Hydroxy- and Amino-Phosphonates and -Bisphosphonates: Synthetic Methods and Their Biological Applications

Phosphonates and bisphosphonates are stable analogs of phosphates and pyrophosphates that are characterized by one and two carbon–phosphorus bonds, respectively. Among the various phosphonates and bisphosphonates, hydroxy and amino substitutes are of interest as effective in medicinal and industrial chemistry. For example, hydroxy bisphosphonates have proven to be effective for the prevention of bone loss, especially in osteoporotic disease. On the other hand, different substitutions on the carbon atom connected to phosphorus have led to the synthesis of many different hydroxy- and amino-phosphonates and -bisphosphonates, each with its distinct physical, chemical, biological, therapeutic, and toxicological characteristics. Dialkyl or aryl esters of phosphonate and bisphosphonate compounds undergo the hydrolysis process readily and gave valuable materials with wide applications in pharmaceutical and agriculture. This review aims to demonstrate the ongoing preparation of various classes of hydroxy- and amino-phosphonates and -bisphosphonates. Furthermore, the current review summarizes and comprehensively describes articles on the biological applications of hydroxyl- and amino-phosphonates and -bisphosphonates from 2015 until today.

Synthetic Procedures Leading towards Aminobisphosphonates

Growing interest in the biological activity of aminobisphosphonates has stimulated the development of methods for their synthesis. Although several general procedures were previously elaborated to reach this goal, aminobisphosphonate chemistry is still developing quite substantially. Thus, innovative modifications of the existing commonly used reactions, as well as development of new procedures, are presented in this review, concentrating on recent achievements. Additionally, selected examples of aminobisphosphonate derivatization illustrate their usefulness for obtaining new diagnostic and therapeutic agents.

[Inhibition of alkaline phosphatase by thioureido derivatives of methylenebisphosphonic acid]

A series of thioureido derivatives of methylenebisphosphonic acid were synthesized by the reaction of aminomethylenebisphosphonic acid with the corresponding isothiocyanates, and their effect on the activity of alkaline phosphatases from bovine small intestine mucosa (BSIM) and human placenta was studied. It was found that (3-phenylthioureido)methylenebisphosphonate is approximately one order of magnitude more effective in inhibiting the activity of alkaline phosphatase from BSIM than the alkyl derivatives of thioureidomethylenebisphosphonic acid with methyl, ethyl, tert-butyl, or cyclohexyl substituents. The introduction of substituents into the benzene ring of (3-phenylthioureido)methylenebisphosphonate decreased the effect of the inhibitor on the activity of the enzyme. The affinity of (3-phenylureido)methylenebisphosphonate to the alkaline phosphatase of BSIM was also weaker as compared with the corresponding thioureidomethylenebisphosphonate. The insertion of thioureidobisphosphonates into the active site of alkaline phosphatase of human placenta by the method of molecular docking indicated that the methylenebisphosphonate residue and the substituted amino groups of the inhibitor are involved in the mechanisms of complex formation with the enzyme. It is supposed that the improvement of the inhibitory activity of (3-phenylthioureido)methylenebisphosphonate toward alkaline phosphatase of BSIM is due to the additional fixation of the phenyl substituent in the active site of the enzyme.

Determination of lesion to normal bone uptake ratios of skeletal radiopharmaceuticals by QARG (quantitative autoradiography)

An animal model was developed to determine the ratio of uptake of different bone seeking radiotracers in skeletal lesions relative to normal bone in rats by dual tracer quantitative autoradiography (QARG). The lesion to normal bone uptake ratios (L/NB ratios) of 99mTc-MDP, 99mTc-HDP and 99mTc-DMAD were compared using 153Sm-EDTMP as an internal standard. The results from these studies demonstrated that the L/NB ratio of 99mTc-DMAD was an average of 54 and 71% higher than 99mTc-MDP and 99mTc-HDP, respectively and 79% higher than measured for 153Sm-EDTMP.

131I labeled diphosphonates for the palliative treatment of bone metastases--IV. Syntheses of benzylidenediphosphonates and their organ distribution in rats

Palliative treatment of bone pain induced by disseminated bone metastases can be performed with osteotropic, beta(-)-emitting radionuclides. Newly developed 131I labeled benzylidenediphosphonic acid (BDP) derivatives show osteotropic characteristics which suggest that they might possibly be useful radiopharmaceuticals for that purpose. Six BDP derivatives were synthesized with H, OH or NH2 in the 4- and alpha-position. Syntheses were performed by the formal addition of 2 mol of phosphorous acid in the presence of PBr3 to 1 mol of the respective benzonitrile. Transformation of the 4-methoxy and 4-nitro substituents, which were stable during the diphosphonate formation, to 4-HO and 4-NH2 was achieved by hydrolytic ether cleavage in boiling HBr and catalytic hydrogenation with Pd/C, respectively. Transformation of the alpha-amino to alpha-hydroxy group was achieved by the action of NaNO2 in HCl. 4-Hydroxybenzylidenediphosphonic acid (9) was formed unexpectedly during the reaction of 4-hydroxybenzoic acid with H3PO3/PBr3. The addition of 2 mol of phosphorous acid to the benzoic acid was accompanied by an additional reduction at the alpha-carbon. The new BDP derivatives were analyzed by HPLC, NMR and elemental analysis. After labeling with 131I the BDP derivatives were tested in female Sprague-Dawley rats to obtain organ uptake and kinetic data. The various substituents showed an influence on the bone affinity and the uptake in other organs. Among the BDP derivatives tested alpha-amino-(3-[131I]iodo-4-hydroxybenzylidene)diphosphonate (4a) showed the best biological characteristics.