulose 5-phosphate reductoisomerase (IspC) of Plasmodium falciparum and for their in vitro antiplasmodial activity against chloroquine-sensitive and resistant strains of P. falciparum . The most active derivative inhibits IspC protein of P. falciparum (PfIspC) with an IC(50) value of 12 nM and shows potent in vitro antiplasmodial activity. In addition, lipophilic ester prodrugs demonstrated improved P. falciparum growth inhibition in vitro.

Synthesis of α-Arylphosphonates Using Copper-Catalyzed α-Arylation and Deacylative α-Arylation of β-Ketophosphonates

Efficient methods for the direct arylation and deacylative arylation of β-ketophosphonates with iodoarenes in presence of a copper(I) or a copper(II) salt as the catalysts were developed. The corresponding α-arylphosphonates were obtained in high yields. A tentative mechanism for the deacylative arylation reaction was proposed on the basis of the experimental data.